75 2.0L C/R, and blowin' smoke

[JP Noonan]

Well I just put 1.5 quarts of oil in my car yesterday because the green oil light came on. I guess I've been busy and didn't realize how much I've driven the car without checking the oil. My friend says that I'm blowing smoke that I assumed was from a rich condition (based on smell and color of the smoke) but must be from the oil control rings. The engine has between 140-150 psi on all cylinders, but sat for a long time after it was rebuilt.

Here is the question(s), is there anyway to tell the Compression Ratio of the engine based only on a compression test (i.e. find the CR without tearing down the motor)? Also if I can't control the smoking with some of the "snake oil" additives on the market I plan on doing a ring job. If I find that the CR is 7.6 not 8.0, has anyone tried removing the cylinder head gasket to increase compresion. I've heard that this is a standard practice on 1.7L big bore kit with no leaking problems. I don't want a leak at the head that will ruin my only pair of 2.0L 914 heads. I would prefer to remove the base shims but since all the head gaskets are the same (vs. the base shims which are different thickness) it would be easier.

BTW while the heads are off I plan on port matching them and polishing up the combustion chamber. I know enough not to change the ports (without a flow bench anyway) but I think the CR is going to drop because of this and I'm trying to figure out a way to compensate.

Any help would be appreciated.

[PeterB]

My 2.0 engine was assembled sans head gaskets, to try to give me a tad more air at Denver's altitude. No apparent problems to date, with about 3000 miles on the engine.

I hope it's not a case of "ignorance is bliss".

-- Peter

[JP Noonan]

Thanks for the advice. Have you done a compression check lately? It should tell you if the heads are leaking. Other than that there would be a tell tale black discharge at the point the head meets the cylinder, only problem is that this area is covered up mostly by tinware.

BTW do you happen to know how much your compression ratio went up with out the gaskets?

[Dave at Pelican Parts]

Jim Thorusen has a spreadsheet that calculates compression ratios. It's in MS Excel format, forget which version. That will let you play around with how much the seals affect CR. If I remember correctly, on a theoretically-stock motor, it was something like half a point.

You can't tell the CR from the cylinder pressure. Pressure within a cylinder is a very tricky and funky thing. Someone who has done a lot more looking into it than I called it "adibiatic" (I know I've got the word wrong), which meant that the pressure reacted in a non-linear way to changes in the system.

The ONLY way to accurately determine your CR is to measure everything. You can assume the bore and stroke are stock, but that only gives you the swept volume. You can probably take someone's word for it about the dish in the pistons. But you have to measure the rest of the fixed volume yourself. That means measuring the distance between the top of the cylinder and the top of the piston (straightedge and caliper), and measuring the volume of all four combustion chambers (using some type of liquid is easiest).

You can't just take someone's guess, because very small changes in fixed volume (e.g., head gaskets vs. no gaskets) can have large effects on the CR.

--DD

[JP Noonan]

Thanks for the insight. About half a point sounds good, I'll find out once I tear the engine apart. I knew there was some difference between CR and PSI (static vs dynamic????) but was told by a friend who is a jet ski mechanic (2-stroke motors) there was some way to transfer one to the other, maybe for two strokers, or maybe he was mistaken.

I plan on making a deck height tool a-la-Bob Hoover. A thick steel plate with a dial indicator in the middle, this way I can torque it down to get a more accurate reading. I have to locate a graduated burette (or at least an old horse syringe) with alchol for the combustion chambers. I can make some plexiglass circles at work to fit into the head.