Compression Numbers


477 Owner
Nov 25, 2022
United States
What is the highest compression that you guys have successfully ran? I recently got my 500cc put together and I’m at 250 psi. With my 477cc variations I was in the 190-220 psi range.
250 seems pretty high. I’ve pulled a decent amount of timing out and it still doesn’t seem too happy. I’m assuming I’ll have to pull it apart again and try to get the compression down a bit. I was just curious if anyone had successfully ran with this much compression.
highest I ever had was 220 but it knocked a lot. My 500 has 200 with only one foot gasket and unmodified head.
I have 2 cheap chinese instruments and they vary by 10% - keep that in mind too.
I took it apart and milled some off of the piston and opened up the chamber some more. It’s at 230 psi now, hopefully I can make that work. I would have preferred to be in the 200-220 range though. Next time I have it apart I’ll probably add a second base gasket.
I am pretty sure the 471 is 230, it's about as high as I would go.
On the edge of knocking, especially when under load.
Working on the assumption it will wear a bit and lose some :giggle:
What condition is your chain? sometimes going up is good slack adjustment.
My cam chain is still pretty tight. The bike only has about 3000 miles. In the past, I have used 2 base gaskets. So I know I have enough slack to squeeze a second one in.

I’m going to try it out with the 230 psi that it has now. I’m definitely going to have to make some significant tuning changes. I’ve only had it idling and revved it up a few times so far and can tell the tune needs quite a bit of adjustment.
2 gaskets must be the way or maybe resurfacing the piston's top
With this high compression , what about your electric starter , does it follow ?
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the one 83.5mm 477 i have has 207 psi, the other 200 psi.
Modify the Combustion chamber and that will serve 2 purposes.
Reduced compression and a better fitting chamber, you can de shroud the valve seats at the same time.
A Camshaft with a Decompressor is handy, at present there is only the HT C that has this Decomp but I believe that an even better Cam is on the way with Decompressor.

That was one of my first mods, exposing the Valve area can improve flow.

The head fitted the 500 and the 443 have a lot more work done to them so my output results are a mix of Cam improvement, Throttle Body, Displacement increase, Combustion Chamber mods as well as the tuning available from a Reprogrammable ECU.
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