Compression Vs Boost
Compression Vs Boost Quick question which motor would make MORE power to the wheels a 10.25:1 compression motor with 10lbs boost or same setup 9.8:1 compression and 14lbs boost? Its decision time. Thanks for the input! The boost figures are guestimates the motor would be fed the most amount of boost it could take without detonation utilizing pump gas.
That is as low as my builder wants to take it? He is confident and highly experienced in the turbo world. These are my choices right now. He does not believe in dished nor dome pistons.
Teching In
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what cubic inch motor are you looking at? i could see the concern with a 408 or bigger due to piston thickness but with something like a 396 you would still have the extra meat for strength and then a dish would not be any concern.even with a 408 its not that bad. some builders i have heard worry about it others not at all. my only thought would be to run a piston with a lowered top ring to help with the extra heat. as for wich one should make more power it depends on cam head combo.but given good flowing heads i'd go 8.5 with a good dished piston (i prefer diamond) and put the 14lbs and enjoy.
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From: Loganville, GA
I've heard for every point of compression (as a rule of thumb) is ~10rwhp (for 346's at least... I think 
). For a pound of boost, it varies on the turbo size. Wait for someone to chime in... Personally, I would keep compression in the low 9's... That way I'd be able to crank up the boost if my fuel system were up for the job...
There have been a lot of high compression motors with boost though...
). For a pound of boost, it varies on the turbo size. Wait for someone to chime in... Personally, I would keep compression in the low 9's... That way I'd be able to crank up the boost if my fuel system were up for the job...There have been a lot of high compression motors with boost though...
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From: Colorado
Originally Posted by 52172
That is as low as my builder wants to take it? He is confident and highly experienced in the turbo world. These are my choices right now. He does not believe in dished nor dome pistons.
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9 Second Club
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From: Lake Orion, MI
For some reason I want to say the math was something like a +/-1% change in NA power for every +/-.3pt change in compression but I don't remember for sure.
Does your builder normally do LSx turbo cars or is he into LSx S/C cars or 4/6 cyl applications?
There is more power potential in the additional boost that you can run when you trade down a point of compression with a given fuel/octane rating.
Running at 9.8:1 is on the high edge for LSx turbo setups. You'll find that anyone bothering to do a forged turbo setup is probably looking at 8:1-9.5:1 in order to allow them to turn the fun **** farther to the right for whatever fuel they want to run.
When you run something in the 9.8:1 range you are capping yourself in the 14lbs range on Cali 91 octane gas. You're going to want to run Torco additive or meth if you want to run anything higher than that safely.
Don't waste your time with anything above 9.8:1.
Rick
Does your builder normally do LSx turbo cars or is he into LSx S/C cars or 4/6 cyl applications?
There is more power potential in the additional boost that you can run when you trade down a point of compression with a given fuel/octane rating.
Running at 9.8:1 is on the high edge for LSx turbo setups. You'll find that anyone bothering to do a forged turbo setup is probably looking at 8:1-9.5:1 in order to allow them to turn the fun **** farther to the right for whatever fuel they want to run.
When you run something in the 9.8:1 range you are capping yourself in the 14lbs range on Cali 91 octane gas. You're going to want to run Torco additive or meth if you want to run anything higher than that safely.
Don't waste your time with anything above 9.8:1.
Rick
I was running 10.9 to 1 in my 346 stock bottom end H/C with the d1sc at only 9psi the thing made over 633 at the rear wheels that was through an auto, no detonation shifting at 6400 rpm. Car felt great on and off boost and a higher cr helped with that and still got 25+mpg. My new set up with the forged 408 is acually going to be a bit more conservative so that I can run more boost but mainly because i want more boost in the midrange so have to have it higher up too so i went with 9.6 to 1.
52172 -
Assuming power is a product of cylinder pressure, then 10 psi is 68.03% of ambient pressure, while 14 psi is 95.23% this differential is 40% greater. While the differential of CRs (9.8 vs. 10.25) is only 4.6%. But cylinder pressure is volume of air X CR, so a 1.68 (ambient) volume compressed using a 10.25 CR, or 1.95 volume compressed using a 9.8 CR the 14 psi set up would clearly produce greater cylinder pressures and therefore greater power, assuming the above figures are correct.
Oh, and yes, I know all about assumptions, but I can’t help it - I am an Engineer!
Assuming power is a product of cylinder pressure, then 10 psi is 68.03% of ambient pressure, while 14 psi is 95.23% this differential is 40% greater. While the differential of CRs (9.8 vs. 10.25) is only 4.6%. But cylinder pressure is volume of air X CR, so a 1.68 (ambient) volume compressed using a 10.25 CR, or 1.95 volume compressed using a 9.8 CR the 14 psi set up would clearly produce greater cylinder pressures and therefore greater power, assuming the above figures are correct.
Oh, and yes, I know all about assumptions, but I can’t help it - I am an Engineer!
The 9.8 combo will make more power.
Kurt made a good example to me a few years ago... 15psi will double your engine HP, that's a good general rule of thumb.
FWIW being against dished pistons sounds goofy, guys on here have done it for five or more years and have been running 7's and 8's (and 6's).
Kurt made a good example to me a few years ago... 15psi will double your engine HP, that's a good general rule of thumb.
FWIW being against dished pistons sounds goofy, guys on here have done it for five or more years and have been running 7's and 8's (and 6's).
9.8@14psi will make better power on the same sized engine for two reasons. More compression makes more power to a point and 9.8@14psi will have a higher dynamic compression ratio than 10.25@10psi. You have higher dynamic compression because you have much more air in the cylinder occupying an only very slightly larger space. Another larger contributing factor is more air and more fuel simply makes more power even at the same compression.
I agree with your engine builder's philosophy although I would be slightly more conservative in the 9.2 range imho.
I agree with your engine builder's philosophy although I would be slightly more conservative in the 9.2 range imho.
Thanks for the input guys. The engine builder was in GMHTP magazine in the Feb 06 issue with the black SS with an ATI d1sc putting down 840 rwhp with C16.
Last edited by 52172; Jan 18, 2007 at 05:29 PM.
Originally Posted by 52172
Thanks for the input guys. The engine builder was in GMHTP magazine in the Feb 06 issue with the black SS with an ATI d1sc putting down 840 rwhp with C16.
Good stuff.
Originally Posted by mahhddgtp
I've heard for every point of compression (as a rule of thumb) is ~10rwhp (for 346's at least... I think ).....
).....But that HAS to be an N/A estimator...