compression test reveals bad?

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cudaboy 67

cudaboy 67

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done a compression test on my car today, a 440ci unknown history, runs nice,no burning oil/losing water,want to eventually change heads to ali as 906s and have difference of over a bar between first two cylinders .whats the chances of my heads being poor valve clearance or bores worn etc. car never really inspired me power wise,bit of a dialema.options i guess are change heads to ali and find out what bores are like then.try so source a 440 block and rebuild,take my heads off and get overhauled.
 
I would not say that 1 bar (14.5 psi) is necessarily cause for alarm, but it's "getting there." Variation of more than 10-15 percent between high and low on a warm engine.

Sounds like these are two adjacent cylinders, it could be a leaking head gasket between.

Try checking the two with plugs out, then recheck with a plug in the one you are not checking and see if that affects the readings

The other thing you do, to help determine if rings or not, is throw a little engine oil into the cylinders and recheck
 
General rule of thumb is all cylinders should be within 10% of each other. I like to tighten that spec up for a performance build.
 
cheers for that, im thinking its a head problem not rings as the compressions not low just bit varied and the engine was laid up for bit before i fitted so possible rust on valves or just heads crap.
 
Repeat Cudaboy, to eliminate piston rings as being the problem squirt a little engine oil in the spark plug hole and redue your compression test. If the deviation between cylinders improve then it's your rings otherwise it's just a guess what's causing it. Also when doing a compression test make sure your carb butterflies are all the way open and all the other spark plugs are removed.
Also before contemplating different heads to increase the performance you need to list what cam is in the engine, what carb are you running, what intake manifold is installed, where is your ignition timing at initial, total and all in at what RPM.
There is nothing wrong with 906's for a 400-450HP build and that installing a high flow set of heads without upgrades to the feed system or exhaust system is jumping the gun in my books.
 
hello! redid test, cold, this time held throttle open, readings anywhere between 9 <130psi>{is that low?} and 9.5 bar. squirting oil in bores made no difference to readings.told my engine had a rebuild 15 years ago probobally using factory pistons etc,with a low spec block is it worth changing to ali heads or wait and get a fresh bored 440 short block,decent pistons etc and change lot together. cheers for the advise! oh, heres my settup, holley 750 vac sec carb,edelbrock performer dual plane inlet,shumacker headers,msd ign, sorry not know timing off top of my head.oh, told cam a purple and propobally just off a standard as idles verry smooth.
 
demon seed said:
Repeat Cudaboy, to eliminate piston rings as being the problem squirt a little engine oil in the spark plug hole and redue your compression test. If the deviation between cylinders improve then it's your rings otherwise it's just a guess what's causing it. Also when doing a compression test make sure your carb butterflies are all the way open and all the other spark plugs are removed.
Also before contemplating different heads to increase the performance you need to list what cam is in the engine, what carb are you running, what intake manifold is installed, where is your ignition timing at initial, total and all in at what RPM.
There is nothing wrong with 906's for a 400-450HP build and that installing a high flow set of heads without upgrades to the feed system or exhaust system is jumping the gun in my books.
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many thanks for the input, im learning all the way...... im now told my engine a "lo comp "ie rebuilt years ago and that my 9 bar or 130psi on compression test about right<correct me if im wrong here>and no real performance possible unless fit better pistons etc. my 440 block an early one and if i do rebuild will build that rather than source another.
 
Yeah, low comp. I have a 70 340 with a purple, factory 1970 pistons that has 150 psi cold.
 
cudaboy 67 said:
cheers for that, im thinking its a head problem not rings as the compressions not low just bit varied and the engine was laid up for bit before i fitted so possible rust on valves or just heads crap.
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u make it sound like u dont know when it last ran...
 
engine in car and been run for year and as half now,only reason me doing the compression test was to see what shape it in prior to me changing heads and cam.the motor was left standing prior to me doing 44o conversion.engine runs nice, no oil/smoke etc just docile.im thinking i got head/compression probs as "440s " were supposed to be 375hp or whatever, mine just a lazy one!
 
cudaboy 67 said:
engine in car and been run for year and as half now,only reason me doing the compression test was to see what shape it in prior to me changing heads and cam.the motor was left standing prior to me doing 44o conversion.engine runs nice, no oil/smoke etc just docile.im thinking i got head/compression probs as "440s " were supposed to be 375hp or whatever, mine just a lazy one!
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375 was for the magnums or 6 parks i think...
 
green1 said:
Yeah, low comp. I have a 70 340 with a purple, factory 1970 pistons that has 150 psi cold.
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150 is pretty low. I had a 8.5 360 with a crane hyd cam that had almost 175.
 
great info guys! im at that point<power quest=P~> where i could either change heads on engine as is along with cam,dial in properly ad see if that picks things up or go full hogg and rebuild block along with new heads and cam, some time ago i posted and quoted somthing like "my 440 doesnt feel much more than my 318" that small block nothing special but 240 hp,seems to make sense now,on the subject of blocks etc, somone mentioned keith black pistons and is it worth going steel crank?
 
805moparkid said:
if the cam is not degreed in and the marks are off, the cyl pressure could be low because the cam is to retarded
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Absolutely. That engine I referenced was blueprinted to the inth degree. Even though all it was was a 71 8.5:1 360. It ran 13.99 first time out in my dart. If you are dilligent, you can make chicken salad outta chicken ****. lol
 
hello, so maybe worth looking at cam timing first? to be honest im in no rush to go spending<who is> if i can liven it up a little i will!:cheers:
 
An old racer's trick is to degree with a compression gauge. You can do it with a mild cam, but as you get more radical, a degree wheel is best. All you do is remove the timing cover, put a compression gauge in number one, get a base reading and advance the cam timing until the pressure drops off, then back it up one. That's your highest cylinder pressure for that combination. It works well, but you need to be careful. You need to call Mopar Performance and see how much advance (if any) is ground into that cam. Usually, most cam grinders grind 4 degrees in. If that's so, then you don't want to go too far. Advancing the cam timing will give big gains in bottom end torque while sacrificing a smaller amount on the top end. It's usually a good tradeoff. Course, I'm sure I'll get slammed with the "you need to use a degree wheel crowd", but that method really works very well for mild cams and it certainly sounds like yours is mild. I've done it more times than I can count. If you can get it at 160 PSI or more, that'll really wake it up.
 
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