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78sharpshooter

High comp piston: any dynos or other data on differences?

13 posts in this topic

I am thinking of putting a high comp piston in my '01 YA426F. I know it has been discussed before qualitatively that the high comp piston when fed race gas will have better low to mid but does anyone have any dynos to show the difference quantitatively?

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Looking at the peak power numbers doesn't tell the whole story. Increases in compression give the greatest gains in power at lower speeds, the gain tapering off as rpm increases. This is because there is more time to completely fill the cylinder at lower rpm than at high. Nevertheless, compression is a good way to pick up power across the board.

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I see. So because I like to be in the lower half of the RPM's quite often then the high comp piston should suit me well, right?

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Absolutely...so would abig bore kit!

Alright alright alright...... I'll send my extra cylinder off next week. :thumbsup:

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I am thinking of putting a high comp piston in my '01 YA426F. I know it has been discussed before qualitatively that the high comp piston when fed race gas will have better low to mid but does anyone have any dynos to show the difference quantitatively?

The 13.5:1 piston makes a big difference in the low end power. Mid range has more snap too, but it doesn't make much difference on the top end.

I wouldn't rebuild mine without a 13.5:1 piston.

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The 13.5:1 piston makes a big difference in the low end power. Mid range has more snap too, but it doesn't make much difference on the top end.

I wouldn't rebuild mine without a 13.5:1 piston.

Pump gas ok?

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Pump gas ok?

Yes it will run off pump gas, but I do use race fuel. The high compression isn't the only reason for my use of the race fuel.

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Yes it will run off pump gas, but I do use race fuel. The high compression isn't the only reason for my use of the race fuel.

Does it run OK on pump?

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Does it run OK on pump?

As long as its not pinging its okay to use pump gas. Lots of newer MX bikes come from the factory with 13.5:1 compression and a 91 octane recommendation.

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Use a ring compressor.

Avoid dislodging the front cam chain guide when installing the head (verify its position and correct as necessary before bolting the head down).

Pay attention to the details, like torque, lubrication, cleanliness.

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