tracking the reason for low compression



tracking the reason for low compression

Postby grumpyvette » September 19th, 2014, 4:16 pm

occasionally I see guys post questions regarding checking engine compression,or doing a leak-down test or guys who find they have unexpectedly low compression, and can,t find the reason.

COMPRESSION/LEAK DOWN TEST
viewtopic.php?f=87&t=332&p=406&hilit=compression+test#p406

start with the basics, on a properly assembled engine the intake valve opens well before top dead center ,on the intake stroke and as the piston drops down the bore outside air pressure rushes into the low pressure above it in the cylinder area caused by the descending piston.that fuel air mix can only exit the engine cylinder past the valves or ring seal surfaces unless theres mechanical damage. yes valves leak and head gaskets fail , but in many case's, if your getting low readings badly adjusted valves or worn parts in the valve train or rings can be or are the problem, if you do a compression test then repeating after squirting a teaspoon of oil into the cylinder between tests and the reading does go up its generally an indication of less than ideal ring seal,if it does not its generally the valves or head gasket.
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keep in mind you can,t compress whats not getting in, so if you wiped a cam lobe or incorrectly adjusted the valves you may not be allowing the engine to ingest air.
if you look at the cam timing figures on a cam card like this one I,ll post below, you see with this particular cam the intake valve timing says it opens 1 degree BTDC BEFORE TOP DEAD CENTER and closes 35 degrees ABDC AFTER BOTTOM DEAD CENTER keep in mind the upward moving piston can,t compress ANY of the gases potentially trapped between the rapidly ascending piston top surface and the combustion chamber UNTIL BOTH the VALVES ARE SEALED, and the later the valve timing or RETARDED it is the further the piston moves up toward the cylinder head combustion chamber above it before it starts to compress those trapped gasses
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at first you might think that it would be logical to close the intake valve AT BOTTOM DEAD CANTER (ABDC ) to maximize the volume of gases trapped above the piston, but the air/fuel mix has mass and inertia and continues to flow into the cylinder until pressure equalizes, and as rpms increase and port velocities speed up this occurs later in the cycle, ,well past BDC.
Now, if your getting a low compression number on a compression test it can be from several causes
gases can generally escape if the valves are badly adjusted delaying or holding the valves open longer than intended,(this is VERY COMMON WITH HYDRAULIC LIFTERS THAT WERE IMPROPERLY ADJUSTED)or it can be the result of leaking valves or piston rings.
DETONATION DAMAGE IS A COMMON CAUSE,ESPECIALLY ON AN ENGINE THAT WAS RUN HOT< readings below about 60 psi usually indicate mechanical damage, but if you find this on a newly assembled engine be aware that an improperly degreed in cam will cause this and other problems and its very possible to index a cam incorrectly if your don,t degree it it, but simply use the common DOT-TO-DOT install if your no careful
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be aware that some crank gears have more than one index slot to index to the crank key and each slot is marked and you must use the correct matching marks indicating (ZERO) that match the crank slot marks
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above is a picture of how a typical timing chain looks with the DOT-TO-DOT install having the crank gear at 12 o'clock ,(B) and cam gear (A)indexed at 6 o'clock, NOTICE THE WOOD RIFF KEY AT 2 O'CLOCK,IF ITS ANYPLACE ELSE YOU HAVE IT INDEXED INCORRECTLY(C) naturally youll need to rotate the engine one full revolution to get the upper gear index to 12 o'clock and the lower gear back to 12 o'clock before dropping in the distributor.
I,ve seen several cases of guys installing cams that were very badly indexed being 70-120 degrees off the correct index and naturally the car won,t run or build compression when the valves open and close at the wrong time

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=90&p=494&hilit=+bushings+advance#p494
since compressed gasses in the cylinder can only exit over limited areas unless theres been mechanical damage like a cracked head or cylinder wall, heres a few areas to check.
1) badly adjusted valves (very common,can easily result in burn valves, bent valves, broken components, its almost always due too overly tight adjustment, if suspected keep backing off each rocker until it clicks mildly at idle then re-test)
2) leaking valve seats (lack of power, IR heat gun can be useful tool for locating bad cylinder as can reading plug condition)
3) leaking rings (usually lots of blow by at the valve cover breathers, fouled plugs and low compression, usually all cylinders have similar compression within 15%)
4)leaking head gasket (usually lose of coolant overheating ,plugs get wet, bubbles in radiator or coolant in oil)
5) detonation damage (usually lots of blow by at the valve cover breathers, fouled plugs and low compression, one or more cylinders far lower in compression)
6) incorrectly indexed cam (if you just installed the new cam and get very low compression theres an EXCELLENT chance its been indexed wrong, even if your 100% sure you did it correctly go back and check using a degree wheel and a dial indicator and your cam card , not the dot to dot install.)

related info yeah I know you would rather be buried in a red ant hill up to your lower lip rather than read links...try it you may learn a few tips
AND IF YOU CAN THINK OF ANYTHING HELPFUL TO ADD PLEASE DO IT!

http://www.youtube.com/watch?v=OGj8OneMjek

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IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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