Just curious...

Two different sets of heads.
First set is C8AE-H, listed as 68-71 CC chambers.

Second set is C6AE-R Listed as 71-74 CC chambers.

What would be the difference in compression ratios between these heads?
(Neither have been milled, or surfaced)
Assuming of course they are used on the same short block, same pistons, etc..?

Also, there seems to be a 3 CC variable in the heads. Does that mean that each chamber could be off 3 CC's? I.E. cylinder 1 be 68 CC's, and cylinder 3 be 71 CC's on the same head?

And if this is the case, what impact would one cylinder having a higher compression ratio than the others have on engine performance?
Let's say that a matched set of C8AE-H heads are cast with that 3 CC variance between the heads. One head has all 4 chambers at 68 cc's, and the other head has all 4 chambers at 71 CC's. What impact would this have on the way that engine runs?

Yeah, I know...I'm full of questions...

But some of you guys probably know this stuff, and I want to learn...

 

Calculator here:

CUBIC INCH CALCULATOR and COMPRESSION RATIO

On a 390 with .030 over bore, the difference from the low of 68 to the high of 74 is approx 9.8 vs. 10.6 .

68 = 10.6 approx
71 = 10.2
74 = 9.8

These could be different in a real example due to head gasket thickness (I used .040) and piston design (eyebrows) and final compression height, overbore, etc.

The difference between cylinders would be slight enough to pass in the world of production engines. You may have heard stories (fewer such these days methinks) of engines that ran great, great mileage, power etc, and the guy down the road has the same one that's a dog. Rough, doggy factory engines are the result of such production variations. Imagine the variation in the heads, compounded by a similar variation in the cam, rocker arm ratio, perhaps even rod length or piston height, and a real crowd pleaser, the bore-line. If enough of those specs go the "wrong" way, you'll have an engine performing below, if not well below, the average, and certainly well below one that is on spec.

"Balanced and blueprinted" engines "fix" the specs so every chamber is the same CC, the pistons & rods weigh the same, crank is straight, bores are in line at spec, rockers are all the same ratio, etc etc. What you get then, even with all else stock, is a smooth running, more powerful engine.

There are many books on engine building. According to many posters here, it's hard to beat this one:

Amazon.com: How To Rebuild BIG-BLOCK FORD ENGINES: Steve Christ: Books

 

Is there a pretty much standard thickness for head gaskets? I noticed you used .040 in your exercise, is that fairly normal?

 

The link for the calculator uses a .040, but they also reference a .020 steel shim gasket. IIRC others here have made reference to that type of head gasket as well.

This from the Summit site for Fel-Pro--seems .040 is what you find a lot of, with .071 down to .021 or so:

Fel-Pro Performance Head Gaskets - summitracing.com

 

Thanks for the info. And I have Steve's book. It's a great resource.

 

The c6ae-r are a nice sleeper performance head and i would use them before a c8ae-h. The dip in compression is worth it. Ford seems to have a 3cc variance for all their heads not just FE's. Valves and valve jobs also play a little role in chamber size.

Most felpro performance gaskets are around .040 thick but your stock replacement permatorque blues are thicker than that. they are .047 for the sbf, and i was trying to figure out the fe's but had no luck the other day.

As for performance, what happens when a cylinder has a better intake manifold runner than the next? Its hard to balance each cylinder, thats what the nascar guys do.

 

OK, here's another question. I just picked up my new heads and they have 77 cc combustion chambers. These have been port matched to the intake. How much would I have to mill these heads to get them down to 72 cc? And more importantly, how would that affect the port match with the intake? Will it move it?

 

about .020-.025 needs milled.

 

I found the answer to my questions. First of all, the head has already been milled .010 so to get it to 72cc it would need an additional .030 taken off. Then the intake, since it was gasket matched, would have to have .040 taken off of it. I think I'll just stick with it the way it is and live with a 9.46 to 1 compression ratio.