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Old 10-24-2005, 03:25 PM
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hp gains from increased cr

hi guys, can anyone give me a general idea of the hp gains that could be seen on a sbc 350 from increasing cr. all things being equal, if you increase the cr from say 9:1 to 9.5: to 10:1 what type of hp would you approximately gain? again, everything stays the same just changing the cr, wether it be pistons, gaskets, decking, etc.
thanks,
keith

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Old 10-24-2005, 03:48 PM
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Well, depends, in some cases you could loose power, diferance between static & dynamic compression. generally speaking going from 9.0:1 to 10.0:1 may be good for 50 ft. lbs of torque, & maybe close to the same HP. This is all assuming that the camshaft your using is versitle enough to work well in both cases.
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Old 10-24-2005, 04:21 PM
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I read somewhere that one point (9.0 to 10.0) was worth about 2-3% in the horsepwer arena. Typically. If you have a 400 horse engine, and increase compression by 10% (say, from 10.0 to 11.0) I think it's unreasonable to expect a HP increase greater than or even equal to the CR increase. So, I'd guess that 40 HP for this kind of an upgrade is probably not achievable without additional mods to take full advantage of that compression.

The previous comment needs to be respected particualrly with regard to the fact that you can lose HP. Particularly if you start seeing ping and/or detonation. In fact, you might lose more than HP. I think raising compression also reduces NOX emissions.

Just one man's opinion.

Pat
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Old 10-24-2005, 04:26 PM
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Quote:
Originally Posted by stepside454
Well, depends, in some cases you could loose power, diferance between static & dynamic compression. generally speaking going from 9.0:1 to 10.0:1 may be good for 50 ft. lbs of torque, & maybe close to the same HP. This is all assuming that the camshaft your using is versitle enough to work well in both cases.
the engine right now is 350 .030 over, piston down .050, gasket .04, 4 relief valves for about 9:1 cr. lightly ported vortecs, roller cam 228 @.050 525 lift on a 108, air gap, 750 holley. rwhp 303 @5500 rpm. some problems with timing and fuel pressure( drops to 2 psi wot). not going to change the pistons right now to increase cr, but combos like mine but with 9.5:1 putting out 410-412 chp compared to my 377 chp. trying to figure out if i'm losing alot of hp somewhere. local speed shop thinks timing issues and fuel delivery along with msd ignition box will put me closer to those other engines. bottom line is that i was wondering what difference there will be when i get these other issues taken care of. if a .5 increase in cr will gain approx 20-25 hp than i can maybe hope that by fiixing the issues i stated then maybe i can expect chp to be closer to 390 chp. sound right?
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Old 10-24-2005, 05:55 PM
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Ive read in a couple magazines that the rule of thumb is 1 point increase in compression yields on average 15 hp. I cant back this up but I did read it.
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Old 10-24-2005, 10:42 PM
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It is in the 2% to 4% range per point.

The gains aren't in the power though. The gains are seen in the idle quality with the cam being the same grind. The vacuum will be higher, throttle response better, etc.
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Old 10-24-2005, 10:57 PM
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Quote:
Originally Posted by lluciano77
It is in the 2% to 4% range per point.

The gains aren't in the power though. The gains are seen in the idle quality with the cam being the same grind. The vacuum will be higher, throttle response better, etc.
Thats what I thought too. I didn't know about how much power, but I know higher compression motors handle bigger cams better, idle nicer, and always seem to run harder. High octane is a killer for me right now though.
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Old 10-25-2005, 05:19 AM
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your current compression ratio is too low for a 108 grind. You need more compression, like 10.5 to take advantage of it. Most cams like your5s have a 112 lobe sep and give greater topend power not to mention a better idle quality. The wider lobe sep will also let the lower compression work better due to less overlap.
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Old 10-25-2005, 07:18 AM
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I personally like about 10.5:1 compression with a somewhat overkill cam ground with about 8 degrees more exhaust and a wide LSA (112-114) if you use a big enough duration at .050 (like 245/255 ish for a SBC) you can still run 87 octane just fine and it seems to have a better over all power curve and a torque curve a flat as Nebraska and pas a sniffer test as well as have a very friendly idle.
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Old 10-25-2005, 12:26 PM
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If I'm reading you right, pistons.050 in the hole and a .040 gasket means your quench is at .090, which is way too wide in my opinion. That may have something to do with it.
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Old 10-25-2005, 05:22 PM
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Quote:
Originally Posted by USA1
If I'm reading you right, pistons.050 in the hole and a .040 gasket means your quench is at .090, which is way too wide in my opinion. That may have something to do with it.
I agree, 0.090 quench is way too much. I believe the target is about 0.040.

I didn't know that pistons could be that far down on a stock block. I am used to seeing +/- 0.025 stock.
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Old 10-25-2005, 09:31 PM
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Might be some extra-short "rebuilder pistons."
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Old 10-25-2005, 10:13 PM
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Quote:
Originally Posted by jimfulco
Might be some extra-short "rebuilder pistons."
yes, they are. i beleive about .020 short. that was a 10 year ago mistake.
gonna have to live with it for now until i can afford my stroker kit.
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