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compression testing

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tahoe healey

tahoe healey

Yoda
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I've read the older posts on compression testing and I'm confused as to how it is done. Some say cold some say after running. I've heard it done cranking the engine and I've heard it done with the engine running. I'm getting 105 psi on all cylinders (Hot) with a gauge from Harbor Freight (cheep). Car runs great. ?????
Thanks
 
I would say that 105 is low. I think the manual says something like 174. With my engine cold, my low cylinder is 160 and my high cylinder is 169. mac
 
I assume that you checked this topic;
https://www.britishcarforum.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=001253

From this it appears that a reading of 150 to 160 is about average.

The test would normally be done with the engine warmed up, all plugs out, & cranking on the starter. Your readings of 105 are pretty low but if they don't vary more than 10 from highest to lowest the engine will still run ok.

Low compression is not a disaster. It just means that power & fuel economy will be less than if the compression were higher.

How low compression & the resulting power can be depends on your personal choice & your pocket book.

Eventually it will get so low that hard starting & rough running result.
D
 
More of a question than a reply. Did you use the type of compression tester that just pushes into the spark plug hole, and relies on a rubber grommet to contain the air? If so, I would expect low readings. The type that has to be screwed in the hole would be better.
 
Thanks for the replys. I used the screw in type. But I only removed one plug at a time. There is no oil on the plugs and I don't burn an excessive amount of oil (just the standard dripping oil loss). She runs smooth with a lots of "pushyouintotheseat" power. I think I'll try to find another gauge to test against mine. I just couldn't find anything on the proper way to use the gauge.
blush.gif
 
Tahoe, as Dave says,1, it should be done warm. This guarantees that all surfaces are lubed, and the bores have gotten to their optimal shape, so the rings should be sealing at their maximum capability. 2. Pulling all the plugs allows the engine to attain maximum revs on the starter, as it's not fighting the compression. 3. Make sure that you crank the engine for the same amount on each cylinder. Usually five revolutions will be plenty. This insures consistency in the readings.
4. This may be personal preference, but I was always taught to block the throttle fully open, to allow maximum airflow into the cylinders. 5. Be sure to disconnect the coil wire at the coil, or you could be in for a nasty surprise if you brush against a plug wire. Trust me on this one! 6. Most references that I have say that a 10% difference between the highest and lowest numbers is acceptable. I have seen 15% quoted, but I like to be on the conservative side.
I certainly hope this helps.
Jeff
 
One topic not mentioned is the effect of the camshaft on compression. With a "performance" cam the readings will be somewhat lower due to the overlap of valve openings. Likewise a cam with worn intake lobes may cause a lower than normal reading. Bob
 
Bob, you're correct in that there are many variables that will influence compression pressures. I won't even bother to mention one of my performance engines that pegged my 300 PSI gauge! Ran strong, not long.
My post was meant to supply a basic overview on the procedures involved in using a compression gauge, as Tahoe said there were no instructions with the gauge.
Jeff
 
How about a leak-down test, how is this achieved? I'd also like to know the significance, are you testing the ring seal, valve seal or both as a working unit?. Thanks.
 
Hi Jim,

Rather than go through a lengthy explanation see these links;

https://www.xs11.com/tips/misc/misc3.shtml
https://www.geocities.com/dsmgrrrl/FAQs/leakdown.htm

Testers are available at the better auto supply stores & one of these references tells how to build your own tester.

The leakdown test is also called a differential pressure test in the aviation industry. Piston type aircraft engines are "required" to be tested in this manner & to meet certain minimum specifications. For a full aviation procedure go here;
https://www.aeas.com/PDF/m84_15.pdf
D

[ 11-30-2003: Message edited by: Dave Russell ]</p>
 
Hello all,
glad to see someone agree with the opinion I made at an earlier discussion on the same subject, that of having the throttles wide open.
In my case the idle is done through a bleed screw with the throttle discs totally closed at idle.
I achieve stable pressure readings with no more than two full revolutions.
I also built a leak down tester; really all it is is a restricted air supply (1\16" orifice) and a pressure gauge. What is akward is having each cylinder absolutely on TDC or the air cranks the engine. It is useful if you are looking to pinpoint the loss of compression but I'm happy to do a standard compression check during services, much quicker.

Alec
cheers.gif
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by piman:
Hello all,
glad to see someone agree with the opinion I made at an earlier discussion on the same subject, that of having the throttles wide open.
Alec
cheers.gif
<hr></blockquote>

Alec,
I'm sure that you will find lots of agreement. To find out how much it really matters, simply try it both ways next time.

To put it in perspective, the throttles are set for the engine to inhale enough air to idle at 500 t0 700 rpm. The starter turns the engine at 100 to 150 rpm. Far less air is required at cranking speed than at idle speed.
D

[ 12-01-2003: Message edited by: Dave Russell ]</p>
 
I agree, Dave, that logic dictates it should make no difference. But, way back in the days when my Grandfather taught me how to do it, that's the way I learned. And,this late in the game, I guess I'm stuck with it, although I do have to admit to doing it either way, depending on where my head is at at that particular time.
I wonder if Tahoe Healey read any of this?
Cheers,
Jeff
 
Also, with the plugs out & the fact that there are always more than one intake valve partially open at the same time, the test cylinder can intake get air from adjacent cylinder ports.
D
 
Yep, I reading all this. It sound like the leak down test gives more info if you get a significant differens in pressures. This all came up because of a fluttering noise in the exhaust at idle. Turned out to be a rich mixture on the front carb but I already bought the pressure gauge and I was curious. I did find in one of my Healey shop manuals that the optimum psi is 175. So you are all low. Now you have something to do this weekend.
 
OK, if we're going to be busy, so are you. Did you correct for sea level? Let's see, Tahoe is roughly at 6250' MSL, so the correct compression pressure at that altitude would be,(and may I have a little drum roll here please?)......................?
Hee Hee.
Jeff
 
Jeff,
You are always stirring the pot! I just can't resist the bait.

Seriously, you have a legitimate point. Atmospheric pressure at 6,000 feet is about 80% of sea level. This means that an engine which reads 160 psi compression at sea level will only read 128 psi at 6,000 feet.
D
 
Dave;

Great info, thank you for the links, they are more than just helpful and much appreciated.
 
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