Air-volumes & making compression tester adapters

[fitterski]

There are different kinds of compression tester fittings mostly according to max anticipated pressure. Mine is good to 1000 psi and has a male fitting like so



I don't know what the format is 'called', maybe its the 1000 psi format?

While trying to devise an adaptor for a Cummins 6bt 12-valve and a Deutz F4L912 I noticed that the compression RATIO on some diesels can be quite high, like 21:1 even. In theory that would approximate 21x14.7 i.e. 309 psi. The practical cranking pressures however are MUCH higher than the calculated ones. Anyone know why this is so?

Another issue is the volume of air OUTSIDE the injector nozzle normal equivalent position. Using very approximate numbers usually something like a liter of cylinder is compressed to like 1/18 liter or 61 to 3 cubic inches. IF only 1/2 cu-in of ADDITIONAL space becomes available in the test gauge fitting, the adaptor and the gauge hose then that will throw the result by 1/6, or a typical 400psi read by as much as 66 psi!!!

Anyone with opinions on this? I'm trying to make adaptors with only pinhole air passages and in such a way that inside the fire-chamber exactly the same volume be taken up by the test fitting as is by the real-world injector nozzle protruding inside, not one cubic millimeter more. I'll probably have my machinist change the black adapter to a gauge-to-1/4" male-plumbing thread fitting that I can screw into any future home made slugs with pinholes in them.

 

[CalG]

Opinion?

Leak down rate is WAY MORE INFORMATIVE. And does not require "high" pressures.

 

[fitterski]

Opinion?

Leak down rate is WAY MORE INFORMATIVE. And does not require "high" pressures.

I'm interested in that too, not much documentation though, I'd do both with a T and a valve on the gauge hose so that once I saw the compression I would next pump air in and time the drop on the gauge. Sofar I've never done that though.

 

[CalG]

How to do a leakdown test | Mobil™ Motor Oils

Obviously, there is a restriction between the two gauges as to produce a pressure differential.
Other references will help with determination of passage size for any home built instrument.

 

[fitterski]

My brother was an airplane engineer and i remember seeing him do a different leakdown test. I think it meant putting xyz psi into the cylinder and then timing how long it took to drop to zyx psi. This week I'm setting up a diy head tester, clamping a deutz head down on the bench on a rubber mat and putting 160 psi into it to listen to the valves leaking if they leak. If they do then out comes the lap kit before going any further.