Compression Test - Cold v. Warm

[schoutenbr]

Greetings,
I'm relatively new to the Model T hobby and have what may be considered an elementary question regarding performing a compression test on my 1926 Touring. In reviewing old MTFCA forum posts on the subject of compression testing, I found reference that a compression test for a Model T should be done when the engine is "cold". My neighbor, a Ford guy but who has never worked on a Model T engine (nor any engine from the 1930s era or before), indicates that it is best to perform a compression test while the engine is "warm". Therefore, my question is should I perform a compression test on my 1926 Model T with the engine "cold", or "warm", or does it not matter either way. Thanks in advance for you assistance to a newbie.

[TWrenn]

Tell ya what buddy...do 'em both ways, and report your findings to us, then we'll all have a good idea of what may or may not really matter. Be interesting to see what the numbers show. Thanks.

[JTT3]

When you do the compression test be sure the throttle is wide open & plugs out of the engine, my way maybe no one else’s

[Rich P. Bingham]

I guess it depends partly on the reason for doing a compression test. You kinda just "know" if you have a well-worn, tired engine, pistons swapping holes without missing a beat, but down on power. If you're testing for leaky valves to "fix" an engine that misses and stumbles, it's best to squirt a little oil in the cylinders to reduce blow-by.

[speedytinc]

I have always done the test on a cooled down/warm motor. Primarily since I dont want to be working with a hot engine. Also the motor has the normal oil it carries while running. This does not preclude the adding of extra oil to test ring condition after the first test is run.
Cant say which way is recommended, but intuitively, A cold, long sitting, dry engine would be prone to giving skewed readings.
I look forward to other opinions.

[JTT3]

John I assumed, you know what that means, that most folks know a warm engine gives a truer reading of the compression of each cylinder. Best John

[Art M]

A warm engine will give higher compression readings than what a cold engine gives. I don't know how much different or which way is correct.

Art Mirtes

[TXGOAT2]

I believe a warm test would be preferable. Cold engine parts with the oil drained off may give a lower reading less reflective of the engine's true condition. Pistons fit looser in the bores when everything is cold, especially aluminum pistons. When an engine is shut down hot, oil drains off parts and settles in the oil pan. Cranking a cold engine by hand probably won't fling oil up into the cylinder bores to relubricate the pistons and rings. Oiling the pistons via the spark plug holes may give a false high compression reading by flooding the rings with oil, and any significant amount of oil in the cylinder will reduce the clearance volume, thus increasing compression. I believe the most represntative reading wold be obtained by running the car a few miles, then shutting it down for 10-15 minutes, then running the compression test with the throttle wide open and testing each cylinder with the engine moving at the same speed, so far as is posssible. Valve leaks can sometimes be heard at the carburetor or tailpipe. Hooking up compressed air to one cylinder at a time with the cyliinder under test at TDC on the compression stroke can reveal ring leakage and valve leakage. If you do this, have the car in high gear and the brakes set tight to keep the engine from moving.

[Charlie B in N.J.]

On a low compression engine like the T a cold comp test done dry then wet is your very best option for finding out how hard cold starting the engine will be. Warm will falsely increase the pressures somewhat but if your on the low side cold starting with even lower pressures could really be tough. Do it cold.

[Mark Gregush]

Having the throttle open or closed on an engine that cranks over as slow as a Model T, I never have found it make any difference.

[TXGOAT2]

Anything that restricts airflow into the engine will reduce compression pressure to some degree.

The point about cold testing reflecting typical starting conditions is valid.

Use of Marvel Mystery Oil in the fuel may improve cold start compression.

An engine in good condition will have adequate compression under all conditions, and that will be reflected in substantial and uniform compression resistance when hand cranking the engine.

[JTT3]

Pat while I might agree on the typical starting situation, I’d believe that the running compression would be more reflective of the conditions to address. Now if an engine would stay cold during operation that might be reflective of the overall compression. I seriously doubt that can happen but it could be insightful if it did. Ha

[Charlie B in N.J.]

Pretty much agree with Pat except for the MMO business. The only way MMO would effect cold starting is if you dumped a coupe of gallons of it into your tank. It's vaguely possible it would seal the rings for a bit then. Of course once the engine got warmed up MMO would do exactly what is normally does. Produce a cloud of smoke out the tail pipe. Not sure I understand where John is coming from though.

[TXGOAT2]

I use MMO regularly in my car, and it does not smoke at all. (4 oz to ten gallons of fuel)
Even a small amount of lubricant in fuel will improve ring seal and reduce friction, as confirmed by air-cooled 2 cycle engines running at very high speeds with very little oil in the fuel, and no other source of lubrication. The car performs slightly better with the MMO, and I believe that upper cylinder wear and carburetor throttle shaft wear are reduced, and valve action improved. The gas cap is certainly easier to remove and replace. 4 oz of light oil in 1280 oz of ethanol-adulterated gasoline isn't going to make any engine smoke.

[Charlie B in N.J.]

None of which is provable. Sorry.

[JTT3]

I’ll try to explain.
Does the engine normally operate cold while in use? No.
You can surely do a compression test cold but cold is not the regular operation temperature so testing cold can provide you data to evaluate but not operational data. It may give you some insight to some possible issues with leakage through the valves, head, block & rings. As you know increased temperature expands metal so if you could measure your pistons & rings cold then measure them at operational temperatures you will find that they have expanded 3 + thou.
Would that change your compression or performance if the valves were sealing correctly, no leaks between block & head? So which is more reasonable to evaluate the overall condition of the engine in use? Just my opinion

[TXGOAT2]

I get good results using Marvel Oil in the fuel, as many people have over many years. I don't use it in the oil, because I believe that synthetic multi grade detergent oil negates any need for it.

If I used straight grade non detergent oil, I would definitely use MMO in the oil in cold weather, especially in a Model T.

I've had very good results using MMO in the crankcase of old sludged engines with noisey hydraulic valve lifters and/or stuck rings. Rislone is another good product for use in such situations.

I use MMO in old stuck engines prior to trying to unstick them.

[TXGOAT2]

I believe Charlie has a point regarding cold compression readings reflecting conditions when starting a car that has not run in some time. It's harder to start an engine under cold conditions, and unexpanded pistons and rings with little lubrication probably contribute to that in an engine that has sat for days or weeks or more. However, I don't know of a specific minimum compression pressure that would be required to start any particular engine under any specific conditions using any specific fuel and cranking speed, but I'm sure that more is better, sinced more compression pressure facilitates ignition and combustion, and a tighter engine will be more likley to keep running once firing commences. You can tell a lot about a T engine just by using the hand crank.

[JTT3]

Well Pat wouldn’t you agree that the variable has changed with respect to a vehicle that hasn’t run for a while? I can’t prove this but from an anecdotal aspect how many times have you found through your own experience or observations of other T owners that initially a car freshly started from a long slumber seems to have improved performance once warmed up? Many times I’ve seen an old engine trying to be revived but the compression was so low you couldn’t gain combustion then a squirt of oil in the cylinders and presto chango you get it to run and as it runs it seems to improve. More importantly it continues to be able to crank & run on rechallenging it.
On MMO, call it voodoo are whatever, I like it. I can’t prove it but from personal experience I believe it helps in the crank case and yes sometimes in the fuel. I know, I know it’s snake oil but I like snake oil.ha

[Charlie B in N.J.]

I have to say I appreciate the admission. Thanks.

[TXGOAT2]

Old engines that have sat for months or years do seem to get groggy. I think this is due to oil having drained off, and perhaps gummy valves, rust on valve seats, and rings that were parked in the lower bores being stuck at the position they had in the lower, less worn bore. Points are often corroded, and carburetors often have issues. Fresh oil and selected additives can assist in getting these engines up and running as least as well as they did when put out of service. I have encountered old, long dormant engines that barely ran and smoked heavily when started, and after a few hundred miles began to run quite well. I've encountered later model engines that were in use in spite of difficult and uncertain starting and very poor running that ran like new after a tune up. Sometimes a little work and minor expense can reverse the effects of years of neglect.

[Craig Leach]

Hi Bruce,
Is there a particular reason for doing a compression test or are you just curious? I warm up a engine before I perform a compression test so
the cylinders are properly lubricated. If the test is being run on a non running engine then by all means do it cold. All plugs should be out of
the engine. If you are using the starter make sure the battery is good & well charged. block the front tires, jack up one wheel. (you are
testing the compression not for a free neutral) There are two reasons for having the throttle fully open, one is to not restrict the flow of air
into the cylinder. The other is so not to draw fuel into the engine while testing. different amounts of fuel in the first to last test can effect
the outcome.
MMO I generally only use it to lubricate my Judson supercharger because the Mfg. says to. My preferred top end lubricant is synthetic Caster
oil. If it can protect a two stroke race engine at 15000 RPM in the Az. heat it's good enough for me. Besides I love watching people stick their
noses up and sniff the air and try to remember what that smell is.
Welcome to the hobby.
Craig.

[schoutenbr]

Thanks to all for the replies and information. The reason for the original question was curiosity after reading an old MTFCA forum post that referenced that a compression test on a Model T should be done cold. I had always heard that a compression test should be done with an engine in a warmer state, but this is the first Model T that I have owned. From the replies, I can see certain advantages to both cold and warm compression testing for a Model T. While at present I have no major known issues with the performance of my T, I will be doing compression testing on my T just to get more familiar with the engine. I now plan on doing cold/dry, cold/wet, and warm compression tests. Thanks again for your replies.

[Norman Kling]

It would be good to test while warm. it would have the amount of oil in the cylinders which would be normal while driving. The clutch would also be freer than when cold, especially if parked with the parking brake on for a long time in cold weather.
However, sometimes the engine isn't running and you are trying to check things out to determine why it doesn't run. In that case a cold test with a few drops of oil in each cylinder would work.
Norm

[TXGOAT2]

You can get a good idea of the engine's condition as relates to compression by using the starting crank.

[Charlie B in N.J.]

No good reason to do the wet test first. Most posting here insisting on a warm engine test are ignoring a possible answer to hard cold starting.