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Discussion Starter #1
I recently did a compression test on my 69 that I just bought, it has a 71 400 small block and the numbers that I got averaged out at about 230 psi. I have have a few people tell me that it must have some type of upgraded piston in it. Is that number high? what should it be if the pistons where stock type? any help would be appreciated. Thanks
 

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That's pretty high. I would guess they have a flat top piston and probably a smaller chamber head like 64cc or so. Compression jumps quick with the larger bore/stroke on the 400. Cranking compression has a lot to do with cam timing also, so a cam with less duration will also elevate things.
I'm going to go out on a limb and say you are going to have to run a race gas mix to keep it out of detonation.
 

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That's pretty high. I would guess they have a flat top piston and probably a smaller chamber head like 64cc or so. Compression jumps quick with the larger bore/stroke on the 400. Cranking compression has a lot to do with cam timing also, so a cam with less duration will also elevate things.
I'm going to go out on a limb and say you are going to have to run a race gas mix to keep it out of detonation.
Yep, that's high. I agree with Sean.
 

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160 on a tight pump gas motor is normal. 140 is getting low. Yours is....wow....it's high.
 

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Mine came in at 190-195 with 10.2:1 compression and a 280H CompCam installed straight up.
 

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A cam with tight lsa around 104 will elevate low rpm cyl pressure.A different grind will bleed off some of that pressure for street usage.A later ivc and say around 110-112 lsa.Contact a cam grinder for detailed info.
 

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Discussion Starter #8
Let me tell you how I did the compression check, maybe I did not do it correctly? First I removed all 8 plugs, I threaded the gage into each cylinder and cranked the engine over until the gage stopped moving (crank time of about 4 seconds). If I do have a small chamber cylinder heads on the engine what would be the correct size to go with to make a reliable driver out of this motor? I really just want something that has a nice muscle car sound but reliable enough to cruise and be dependable, I dont want a full maintence race motor where I am wrenching on it more that driving it. Any response would be appreciated. Thanks
 
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