Super Jet Compression

[JBattag121]

Looking for advise on how high I can raise compression and still run 93+ octane. I have a fresh built 61x with a pro-tec race cylinder bored over to 753cc. ported case and twin 44mm carbs. the factory pipe is opened up to 47mm, and I have a re-machined oem cylinder head. group-k did all the mill work and everything seems good accept the compression. I specified 180psi and after break in, it's reading 160psi both cylinders. Group-k says it could be my gauge, but I've used my snap on gauge for years and it seems accurate. I said I'll compare to to another gauge and call them back. I'm looking for a little more torque on the bottom end.

Thanks,
Jim

 

[Fro Diesel]

Are you holding the throttle wide open whilst testing?

 

[Mark44]

I run 185 compression on 93 octane without issue.

 

[JBattag121]

I check it with the throttle wide open, Finger on the stop button of course, opposing spark plug still installed and the engine is cold. the battery is new and fully charged, sounds normal while cranking and I tun it until the needle appears to be holding.

 

[JBattag121]

That's what I thought, its been a while but I thought you could run up to 185 psi without a detonation problem. are you using an aftermarket head?

 

[Schmidty721]

Cylinder porting can skew compression numbers during cranking.
Did group K have the cylinder as well, or just machine the head?
Have you checked your squish and what ignition / timing are you running.
Lots of different factors will determine what you can get away with.

 

[SXIPro]

I've got ported cylinders and pull 205 compression and run 93. No one believes me, but I'm ok with that.

 

[txsuperjetfreak96]

I've got ported cylinders and pull 205 compression and run 93. No one believes me, but I'm ok with that.

X2 was running 200 psi when i was in houston couple yrs back on 93 octane zero issues as well


Jeremy

 

[JBattag121]

group-k did all the mill work, the cylinder is pro-tec's old triple port exhaust. the cylinders were thick enough to bore it over from 701, to 753. group-k sold me an oem head and re-machined it, and I'm running stock timing. I haven't checked the squish yet, do you know the minimum specs on squish clearance? I don't remember them.

 

[JBattag121]

the guys who were running 200+ psi, was the bottom end throttle response crisp, and did you experienced no knocking? I assume you were running stock timing?

 

[txsuperjetfreak96]

Stock timeing no advance, msd enhancer, bpipe was pretty crisp, didnt hear no lnocking


Jeremy

 

[Vumad]

Didn't Jetmaniac say they have a motor running 230psi or something crazy like that on pump gas?

 

[txsuperjetfreak96]

Yeah ive heard that


Jeremy

 

[JetManiac]

Alot depends on how your tight your squish is, how you advance/retard timing, and what type of riding. With freestyle brap brap riding you can run more compression on pump gas and be fine, but if you want to ride WFO, you will blow up the same motor from heat/detonation at sustained high rpms.

As an example, my last BB ran 215psi on prem pump gas for 1 1/2 years without issue. Compression gradually dropped to 210 over that time period and was torn down for inspection and refresh recently just as precaution and to see how SBT crank was holding up.