ck out my compression...
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Posting Guru
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From: western montana
ck out my compression...
remanned 351 with about 50,000 miles on it.
cyliders 1,4,7,8 start at 60+ and bounce up to 120-123 real fast.
2,3 start at 35-40 then bounce up to 120 by 3-4 cranks
5,6 start at 60+ and bounce up to 110-112 real fast
I'm wondering how that measures out
thanks
cyliders 1,4,7,8 start at 60+ and bounce up to 120-123 real fast.
2,3 start at 35-40 then bounce up to 120 by 3-4 cranks
5,6 start at 60+ and bounce up to 110-112 real fast
I'm wondering how that measures out
thanks
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Joined: Feb 2002
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From: East Bend, Carolina, U.S
You aren't supposed to watch for a fast or slow increase in pressure readings. The maximum reading that each cylinder can build up after several revolutions is what you need to look at. The compression gauge will only indicate a maximum no matter how many revolutions an engine is turned. Just try it by continueing to turn the engine over and the maximum you have seen so far will not vary much if any.
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From: western montana
the book mentioned being able to diagnose different possible problems based on how fast the pressure builds as well like the difference between bad rings and valve parts etc. I'm not even a qualified backyard mechanic...yet.
I was wondering how this stacked with a properly built 351/400. I know it was remanned stock by some nitwit. (who in the world would reamn a 351m stock??? unless you were going to use it as a boat anchor.)
So any additional input would be appreciated.
I was wondering how this stacked with a properly built 351/400. I know it was remanned stock by some nitwit. (who in the world would reamn a 351m stock??? unless you were going to use it as a boat anchor.)
So any additional input would be appreciated.
Elder User
Joined: Feb 2002
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From: East Bend, Carolina, U.S
Never heard of anything like that. If a difference is discovered then to me it just comes natural to find the cause I guess from having done it my way for so long. Either way you look at those pressure readings, in the end, either by book or experience, our engines are treated equal.
It is nice to know that there are more books out there giving that kind of detail. I learned the hard way and wish I had had a book like the one you have way back when.
It is nice to know that there are more books out there giving that kind of detail. I learned the hard way and wish I had had a book like the one you have way back when.
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From: western montana
I don't think the difference is big enough to worry about- but was wondering. I was wondering how the pressure stacked against a typical 400 built with higher cr. I will be putting on a dual fuel propane system that I just got- used- to experiment and knowing how the compression stacks up might (or might not) give me some useful info as i decide what to do.
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From: western montana
not really (?)- propane guy sells cams for 9.0 for dual and 9.5 for dedicated. Some say that 12:1 + is best and it probably is ...but I'm willing to bet that 9.5 will do pretty good and may be more practical and dependable.
I've read a whole bunch and will continue to press on. I've drained the alternative fuel section.
I've read a whole bunch and will continue to press on. I've drained the alternative fuel section.
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From: Kentucky
Originally Posted by roger dowty
I don't think the difference is big enough to worry about- but was wondering. I was wondering how the pressure stacked against a typical 400 built with higher cr. I will be putting on a dual fuel propane system that I just got- used- to experiment and knowing how the compression stacks up might (or might not) give me some useful info as i decide what to do.
Just performed a comp test on my newly rebuilt 79 ford 351m converted to a 400. Five crankovers with all plugs removed, with throttle plate on carb open. Each cylinder read 148 to 150psi .
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Roger's compression ratio is down around 8:1 or lower, not good for propane. He has some serious wear or a slow cranking speed that is causing his cranking compression to show up to be in the 7:1 range. At least it is fairly even. His dynamic compression may be better but the engine is tired which is strange after only 50K miles. A valve job may help.
Greg your engine works out to be about 8.5:1 according to my ancient compression gauge.
I posted this in another of Roger's threads:
Approximate cranking speed pressure:
8.0---140PSI
9.0---160
10.0--180
11.0--200
These are approximate and will depend on the cranking speed etc.
Greg your engine works out to be about 8.5:1 according to my ancient compression gauge.
I posted this in another of Roger's threads:
Approximate cranking speed pressure:
8.0---140PSI
9.0---160
10.0--180
11.0--200
These are approximate and will depend on the cranking speed etc.
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From: western montana
I'm not expecting great things from my dual fuel with current motor and 7:1 compression. The truck doesn't seem to burn much oil- hard to say as the rear main leaks a bit. Also keep into consideration that the engine builder built it to stock specs which isn't very impressive to begin with. I am looking for a 400 to rebuild.
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400's were available in full size cars since 1971 as well as 77.5 and up trucks. You will need to keep the compression down to run on pump gas for a dual fuel engine. Try as close to 9.0:1 as possible. All the 351M's were low (8:1) compression engines.
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