REAL Detonation.....can valve train cause this?

[turbosam6]

He did check the cam timing, read the above posts.

You don't have to pull the whole intake off to check lobes. If it is the cause, I would think it would be obvious by the rocker arm movement. You could use a dial indicator on each rocker to measure travel. Just trying to save you some labor. How 'bout them plugs?

 

[snodgrass]

I don't see that Grandnat has actually checked the cam timing. ILBCNU6 posted that he has degreed his cam in on his post. Advanced cam timing creates tremendously high cylinder pressures. All of his symptoms go back to when he replaced the cam. He's getting detonation at 6 lbs boost with good gas and a good A/F ratio. His timing is correct at 20*. All this leads back to too much cylinder pressure. So, my suggestion to Grandnat would be to check your cam timing with a degree wheel and dial indicator. Don't just go by the dots on the timing set.
-Jeff

 

[Lee_Burough]

Any changes in your data logging when you run race gas? Any change in KR? Just asking the obvious....

 

[steve hughes]

I had a problem with a Comp 212-212 a few years ago. Car would run great up to 17 #'s. Anything over that it would miss and blow head gaskets. I replaced everything electrical in the car and even changed from the stock ecm to a gen 6 dfi . Nothing improved the problem. After 2 years of chasing my tail i put my old Modern Muscle 208 back in the car. Problem solved.

 

[TurboJim]

Originally posted by Grandnat
my question is this.

can weak springs or an improperly ground cam cause this?


I am really fed up because it looks like I am pushing out the passenger's side HG again!!!

Too small a margin on the valve faces can do this...

Have they ever been cut?

Jim

 

[Grandnat]

Yes Jim,
I did have a valve job done on the heads before I installed them If I pull the head what do I need to look for to see if that is a problem.


I checked the plugs but couldn't get alot of useable data since they had been idled, but I didn't see redeposited aluminum (detonation). They were dark brown. the startup tables in FAST are probably too rich....still working out the tuning bugs on that.

I also did a compression check
and they were as follows:

cyl #1 155 psi
cyl #2 155 psi
cyl #3 155 psi
cyl #4 165 psi
cyl #5 160 psi
cyl #6 160 psi

I pulled a spring last night and will have it checked to see if it is what it is supposed to be.

and if I have time I will check my pushrod length

basically I am going to make sure I have 50 thousandth between the rocker and valve stem surface.

 

[Reggie West]

Flat cams will do all kinds of neat things to your motor.

Stage motor I just saw wiped out 4 lobes and the car still ran OK.

The valve springs had 100 ft lbs on the seat when they should have been 150 at least.

What the lower seat pressure does is blow open the valve when it should be closed. If you are seeing this happen at 6 psi and better that would explain a couple of things.


I would be pulling the cam out and looking at that.

Good luck

 

[Grandnat]

I checked a valve spring and it is actually a bit stronger than what comp suggested for my cam, but not enough to worry about IMHO.

HOWEVER, my installed spring height was ~1.8"

which probably makes my seat pressure really low.

I know at 1.88" the seat pressure was only 60lbs


I need to pull the heads and make sure the installed height is universal on all the valves and that they are shimmed to the appropriate height.



so can someone tell me what seat pressure I need?
or what the installed spring height should be?

 

[Reggie West]

If your spring height is lower than it should be that would tell me the lifter is resting on a worn lobe.

 

[Grandnat]

actually valve spring installed height is independant of the cam.


it is simply a measure of the spring height when there is no pressure applied to the valve.

 

[turbosam6]

Sorry, I thought you had a roller cam. Definitely check for worn lobes, use the dial indicator if you don't wanna pull the cam. If you only have 60 lbs. seat pressure closed, then you may be hanging valves open, but I'm not sure how that would blow a head gasket. I'd say at this point the cam has to be the culprit.

 

[chevyII]

heads asssembly

the valve height measurment is different for each vale depending on how much seat was ground and if the valves are ground. If you heads dont have the correct height,you spring tension will be off. your lifters might be not staying planted on the cam. not good

 

[TrboHead]

Check your installed height with your retainer and then measure your spring pressure at that height. Better yet, check all your valves for the installed height. If you have a cylinder giving you detonation then you may have an unequal installed height.
Also I would degree your cam against each piston and make sure your cam is not ground funky. They will be off sometimes a couple of degrees which is ok. You will be looking for 20 degree differences. It's a pain but it needs to be done.
If your cam has any pits or if the tip of your lobe has a nice wide shinny area all the way across from edge to edge then I would pitch it for a new one along with new GM lifters.

Am I wrong guys?

 

[Grandnat]

Just a little update here.

While inspecting the pistons I noticed that the #1 and #5 cylinder had cuts in them where the exhaust valve had indeed kissed the piston.


so it looks like I may be on the right track regarding the installed height of the springs. (low seat pressure)


I will keep you all informed.

 

[Big Mike]

Originally posted by steve hughes
I had a problem with a Comp 212-212 a few years ago. Car would run great up to 17 #'s. Anything over that it would miss and blow head gaskets. I replaced everything electrical in the car and even changed from the stock ecm to a gen 6 dfi . Nothing improved the problem. After 2 years of chasing my tail i put my old Modern Muscle 208 back in the car. Problem solved.

Steve,That is the same problem I am having .Put everything new ,changed to fast,and lastly this fall I put new Hg in.Runs great at 18lbs with c16 but when I put the boost up,it would pop and miss.Have a comp 212 212 cam not sure on the v springs.2 years of chasing also.Looks like I will check the springs!

Let us know what you find!
Good Luck