When is compression too low?

[KenVoodoo]

HI,
At the end of last year I did a compression check and all cylinders were 70 PSI +-5 psi. I only had one more day with the boat, so couldn't play more to see what was going on.

I just got back to the boat, and I will start putting the boat together again to launch in a week. My question is, if I find the compression is still this low, would it help to replace the two headgaskets I have on it now with a single head gasket? Or should I live with this compression. My understanding is I need to let the engine warm up before I can do a valid compression check...I will be starting the engine in a few days I could probably check the compression again at that point.

But what (even) compressin is too low?
ken

[romantic comedy]

I am wondering if your gauge is reading low. But even if it is accurate consistency is very important.

As far as the head gaskets, what is you thinking? In general, if the gaskets are the same, two would lower the compression as compared to only one.

But I doubt that the head gasket is bad. How does the engine run and start. Yes, you should have it at operating temp, open throttle, no choke when doing the test. Also all plugs removed.

You can squirt oil in the cylinder then test again. Turn it over a few times to get the most of the oil out. The oil can help seal the piston rings and help diagnose if the rings are leaking.

You might want to check your gauge with another also.

[KenVoodoo]

I was thinking

I was thinking that 1 gasket would increase the compression....maybe even into the 90 PSI range...and it would be good for power....

The engine is probably 50 years old..so at some point I will have to do something...but at 2-3 months a year in the water..how long can/should I put it off?
Ken

[sastanley]

Ken, One gasket would increase the compression ratio for sure.

Also, if I wanted a real world compression value, I would test it both cold and hot...then you can do an guesstimated analysis of what causes the compression numbers to (hopefully) rise after running. A cold compression test would be a true 'what am I up against'? scenario.

I keep babying my abused 1976 engine that I know had seawater and rust in #4 more than once before I owned it. The last time I did compression numbers several years ago (which if you dig deep enough are posted on this forum, so this is from memory) were 95-95-90-75 or so.

I'll post up some current numbers in a couple weeks (after I find my compression gauge) if you want to compare for analysis purposes.

[Al Schober]

When talking about one vs two head gaskets, it's helpful to consider the history of this engine. Engine originally had a single head gasket, but this was a thing of beauty - it had copper upper & lower surfaces and an asbestos core. After being compressed, used for years, and then removed, this gasket would still measure over 1/16 thick. I know folks that have done a valve job and re-installed the old gasket!
Head gasket technology has changed. The gaskets currently available are only 1/32 thick, hence the need for two. If the block for head has been milled a bit, 3 might be in order.
What should compression be? Spec says 6.3:1 compression ratio - 6.3x14.7psi comes out to almost 93 psi. Numbers over and under this are both suspect, but compression gauges aren't really 'calibrated' instruments.
In your case, I suspect the gauge is reading low. The consistency of the numbers indicates there's nothing 'strange' going on with the engine. Might be instructive to try a different gauge.

[edwardc]

Also, be sure to have the throttle wide open while testing. A closed throttle will decrease the numbers.

[Dave Neptune]

Ken, the question should first be "why is the compression low" not when. These were a low compression engine and that is WHY WE SHOULD RUN REGULAR GRADE GASOLINE which is for LOW COMPRESSION engines.

Now to the 70 + 5 psi. Was the throttle fully open, batteries fully charged? A low battery or closed throttle is not how to check compression. However since you state the compression is somewhat even I would suspect something above or a faulty gage. The good thing is that the compression is even across the board. Are you using a lot of oil?

The head gasket single or double would show variations in compression if compromised so I don't think the gasket(s) is the issue. Through some testing I participated in with another A-4 provider we gleaned some interesting findings. On the direct drive engines we made more power with one gasket than two however on the engines with the gear reductions that could rev to the A-4's rated 30+HP we got more power with the two gaskets and lower compression due to the breathing advantage of the larger plenum that the additional space provides.

Also valve adjustment can have a lot to do with compression findings at cranking speeds.

Dave Neptune

[Ken Rockwell]

Al is 100% correct. Also, you will tear out the bottom end. You must have the full 1/16. The compression ratio would be way too high. However, if you replaced all the rod and main bearings and rings and maybe pistons. Then and only then, could you increase compression. And for the record, I would turbo charge my A4 if it had a third main bearing.
My buddy had a yamha blaster. He was getting blow by and needed rings. He decided to change the piston.....to a high compression piston. The engine ran like a champ.........for about an hour. It looked like someone put a hand grenade in the crankcase.

[romantic comedy]

If the engine is worn and has low compression, the last thing to do is to increase the compression ratio. That is not a fix.

Find out what the problem is, and fix that.

We could do the figures to find out what the compression ratio would be comparing one gasket to two gaskets. I am guessing it would not make the ration that much different.

The last air cooled VW engine I rebuilt I lowered the compression ration from 8:1 to 7:1.

[KenVoodoo]

the winner is the meter

HI,
Was able to get the engine together and working. I noticed the meter I had used last year wasn't holding the peak...so bought a new one. After getting the engine warmed up I tried the compression again (all spark plugs out, throttle open...never had done that before)...now I have 100psi +-2 psi....so looks like I am good.

As an aside, before I started the engine I tried the compression with the engine cold...ended up with 90psi, 100 psi, 110 psi and 105 psi....but there was left over miracle oil in the cylinders from putting it away last summer....

thank everyone for the help
Ken