Ive been curious what my truck is doing as far as compression so I decided to do a test. I have heard you need to disconnect the ignition. Is this true? If so, for what reason? Is it possible that the arc from the spark plug wires could ignite incomming fuel from the cylindars and cause an underhood fire? That is the only thing I could think of. Can I just remove the wire that lead from the coil to the distributor or do I need to do something special? Last thing is anyone know what kind of compression I should be looking for? maybe like what they come brand new compared to what mine could be. Reason Im doing the test is because I don't want to leave out any other alternatives since im planning on a 351 early next spring. OH, and my engines doesn't burn any oil that I know of, just leaks. Thanks a bunch!!

 

Disconnect the coil wire at the coil.

For the most accurate test, do your compression test after the engine is warmed up and at normal operating temp.

 

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 temp. Turn ignition off, then remove all spark plugs.

2. set throttle plates in WOT position.

3. Install a compression gauge in No. 1 cylinder spark plug opening.

4. Should disconnect the plug at the coil. Crank the engine at least five compression strokes and record the highest reading. Note the approximate number of compression strokes required to obtain the highest rating.

5. Repeat test on each cylinder cranking the engine approximately the same number of compression strokes (i think 5 seconds each cylinder will do).

Test results:

The indicated compression pressures are considered within spec if the lowest reading cylinder is within 75 percent of the highest.



Compression readings-interpreting

Post me the numbers, and I will compair them with the chart in my book.

"If one or more cylinders reads low, squirt approx. 1 tablespoon of SAE 50 weight on top of the pistons in the low reading cylinders. Repeat compression test."

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 does not increase compression, cause may be a cylinder head gasket leak between cylinders. Engine oil and/or coolant in cylinders would result, but you'd see that in the exhaust so I don't think thats happening.

Again, post me the results and I will interpret them for you, most of this imformation came from my ford dealership manual, so there is no question that it's the right information.

Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.

Then theres:
99' Mustang GT 4.6L
88' F-250 Heavyduty 4x4 351/c6
95' Mercury Cougar 4.6L V-8
80' E-350 300/6 with a
3 spd column shifter. Weighs around 7,000lbs w. 65mph top speed, who wants to race me!

 

"2. set throttle plates in WOT position."

What do you mean by WOT ?

 

WOT means Wide Open Throttle.

Thanks a bunch. I will try and get it done today and post the numbers. My chiltons listed pretty much the same procedure for it. They just never explain WHY you need to do something.

 

Silly me......thanks, I couldn't figure that one out:+

 

After warming up the engine and removing the sparkplugs, I leave the ignition key off and jumper the starter relay (next to the battery) to turn over the engine. No ignition and no fuel. I bought a switch made for doing this at a parts store for less than $10.

'94 302 with 118,000, my results were all within five percent of 160 PSI.

Jim

 

Thats a good tip. The manual tried to explain about putting in an external key switch and crap, and i know you don't have to, so i bypassed that. But yah, thats a good suggestion!


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.

Then theres:
99' Mustang GT 4.6L
88' F-250 Heavyduty 4x4 351/c6
95' Mercury Cougar 4.6L V-8
80' E-350 300/6 with a
3 spd column shifter. Weighs around 7,000lbs w. 65mph top speed, who wants to race me!

 

OK, I finally got the compression test complete!! By the looks of things the numbers are good. Here is what I got:

#1 cylindar= 162psi
#2 = 155
#3 = 161
#4 = 155
#5 = 155
#6 = 148
#7 = 162
#8 = 150

148/162= .9135 =91%

That is less than 9% tolerance

Any ideas on how this rates?

 

That is considered within spec, and are decent numbers. My book lists a bunch of max/min numbers (basically does the math for you).


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.

Then theres:
99' Mustang GT 4.6L
88' F-250 Heavyduty 4x4 351/c6
95' Mercury Cougar 4.6L V-8
80' E-350 300/6 with a
3 spd column shifter. Weighs around 7,000lbs w. 65mph top speed, who wants to race me!

 

May I tack on to this thread? My dry compression is notably less than Gamehunter’s, wet comparable. Is it still within spec? Does it imply significant ring wear? or?

1984 351W HO
COMPRESSION: (WOT;engine warm but less than operating temp)

CYL DRY WET
#1 130 notest
#2 135 158
#3 145 160
#4 145 155
#5 130 150
#6 130 160
#7 145 168
#8 130 152

Thanks,

Charlie Lincoln
Portland, Oregon USA
clincoln@spiritone.com

 

I didn't even bother with adding oil to the cylindars and test again. I figured I had spend enough time as it was and my numbers were high enough to were I didn't think much would change.

Those numbers of yours might be a little low. The good side of things is they are very close to each other and not having one 170 and another 60 or something like that.

 

The numbers are low, i don't know what you mean by dry/wet, but they are low. They are considered satisfactory if they are within 75% of eachother, so you passed. You will probably need an engine rebuild in the somewhat near future. Right now, you probably see an amount of oil burned in the exhaust. But, drive it until that number is unsatisfactory, thats all.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.

Then theres:
99' Mustang GT 4.6L
88' F-250 Heavyduty 4x4 351/c6
95' Mercury Cougar 4.6L V-8
80' E-350 300/6 with a
3 spd column shifter. Weighs around 7,000lbs w. 65mph top speed, who wants to race me!

 

Thanks for your comments. By dry/wet I was refering to initial testing, then with a few squirts of oil in each cylinder.

- Charlie

 

Stang, can you tell me what a brand new 5.0 runs at?