Compression testers

[JPark]

I've seen many references to compression test figures on this site and have had to wrestle with some odd readings of my own. I've had the same tester for over four decades and it's been quite accurate, giving the expected readings, but with an Ace 100 there is a pitfall.

The fitting is for a 3/4" reach plug. It has an internal profile to mimic a plug interior and the valve is in the threaded fitting on the hose just behind that. Essentially, it only really works for 3/4" reach plug holes. If you put it in a 3/8 reach hole the fitting protrudes into the chamber and you can get maybe 25 psi more phantom compression in your reading. You can take the adapter off and just screw the hose into the hole, but that disregards the internal volume of the plug and you still get a high reading. On a 1/2" hole you're in limbo. I suppose there should be fittings available for this gauge that correspond to the plug reaches but I never got them.

There are the rubber tipped gauges that press into the hole but those seem to be disregarding the discrepancies of the plug hole length entirely. On a typical 5+ liter V8 each cylinder is about seven times the 100cc displacement so I can see why this isn't really going to matter much, especially if you're just comparing cylinders. On a Hodaka with maybe a 10cc combustion chamber, the 5 lb difference becomes 35 lbs and you are having to guess what is really going on.

I used a stack of old plug washers to take my fitting down to 3/8" depth and tested a recently broken in Ace 100 and it came out at 148 lbs. The manual says 80-150 psi so that's to be expected, but I got a reading of 175 with the 3/4 reach fitting. With just the 3/8" bare hose end it was again similar, so the internal volume of the hollow end is a significant part of combustion chamber volume.

I'm posting this because it may not occur to people that these small cylinders are pretty sensitive to small volume changes. It also shouldn't matter if you have the throttle closed or open to test a two stroke as the cylinder will fill to atmospheric pressure from the exhaust port anyway. On a four stroke it matters, as only the intake valve is open at the bottom of the stroke, but I still hold it open just to stay in practice; just a habit.

[taber hodaka]

Me thinks the engine is built to draw from the input side not the exhaust port. It is probably relative to the standard of the industry. Break in can make a very big difference on compression and if someone else did a little head shaving. It is really good to see folks working with the variables, that do make differences, many of just take for granite. ------------------------ Clarence