compression testing

[JIM NEWMAN]

OK. I've looked in all the obvious places (including prior threads in this forum) and can't find the answer. For a stock 3000 engine, what are "good" compression numbers? In other words, how low do they need to be before one should start thinking about a fix? And what is the most likely nature of the fix - rings, valves,.....??
Thanks for the insight. Mine are running around 100psi!

 

[aeronca65t]

There may be other opinions here, but generally, a compression difference of 15% to 20% or more (from highest to lowest cylinder) would be cause for concern.

[ 09-22-2003: Message edited by: aeronca65t ]</p>

 

[Dave Russell]

Jim,

This is a hard question to answer with exact numbers. The variables are - exact compression ratio, condition of the engine, calibration of the gages used, cam timing & valve lash setting,starter cranking speed,engine temperature, throttles open or closed, & altitude of test site.

As Nial said "generally, a compression difference of 15% to 20% or more (from highest to lowest cylinder) would be cause for concern."

Cylinder to cylinder differences of a given engine are not subject to to these variables & are thus the most reliable indicator. However if an engine has very low compression averages even with little C to C variation , it can still be worn.

In search of more information, I did a survey on another list , & got the following results which are quoted below;
------------------------------------------
"My query on Hundred Four compression readings brought some good responses & several explanations of why there were too many variables involved to give accurate answers. I expected variables as there are many. The more responses the more that the variables will average out.

To summarize:
Six people responded with actual numbers, the individual engine averages (all cylinders of a particular engine) ranged from 111 to 138. The average of the five responses was 125 psi. This is what I personally would consider a reasonable target number. The 100 spec & the 100M spec engines were not notably different. This is likely explained by the higher compression ratio of the M spec engine being offset by the later intake valve closing of this engine."
----------------------------------------

I would say that your engines compression is pretty low if the average of all cylinders is 100 psi. Be sure to check it with a different compression tester to be sure, & still note the C to C differences as above.

A fairly reliable test to isolate the problem is to take the first set of readings as usual & then a second set with a couple of cc's of oil squirted into each cylinder. If the rings are leaking the oil will temporarily seal them, & the low readings will be due to valve or gasket problems.

The "definitive" test is to use the differential pressure or so called leak down test, but this is a much more involved test. A leak down reading of 10% or more is considered to be defective. Most race engines are rebuilt when the leakage is more than 5%.

If we can get more feedback from others on this forum it will help add to the data base. The above averages were based on a compression ratio of 7.5 to 8.3 to one.
D

 

[ThomP]

These compresson readings were taken on my BJ8 in May, 2000 about 5,000 miles after the engine was rebuilt and recently tuned. The head was shaved enough during the rebuild to ensure it was flat, nothing more. These numbers were taken while the engine was cold. Compression was between 150 and 165 pounds (~10% variation). I took these readings for future reference. I read somewhere a LOOONG time ago that this was the best way to use comperssion numbers and it seemed like fun at the time to collect and record this info. Thankfully, I haven't had a need to use them (Yet!)

Again, the number of variables involved makes a direct comparison impossible, but if some more 3000 owners publish numbers, we'll at the least, have an average for comparison like Dave has for his 100-4.

Cheers

 

[John Loftus]

Jim,

The P.O.'s records showed a compression check done on my 6 cyl. motor at 61,000 miles. The range was 157 to 166.

Just before I pulled the motor at 87,000 miles I was getting readings averaging 155. The engine was running strong but low oil pressure and oil consumption of 1qt/500 hwy miles convinced me that it was time.

Cheers,
John

 

[jjbank]

Jim,

Did you drive your car or warm it up to operating temperatures before you did your compression test. Make sure your gauge is accurate.I believe from past knowledge that 140 to 160 psi with only a max 10% deviation is good.

 

[piman]

Hello Jim,

did you open the throttle when you did your check, I understand that compression testing should be done with the throttle fully open, and that variation between cylinders is more important than actual figures, within reason.

Alec

 

[Dave Russell]

Hi Alec,

I think that at cranking speeds The idle throttle setting is sufficient To allow good compression readings without opening the throttles. Throttles closed are not likely to lower the readings by more than 5 psi or so.

So far in this thread, for the six cylinder engines, we have an overall average of 156 psi reported. This is for unstated or unknown conditions of throttle settings, gage calibration & the other variables which come into play. As stated before, if we can get even more compression reading reports, the accuracy of the data will be improved.

Jim's readings of 100 psi (uniform or not) are clearly way below average & not "within reason". I think that Jim would experience a large gain in power & fuel economy by doing a rebuild at this time.
D

 

[piman]

Hello Dave,

I have always done compression tests on full throttle as that is how I was taught. So I have no experience of 'throttle closed' readings. The reason I asked is because it seemed odd that the very low readings obtained are even. I would expect a badly worn engine to give a more varied spread of figures. Obviously it is worth checking the accuracy of the instrument before carrying out a rebuild.

Jim,
you make no mention of how the car is running, or if it is smokey, using a lot of oil, all pointers that things are not as they should be?

Alec

[ 09-29-2003: Message edited by: piman ]</p>

 

[NW_SpriteGuy]

If it helps any, I have a '63 BJ7 and I haven't done a thing to it in many years. It's compression averages 155 across the board with one cylinder low at 152, and one high at 158. Other remaining four are right at 155. Oil pressure is good and the motor runs strong, but it still uses a lot of oil (mostly dripped onto the garage floor from leaking seals), not through combustion!

 

[Dave Russell]

Hi Alec,

I completely agree - Check with another gage.

As far as a "more varied spread" I have seen very worn engines with low variation & some with a great deal of variation.

I would speculate that compression ring leakage might result in little spread since there is little reason for variations in ring/cylinder wear. On the other hand, valve & gasket problems are much less likely to be even.

To repeat earlier advice, (the "definitive" test is to use the differential pressure or so called leak down test, but this is a much more involved test. A leak down reading of 10% or more is considered to be defective) This would quickly define the problems. For folks who are interested in further info see this linl;
https://www.team.net/html_arc/healeys/200309/msg00578.html
D

 

[Blonde Healey Girl]

Hey guys here's one more for your stats, We have a 61 3000 with 1500 miles SMOH and it has 158+/- 2psi on all 6 tested with with cold engine and closed throttles.
Has Low oil pressure at idle and 60lbs.at 3000. Very few leaks and behaves itself.
shelly

 

[Randy Forbes]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Blonde Healey Girl:
Hey guys here's one more for your stats, We have a 61 3000 with 1500 miles SMOH and it has 158+/- 2psi on all 6 tested with with cold engine and closed throttles.
Has Low oil pressure at idle and 60lbs.at 3000. Very few leaks and behaves itself.
shelly

<hr></blockquote>

Is that the red car?
My wife & I spent about an hour looking at it when Winn brought it over the the Chateau Elan for us to see.
Hope we get another chance to see you guys!

 

[KenG]

Does anyone see any concern with compression readings between 130-140 on 67 BJ8? (avg. 132.8) My numbers are a little lower than what I saw above.

 

[Healey Nut]

Does it run , does it burn excessive amounts of oil ? If the answer is yes/no then you have answered your question . Gas er up and drive it like ya stole it .

 

[Deleted member 8987]

10% was always the tolerance. Throttle open to allow full flow of air into the cylinders.

Now, look at the face of your compression gauge and tell me the calibration date.




Oh, right. None are. It's really a guess anyway. You try three different gauges, you will get three different readings.
Plus ALWAYS the same number of compression "hits". I don't care if one get "normal" after 100 hits, some folks do three, some 5, some ten...but the same on each hole.
Then ONE pump of oil in each hole, crank it a couple of spins, and re-do it.
You put a half a quart in the hole, you will have unbelievable readings (high), so just enough to hit the top ring and spin it before you do it.
Using lots of oil (1 quart in 500 miles) with good readings makes me think valve guides.