would someone know what the "compression maximum" is for a '81 400? thats what chilton refered to it as. i assume they mean what the compression was when it was new. chilton says " a cylinders compression pressure is usually acceptable if it is not less than 80% of maximum". no more than 12 to 14 lbs. difference. all my 8 cyls. test between 150 to 155 lbs. so to go by the book, if max was 190, then i'm less than 80%(bad). if max was 170, then i'm not less than 80%(good). but i don't know what max was. the books don't tell me, the parts stores don't know either. another good question is, an i understanding chilton correctly?

 

Re-read that, they say the lowest cylinder shoudl be no less than 80% of the highest cylinder. I'd say that with 150-155, you have a pretty healthy 400 on your hands.

 

82F100SWB.......i re-read and acually got the quote correct word for word (chilton book company part no. 6913). the "maximum" chilton refers to is suppost to be in their tune-up spec. chart but it's not. maybe a typo on their part. but anyway, with an endorsment from someone such as yourself, i'll proceed with the tune-up, with the knowlege that i have a " pretty healthy 400". thanks for your help. FTE ROCKS

 

82F100SWB is giving you good info. The spec is for the lowest cylinder pressure to be within 80% of the highest indicated pressure. And 150-155 sounds normal and healthy. The bowtie boys were given actual cylinder pressures to shoot for, we were given the "80% of max". Probably because the engines were available with many different head and piston configurations. This spec works fine to diagnose burnt valves, holed pistons or broken rings. You'd probably be smoking like a mosqito fogger before the rings evenly wear enough to cause measurable loss of compression.

 

Sport45..... i beleive he is giving good info and i was paying the man a complement. i have a lot of confidence in you guys, thats why i ask you guys for advice. as far as the book thing goes, i was just quoting the paragraph that was confusing the heck out me. so the book is wrong - it wouldn't be the first time. if my scanner was here i would post it for ya. when it comes to chosing between the working experience you guys have and a book - i say f___ the book. if i have offended, it was not my intention and i do apoligize.

 

I didn't think for a minute you offended anyone. I was just seconding his advice and offering a bit as to why the spec is so. The % of max thing confused me at first (in 1976 when I first read the spec's). I believe my '75 and '83 Ford Truck Shop Manuals say it the same way as your Chilton.

 

If I dig around I have that Chiltons manual around somewhere... LOL
This is from one of my Ford shop manuals(93, first one I had handy):

Compression Test


Compression Gauge Check

1. Make sure oil in crankcase is of the correct viscosity and at proper level and battery is properly charged. Operate vehicle until engine is at normal operating temperature. Turn off ignition switch, then remove all spark plugs.

2. Set throttle plates in wide-open position.

3. Install a compression gauge such as Rotunda Compression Tester 059-R0009 or equivalent in No. 1 cylinder.

4. Install an auxiliary starter switch in starting circuit. With ignition switch in the OFF position, and using auxiliary starter switch, crank engine at least five compression strokes and record highest reading. Note the approximate number of compression strokes required to obtain the highest reading.

5. Repeat test on each cylinder cranking the engine approximately the same number of compression strokes.


Test Conclusion

The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest. Refer to the chart below.



If one or more cylinders read low, squirt approximately one tablespoon of SAE 50 weight or equivalent engine oil on top of the pistons in the low reading cylinders. Repeat compression pressure check on these cylinders.

1. If compression improves considerably, piston rings are at fault.

2. If compression does not improve, valves are sticking or seating poorly.

3. If two adjacent cylinders indicate low compression pressures and squirting oil on pistons does not increase compression, cause may be a cylinder head gasket leak between cylinders. Engine oil and/or coolant in cylinders could result from this problem.

It is recommended the Compression Pressure Limit Chart be used when checking cylinder compression so that the lowest reading number is 75 percent of the highest reading.


Example

If, after checking the compression pressures in all cylinders, it was found that the highest reading obtained was 196 psi and the lowest pressure reading was 155 psi, the engine is within specification and the compression is considered satisfactory.


I hope that helps clear things up

 

Operative word is HIGHEST.

 

YES.....this does help me to understand the test. this is what my chilton doesn't tell ya. guess ya get what ya pay for, with repair manuals. so with a difference of 5lbs. between cyls., you could say that the lowest reading is within 97% of the highest in this engine which is well within spec. thanks again, you are the man. ya know i,ve been hangin out on the net for about 13 yrs. now and i can't beleive i didn't come across FTE until just a few weeks ago. it's like being in a room with a hundred (maybe a thousand) guys with all kinds of info and exper. and they're willingness to share and help, it's unreal. i tell everyone i come in contact with about FTE. it's a privilegde to be let in the door.