Compression Ratio Info?

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Apr 19, 2021 | 05:14 PM

Folically Challenged

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From: CVG

Compression Ratio Info?

I'm looking to beef-up a 107 M8. Some builders claim they begin a build by targeting a certain CR, then building the rest of the motor to/around that number. OK, great. Why?

From what I've read, higher CR offers more power + torque, and a more complete fuel burn. So why not go as high as the fuel that's available where the bike will be ridden? What would be the downside? Detonation (available fuel), hard starting, & extra heat get mentioned, but would a higher CR affect engine reliability / length of service, or make it run less smoothly? Does it move the power band higher or lower? I see big bore kits offered in, "low compression" models, so there must be some reason for it.

For being such an important part of building an engine, I'm sure not finding much info (broken-down to my level of understanding) about the issue. Can anyone point me to a decent primer on the subject?

Thanks,

FC

Apr 19, 2021 | 09:20 PM

98hotrodfatboy

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From: Poolville

You never start with compression.. Start with your riding style and where you want your power band. Once you figure that out then the comp will be set to maximize the cam...

Apr 19, 2021 | 11:19 PM

skills

Road Captain

Joined: Oct 2018

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From: Coldstream B.C. CA.

Quote:

Originally Posted by Folically Challenged

Like 98hotrod said, start with your type of riding. I suggest you do a study on "compression" and come to understand static compression verses combustion pressure. For instance, the camshaft you choose can increase your combustion chamber pressure without changing your static compression (the cr of your pistons and cc's of your heads). In other words, you can end up with detonation problems by using a cam that is intended for lower "static compression" pistons if you have higher comp pistons than recommended. And the flip side of this equation would be- using low comp pistons with a cam that is intended for higher comp, they just don't play well together and you'd have a dog for performance. Example, 551 S&S cams were great in my stock low comp 88, and I loved how they worked. I now have a 103 with 10-1 comp. The 551 cams would have been a bad match and created too much combustion pressures and detonation with my new 10-1 pistons, so I went to the 585 S&S cams (intended for higher static comp). So, if you like low end Torque, research what combination of components do that best. Same thing if you want HP & higher rpm performance. This has all been studied and experimented at great length by the manufacturers of HD components, so again, I suggest you do research on what components work well together that fits your style of riding. Check out the S&S website and watch the tutorial on camshafts and compression, they do a good job of explaining all this. Good luck!

Apr 20, 2021 | 09:26 AM

OldMike

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Joined: Feb 2009

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From: Cleveland Zoo

Quote:

Originally Posted by 98hotrodfatboy

You never start with compression.. Start with your riding style and where you want your power band. Once you figure that out then the comp will be set to maximize the cam...

This.

Apr 20, 2021 | 10:18 AM

Max Headflow

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From: poway

Personally I start with corrected compression based on engine design and available fuel, then figure desired power band (knowing the exhaust pipe helps here) select the cam and calculate final compression ratio.

Apr 20, 2021 | 02:38 PM

djl

HDF Community Team

Joined: Jun 2006

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From: san antonio

Quote:

Originally Posted by Max Headflow

So do I and I find it is always a compromise; the first setup usually doesn't work for me and compression or cam, or both have to be juggled.

Apr 20, 2021 | 03:04 PM

TriGeezer

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Joined: May 2012

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From: Carlsbad, CA

Seems to me the stock M8 is already running as much compression as it can for 91 pump gas. Under load, most bikes with 91 get a slight ping indicating the anti knock control is kicking in.