I'm sure a lot of you have read about my low compression problems that i've posted before. Some answered, thanks. After talking to a stangnetter the other night, we thought the low compression might be caused by timing being messed up, thus keeping valves open when they should be closed. 1 cyc. was good (160) and 7 were bad...110, 90, 40's, etc. He said to test it, check a cyc. w/ low compression and pull the rockers off for that cyc. and test again. He said if it's out of time, the compression should increase because the absense of a rocker would allow the valves to close. Is this a valid assumption? I did this and before w/ rocker comp. was 90 and after w/ rockers removed, it was like 70, so i'm not sure what to think. Any ideas?
Is Compression influenced by rockers off?
- Thread starter TheUser
- Start date
The only thing timing affect is when the spark hits. It does not affect when the valves open or anything that has to do with the valve train. An ill adjusted rocker may because tightened too much, it might leave the valve open which would kill your compression and also make it run like crap. How does the car run? Is it burning oil? How many miles are on it? There are a few things that can cause you to lose compression. The most common reasons are bad valves or bad rings. Both should cause massive oil burning and foul your plugs. After testing the compression, pour some oil into the piston and redo the test. If the compression goes up you have bad rings because the oil will help seal better, but if it stays the same its probably the valves. Another reason could be a melted piston. Is it running like crap or were you running a power adder? Also, you cant remove the rockers and test it because no air will get into the chamber to compress. The piston will go down and have no air to suck in because the valves are closed. Just make sure the rockers are a little loose so they dont keep the valves open, then test. But I'm saying its either the rings or valves. Hope for the valves, lol.You didnt say how many miles are on it or how its running so who knows.
Dark Knight
Founding Member
pulling a rocker off wont tell you much.. if you arent pulling any air in thru the closed valve, there's nothing to compress..
valves being adjusted to tight WILL cause a loss of compression though.. as well as the cam being installed retarded..
the only thing the dist. does is change when the spark plug fires, nothing to do with compression...
valves being adjusted to tight WILL cause a loss of compression though.. as well as the cam being installed retarded..
the only thing the dist. does is change when the spark plug fires, nothing to do with compression...
thumper460
Founding Member
Timing.... Ign?? or Cam?? Ign timing will have NO effect on compression, as it is electrical pulse to the cylinders to fire the fuel. Cam timing WILL effect the compression!! the more the Cam timing is retarded ( like a streached timing chain) the LOWER the compression!! the more advanced ( 2*-4* ect..) the HIGHER the compression, as the intake valve will close sooner and allow the piston to build more compression!! COOL?? Pulling rocker arms off , to test the compression...?? With the valve shut..( rocker Off) how does the cylinder GET the air in or out???
Low compression is MECHINACAL !! bad rings, valves , or gasket ( or pistons if Boosted). Ign timing will do nothing !! Run the compression test Dry ( engine warm) record the readings... ie 120 lbs. then squirt some oil in the cylinder, run the test again!! IF the pressure increases..( 145 lbs) then "More than likely" it is rings, and the add'l oil helps them seal and the pressure can build. If the pressure doesnt increase ( 125-130) then " more than likely" there is a hole in there and the oil cant seal it!! Like a head gasket, burnt valve, blown piston!!
Now.. you know there is a problem!! gotta pull the heads to find out exactly what is wrong!! FYI.. if all the cyl are low and within 10% of each other?? then it is the engine its self!! you can increase the compression by: heads w/smaller CC's, Deck the block, or as easy as Advance the cam timing!! cool??
Just me..............................
Thumper
Low compression is MECHINACAL !! bad rings, valves , or gasket ( or pistons if Boosted). Ign timing will do nothing !! Run the compression test Dry ( engine warm) record the readings... ie 120 lbs. then squirt some oil in the cylinder, run the test again!! IF the pressure increases..( 145 lbs) then "More than likely" it is rings, and the add'l oil helps them seal and the pressure can build. If the pressure doesnt increase ( 125-130) then " more than likely" there is a hole in there and the oil cant seal it!! Like a head gasket, burnt valve, blown piston!!
Now.. you know there is a problem!! gotta pull the heads to find out exactly what is wrong!! FYI.. if all the cyl are low and within 10% of each other?? then it is the engine its self!! you can increase the compression by: heads w/smaller CC's, Deck the block, or as easy as Advance the cam timing!! cool??
Just me..............................
Thumper
First of all, thanks for the responses. I get the feeling most people don't read previous responces, Smock, lol, so that's prob. the reason for the same responses.
I'm sorry, yes I know ignition timing fires the plugs, I did mean mechanical timing as in timing chain, etc. I figured maybe the timing chain slipped or something, but a shop checked it and it looked to be fine. No broken teeth on the gears; I put the front half back on at home instead of having them do it. The car does not run at all. I bought it, drove it for like 3 weeks, less than 300 miles and it quit running one day and hasn't ran since June. It is supercharged, i'll throw my sig up after this post. The motor was supposed to be rebuilt with like 5000 miles on it, but i'm not positive of that...no documentation. It does have one good cyc (160 lbs) and I think that's cyc. 4....the others are not all within 10% of each other. I pretty much figured either bad rings, gaskets or valves. The shop said there is NO possibility of a bad or broken valve in that car, but i'm not sure why. Head gaskets would most likely give a white smoke correct? And what are the chances of blowing both head gaskets at once? Also, I noticed no anti-freeze in the oil. If you throw gas in the throttle body and crank it, it will try to start, but can't quite do it. Before it quit running, it stalled at lights and stuff possibly due to the blower w/ no bypass valve, but i'm not sure. Also, it was blowing black smoke on takeoff=rings, right? This is gonna cost a fortune!
I'm sorry, yes I know ignition timing fires the plugs, I did mean mechanical timing as in timing chain, etc. I figured maybe the timing chain slipped or something, but a shop checked it and it looked to be fine. No broken teeth on the gears; I put the front half back on at home instead of having them do it. The car does not run at all. I bought it, drove it for like 3 weeks, less than 300 miles and it quit running one day and hasn't ran since June. It is supercharged, i'll throw my sig up after this post. The motor was supposed to be rebuilt with like 5000 miles on it, but i'm not positive of that...no documentation. It does have one good cyc (160 lbs) and I think that's cyc. 4....the others are not all within 10% of each other. I pretty much figured either bad rings, gaskets or valves. The shop said there is NO possibility of a bad or broken valve in that car, but i'm not sure why. Head gaskets would most likely give a white smoke correct? And what are the chances of blowing both head gaskets at once? Also, I noticed no anti-freeze in the oil. If you throw gas in the throttle body and crank it, it will try to start, but can't quite do it. Before it quit running, it stalled at lights and stuff possibly due to the blower w/ no bypass valve, but i'm not sure. Also, it was blowing black smoke on takeoff=rings, right? This is gonna cost a fortune!
Well the first 2 things I see are cast iron heads and 11 pounds. You've also admitted to not having a blow off valve. Basically your saying you street drove a used combo with iron heads, 11 pounds of boost, no intercooler, no BTM, and no blow off valve. You'd be lucky if it was just the rings. Have you ever taken a head off before. I think you just need to pull one and look for chucked pistons or do the oil on the rings test to see if they seal up a bit. I would say screw this pile. You really need to try the oil comp. test to see if the rings are bad. If that doesnt help, pull the head and check for melted pistons which could easily be a problem. This will also give you a chance to get the valves checked and inspect the gaskets. If none of this helps, get it cleaned up, bolt it back together with new gaskets and restab the cam with a new chain. I know its shotgun maintenance but at least in steps. It should also eliminate your problem or let you know what the problem is. Also, black smoke is generally more gas than oil. Oil burns blue and water is white.
Thanks smock. I didn't know cast iron heads were bad? I think I did the oil in cyc compression test, but i honestly can't remember because i've done a few compression checks at different times just messing around and this process has been months; i'll do it again. I've never pulled a head, but I want to and just see. If the weather were better here, i'd for sure do it now since i'm on Christmas break, but hopefully I can get it done anyway. I'm definately gonna get a bypass valve. Thanks for all your help....really appreciate it man.
