Compression Test On 400 Small Block - Team Camaro Tech
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post #1 of 9 (permalink) Old Jan 16th, 08, 10:37 AM Thread Starter
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Compression Test On 400 Small Block

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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post #2 of 9 (permalink) Old Jan 16th, 08, 11:53 AM
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Sean
 
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Re: Compression Test On 400 Small Block

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.

Sean

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post #3 of 9 (permalink) Old Jan 16th, 08, 02:52 PM
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Patrick
 
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Re: Compression Test On 400 Small Block

Quote:
Originally Posted by 68rs406 View Post
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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post #4 of 9 (permalink) Old Jan 16th, 08, 03:33 PM
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Re: Compression Test On 400 Small Block

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

Dave
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post #5 of 9 (permalink) Old Jan 16th, 08, 03:42 PM
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Re: Compression Test On 400 Small Block

well.... 180-200 is nice. 230 is.... well, I built one like that once. a 400 with flat tops and 64cc heads. It was like 12:1





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post #6 of 9 (permalink) Old Jan 16th, 08, 04:07 PM
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Re: Compression Test On 400 Small Block

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

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post #7 of 9 (permalink) Old Jan 16th, 08, 04:39 PM
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Re: Compression Test On 400 Small Block

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.

George
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post #8 of 9 (permalink) Old Jan 16th, 08, 08:27 PM Thread Starter
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Re: Compression Test On 400 Small Block

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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post #9 of 9 (permalink) Old Jan 16th, 08, 09:11 PM
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Re: Compression Test On 400 Small Block

Have you run it?
How does it run and sound?
How lumpy is the idle?
Do you know what cam is in it?

How 'bout pulling a valve cover and getting a casting number off a head?





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