Just bought a superjet with a 760 big bore in it. It has 85.5 mm flat top pistons with an ADA girdled head, novi 48 carbs, with speedworx pipe. The compression is about 230psi per cylinder. It seems a little high. I was wondering if I could lower it to try and run pump gas or a mixture of pump and race gas together. I would like to get it to around 200. Should I try and lower it or just leave it alone. Any info would be helpful. Thanks
Superjet big bore compression
- Thread starter sparky
- Start date
You can lower compression by changing dome size
- Location
- Hokesbluff,al
compression makes hp. 200lbs is still gonna need something stouter than pump gas. some in our group run that much compression but they also run total loss and a lot of timing
Its only high if you don't use proper octane. With 230 you should be looking at 112 octance or higher.
- Location
- Hokesbluff,al
sounds like you have a good set up. wanna sell the total loss?
- Location
- Laughlin, NV
Actually porting a cylinder does'nt demand higher octaine or an after market ignition, higher comp. and ignition upgrades just add to the performance. You can always switch to larger cc domes to lower your compression if your looking for reliability and longer engine life. Most of the riders on this forum seem to be freestyle riders and higher compression gives major gains in bottom end hit and arm stretching midrange pull for that riding style and even if you race you know the holeshot and being quicker around the bouys is more important than top speed in the back strech, unless off course your doing long distance or endurance type races. If you want a reliable ski but still crave that extra power try keeping your compression between 175 and 195 psi and run 104 VP race fuel. One note on octaine, I would never recommend running less than 91 even on a stock ski. Some people dont realize a two stroke engine is a modified liittle powerplant even with stock compression and are more vunerable to detonation and preignition than four stroke engines. I'd say most of my engine rebuilds for customers over the years were from not swapping out old fuel that has lost its octaine points after sitting through the off season or getting bad gas from a marina or station that was supposed to be 87 octaine and was'nt.