i checked my compression recently, and had the following numbers...

1) 100 psi
2) 125 psi
3) 125 psi
4) 135 psi
5) 125 psi
6) 125 psi


are these readings normal, and if not, what should my compression readings be? also...how does the compression ratio work? i.e. 8:1 ...... 8 of what to 1 of what???? this has confused me for a while...an explanation would be helpful...

MontanaFord

 

Ok the compression ratio is a calculation and the compression pressure is a psi. One does not determine the other due to the differences in wear as engines age. To determine the compression ratio measure in cc's the volume of the cylinder with the valves closed and the piston at bottom dead center, this includes the cylinder volume and the head volume for the cylinder. Then measure the cylinder and head volume, valves closed with the piston at top dead center. Divide the volume measurement of the cylinder at bottom dead center with the volume measurement of the cylinder at top dead center. So if the cc at bottom dead center is 1000cc and the cc at top dead center is 100cc then the compression ration is 10 to 1.

 

I forgot to mention, I don't know what the readings should be for this motor but I do know that at healthy motor is within 10% in readings between each cylinder. Cylinder 1 looks weak, at a little oil through the spark plug whole and try the reading again, if it goes up then the rings are worn if not then the valves are definately worn and the rings might be as well.

 

From my experience, a newly rebuilt I-6 engine should give around 125 PSI all across, 120 if is moderately worn, and if worn smaller numbers. Your number one needs to be checked, maybe the head gasket on top of that cylinder has a leak thereby losing compression, that happened to me once. There might be some wear at the piston rings on that #1. Other than that you engine looks pretty good to me...

 

could it be possible that only #1 has bad rings, causing it to blow blue smoke? or would it be all or most of the cylinders? and why would #4 be higher than the rest? i'm just curious as to why this might be...

also...i have a 300-6 that i bought from a guy i know, and he said that it ran good before it was pulled, but the reason it was pulled was because of a stripped cam gear...i'm half tempted to just put on a set of gears and throw the motor in, but i also don't want to go thru what a friend has gone thru w/ his truck...4 diff. motors inside of 3 months, because each motor had or developed either a tick or a clack...i'm considering tearing my spare motor down to the bare block and having some machine work done on the block and head, then putting a complete rebuild in, along w/ a few other goodies before putting the motor in my truck...or would i be better off for the time being to just put a stock rebuild kit in it and build the motor i pull out? any suggestions/ideas/opinions would be great....thanks for all the help...

MontanaFord

 

Why not leave it? It runs pretty well, right?

 

The engine seems OK and might only need some minor work. Just need to take it apart and see if it has some irregular wear in some of the moving parts. I believe a stock rebuild would not be needed, maybe just check the conflicting cylinders, perhaps it's just a matter of changing a piston ring. Just to make sure, have someone else check your compression for the readings may vary. I still insist on checking if the head (metal one) gasket to see if is worn or broken so it can be replaced, if needed, usually due to overheating.

 

Looks like all of these numbers are a little Low. After replacing the head on my I6 (210K miles) I was getting like 165 psi, The lowest number in the ford spec book table of Compression Test Precentages is 134 psi.

 

I was getting figures from a smaller engines, maybe your figures are low. That could be a call for a rebuild but it depends on how much exhaust and what color smoke you're getting and oil comsumption. Maybe you just need to change piston rings, bearings and gaskets as opposed to a full rebuild...

 

Figures are dependent on altitude, but, those numbers are kind of low from what I'm used to my 300 spitting out, it had 165 across the board last time I ran a compression test on it.

 

Montana Ford; During my first compression test I forgot to block the throttle wide open. The result was all six cylinders with reading low similar to yours. I re-tested following the Ford manual and the compression readings were considerably better. By the way, there was a thread over at fordsix.com that talked about the number one cylinder being the first to have compression problems because it’s the coolest being that it’s the closest to the radiator.

The following is a page scanned from the shop manual:
Positive Closed-Type Crankcase Ventilation System
A malfunctioning closed crankcase ventilation system may be indicated by loping or rough engine idle. Do not attempt to compensate for this idle condition by disconnecting the crankcase ventilation system and making carburetor adjustments. The removal of the crankcase ventilation system from the engine will adversely affect the fuel economy and engine ventilation with resultant shortening of engine life. To determine whether the loping or rough idle condition is caused by a malfunctioning crankcase ventilation system, refer to Emission System testing and diagnosis in the Engine/Emissions Diagnosis Manual. .

Compression Test
The following procedure is to be used on all engines when checking compression:
1 . Be sure the crankcase oil is of the correct viscosity and make sure that the battery is properly charged. Operate the engine for a minimum of 30 minutes at 1200 rpm, or until the engine is at normal operating temperature. Turn the ignition switch Off; then remove all the spark plugs.
2. Set the carburetor throttle plates in the wide open position.
3. Install a compression gauge in No.1 cylinder.
4. Crank the engine at least five pumping strokes and record the highest reading indicated. Note the approximate number of compression strokes required to obtain the highest reading.
5. Repeat the check on each cylinder cranking the engine approximately the same number of compression strokes.
Test Conclusion
The indicated compression pressures are considered normal if the lowest reading cylinder is within 75 percent of the highest. Refer to the quick reference chart, (Fig. 3), for pressure limits between cylinders. Variations exceeding 75 percent implies an improperly seated valve or worn or broken piston rings. If one cylinder reads low, squirt approximately one tablespoon of engine oil on top of the pistons in the low reading cylinders. Repeat compression pressure check on these cylinders.
a. If compression improves considerably, the piston rings are at fault.
b. If compression does not improve, valves are sticking or seating poorly.
c. If two adjacent cylinders indicate low compression pressures and squirting oil on the pistons does not increase the compression, the cause may be a cylinder head gasket leak between the cylinders. Engine oil and/or coolant in the cylinders could result from this problem.
Example
After checking the compression pressures in all cylinders, the highest reading obtained was 140 psi and the lowest pressure reading was 100 psi. By locating 140 in the Maximum column it is seen that the lowest allowable pressure listed in the Minimum column is 105 psi. Since the lowest cylinder reading was 100 psi, the engine is not within specifications and the compression is not considered satisfactory.

 

Just rebuilt my 300.
Haven't fired up engine yet but checked the compression on each (cold)
and read 150-170 If I figure out the egr crap will actual use procedure above tomorrow.

 

If I did everything correctly I got 140 on all but one, which was 135. The block was bored .030 over? what ever the machine shop does to fix the block.

 

here's what i know about my truck since i've had it...

1984 F-150 4x4 300-6 T-18 4 spd

-the truck supposedly has almost 300K on it, which is a possibility depending on how much it was driven by previous owners
-a friend and i set the timing approximately using a vaccume gauge, and it does run better than it had before (at one point, my max speed was 50 mph in 4th gear)
-i did a tune-up earlier this year (plugs, wires, cap, rotor, fuel filter, air filter)
-i replaced the spark plugs a couple days ago, and all but #3 looked pretty good, other than a little too black from the carb over-fueling...#3 had some nasty build-up on it...
-i get roughly 7.7 mpg, but i believe it'd due to the carb over-fueling, as when i pulled the carb off to swap intake and exhaust manifolds, there was liquid fuel just sitting in the bottom of the intake
-it blows the dipstick out of the tube about 1/2" on occasion
-the pcv valve is new this year and still good
-i replaced the large hose from pcv valve to intake spacer
-it blows blue smoke out the exhaust upon acceleration and also if i just let off the gas going down a hill
-if i park on a hill of any angle and put it in gear (any gear), it will not hold itself in place (kinda chugs downhill a little at a time) so i hafta block a tire as my e-brake doesn't work (pedal won't even push down)

this's everything i can think of offhand...any ideas/opinions would be helpful...

MontanaFord