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Author Topic: Compression and Power  (Read 798 times)


  • Bulleteer
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Compression and Power
« on: December 23, 2012, 10:25:00 am »
 How much does compression influence the "power" or toque output of an engine? I'm not taking about compression ratio of the piston  but instead what you measure with a gauge when the engine is cold.

The stock  AVL  500 engine has a cold compression of 145 - 150 psi. I personally have run compressions of up to 175-180 psi with good success ( ie no pinging, but with lots of other mods). Just wondering how much of the performance gain is due to the increase in compression and how much the rest of mods.

Just out of curiosity, say we have two identical engines: one running at a lowly 145 psi and the other at max compression possible without it knocking , say around 175 psi. Would the performance be dramatically different?

Let's also suppose that we have the best bottom end installed so that's not an issue.

One thing I did notice is that the oil turns black very fast, even with a proper run in barrel and piston. This  is obviously due to blow by, yes? That can't be good for the engine breather system, can it?  Having to work more? Is there anyway to gauge what's the highest usable compression to achieve optimum power with out the ill effects of blow by etc?


  • Grand Gearhead
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Re: Compression and Power
« Reply #1 on: December 23, 2012, 01:35:39 pm »
The compression is a big influence on torque, so it has a lot to do with the overall output. It's probably the biggest factor on power below the torque peak rpm, and the breathing becomes the most important at rpms above torque peak.
30 psi of compression is probably going to make a difference of maybe 5 hp, which is quite a bit on a Bullet. But you have to be sure that there's no pinging, or it will hurt, and not help. The quality of the fuel is key to making this work. You must use premium grade high-octane fuel to run high compression, and you may even need to use octane booster additive.

You can help to reduce blow-by with a really good set of rings that are run-in properly on a very well bored/honed barrel to the correct clearance. You might even help further by using gapless rings.


  • Grand Gearhead
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Re: Compression and Power
« Reply #2 on: December 23, 2012, 04:42:15 pm »
You can have a high PSI and a low compression ratio the ratio is what affects whether the motor will detonate or not and along with other factors gives more HP.  ERC
2-57 Apaches, 2-57 Trailblazers, 60 Chief, 65 Interceptor, 2004 Bullet, 612 Bullet chopped.