Compression Test

[f117rt]

Just had a compression test don on my 351C 2V.

The Mechanic said that it they were kind of low.

Anyone know what the range should be and when would I need to be concerned?

 

[jeff8877]

Just had a compression test don on my 351C 2V.

The Mechanic said that it they were kind of low.

Anyone know what the range should be and when would I need to be concerned?

What you are really looking for is consistency among cylinders, and a reasonable value (130 up?).

I've seen folks quote no more than 5 to 10%, so if you are w/i 5% after testing all, I'd say you are real good.

Post the values you get showing each cylinder and I'm sure you will get a better diagnosis than this.

Jeff

 

[f117rt]

Just had a compression test don on my 351C 2V.

The Mechanic said that it they were kind of low.

Anyone know what the range should be and when would I need to be concerned?

What you are really looking for is consistency among cylinders, and a reasonable value (130 up?).

I've seen folks quote no more than 5 to 10%, so if you are w/i 5% after testing all, I'd say you are real good.

Post the values you get showing each cylinder and I'm sure you will get a better diagnosis than this.

Jeff

The Mech told me that the readings were

85, 60, 80, 90, 85, 80, 70 and 90!

Maybe I should get my own tester and do it myself!

Two seem kind of low.

 

[Kermit 460]

I"ve had 2 different testers give drastically dif. results.If thats his go to tester & occupation he may know what he's talkin about.For a piece of mind ,you could do your own,but even if his tester is off sounds like more than 10% dif.

 

[jbojo]

Just had a compression test don on my 351C 2V.

The Mechanic said that it they were kind of low.

Anyone know what the range should be and when would I need to be concerned?

What you are really looking for is consistency among cylinders, and a reasonable value (130 up?).

I've seen folks quote no more than 5 to 10%, so if you are w/i 5% after testing all, I'd say you are real good.

Post the values you get showing each cylinder and I'm sure you will get a better diagnosis than this.

Jeff

The Mech told me that the readings were

85, 60, 80, 90, 85, 80, 70 and 90!

Maybe I should get my own tester and do it myself!

Two seem kind of low.

A 1:1 ratio is equal to 0 PSI. 14.7 PSI is equal to a 2:1 ratio. Just multiply your ratio by 14.7 to get PSI, or divide PSI by 14.7 to get ratio. So for a 8:1 ratio you would see 117psi under ideal conditions . Looking at the Ford shop manual you want your lowest reading to be within 75% of the highest reading which in your case would be 67.5psi.

By the formula your low cylinders are at 4.59:1 and high at 6.1:1

-jbojo


Just to add what you will see has alot of variables, engine hot or cold, valve timing etc. Here is what range you would normally see:

8>1 = 102.9 psi

8.5>1= 102.9 - 110.25 psi

9>1= 110.25 - 117.6 psi

9.5>1= 117.6 - 124.95 psi

10>1= 124.95 - 132.65 psi

10.5>1= 132.3 - 139.65 psi

11>1= 139.65 - 147 psi

11.5>1= 147 - 154.35 psi

12>1= 154.35 - 161.7 psi

 

[jb73Mach]

+1 on within 75%. So 60 and 90 are out of tolerance. All of the numbers appear a bit low also. I would expect 110 or so as an average. If you run the test again and the numbers are low, put a teaspoon of oil into the cylinders and see if that improves the compression. If the compression goes up - check rings. If there is no change - check values. Based on the numbers, I would suspect rings.

 

[hyena429]

+1 on within 75%. So 60 and 90 are out of tolerance. All of the numbers appear a bit low also. I would expect 110 or so as an average. If you run the test again and the numbers are low, put a teaspoon of oil into the cylinders and see if that improves the compression. If the compression goes up - check rings. If there is no change - check values. Based on the numbers, I would suspect rings.

+ 2 on that..if your hitting in the 60's too 90's you got a issue..Specialy across the board like that.....Mine did the same thing..Was not smoking or anything on start up..Was runing a big ruff and i went nuts and thought it was the old carb...replaced it...Replaced the points out of the dual point...new plugs and wires...finaly gave it a vacume check and it was low...gave it a compression test...just like your car each was hitting around the 80's and 90's on the high end.....I tore it down to find 2 things wrong...1 some one forgot to cross hatch the cilinder...2..put a mic on the cam and it did not have consistent numbers was a little worn....Who knows what cam the rebuilder used {shakes head}..But i would agree that its most likely rings...Specialy when you have compression loss in each piston.

 

[droptop73]

Do a compression test to get baseline readings. Then do another compression test after squirting oil into the cylinder. Check for an increase. This will verify the ring conditon. It's called a wet compression test.

 

[don29163]

100,000 + miles, it needs a rebuild, he really doesn't give us enough info. if this is a stock engine...never rebuilt, and 35 years old, it needs a rebuild. The slack in an old timing chain could be part of this along with worn rings, bad valve seating, etc.

 

[marks73]

Do a compression test to get baseline readings. Then do another compression test after squirting oil into the cylinder. Check for an increase. This will verify the ring conditon. It's called a wet compression test.

+1 to that. If your low and high #'s are still low and high, time for a rebuild.

 

[Don C]

This discussion made me think about something that I realized many years ago. I lived at altitudes of more than 4,000 feet, and checking manifold vacuum and compression on the cylinders always resulted in low readings. I finally realized in Physics class in high school it was a result of atmospheric pressures (or lack thereof).

This discussion also prompted me to put together the attached spreadsheet so that anyone can enter the altitude where they are and barometric pressure (if known), the compression readings, and calculate the highest compression to each cylinder percentage and the compression ratio of each cylinder.

This is also an easy way to play around with pressures and see how much altitude and barometric pressures change the absolute pressure, giving you an idea of how it messes with carburetor air/fuel ratios.

If anyone sees any errors or flaws, please let me know.

Compression Calculations.xlsx

 

[f117rt]

100,000 + miles, it needs a rebuild, he really doesn't give us enough info. if this is a stock engine...never rebuilt, and 35 years old, it needs a rebuild. The slack in an old timing chain could be part of this along with worn rings, bad valve seating, etc.

128k miles, never rebuilt, all stock, engine does not smoke.

I was told that it's probably the valves and not the rings since it does not smoke

 

[hyena429]

100,000 + miles, it needs a rebuild, he really doesn't give us enough info. if this is a stock engine...never rebuilt, and 35 years old, it needs a rebuild. The slack in an old timing chain could be part of this along with worn rings, bad valve seating, etc.

128k miles, never rebuilt, all stock, engine does not smoke.

I was told that it's probably the valves and not the rings since it does not smoke

Mine did not smoke either...Yet the rings where toast...Heads and valves ok....There is a way to tell if its your valves ....if you got help....By using a spray bottle of water...And misting water into your carb while some one keeps it runing so the gas and water mix well.....That will boost your compression..and if a big puff of smoke and carbon comes out...You will know its built up carbon on your valves....The water will steam clean the valves and break things loose..And will not hurt your motor long as it runing..Water injection was used in few race cars ...rv's and such......it is not a fix...Just a way to diagnose what is going on inside...I know it sounds crazy..But my cousin the engineer did it too my stang...How i figured out it was not my vavles...i still didnt get any smoke..lol

But with that many miles on original motor... age will get them too....Cam could of went flat on you...timing chain like some one mentioned.piston rings collaps....Sounds like she needs a go threw anyways...40 year old motor.

 

[f117rt]

So, any one have any idea on what this is going to cost me for a full rebuild?

Also, what should I expect to pay for the labor on the engine removal and install?

 

[droptop73]

It depends on what you can do yourself and the deals you can get on parts and what you want done.

 

[TommyK]

To determine if the valves are not seating fully you can hook up an air hose to each spark plug hole using the adapter from the compression tester. You will hear the air escaping through the exhaust system if the exhaust valve is hanging open and through the carb if the intake valve is hanging open.

Multiplying SCR by 14.7 is not a very accurate prediction of cylinder pressure because is does not take into account the intake valve closing point which greatly influences actual cylinder pressure.

 

[marks73]

Just my opinion mind you. But if my motor was that old it would earn a total rebuild. Would hate to see you rebuild the bottom and then begin to have valve issues (broken rocker, broken spring holder, ect) that could do damage when they fail.

 

[f117rt]

I now need to make a decision on whether to go with a complete rebuild or go with a crate engine. Seems like the costs are about the same. Need to do a little more research.

Anyone have any experience with Promar engines in Paterson NJ?

They both rebuild and sell engines.

If I go with a rebuild I'll switch to a 4v setup.

Is the only other thing that should be considered is a different cam?

Any suggestions?

 

[hyena429]

I now need to make a decision on whether to go with a complete rebuild or go with a crate engine. Seems like the costs are about the same. Need to do a little more research.

Anyone have any experience with Promar engines in Paterson NJ?

They both rebuild and sell engines.

If I go with a rebuild I'll switch to a 4v setup.

Is the only other thing that should be considered is a different cam?

Any suggestions?

Ya.if you go 4v you will need a dual pattern cam.....Im putting a dual pattern plain old hydrolic lift cam in mine...5.30 lift..2.80 duration...Should be pushing around 430ish hp...And thats not a very big cam at all compared too some.....Need a matching 750 holley to go on top...And long as you dont go over 10.1 compression.. keeps you in the regular pump gas area and wont be too tempermental...Im building mine for street power and a bit of reliabilty.

Some crate motors are scary sometimes...So watch out!! My freind got a motor from a place called..M and M...out of cali...500hp+ nitro motor...He got a good deal on it..saved him some time rebuilding...But in the end..It didnt...M and M went out of bis....online decent rep too.....then he started hearing about the horrible motors they did for some people....he was scared...So we tore down the little small block...Sure enough...The block had a stroker kit it in....But they didnt notch the block..and when you turned it over...the crank was hitting side of the block...They was suposta gap the rings for nitro...Nope...This 4,000 dollar motor would of lasted about 5 secs runing how they set it up....and this is just one of a few i have seen with my own eyes....Im not sure about that place your talking about...But defenatly do your home work on who you get to rebuild or where a crate motor comes from...Mite end up costing you more than you wished...Or mite save you a ton too if you find a good rebuilder.

Like most say...You can put a price tag on it till it is tore down and you see what you need...usualy cost a bit more than you expect..You got to make sure the block is ok and the crank...make sure the heads are good and rebuilt...it will cost some bank..i would say 1500 too 2000 will get you a long ways on a rebuild...or should..lol