I have one of the last of Andy's kits and will be be having it installed about 11/04/05. How high can the effective CR be on pump gas, intercooled and not intercooled? Conventional wisdom dictates staying at or lower than 12.5:1 but I've seen some references to as high as 16:1 or 18:1. 5 pounds of boost with a 10:1 CR yields an effective CR of 13.4:1. Has anyone calculated dynamic CR?
Also should there be any while I'm in there I should do this?
Thanks,
Rick
85 32v (currently 291 rwhp)

 

Nice! When did you get yours shipped to you?

I guess effective or dynamic CR might be important.
I am going with what field tests have been done and watching my guages closely to avoid problems. I will be listening for pinging and that rules out a loud exhaust for engine safety purposes.

I am not sure if there is any way to use guages for detonation on 85-86's. Usually 12 to 1 requires race gas, so I am not sure if CR ratios all add up like you would initially think. Many people are running 9 psi on their SC's with premium 91 gas.

IF they are running less octane than 91, I would like to hear from them.

My kit is supposed to be able to run on premium gas at 6.7 psi, and no IC. Testing it will be the only way to find out in my opinion. So many variables on each car that could change the point of detonation.

 

Warren,
I got it about two weeks ago.The 12.5:1 effective CR is for pump gas. There is not a lot of solid information that I've been able to find on how people are able to combine high boost and high CR. Twin Screw and Snail SC have both higher volumetric and adibatic effeciency than Roots blowers but the air temp still goes up about 8.6 degrees per 1psi. Some say intercooling will help even with low boost, others say it restricts air flow resulting in more inefficiency until about 8 pounds of boost. I've lurked on the Mustang and Corvette chat rooms. They don't seem to know much more than we do. It seems to be Joe did this and got this much on the dynoor 1/4 mile etc., so it must be OK. The developers and installers are pretty tight lipped. Seem familiar? I'll post what I can after I'm done.
Rick

 

Hey Rick
I have an I/Cd autorotor and it has held up in the summer out here. I will say if i turn my intercooler pump of in the heat of the summer i can get it to detonate. Stock S4's are appx 10:1 CR...when combined with the cams/timing, im really not sure what the effective CR becomes.
There was a guy name Lagavulin aka LAG'S who was pretty knowledgable on this sort of stuff, but he was "banned" saddly. You could probably search the word "effective" and you may find some leads.


While your in there you will have a great oppurtunity to clean out the "vee" of the engine.
also check condition and just replace ALL fuel lines if you can.
Blue loctite the crank shaft bolt after cleaning the threads out. If you can get one , it wouldnt hurt to get a longer and stronger crank bolt...however, that beign said, it really doesnt seem to be an issue these days with folks. Just makes sure you loctite it and use a propr TQ wrench, nto just a "good pull" on a cheater bar.

Air leaks will be your BIGGEST gremlin. seal everythign as good as you can. In theory if you start your car and unplug the idle stabilzer and it quits, you should be OK. If it runs you are obviously drawing air for combustion from some where as teh throttle plate is a good tight seal as is the idle stabilizer when it slams shut. Thats a raw test that i did.

before you start the car, jump the fuel pump relay and check for leaks. perhaps even turn up the regulator to get the max pressure you can out of the lines....at least set the pressure you will be under boost. Best to find a leak then than when the engine is hot!

perhaps Bill Ball will read and can fab up another throttle cable system for you if you request. He's made a vast improvement over the old one, Derek 86.5 can atest to that (as will his burn out )

Other than that, take your time. To do it right plan more than 2 days...little things will crop up

 

Nice rwhp from an 85. I was going to suggest replacing the knock sensors but, ummm, ya don't got em.

 

Lag is still banned?? There are a few very knowledgable guys that don't come here anymore and it's a shame.

 

Loctite makes a "red" version that is for high-strength, high-heat applications--like crank bolts. I got mine at Napa, but they had to order it. It was pricey, like 14 for a little bottle. There are two different red versions, but you want the high heat one.

 

Tony
Which size Autorotor do you have...1.7L or the 2.0-2.2? I know the 928 specialists kit is talking about the 1.7 or 2.0L size....I think Kenne Bell uses the 1.7 on the 4.6 V8 mustangs with very good results up to 650rwhp?

 

Richard:

You say you are having the kit installed. Who's doing it? Tony made some good points about the install. Print out the installation guidelines from Andy's forum too.

I run on 91 pump gas and have no problems with detonation even when flogging it in the desert for a 100 miles.

As Tony mentioned, I have made a bracket that allows an optimal throttle cable angle. I could make one up for you. PM me your address.

 

Tony,
I searched and found this by Lag. Thanks for the tip.
Rick
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?
__________________
87 5-Speed, Black/Can Can: 513 rwhp
MURF928 32v SuperShark #2
18" HRE 540R's: 9.5 x 68mm, 10.5 x 59mm; Sumitomo HRZ II's: 265 x 35, 285 x 35

96 993 Carrera 6-Speed, Black-Metallic/Red
Cargraphic 18" Wheels, Bridgestone SO3's
Fabspeed MaxFlo's

Last edited by Lagavulin : 03-31-2004 at 12:34 PM.

 

Yup, thats it Rick.

2.2 on mine. Plenty of blower for what i need, its not even working at 12600 rpm!

The ford guys with the 4.6L are turning them over 18-21k!!!

 

Rick, your car should be a real beast with some boost. Definitely quick as it is. Good luck with the install. Let us know how it turns out.

 

The BB Engineering (Bill Ball) throttle cable bracket is a MUST.

That is...if you want control over your throttle.


According to Lag's compression ratio thread, the 85-86 "actual" compression ratio is really 8.74:1 ?!?!

Does this mean it's time to turn up the boost?

 

Yes

 

Rick, your car should be a real beast with some boost. Definitely quick as it is. Good luck with the install. Let us know how it turns out.

__________________
Chris Lockhart
'89 928 GT
"The Wild Beast"

Chris,
I just dropped off the SC & peripherals. I'll probably go with 8 pounds of boost, shoud be plenty for a street car with occasional DE. I think you'll have fun with the rear turbo.
Rick