Thread: Techinspector need your help with compression ratio!

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But I will be checking the cranking pressure as well, thanks for that post by the way. Thanks for all your posts. The engine is already in the car and getting ready to turn over, so I won't be able to CC the cylinders. Next thing to do is post all my specs of the engine and see what the desktop dyno will think of it. This engine was a hard one to put together, since no one builds 396's anymore. I basically went off what I have learned over a couple years of reading etc., and put the whole combination together myself. My engine isn't based off of any other engine off the internet or anything, so I will be happy if it runs good. The heads were ported by me, one of my first port jobs. A lot of credit goes to David Vizards porting books. They turned out good, flowing 271cfm at .500" lift, with a 68% intake/exhaust flow ratio. But if you want all the good stuff, I'm going to post another thread and see if someone has a desktop dyno. Anxious to get this thing on the road!

I haven't been on here for several days, so sorry about the delayed response.

Here's my take on it........
cylinder cc's 823.42
chamber cc's 96.4
0.015" gasket 3.57 cc's
piston dome 17 cc's
0.025" piston deck height 5.47 cc's

On the intake stroke, you'll be filling the cylinder, the chamber, the gasket and the deck, but the dome will have to be deducted from the cylinder volume because it just takes up space and is a negative value for our computations. Cylinder, chamber, gasket and deck equal 928.86 cc's, less the 17 cc's on the dome equals 911.86 cc's. Now, when we compress the mixture, we have to deduct the 17 cc dome from the combustion chamber volume, so the chamber volume will be 79.4, plus the gasket of 3.57 and the deck of 5.47 for a total compressed volume of 88.44 cc's. Dividing 911.86 by 88.44 finds a 10.31 static compression ratio.

The 0.050" intake closing point of your cam is 48*ABDC. Add 15 degreees to that and enter 63 into the calculator. The indicated dynamic compression ratio is 8.34:1 and is perfect in my opinion.

Nice combination elko.

Cool thanks, now I just have to take the heads off and find the true deck height. I have a .038" head gasket on right now, if I really do have .020-.025 deck than I will have to put a .015" head gasket. And that will get me right in the squish zone.

find you true cc of the heads and find your true cc of the head gaskets as far as i know you can not get a 396/402 head gasket any more. so it a 454 gasket and some are bigger to fit the 502 from 9.7 to13.7 cc.i never look up your gaskets. did you check piston to valve ? on the piston with the 108 lsa cam .i would be surprised that it fit .going to a thinner gasket and cutting down any thing deck or gasket you will loose piston to valve . do you have any on the intake side to give up????????

Originally Posted by pat mccarthy

find you true cc of the heads and find your true cc of the head gaskets as far as i know you can not get a 396/402 head gasket any more. so it a 454 gasket and some are bigger to fit the 502 from 9.7 to13.7 cc.i never look up your gaskets. did you check piston to valve ? on the piston with the 108 lsa cam .i would be surprised that it fit .going to a thinner gasket and cutting down any thing deck or gasket you will loose piston to valve . do you have any on the intake side to give up????????

I checked the valve/piston clearance when I had heads with 2.25 valves. The valves came right to the edge of the piston eyebrows. Clearance wasn't a problem in the middle so I put on some heads with 2.065 valves and called it a day.

I'm pretty sure the head gasket I got was 4.370 bore. And you're right, they don't make any gaskets smaller than that. Wonder if that will effect anything besides compression?

I'm pretty sure the head gasket I got was 4.370 bore. And you're right, they don't make any gaskets smaller than that. Wonder if that will effect anything besides compression?[/quote]no i just built a 402 and that is all they have it would be nice to have them smaller i do not like the fire ring back that far but have done it on blower engines it works just not happy with it