Compression for 2.8SS - did I do it right?

[Walt Fricke]

I rechecked the piston/head clearance, this time with rings installed to stabilize the piston. With two 0.5mm shims I got 0.032". That's a little tight, but I think I can live with it.

Then I set things to TDC, put in a spark plug, and filled the combustion chamber via the second plug hole with Marvel Mystery Oil via a burette, with the engine tilted on the stand. I got 41.7ccs.

I used a formula of compression ratio = swept volume (467.82357) plus compressed volume (41.7) divided by 41.7 . CR= 12.21879 (12.2 to one). That's a nice figure, and should help in the quest for 300 nominal flywheel horsepower.

Query: Is this a legitimate way to measure CR?

The formulae one finds are all based on measuring the head volume, and then the piston dome volume, and deck height. But I think the method I used, with the head on the partially assembled motor, is correct. If not, I'd like to know it.

I overfilled things a bit (hard to see down that hole) and took some back out and back into the burette, so I need to redo this and see if it comes out about the same. I rotated the engine back and forth some and didn't see any oil move back down, so I don't think I had any air pockets.

Query #2: So what would be a no spill way to get the oil back out without making a big mess, inquiring minds want to know? I don't have a convenient suction method.

Walt

I've yet to come to grips with $8/gallon race gas. For a 15 gallon tank, that's $120/tank. I burn about 12-13 gallons per hour, so there is another minimum $250 per race or DE weekend.

[Flieger]

Yes, your method is theoretically correct, though there can be the extra experimental error of leaks past rings and not having the second plug installed to take up some volume. When CC'ing the heads with a lexan plate bolted to it you can put in the spark plugs and valves to get actual head volume, but then you need to find the actual dome volume and mess with deck height and such, so I think your method is simplest and best in the end.

[PFM]

Walt,

A little grease on the rings will help seal it up tight so nothing leaks. Ant fluid past the rings will make your numbers go wrong.

[Jon B]

Walt,

Here's a photo from the Porsche 4-cam Carrera manual, using the method you describe. Even so, it's not the most reliable way to check your volume, as it's difficult to be absolutely sure there are no air pockets. To remove the fluid, just run the piston down, remove the head, and tilt the engine over to drain.

[304065]

You can tell who the apprentice is in that photo. "Halten sie bitte!"

[cgarr]

Walt: This is what I made, its screws right into the plug hole, less mess and each mark is 1cc then you can drain it back out. It also lets you jiggle the engine around a bit to make sure all the air is out.

[Cupcar]

Originally Posted by 304065

You can tell who the apprentice is in that photo. "Halten sie bitte!"

Wow, one holding the engine steady looks like a gal to me. How many gals worked on 4 cam engines I wonder?