Thread: Fuel

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Hey guys i've been calculating (or trying, i'm more of an english guy, horrible at math) all day trying to figure my compression ratio.

What I get with a 4.155 bore, 3.5 stroke, 67cc heads, and flat top pistons is about 10.6:1...

My question is, can I run at that lvl on 93 octane gasoline? If not, is there any thing I can do without changing my hardparts to make it run on pump gas without castrating it?

How far are the pistons down in the bore at TDC?
What gasket thickness and bore?
Where are you gonna get a 3.5" crank?

I already have a 3.5" crank, Scat makes it especially for the 377.

I based the deck at Zero, and I used a .045 Gasket thickness.

I also came up with an 8.76:1 Dynamic CR if that makes a difference...

I haven't mocked up the motor yet. All my parts are still in the boxes. The machinist mocked it up when he did the decking and boring but it's been almost a year since I had it done and I can't remember if I had him zero deck it or not.

.7854 times 4.155 times 4.155 times 3.5 times 16.387 equals 777.7 cc's. You didn't say what the gasket bore is, but assuming 4.200, .7854 times 4.2 times 4.2 times 0.045 times 16.387 equals 10.2 cc's. At zero deck, add 777.7, 67 and 10.2 for a total swept volume of 854.9 cc's. Add 67 and 10.2 for a compressed volume of 77.2......divide 854.9 by 77.2 and you'll find a static compression ratio of 11.07. That's too high to run on pump gas even with a tighter squish than 0.045" and a long cam.

Oops, forgot about the piston eyebrows, good for about 5 cc's. Final c.r. with 0.045 gasket 10.46
Use a 0.039" gasket for 10.61 c.r. and the appropriate cam and you should be able to use premium pump fuel because of the tight 0.039" squish. See, you had it right all along.

Well Tech...it's a lot easier with the engine calculators on wallaceracing.com

I'm literally a math retard. If they could give awards in college for worst performance in Algebra, I'd get it...I worked really hard to earn a zero on my college algebra final. Still got a B in the class though

Thanks a lot for your help. I think I'll run the .045 gasket. Would retarding the timing do anything for fuel?

My final compression will be actually lower because I honestly don't think that I had the machinist zero deck it. I think I just had him take off enough to square up the banks.

"Would retarding the timing do anything for fuel?"

Might allow you to run worse fuel, but it'll also take the "snap" out of the motor. The motor will run better on pump gas with a tighter squish even though it'll raise the static c.r.

Ok thanks

I'm planning to run a 268H comp cams hydraulic flat tappet cam, either that or the 270. They are almost the same cam from what I can see.