What are normal compression test values? Options

[Barry Brisco]

I've run a search for posts talking about "compression test values" but can't find an answer to my question, and my Owner's Manual and Haynes repair manual don't seem to tell me either: what are normal compression test values in a 914?

These numbers are for a 1.7 engine with an unknown number of miles, it's in a 914 that is for sale:

#1 130
#2 150
#3 140
#4 150

I am concerned about the #1 cylinder. That is a 13% drop from the highest cylinders.

Here are some online references about how much of a difference is acceptable between cylinders, and what indicates a potential problem:

http://www.automedia.com/Engine_Compressio...ccr20050801cc/2

If all the cylinders check out within 10 or so PSI of each other, and those numbers sync up with the factory specifications, then you're good to go.
If one or more of the cylinders show a difference of 15 or more PSI, then there are problems inside

http://www.freeengineinfo.com/compression-test.htm

The cylinders should all read within 5-10 psi of each other.

http://www.aa1car.com/library/compression.htm

As a rule, most engines should have 140 to 160 lbs. of cranking compression with no more than 10% difference between any of the cylinders.

So, at this point I'm thinking that this 1.7 engine is a bit tired and in the near future it will likely need some major attention: maybe a bad exhaust valve, maybe worse.

And yes, a leakdown test would be helpful.

Comments?

Thanks,

Barry

[Jake Raby]

Were these numbers gathered with the throttle held open or closed? engine hot or cold? Did you follow up with a leak down test?
What was the elevation at the test location? Generally a 10% differential lowest to highest is the max acceptable, but I have seen engines tested with the throttle closed that had differentials due to chambed filling differences with the throttle closed.

[Barry Brisco]

Hi Jake,

Thanks for your reply. According to the owner of the car (not me) the test was done by his mechanic just over a year ago. I do not know if the engine was hot or cold or the throttle position (I thought the standard way to do it was with the throttle wide open). The elevation was close to sea level.

Barry

Were these numbers gathered with the throttle held open or closed? engine hot or cold? Did you follow up with a leak down test?
What was the elevation at the test location? Generally a 10% differential lowest to highest is the max acceptable, but I have seen engines tested with the throttle closed that had differentials due to chambed filling differences with the throttle closed.

[Jake Raby]

You might want to re-do it and do a leak down test as well. I do both on a warm engine.. Butterfly open for the compression test..

[tod914]

One time back in band camp, I had a shop do a compression check on my 75 2.0. They gave me numbers in the 60's for compression. I was wondering how the car even moved with those low numbers. Low and behold, they didn't have the buttler fly open when they did the test. I redid the test and it was up in the high 150's. Listen to Jake.. he's da man.

[Barry Brisco]

I certainly agree that Jake is an expert on 914 engines. But the fact that the compression test numbers I described ranged from 130 to 150 makes it seem likely that the test was done with the throttle open. Of course it is possible that it was not fully open. I don't know, I wasn't there when it was done.

Whether the seller of the car I am looking at is willing to do another compression test, and a leakdown test, I don't know yet.

Thanks,

Barry

One time back in band camp, I had a shop do a compression check on my 75 2.0. They gave me numbers in the 60's for compression. I was wondering how the car even moved with those low numbers. Low and behold, they didn't have the buttler fly open when they did the test. I redid the test and it was up in the high 150's. Listen to Jake.. he's da man.

[Jake Raby]

I have seen numbers that high with the butterfly closed.. on engines set up with a high idle speed and more bypass air than normal.. Or the engine was really strong.

I have also seen differentials of the questionable percentage that come from a close throttle and less chamber filling on one cylinder than the rest.

With the lower numbers coming from the most problematic cylinder I'd say that cylinder could have a tight valve.

Run through the valves and repat the test at WOT then back it up with a leakdown and report back.

The numbers aren't bad but are out of the tolerance for a completely healthy engine if they are correct.

This post has been edited by Jake Raby: Sep 26 2010, 06:28 PM

[naro914]

so you're saying that my 225 on 4 of my 6 cylinders, WITH slightly blown head gaskets may be a bit high??? (IMG:style_emoticons/default/smile.gif)

[Krieger]

Buy the car based on how much rust it has. This is the most important and potentially expensive issue. If it drives well, perhaps you could build an engine on the side for this car, or buy parts and just do the top end of this motor until you have more $ If it is rusty and has a bad cylinder walk away.

[brant]

Buy the car based on how much rust it has. This is the most important and potentially expensive issue. If it drives well, perhaps you could build an engine on the side for this car, or buy parts and just do the top end of this motor until you have more $ If it is rusty and has a bad cylinder walk away.

I agree...
motors can be built easier than rust can be repaired.
motors come and go... (with race motors I've had 9 of them in my oldest chassis)... But rust is really really hard to get rid of.
a 1.7 that drives well is fine...
if you wanted an amazing motor, you'll be rebuilding it to a raby motor or -6 conversion... a little low compression on a 1.7 doesn't affect the driveability much.

brant

[Barry Brisco]

What are typical compression test values in a 914 engine in good condition? Or phrased a different way, what numbers would one expect to have seen when the cars were relatively new? 170? 160?

I'm trying to get a feel for how much below "new" condition the engine is in the car I am looking at. A year ago the seller had his mechanic do a compression test and got values ranging from 130 to 150.

Thanks,
Barry

[Jake Raby]

150-165 is a good number at sea level for a bone stock engine..

[Barry Brisco]

150-165 is a good number at sea level for a bone stock engine..

Thanks Jake, that's good to know as a reference. My concern with the numbers I got from the seller is that they are all round numbers, as if they are just estimates:

#1 130
#2 150
#3 140
#4 150

The mechanic who did the test should have written down the number showing on the gauge. Instead he seemed to have rounded off; I'm skeptical that the gauge actually showed the numbers reported to me. What if the "150" was really "154" , and the "130" was really "126"? Then the difference between the high and low numbers would be even greater.

As you said earlier, this test should be repeated to confirm, and a leakdown should also be done.

Thanks,
Barry