So I am starting to burn a bit of oil. does not smoke all the time just when I let off it or when I first start it so I am thinking valve stem seals or guides. For giggles I pull out the compression tester and pull a compression test on all eight After a quick three rises of the needle all cylinders are 190-195.I read someplace that you need to spin the engine until the needle no longer moves.
Ha so ill try this. My helper hits the starter and we let the needle bunce up to its max on the gauge 250psi.about 4 revolutions. While in there I decided to install new plugs yup time to spend some money on the old girl figured what the hell its been about 4 years. Motors had the crap knocked out of it and it deserved it. (even though I knew they could be cleaned. I just happened to have a new set sitting in my 496 in the corner.so I saved a few bucks.

Any ideas on what the pressure should be???

 

That 250 number sounds very high, what is your compression ratio?

 

Agree 250 is very high. Question the accuracy of your compression tester.

190 is even very good. Depends on CR, cam specs and battery strength/ speed of rotation.

 

lets see battery is a big Optima fully charged-starter is a high torque gear reduction starter-and the tester is a Snap-On.... never had issues with the gauge.
cam is a solid roller 110 separation .576 lift 243 duration@.50

 

You never mentioned compression ratio and a big cam will lower the readings.
Borrow or rent another gauge.

 

11.2:1 forgot that sorry. yeah maybe ill try another gauge.

 

Good idea.....180 is the threshold of pump gas.
250 would require like C16 race fuel.....
The gauge is garbage....

Jebby

 

So how would you do a compression test out of curiosity. I have always done them with turning the engine over just three two four times that's the way I was taught.
My gauge is a Snap on MT 308KB. For giggles I did hook it up to my air compressor and with the regulator I was able to bring the pressure up to 120 in 5 pound increments. So i hguess I know the gauge is good to 120 psi.
So the highest octane I can get on the street here is 94. the car likes that and I do need to add fuel additive or retard it.
Now I know a high compression engine will give higher cylinder pressures but that is usually only at higher RPM s due o the design of the cam. At lower engine speeds and slower cranking speed a lot of the pressure should be getting bled off also due to the overlap unless you have the cam retarded a few degrees. But cam discussion is a whole different topic.
I read a one point that the correct way to do a compression test is to crank it until the needle no longer rises, I believe I read it from the forum.
so 195 sounds about right but what is anyone's response to how they do it.

 

Im wondering if the PCV system may be sucking oil into the carb when you decelerate, maybe all that pressure is giving you a lot of blow by and your sucking it out.........

 

only way to have that high of a crank pressure is buildup on the pistons and chambers reducing actual chamber volume.

 

Well, I guess what you really want out of this compression test is to compare the cylinders to each other in this case. So if the tester is consistent at least you can compare cylinder pressures to each other.
Leak down test is the way to test valve and ring seal.

What pressures have other compression tests given you? Or is this the first time?

My 350 CI 10.6:1 with a 219/219 @ .050 108 LSA cam gives me 193 average cylinder pressures cold @ 4500 Ft density altitude.

So it's conceivable you could blow 250 psi, but with that cam duration I would think it would be less.

 

He averaged 190 across all 8 with 3 strokes. The 250 was at 4 to 5 strokes. The question was related to a possible oil burning issue. All cylinders are pretty much the same so i would think it's on the exhaust side pulling in the oil past the seal as the spent fuel is exhausted. But I have read that some builders only use seals on the intake side to prevent oil being drawn past the valve. I could also argue a stuck pcv valve and the oil burning being on throttle let off and it being pulled in on the vacuum increase. But what do I know. I'm just the guy with the shovel

 

The cylinder pressures really are not important. I am okay with 190-195 psig. The question comes how many times do you turn the engine over. The oil burning is a different issue all together.
I do puff smoke out the right side on sudden rpm drop. Now I do have aftermarket valve covers but they do have baffles in them.

 

I turn it until the needle stops rising. Always cold. Don’t like trying to remove hot plugs near hot header tubes.

 

Thanks to all the replies so far. I've done many compression tests but I just wanted to through the question out to the forum.
I am actually looking to edumacate
so when are you guys are talking compression ratios are you all talking static or dynamic?
Lets say mine is more of 11.67:1 static and and I have a cam that does bleed off some air because of the over lap. Now most cam cards do read valve timing at .050 and others read it at.006. My compression ratio could be extremely high or low depending on the cam.
So lets keep the static at 11.67:1 and just play with some cam numbers.
If we use a comp 12-422-8 we will actually have a dynamic ratio of 10.25:1 That cam if set up right will make roughly 450hp at the flywheel.
If we use a comp 12-771-8 our dynamic ratio will actually drop to 9.48:1 we should make roughly 25-30hp over the previous cam.
Don't ask how I know any of this junk in my head.
No I am not trying to show how dumb I am but just showing how different cams actually do affect readings. yes a battery and starter choice will play a roll here. The faster it turns over the quicker the valves will shut so we can make pressure-to a certain amount.

 

This is why pressure tests are inaccurate as hell.....for any kind of real diag. It is only used as a cylinder to cylinder comparison.......
Like I mentioned.....185-190 psi is the threshold of pump 93 octane......this psi. number does not care about cams or dynamic or static.......at around 200 psi, pump fuel will ignite...with bad consequences.
So......a large cam with a lot of overlap can bleed off enough pressure to make a high compression engine live, but there is no exact recipe for this as each engine will make a different VE (Volumetric Efficiency)....which is the amount of total charge in the cylinder relative to the size of the cylinder at BDC.......there is a buttload of science behind this and too much to get into here.

For the sake of this thread and your original question.......toss the Compression Tester and do a Leak Down test.....OTC makes a nice leak down kit for $70.

Jebby

 

Jebby absolutely correct. leak down testis the best way to test I absolutely agree. I knew you would be there. Yes getting into cam science is too much too get into here and requires a lot of know how unless you can do all the calculations. I am no expert by any means, just some dummy.
But please i see so many things about how many revs you need to do to get the right number so yeah I'm kind of stuck on it right now.
oh yeah by the way no I cant run pump fuel, motor pings like hell unless it has additives or a different fuel mix

 

Be a good idea now to check your total timing and curve.....

Jebby

 

Given that a compression has limited diagnostic value I still consider it useful information. As long as you do it the same for each cylinder and each time you complete one, you will have a comparison basis. That is what you're looking for.

If next time you do one and lets say cylinder #3 is much lower than the last one you did or the other cylinders, then you got a problem developing. You can see it's an issue due to previous compression tests and compared to the other cylinders on the current test. So in comparison it has a problem. And it's time to do a leak down on that cylinder to see what's going on.

In addition to cam dynamics altering pressure reached during a test there is also air density due to elevation or temp and humidity or both.

Take your car up to 8000 feet on a summer day and I bet your pump gas problems will be gone. Loss of cylinder pressure to auto ignite the fuel.

 

Jebby total timing and curve?? now that's really going to raise a few eyebrows here my base timing is set at 10* before TDC if I remember correctly. I have the vacuum advance locked out and just use the mechanical advance. Yeah I know its not the best for a street car but it works.
My detonation is not timing related. On the dyno with the engine running we moved the distributor around first forward 2* and then back to 36* then retarded 2*. It liked the 36* where it was set. There it gave the highest hp and torque numbers the AFR was right on up until 1800 then it went fat 13.6 or so

ok ok so i found out doing my test what I needed to find. Yup all numbers are even across the board and the next would be a leak down test. Frankly I just wanted to go cruising and didn't want to take a lot of time but quickly find out and put my mind at ease. The plugs all looked ok but a dark tan probably from all the different fuels and additives I run. I will sometimes run a fuel fragrance to hide the race fuel smell.
Yes I already have a leak down tester and that will be the next step.

I know I was vague in the begining but I was figuring on someone to say yeah 3,4, 10 whatever revs for the test. Yes my cylinders are really that high with 3-4 revs and and it will peg the gauge with multiple turns.