high compression

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jacksdodges

jacksdodges

jacksdodges
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Little Rock Ar,
I was checking the compression on my stroked 340 and here are the mumbers 1/200,2/200,3/190,4/205,5/150,6/185,7/190,8/185. we checked this with two gauges.
and came up same. I have adj. hyd.roller rockers if that matters. This is not a new motor and runs pretty good. I am not running high comp. pistons 9.7 I think, and X heads.
I have been told the exhust valve isn't opening enough ?? I will pull the valve cover off tomorrow and do some checking. Just wondering about so much comp.
 
Valves are either open or closed on a comp test (amount matters very little)
Length of open and closed times could, but I don't see the valve timing changing without there being obvious reasons why.

Considering a standard 5.9 magnum can run 175 or so normally, it doesn't seem that high.
 
Number 5 either has a popped head gasket, valve going, cracked ring or notches are lined up, in that order. You never did tell us what stroke, 4.00, 3.85, 3.75, 3.58 and Bore 4.04, 4.07, 4.08. The standard 340 with the slight dome, 4.04 bore, 68cc chambers is 10.5 to 1, with a same pistons and a 4.00 crank is a 13.5 to 1 or 14.0 to 1. So we need to know 0 deck or not, flat tops or dome and if domed what type and shape, stroke, heads, and ring position or type of piston. But I would bet on head gasket looking at the numbers (Between 3 and 5), either way heads are coming off. I wouldn’t play with it take them off and look, the last thing you want to do is hydraulic it, then its junk. If it is the head gasket resurface both heads and block.
 
200psi is very definitely high cranking-compression.

I plugged some numbers into the Wallace calculator.

Suppose you had a cam like about 292/108,with an ICA of 70*
It would take an Scr of 12.5, to make a Dcr of 9.6, to make 200psi of cranking compression.
But suppose you had a 284/112/ICA of 70*;same results.
But suppose you had a 276/110 cam. ICA of 65*. Compressions come out to 12.2/9.58/200psi

You are in racegas territory. Even with aluminum heads.
Ima thinking you burned it down. I'd put a scope in that #5hole.
 
jacksdodges said:
I was checking the compression on my stroked 340 and here are the mumbers 1/200,2/200,3/190,4/205,5/150,6/185,7/190,8/185. we checked this with two gauges.
and came up same. I have adj. hyd.roller rockers if that matters. This is not a new motor and runs pretty good. I am not running high comp. pistons 9.7 I think, and X heads.
I have been told the exhust valve isn't opening enough ?? I will pull the valve cover off tomorrow and do some checking. Just wondering about so much comp.
Click to expand...

My 408 is 9.9 to 1 compression and the compression runs in the 190's across the board. The only problem I see with yours is #5 has an issue and is real low compared to the rest suggesting a problem with that cylinder.
 
my 416 is 10.03 to 1 and is 170 psi in every hole . so the cam determines a lot when it comes to compression .
 
Yes. ICL makes a huge difference in cranking compression. We need cam specs from the OP to figure anything out. Eric L
 
Oh oh, science time!

Ok! AJ, take over!
 
Heck yeah, I hear AJ is gonna go on a science rant? Ears perked.
 
Naw,
Just check out the Wallace calculator. Plug in all same numbers except play with the ICA. Then watch the Dcr and cranking compression numbers.
And the V/P index(I love that V/P index)
Bring pencil and paper. And coffee.Build a graph. Be prepared for surprises.Don't be in any hurry, cuz it's addictive.
 
Well I have done all the things I have been told to try and I guess I just got a weak cyl. Did the leak down test today and it didn't leak down, put a camera in the cyl. couldn't see anything, took the rad. cap off with motor warmed up did the leak down test no bubbles in rad. plus it didn't leak down.
got all the water leaks stopped, new pvc like someone suggested. I guess I got it all done, all I have to do now is drive it. Thanks for all the help.
 
U could drive it hard for a few weeks then check cyl. 5 again for laughs. sometimes U get a weak cylinder, drive it
 
When that happens to me, this is what i do;
I put the piston at the bottom and remove the rocker gear. I pressurize the chamber with the LeakDown tester. Then I tap the ends of the valve stems, while watching the gauge, and listening to the sound. If you do this to one of the good cylinders first, you will get to know the sound that a well-sealing valve makes, and how the LD gauge responds.For this test, you can use full compressor pressure up to something like 100 to 120psi.

What I am looking for is ever-climbing pressure, and what I am listening for is the very specific sound that a well-sealing valve makes.

And here is the why of it. Sometimes carbon can build up on the seat or the valve face, especially if your carb is running fat.Then it can hold the valve off the seat, and the compression suffers. The LD test should have caught it, but when not using full compressor pressure, it sometimes sneaks by.The tapping on the valve stem allows the spring to smash the carbon to powder, and the air pressure blows it outta there.
..... DO NOT TAP ON THE RETAINER.......
If you dislodge the keepers, the spring will send the retainer into outer space, and half your face too,if it gets in the way.
Now there is a chemical alternative to this. You pour this stuff down the carb throats in a very specific way, and then let it ferment overnight. Next mourning you start it up and clean it out by going for a spirited drive. I think there are at least 3 long-time suppliers of this wonder chemical, but I'll be durned if I can remember the name of it.
In any case it's a carbon cleaner.
 
OK I have been doing this for a long, long, long, time, just pull the heads. You read the spark plugs, and you read the valves with a scope then there is nothing else. If your leak down went OK, plugs look good, I would bet you popped the gasket between 3 and 5 on the thin spot between the two. I would get a straight edge, pull the heads, and check the block and have the heads magnafluxed. The PSI doesn’t concern me it’s the drastic difference between all 8 that have me concerned. You can pop the gasket in the thin area and never see a bubble in the coolant. When you reassemble the engine start an engine log, thickness of head gaskets, piston type (Flat, two valve reliefs or four, small dome, medium, large), head casting numbers, valve size (gasket type, head type), CC heads (It is easy to do with Plexiglas, a $3 dollar Beeker, and oil “not alcohol”, and a little grease, cost $20 bucks). The point is you should have a book with everything you have done to the engine, and all your experiences. It will save a lot of headaches for a few minutes and a $3 book, just saying. When you put it back together use a tri-metal-gasket they are expensive but worth every dime.
 
If you passed the leak down test, my opinion is that it is in the valve adjustment. You should go over them ONE AT A TIME very carefully and make SURE they are all adjusted the same amount of lifter preload. I have even gone so far in the past as getting the cam lobe on the base circle, marking a scribe on the pushrod even with the valve cover rail, making my adjustment, making another scribe and measuring the distance between the two, then continuing on and making each of them the same.

If you have decent compression numbers......and you do, and it passes a leak down test......and you say it has, the valve adjustment is pert nearbout the only thing left.
 
340BABY said:
If your leak down went OK, plugs look good, I would bet you popped the gasket between 3 and 5 on the thin spot between the two.
Click to expand...

Umm, no offense but how the heck could the leak down test pass if the head gasket was blown between cylinders? No possible way IMO
 
I've seen some weird chit before.....
 
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