By the numbers-Real world compression ratio

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dartjack

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I'm thinking of using 360 heads on a 318 build. Without decking or milling the heads what would be a realistic compression ratio? The original pistons are
0.104 down in the block. Heads are stock size with a valve grind done. Pistons have dish and valve notches. My abilities to measure compression volume are next to nonexistant. My guess would be about 7.5 but from what I've read on here that might be generous. Anyone got any hard numbers?

Keep reading. :evil2:
 
Which 360 heads? LA style or magnum's? The only hard #'s I have is I've cc'd LA 360 heads and their chambers were generally around 70-72 cc's (even though they claim to be 68 cc's). If that's what you have and considering yours have had a valve job, which lowers the valves a little and raises the chambers 1-2 cc's, and dished pistons .104 in the hole with valve notches I'd say you'll be real lucky if it's above 7 to 1 using a standard thickness replacement gasket. Should run on kerosene at that level, LOL... If their magnum heads they have a smaller chamber. Haven't cc'd any of them but read reports of guys saying they have and their about 62 cc's. If your heads are LA style you'd be much better off trying to find some closed chambered 318 heads like 302's or magnums although watch magnum heads real close cause they are very crack prone. Those should get you above 8 to 1, possibly even 8.5 to 1. The big problem is those pistons. Those are about the worst you can find for compression and no heads will give you very much compression unless you cut them a lot.
 
I don't think the 360 heads are worth it on a 318. I'd go with '302's of a small chamber aftermarket aluminum head.

Just my $0.02.....
 
Thanks for the input guys. The real reason I'm needing hard numbers is because the teen will have a 360 stroke crank. I can figure the ratio by adding the cubes and sutracting the deck height. Without a real world 318/360 head number the best you can get are fuzzy numbers. Best I can calculate it will come in somewhere between 9.75 and 11.50. The lower half of that scale would be good. Pump gas with a little "help". Over 10.50 would be a little much for the street with my stingy *** wallet for exotic fuel. I'm thing(hoping) valve clearance won't be a problem with 1.88 valves. Been down that road. Wasn't a pretty sight or pleasant memory. I didn't want to start a flaming thread because almost everyone would want to jump in and talk about go big or go home. It is just something I WANT to do. I have 360's,
400's and 440's complete sitting in the garage. Forgot the 383. I'm thinking it can be done for under $1k. I'll let everyone know as time goes by. Looking at spring to be ready to light the fire.

Again thanks.
 
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