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 |
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...
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Originally Posted by Folically Challenged
(Post 19875498)
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 |
Originally Posted by 98hotrodfatboy
(Post 19876027)
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...
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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.
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Originally Posted by Max Headflow
(Post 19876822)
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.
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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.
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Originally Posted by TriGeezer
(Post 19877387)
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.
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