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#1
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Does any one know what cranking compression a 340 with 10.5:1 pistons would have when using stock '915' heads? I am trying to deduce what pistons are in my 340 which I do not have all the specs for. I am getting compressions of 150 +/- 10 in all cylinders. This engine was rebuilt about 10,000 street miles ago.
I know actual compression was often less than specs. Was this due to extra volume in combustion chambers, and if so could this mean this engine was rebuilt with 10.5:1 pistons? I believe the cam is a stock profile and the heads have been milled lightly just for a good sealing surface. I'm trying to decide what to do with the 340 which is in a 67 cuda to wake it up a bit, but don't want to pull the heads until I get serious since this is my daily ride in the summer. Thanks for your help, Keith |
#2
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Keith, I think you should get at least 160, more like 180. Just because you have been told that it has 10.5 pistons doesn't mean it all adds up to that. I believe it should have the thin steel head gaskets to be close. I bet it has a thicker composite gasket that will lower it .5 to .7 points. And of course the 10.5 is a blue print ratio. Few motors are at blue print numbers from the factory. My cude formula of compression ratio times 18 is close to what you may see. Also 10 to 1 will need racing gas mixtures to run on the street unless you REALLy know how to tune it!
fox |
#3
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fox,
Thanks for the reply. Actually, I have no idea what pistons are in there, I just know the cranking compressions I measured. I'm wondering if all those things you mentioned could mean that it was actually rebuilt with 10.5 pistons and still acount for my measured compressions. I'm thinking of doing a head and cam swap, with the heads most likely being edelbrock. They have a different head for the high compression 340's. Keith |
#4
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keith make sure you checked your comp with engine warm throttle wide open.if you did then your comp is normal. for a little more hp bump the cam up just a little over stock. dont mess with the heads a whole lot. youd be surprized what those heads will handle. best to work on that trans a little[ ie. converter, shift kit ect.] good luck.
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