HP loss/gain with compression ratio
#929339
02/15/11 01:32 AM
02/15/11 01:32 AM
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Joined: Oct 2003
Posts: 4,013
South Park, Pa.
68LAR
OP
master
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OP
master
Joined: Oct 2003
Posts: 4,013
South Park, Pa.
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I know this topic has been posted several times. I tried a search, but couldn't come up with anything.  So, what effect does changing compression ratio have on horse power? Thanks for any help.
4 speed street legal. Best time 10.99 @ 124 mph on 93 octane pump gas @ 3926# total weight
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Re: HP loss/gain with compression ratio
[Re: 68LAR]
#929346
02/15/11 01:39 PM
02/15/11 01:39 PM
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Joined: Jan 2003
Posts: 5,138
East Aurora (Buffalo) NY
RoadRunner
master
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master
Joined: Jan 2003
Posts: 5,138
East Aurora (Buffalo) NY
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I am interested in this. I know I went from a 11.5:1 down to 9.1:1 by changing heads and thicker head gasket. I did it to run pump gas. The difference was very noticible. The cam is probably to far off for the 9.1 combination so I can't say that compression is the sole reason.
68 Road Runner (383/4speed, post car w/decor pkg) - Major Project
69 Road Runner w/472 Hemi & 4 speed.
70 Challenger R/T SE EF8 w/ V9J, U - A32 - Major Project
2023 Ford Mach 1
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Re: HP loss/gain with compression ratio
[Re: mopar dave]
#929348
02/15/11 09:56 PM
02/15/11 09:56 PM
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Joined: Jan 2003
Posts: 16,925
NC
440Jim
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 16,925
NC
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Quote:
theirs a calculator on wallaceracing.com for this.
I thought it was informative using that calculator to show the difference
changing 1 percentage point in CR starting from different levels:
Quote:
Your old Compression Ratio of 13.5 and HP of 750 is now calculated
as a Compression Ratio of 12.5 and 737.20 Horsepower.
(750-737.20)/750 x 100 = 1.71 percent
Your old Compression Ratio of 12.5 and HP of 737.2 is now calculated
as a Compression Ratio of 11.5 and 722.88 Horsepower.
(737.20-722.88)/737.2 x 100 = 1.94 percent
Your old Compression Ratio of 11.5 and HP of 722.88 is now calculated
as a Compression Ratio of 10.5 and 706.71 Horsepower.
(722.88-706.71)/722.88 x 100 = 2.24 percent
Your old Compression Ratio of 10.5 and HP of 706.71 is now calculated
as a Compression Ratio of 9.5 and 688.22 Horsepower.
(706.71-688.22)/706.71 x 100 = 2.62 percent
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Re: HP loss/gain with compression ratio
[Re: Crizila]
#929351
02/15/11 11:52 PM
02/15/11 11:52 PM
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Joined: Jan 2003
Posts: 16,925
NC
440Jim
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 16,925
NC
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Quote:
Quote:
Read my article:
http://victorylibrary.com/mopar/otto.htm
 People don't seem to realize just how much the HP gain drops off at higher ratios. - and as the HP curve flattens out the $ expendature curve takes over 
That is one nice thing about running alcohol. It supports 14-15 CR with no more fuel cost. Unlike racing gasoline that gets real expensive to support 15 CR, even the 110 that supports 12-13 is expensive in comparison to alcohol.
But like you mention, things like head gaskets to seal that extra compression go up in cost compared to a 10 CR motor.
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Re: HP loss/gain with compression ratio
[Re: 5spdcuda]
#929354
02/16/11 01:49 AM
02/16/11 01:49 AM
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Joined: Oct 2007
Posts: 4,206
New York
polyspheric
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master
Joined: Oct 2007
Posts: 4,206
New York
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Yah - all benefits are based on a non-intrusive piston dome, no change in recip weight, no weird valve relief cuts, and no closing off the chamber walls to lose a few CC.
For us flathead owners, the real big gains are going from 5:1 to 6:1.
In some cases the extra pressure will allow a bit later IVC which also adds some power, but certainly not always and not a predictable amount.
Boffin Emeritus
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