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Old 11-07-2002, 03:42 PM
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Post Low compression causing poor performance?

I thought I did everything right on my first engine build. My 283 sbc runs great in the garage with no load. On the road it has zero balls. This engine has 35 minutes of run time since rebuild. I gave the machine shop my block(283) and heads(76cc), I bought everything except pistons which I left to him. 8.5 to 9.0 crwas what I wanted. He installed flat top pistons which I didn't think twice about( he is the expert). Now my car has no guts and the compression in the cylinders are all at about 100psi. I have since estimated my ratio at about 7.5. Am I right in guessing this is my problem? Are my valves too tight? Any brainstorming would be apreciated. JP

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Old 11-07-2002, 03:57 PM
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Those 76cc heads are killing you! The 283s ran with about 58cc heads.
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Old 11-07-2002, 04:32 PM
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even with the large chamber heads you should have good cranking compression. what cam is in this motor? was it a rebuild or a rering?
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Old 11-07-2002, 04:34 PM
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That is not the problem.....Don
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Old 11-07-2002, 04:37 PM
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7.5:1? Cool great compression ratio for a blower!

I'm sure it's higher then that. Advance your timing until you start to get some detonation. Then back it off a couple of degrees and take it for a run.
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Old 11-07-2002, 05:06 PM
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I love the blower idea.
I realize now that the heads are huge, but a domed piston should off set that right?
I'm running a power package from edelbrock, the cam is 204 In ,214 Ex @50.
I'm planning on running the valves again as my next action. Then I'll give the timing advance a try.
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Old 11-07-2002, 05:25 PM
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100 psi? that's very low. Is it 100 across all the cyl's? Does the addition of oil into the cylinder increase the pressure? How much? Were the heads rebuilt?
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Old 11-07-2002, 05:39 PM
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I bought the heads already rebuilt at the auto swap meet.
The compression test was only done on #3 and #4, didn't add oil to the cylinder. Is that to check the rings?
I'm going to do what it takes to find the problem before buying new heads. Maybe some s/r torquers
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Old 11-07-2002, 05:41 PM
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Unless you just don't feel good about the valve adjustment, try advancing the timing first. What is the lobe seperation angle of the cam? Seems to me the duration of that cam will cause a little lower reading. I know this is a dumb question but, are you performing the compression test correctly (wide open throttle, all plugs removed, properly charged battery)?
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Old 11-07-2002, 05:46 PM
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I appreciate the "dumb question" Not all the plugs were out. How much of a difference should that make? That is my dumb question
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Old 11-07-2002, 06:04 PM
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If it is a real hot cam, and with the those big chambers that could expalin the problem
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Old 11-07-2002, 07:01 PM
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Evan at 7.5:1 thats still enough compression for that cam.Try pulling as much timing as you can w/o detonation.I'll bet its a tuning thing.
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Old 11-07-2002, 07:38 PM
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Well, I have not seen anyone say it....YES, that low compression is probably hurting the performance. Once had a 305 with similiat issues, and a set of 58 cc heads helped a ton and the miliage went up by about 3.

Chris
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Old 11-08-2002, 06:46 AM
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Yes, your low compression is killing you....you asked about a domed piston.....in your original post you said you used flat top pistons. ???
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Old 11-08-2002, 07:03 AM
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Pull all the plugs and run the test again with the throttle wide open. Oh yeah, not having all the plugs out will make a difference, sometimes a huge difference.

[ November 08, 2002: Message edited by: BstMech ]</p>
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