cam timing affect compression?
12-24-2012, 01:16 PM
#1
cam timing affect compression?
I recently broke a bank 2 intake cam and I replaced both intake cams with revup intake cams. and im having trouble getting the engine timed right.
with the timing set per the fsm the car runs but when I compression test it. bank 1 reads 240psi. and bank 2 reads 90psi. and I can play with the timing of the gears and I can get it to 180bank 1. but bank 2 will drop to 60ish.
I have had the front cover off 7 times now and I cannot find any reason for the compression to be off I have set the timing over and over again. I took the head off to make sure the valves were ok because one of the spark plugs came apart. the top of that cylinder had some dings and the head had some dings. but the valves were fine and the compression of that cylinder reads the same as the one behind it.
with the timing set per the fsm the car runs but when I compression test it. bank 1 reads 240psi. and bank 2 reads 90psi. and I can play with the timing of the gears and I can get it to 180bank 1. but bank 2 will drop to 60ish.
I have had the front cover off 7 times now and I cannot find any reason for the compression to be off I have set the timing over and over again. I took the head off to make sure the valves were ok because one of the spark plugs came apart. the top of that cylinder had some dings and the head had some dings. but the valves were fine and the compression of that cylinder reads the same as the one behind it.
12-24-2012, 02:22 PM
#3
you have something crazy going on, need pictures and more info though the one bank seems crazy high if thats a dry compression test and bank 2 is a good bit low. not sure what you mean by playing with the timing of the gears unless your putting it a tooth or two off which could potentially damage the engine.
how did you check the valves and seats? if it dint involve disassembling the head and measuring with precision tools its the wrong answer
how did you check the valves and seats? if it dint involve disassembling the head and measuring with precision tools its the wrong answer
12-24-2012, 02:28 PM
#4
When bank 2 cam broke, the valves on at least 2 cylinders would have kissed the pistons. Hence the low compression on bank 2
Last edited by Nathan; 12-24-2012 at 02:29 PM.
12-24-2012, 02:32 PM
#5
pictures of what? I have moved them a tooth or two each way. and messed with the timing between the each intake and exhaust cam. and yea dry compression test. cold and with tb closed.
as far as the head goes. my family owns a machine shop and I had my dad check it out.
as far as the head goes. my family owns a machine shop and I had my dad check it out.
12-24-2012, 02:53 PM
#7
Unlike most cam-belt motors whose combustion chambers & pistons are designed not to interfere when a cam belt brakes the VQ is not
Last edited by Nathan; 12-24-2012 at 03:03 PM.
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12-24-2012, 03:01 PM
#8
Just looked again at the picture & can see that the cams are turning & this breakage would only effect that one valve.
If your camshaft timing is correct the compression can only disappear 2 places. Past the pistons or past the valves.
If your camshaft timing is correct the compression can only disappear 2 places. Past the pistons or past the valves.
Last edited by Nathan; 12-24-2012 at 03:04 PM.
12-24-2012, 03:02 PM
#9
When a camshaft stops, the 6 valves actuated by that camshaft are in various positions from full open to full closed or anything in between. You were just lucky the 1st time that the cam stopped with the valves in a good position.
Look at the photo you posted. If it had been the exhaust cam that had broken, those 2 back exhaust valves are fully open & the piston would certainly hit these as it came up.
Unlike most cam-belt motors whose combustion chambers & pistons are designed not to interfere when a cam belt brakes the VQ is not
Look at the photo you posted. If it had been the exhaust cam that had broken, those 2 back exhaust valves are fully open & the piston would certainly hit these as it came up.
Unlike most cam-belt motors whose combustion chambers & pistons are designed not to interfere when a cam belt brakes the VQ is not
not trying to argue with you about the cam breaking. this thread was to inquire how cam timing affects compression.
12-24-2012, 03:02 PM
#10
im willing to put money on bent valves or damaged valve seats. on top of that there has to be something going on causing the cams to break i know people have broken em before but im running stock cams to 7k on bc springs and retainers with 0 issues. on this engine im betting your "messing" with the cam timing by moving the teeth on the timing was causing an issue with the pistons hitting valves at high rpm and thats what is breaking your cams.
12-24-2012, 03:06 PM
#11
depends which way you go and how it interacts with the exhuast, by adjusting timing you can raise or lower your dynamic compression(actual compression the engine is seeing.) only way to tell is a **** ton of math or one of the online calculators. however do not adjust cam timing on the chain you will continue to break things because you could be advancing things to the point it hits the pistons. honestly im betting you already did, and since the compression is way off then obviously the valves are not fine.
12-24-2012, 03:35 PM
#13
Did you find out what caused the dings on the piston & head?
Last edited by Nathan; 12-24-2012 at 11:15 PM.
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