Abnormal compression test, Chevy 454

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2000 7.4 liter 454, L29 based engine. I recently did a compression test, both dry and wet, and all cylinders were about 135-145 except cylinder #1 which reads 160 psi. I have my records from doing a compression test on the same motor about 10 years ago, and the same cylinder had the same high reading. Its actually a boat engine- the first reading was at about 200 hours runtime, the current compression test was at 600 hours. Comparing to a car, think 1 hour runtime is about 100 miles, so first reading at 20k miles, current test at 60k miles. Engine runs fine. No knocks, no oil burning, makes full power. 2 different compression test tools were used. I cannot explain 1 cylinder having abnormal high pressure on a compression test but otherwise running fine. Help me understand how this can occur ?
 
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Could simply be a slight difference in machining of the chamber in the head or top of the piston. It would not take a large deviation to cause a 15# or so difference.
 
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Originally Posted By: 2015_PSD
Could simply be a slight difference in machining of the chamber in the head or top of the piston. It would not take a large deviation to cause a 15# or so difference.
I have to agree with this. Are there aftermarket heads or pistons in the engine? Was there any head work done to the stock heads if they are stock heads? I would normally think if only certain cylinders would have carbon build-up on a carb'd engine it would be the center 4 due to unequal fuel dispersion. How did the spark plugs look? Might be worth trying to get a borescope in the spark plug hole and see what it looks like compared to the others.
 

rubberchicken

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Engine is bone stock, never been opened up. What caused me to run a compression check is that, although currently runs fine, but is suffering from an exhaust manifold coolant leak. The manifolds need to be replaced, which is generally a standard job. When a manifold leaks coolant, it can be an external leak, or an internal leak, in which case you need to flush any water out and fog the cylinders to prevent rust, then run a compression test. The problem is I used ARP stainless bolts which now appear somehow welded in place: original torque was 20 ft lbs, but at 150 ft lbs the heads are striping. I used high temp anti-seize . I called ARP and they were stumped and suggested using a heat hammer and impact gun which resulted in rounded heads. Next step is remove the engine to get room to cut off the heads of the bolts.
 

rubberchicken

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Originally Posted By: bdcardinal
Originally Posted By: 2015_PSD
Could simply be a slight difference in machining of the chamber in the head or top of the piston. It would not take a large deviation to cause a 15# or so difference.
I have to agree with this. Are there aftermarket heads or pistons in the engine? Was there any head work done to the stock heads if they are stock heads? I would normally think if only certain cylinders would have carbon build-up on a carb'd engine it would be the center 4 due to unequal fuel dispersion. How did the spark plugs look? Might be worth trying to get a borescope in the spark plug hole and see what it looks like compared to the others.
I have a borescope so that is a good suggestion.
 
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Long, long time ago I took the head off an 1800 Marina (BMC B series petrol engine), which had dished piston crowns, except the middle two were slightly convex. They seemed to be filled in/topped off with fairly hard, grey, semi-metallic stuff which was quite difficult to remove. Although running on unleaded fuel at that time, the engine had run on leaded for most of its life. I didn't do a compression test but the effective compression ratio on these cylinders must have been higher.
 
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Warm engine at or close to operating temp, WOT or did you do it cold? Cold numbers are almost worthless unless you are looking for a dead hole.
 
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If it really bothers you.... http://www.mgexp.com/article/tuning-the-1800cc-mgb-engine.html "Aside from matching the weights of the reciprocating components and dynamic balancing of the crankshaft and the flywheel, perhaps one of the best ways to create a smooth engine is to equalize the compression and thus the power impulses occurring in each cylinder. Once the crankshaft and the connecting rods have been indexed, this can be accomplished by making sure that the combustion chambers are of equal volume so that the compression ratio in each cylinder will be the same. The volume of each combustion chamber can be measured after the head has been skimmed flat by using a clear piece of sheet plastic with a small hole drilled in it. Simply put a bead of chilled grease around the edge of a combustion chamber and press the plastic down onto it so that the grease forms a seal. Using a syringe or an eyedropper with a scale of measurement on it, carefully fill each combustion chamber with light oil, keeping a record of how much is necessary to fill each one. Next, use a Dremel tool to gently remove small amounts of metal from the smallest combustion chamber. Work slowly. "
 

JC1

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Originally Posted By: rubberchicken
Engine is bone stock, never been opened up. What caused me to run a compression check is that, although currently runs fine, but is suffering from an exhaust manifold coolant leak. The manifolds need to be replaced, which is generally a standard job. When a manifold leaks coolant, it can be an external leak, or an internal leak, in which case you need to flush any water out and fog the cylinders to prevent rust, then run a compression test. The problem is I used ARP stainless bolts which now appear somehow welded in place: original torque was 20 ft lbs, but at 150 ft lbs the heads are striping. I used high temp anti-seize . I called ARP and they were stumped and suggested using a heat hammer and impact gun which resulted in rounded heads. Next step is remove the engine to get room to cut off the heads of the bolts.
That doesn't sound like a fun task. Let us know how it goes. Post some pics of you can.
 
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Originally Posted By: rubberchicken
Next step is remove the engine to get room to cut off the heads of the bolts.
Which is easier - - removing the whole engine, or just the heads?
 

rubberchicken

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Originally Posted By: Linctex
Originally Posted By: rubberchicken
Next step is remove the engine to get room to cut off the heads of the bolts.
Which is easier - - removing the whole engine, or just the heads?
Removing the whole engine is easier, as it gives you more room to work on the various bolts. BUT in the end I replaced the engine with a brand new marine 454 long block.
 
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