compression test

[ken1007]

Most manuals call for engine to be warm to hot for compression test. Does anyone have a best guess estimate of what per cent difference between cold vs warm/hot. For example 125 cold might be ___145____ when tested warm/hot. Ken

Thanks to all, I got my answer.

Edited February 23, 2013 by ken1007 (see edit history)

[Bleach]

Why would you check it cold? It's meaningless.

[Paul Dobbin]

"Why would you check it cold? It's meaningless." , he said.<!-- google_ad_section_end -->

If you are buying a non running engine, how could you check it hot?

[NTX5467]

Hopefully, the "compression test" would be made with the engine temperature stabilized at normal operating temperature. Like ten miles of highway driving, then letting it cool until you can safely and somewhat comfortably be close enough to the exhaust manifolds to remove the spark plugs and safely stow the plug wires "out of the way". The desire is to mimic the "normal operating conditions" of the motor, which means a certain level of "heat" in the block and innards which move around . . . including the oil and coolant.

If the engine hasn't been running, but will turn over, then it might be best to just put a hot battery on it and turn the starter for several revolutions, to get oil pressure built, once again. Then, as long as it turns over "smooothly", with no "speed-ups" as particular cylinders are on their compression stroke, you can be reasonably sure there are no valve train issues or "perforated" piston crowns. Past that, the total number of psi in the compression check are more for cylinder-to-cylinder differences than how "good" the motor might be. PLUS, you don't need any fancy gauges or such to LISTEN to what's going on, hopefully. Key thing is that it HAS compression and the amounts are within about 20% of each other -- highest to lowest.

That's the way I'd look at it. Additionally, all the compression test will do is test the upper "compression" rings'/cylinder wall interfaces. NO indication of integrity of the oil rings or how much oil consumption the engine might have . . . from checking compression.

Good compression can also mean the valves are seating nicely . . . at this time. But it might not indicate which stems/guides are getting ready or might currently have clearance issues that can ultimately lead to "burnt valves" or oil consumption coming from the valve guide area (regardless of how well the stem seals might be).

Just some thoughts,

NTX5467

[huptoy]

You can add some oil in the spark plug hole to increase the oil ring seal compression when the engine is cold.

I was having a problem with a 1931 6 cyl engine not running well with a loss of power. I had a repair shop do an "Engine Check" for compression. I knew 2 of the cylinders had low compression but not sure of the cause. The shop pulled all the spark plugs, and set the first cylinder on the power stroke with both valves closed. They put a pressure gauge on the tail pipe. By pressurizing the cylinder at the plug they were able to identify which of the two valves was leaking and how much. They did this on each of the cylinders. With two bad cylinders and two others being border line, I elected to replace all 12 valves.

About a year after the valve job, it would loose power as it warmed up. The valves were set too close and as the engine heated up, the valve stem would expand and leak. After having a different shop re-adjust the valves, the problems went away. When I purchased the car, the engine ran very quiet. A retired guy in his late 80's later told me it is better to hear the valve lifters than too quiet and burnt valves.

Car has excellent compression on all cylinders, starts with very little difficulty, and pulls hills nicely. Steep hills and 3rd gear can be a problem but it will run 30 in 2nd on almost every hill.

[misterc9]

125 pounds cold MIGHT be 145 pounds hot. The older and more worn out the bigger the difference between hot and cold. It's difficult to get meaningful readings from something that hasn't run in a long time due to dry or stuck parts. You should oil the cylinders first to prevent damage.

[Bleach]

"Why would you check it cold? It's meaningless." , he said.<!-- google_ad_section_end -->

If you are buying a non running engine, how could you check it hot?

That is a point indeed. If it isn't running in the first place, I would be ready to expect the worst. The leak down test is probably the best method but again only worthwhile on a running engine. A compression test or leak down test won't reveal a bad bottom end if it's not running in the first place.

I had a car that had a bad rod but it only knocked when it was fully warmed up. The compression was still good on all the cylinders and it otherwise ran good.

[Dandy Dave]

You can add some oil in the spark plug hole to increase the oil ring seal compression when the engine is cold.

I was having a problem with a 1931 6 cyl engine not running well with a loss of power. I had a repair shop do an "Engine Check" for compression. I knew 2 of the cylinders had low compression but not sure of the cause. The shop pulled all the spark plugs, and set the first cylinder on the power stroke with both valves closed. They put a pressure gauge on the tail pipe. By pressurizing the cylinder at the plug they were able to identify which of the two valves was leaking and how much. They did this on each of the cylinders. With two bad cylinders and two others being border line, I elected to replace all 12 valves.

About a year after the valve job, it would loose power as it warmed up. The valves were set too close and as the engine heated up, the valve stem would expand and leak. After having a different shop re-adjust the valves, the problems went away. When I purchased the car, the engine ran very quiet. A retired guy in his late 80's later told me it is better to hear the valve lifters than too quiet and burnt valves.

Car has excellent compression on all cylinders, starts with very little difficulty, and pulls hills nicely. Steep hills and 3rd gear can be a problem but it will run 30 in 2nd on almost every hill.

That 80 year old guy knew what he was talking about. Much better a little loose than too tight. Dandy Dave!

[Guest Magoo]

The compression values will typically be greater on a cold engine than a hot one. As the engine warms up, the wear-related dimensional issues show themselves. Presuming the engine can run, it doesn't hurt to run the compression test both hot and cold as a diagnostic aid.

And has also been pointed out, the leakdown test is also a great tool--arguably more useful than a compression test in some ways.

More important than engine temperature in a compression test, and often overlooked: the throttle must be blocked 100 percent open.

[Dandy Dave]

Hopefully, the "compression test" would be made with the engine temperature stabilized at normal operating temperature. Like ten miles of highway driving, then letting it cool until you can safely and somewhat comfortably be close enough to the exhaust manifolds to remove the spark plugs and safely stow the plug wires "out of the way". The desire is to mimic the "normal operating conditions" of the motor, which means a certain level of "heat" in the block and innards which move around . . . including the oil and coolant.

If the engine hasn't been running, but will turn over, then it might be best to just put a hot battery on it and turn the starter for several revolutions, to get oil pressure built, once again. Then, as long as it turns over "smooothly", with no "speed-ups" as particular cylinders are on their compression stroke, you can be reasonably sure there are no valve train issues or "perforated" piston crowns. Past that, the total number of psi in the compression check are more for cylinder-to-cylinder differences than how "good" the motor might be. PLUS, you don't need any fancy gauges or such to LISTEN to what's going on, hopefully. Key thing is that it HAS compression and the amounts are within about 20% of each other -- highest to lowest.

That's the way I'd look at it. Additionally, all the compression test will do is test the upper "compression" rings'/cylinder wall interfaces. NO indication of integrity of the oil rings or how much oil consumption the engine might have . . . from checking compression.

Good compression can also mean the valves are seating nicely . . . at this time. But it might not indicate which stems/guides are getting ready or might currently have clearance issues that can ultimately lead to "burnt valves" or oil consumption coming from the valve guide area (regardless of how well the stem seals might be).

Just some thoughts,

NTX5467

I have to agree that Listening to an engine, and being tuned to the music it makes, is the first thing to diagnosing a problem. Nine times out of ten I can tell a problem without a compression test or other tools. Confuses Say, "He must be One with the Motor." Using your first five senses usually leads to a sixth sense of trouble shooting.

[Rusty_OToole]

I would rather listen to the factory that made the car than some old boob who thinks he knows better than they do. I have heard their engines, rattling away, the valve stems and rockers getting all mushroomed. The problem is, the valve mechanism takes such a beating that even if you adjust everything correctly it will never be as quiet as it was before Mr I like to hear em working got hold of it.

Once I bought at 1970 Dodge slant six pickup truck with 160,000 miles on it. When I adjusted the valves it was dead silent, I mean as quiet as a brand new car with hydraulic lifters if not quieter.

The old farmer I bought it from used it hard but always maintained it by the book. It was the only slant six I ever owned that was completely quiet. On the others, I could get them pretty quiet but not totally quiet, not after they had been run with too much clearance for a while.

Like I say, the truck had been used hard. You could see by the way the inside of the box was pounded that it had carried a lot of heavy loads in the 22 years before I bought it. The engine NEVER had any more valve clearance than the factory called for, and all the hard work never did the motor any harm.

[NTX5467]

On a mechanical flat tappet camshaft, there is an additional "clearance ramp" which is there to gently take all of the clearance out of the valve train before the real task of openning the valves is attempted. Hydraulic lifter camshafts don't have or need that more gradual "ramp", though. If the mechanical lifters are adjusted a little too tight, it can also result in the effective duration of the valve timing events being a few degrees longer, too.

The old Ford 223 6-cylinders had a special rocker arm, with a "slack adjuster" on the rocker arm end. It was there to keep things quieter and compensate for things as the engine heated up from cold, valve adjustment-wise. From what the service manual said, you had to compress the compensating spring to do the valve adjustments.

To me, a mechanical lifter "sound" is just another "note of the music" and engine makes as it runs. Same with the roller chain timing chains I like to use. I know what they are and why they're there.

From what I've seen, erring on the loose side of valve adjustments on a stock motor, just a "rch" more (maybe .002" at the most), for good measure, can be good insurance in ensuring top performance for a long time . . . especially on the old VW flat-four air cooled motors . . . which get that wheeeezing sound as their valves tighten in use, rather than become looser (as other motors tend to do).

Certainly, having the valve clearances a good bit too loose can hammer the parts, as mentioned, but this extra looseness can also shorten the cam timing a little bit, which might make just enough low-rpm torque for the many "idling" conditions of farm truck use. Like letting the engine idle in gear as a helper chunks hay bales into the bed or distributes hay onto the ground for the cows to eat. In that case, a little extra noise was a small price to smooooth running under those conditions . . . without "chugging", "chuggling", or "lugging too much".

Personally, I like hydraulics for their maintenance-free aspects, but even they have to be adjusted. I like to do the Chevy adjustable lifters about 1/4-1/2 turn pre-loaded, rather than the factory spec of about 1-1.5 turns pre-loaded. Just enough pre-load to keep every thing quiet on the coldest mornings, but also extended enough to allow for higher rpm activities, too . . . the best of both worlds to me. Then I can hear that timing chain sound, IF you know what to listen for.

Respectfully,

NTX5467

[Dandy Dave]

I would rather listen to the factory that made the car than some old boob who thinks he knows better than they do.

I once knew a smart girl. Problem was she always had two Boobs hanging around. That usually lead to trouble. :eek: Valves that are too loose are not good either. But one or two thousands loose is much better that a few thousands tight. I've done a lot of valve work and rarely see mushroomed valve stems from the valves being adjusted several thousands more than called for it the book. After doing a valve job on an early soild lifter engine it is not uncommon to have to re-adjust the valves after several hundred miles. Most of the mushrooming I have seen is on very early engines with soft valve stems. (Typical on one lunger, hit and miss igniter fired engines.) The other mushrooming problem is when the valves are too tight in a valve Interference engine. Makes a lot of noise and a big mess of things. Worse when the timing belt or chain gives up while heading down the highway at 65 miles an hour. Dandy Dave!

Edited February 24, 2013 by Dandy Dave (see edit history)

[Dandy Dave]

On a mechanical flat tappet camshaft, there is an additional "clearance ramp" which is there to gently take all of the clearance out of the valve train before the real task of openning the valves is attempted. Hydraulic lifter camshafts don't have or need that more gradual "ramp", though. If the mechanical lifters are adjusted a little too tight, it can also result in the effective duration of the valve timing events being a few degrees longer, too.

The old Ford 223 6-cylinders had a special rocker arm, with a "slack adjuster" on the rocker arm end. It was there to keep things quieter and compensate for things as the engine heated up from cold, valve adjustment-wise. From what the service manual said, you had to compress the compensating spring to do the valve adjustments.

To me, a mechanical lifter "sound" is just another "note of the music" and engine makes as it runs. Same with the roller chain timing chains I like to use. I know what they are and why they're there.

From what I've seen, erring on the loose side of valve adjustments on a stock motor, just a "rch" more (maybe .002" at the most), for good measure, can be good insurance in ensuring top performance for a long time . . . especially on the old VW flat-four air cooled motors . . . which get that wheeeezing sound as their valves tighten in use, rather than become looser (as other motors tend to do).

Certainly, having the valve clearances a good bit too loose can hammer the parts, as mentioned, but this extra looseness can also shorten the cam timing a little bit, which might make just enough low-rpm torque for the many "idling" conditions of farm truck use. Like letting the engine idle in gear as a helper chunks hay bales into the bed or distributes hay onto the ground for the cows to eat. In that case, a little extra noise was a small price to smooooth running under those conditions . . . without "chugging", "chuggling", or "lugging too much".

Personally, I like hydraulics for their maintenance-free aspects, but even they have to be adjusted. I like to do the Chevy adjustable lifters about 1/4-1/2 turn pre-loaded, rather than the factory spec of about 1-1.5 turns pre-loaded. Just enough pre-load to keep every thing quiet on the coldest mornings, but also extended enough to allow for higher rpm activities, too . . . the best of both worlds to me. Then I can hear that timing chain sound, IF you know what to listen for.

Respectfully,

NTX5467

Agreed. Dandy Dave!

[Guest]

From what I've seen, erring on the loose side of valve adjustments on a stock motor, just a "rch" more (maybe .002" at the most), for good measure, can be good insurance in ensuring top performance for a long time . . . especially on the old VW flat-four air cooled motors . . . which get that wheeeezing sound as their valves tighten in use, rather than become looser (as other motors tend to do).

I have worked nearly all my life on VW engines. VW valves do not tighten up between valve adjustments, they may loosen up just a bit because of rocker to stem wear just like on any engine. If you are using some inferior valve that's made in some developing country where the metallurgy or the hardness is questionable this may be the case, but if you are using German ATE valves or TRW valves made here there is no stretching of valves. Want to run the wheezing sound by us again. As a trained VW line and unit repair tech from my misspent youth I have never heard that one before. Perhaps you heard someone in the back room who couldn't play the zither correctly.

[MikeC5]

Perhaps he meant decreased clearance due to thermal expansion of the valves, etc. I always set the cold clearances a little on the high side for this reason.

[Guest]

Perhaps he meant decreased clearance due to thermal expansion of the valves, etc. I always set the cold clearances a little on the high side for this reason.

There is no hot adjustment on a air cooled VW. Only cold, and stone cold the better.

[F&J]

From what I've seen, erring on the loose side of valve adjustments on a stock motor, just a "rch" more (maybe .002" at the most), for good measure, can be good insurance in ensuring top performance for a long time . . . especially on the old VW flat-four air cooled motors . . . which get that wheeeezing sound as their valves tighten in use, rather than become looser (as other motors tend to do).

I have worked nearly all my life on VW engines. VW valves do not tighten up between valve adjustments, they may loosen up just a bit because of rocker to stem wear just like on any engine. If you are using some inferior valve that's made in some developing country where the metallurgy or the hardness is questionable this may be the case, but if you are using German ATE valves or TRW valves made here there is no stretching of valves. Want to run the wheezing sound by us again. As a trained VW line and unit repair tech from my misspent youth I have never heard that one before. Perhaps you heard someone in the back room who couldn't play the zither correctly.

You are misinformed. VW A/C valves do get tighter with mileage/age, even with factory parts. VWoA was well aware of this and even changed the cold setting from .004 to .006 in the early 1970's... and this new setting was also to be used on all of the previous years that were spec'd for .004 on the sticker on the fan shroud. From that point on, no VW dealers were setting any years at .004.

I too, was a VW dealer line mechanic, starting in Jan 1970 and attending VW School at World Wide VW Corp in Orangeburg, NY. WW was/is the northeast distributor for VWoA

Edited February 25, 2013 by F&J (see edit history)

[Dandy Dave]

You are misinformed. VW A/C valves do get tighter with mileage/age, even with factory parts. VWoA was well aware of this and even changed the cold setting from .004 to .006 in the early 1970's... and this new setting was also to be used on all of the previous years that were spec'd for .004 on the sticker on the fan shroud. From that point on, no VW dealers were setting any years at .004.

I too, was a VW dealer line mechanic, starting in Jan 1970 and attending VW School at World Wide VW Corp in Orangeburg, NY. WW was/is the northeast distributor for VWoA

Ahhhh, .002 of an inch. My gut feeling has been right all these years. I have never re-built a bad running engine. Some are just better than others. Dandy Dave!

[Guest]

You are misinformed. VW A/C valves do get tighter with mileage/age, even with factory parts. VWoA was well aware of this and even changed the cold setting from .004 to .006 in the early 1970's... and this new setting was also to be used on all of the previous years that were spec'd for .004 on the sticker on the fan shroud. From that point on, no VW dealers were setting any years at .004.

I too, was a VW dealer line mechanic, starting in Jan 1970 and attending VW School at World Wide VW Corp in Orangeburg, NY. WW was/is the northeast distributor for VWoA

I can only tell you about my own two cars and from some close friends that I wrenched on their car regularly that I have never had a valve stretch. From the many cars I worked on in the dealer I have no data, and I worked in Unit repair as well as line. What is your data?

You know as well as I do (and the factory) that when a car came in for a valve adjustment that even with the blowers we used to cool the cars down with didn't completely cool the engine. That is the reason the cold valve setting was changed from .004 to .006. You may or may not also know that before the doghouse oil cooler showed up the valve setting after TSB's came out in the middle 60's that # 3 was to be set at.006 and the rest at .004.

I too went to VWoA school called on the west coast VPI. We were told and were aware of these problems , but were told it was OK and the engine would like it better if as long as the engine was stone cold .004 was perfectly fine. If you were in Unit repair .004 was the norm.

Edited February 25, 2013 by helfen (see edit history)

[Dandy Dave]

Not for nothing, but I also come across faulty information in factory shop manuals and other repair books from time to time. Just a few days ago Looking up torque specs on the connecting rods of an engine. First page I looked up was in the specs section and 54 foot pounds were listed. Years of experiance had me thinking this was too high for the size of the bolt and nut. In the same factory written book I looked up the assemble section and 38 foot pounds was listed. That was closer to my original thought of 35 to 40 for the fastener at hand. These Books are written by people and people do make mistakes. Not all information written down can be relied on as correct.

In the old days, on solid lifter type engines we use to figure .0015 to .002 thousands of an inch for every inch of the length of the valve for a cold valve adjustment to allow for expantion. So, if you had an exhaust valve that was 7 inches long the at .002 the adjustment would be .014. Intakes were generaly set a little closer. .0015 times 7 would be 10.5. Even if the intakes were set at .011 or .012 everything would work fine. Dandy Dave!

[F&J]

I can only tell you about my own two cars and from some close friends that I wrenched on their car regularly that I have never had a valve stretch. From the many cars I worked on in the dealer I have no data, and I worked in Unit repair as well as line. What is your data?

You know as well as I do (and the factory) that when a car came in for a valve adjustment that even with the blowers we used to cool the cars down with didn't completely cool the engine. That is the reason the cold valve setting was changed from .004 to .006. You may or may not also know that before the doghouse oil cooler showed up the valve setting after TSB's came out in the middle 60's that # 3 was to be set at.006 and the rest at .004.

I too went to VWoA school called on the west coast VPI. We were told and were aware of these problems , but were told it was OK and the engine would like it better if as long as the engine was stone cold .004 was perfectly fine. If you were in Unit repair .004 was the norm.

Well we are way off the original posters question, but whatever...

As my recollections go, the .006 came out in 71 when several nasty things happened. Smog certs were tough to meet, and VW had to reset the timing specs to 5 degrees After TDC. The other reason is likely due to the 71 final drive ratio change. Both made the car run even hotter, despite that 71 was also the year they moved the oil cooler that was pre-heating #3 and #4, to outside of the fan shroud.

The earlier pre 71's did have problems with #3 ex valve being tight. Guys that cheated on the line doing a full maintainance, would do the comp test, and only adj whatever cyl was lower. I honestly don't recall if the 71-up had the same issue after the cooler was moved.

We had very few "waiters" on reg maint, the service writers made them leave it for the day, so we did not have any external cooling fans. So the majority were adjusted cold. Comp could, and did raise, on some of those low cyls, mainly because the valve was not sealing well cold with -zero clearance, but did have a tiny bit of lash when at driving temps...which saved the valve from burning. The unit repair guys always claimed those were the valves that would break eventually, but had no real proof.

I must have seen 1000's with tight #3, and seem to recall a few on #1. Being a line mech, I lost track of how many cars I adjusted valves on, just in the first week.

Then we can go way back to the earlier version of the 40hp where they had super long rocker arms studs...which I believe made for less clearance when hot? We set those at .008 cold I think. The later 40 had short studs like the 1300-1600.

[Rusty_OToole]

Why do you buy such lousy cars? Do you really think the engineers who designed the engine are so stupid they do not know how to adjust valves, or they did not test the motor under all conditions from red hot to stone cold?

I have had old motors with solid lifters that were dead silent even though they had substantial miles on them. I sincerely believe that a solid lifter engine, that has been adjusted to factory specs and maintained by the book , is as quiet when new, and quieter than a hydraulic lifter engine once it gets 50000 miles on it.

My policy is to listen to the factory that designed the car and built it. I believe they know better than I do. Usually when I go getting ideas of my own, or listen to some old boob, it turns out the factory was right.

On the VW engine. The valve adjustments definitely will tighten up, especially the #3 exhaust valve, as the valve wears or the stem stretches. If you find you have to adjust the #3 exhaust valve at every tuneup, better plan on a valve job quick before the head breaks off the stem and smashes your piston, and cylinder head.

Much as I like the old air cooled VW they were basically a 1930s design and required a LOT of maintenance if you expected reasonable engine life and long service. They usually didn't get it.

Edited February 25, 2013 by Rusty_OToole (see edit history)

[Rusty_OToole]

Speaking of the factory being aware of the correct valve clearance.. Chrysler spec's an extra .002 clearance on slant sixes used in trucks and vans vs the same engine in passenger cars. Evidently they are aware that trucks may be used hard, and the valves may run hotter, and benefit from a little extra clearance.

I have set them both, and the extra .002 makes little or no difference in noise. Little difference on a neglected engine with pounded valves and rocker, no difference on a well maintained engine. If the valve train is in good shape the valves will be dead silent whether the clearance is .010 and .020 or .012 and .022.

This is a completely different situation from Mr "I like to hear 'em working" who sets them so wide you can hear the valves hammering from 50 feet away, with the hood closed and the engine idling. It is also completely different from an engine that has NEVER had the valves adjusted in 100,000 miles and sounds like a diesel.

In both those cases it is impossible to adjust the valves to be quiet, once the mechanism is pounded to oblivion. Believe me I have tried.

[Dandy Dave]

Be careful when you call old folks, "Boobs". :eek: You may be one yourself someday. Dandy Dave!

[Guest]

This week it's all the cars bi-annual oil changes and once overs for the summer season. First car was my 64 113. This car has a 1200 40hp KD engine that was put in by the original owner in 1965 after he blew the original engine up because he didn't know how to keep VW and Porsche engine alive. After learning his lesson the KD engine lasted him through 1974 when I bought it from him. That engine lasted me until two years ago when it burned 4 exhaust valves at the same time, although I still had 80psi on all four a leak down confirmed it. At that time the car had turned just 170,000 miles...the heads had never been off since the Kassel plant. I rebuilt it two years ago and I am proud to say that before or after the rebuild and after checking yesterday I don't have any tight valves, I did have two intakes at .005 instead of .004.

Today I will do my 65 111. I rebuilt it's 36 hp engine in 1972 after 193,000 Kilometers, it has 289,000+ kilometers on it now. I have never had a tight valve in that engine ever. I bought that car in 1968 and knew the original owner also. I will let you all know how today goes tomorrow.

Yes VW engines are maintenance heavy if you call a change of points at 12,000 miles, inspecting plugs every 6,000, adjusting or checking valves every 3,000, changing oil at 1,500 miles. But then again you have a car with ease of maintenance in mind. I was a flight engineer and radial and jet engine mechanic in the navy so I know all about air cooled 3,350" 18 cylinder radial engines and maintenance. Now those can be labor intensive, but they were designed to come apart too.

Update 2/2713 No valve stretching on the 65 either! Just like they were a year ago .004

Edited February 27, 2013 by helfen (see edit history)