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compression
I have a 390 bored 30 over with 10:1 CR. Engine has 700 miles on it, did a compression check and numbers I got were 125 psi for the low cylinders and 140 for the high. Check was done at WOT, all spark plugs removed, battery charger running and engine at operating temp(160 thermostat). Right at the 10% mark.
Is this normal for a FE? |
I think you rings still need more mileage on them to seat better. Your numbers are fine as along as the are consistant with one another
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The absolute numbers look a tad low based on what you'd expect - but I don't get too concerned about those. Your cam selection will havea huge impact on cranking compression. I have had 428 engines with lower numbers that were smooth, tight and dry - and ran into the 11s in a 3600 pound street car.
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compression is one thing....................leak down might tell you different.
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I agree with Jeff - (csxwnab) -- Diego are you feeling something is a miss with your FE >?
they look a little low if it is supposed to be 10:1, are you sure they have known values for cc of the heads, pistons (ie compression with a specific cc of head) and of course head gasket compressed thickness. Steve |
Compression ratio and cranking compression are two different animals.........
The cranking compression has a lot to do with the camshaft profile, probably more so than anyhting else...(assuming a good ring seal and good valve seal) the amount of overlap in the profile has a ton to do with cranking compression.... My 10.5 to 1, 351-W cranks out at about 125 psi, but my camshaft has a lot of overlap and at those low cranking speeds, bleeds off the compression as it is designed to do. by contrast, I checked a bone stock 351-W and it's cranking compression was 145 psi!!!!!!!!!!! David |
Compression ratio and cranking compression are two different animals.........
The cranking compression has a lot to do with the camshaft profile, probably more so than anyhting else...(assuming a good ring seal and good valve seal) the amount of overlap in the profile has a ton to do with cranking compression.... My 10.5 to 1, 351-W cranks out at about 125 psi, but my camshaft has a lot of overlap and at those low cranking speeds, bleeds off the compression as it is designed to do. by contrast, I checked a bone stock 351-W and it's cranking compression was 145 psi!!!!!!!!!!! doing a cranking compression test is a good way to check for ring sealing and valve seat wear/sealing, not a good way to determine overall compression ratio... David |
Not enough miles on the motor for a good test
Diegokid Your readings are a little low for my taste but you have left out some of the important things. Was this test done wet or dry? What piston rings are on the pistons? Normal or low tension? Cam shaft, how big and what is the LSA? Also how must overlap does the motor have? As said by others, I believe a leak down check is a better test than compression. You are within the 10% high to low for 8 cylinders but didn't break each one down withthe numbers. I run 180 thermostats is what I run in all my cars and trucks. I know some FORD motors like alot of heat in them. Your engine oil may not be getting up to temp if the motor is running this cool. This will effect breakin of he rings and valves. You also my cause a excessive carbon build up on the valves. What color are the spark plugs when you checked them? We need to do the basics first before going crazy. Cam specs would help also. Rick L.
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Lads, overlap has nothing to do with compression pressure while cranking.
The valve event that influences cranking pressure is inlet valve closing point. After all, compression can't start until the inlet valve is shut. So if the closing point was example 90 ABDC, the piston would be halfway up the bore on the compression stroke and no pressure will build at 300 rpm (cranking) until the inlet is shut. Static compression ratio could be 12:1, dynamic would be 6:1. |
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A while back, I built a spread sheet to calculate piston position at a given crank position. At the time I was interested in the the thrust vector angle that put power to the crankshaft with different rod ratios, but I digress.
To my surprise, I found that at 90 deg BTDC or ABDC (same position) the piston is not half way in the bore. It is actually more than half way down. Depending on the rod/stroke ratio, the half way point occurs around 80-83 BTDC which is 97-100 ABDC. On a 4.0" stroke 6.2" rod at 90 deg ABDC the piston is 2.331" down. With a 4" bore and 10:1 static compression ratio there is 5.585 ci above the piston at TDC. So if the intake valve closes at 90 deg ABDC, you would have 34.883 ci above the piston. So the dynamic compression would be 6.25:1 not the 5:1 that you would expect (thinking the piston is half way). This all assumes that I did all the geometry correctly, and I think I did, but I have made mistakes in the past. I hope this is useful to the subject. |
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When you do a compression test, you're measuring Cold Cranking Pressure (AKA Dynamic Compression.) The static compression ratio (in your case, 10:1) is one of the facters in the equation. The three things that seem to make the most differance are static compression, intake valve closing point, and altitude. But it also includes bore, stroke, rod length and boost pressure.
Here you'll find a calculater. It's for Harley engines, but works on any 4 stroke: http://www.rbracing-rsr.com/comprAdvHD.htm |
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