LOW Compression Readings! Need HELP
07-15-2005, 10:08 AM
#21
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Re: LOW Compression Readings! Need HELP
Originally Posted by rmbuilder
There are two mechanical parameters that will alter cranking pressure. They are
1) Static compression ratio or the swept volume of the cylinder.
2) Effective or dynamic compression ratio, which is determined by the Intake Valve Closing angle. (IVC) Altering the LSA or overlap does not affect cranking pressure unless the changes made move the IVC angle.
Because the intake valve does not close until well into the compression cycle, you cannot build significant cylinder pressure until that point of closure. That means the total swept area is not compressed, only the volume above the piston when the intake is completely closed. An early IVC will increase cranking pressure and a later event will decrease it. Be careful not to confuse cranking pressure with running cylinder pressure. Advancing or retarding the ICL can also accomplish this after the cam is ground. Matching the IVC to your static compression ratio is critical in determining the torque curve you arrive at.
In this case the IVC point would not alter your cranking numbers to the degree shown in your results. I hope this helps.
Bob
1) Static compression ratio or the swept volume of the cylinder.
2) Effective or dynamic compression ratio, which is determined by the Intake Valve Closing angle. (IVC) Altering the LSA or overlap does not affect cranking pressure unless the changes made move the IVC angle.
Because the intake valve does not close until well into the compression cycle, you cannot build significant cylinder pressure until that point of closure. That means the total swept area is not compressed, only the volume above the piston when the intake is completely closed. An early IVC will increase cranking pressure and a later event will decrease it. Be careful not to confuse cranking pressure with running cylinder pressure. Advancing or retarding the ICL can also accomplish this after the cam is ground. Matching the IVC to your static compression ratio is critical in determining the torque curve you arrive at.
In this case the IVC point would not alter your cranking numbers to the degree shown in your results. I hope this helps.
Bob
I think all the replys you have here especially this one from Bob should leave no doubt that something is wrong with the compression numbers you have been given. I have checked motors with blown head gaskets and seen higher numbers.
If it is possible you should witness the test with a guage supplied by you. If the numbers are the same, then there is something not right. Make sure the guage has the correct adapter for that type head and that the o-ring is lightly greased where it seats against the sparkplug hole.
07-15-2005, 10:12 AM
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Re: LOW Compression Readings! Need HELP
C'mon Bob, enough with the stats. Tell him your opinion of a motor with those numbers. 
BTW, very good explanation in your earlier post.
Gary
BTW, very good explanation in your earlier post.
Gary
Originally Posted by rmbuilder
Throttle open/closed +/- 10#. Warm/cold +/- 10#. This could swing the delta approx 20# in either direction. Not the correct way to perform the check, but it wont alter the results by 100#. This cam (396271/281-14) specs at:
270*/280* Adv
221*/232* @ .050"
.542"/.565" valve
114* LSA
This cam set @ 114* ICL will have a IVC value of
69* ABDC
112* ICL
67* ABDC
110* ICL
65* ABDC
Bob
270*/280* Adv
221*/232* @ .050"
.542"/.565" valve
114* LSA
This cam set @ 114* ICL will have a IVC value of
69* ABDC
112* ICL
67* ABDC
110* ICL
65* ABDC
Bob
07-15-2005, 10:37 AM
#23
MarineKinetics
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Re: LOW Compression Readings! Need HELP
Gary,
Thanks, we have done alot of work with cylinder pressure as it's one of the keys to making power. That engine with an approx 9.25:1 SCR, in healthy condition, with that cam, should be making a bare minimum of 100#+ more on every cylinder. It seems suspect that with hundreds of these cams installed the cranking pressure has not been recorded. There a many potential events or combinations that could account for these readings, some very expensive and some not so. I would personally retain someone competent to determine the cause/causes and the expense to repair them prior to any purchase. These numbers are not within tolerance, and the cam is not a contributing factor. Anything else at this point is speculation.
Bob
Thanks, we have done alot of work with cylinder pressure as it's one of the keys to making power. That engine with an approx 9.25:1 SCR, in healthy condition, with that cam, should be making a bare minimum of 100#+ more on every cylinder. It seems suspect that with hundreds of these cams installed the cranking pressure has not been recorded. There a many potential events or combinations that could account for these readings, some very expensive and some not so. I would personally retain someone competent to determine the cause/causes and the expense to repair them prior to any purchase. These numbers are not within tolerance, and the cam is not a contributing factor. Anything else at this point is speculation.
Bob
07-15-2005, 10:59 AM
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Re: LOW Compression Readings! Need HELP
BTW,
I have a 36 year old, 10,000 hr, flathead, 4 cylinder, 6 1/2:1 CR, ford generator motor with better psi numbers than that motor.
But maybe if I put in one of them thar' hi performance Isky cams, the numbers would go down.
Gary
I have a 36 year old, 10,000 hr, flathead, 4 cylinder, 6 1/2:1 CR, ford generator motor with better psi numbers than that motor.
But maybe if I put in one of them thar' hi performance Isky cams, the numbers would go down.
Gary
07-15-2005, 11:00 AM
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Re: LOW Compression Readings! Need HELP
This mechanic that tested this needs to either toss his gauge in the trash or learn how to read!These 2 motors wouldn't even want to start if the numbers were really that low and it would blow by so bad it would look like a oil bomb went off in the engine compartment,Smitty
07-15-2005, 11:05 AM
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Re: LOW Compression Readings! Need HELP
Originally Posted by articfriends
This mechanic that tested this needs to either toss his gauge in the trash or learn how to read!These 2 motors wouldn't even want to start if the numbers were really that low and it would blow by so bad it would look like a oil bomb went off in the engine compartment,Smitty
Smitty, quit "beating around the bush" and give us your honest opinion
07-16-2005, 08:17 AM
#28
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Re: LOW Compression Readings! Need HELP
Originally Posted by articfriends
This mechanic that tested this needs to either toss his gauge in the trash or learn how to read!These 2 motors wouldn't even want to start if the numbers were really that low and it would blow by so bad it would look like a oil bomb went off in the engine compartment,Smitty
I have never had a motor read lower than 129 psi.
On the leak down I guess it could be how he did it for the higher #'s???.But I would consider anything above 10% would need re ringing .But I am use to Drag car engines .Mine would be 10% or less or 50% and above being a burned piston
It does sound like something it up with the compression test. its to far off!
If the cranking compression in that low it would spin over like it doesn't have the spark plugs in it
Rob
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07-16-2005, 01:16 PM
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Re: LOW Compression Readings! Need HELP
If these are the numbers, that the mechanic for six years has given you, and he comes up with the cam as the reason,
RUN, DON'T WALK away from this deal AND the mechanic.
If he has not omitted a '1' from in front of these numbers, either he is an idiot, the motors are junk, or both.
I cannot envision a competent mechanic even GIVING someone these numbers to support a claim of motor health.
He's not doing the test properly, or his guage is junk or both; but to not recognize these numbers as unacceptable, and track down the cause, is totally incompetent.
The cam is super mild anyway...
There's lot's of boats.
RUN, DON'T WALK away from this deal AND the mechanic.
If he has not omitted a '1' from in front of these numbers, either he is an idiot, the motors are junk, or both.
I cannot envision a competent mechanic even GIVING someone these numbers to support a claim of motor health.
He's not doing the test properly, or his guage is junk or both; but to not recognize these numbers as unacceptable, and track down the cause, is totally incompetent.
The cam is super mild anyway...
There's lot's of boats.
07-18-2005, 08:00 AM
#30
Re: LOW Compression Readings! Need HELP
OK, OK. I've read all the results and offer the following questions.
Would these engines be capable of getting a 8000# boat on plane, much less even run?
Would these engines be capable of pushing said boat near 60mph at 4000 rpm?
I do not profess to be an engine guru. That's why I came here for answers and I have no earthly idea as to why the compression readings were so low. But, I can tell you that this boat DOES run, and RUN well.
My guess is that the test was mis-read some how some way. About the time the previous post was posted, we were cruising Lake Michigan at 60 mph.
Plain and simple the compression test results do not match the ability or health of these motors.
Bob - you want to chime in here?
BTW, I need suggestions for a user name change! Anyone want a ride?
Would these engines be capable of getting a 8000# boat on plane, much less even run?
Would these engines be capable of pushing said boat near 60mph at 4000 rpm?
I do not profess to be an engine guru. That's why I came here for answers and I have no earthly idea as to why the compression readings were so low. But, I can tell you that this boat DOES run, and RUN well.
My guess is that the test was mis-read some how some way. About the time the previous post was posted, we were cruising Lake Michigan at 60 mph.
Plain and simple the compression test results do not match the ability or health of these motors.
Bob - you want to chime in here?
BTW, I need suggestions for a user name change! Anyone want a ride?
