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Compression results on an unfired rebuild??


Guest gbvol54

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Guest gbvol54

What should compression and leak-down tests results on a rebuilt (but yet to be started) long block look like?

 

I just finished assembling the long block (SBC 350) for my daugther's Trans Am (yeah, the Z has to wait again.....). I did an economy rebuild; honed cylinders, new rings, bearings, timing chain, oil pump and a mild/medium cam (couldn't resist). Got some "rebuilt" heads off e-bay which yeild 9-1 compression. I decided to check out my work prior to installing in the car. A compression check shows 130ish PSI in most cylinders with #6 at a low of 110. A leak-down test shows consistent measurements around .95 with (again) #6 at a .9 reading (leaking through the exhaust valve).

 

How much is too much? I'm afraid I should at least pull the head and lap the exhaust valve on #6 to get it consistent with the rest, but even then, is 130 PSI reasonable for a unfired rebuild?

 

(Moved from Chevy forum at moderator's request)

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What you are asking about is [break Mean Effective Pressure], or simply BMEP which should not be confused with BSFC, Break Specific Fuel Consumption. Both the BMEP & BSFC will be highest during peak torque.

 

Typical BMEP figures are as follows:

1) 130-145...typical passenger cars

2) 165-185...Hipo Cars & Sports Cars

3) 185-210...Racing engines

 

Surely you will gain a few points after the engine has been broken in.

 

As for your low cylinder; was your cyl.head rebuilt by a machine shop? Did you rebuild the engine-who installed the rings: you or the shop?

 

Dont be so quick to tear it down, break the engine in and then if that cylinder is still low then decide what you want to do, but not until the engine has been broken in.

 

I did an econo build once on an Olds 350, simply honed the cyl's and slapped it together as a re-ring. Upon breaking the engine in the engine on both banks smoked for about 15 minutes like the City Mesquito Killer, then mysteriously as the rings set in the smoking completely disappeared.

 

You never know how a rebuild will run after it has been broken in; perhaps someone else could put in their .02c's worth?

 

Kevin,

(Yea,Still an Inliner)

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Guest gbvol54

Thanks Kevin.

 

The heads were a "bargin" deal off e-bay. Claimed to have been rebuilt long ago then set on the self. Looked fine externally, but I didn't tear them down. Number 6 cylinder has a noticable hiss through the exhaust valve during a leakdown test (much worse that the other cylinders) with a leakdown reading of .9 compared to a .95-.97 for the others. I'm considering pulling the one head and lapping number 6 exhaust.

 

Anyone still lap valves?

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