anyone have any nominal compression numbers in pounds per sq inch on a 1967 mk2 3.4l engine? as in acceptable...

 

The usual rule is no more than 10% variance from lowest to highest cylinder. As for absolute numbers, those are hard to give. The reason is it can change with calibration of your gauge, how warm the engine was when the test was done, grade of oil in the crankcase and elevation.

I don't think I have ever seen a specific number given in the manual. As a very general rule of thumb, for an 8:1 compression engine I'd expect to see something over 125psi. for an engine in good health, perhaps 160psi for a 9:1 engine. That number could vary quite a bit, for example a V12 HE I'd expect to see 220 psi or more.

 

thanks for the info.

 

For an 8:1 c.r., it's nice to see a little over 140 for a cold engine and over 150 hot, but as Jagboi says, there's lots of 'it depends'. The main thing is to be at or above 120 and not have too much difference between cylinders. If one is more than 10% lower than the others, it's good to find out why, rings, bore wear, valves - repeat the test with a little oil in each cylinder, leak down tests ... .

 

Leakdown tests are most effective IMHO. In countries like SA altitude can make a significant difference all things remaining equal. Why Boeing still test here at altitude.

 

thank you everyone, for your input. your responses were exactly what i was looking for. much appreciated.

 

When I bought my 3.4s 8:1 compression ratio I did a compression test and found all the cylinders were between 115 and 125 psi. 75000 miles on the clock and the engine had been sat for ten years. Did not think this was good enough as I would have expected 140 psi to be acceptable so engine came apart.
Looking inside the engine when I stripped it down there were wear/score marks in the bores and a lip so decided on a rebore and engine rebuild.
New 9:1 compression pistons fitted after a rebore with new rings and valves and now the compression test shows 165 psi across the board.

 

Rob ~ Your car is big bore, high compression. You obviously shaved the head. Bit of a rocketship.

 

My 1966 3.8 engine is the high compression version and at 26,000 miles (yes, original miles) the psi on every cylinder was/is 170 even.

 

The leak down check is the standard used in light aircraft compression assessment.
Trying to do a standard compression check with a 50-pound prop whizzing around a foot or two away is not ideal.
80 PSI input is the general standard used.
Even without a calibrated leak down gauge a leak down check will show up leaky valves (listen at the carby inlet or exhaust pipe) or bad pistons/rings (listen at the oil cap).

Another point is how well is the compression gauge calibrated. I have seen two different gauges giving readings which are 10% apart.
The important point is just how even are the compressions across all cylinders.

 

I will not deny that it goes well when the pedal meets the metal but very rarely do I do that. Who needs a V8!

With the rebore I must be around 3.7 and the Engine number ends in 8 so don't believe what is written on the outside of the tin.

 

Hey Huey Hoolihan
We don't mind answering/advising on your queries but do wonder as to your location.
Is it in a cave in the Catskills or on a mountain top in Colorado or in some other part of this planet? Possibly outer Mongollia?
Please reveal your location.
It really does affect the sort of information you may require or allow us to ponder your specific need.
Serious differences in climates such as the sub tropics (where I live) and possibly extreme cold such as Canadian winters will affect factors like carbies, thermostats, heaters etc
Cheers

 

Indeed, tomorrow night is forecast to be a low of -27°C and a high of -21°C tomorrow. Needless to say, my S Type is tucked away for the winter.

 

Canada is a country of extreme temperatures, and even more so because of global warming.
I was able to swim in our lake right into the middle of October this year for a good hour and a half with out getting cold.

So far our winters don't get any colder then about -7, the worst its been is around -15.
Our summers are hotter and longer with temps this year reaching to +41C for 3 days _ that's never happened in recorded history along with a 3-1/2 month drought.

 

Daytime summer temps hovering in the 37 to 42 C (99 to 108 F) are normal in the far west of Queensland in places like Birdsville and Boulia.
Paroo shire in western Queensland has just been taken off "declared drought" status which had lasted for 9 years.
We do get a light pattering of snow in the states southeast corner highlands around Stanthorpe about once every 3 to 5 years.
Most Jaguar enthusiasts outing take place in autumn, winter and spring. Summer is generally too hot

All this is to simply explain why geographical and climatic differences can have a bearing on advice to forum members.
Cheers

 

IIRC "hueyhoolihan" is located in the San Francisco Bay area, which has a pretty mild, Mediterranean climate. His location has been asked before, but this has not resulted in a profile update...

Mild climate notwithstanding, his area features some of the most crowded, miserable freeways in the state, where "rush hour" begins before 3:00 PM and crawling along for extended periods is the order of the day. Cooling systems must be up to snuff, or trouble will ensue.