what is a good compression test reading for a 308 ?

I have a 1982 I am restoring, and the compression is 150 to 170 psi for cylinders 2-8, but alas, #1 is only 100 psi.

What is the new compression (in psi) of the 308, and what is the nominal (below which a valve or ring job is inevitable)...

BTW, My reference is my 1963 Ford 390 is 170 psi per cyl.

thanks

Andy

 

Andy, I have a 82 also and I don't pretend to be an authority. I can tell you that my engine had a healthy 175psi on all but #7 and it registered 25. I did fear the worst and pulled the engine because I don't think a major had ever been done. I was delighted to find that ,for whatever reason, the EX valve clearances were all over the place with #7 -.2mm. After reading allot of posts on this forum (the best info on the net as far as I'm concerned) I decided to change out the sodium valves and detail the engine bay. I hope this helps.

 

Our local F-mechanic indicated 170-190 w/ 10% or less variation btwn all cylinders and @ TDC a leak-down no greater than 5-0% is within spec.

 

I think the answer to that is that it depends on how you conduct the test (i.e. cranking speed, whether the thing is warm or cold etc etc).

The key thing though is that however it is conducted you want all cylinders coming up as close as possible to the same number with a max variance of around 10%.

I would suggest you do a leakdown test on #1 & see what that tells you because what you have doesn't look good if the result is right.

 

A 10 percent variation in compression readings from lowest to highest cylinder has been "within specs" on virtually every engine ever made as far as I know. But remember, that spec is so the factory doesn't have to fix engines!

Most engines compression check at around 150 psi though this depends on cam timing as well as actual compression ratio. Using that (150) figure, we would not let an "normal" engine out of the shop that had more than 10PSI difference from hi to low and most commonly the difference was around 5-6PSI. A blueprinted engine would have virtually no difference between compression readings. When I bought a new 996 Porsche 911 in 1999 all cylinders were within 4 PSI from the factory - I was quite impressed!

As is often stated, the actual compression number is less important than the variation. The psi reading will change depending on how the test is performed. It should be done on a warm engine with the throttle held at WOT and each cylinder rotated through the same number of "puffs" on the gauge. Most recommendations are for at least 4 - we always did 6.

Don't expect a high compression engine to necessarily show higher PSI than a lower compression engine. As noted earlier, cam timing/profile have a huge impact on static compression pressure. As an example, a 13.5:1 Mopar max wedge engine had a static PSI of around 150; a Dodge Daytona turbo with a compression in the mid-8's had a PSI reading also around 150.

Also, because of the differences in the way a person in their back yard may perform the test, don't compare YOUR 328's (or whatever) compression PSI reading with another 328 unless you did them both under identical conditions/methods/, including weather. Atmospheric conditions will change the readings.

 

Who cares?

A big variation in compression (great than 10%) tells you there's a problem. On the other hand, a leakdown test will tell you what the problem actually is.

Best,
David

 

Just by chance, did your helper who was cranking the engine happen to let the throttle close when you did #1?

 

Thought I'd also add...

A compression check can be misleading. Let's say that a brand new engine shows a static compression of 150PSI with all cyls within a reasonable range (personally, as I indicated earlier, 10% is too far apart IMHO but it IS the common spec). Somebody has an older version of the same engine with quite a few miles and, under the same test conditions, it shows 165PSI. Good, huh? MAYBE... It MIGHT be that that engine just ended up with more compression out of the factory. But it could also be caused by carbon deposits (oil burning) on the pistons. If there are deposits on the pistons, there are deposits on the intake valve stems. If there are deposits on the valve stems, the airflow will be disrupted/reduced under high load conditions but this reduced airflow will not show up at all in a compression test. SO the engine with the higher PSI readings will actually put out measurably less power.

Personally, I have never been much of a fan for compression testing as a stand alone. It CAN be useful but it is better used in conjunction with a series of tests to really determine engine condition. I'm not saying I didn't or wouldn't do a comp check alone; I would/did on many occasions. To me the value of them is to get a quick indication of a possible problem. For example, you do a check and two adjacent cyls show very low or no compression. It takes no further testing to make a pretty darned positive diagnosis of a blown head gasket. But the bad part of compression checks alone is that they may not give any indication of a problem when one exists. That's why most of the time we considered it just 1/2 of a test. THe other half was a leakdown. If both show healthy readings, then you don't have a cylinder-related problem.

As stated in other posts, leakdown is a better test. But the leakdown often won't give an indication of carbon buildup on the heads and valve stems whereas the artificially high compression will. Admittedly, if there is sufficient buildup to be a problem, the ring sealing will often be compromised due to the same problem (carbon buildup in ring grooves), which WILL show up with the leakdown. But I strongly feel that compression check and leakdown compromise two parts of one test.

 

I'm looking at three 308's for sale and will do compression tests on them. I've done it many times on cars and motorcycles so I know the procedure.
I was told that the four cylinders facing back are easy to get to but the ones facing forward are difficult. Is this true? Anything special I need to do the get the cylinders?

I'm planning to test all the electrical switches and get a short test drive.
Any thing else I should do for a pre purchase inspection?

Thanks
Jim

 

Hi Mike
Your advice sounds sensible.
How is a leak down test performed?

 

The 5-8 bank can somewhat easily be accessed by slipping your hand between the open rear deck lid and the rear glass. Using a 3" extension and appropriate spark plug socket you can remove the plugs, the trick is to drop the 3" extension and plug socket in first, and then attach the rathet after the extension is in the plug hole.

Once the plugs are out you can insert the compression test equipment be accessing the same way although it is a bit of an unweildy process with a flexible compression test hose....

For all 2v engines, the dynamic stroke is about the same for all of them even though the cams are all different. Dynamic or effective stroke is dictated by when the intake cam closes, which on the 2v engines is about the same. With a good battery, expect 150psi+ and above all else consistency among cylinders. If it's a QV engine, expect more than that, say 165psi+.

 

Ferrari Pilot
Thanks a lot. I have to do a similar thing on one of my motorcycles.
Drop the socket and extension on and then attach the ratchet.
Looks like the wrench size is 13/16 inch and the thread is 14 mm.
Same as many motorcycles.
But I will take several wrench sizes and compression adapters with me.
Good to know that I don't have to take off some parts to get to the plugs.

Jim

 

"How is a leak down test performed?"

It's considerably more time-consuming and involved than a compression check but it provides pretty much THE data on what's going on in the engine.

There are a lot of web-site explanations for leak down testing. This one is excellent (I used to work extensively with Car Craft and always considered them the best of the mags in their field.) How to do Cylinder Leak Down Testing- Car Craft Magazine

 

If you are compression testing, make sure you disable the ignition boxes properly - especially if its a QV. If you don't you can fry them - which would be bad, especially on someone else's car!

 

That's a great article. Very real information.

 

I've read that earlier produced cars from new had a habit of burning oil.
Would this have a effect on the leak down test or was this issue due to valve stem seals?

 

Early cars much of the time ran larger stemmed valves, usually 8mm. Additionally, valve guide material at the time wasn't that great meaning the correct guide to stem specification of around .0015-.002" I conjecture wasn't held for all that long which leads to oil consumption regardless of the valve stem seal condition. I've pulled apart a few sets of heads now and each of them burn about a quart every 500-1000miles depending on use. The original guides all had about .005-.006 clearance. Strangely, the engines ran fine before the heads were pulled! They just consumed a bit of oil and needed refresh. It's amazing the amount of work that can be done to refresh worn parts in these engines, but little in the way of power is picked up regardless.

Install new guides made of good modern materials with good stem seals and oil consumption goes way down but to some extent there will always be a bit of oil used as these are high revving engines and the oil scraper rings on the pistons can only ever control 'so' much.

 

I've always found it amusing that the Ferrari 3x8 "spec" for oil use allows more oil burning than the specs for the late '60s 426 Hemi Mopar. That sounds to me, frankly, like really poor engineering on Ferrari's part since the engine is less than half the size of the Mopar and with lower compression.

OTOH...I drove my 328 from Vegas to MD - 2700+ miles - and at the end of the trip it was down a bit over a quart which I'd call pretty decent. So if using a quart at 500 miles was considered acceptable for a factory engine while another factory engine could do nearly 3k miles on a quart, clearly the factory "tolerances" for production was large enough to drive a (oil) truck through!

 

Oil rings can be re-awakened: Add Marvel Mystery Oil to a car that sat a lot, and drive it regularly. The "pep" the engine has will pick up and usually oil consumption will go down, if it is escaping past the oil rings. I am not saying all things in a can will really help all engines, But I have owned several cars re-awakened with MMO.

 

What is the proper method to disable the ignition boxes (on a QV) for a compression test?

 

Do you add it to the gas, the oil, or both?

Any idea what makes it work so well? I have heard many people swear by MMO.