safe compression

[KreagerM]

Looking to build a new motor (347) with higher compression. What compression ratio should i shoot for, to run a max of 92 octane? I was thinging 10.3:1, but I want your input. I will be running aluminium heads.

 

[MitchGT]

I run 10.4 to 1 in my 331 c.i. motor, also with aluminium heads. If you keep the motor cool enough, and don't go too crazy with the timing advance, you can run 10. to one or higher safely. Most of the manufacturers are going at least that high on new performance cars. Of course fuel mixture distribution is important (no stock manifolds...), and a long duration cam allows (needs) more compression as well. The more you are willing to rev up the motor, the more compression you will want and need.

 

[mikemustang289]

the Static compression ratio doesn't mean much, your cam will determine the dynamic compression ratio and the resultant cylinder pressure, which will dictate whether or not you'll be able to run pump gas

Just because one guy might run 10.5:1 on pump gas doesn't mean you will be able to if you choose a different (smaller) cam.

Big cams, with late intake valve closing points will need a higher SCR to build cylinder pressure and make power at lower rpm.

I would first select which type of cylinder head you plan to run, then make a cam selection from there to match the heads. You'll have room to play with the compression ratio when you buy pistons.

 

[D.Hearne]

10.4 to 1 in my 331 with a Ford Racing Z303 hyd roller (228*@.050 and .587 lift) Canfield heads, -.003 deck clearance, SRP pistons, .042 gasket. 57 cc chambers. Does fine on 93 octane with 12 degrees initial timing. In a pinch, you can run it on 89 octane, but you got to watch the right foot.

 

[SoCalCruising]

Ditto, Mikemustang289 for theory. D.Hearne has a good real-world example. You may note that some manufacturers of cams will specify a compression ratio (static) range for a particular cam. Say 9.5:1 - 10.5:1 for a certain cam in a certain displacement engine. That's because that cam needs that SCR in order to have appropriate dynamic compression. So, your combo has to be properly matched. Start with the power band you want and then spec the components to match that power band. Given a particular octane rating, the cam and SCR will fall out of that decision.