1967 Dauntless Compression Pressure

[67 Kaiser]

Hello, First post for me.

I am looking at a 67 CJ5 with a Dauntless V6. I have not yet actually looked at the vehicle. The owner claims the engine was recently rebuilt and runs well, but the compression across all cylinders is low, but all within 10 psi. I can not find specs on the correct compression pressure in any literature. I found the compression pressure for a F134 four cylinder at 120-130 psi. I found this in 53-71 Service manual in section A.8 page 6 general specifications, but it does not list the pressure for a v6.

I may be way off, but I think the low compression is one of three possibilities considering he had farmed out the rebuild. they are in no particular order

1. loose timing chain, or crank and cam not properly aligned
2. did not lap valves when doing the work
3. did not install rings correctly (lining the ring gaps up instead of proper stagger).

It could be a few other things, but I suspect these to be the primary culprits

Does anybody know the correct (target) compression for this engine.

Thanks for your help. Will make a difference when I talk to the current owner about purchase

[gmwillys]

Welcome 67 Kaiser,

Kaiser bought the tooling from Buick, so you will have to look at the 225 V6 specs. 10 pounds difference across the board would lead me to think you are right on the rings being lined up.


http://www.earlycj5.com/xf_cj5/index...ession.125641/

[bmorgil]

67 Kaiser, You didn't mention how low but, most engines running on gas wont fire when cranking pressure drops much below 70psi. The real check is a leak down. More than 10% indicates valves or rings. It should leak down within 3% to 5% between cylinders.

A quick check would be to check cranking compression with a dry cylinder then, squirt in a half a teaspoon of oil and check it again. If it comes up significantly or, improves on the the leak down it is the rings. If it remains low or, leaky, it is the valve train, head gasket or a crack. Make sure the valves aren't hanging open if it fails the oil test.

[67 Kaiser]

Thanks for the replies.

Without looking at the engine, and only talking to the current owner (long distance), I pretty much ruled out head gaskets, or weak valve springs. or individual piston issues. Since on a V6 there are two banks of cylinders, someone would have to made the same reassembly on both sides to get within 10 psi across all cylinders, and still have low compression. The seller is stating a number that is above 100 on all cylinders, but I was hoping someone knew what the correct PSI should be so I know how much it is below specs. I just can't find Compression specs for a 225 Jeep, Buick or otherwise.

If the valves are hanging open, I would lose compression on those cylinders only, unless all intake, or exhaust were partially open which I think would be a cam timing issue or loose chain. Weak springs, or bent valves is doubtful across all cylinders, but anything is possible.

I know a leak down test will show a ring problem, but since it is all cylinders, I suspect it was a sloppy rebuild.

Just looking for the Factory Specs on compression, If anyone knows this it would answer my question.

Thanks

[bmorgil]

67 Kaiser, there is an old engineering rule of thumb for cranking compression. It should be 15 to 20 times the compresion ratio. I don't know if you can find the ratio however. That is a pretty good rule if you can get the ratio. At 15 times an 8.5 to one motor calcs out at 137 psi. that is about what you would see on a used engine in good shape. A new motor would be around 150 psi. When 8.5 to one motors dropped to around 120 psi, we rebuilt them. I doubt it has a lower compression ratio than 8.5 to 1. The F134 had a ratio of 7.5 to 1. 115 psi would be the bottom number accepted before a rebuild.

If it is below 120, I am thinking it is not enough for a freshly rebuilt 1967 motor. On my freshly rebuilt 6.5 to 1, L134 the cranking compression is 132 within 2% on all cylinders.

[67 Kaiser]

Thanks Bmorgil!

That is a helpful answer. I know the factory spec for the Dauntless is 9.0:1, so if I have factory Specification compression of 9.0 X 15 = 135 PSI. and 9.0 X 20 = 180 I am looking at 135-180. Splitting the difference gives me 148 or 16.38 times the stated compression. Plus or minus 10% gives me a range of 134 -162. These numbers sound reasonable, but possibly a little high for the top.

All in all, this has been extremely helpful and without having actual a factory spec sheet, it has answered my question.

I cross referenced this to a f-134 6.3: to 7.8: depending on altitude and early/late production or options, all showing 120-130 compression pressure, and it pretty much falls in the 15 to 20 times grouping.

Thanks again to Bmorgil for the great answer.

[bmorgil]

Yes the 20 times number is high. The thought being if you are getting that high of pressure, the cam timing is either very short or, off.

Thanks for the kudos! Good luck on your endeavor.

[90 degree]

after reading the universal manual for the dauntless i don't think there is a set pressure, it is a look at all cylinder pressures. and they all have to be more than seventy five percent of the highest one. the book also has an example. jeep universal series manual C-9 b

[bmorgil]

Yes you are correct The service manual focuses on how well the motor compresses during the first four strokes. If compression jumps up "quickly" it indicates good pressure. If it is slow to come up and needs all four strokes to peak, it is getting "weak". If it is varying between cylinders, something is up. On these motors this is still a good way.

This is vague to say the least! Back in the 40's and 50's however it did the job. Basically back then, if it didn't pump up quick, was hard to start and smoked all the time the rings were bad. If it did all that and didn't smoke all the time, the valves were bad. These little guys "wear their heart on their sleeve". Now in these days we do like to know the numbers. The specification for the L and F motor is 90-110 and 120-130 respectively. Bellow those bottom numbers and you would start to watch for symptoms of low compression. The top numbers are a best case, engine warm and all new. Using a leak down and/or a compression tester remains a bit of a mechanic's call and technique derived from experience. Performance drops off as compression falls. As long as it is running acceptably, some are hesitant to touch it. Most builders are going to start looking when the pressure on a warmed up engine is off by 15% on a leak down or 25% on a cranking compression test. Power is going to be off at this point. It will be slow to start. It is probable it is going to start fouling soon.