High Compression

[King68]

The car in question is a 1968 Monza with the garden-variety engine. The number 5 cylinder indicates 180 psi when compression tested. (All the others seem okay.) The carburetor on the passenger side bank of cylinders appears to have been really out of whack, likely running 'way rich for who-knows-how long before this car became MY car. (If anyone is interested, I can relate to him the events leading up to the subsequently ruptured muffler.) I have since rebuilt both carburetors, replaced the gas tank and blown out all the intervening fuel lines. After that, the engine seems to run much better.

Now I want to replace the push-rod "tube seals." Before I do the obligatory adjusting of the rocker arms with the engine running, I'd like to address this high-compression issue. I have read in a number of places of a technique (intended to treat high compression due to carbon in the cylinder heads) which involves pouring water into the carburetor on the afflicted bank of cylinders while running the engine at idle (?).

I am skeptical. Can anyone out there give me a hearty "Do this; it really works! Here's how." or, a likewise definitive, "This is truly bogus! Remove the head and start scraping!"?
Thanks.

[Jerry Whitt]

What compression readings do you have for the other cylinders?

[King68]

They vary from 135 to 150 psi.

[Jerry Whitt]

My tech book suggests 130 lbs would be about right. Your numbers all seem high. Several ideas come to mind.
1. High compression pistons were installed.
2. Super thin head gaskets.
3. Modified heads
4. Carbon build up, on all cylinders, with one having even more build up than the others
5. Faulty compression gauge.
6. Improper test procedure
Atmospheric pressure is around 15 lbs per square inch at sea level. Normal compression ratios are around 8.5 to one. Your numbers all around, are high.
If you have available a bore scope, an interior inspection might confirm pistons with numbers on the top showing high compression, or you may be able to see a really high build up of carbon

A method to remove carbon used in the past includes, fast idle, then pour small amounts of water down the carburetor. Just a little bit at a time. Brake fluid has
been used also. Let the fluid of choice just dribble a few drops at a time. If you pour too much, water does or brake fluid does not compress well and serious
damage could occur.

[66vairguy]

They vary from 135 to 150 psi.

Gauges and re-worked heads vary so the rule of thumb on Corvairs is about 130 to 140PSI, but the real test is no more that 15PSI difference between cylinders. Yes your 180PSI is out of range.

I DO NOT RECOMMEND USING WATER TO REMOVE CARBON!! A lot can go wrong. Many use a product called "Seafoam Engine Treatment". Typically no more than a third of a can is poured into the engine per minute (half that for just one side of a Corvair engine) at 1,500 to 2,000 RPM. Too much liquid in an engine can cause thermal damage, or worse, hydraulic lock that can break pistons and bend rods. I've never used Seafoam, but many have. Expect a tremendous amount of smoke to come out of the exhaust - so don't use it were the neighbors will get upset and call the authorities.

Finally - there is only one correct way to remove very excessive carbon - pull the heads and clean them and the top of the pistons. Soda blasting will clean away head carbon without removing metal. Scrapping works, but if you leave any sharp edges you risk detonation problems.

[King68]

Here's an update: I found my original compression test data sheet from last spring (!?!) All values in psi: #6 -135, #4 - 135, #2 - 150, #5 - 180, #3 - 150 and #2 - 150. I may try the SeaFoam idea first (carefully) before I pull the heads, if only just to see if it is effective at all.

So okay, I'll bite: What else should I do while I have the heads off? I hope to accomplish this without completely yanking the engine out. Good idea/bad idea?

Also, if I understand Jerry's comment regarding domed pistons correctly, I should see numbers stamped on the tops of the pistons when I bore scope the heads? (I presume that this scoping is done through the spark-plug openings.)

[Jerry Whitt]

The bore scope is used through the spark plug hole. Many, but not all, pistons will have some numbers on the top of the piston. Some will say .030, meaning
thirty thousands over size. Some will have other numbers related to compression ration.

The heads can be pulled without a complete engine tear down. If you choose this method, consider a valve grind and possibly valve guides. The machine shop that does the valve work will measure the guides and advise you. Please consider adding intake valve stem seals. The cost of the seals is minor. Installation is
usually done at the time any valve work is done.

Good luck!

[66vairguy]

I didn't notice what engine you have. In 1968 all the engines EXCEPT the 140HP engine had doomed pistons. Of course by now it hard to guess what engine is in your car. Prior to the SMOG engine design all the pistons were flat top. Many folks rebuild SMOG engines with the flat top piston and older closed chamber heads. Lots of pros and cons and difficult to separate the myths from the facts on SMOG engines.

I would advise against a valve grind. Unlike cast iron heads the Corvair aluminum head had durable valve seat inserts. Unless a valve and seat leaked and are burned the seats are usually fine. The main issue is a seat coming loose and that's mostly an issue in the large valve 140HP heads. For whatever reason many have reported valve seats that dropped out shortly after doing a valve grind - not sure why, but it's been reported numerous times over the years. Now it's leave them alone or replace the seats with a deeper seat.

Installing new valve seals is a great idea, only on the intakes. If the guides are badly worn then it's time for a complete head rebuild - new deep seats, new guides, new valves, new valve springs -- $$$$$$$.

Note that the books say GM had an issue with guides slightly off center and would machine the guide bores to line up with the seats. Now most shops install new guides and seats and machine the seats to line up with the valve guide bores.

[Joe B]

In 1968 all the engines EXCEPT the 140HP engine had doomed pistons.

Wow! If that's true I can see why there aren't too many '68 engines out there. Sounds ominous! Too bad they didn't have domed pistons, that would been better!

[bbodie52]

I believe that these pistons were doomed, since they were low production items and nobody makes them as replacement pistons anymore...

We all just have to live with flat tops in the 21st century!

[Jerry Whitt]

Is this Brad's picture?

[Wagon Master]

It is my under standing that only the 68-69 110 h.p. motors received the domed pistons. 95 h.p. and 110 h.p. units shared the same open chambered no quench heads and the higher compression in the 110's was achieved by using the pop-up pistons. 95's and 140's used flat tops and the only real change to the 140 head design from earlier years was the addition of stainless steel exhaust tubes to deal with the increased heat from the a.i.r. injection pump.

My opinion only. Your actual mileage may vary

[chris]

Here's an update: I found my original compression test data sheet from last spring (!?!) All values in psi: #6 -135, #4 - 135, #2 - 150, #5 - 180, #3 - 150 and #2 - 150. I may try the SeaFoam idea first (carefully) before I pull the heads, if only just to see if it is effective at all.

So okay, I'll bite: What else should I do while I have the heads off? I hope to accomplish this without completely yanking the engine out. Good idea/bad idea?

Also, if I understand Jerry's comment regarding domed pistons correctly, I should see numbers stamped on the tops of the pistons when I bore scope the heads? (I presume that this scoping is done through the spark-plug openings.)

Your #5 being higher than the others sounds like mine. I want to say it has something to do with the heater hose nearby and possibly air leaks? Hopefully someone more knowledgeable than me will weigh in.