Higher Compression Ratios= More HP?
Thread Starter
Melting Slicks
Joined: Jul 2004
Posts: 2,695
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From: Chicago Illinois
Higher Compression Ratios= More HP?
My 93 vette is in for heads and cam and the installer uses Fellpro 1074 head gaskets. He is telling me to tell my head porter to shave down the deck enough to gain higher compression. My porter is shaving the deck down butt doesnt want to take to much off incase I ever decide to go with a 383 set up. My porter recommends using Cometic MLS gaskets around $100 pair. With the Fellpro I will see 10.8 to 1 compared to Cometic MLS will give me 11.3 to 1. What do you guys think? Any HP gains by going with the Cometic? How does higher compression change the characteristics of the car?
Race Director
Joined: May 2001
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The LT1 will allow you to run a higher compression ratio without trouble due to the reverse flow cooling. However this will make it that much more important that you always run premium fuel. If everything else is held constant an increase in the compression ratio will give you an increase in power but it will not be as big an increase as you might expect.
We had some information about this in my Thermodynamics class last semester, so I started playing with the numbers and I found that the power increase found by raising the comrpession was probably not even worth the effort.
We had some information about this in my Thermodynamics class last semester, so I started playing with the numbers and I found that the power increase found by raising the comrpession was probably not even worth the effort.
Team Owner
Joined: Mar 2001
Posts: 54,124
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From: Cincinnati, Oh USA
Increasing compression to increase hp also forces you to use high octane fuel so you don't have knock. You cannot just increase compression. I don't know what you can expect from 10.8 to 11.3, but I doubt its worth it.
Thread Starter
Melting Slicks
Joined: Jul 2004
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From: Chicago Illinois
Originally Posted by Nathan Plemons
The LT1 will allow you to run a higher compression ratio without trouble due to the reverse flow cooling. However this will make it that much more important that you always run premium fuel. If everything else is held constant an increase in the compression ratio will give you an increase in power but it will not be as big an increase as you might expect.
We had some information about this in my Thermodynamics class last semester, so I started playing with the numbers and I found that the power increase found by raising the comrpession was probably not even worth the effort.
We had some information about this in my Thermodynamics class last semester, so I started playing with the numbers and I found that the power increase found by raising the comrpession was probably not even worth the effort.
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
Hmmmmmmmm! we used to always shoot for higher compression ratios-
but the only thing I can say-is mess with octane ratings-actually the lower the better-if you expierience higher knock counts-try a higher octane rating. In the "old days" we did everything we could to increase
the compression ratio. Maybe your mechanic came from the same school of thought! BTW-anyone going for the highest performance you can get from a SBC goes- first with heads-then with carb-then with cam-then with exhaust!
but the only thing I can say-is mess with octane ratings-actually the lower the better-if you expierience higher knock counts-try a higher octane rating. In the "old days" we did everything we could to increase
the compression ratio. Maybe your mechanic came from the same school of thought! BTW-anyone going for the highest performance you can get from a SBC goes- first with heads-then with carb-then with cam-then with exhaust!
Burning Brakes
Joined: Nov 2004
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Do It!
Relatively speaking a 350's 4 inch piston/bore is not that big, which gives it decent octane tolerance. Increasing compression generally translates into more torque, which tends to show up as more efficient fuel usage as engine speed increases. Done correctly, this will create a stronger, useable power curve somewhere in the lower midrange on up to peak power. As to a measureable difference, anything less than a half of a point (0.5) is hard to quantify. However, a definite half of a point, or more, is worth adding if it is not going to cause trouble in a specific engine combination. In your case, since you run 'good' gas and were going to use quality head gaskets either way, I would do it. Don't forget to run a 160 thermostat and get your fans programmed to suit your type of driving.
Large Impressive Member
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From: Good health is merely the slowest possible rate at which one can die
St. Jude Donor '04-'05-'06-'07
I'm running precisely 11.3 and like you, I always run 93 octane. It's been a great combination. If all it takes is a gasket, why not? Even if you build a 383 later, this should not be a problem. I'm building something similar to a 383 right now with a zero deck height. Obviously, piston selection enters into this. Quench will be snug by design.
Melting Slicks
Joined: Oct 2002
Posts: 3,193
Likes: 1
From: Cincinnati Oh
Don't quote me on it - but I vaguely remember something like a 3% power increase for every 1 point in CR (or something similar).
It IS benificial to increase the CR of a combination, but the dynamic CR is a more accurate indicator of how much CR you can run for a given setup and cam. Afterall, the cam duration is what makes or breaks a setup with reference to CR and detonation tolerance. There are some easy calculators out there (and do a search in C4 Tech, I know I've posted on it in the past).
So for the price of a headgasket and all other things remaining equal... I would do it.
It IS benificial to increase the CR of a combination, but the dynamic CR is a more accurate indicator of how much CR you can run for a given setup and cam. Afterall, the cam duration is what makes or breaks a setup with reference to CR and detonation tolerance. There are some easy calculators out there (and do a search in C4 Tech, I know I've posted on it in the past).
So for the price of a headgasket and all other things remaining equal... I would do it.
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Thread Starter
Melting Slicks
Joined: Jul 2004
Posts: 2,695
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From: Chicago Illinois
Originally Posted by Ramanstud
Don't quote me on it - but I vaguely remember something like a 3% power increase for every 1 point in CR (or something similar).
It IS benificial to increase the CR of a combination, but the dynamic CR is a more accurate indicator of how much CR you can run for a given setup and cam. Afterall, the cam duration is what makes or breaks a setup with reference to CR and detonation tolerance. There are some easy calculators out there (and do a search in C4 Tech, I know I've posted on it in the past).
So for the price of a headgasket and all other things remaining equal... I would do it.
It IS benificial to increase the CR of a combination, but the dynamic CR is a more accurate indicator of how much CR you can run for a given setup and cam. Afterall, the cam duration is what makes or breaks a setup with reference to CR and detonation tolerance. There are some easy calculators out there (and do a search in C4 Tech, I know I've posted on it in the past).
So for the price of a headgasket and all other things remaining equal... I would do it.
Burning Brakes
Joined: Jun 2003
Posts: 937
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From: Bedford Tx
at part throttle, the gain in output
[really efficiency] is three percent, for one 'point'
of compression, ie, going from nine to ten in CR,
and assuming best spark timing, in both cases.
at full throtte, the issue is more difficult,
as most fuels are 'knock-limited'.
Changing from 87 to 93 octane, adjusting both
CR and timing for best in both cases,
I would expect 4 percent gain in power,
some of that being better cylinder evacuation
of exhaust.
IMO, reported gains of more than that,
result from not trying hard enough,
with the starting 'low CR' condition.
[really efficiency] is three percent, for one 'point'
of compression, ie, going from nine to ten in CR,
and assuming best spark timing, in both cases.
at full throtte, the issue is more difficult,
as most fuels are 'knock-limited'.
Changing from 87 to 93 octane, adjusting both
CR and timing for best in both cases,
I would expect 4 percent gain in power,
some of that being better cylinder evacuation
of exhaust.
IMO, reported gains of more than that,
result from not trying hard enough,
with the starting 'low CR' condition.
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
Do it as well!
Increase in compression is tried and true way to gain power...how much totally depends on situation.
Until you exceed the 'limit' there are no negatives...if you drive a Corvette one pretty much assumes you run the best octane fuel otherwise why would you be modifying your car...
Another option is set of Impala head gaskets...they are thinner than OEM LT1/LT4 gaskets and cost less than the Cometics. Many have run those on the LT1 with the resultant 11.3 or so number.
I run 11.9 on 92 octane...I also have a somewhat large camshaft, but look at the LT4...raised CR from LT1's 10.5 to 10.8...it just makes sense.
By the way, the 383 issue is one that could impact you on the next build if you deck it too much. The LT1 chambers are already on the small side and then you add a longer stroke...you almost have to switch to dished piston to keep a lower number. My example...54.6 cc chambers, SRP flat top (5cc reliefs) and 3.75 stroke...Fel Pro gasket...11.9. And that wasn't even a zero decked block!
Until you exceed the 'limit' there are no negatives...if you drive a Corvette one pretty much assumes you run the best octane fuel otherwise why would you be modifying your car...
Another option is set of Impala head gaskets...they are thinner than OEM LT1/LT4 gaskets and cost less than the Cometics. Many have run those on the LT1 with the resultant 11.3 or so number.
I run 11.9 on 92 octane...I also have a somewhat large camshaft, but look at the LT4...raised CR from LT1's 10.5 to 10.8...it just makes sense.
By the way, the 383 issue is one that could impact you on the next build if you deck it too much. The LT1 chambers are already on the small side and then you add a longer stroke...you almost have to switch to dished piston to keep a lower number. My example...54.6 cc chambers, SRP flat top (5cc reliefs) and 3.75 stroke...Fel Pro gasket...11.9. And that wasn't even a zero decked block!
Safety Car
Joined: Apr 2004
Posts: 4,532
Likes: 7
From: Lahaina Hi
The newer blocks are for smog reasons(higher deck).
I would rather cut the block ,you won't have as many problems in the future.But you could alway's port the combustion chamber to lower compression later.
I would rather cut the block ,you won't have as many problems in the future.But you could alway's port the combustion chamber to lower compression later.
Race Director
Joined: Oct 2001
Posts: 19,653
Likes: 428
From: Melbourne Florida
Cruise-In IV Veteran
I've discussed this with a couple of pretty good engine builders who specialize in Corvettes...they both asked right off how many miles I had on my motor...apparently the higher the miles there is a concern for the motor when raising the CR...
I have 113,000 miles on mine and am looking into H/C as well...just passing along info...good luck
I have 113,000 miles on mine and am looking into H/C as well...just passing along info...good luck
Le Mans Master
Joined: Oct 2003
Posts: 6,130
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From: seattle WA
Of course the internals-rings, bearings condition are always a concern when modifying-their probably focusing more on the rings than bearings,
I don't think I'd be to concerned-unless you're already burning oil-and if that's the case-I'd go in with new rings and bearings.
I don't think I'd be to concerned-unless you're already burning oil-and if that's the case-I'd go in with new rings and bearings.
Burning Brakes
Joined: Sep 2003
Posts: 875
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From: Louisville KY
I hope your builder is ensureing your quench distance is ~.040-.042... especially if you are running a higher compression. I have a cam similar (Cranes version) and a static compression of around 11:1... my dynamic compression ratio was around 8.08:1 wich is well within the streetable area... you just don't want to go past 8.3 on the DCR unless you pay real close attention to everything.
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
This CR issue comes up from time to time, yes, 1 point in CR translates in to between 3 % and 4% power difference.
The wrinkle is that percentage isn't across the board. What I mean by that is from 11:1 to 10:1 there will be a certain percentage decrease, but from 9:1 to 8:1 a slightly different percentage decrease results.
I have an article that shows the dyno results on changing CR in 1 point increments starting at 11:1 and going down to 8:1. Shows how power is effected.
I posted it on this Forum some time back and will again if you REALLY want to know the results. The tests were done on a NON Chevy (SB Chrysler I believe) engine, but the results can be extrapolated.
Let me know.
Jake
The wrinkle is that percentage isn't across the board. What I mean by that is from 11:1 to 10:1 there will be a certain percentage decrease, but from 9:1 to 8:1 a slightly different percentage decrease results.
I have an article that shows the dyno results on changing CR in 1 point increments starting at 11:1 and going down to 8:1. Shows how power is effected.
I posted it on this Forum some time back and will again if you REALLY want to know the results. The tests were done on a NON Chevy (SB Chrysler I believe) engine, but the results can be extrapolated.
Let me know.
Jake
Race Director
Joined: May 2001
Posts: 14,165
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The thermodynamics book shows graphs and such that illustrate the point. It starts as a very sharp angle and then tapers off as compression ratio continues to increase so it is in no way linear. For very short jumps you MIGHT make a fair approximation.
It's kinda like the drivetrain loss expressed as a percentage, it's something that is greatly misunderstood.
It's kinda like the drivetrain loss expressed as a percentage, it's something that is greatly misunderstood.