johnnyjaws

what should a good 1970 350 olds motor with 132000 miles on it have for compression when u do a cyclinder test on the motor?

hookem horns

Really looking for consistency between cylinders rather than absolute value, but readings will likely be in the 120-130PSI range. Any specific issues like blowby or oil consumption that are causing you to ask? Look at the condition of the plugs as you pull them to do the test too.

johnnyjaws

just got the car have been waiting 4 months for delivery until I got a phone call from the guys sister that I bought the car from he died but she told me ill still get the car and delevered I bought the car not running . was in such good shape he said he drove it in to the building were it sat for ten years . I put a starter in it lubed up the cyclinders and put 1/2 a bottle of mmo down the carb and let it sit a day then I hand spun the motor , put new points in it and it fired right up smoked like hell for a couple of minites then cleared up runs great . did a compression check on it. with the motor warm and got a reading of 100lbs per cyclinder except the middle one on the right side 78lbs haven't driven the car yet have it on jack stands and in gear sounds great put new plugs in it I got the compression tester snap, on from a pawn shop. I wonder if its accurate also had a new trans in it before he parked it ive had past cutlass s and they had 190lbs to 200lbs per cyclinder this motor is a 1970 cutlass s 2 bl the other one was a 4 bll this car has 136000 miles on it don't smoke and idles smooth

jaunty75

This can be figured out for any engine with knowledge of the engine's compression ratio and a simple formula:


compression (in psi) = (N-1) x 15

Where N is the compression ratio. "15" is atmospheric pressure, and 14.7 can be used for a more exact number.

So if the engine's compression ratio is 8 to 1, then the pressure in the cylinders at the top of the stroke should be (8-1) x 15 = 105 psi.

For 9 to 1: (9-1) x 15 = 120 psi
For 9.5 to 1: (9.5 -1) x 15 = 127.5
For 10 to 1: 135 psi


Now, no engine is perfect, and you wouldn't expect to get exactly these readings. But I would expect a new engine's readings to be close to these. As the engine ages, compression values will decrease, and if they get too low, it's time for an overhaul.

As noted, what matters as much as anything is consistency from cylinder to cylinder. This apparently 6-cylinder engine you're talking about, with readings of 100 psi except for 78 in one of them, has a problem as that's a significant difference. Something is up with that one cylinder.

jaunty75

200 psi means a compression ratio of more than 14 to 1. I don't think any Olds engine ever had a compression ratio that high.

By the way, your posts would be far easier to read and understand if you'd take a moment and put in a little punctuation. It's sometimes difficult to figure out what engine you're referring to from time to time as you seem to be talking about several all at once.

johnnyjaws

ok im talking about a 1970 350 9.5 to 1 v8 motor not a 6 cyclinder. it does have #6 heads though so 100lbs per cyclinder is good ?

jaunty75

I assumed you were talking about a 6-cylinder engine because you said this earlier:

A V-8 engine has FOUR cylinders on each side, so there can't be a "middle" one. But there would be only three on each side on a V-6, meaning there could be a middle one, so I assumed V-6.

I think a reading of 100 psi is low. It should be in the 120 range. But if 7 of the 8 read about 100, your engine is worn, but it's wearing pretty much evenly except for that one cylinder reading 78.

MDchanic

Was that compression test with the throttle open or closed?

- Eric

Magna86

Yes as Eric pointed out did you have the throttle opened or closed for the test. Also if it sat for a long time as you stated the you could have some sticking rings that need to reseat. Using MMO to soak the cylinders helps and using it in your oil. With over 100K the engine its still reading pretty good except that low cylinder. Need to a leak down test to see if its rings or a valve issue.

Macadoo

Have you tried putting a spoonful of oil in each cylinder before the test to see if the pressure comes up?
My readings were a lot higher than jaunty's calculations but I had to run my tests cold. Would that give higher readings?

MDchanic

It should give lower readings.

- Eric

Macadoo

I had 155 on my highest cylinder on an 8.something CR engine. Maybe a faulty gauge.
Johnny, try adding a little oil to that low cylinder and see if the compression comes up.

MDchanic

Well, on my 10.25:1 motor, (10.25-1) 14.7 = 136, and my highest cylinders were around 172psi, on a 106,000 mile motor, so I think that formula may be imperfect.

- Eric

Macadoo

Roger that.

johnnyjaws

I had the throttle open ill let it run for a little while longer

jaunty75

It is. There is no direct formula for calculating cylinder compression based solely on compression ratio. The formula I gave neglects a number of things.


I've read the following three things:

1. Typical combustion engine cylinder compression pressures are between 150 and 200 psi.

2. Below 100 psi is a problem.

3. A difference of less than 15% from cylinder to cylinder is OK. More than that and you have a problem.

pogo69

if your going thru all that work you might as well think about doing a leak down test if you have a compressor...it will give you better info on your engine

Fun71

Yes, as in it does not (can not) take into account inertial filling and the intake valve closing events. Earlier intake closing "traps" air as the piston is rising whereas later intake closing allows the air to escape as the piston is rising. So two engines with identical compression ratios but different camshafts can have different cylinder pressure readings.

johnnyjaws

thanks for the info

Ozzie

If you'd like the Oldsmobile recommendations, see page 6C-5.

http://wildaboutcarsonline.com/membe...Section_6D.pdf

johnnyjaws

I changed out the fuel pump timeing chain was very loose maybe that should help it out

Billyaxe

Hey guys, I just had a compression test done on my 68 442. There’s a lot of mystery in what was done to this 400 engine by the previous owner. I’m new to this so can anyone offer an option on these numbers my mechanic have me.
Thanks. —Billy

1. 230, 2. 190, 3. 190, 4. 200, 5. 235, 6. 200, 7. 210, 8. 230.

Ozzie

Oldsmobile's criteria was the same for 1968 as it was for the 1970 shown in the 6 year old thread. (Lowest no less than 80% of highest and none less than 100) So assuming the test was conducted properly, your numbers satisfy Oldsmobile's criteria as being O.K.

matt69olds

Even though I’m replying to a 6 year old thread, I think this is still useful info.

The actual compression number is influenced by many things. Cam timing, elevation, cranking speed, and engine condition, etc. The actual number isn’t really a factor, as long as they are constant.

The textbook compression test method is to warm the engine. Remove all the plugs, disable the ignition. Wire the throttle wide open. This is to allow maximum cranking speed, with as little airflow restriction as possible.

Install the compression tester into each cylinder, crank the engine the same number of times. The first full pump reading on the gauge should be at least 1/2 the final reading. The difference between the highest and lowest cylinder shouldn’t be more than about 10%, and each cylinder should be fairly even.

If the reading are even but low, a stretch timing chain could be the cause. A little oil in each spark plug hole will increase the number, but if ring seal is good it should be a minimal increase.

If you have a cylinder with low numbers, a leak down test will help pinpoint the cause.

Miles71

Wow, those are high compression numbers for your 68-442.
Must be some interesting engine work.

I had a 1969 442, 400ci, 4bbl, 300hp, 10.25 compression.
Near as I could tell the matching numbers engine was unmodified.

In 2018, when the car had 130k miles I did a compression check.
All 8 cylinders were between 168 - 175.
I was very pleasantly surprised.

The engine purrs like a kitten.

https://classicoldsmobile.com/forums...e-117262/page3

69CSHC

Miles71 thanks so much for that post. I was starting to wonder if my engines numbers were a fluke.

My 69 10.25.1 350 CID tested at 180+ and 190 in 2016 at 110,000 miles. All cylinders not tested, mechanic was just curios. I was pleased but figured it was on the healthy side by the way she ran...

Noel Anderson

I was also thinking my tester was off. My 1969 Cutlass Supreme Holiday Sedan with only 64,000 miles and a 10.25:1 compression 350 V8 showed between 205 to 210 PSI on all cylinders.

Noel

69CSHC

Very nice, so I am seeing a bit of a pattern between our 3 cars. All 1969s, all original unmoded engines, all no rebuilds.

64,000 miles 205-210 PSI
110,000 miles 180-190 PSI
130,000 miles 168-175 PSI

Sugar Bear

69CSHC, Looking at the #'s provided...are all the timing chains original? I'm wondering if late valve timing is the culprit. SB

Noel Anderson

My timing chain was original when I took those numbers. I have since replaced the timing set because I didn't trust my luck with the nylon teeth. When I tore it down, the chain was hardly stretched and I was only missing nylon on two teeth. The nylon was heavily cracked though. I have not checked my compression with the new chain. I thought a timing chain would not affect a compression test because both intake and exhaust valves would be closed at the same time allowing compression to build. Am I right or wrong?

Noel

Sugar Bear

I believe it would as it makes both valves open and close late in relation to where the piston is in its travel. If you were to do a compression test on a motor with a jumped chain it would probably be around 60 psi and not start.

It would be good if you could retest using the same gauge.

Good luck!!!

69CSHC

Mine was not, it was the timing chain I bought the car with but it was already replaced. (All metal no nylon not that worn with some stretch.) I had that one replaced sight unseen because I couldn't verify when or if it was ever done. The previous owner did not have it replaced but he was third owner I am fourth. I needed piece of mind so I had it done at around 120,000 miles. I have not had the compression tested since.

Interesting, so you are implying our numbers could be better or at least mine could ?

I took the numbers to imply that our engines look to be losing about 5 PSI give or take, every 10,000 miles, at least between 64,000 miles and 130,000.


Very interesting, since I had the timing chain replaced my car feels slightly more powerful or more peppy. Butt dyno observation only so far. I can now get the tire to start screaming at a slow roll maybe 10-15 MPH with throttle pops. I never noticed that before.

My idle sped up 25 RPM immediately after timing set replacement with absolutely no other adjustment...

matt69olds

The idle speed increase is due to the ignition timing becoming advanced due to the chain replacement. The ignition timing was set with the slop in the old chain, removing the slop advanced the timing without moving the actual distributor location.

69CSHC

Thanks Matt69Olds I appreciate that. I figured the increase in RPM was proof that the new timing chain set made a difference.

The car feels great, no question the best since I bought it. Just took her out for a healthy spin yesterday. Once she is fully warmed up and settles out, 600 RPM in drive and 700 RPM in park. Vs the factory recommended 575 and 675 I had prior to the new chain. I chose to not adjust anything as of yet. Closing in on 2000 miles since replacement.

P.S. does anyone know what the compression readings on a new Olds 10.25:1 V8 were ? Going by how healthy Noels engine is I'm figuring 220 PSI ?