Exhaust valve or cam reversion

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hughesboysvaliant

hughesboysvaliant

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Hey guys , got a question. I was checking the old car out the other day and tried the paper to the tailpipe trick to check for a burnt valve and sure enough the paper would suck up to the pipe and out very quickly. Well I just knew I had a burnt valve or two on the exhaust. It thought I would go ahead and check the intake valves with a vacuum gauge and they all looked good about 14" - 14.5" vacuum with very little slow needle movement in that range. So I moved on to the leak down test . Cranked each cylinder to TDC and set the regulator to 100 psi and went threw the cylinders the worst one was 89 psi the rest was between 90 to 92 psi. So 8 to 10% leak and one with 11 %. Check the tail pipes with my ear every cylinder heard nothing, intake nothing, crankcase was where I heard any air , normal at the % leak I saw. Scratching my head now. So I think cam reversion maybe. The motor is LA 360 with a comp XE268 cam and lifters. The cam is 234 @ 50, 110 LSA, 477 -480 lift with 54 degrees of overlap. Installed straight up dot to dot no advance. The heads are 360 cast iron non smog that has 2.02 1.60 SS valves, shaved 60 thousand mopar performance valve springs shorter push rods and some port work with gasket matching. They run into TTI step headers 1 5/8 to 1 3/4. My question is what do you think is there a problem. Could the springs be weak, could a valve be sticking or would you see that at idle with that cam ?
 
I think you are thinkin too much. Drive it and enjoy it.
 
Yep, your probably right, my wife says I can be obsessive over anything. Can't help it, you speed three years fixing one up then you worry about every little thing.
 
I never saw you describe a problem. I read it four times. Cranking pressure is a little low, but that's a pretty big cam for an engine with low static compression.
 
I agree with Rusty that your overthinking this. And just so you know, having a burned valve will mean either zero or extremely low compression on that cylinder. Since all your cylinders have compression (although it's somewhat low) no valves are burned out.

As for that old "paper to the exhaust test" goes, it's not accurate to say that if it sucks up a paper like you describe that the engine has a burned out valve. If the engine misfires for nearly any reason at all it will suck the paper up. About 25 yrs. ago a guy I knew bought a Caprice convertible real cheap cause it was advertised as having a couple burned out valves. It misfired and ran like crap. I pulled the plugs and there was hardly nothing left of the electrodes. Must have been original. Put in a new set of plugs and it ran smooth as silk. Man was he happy
 
The compressions not that low it's not high either . With the head shaved 60 thou the bigger valves in it and a .028 head gasket we were shooting around 9 to 1 maybe a bit more. The numbers were the leak down pressure with 100 psi on the primary gauge to get the cylinder leak down %. The compression the last time I done it was in the 143 to 146 range that was with a even bigger cam than this comp xe268 I put in it back in the winter. The new comp cam intake valve shuts at 63 degrees which is 8 degrees sooner than the old cam that was building compression in the 140s so I should have higher cylinder pressure now. I think I will do a comp test today to see what I get with the comp cam. If it's a miss it's random got new plugs wires firecore distributor MSD 6a ignition box. I've wondered about the MSD box it was used but they either work or don't usually.
 
My FormulaS has a 232* cam, idles at 700, fairly smooth, and does not do what yours does. I used to leak mine down every once in a while. It was always 4% or less.With new file-fits it was close to zero LD

Bad ig timing can cause misfires,which in a small pipe can suck the paper in. So can sticking or burnt exhaust valves. Reversion has nothing to do with that. Reversion happens on the intake side. The reverse pulse in the exhaust is a function of the header.Thats kinda how it works. It really shouldnt make it all the way out the tailpipe.
The misfire can be interpreted as normal on a big cam engine. 230*@050 is not big. When correctly tuned, it should idle at 750,almost dead smooth, at around 11/12 inches.
Yours, Idling at 14plus inches: either the ignition timing is over-advanced, or the idle speed is too high,or the cam is not a 230*,or the gauge is bad, or some combination. In any case the tune may not be there yet.
10/11% leakdown in my book is inspection time. You didnt say where the pressure was going. Hopefully its exhaust valves, cause that would explain it.
Theres also a good chance that the valves are just carbon fouled,so before you tear it apart,do a plug-check and adjust the A/F accordingly. Then put some top-end carbon cleaner through it, and take it down the highway and clean her out. Then redo the leakdown and see where thats at. At 11% LD youre giving away a lot of performance.
Good luck
 
Thanks for the reply. My vacuum reading are in park at about 1000 rpm I idle in gear at 750 prob would be around 11/12at that rpm. My base timing is 18 with all in at 34. My distributor has adjust plate and the lower compression like the advanced initial timing. When I done the leak down engine was warm not at full operating temp and everything I heard was going in the crank by the rings nothing out pipe or intake. Your engine with a 4% leak down loss is awesome everything I've read new race engine has 4to5% leak . most engines average around 10 to 12 with everything normal.
 
I guess thats why my S goes 93 in the 1/8 at 3600plus pounds with that little cam.
If the pressure is going into the pan, thats too bad.
Your timing sounds real close. Just because it likes 18* at 750, doesnt mean its correct. Too much idle timing can put the blades too far down, and out of the transfers. Then you get idle tip-in problems and/or off idle stumbles, which pump-shot cannot always overcome. However,since I have no idea as to your tuning skills, I may not be telling you anything you dont already know.If thats the case,just ignore me.You wont hurt my feelings.lol.

And is it pulling paper at that 1000rpm?One side or both? X-over? If yes, one and no; there is a problem.
Why are you idling it at 1000/park?
 
I think it's worth mentioning that you should NOT take compression / leakdown readings "literally" across the internet. Here we have a guy throwing around figures of 4% and stating that more than 10% should be dealt with.

THE FACT IS that you can easily obtain a 10% variation by doing nothing more than using different leakdown testers on the same engine!!! Further "at least some" (maybe still do) of the Horrid Freight testers were no where NEAR the accepted "standard."
 
hughesboysvaliant said:
The compressions not that low it's not high either . With the head shaved 60 thou the bigger valves in it and a .028 head gasket we were shooting around 9 to 1 maybe a bit more. The numbers were the leak down pressure with 100 psi on the primary gauge to get the cylinder leak down %. The compression the last time I done it was in the 143 to 146 range that was with a even bigger cam than this comp xe268 I put in it back in the winter. The new comp cam intake valve shuts at 63 degrees which is 8 degrees sooner than the old cam that was building compression in the 140s so I should have higher cylinder pressure now. I think I will do a comp test today to see what I get with the comp cam. If it's a miss it's random got new plugs wires firecore distributor MSD 6a ignition box. I've wondered about the MSD box it was used but they either work or don't usually.
Click to expand...

Trust me, if milling the heads is all you did, the compression is still low. These engines blueprint down into the high 7s from the factory. Throw the factory rating out the window. It was just that. A rating.

Chrysler had poor machining tolerances with block decks. They were notoriously tall, adding to the cylinder volume. They also routinely cast head chambers big, adding to the combustion area. All of this coupled together made for a dismally low factory static compression ratio. Generally speaking, around a .070" cut on the heads will add one full point of static compression. That means you are probably right on top of 9:1 of a hair lower.

Your cranking pressure points to it even more. If your compression was 9.5, IMO your cranking pressure would be somewhat higher. Anyway, cranking pressure is only a number. Maladies such as yours can be worked around. That cam is ground on a 110 LSA and installs at a 106 intake center line. My guess is it was never degreed. You can advance it down around 102 or even 100 intake center line and that will wake it up tremendously with no ill effects. Then re curve your distributor so you have around 20 degrees initial timing and about 34-36 degrees total all in around 2500 RPM and that thing will be a beast for what it is.
 
hughesboysvaliant said:
The compressions not that low it's not high either . With the head shaved 60 thou the bigger valves in it and a .028 head gasket we were shooting around 9 to 1 maybe a bit more. The numbers were the leak down pressure with 100 psi on the primary gauge to get the cylinder leak down %. The compression the last time I done it was in the 143 to 146 range that was with a even bigger cam than this comp xe268 I put in it back in the winter. The new comp cam intake valve shuts at 63 degrees which is 8 degrees sooner than the old cam that was building compression in the 140s so I should have higher cylinder pressure now. I think I will do a comp test today to see what I get with the comp cam. If it's a miss it's random got new plugs wires firecore distributor MSD 6a ignition box. I've wondered about the MSD box it was used but they either work or don't usually.
Click to expand...

This is sounding more like a faulty compression reading at some point. Either with the old cam or with the 268.

Or something has since gone down hill with the heads and or rings......which is unlikely on all 8 holes.
 
Thank guys for all the info. I will do another compression test soon and see if the numbers has changed from the last time. I haven't done one since I put the xe268 in the others was old cam back in winter. The leak down test was this week. I agree with you on the few more degrees in the cam, and no I didn't degree it in. The engine has always had a rough idle I hoped with the new cam It would smooth out some while still have a good sound and performance. She's no beast but I run 8.7 to 8.9 around 86 mph in 1/8th fun but could be better.
 
The only issue I see is going "dot to dot" without degreeing it. But if the cylinder pressure is in the 140+ area, you're fine. Your timing is good, but it should not have to idle at 1000. So I'd say check the carb settings and check for a vacuum leak. How well did the intake fit with the head milling? RRR's right - you have less than 9:1, but that cam should still run decently.
 
Well I guess I shoulda qualified my opinion with:
I assume your guages are accurate, and the engine was not sitting on the top of a mountain, during testing.
I assumed when you said the pressure was escaping into the crankcase, that you meant the pressure was escaping into the crancase. At less than 4% there is almost no sense of where the pressure is going.
But dont take my word for it;
Smokey Yunick himself says at 8%LD "you dont have an engine, at least not one that will win". He also says "if 8% of the cylinder pressure is leaking away, you are theoretically losing 24% of the recoverable flywheel horsepower.I dont think many guys can win a race with only 3/4 of an engine."Smokeys Power Secrets,SA Design, p50
 
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