Gear-Heads! Altering Compression Ratio Another Way...
02-14-2004, 09:24 PM
#1
Three Wheelin'
Thread Starter
Gear-Heads! Altering Compression Ratio Another Way...
Everyone it seems is building lower compression ratio blower motors, and I’m feeling left out. How can I accomplish the same result without rebuilding an otherwise perfectly healthy 10:1 engine?
One day while musing, an idea finally dawned upon me, and the gist of it revolves around camshaft timing, specifically, the point in time when the intake valve closes where actual compression pressure can finally begin building on the compression stroke.
First, a compression ratio refresher.
‘Static’ compression ratio is simply the volume of the entire cylinder above the piston at BDC, divided by the volume above the piston at TDC. Using those measurements will give one the engine’s static, or factory rated compression ratio; the S4’s static cr is 10:1.
Let’s calculate a simple static compression ratio.
To visualize the process, draw a cylinder with a top and bottom, and let’s say it displaces 9 cubic inches. Now draw on top of the cylinder a ‘hockey puck’ representing the combustion chamber and piston dish whose total volume is 1 cubic inch. Now when the piston is at BDC of our imaginary cylinder, the resulting compression ratio at that point is the entire cylinder volume of 9 cubic inches, plus the head and piston volume of 1 cubic inch, divided by they head and piston volume, or:
cr = (cyl vol + head vol) / (head vol)
cr = (9ci + 1ci) / (1ci) = 10:1 cr
As one can see, static compression ratio is pretty simple to calculate. In reality though, it is not representative of an engine’s ‘true’ generated compression pressure. The engine’s ‘true’ compression ratio, however, can be calculated, and it is sometimes referred to as an engine’s ‘dynamic’ compression ratio, or ‘actual’ compression ratio. Just what is this ‘dynamic/actual’ compression ratio, and why is it important?
The ‘actual’ compression ratio of an engine is primarily a function of when the intake valve closes after BDC during the transition from the intake stroke to the compression stroke.
Once the intake closes, compression pressure can finally begin, and is calculated from where (..how high above BDC) the piston is in the bore at that time; this is the ‘actual’ compression ratio of an engine with it’s currently installed camshaft. The longer the intake valve is held open, the higher in the bore the piston travels which decreases the volume above the piston that can be used to subsequently compress the air/fuel charge once the intake valve closes.
This is important to recognize because virtually all camshafts hold the intake valve open past BDC to aid cylinder filling due to the charge’s inertia at higher speeds allowing more fuel into the cylinder, despite the fact the piston is moving upward in the bore.
The longer the intake is held open, the more top-end horsepower is improved, and likewise, low-end power is compromised because of the air/fuel charge being pushed out the intake valve at lower engine speeds which means there’s less air/fuel to combust and push on the piston on the power stroke.
Let’s do another compression ratio calc to illustrate dynamic compression ratio.
Take that same cylinder and head drawing made earlier, and this time draw a line that’s 3/10’s the way up from the bottom. That line represents where the top of the piston is in the cylinder bore when the intake valve closes. Let’s say that at that point, the remaining cylinder volume above the imaginary piston is 7 cubic inches. Using the same calcs as before, we can calculate the dynamic compression ratio, and we get:
cr = (cyl vol + head vol) / (head vol)
cr = (7ci + 1ci) / (1ci) = 8:1 cr
That’s quite a difference from the static compression ratio of 10:1, although it’s just an example.
So let’s put this information into perspective by comparing the various dynamic compression ratios produced by the factory Porsche 928 32v camshafts.
In order to do that, we’ll need to calculate where in the bore the piston is when the intake valve closes for the various cams. In order to do that, we’ll need to know the intake cam timing specs, and, make trig do some work for us.
Cam specs, intake closes ABDC in crank degrees:
36 degrees ABDC – S4
42 degrees ABDC - GT
50 degrees ABDC – 85-86
61 degrees ABDC – GTS
Right away, one can see the great disparity of 25 crank degrees between the S4 camshaft and the GTS’s. To help picture the significance of 61 degrees of crank rotation before the intake closes, the piston has already traveled 1/3 of the way up the bore without compressing the charge! On the other hand, the piston traveled only roughly 1/6 of the way up the bore with the S4 cam. With that thought in mind, what will the difference yield in the form of actual compression ratios between the two?
**** EDIT - Corrected the Paragraph Above: ****
Right away, one can see the great disparity of 25 crank degrees between the S4 camshaft and the GTS’s. To help picture the significance of 61 degrees of crank rotation before the intake closes, the crankshaft has already rotated 1/3 of the way on the compression stroke. On the other hand, the crankshaft rotated roughly only 1/6 of the way on the compression stroke with the S4 cam. With that thought in mind, what will the difference yield in the form of actual compression ratios between the two?
**** END EDIT ****
Without boring you more with how the calcs are performed, the calcs were made using the stock specs of an S4 of a 100mm bore, 78.9mm stroke, and 150mm rod length.
The actual compression ratios for the 928 32v camshafts in an S4:
36 degrees ABDC yields an actual cr of 9.35:1
42 degrees ABDC yields an actual cr of 9.11:1
50 degrees ABDC yields an actual cr of 8.74:1
61 degrees ABDC yields an actual cr of 8.14:1
Notice the difference between the S4 and GTS cams, a 1.21 point difference of actual compression ratios, a HUGE difference!
This well-known phenomenon is the reason why the aftermarket cam manufacturers recommend increasing the compression ratio of the target engine if it is not already high enough when using one of their bigger grinds which has a late closing intake.
The ‘downside’ is that the later closing intake reduces the thermal efficiency of the engine because of the reduced actual compression ratio, and is one reason why installing a big cam on a 8:1 static cr engine ends up making the engine a dog since besides reducing low-speed power due to the intake charge being forced back into the inlet tract (..intake reversion), the resulting actual compression is now even lower than before on an already low 8:1 static compression ratio engine.
However, I am counting on this ‘downside’ to manifest itself on my 10:1 static cr engine when I install the GTS intake cams in my car (..as well as 85-86/GT exhaust cams). Once installed, I should be able to safely run quite a bit more boost without tearing down my engine!
On the other hand, let’s suppose one has a 10:1 static cr engine and an attempt is made to alter the cam’s timing to gain a little more low to mid-range power by advancing the cams. Advancing a camshaft will cause the valves to open sooner, and likewise, close sooner. Can you see where I’m going with this? Since the intake valve is closing sooner, the engine’s actual/dynamic compression ratio increases, and the end result may very well be that the engine will be more susceptible to detonation. As you can see, it goes both ways when altering intake valve timing.
Furthermore, if an engine is experiencing detonation, it is quite possible to eliminate it by retarding the camshaft timing which will cause the valves to open later, thus close later, which will reduce the engine's actual/dynamic compression ratio. Going after detonation this way versus taking timing out of the ignition system is a much better way to go from a power-production-perspective.
Hold on just a second...
Snap!, ..snap!, ...zzzzzzz-zip!, ...buckle!, buckle! Alright, flame-suit on and, *beeeep!* ..activated!
Well, that is the plan; what do you guys think?
One day while musing, an idea finally dawned upon me, and the gist of it revolves around camshaft timing, specifically, the point in time when the intake valve closes where actual compression pressure can finally begin building on the compression stroke.
First, a compression ratio refresher.
‘Static’ compression ratio is simply the volume of the entire cylinder above the piston at BDC, divided by the volume above the piston at TDC. Using those measurements will give one the engine’s static, or factory rated compression ratio; the S4’s static cr is 10:1.
Let’s calculate a simple static compression ratio.
To visualize the process, draw a cylinder with a top and bottom, and let’s say it displaces 9 cubic inches. Now draw on top of the cylinder a ‘hockey puck’ representing the combustion chamber and piston dish whose total volume is 1 cubic inch. Now when the piston is at BDC of our imaginary cylinder, the resulting compression ratio at that point is the entire cylinder volume of 9 cubic inches, plus the head and piston volume of 1 cubic inch, divided by they head and piston volume, or:
cr = (cyl vol + head vol) / (head vol)
cr = (9ci + 1ci) / (1ci) = 10:1 cr
As one can see, static compression ratio is pretty simple to calculate. In reality though, it is not representative of an engine’s ‘true’ generated compression pressure. The engine’s ‘true’ compression ratio, however, can be calculated, and it is sometimes referred to as an engine’s ‘dynamic’ compression ratio, or ‘actual’ compression ratio. Just what is this ‘dynamic/actual’ compression ratio, and why is it important?
The ‘actual’ compression ratio of an engine is primarily a function of when the intake valve closes after BDC during the transition from the intake stroke to the compression stroke.
Once the intake closes, compression pressure can finally begin, and is calculated from where (..how high above BDC) the piston is in the bore at that time; this is the ‘actual’ compression ratio of an engine with it’s currently installed camshaft. The longer the intake valve is held open, the higher in the bore the piston travels which decreases the volume above the piston that can be used to subsequently compress the air/fuel charge once the intake valve closes.
This is important to recognize because virtually all camshafts hold the intake valve open past BDC to aid cylinder filling due to the charge’s inertia at higher speeds allowing more fuel into the cylinder, despite the fact the piston is moving upward in the bore.
The longer the intake is held open, the more top-end horsepower is improved, and likewise, low-end power is compromised because of the air/fuel charge being pushed out the intake valve at lower engine speeds which means there’s less air/fuel to combust and push on the piston on the power stroke.
Let’s do another compression ratio calc to illustrate dynamic compression ratio.
Take that same cylinder and head drawing made earlier, and this time draw a line that’s 3/10’s the way up from the bottom. That line represents where the top of the piston is in the cylinder bore when the intake valve closes. Let’s say that at that point, the remaining cylinder volume above the imaginary piston is 7 cubic inches. Using the same calcs as before, we can calculate the dynamic compression ratio, and we get:
cr = (cyl vol + head vol) / (head vol)
cr = (7ci + 1ci) / (1ci) = 8:1 cr
That’s quite a difference from the static compression ratio of 10:1, although it’s just an example.
So let’s put this information into perspective by comparing the various dynamic compression ratios produced by the factory Porsche 928 32v camshafts.
In order to do that, we’ll need to calculate where in the bore the piston is when the intake valve closes for the various cams. In order to do that, we’ll need to know the intake cam timing specs, and, make trig do some work for us.
Cam specs, intake closes ABDC in crank degrees:
36 degrees ABDC – S4
42 degrees ABDC - GT
50 degrees ABDC – 85-86
61 degrees ABDC – GTS
Right away, one can see the great disparity of 25 crank degrees between the S4 camshaft and the GTS’s. To help picture the significance of 61 degrees of crank rotation before the intake closes, the piston has already traveled 1/3 of the way up the bore without compressing the charge! On the other hand, the piston traveled only roughly 1/6 of the way up the bore with the S4 cam. With that thought in mind, what will the difference yield in the form of actual compression ratios between the two?
**** EDIT - Corrected the Paragraph Above: ****
Right away, one can see the great disparity of 25 crank degrees between the S4 camshaft and the GTS’s. To help picture the significance of 61 degrees of crank rotation before the intake closes, the crankshaft has already rotated 1/3 of the way on the compression stroke. On the other hand, the crankshaft rotated roughly only 1/6 of the way on the compression stroke with the S4 cam. With that thought in mind, what will the difference yield in the form of actual compression ratios between the two?
**** END EDIT ****
Without boring you more with how the calcs are performed, the calcs were made using the stock specs of an S4 of a 100mm bore, 78.9mm stroke, and 150mm rod length.
The actual compression ratios for the 928 32v camshafts in an S4:
36 degrees ABDC yields an actual cr of 9.35:1
42 degrees ABDC yields an actual cr of 9.11:1
50 degrees ABDC yields an actual cr of 8.74:1
61 degrees ABDC yields an actual cr of 8.14:1
Notice the difference between the S4 and GTS cams, a 1.21 point difference of actual compression ratios, a HUGE difference!
This well-known phenomenon is the reason why the aftermarket cam manufacturers recommend increasing the compression ratio of the target engine if it is not already high enough when using one of their bigger grinds which has a late closing intake.
The ‘downside’ is that the later closing intake reduces the thermal efficiency of the engine because of the reduced actual compression ratio, and is one reason why installing a big cam on a 8:1 static cr engine ends up making the engine a dog since besides reducing low-speed power due to the intake charge being forced back into the inlet tract (..intake reversion), the resulting actual compression is now even lower than before on an already low 8:1 static compression ratio engine.
However, I am counting on this ‘downside’ to manifest itself on my 10:1 static cr engine when I install the GTS intake cams in my car (..as well as 85-86/GT exhaust cams). Once installed, I should be able to safely run quite a bit more boost without tearing down my engine!
On the other hand, let’s suppose one has a 10:1 static cr engine and an attempt is made to alter the cam’s timing to gain a little more low to mid-range power by advancing the cams. Advancing a camshaft will cause the valves to open sooner, and likewise, close sooner. Can you see where I’m going with this? Since the intake valve is closing sooner, the engine’s actual/dynamic compression ratio increases, and the end result may very well be that the engine will be more susceptible to detonation. As you can see, it goes both ways when altering intake valve timing.
Furthermore, if an engine is experiencing detonation, it is quite possible to eliminate it by retarding the camshaft timing which will cause the valves to open later, thus close later, which will reduce the engine's actual/dynamic compression ratio. Going after detonation this way versus taking timing out of the ignition system is a much better way to go from a power-production-perspective.
Hold on just a second...
Snap!, ..snap!, ...zzzzzzz-zip!, ...buckle!, buckle! Alright, flame-suit on and, *beeeep!* ..activated!
Well, that is the plan; what do you guys think?
Last edited by Lagavulin; 03-31-2004 at 01:34 PM.
02-15-2004, 02:31 AM
#2
Addict
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Well, that is the plan; what do you guys think?
Thats my approach to all this so far, cuz nobody's gonna tell ya.
Ive read up on that stuff as well, should work. If not, swap the cams back!
simple enough and you still havent "torn" into the motor.
Better start before the "thaw" comes in...uh..June?
02-15-2004, 02:39 AM
#3
Inventor
Rennlist Member
Rennlist Member
I did a portion of reading on cams when I was thinking about doing this:
From what I could gather, advancing the cams increases the effective compression which in turn creates more torque, at a lower engine speed. But, as there is less time to fill and empty the cylinders at high rpms with the cams advanced, overall horsepower is reduced.
On the dyno, with my cams advanced ~3 degrees, I traded 5 horsepower for torque, but the torque peak came ~600RPM sooner.
At ~6 degrees advance, I would bark the tires at almost every stoplight if I wasn't careful with the pedal.
I found that it was hard to keep a smooth idle with a lot of advance so I run ~4 degrees now.
...
I have thought that if/when I get a supercharger, I would try retarding the cams, the theory being that there will more time to exhaust, and the supercharger will make up for the lost low end power.
...
As to the topic; you could just retard the intake cam to reduce the effective compression to run higher boost?
From what I could gather, advancing the cams increases the effective compression which in turn creates more torque, at a lower engine speed. But, as there is less time to fill and empty the cylinders at high rpms with the cams advanced, overall horsepower is reduced.
On the dyno, with my cams advanced ~3 degrees, I traded 5 horsepower for torque, but the torque peak came ~600RPM sooner.
At ~6 degrees advance, I would bark the tires at almost every stoplight if I wasn't careful with the pedal.
I found that it was hard to keep a smooth idle with a lot of advance so I run ~4 degrees now.
...
I have thought that if/when I get a supercharger, I would try retarding the cams, the theory being that there will more time to exhaust, and the supercharger will make up for the lost low end power.
...
As to the topic; you could just retard the intake cam to reduce the effective compression to run higher boost?
02-15-2004, 03:09 AM
#4
Inventor
Rennlist Member
Rennlist Member
What's the difference in lift between the different model cams?
(An aggressive lift will reduce the limit of safely advancing or retarding the cams.)
(An aggressive lift will reduce the limit of safely advancing or retarding the cams.)
02-15-2004, 03:17 AM
#5
Burning Brakes
Join Date: Aug 2001
Posts: 1,117
Likes: 0
Received 0 Likes
on
0 Posts
Lag,
If I'm reading you correctly...
You have (basically) found your way to 'Miller Cycle'. It has only been used (IIRC) on a production car on the Mazda Millenia V-6(?) which has factory forced induction,
Blower solves the low-end power issues inherent to that way of managing intake/exhaust flows
Google it, I would imagine you will find some useful data.
Greg
If I'm reading you correctly...
You have (basically) found your way to 'Miller Cycle'. It has only been used (IIRC) on a production car on the Mazda Millenia V-6(?) which has factory forced induction,
Blower solves the low-end power issues inherent to that way of managing intake/exhaust flows
Google it, I would imagine you will find some useful data.
Greg
02-15-2004, 09:26 AM
#6
Banned
Join Date: Feb 2002
Location: Ft. Lauderdale FLORIDA
Posts: 5,248
Likes: 0
Received 4 Likes
on
3 Posts
Lagavulin:
Interesting idea…and it makes me wonder if part of the logic of Porsche’s Vario-cam system might be based upon this! If not, it might be possible to rig some sort of Vario-cam system to the S4 engine that uses an input from the knock sensors….
Anyway, I had some questions about your math. When I did the numbers, I discovered that the pistons in the GTS engine had not moved 1/3 [33.3%] of the way up the bore when the intake valves had closed at 61 degrees after bottom dead center. In fact, when I did the trigonometry, I discovered the following:
Pistons have moved the following amounts up the bore at intake valve closure-
S4: 9.4%
GT: 12.5%
85: 18%
GTS: 26%
[I think! There is a thing called "dwell time", which is caused by long connecting rods. It is possible that the pistons have moved even less than that]
-If you then go even farther into trigonometry [twisting my brain into even worse knots!] then you discover the following:
Effective chamber volume at intake valve closure-
S4: 71.5 mm stroke = 561.56 cc
GT: 69.0 mm stroke = 541.92 cc
85: 64.7 mm stroke = 508.15 cc
GTS: 65.1 mm stroke = 511.45 cc [applied to 88.0 mm stroke]
GTS: 58.4 mm stroke = 458.67 cc [applied to 78.9 mm stroke]
I guess I don’t completely understand how you are computing compression ratio- my “hockey puck” volume came out to 68.85 cc’s, but in the end my computed compression ratios suggested that by going to the GTS intake cams your compression drop would actually be 1.5 points instead of 1.21.
?
Anyway, there is one other think to think about- the centrifugal-type blower that you’ve mounted. This device works by increasing the velocity of the intake air- compression takes place farther down the system [actually, in the scroll] when the air slows down and velocity is exchanged for pressure. The blades of this device need to have the air smoothly flow in….and smoothly flow out. Any disruption in the flow causes the blades to aerodynamically stall, and instead of moving air, it is now simply churning it. This is why you’ve installed that blow-off valve- to prevent compressor stalls during shifts.
Believe me, from flying jets that are often times as old as I am…I’m well acquainted with compressor stalls~
The 2.3 liter Miller-Cycle V6 in the Mazda Millenia used [I think….] a Whipple-type blower in order to get around this problem. These are not subject to compressor stalls.
What I wonder about is that if you change the cam timing such that you now have quite a bit of back pressure applied to the intake charge by the rising piston [because the intake valve is open longer], if you might run into these compressor stalls, or wind up having your pop-off valve opening all the time. Food for thought…
B safe!
N!
’85 S2 5 Speed
Interesting idea…and it makes me wonder if part of the logic of Porsche’s Vario-cam system might be based upon this! If not, it might be possible to rig some sort of Vario-cam system to the S4 engine that uses an input from the knock sensors….
Anyway, I had some questions about your math. When I did the numbers, I discovered that the pistons in the GTS engine had not moved 1/3 [33.3%] of the way up the bore when the intake valves had closed at 61 degrees after bottom dead center. In fact, when I did the trigonometry, I discovered the following:
Pistons have moved the following amounts up the bore at intake valve closure-
S4: 9.4%
GT: 12.5%
85: 18%
GTS: 26%
[I think! There is a thing called "dwell time", which is caused by long connecting rods. It is possible that the pistons have moved even less than that]
-If you then go even farther into trigonometry [twisting my brain into even worse knots!] then you discover the following:
Effective chamber volume at intake valve closure-
S4: 71.5 mm stroke = 561.56 cc
GT: 69.0 mm stroke = 541.92 cc
85: 64.7 mm stroke = 508.15 cc
GTS: 65.1 mm stroke = 511.45 cc [applied to 88.0 mm stroke]
GTS: 58.4 mm stroke = 458.67 cc [applied to 78.9 mm stroke]
I guess I don’t completely understand how you are computing compression ratio- my “hockey puck” volume came out to 68.85 cc’s, but in the end my computed compression ratios suggested that by going to the GTS intake cams your compression drop would actually be 1.5 points instead of 1.21.
?
Anyway, there is one other think to think about- the centrifugal-type blower that you’ve mounted. This device works by increasing the velocity of the intake air- compression takes place farther down the system [actually, in the scroll] when the air slows down and velocity is exchanged for pressure. The blades of this device need to have the air smoothly flow in….and smoothly flow out. Any disruption in the flow causes the blades to aerodynamically stall, and instead of moving air, it is now simply churning it. This is why you’ve installed that blow-off valve- to prevent compressor stalls during shifts.
Believe me, from flying jets that are often times as old as I am…I’m well acquainted with compressor stalls~
The 2.3 liter Miller-Cycle V6 in the Mazda Millenia used [I think….] a Whipple-type blower in order to get around this problem. These are not subject to compressor stalls.
What I wonder about is that if you change the cam timing such that you now have quite a bit of back pressure applied to the intake charge by the rising piston [because the intake valve is open longer], if you might run into these compressor stalls, or wind up having your pop-off valve opening all the time. Food for thought…
B safe!
N!
’85 S2 5 Speed
02-15-2004, 05:25 PM
#7
Have you acquired the GTS and 85/86 cams yet Lag? I'm still trying to get a good idea of what needs to be changed to make them fit.
This is a great idea too. Maybe I can get Forged pistons in a higher compression ratio - like 9.2 or more instead of my 9.0 or less. I was planning on running the 85.86 cams throughout, as GTS cams are a bit more difficult to get, from what I have seen.
This is a great idea too. Maybe I can get Forged pistons in a higher compression ratio - like 9.2 or more instead of my 9.0 or less. I was planning on running the 85.86 cams throughout, as GTS cams are a bit more difficult to get, from what I have seen.
Trending Topics
02-15-2004, 07:27 PM
#8
Well Lag, I think I posted something about the mixing of the GtS and Gt cams so time ago, but in a high rpm normally aspirated motor. You are of course correct about the bleed off of cylinder pressuse. You have effectively also created an engine with a wide lobe displacement angle. These engines do best with great flowing heads.
Well we have these and of course they can be made better. On top of this you have a supercharger. Sounds good so far. The issue I would raise is this: What happens when the revs rise? The cylinders start filling better and the charge is not pushed back out through the intake valve. The whole idea of a late closing intake is for better volumetric efficiency at high engine speeds. So then doesn't this cause trouble higher up in the rpm range?
Well we have these and of course they can be made better. On top of this you have a supercharger. Sounds good so far. The issue I would raise is this: What happens when the revs rise? The cylinders start filling better and the charge is not pushed back out through the intake valve. The whole idea of a late closing intake is for better volumetric efficiency at high engine speeds. So then doesn't this cause trouble higher up in the rpm range?
02-15-2004, 08:40 PM
#9
Burning Brakes
Join Date: Jul 2003
Location: IL
Posts: 1,166
Likes: 0
Received 0 Likes
on
0 Posts
Excellent idea!
This is a very good thread.
Thanks to Lag and others for useful insight!
So what kind of cam timing changes would it take to reduce an 86' 32v engine down enough to run 15 psi? Being that I have an automatic, I will probably be running an Eaton/Jag SC unit for more low end torque.
I guess the answer starts with:
How much Intake/exhaust timing adjustment will give 1 point below 10:1?
What detrimental effect will it have, and how badly will it affect the situation?
How far could you change the timing before it substantially effects the engines operation?
Thanks to Lag and others for useful insight!
So what kind of cam timing changes would it take to reduce an 86' 32v engine down enough to run 15 psi? Being that I have an automatic, I will probably be running an Eaton/Jag SC unit for more low end torque.
I guess the answer starts with:
How much Intake/exhaust timing adjustment will give 1 point below 10:1?
What detrimental effect will it have, and how badly will it affect the situation?
How far could you change the timing before it substantially effects the engines operation?
02-16-2004, 12:06 AM
#10
Three Wheelin'
Thread Starter
By Porken:
From what I could gather, advancing the cams increases the effective compression which in turn creates more torque, at a lower engine speed. But, as there is less time to fill and empty the cylinders at high rpms with the cams advanced, overall horsepower is reduced.
From what I could gather, advancing the cams increases the effective compression which in turn creates more torque, at a lower engine speed. But, as there is less time to fill and empty the cylinders at high rpms with the cams advanced, overall horsepower is reduced.
By Porken:
What's the difference in lift between the different model cams?
What's the difference in lift between the different model cams?
205/195 - .354/.314 = S4
219/205 - .393/.353 = 85
219/205 - .393/.353 = GT
228/196 - .374/.334 = GTS
Note that the 85 and GT cams look identical; however, the intake timing on the GT cam is advanced 8 degrees which should give it more low-end at the expense of some top-end versus the 85 cam.
By gbyron:
You have (basically) found your way to 'Miller Cycle'.
Google it, I would imagine you will find some useful data.
You have (basically) found your way to 'Miller Cycle'.
Google it, I would imagine you will find some useful data.
By N!
Anyway, I had some questions about your math. When I did the numbers, I discovered that the pistons in the GTS engine had not moved 1/3 [33.3%] of the way up the bore when the intake valves had closed at 61 degrees after bottom dead center.
Anyway, I had some questions about your math. When I did the numbers, I discovered that the pistons in the GTS engine had not moved 1/3 [33.3%] of the way up the bore when the intake valves had closed at 61 degrees after bottom dead center.
When I got home tonight, the first thing I did was reread my post to look for any errors, and cringed when I read that part again!
Then I read your post and saw that I was officially busted! 
What I meant to say was that the crank had already rotated 1/3 of the way on the compression stroke before the intake valve closed.
Of course, your observation is correct, and thanks for pointing it out.
By N!
Effective chamber volume at intake valve closure-
S4: 71.5 mm stroke = 561.56 cc
GT: 69.0 mm stroke = 541.92 cc
85: 64.7 mm stroke = 508.15 cc
GTS: 65.1 mm stroke = 511.45 cc [applied to 88.0 mm stroke]
GTS: 58.4 mm stroke = 458.67 cc [applied to 78.9 mm stroke]
Effective chamber volume at intake valve closure-
S4: 71.5 mm stroke = 561.56 cc
GT: 69.0 mm stroke = 541.92 cc
85: 64.7 mm stroke = 508.15 cc
GTS: 65.1 mm stroke = 511.45 cc [applied to 88.0 mm stroke]
GTS: 58.4 mm stroke = 458.67 cc [applied to 78.9 mm stroke]
Here are my numbers using the connecting rod length of 150mm:
‘Effective chamber volume at intake valve closure-‘
_S4: 73.17 mm stroke = 643.52 cc
_GT: 71.11 mm stroke = 627.33 cc
_85: 67.88 mm stroke = 602.00 cc
GTS: 62.60 mm stroke = 560.49 cc [applied to 78.9 mm stroke][/QUOTE]
By N!
I guess I don’t completely understand how you are computing compression ratio- my “hockey puck” volume came out to 68.85 cc’s,..
I guess I don’t completely understand how you are computing compression ratio- my “hockey puck” volume came out to 68.85 cc’s,..
By N!
Pistons have moved the following amounts up the bore at intake valve closure-
S4: 9.4%
GT: 12.5%
85: 18%
GTS: 26%
Pistons have moved the following amounts up the bore at intake valve closure-
S4: 9.4%
GT: 12.5%
85: 18%
GTS: 26%
_S4: 05.73mm / 78.9mm = .0726 = 07.26%
_GT: 07.79mm / 78.9mm = .0726 = 09.87%
_85: 11.02mm / 78.9mm = .0726 = 13.97%
GTS: 16.30mm / 78.9mm = .0726 = 20.66%
By N!
but in the end my computed compression ratios suggested that by going to the GTS intake cams your compression drop would actually be 1.5 points instead of 1.21.
but in the end my computed compression ratios suggested that by going to the GTS intake cams your compression drop would actually be 1.5 points instead of 1.21.
By N!
What I wonder about is that if you change the cam timing such that you now have quite a bit of back pressure applied to the intake charge by the rising piston [because the intake valve is open longer], if you might run into these compressor stalls, or wind up having your pop-off valve opening all the time. Food for thought…
What I wonder about is that if you change the cam timing such that you now have quite a bit of back pressure applied to the intake charge by the rising piston [because the intake valve is open longer], if you might run into these compressor stalls, or wind up having your pop-off valve opening all the time. Food for thought…
By Brendan:
Have you acquired the GTS and 85/86 cams yet Lag? I'm still trying to get a good idea of what needs to be changed to make them fit.
Have you acquired the GTS and 85/86 cams yet Lag? I'm still trying to get a good idea of what needs to be changed to make them fit.
By Brendan:
This is a great idea too. Maybe I can get Forged pistons in a higher compression ratio - like 9.2 or more instead of my 9.0 or less. I was planning on running the 85.86 cams throughout, as GTS cams are a bit more difficult to get, from what I have seen.
This is a great idea too. Maybe I can get Forged pistons in a higher compression ratio - like 9.2 or more instead of my 9.0 or less. I was planning on running the 85.86 cams throughout, as GTS cams are a bit more difficult to get, from what I have seen.
By Greg Gray:
Well Lag, I think I posted something about the mixing of the GtS and Gt cams so time ago, but in a high rpm normally aspirated motor.
Well Lag, I think I posted something about the mixing of the GtS and Gt cams so time ago, but in a high rpm normally aspirated motor.
By Greg Gray:
The issue I would raise is this: What happens when the revs rise? The cylinders start filling better and the charge is not pushed back out through the intake valve. The whole idea of a late closing intake is for better volumetric efficiency at high engine speeds. So then doesn't this cause trouble higher up in the rpm range?
The issue I would raise is this: What happens when the revs rise? The cylinders start filling better and the charge is not pushed back out through the intake valve. The whole idea of a late closing intake is for better volumetric efficiency at high engine speeds. So then doesn't this cause trouble higher up in the rpm range?
By Warren928:
So what kind of cam timing changes would it take to reduce an 86' 32v engine down enough to run 15 psi?
So what kind of cam timing changes would it take to reduce an 86' 32v engine down enough to run 15 psi?
By Warren928:
I guess the answer starts with:
How much Intake/exhaust timing adjustment will give 1 point below 10:1?
I guess the answer starts with:
How much Intake/exhaust timing adjustment will give 1 point below 10:1?
Instead, let’s treat the S4 actual compression ratio as a baseline, and see what it’ll take to get the 85-86 cam 1.0 point below the S4’s 9.35 actual compression ratio.
The answer is 57 degrees ABDC, so one’ll have to retard the cam 7 degrees.
By Warren928
What detrimental effect will it have, and how badly will it affect the situation?
How far could you change the timing before it substantially effects the engines operation?
What detrimental effect will it have, and how badly will it affect the situation?
How far could you change the timing before it substantially effects the engines operation?
I’m glad you guys are asking questions since it will help us all think this through more thoroughly.
02-16-2004, 03:34 AM
#11
Nordschleife Master
What happens if you keep the stock intake mannafold?
The intake mannafold is designed to cram air into the cylinder through the use of harmonics.
As the revs. go up, the harmonics get more and more powerful. This results in the wide lobes allowing more and more air to get into the cylinder. At the end of the day, on high reveing cars, it actualy result in more air being cramed into the engine at high loads, and if the valve was closed early.
It sounds like a dubious concept to me.
Not to mention the problems you'll have of it blowing some of the fuel back up the engine intake mannafold. That and the a bit of the hot exaust gases that remain in the cylinder. This can't be good.
The intake mannafold is designed to cram air into the cylinder through the use of harmonics.
As the revs. go up, the harmonics get more and more powerful. This results in the wide lobes allowing more and more air to get into the cylinder. At the end of the day, on high reveing cars, it actualy result in more air being cramed into the engine at high loads, and if the valve was closed early.
It sounds like a dubious concept to me.
Not to mention the problems you'll have of it blowing some of the fuel back up the engine intake mannafold. That and the a bit of the hot exaust gases that remain in the cylinder. This can't be good.
02-16-2004, 08:04 AM
#12
lag in reference to my post and I think it was "Had a crazy idea about camshafts" No I did not dyno it as I don't have a 4 valve motor yet. It is coming though! I was just suprised that nobody has tried it. They are the biggest cams that came from the factory, seems logical to try it.
As far as your question what do I think in regard to the better cylinder filling at high rpms causing a problem with detonation. I think this, I just would say that a certain amount of cylinder pressure will cause detonation through heat.
I will do some research on this matter and get back to this when I can give a better answer. Basically though this is what I think, that the best chance to make it work is with a great intercooler, very clean combustion chamber and and oil cooled pistons. The maximun attention to eliminating hotspots in the chamber will be of vital importance.
As far as your question what do I think in regard to the better cylinder filling at high rpms causing a problem with detonation. I think this, I just would say that a certain amount of cylinder pressure will cause detonation through heat.
I will do some research on this matter and get back to this when I can give a better answer. Basically though this is what I think, that the best chance to make it work is with a great intercooler, very clean combustion chamber and and oil cooled pistons. The maximun attention to eliminating hotspots in the chamber will be of vital importance.
02-19-2004, 07:11 AM
#13
O.k done some homework on this subject and here is what I researched. I looked at Porsches latest hot offering the GT2 and found that it's engine dimensions are very similar to the 928.
Here is the cam timing for a GT2 all measurements are 1mm zero stroke play like the 928.
Inlet opens at 10 degrees BTDC
Inlet closes at 20 degrees ABDC
Exhaust opens at 41 degrees BBDC
Exhaust closes at 9 degrees BTDC
The static comp ratio is 9:4 to 1
Boost is 1 bar
If you retarded a s4 intake cam you would get roughly the same spec as the gt2
The biggest exhaust cam is the GT's but it's not that similar and it is still smaller that the gt2s, From these specs I think you can see that the boosted inlet timing is mild and the exhaust considerably larger. The one way to overcome this problem is to port the exhaust side of the head and have a very nice flowing exhaust system.
Anyway I hope this was interesting, not that there might be many more supercharged 928s anymore.
Here is the cam timing for a GT2 all measurements are 1mm zero stroke play like the 928.
Inlet opens at 10 degrees BTDC
Inlet closes at 20 degrees ABDC
Exhaust opens at 41 degrees BBDC
Exhaust closes at 9 degrees BTDC
The static comp ratio is 9:4 to 1
Boost is 1 bar
If you retarded a s4 intake cam you would get roughly the same spec as the gt2
The biggest exhaust cam is the GT's but it's not that similar and it is still smaller that the gt2s, From these specs I think you can see that the boosted inlet timing is mild and the exhaust considerably larger. The one way to overcome this problem is to port the exhaust side of the head and have a very nice flowing exhaust system.
Anyway I hope this was interesting, not that there might be many more supercharged 928s anymore.
02-19-2004, 07:41 AM
#14
Rennlist Member
From years ago, I remember that the general rule of thumb was that a belt driven supercharger "tames" a cam. Well, seems what you're saying makes sense with this, so slap in a GTS cam which is "wilder" than a S4 cam and have some fun with a supercharger!
(Would custom cams be cheaper and more suited to your purpose?)
But IMOSHO I'd bet you get more anti detonation and potential power from messaging the edges out of the combustion chamber then from this topic.
(Would custom cams be cheaper and more suited to your purpose?)
But IMOSHO I'd bet you get more anti detonation and potential power from messaging the edges out of the combustion chamber then from this topic.
02-22-2004, 10:10 PM
#15
Three Wheelin'
Thread Starter
Originally posted by ViribusUnits
It sounds like a dubious concept to me.
It sounds like a dubious concept to me.
Now it shouldn’t sound so ‘dubious’ to you anymore.
gbyron, thanks for the heads-up on the Miller-cycle.
By Greg Gray:
O.k done some homework on this subject and here is what I researched. I looked at Porsches latest hot offering the GT2 and found that it's engine dimensions are very similar to the 928.
Here is the cam timing for a GT2 all measurements are 1mm zero stroke play like the 928.
Inlet opens at 10 degrees BTDC
Inlet closes at 20 degrees ABDC
Exhaust opens at 41 degrees BBDC
Exhaust closes at 9 degrees BTDC
The static comp ratio is 9:4 to 1
Boost is 1 bar
If you retarded a s4 intake cam you would get roughly the same spec as the gt2
O.k done some homework on this subject and here is what I researched. I looked at Porsches latest hot offering the GT2 and found that it's engine dimensions are very similar to the 928.
Here is the cam timing for a GT2 all measurements are 1mm zero stroke play like the 928.
Inlet opens at 10 degrees BTDC
Inlet closes at 20 degrees ABDC
Exhaust opens at 41 degrees BBDC
Exhaust closes at 9 degrees BTDC
The static comp ratio is 9:4 to 1
Boost is 1 bar
If you retarded a s4 intake cam you would get roughly the same spec as the gt2
10 degrees BTDC – Intake opens
15 degrees ABDC – Intake closes
One can see that the duration is 5 degrees short though.
By Greg Gray:
The biggest exhaust cam is the GT's but it's not that similar and it is still smaller that the gt2s,
The biggest exhaust cam is the GT's but it's not that similar and it is still smaller that the gt2s,
40 degrees BBDC – Exhaust Opens
15 degrees BTDC – Exhaust Closes
This time the exhaust duration is short 6 degrees.
Unfortunately though, re-timing the 32v cams are not as flexible as one would hope. Timing the cams independent of one another is not possible as it is with other 4 cam engines, unless of course, one skips a tooth on the cam chain sprocket which will change the timing +/- 20 degrees in relation to the other. The granularity is much too coarse.
When I do install the GTS intake and GT exhaust cams, I will advance them one cam-belt-tooth for 7.5 degrees.
Of course, that will cause my intake valve to close sooner at 53.5 degrees ABDC for a dynamic compression ratio of 8.56:1, which is still quite a bit better by 0.79 points over the S4’s 9.35:1.