compression ratio and various boost levels
10-24-2005, 06:57 PM
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compression ratio and various boost levels
I recently purchased a 3.0 liter 6 spd track car (951) from the wife of a good friend after his untimely death. I sold my 2.5 liter track car that I built over the last three years. I mention this to establish that I am pretty familiar with these cars. My buddy's car has 2 hours on the engine. I remember him telling me that after the rebuild, all of the cylinders were reading 120 on a compression test. From this, he determined that the builder removed to much material from the tops of the pistons, and was giving him a compression ratio of only about 7:1. I wanted to be sure about the afr before taking this car to the track. We dynoed on Sat. (dynojet) rwhp/tq. were both right at 350 at 18lbs. of boost. This, with the flatest tq. curve and nicest a/f ratio you are likely to see. This engine has an o-ringed 2.7 liter "S" head with the larger valves, raceware head and rod bolts, knifed crank, Kokeln stage V with #10 hotside, 84lb. injectors, open side exiting exhaust, etc. At this boost level, this motor should be making another 75hp I believe.
My questions: would a 7:1 compression ratio at this boost level be equivalent to 8:1 at a much lower boost level? Said another way, would the stress on this motor/compression ratio be equivalent to running 18lbs boost on a higher compression engine?
This is enough power while I get used to shift points etc. on the 6spd. but I know that I get used to a certain power level after awhile, and will be wondering about more. If I later run 20 or more pounds of boost, is this really stressing this motor at this compression ratio?
Thank you for putting up with this lengthy post and for your input in advance.
Kevin
My questions: would a 7:1 compression ratio at this boost level be equivalent to 8:1 at a much lower boost level? Said another way, would the stress on this motor/compression ratio be equivalent to running 18lbs boost on a higher compression engine?
This is enough power while I get used to shift points etc. on the 6spd. but I know that I get used to a certain power level after awhile, and will be wondering about more. If I later run 20 or more pounds of boost, is this really stressing this motor at this compression ratio?
Thank you for putting up with this lengthy post and for your input in advance.
Kevin
10-24-2005, 08:54 PM
#2
Three Wheelin'
Good questions..
My engine spreadsheet says that you will make 365 hp running equal boost but at a 8 to 1 compression ratio. I do not however know what factors this takes into account, it is really just an estimate. As a general rule people overexaggerate the effect of compression ratio, somewhere there is a graph of cr vs thermal efficiency and it has a positive but decreasing slope. (less than linear)
I would suggest running higher boost (make sure your turbo is still efficient, maybe replace it if necessary) and see how much power you can make with some good tuning. Stress on the motor is more related to torque/power than anything else, assuming you are not detonating. Replacing the turbo for one efficient at higher boost is easier than making new pistons.
Let us know what happens, its an interesting situation
My engine spreadsheet says that you will make 365 hp running equal boost but at a 8 to 1 compression ratio. I do not however know what factors this takes into account, it is really just an estimate. As a general rule people overexaggerate the effect of compression ratio, somewhere there is a graph of cr vs thermal efficiency and it has a positive but decreasing slope. (less than linear)
I would suggest running higher boost (make sure your turbo is still efficient, maybe replace it if necessary) and see how much power you can make with some good tuning. Stress on the motor is more related to torque/power than anything else, assuming you are not detonating. Replacing the turbo for one efficient at higher boost is easier than making new pistons.
Let us know what happens, its an interesting situation
Last edited by mark944turbo; 10-24-2005 at 08:56 PM. Reason: more details
10-25-2005, 11:55 PM
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There's at least a 10% difference in power (probably a little more) when considering 7:1 or 8:1 CR; boost being the same.
Now, like Mark said, there will be less of a difference going from 8:1 to 9:1 CR.
I personally do not like low compression, for several reasons.
But, the bright side of a low CR (7:1) situation is you can make more power (via boost) with much less likelyhood of detonation,( which is one type of serious stress ), than a higher CR engine.
The overall stress is directly related to the power being made per displacement, irregardless of compression or boost.
Now, like Mark said, there will be less of a difference going from 8:1 to 9:1 CR.
I personally do not like low compression, for several reasons.
But, the bright side of a low CR (7:1) situation is you can make more power (via boost) with much less likelyhood of detonation,( which is one type of serious stress ), than a higher CR engine.
The overall stress is directly related to the power being made per displacement, irregardless of compression or boost.
10-26-2005, 01:00 AM
#5
Three Wheelin'
Where is that 10% number calculated from? I ask because the spreadsheet I have predicts less. Are you talking about an optimized setup for a given boost level w 2 compression ratios or what?
10-26-2005, 02:42 PM
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From a purely thermodynamic standpoint the relationship between compression ration (r) and efficiency (n) is:
n = 1 – (1/r) ^ (k-1)
k = 1.4 for an ideal gas
From that, the efficiency relative to an 8:1 compression ratio is -4%, +4%, +7% and +9% for 7, 9, 10 and 11 compression ratios, assuming all other parameters remain the same (boost, VE, ignition timing, etc.). That translates into to same power and torque loss/gain.
Laust
n = 1 – (1/r) ^ (k-1)
k = 1.4 for an ideal gas
From that, the efficiency relative to an 8:1 compression ratio is -4%, +4%, +7% and +9% for 7, 9, 10 and 11 compression ratios, assuming all other parameters remain the same (boost, VE, ignition timing, etc.). That translates into to same power and torque loss/gain.
Laust
10-26-2005, 02:52 PM
#7
Burning Brakes
I didn't think there is any particular relationship between compression and compression ratio. A low compression reading just means the cylinders may be leaking (or the gauge may be off). Compression ratio has to be calculated from the volume.
Tony
Tony
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10-26-2005, 02:55 PM
#8
Race Car
There is most definately a relationship between compression and compression ratio. compression is a result of the compression ratio, and a comparison between theoretical compression numbers and actual compression numbers, in conjunction with a leakdown test tells you a lot about the condition of your engine.
10-26-2005, 03:43 PM
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I believe the 968 turbo ran a 7.5:1 compression ratio, which isn't that different than what you have. It's also worth pointing out that the relationship between measured cylinder pressure and c/r is not 1:1 - the air heats when compressed, creating additional pressure due to this compressive heating. The stock spec. for a 951 is 10 bar of pressure which would be 145 psi. At 8.0:1 c/r, this is 14.7 psi (atmospheric, standard) x 8.0 = 117.6 psi as a result of only compression. The remaining 30-ish psi is solely attributable to compressive heating raising the pressure.
Just guessing, I'd say that a 120 psi number probably is about a 7.0 (maybe a little more) ratio. I'd say raising the boost would be fine although you'll be more dependent on the turbo for performance (boost lag!) and lose a lot of the benefit of the 3.0L engine at the low end with such low compression. . . Just my $0.02. Some of the low-end loss might be made up by gearing choices or creative turbo selection/plumbing.
Just guessing, I'd say that a 120 psi number probably is about a 7.0 (maybe a little more) ratio. I'd say raising the boost would be fine although you'll be more dependent on the turbo for performance (boost lag!) and lose a lot of the benefit of the 3.0L engine at the low end with such low compression. . . Just my $0.02. Some of the low-end loss might be made up by gearing choices or creative turbo selection/plumbing.
10-26-2005, 07:26 PM
#10
Burning Brakes
Well how do you know if the 120 psi is a result of a low compression ratio or leaking rings or valves?
(apart from an even balance across the cylinders?) Obviously a leakdown test will tell you more.
Tony
(apart from an even balance across the cylinders?) Obviously a leakdown test will tell you more.
Tony
10-26-2005, 07:36 PM
#11
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Mark,
as you know , there's a certain amount of thermal efficiency an engine can get out of the fuel used, and this goes up with CR.
I believe the thermal efficiency for 7:1 is about 25%, and 8:1 is about 28% (I don't remember the exact numbers).
At girst glance, this seems like a gain of 3%.
But, getting 3% more efficency out of the fuel means the power increase would be 28%/25%.
Another way of illustrating this relationship:
if it was somehow possible to have 100% thermal efficency, all the heat energy from the fuel would be used to push the piston down, you wouldn't need a cooling system, and you'd have four times the power!
as you know , there's a certain amount of thermal efficiency an engine can get out of the fuel used, and this goes up with CR.
I believe the thermal efficiency for 7:1 is about 25%, and 8:1 is about 28% (I don't remember the exact numbers).
At girst glance, this seems like a gain of 3%.
But, getting 3% more efficency out of the fuel means the power increase would be 28%/25%.
Another way of illustrating this relationship:
if it was somehow possible to have 100% thermal efficency, all the heat energy from the fuel would be used to push the piston down, you wouldn't need a cooling system, and you'd have four times the power!
10-26-2005, 11:59 PM
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Guys,
Thank you for the replies so far. The 120 is even across all cylinders and the motor has just over 2 hours total run time. It is not burning oil. I am pretty confident that it is in good shape.
Kevin
Thank you for the replies so far. The 120 is even across all cylinders and the motor has just over 2 hours total run time. It is not burning oil. I am pretty confident that it is in good shape.
Kevin
10-27-2005, 12:23 AM
#13
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Kevin,
The 10% differnece on a 1 point compression difference is not correct (not even close). It will be much less. You have to remember that you can easily (turn the boost ****) get the same effective compression ratio with a slight tweak in boost.
Furthermore, the VE of any engine, apples-to-apples, will be different since the higher compresion engine has a higher VE.
Lastly, did you bother to check if the engine had a stock cam? A larger cam with closer-than-stock lobe centerlines will for sure result in a static compression reading similar to what you've quoted.
I used to only get 125psi across all 4 on my last 951 running a "middle of the road" JME cam.
What's the vacuum reading at idle?
TonyG
The 10% differnece on a 1 point compression difference is not correct (not even close). It will be much less. You have to remember that you can easily (turn the boost ****) get the same effective compression ratio with a slight tweak in boost.
Furthermore, the VE of any engine, apples-to-apples, will be different since the higher compresion engine has a higher VE.
Lastly, did you bother to check if the engine had a stock cam? A larger cam with closer-than-stock lobe centerlines will for sure result in a static compression reading similar to what you've quoted.
I used to only get 125psi across all 4 on my last 951 running a "middle of the road" JME cam.
What's the vacuum reading at idle?
TonyG
10-27-2005, 12:34 AM
#14
Banned
You will most likely pick up some compression after a full breakin. I don't think you will have a problem with 7:1. There is also the added benefit of being able to run higher boost without detonation on pump gas.
