I'm using an older 350 from the 70s. It has what looks to be stock pistons with the 4 valve reliefs and I'll be using vortec heads. How would I go about figuring out the comp ratio? It's a non roller motor and I plan on using quadrajet. For a cam would the xe274 work out alright? For head gaskets would the fel pro 1094s be ok? This is just a weekend street car but and I'd be happy with around 350 rwhp.

 

Most of the 70's low compression engines had 12cc dished pistons and 76 or 78cc heads.

The factory stock spec for piston down in the bore at TDC is .025"

Factory vortec heads are listed as 64cc but all the ones a checked have been 62/63cc

So with the fel pro 1094 gaskets (4.100 x .015), 4.000" bore, 3.48" stroke, .025" in hole..

62 cc heads 9.697:1
63 cc heads 9.593:1
64 cc heads 9.491:1

I think the XE274 cam has a little too much duration at .050" for your compression. I would want to see 10:1 or a little better. Drop the .050" duration about 6-8* for your compression.

Also the vortec heads likes a wider split. They do the best with 10 to 14* more exhaust duration than intake. They vortec heads also likes more lift on the exhaust side than intake.

 

Appreciate it. Could you recommend a better cam for my application? What about a xe268?

 

http://www.wallaceracing.com/Calculators.htm

 

Here would be some of my picks. In no certain order
http://www.summitracing.com/parts/cr...make/chevrolet
http://www.summitracing.com/parts/hr...make/chevrolet
http://www.summitracing.com/parts/lu...make/chevrolet

 

I run the xe274 in my 383 at only 9.3 compression but it is a stroker motor. I think with some small heads the XE 268 would work good.

That voodoo can looks like it would be a good choice tho.

 

The odds of a 70s 350 still having the "stock" pistons in it, are negligible. Quantum-mechanically tiny.

More likely, they are just typical "rebuilder" pistons.

Virtually ALL such pistons are an extra .020" "down in the hole" at TDC (aka deck clearance), compared to stock. The stock # is .025" +.010" -.000", with any given block often have the full .010" of variation between cyls and sometimes more. That means yoru motor likely has .045" - .055" of deck clearance.

It doesn't matter what "calculator" you use, any more than it matters what "calculator" you use to add 2 + 2. They'll all come up with essentially the same answers if you put in the same #s. They'll all also be WRONG if you put in WRONG numbers (i.e., numbers that don't match the reality of your parts). The key to getting the right compression calculation therefore is, spend less effort on what "calculator" you use, and more on getting accurate measurements of the inputs.

Any time I see those given with precision out to 3 decimal places, I laugh. That's like going out in your yard and claiming to know the height of the old oak tree to within .001". Yeah right. 2 decimal places is bad enough. 1 decimal place is usually more precision than is actually possible under the real-world circumstances. Just the variation in as-cast head chambers ALONE, on a SINGLE HEAD, is often more than a whole decimal place of variation.

To give you some idea of the amount of variation caused by lack of precision in the inputs, a .030" over 350 w 64cc heads, .039" gaskets, 12cc pistons, and .055" deck clearance, works out to about 8.6:1. At .045" it's closer to 8.8:1. The same motors with .016" gasket works out to 9.0:1 and 9.2:1 respectively. With 62cc heads instead of 64cc they are then about 9.2:1 and 9.4:1. With .025" of deck clearance the "calculator" gives 9.8:1.

See how much variation there is, just from using "fuzzy" numbers? How accurate are your numbers?

In reality, your build is more likely to be closer to one of the first sets of #s than one of the later ones; meaning your ACTUAL compression is more likely to be in the high 8s or very low 9s, than anywhere else.

Go get some accurate measurements, especially of the deck clearance, and then you can pin it down a little more closely.