Compression ratio vs. octane rating

[xj4life]

I'm trying to plan a stoker motor and I want to build it so that I can run it on regular gas with no problems. Any idea of what compression ratio i need to stay under to keep it running on the cheap stuff?


Thanks

[Eagle]

Under 9:1 would be good, but it's not that simple. There's the issue of "quench" (the clearance between the top of the piston and the bottom of the head) and also cam overlap and how much cylinder pressure is bled off by the cam.

I suggest that you log onto the Strokers discussion group at Yahoo groups and read up on their info. Also, Dr. Dyno -- who posts here on the NAXJA Street & Performance forum -- has a personal web site with good info on stroker "recipes." Search on his name, drop him a PM, and he'll give you the URL for his site.

[xj4life]

Yeah i've been researching the stroker quite a bit and have gotten a lot out of Dr.Dyno's stuff. Just haven't seen anything about what fuels to run with the various combinations out there. I don't want to build it up and then realize that i have to run premium.

[BajaXJ92]

I have to run high octane, as my stroker's CR is 10:1.

It runs decent on 89 though, but I do get the occasional pinging at lower revs....

[Dr. Dyno]

Yeah i've been researching the stroker quite a bit and have gotten a lot out of Dr.Dyno's stuff. Just haven't seen anything about what fuels to run with the various combinations out there. I don't want to build it up and then realize that i have to run premium.

My own stroker has a 9.24:1 CR, Crane 753905 cam, and a highish 0.088" quench height. When I built it, I sanded the piston tops smooth and feathered the edges of the combustion chambers so that there weren't any sharp edges that could act as detonation-inducing "hot spots".
The only fuel available here is 91 octane and the engine wouldn't ping on it even when it was 117*F outside in mid-summer. I've no doubt that it would run fine on 89 octane if I could get it. Make sure that the engine doesn't run lean, keep it running cool (180* t'stat), and feed it cool air.
_______________________________________
Dino's "Mean Green Machine"
1992 XJ Laredo 4-dr - 4.6L I6 HO Stroker - 178k miles - AX15 (http://www.off-road.com/jeep/tech/trans/ax15.html), NP231 (http://www.off-road.com/jeep/tech/xfer/np231.html), D35c (http://www.off-road.com/jeep/tech/axle/d35c.html), D30 (http://www.off-road.com/jeep/tech/axle/d30.html)
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258hp@4800/320lbft@3200, 1/4 mile = 14.61@94.9, 0-60 = 5.9secs
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