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Something I learned on engine compression

PeteInCT

#LS-378 - So many Porsche's, so little time....
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I recently wanted to check the compression on all cylinders of my Boss given the issues I've been having with limp mode, presumably caused by a high misfire count. My concern was that if the misfires were caused by a lean burn at WOT, I may be doing damage to the rings and valve seals. Long story short I learned that:

1) The pressure (in PSI) in the cylinders has a linear relationship to the compression ratio. The formula is RATIO x 14.7 = PSI. Since our compression ratio is 11:1, 11 x 14.7 = 161.7 PSI. On my Boss the readings ranged fom 155 to 160 PSI, so the compression on all cylinders seems fine.

2) I also read that you could see up to a 15% difference between the highest and lowest cylinder. For me that would mean the lowest pressure I could see would be 160 - 15% = 136 PSI. I am not sure if that 15% value is really a good standard to go by, but in any event I would suspect that on a new car with low miles, even if it's tracked, you would not want to see a deviation in pressures that high.

3) No knock on Harbor Freight at all, but if you are considering purchasing their engine compression kit then be forwarned that it will not work with the Boss. The spark plug threads in out heads are too deep for both the adapters on the hose option as well as the option to use the solid shaft (which does not screw in like a spart plug but rather has a large rubber grommet that when held up against the cylinder head creates a seal for the pressure reading to be taken). The key here I believe is that if you plan on purchasing a compression gauge, make sure the adapters have a threaded section about as long as the threads on our plugs (about 1"). On the Harbor Freight kit they are only about 1/4" long and since the threading is recessed (to help insure we dont cross thread the plug on installation) it cannot reach the threaded portion of the head.
 
I've seen it posted a few times that there are concerns about our cars running lean especially with TK. My thought was don't all modern cars run lean to meet emissions? Anyway I'm glad your compression is fine. What's next new sensors?
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
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FRPP has been thinking about it for 3 days. Don't know yet. Need to call dealer again tomorrow.

On running lean, the real questions are

1) How lean does it run at WOT? I want to use the Aerofirce to ascertain that maybe today. Alert the CT state troopers... ;D

2) What design criteria does our engine design have to allow lean burns, if any. And are they really lean ????
 
Good info on the compression testing on these motors. I guess I need to start looking for a new set up.

I don't think it is intended that the formula is used to calculate the expected psi from a compression test knowing the nominal compression ratio (mechanical compression, calculated by dividing the volume of the chamber by the volume at bottom dead center). I believe that the formula is intended to calculate the effective compression ratio ( the ratio of atmospheric pressure to the chamber pressure in the cyclinder at peak pressure). The reason that the nominal and effective compression ratios will not be the same is because of valve timing events and the dynamic cylinder filling that occurs based on pumping mixture into and out of the cylinder. The pressure you measure even on the same engine will vary by altitude and barometric pressure. Manufacturers usually specify the effective compression in psi that takes into account the characteristics of the motor. It is usually specified as a range, say 150-160 psi for example. This accounts for manufacturing and assembly variation and different testing conditions that might be encountered. The compression test is useful for determining if there are particular problems due to wear (ring seal, valve seating for example) or damage (broken piston, head gasket leak, burnt valve, bent valve, broken ring for example). If a cylinder or more deviate from the average by more than 5%, it may be something that is damage related. If the average is 10 to 20% below the factory range, it probably means general wear. A leak down can tell whether the issue is ring/cylinder seal or valve seal.

Anyway that's my understanding. Hope it helps. Might be interesting to know what others have recorded for compression testinhg. I don't know what Ford specifies for this motor.

Mike
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
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Mike - I fully get your good comments on nominal/effective as well as impacts based on current atmospheric pressure. The site where I got it from made it sound like it was intended to be used for expected PSI, with the assumption of being at sea level. Then again who knows where they cam e up with the 14.7 multiplier. It seemed like a feasible value since I measured 155 - 160 PSI and the calc came up with 161.7 PSI. I spoke to my buddy Rick who runs a local shop and he said he wasn't sure exactly what the Boss motor should read in a compression test but it's been his experience that anything over 150 PSI usually means there are no issues with compression.

I'm going to do a little more research on the web to see if I can come up with any more info. It would be great that if one of us has a gauge they could throw it on the engine and see what they come up with.
 

TMSBOSS

Spending my pension on car parts and track fees.
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Pete

"Then again who knows where they cam e up with the 14.7 multiplier."



14.7 is atmospheric pressure at sea level.....give or take. :)


14.7 times compression ratio does give you a good SWAG for what the pressure should be. Yep valve timing, atmospheric pressure (DA) and a whole bunch of stuff will effect actual compression PSI.


The older Chilton and Motors Manuals listed cylinder pressure in the tune up specs tables. Probably went away with the common tuneup.

Good Luck working through your issues.

I enjoy reading your posts/replies. Great information, outstanding sense of humor. Thanks
 
PeteInCT said:
3) No knock on Harbor Freight at all, but if you are considering purchasing their engine compression kit then be forwarned that it will not work with the Boss. The spark plug threads in out heads are too deep for both the adapters on the hose option as well as the option to use the solid shaft (which does not screw in like a spart plug but rather has a large rubber grommet that when held up against the cylinder head creates a seal for the pressure reading to be taken). The key here I believe is that if you plan on purchasing a compression gauge, make sure the adapters have a threaded section about as long as the threads on our plugs (about 1"). On the Harbor Freight kit they are only about 1/4" long and since the threading is recessed (to help insure we dont cross thread the plug on installation) it cannot reach the threaded portion of the head.

1) Purchase or acquire one stock spark plug.
2) Remove ceramic insulator and center electrode.
3) Weld body of plug to solid shaft from kit.
4) Profit.
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
Moderator
2,848
14
Connecticut
I though of that but I never welded anything in my life. Its on the bucket list ;D
 
I'd say that your motor is healthy based on those values. My '69 Boss with 10.5:1, stock cam and about 25,000 miles on the motor gets 170-175. That is above the theoretical value and it leaks down in the 10% range so there is some blow by (mostly rings). Apparently, even at cranking speed, it has some pumping effects that charge the cylinders. I am at 300 ft asl. Since I have less than 5 psi variation plus or minus and it is within 5-10 psi of what it was when it was built, I figure it is fine. It would be nice to know what Ford uses as the range on these motors. I haven't seen that number.

Mike
 
one can alter the cranking compression with advancing or retarding a camshaft.

keep that in mind also. its not just a simple formula. each cam will generate more or less cranking compression depending upon open and closing events
 

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