mau

Last time the compression was checked it read 140,140,140,135. Is it normal for an ATW motor? Lately the car feels sluggish. Does anyone know?

Mau

Audiboy

That's really on the low side. How many miles? Any mods to your compression ratio? cams?

squidix9

You can further check by pressing the release valve on the gauge, and see how long it takes to bleed down too.

Tricky part is getting it w/ the valves closed and at TDC.

You can pull the cap off the dist, if ya got one, and watch to see it line up with the cylinder you are testing.

Audiboy

most common is the one you stick in the plug and let the engine do it's cranking. The norm for cars these days is 170psi's and up after 4 or so cranks using this tester. On a couple of 1.8T's, I've measured anywhere from 200 to 220psi. My 2.8 is still at a healthy 180-190 range.

Also no distributor cap since early 90's, but there are marks on the cam gear and bottom pulley of the 1.8T

squidix9

180-190 psi tells me you've got around 9-10:1 or so compression. Those are good numbers. An 8.5:1 motor will give you closer to 150-170 or so. My motor is 14:1 and should be around 225-240.

A lot depends on the valves and how the rings are gapped. On our old 1 cylinder race motors we would open the gap on the oil scraper, and #2 ring so they were real wide, then heat them and compress them into the groove. Effectively it was like they weren't there. Then on the top ring we'd make it as tight as we could, depending on the brand of ring, just so it wouldn't crack. Super tight seal, with minimum friction.

But back to the topic, 140 all round should be fine.

Audiboy

Yeah, I agree 140 is fine. Just on the low side, although I didn't experience cars having problems with it. On the old VW's bugs, I measured 140 being on the high side he he.

Nifty trick you got there though. Maybe I can try something if I rebuilt the bottom end. Less friction sounds good

squidix9

BUT, we would tear it down after each event, and slap in a new ring. It would only last for 30-40 laps. At $4-5 a ring/per event it was OK. And at only an hour or so to teardown, and reassemble it wasn't bad. I wouldn't do it on a car.

For a bleed down, 125 psi should be fine, you want to be checking for leaks.

Remember in a bug, you were dealing with an air cooled motor too. A lot of expansion and contraction, those rings were sloppy as anything. Popping the jugs on and off was a family night in the backyard.

xr4tic

my new rebuilt motor is around 160psi.

The important thing is they are all within 10% of each other. My old 1.8 was around 145-150 before the rebuild.

Audi2ptzero

The dbw throttle plate will not open 100% during cranking, which means they might not get enough air in the cylinder while doing the test.

201-A4

including the part# of the gauge require