I have the HPE 809 with the raised up 760 cylinder and the 5mm crank. This engine is fresh and runs very well. I believe that the exhaust port timing is changed with the porting, however it is done. I have the 40cc domes and am getting about 153 psi of compression on a reliable gauge. I was thinking of going to some tighter domes to give my setup a bit more snap. I am limited to 91 octane gasoline and am at 2700 feet elevation. I see that the 38cc domes give +15 psi additional compression. This would put me at about 168 psi, roughly. the 37cc give +22, to 175 and the 36cc +28 to 181. This all assumes that my baseline of 153 psi with 40cc domes would scale up properly based on ADA's ratings. I'm wondering what I can get away with given my fuel and if I would notice much of a difference. Thanks for any advice.
Let's talk compression.
- Thread starter bernard
- Start date
Rushford_Ripper
Site Supporter
- Location
- New York
I would say that you would be safe at the 37cc domes for 175psi
What type of riding? Short burst freestyle riding or boat chasing and lake riding with long pulls? The duration of the wide open throttle will affect the reliability of the engine as related to octane, cylinder pressure and ignition timing advance. Either way if you are at 2700’ and 180+/- cranking pressure you should be perfectly fine on 91 non oxy unless you’re making long WOt passes
- Location
- Bozeman, MT
Storbeck is right, you need custom cut domes for HPE kits.
- Location
- Connecticut
I'm at 185psi, running 93 (sea level). I have done multiple 5-6 second WOT pulls. That doesn't seem long, but it gets you back in front of a boat lol
SXIPro
JM781 Big Bore
- Location
- Here on the 'X' mostly
Yeah, I wouldn't be running advanced timing, but I'm generally a Risk vs Reward guy. And for minimal gains at best vs a far greater chance of toasting your motor.....