Compression test - pleasant suprise!

[Subzilla]

Had my Sub for 14 years or so and have never checked the compression on it. It has 295k miles and I've never done anything to its innerds since I bought it with 130k miles. I'm still planning to throw on my used ATS turbo kit but know I need to replace head gaskets just because of their age (and advice from you guys!).

I finally removed the glow plugs and threw on the gage to each hole this past weekend. 420 was the high and 400 was the low with a warm engine and 8 puffs. That's a 5% spread and good higher numbers! I guess I wasn't expecting these numbers with this many miles. Just my pessimistic nature, I guess. I'm happy.

What's the compression numbers for a new engine??

[john8662]

Great compression, new engines ar the same at that, but at the 420 level and at 6-puffs.

[DmaxMaverick]

Looks good. One less thing to worry about. New engine compression numbers can be very misleading, so they are not normally a bench mark to stand on. A good newer engine should have a range very similar to what you have, perhaps slightly higher, but not much at all. A good running engine with no apparent issues can test as low as 320. The key is the trend between cylinders, and yours are well within range (15% max). 5% is ideal, and within the margin of error.

BTW...
6 puffs is the standard on an engine up to operating temp. Unless you saw a significant difference between 6 and 8 puffs, it probably won't change your results. The maximum pressure is usually seen by 4-5 puffs. Much depends on the test equipment, test conditions, and a few other factors. Keep in mind, a running engine only gets one puff to reach its max pressure per combustion event.

[Subzilla]

OK, so I'm trying to understand....why does the total compression available cumulatively build up with each cylinder stroke then come to a max?? And I'm not sure if I asked that question correctly! I did notice the guage bouncing up with each stroke and it seems it did max out at puff 4 or 5 as Maverick suggested. Is this test just for measuring purposes? So you're saying the actual compression for each stroke will be less when the engine is running?

And I will say the equipment used is not the most expensive brand out there, on the contrary - can you say Harbor Freight?? So, I was a little skeptical about the actual reading. But I did 2 tests on each cylinder and got very consistent readings.

[DmaxMaverick]

It takes a few strokes to build target pressure because the engine is spinning relatively slow (~200 RPM's). During this slower time, some air bypasses the valves and rings (an acceptable/accountable amount). Doing several puffs allows the compression stroke to maximize the top of the stroke, where the pressure builds. If (hypothetically) there were no bypass, the pressure would blow most gages. And, the equipment has a lot to do with it. The hose attached to your gage adds to the compression ratio (reduces the ratio figure) due to its volume. And, yes, compression is essentially a trend measuring process, more than an actual bench mark figure. It can eliminate obvious failures, and offer a comparison across the banks.

Nothing wrong with HF compression testers. I use them. I've used the big dollar stuff, and they are just as good for the purpose at hand. Don't know if they'll last as long, but I haven't wore one out, yet. I haven't calibrated my sets, or tested them against known calibrated instruments, but accuracy isn't the goal. The trend is the target data, and it's well within any margin of error to do that. I also use the HF cylinder leakdown tester. IMO, it's more stable and easier to use than the name brand sets costing 10X as much. If they do last only half as long, I'm still well ahead in the bargain. Not to mention, HF offers a nice warranty on these types of tools (no frills, no questions asked, like Craftsman hand tools), while the name brands offer a warranty that expires the first time you use it. Sure, it may be made in China, but so is our money these days (so to speak). With such a thin bottom line lately, I can't afford to do it any other way.