Compression Test Pressures?

[Neville Heal]

I have rebuilt my engine with 1720cc capacity, NeuTek ZH-1 cam and compression ration of 8.8:1.
The engine has been run (well) in a test stand for a few hours and compression test results of;

Cylinder 1 = 125psi
Cylinder 2 = 120psi
Cylinder 3 = 100psi
Cylinder 4 = 100psi

There is obviously a problem with 3/4 cylinders/head which I need to investigate?

But can someone please advise what pressure (psi) I should expect for my compression ratio?

Thanks

[Greg Bryan]

It requires a special tool, but a compression leak-down test may give you better results and more info to go on. A regular compression test has a number of variables, such as cranking speed, did you hold the throttle open when cranking, valve adjustment, ring seating, etc. A leak-down test will narrow down the problem, if you have one ...
But, if it's running good, don't worry yet about the compression. It may stabilize after you put some miles on it.

[Neville Heal]

Hi Greg

Thanks for the reply. A leak test was my next step, as I am getting a small oil leak/weep at the cylinder to head joint. Possibly different cylinder heights and not getting a good seal at the cylinder to head.
I did the tests with all the plugs removed and the carbs wide open and cranking speed only.

I was hoping for rule of thumb or guide, for compression pressure vs compression ratio? Is the 125/120 psi I am getting on the good cylinders inline with my compression ratio?

By the way thanks for all your excellent "electrical" articles in the Registry magazines, they have been an enormous help in rebuilding and rewiring my Porsche.

Regards

[Martin Benade]

Some compression gauges are not very accurate, but uniform numbers will still indicate things are ok.

[cory cotherman]

It is more important that they all be within 10% of each other as a general rule.

[George Walling]

Hi, I would think a good run in the car would be my first step. Also did you re-torq the heads after your running on the stand. I'm no expert on the Porsche engines but on my VW engines and American engines I always check head bolts after 500 miles and then again after 2500. Some of the engine rebuilders here may have better advise. Also if you are still getting a leak between those 2 cylinders and carb is setup right on that side why might need to check cylinders height but this would be my last check. Good luck.

[Vic Skirmants]

I have NEVER found a need to re-torque the 356 heads; race or street.

[Dick Weiss]

I agree w/Vic;

As long as the heads have been flycut properly(?) w/the cylinder seats w/in .0003"/.0004" depth in both planes
and heads torqued alternately, I use 7, 15, 22, & 28 ft.lbs. & recheck twice more; This stresses the case evenly.
If you don't, the 1st head won't have the final/proper torque--'old racer' practice found by the late Ray Litz of Comp. Eng.
Of course, the cylinder heights must match to each other.

Dick

[Ron LaDow]

Oops. Once is enough

[Ron LaDow]

Hi Greg
[...]I am getting a small oil leak/weep at the cylinder to head joint.
Regards

This is VERY bad news. It is dumping carbon monoxide into your heating system and therefore into the cabin. BAD ju-ju!
And I agree with Vic; that practice is aimed at engines with gaskets which 'take a set', AFAIK.
And Dick.

[Rusty Ferrell]

Leak down is a good next step, but if it were me and that is a big if, I would focus on loading the engine, get cylinder pressures up this will help seat the rings, no load results in the rings floating on the pistons, they need to be forced onto the ring lands. After about an hour of operations, then do a re check of the compression test or leakdown.
Good luck,
Rusty

[George Walling]

Well I stand corrected thanks to Vic, Dick and the rest of you for setting me straight on the need for re-torq the head bolts on Porsche engines. All of you are great for keeping us new folks straight and giving us sound advise. Like stated earlier deck height is very important and from my limited experience I have found it is important to have the engine under load and change load from time to time if you want to set the rings. At any rate Good Luck.

[Steve Proctor]

I would think any leakage at all between the cylinder and head is a red flag. Must be scrupulously clean before installing the head. Also, I like to lap the cylinders into their respective head chambers to make sure there would be a good seal. Don't know if this is a usual or even recommended process, but made sense to me.

STP