[i82much]

Folks, been wondering about something, I'm not swapping cams or heads or anything, but just curious. You frequently hear that you need 10:1 compression for a certain cam, 11-1 for a little bigger cam, etc. You also hear that you need about a full point more compression for an aluminum head motor to make the same power as an iron head motor.

Does it stand to reason that if you're looking at a cam catalog and it says you need 9.5 to 1 compression for cam X, that if you have aluminum heads you really need 10.5 to 1?

[Will]

That's a fair assumption.

[455Dave]

Hmmm...that's pretty interesting, I never thought of looking at compression that way. I'm running 9.6:1 static compression with a cam rated for 10.5 to 11.0 compression with open chamber aluminum heads, no problems so far.

[i82much]

I just thought maybe that alum heads may take higher compression, but at lower compression the heads act more alike, maybe they would heat up less so iron & alum would act more alike at, say 8:1, than at, say, 11:1.

Which wouldn't explain why cam co's don't specify a head material when they give comp ratios for cams, but would maybe partially explain why your car runs well, Dave. I mean think about it this way, if the cam co's base it on iron heads, you subtract a point for alum, your car is 8.6 to 1 and runs ok with a cam for a 10.5:1 car? That can't be right, can it? Or maybe the cam co's base on alum, and you have to subtract for iron?

[i82much]

Do you think the reference point for the cam manufacturers is alum or iron? If it's iron, that can't make much sense. Dave your car would be 2 points below recommended compression yet you say it runs well. So maybe you need to subtract 1 for recommended comp if you've got iron heads?

[BILTIT]

Or is some of it based on cylinder pressures and not actual compression but that is the best way to reference it for the general public? Just a thought.

[i82much]

That would make sense, maybe there is some floor on cyl pressure for a motor to run well regardless of head material.

[chiphead]

By that same logic, the cam mfgs would have to specify a displacement as well as iron or alu heads. Remember, a cam that acts "big" in a 400 will act "smaller" in a 455 with the same compression ratio.

I postulate that the cams are rated for a aggregate average of engine parameters for a given engine family. You have to "know" pontiacs to pick a suitable grind for the combo you're building. For example, most SBC cams are referenced to the behavior of a 350 CID engine as a reference. If I'm building a 327, I know the hot 350 cam is gonna be wilder in a 327 with the same compression ratio and cylinder heads.

[i82much]

Good point, chiphead. I wonder why they even bother to rate them that way.

[Region Warrior]

Ok, time to stir the pot.
It's a smoke screen.
The cam is just making the engine work at a higher rpm.

More rpm's=more power.
More comp.=more power.
Put both together...

I'll duck out for lunch now, and check back later

[steve]

i thought aluminum heads did not NEED more compression they can just HANDLE it because of better heat dissipation......