Topic: Not all compression gauges are equal...

[fendersrule]

Many times I've searched for information on whether small engine compression testers will produce more favorable results. You will find that some people will say that compression gauges that are designed for these engines will read higher than regular "automotive" gauges. You will find another group who say that this is a myth and that compression equals compression. There is no other place documented on the web that has done formal testing of compression gauges.  I won't get into the technical details of "why" smaller gauges should product better results, but it has lots to do with air volume.

I put four compression testers to test.

A) Harbor Freight "Push in" type gauge (comes in a kit with a carrying case)

B) Older brand automotive "screw" in gauge

C) Older brand automotive "screw" in gauge

D) Actron CP7828 gauge which is marketed to be used with smaller engines.

I tested these gauges on a 1979 CB750, 1978 CB750, and a 1974 CB450.

A, B, and C produced consistent results without variation.

D however produced results that were +7 PSI greater (CB450) and up to 15 PSI greater (CB750s).

So, gauges that are designed for smaller engines will produce more accurate PSI numbers is NOT a myth. It is true. The Actron CP7828 is only $40 via Amazon, and should be in your collection.

I would like to hear more testing that has been done with this, but so far I am finding consistent results that are proving that this is not a myth.

[raymond10078]

Hmmm. Four gages, three yield the same results, and yet the odd man out is the accurate one?  What is your basis for this conclusion?

Did you test all four gages against a known, calibrated pressure source or compare to a calibrated gage?

[fendersrule]

The three automotive gauges are NOT the same. They read the same PSI, which is what you probably meant.

I understand your point that more testing is needed to validate. But you're also already jumping to a conclusion that the Actron is wrong, but I would actually bet otherwise. You can find out for youself by picking one up if you have a calibrated pressure source, which I don't. If you notice in the Honda workshops that Honda uses a very small compression gauge that is also designed for smaller engines.

[raymond10078]

The Actron CP7828 looks to be a normal "automotive" type gage.  Actron itselfs describes that gage as:  The Professional Compression Tester kit works on most foreign and domestic vehicles, small engines, motorcycles, snowmobiles, and ATVs.

So, not sure what makes this gage different - unless . . . you're trying to make a case for some gages provide a more accurate indication of actual "in cylinder pressure", then you are of course correct.  If the goal is to determine the pressure closest to that in the cylinder, then you'd need a gage that connects the smallest volume possible to the combustion chamber.  Think of it as replacing the head with one with a larger combustion chamber volume - the compression values would of course drop.  The longer the hose, the more volume within the hose, the more volume in the gage itself - all contribute to a lower indicated value in psi.

[fendersrule]

Good info Kandrtech.

Motorcycle Compression Tester Actron CP7828

[fendersrule]

Regardless of the gauge, you still have to factor out the air volume of the hose.

Has anyone been able to find a good gauge that comes with a small hose to accurately test motorcycle engines? As I've mentioned above, I believe that the Actron is closer than anything I've seen while using the long included hose. I'd like to give another gauge a shot using a very short hose.

Thinking about giving this a shot and only using the small hose...http://www.craftsman.com/craftsman-compression-test-kit/p-00947089000P?sLevel=0&redirectType=SKIP_LEVEL

[raymond10078]

In thinking about this more . . . .

Most gage sets have a check valve at the end of the hose.  Provided that the hose end threads directly into the head - and has this check valve, the change in combustion chamber volume should be negligible.  So . . . if you have a gage set that threads directly into the head - and it has a check valve in the end - your results should be as close as your going to get.

I've seen other gage sets where the an adapter is used to fit into other various heads - this is the type that may give altered results.

[fendersrule]

Actually, most have the check valve near the gauge...after the hose...the snap on is one that doesn't.

The snap on is considered the most accurate gauge on the market. I'll pick one up and report back.

[raymond10078]

Well . . . most that I have seen have a check valve at the end of the hose - but I certainly have not seen them all.

My cheap Craftsman set has one at the end (item 4 on the attached figure) - with another one at the gage to release the pressure (second item 4 closest to the gage).

The problem of inaccuracy comes into play if an adapter is used (see item 7) - because this part does not have a check valve - so this volume becomes part of the combustion chamber volume.  Any other adapter/hose without a check at the end also adds volume.

Based on rough dimensions of the adapter I have (item 7) - doing a quick calc shows that the drop in indicated compression would be about 15 pounds.

So . . . assuming that the pressure gage itself is accurate, what is needed is a connection at the head that has an integral check valve.

[fendersrule]

Yep. One other variable that you left off is that many times, #7 won't have the same amount of threads as the spark plug, therefore increasing the combustion volume.

Snap on will be ordered shortly. I've love to see how it stacks up against the Actron ($40).

Supposedly, the Snap on doesn't have the item #7. The hose just goes straight into the head and it has a check valve at the very tip.

[mrbreeze]

The thing is.....they can all differ between the different brands but what you are looking for is equality between the cylinders. Just like doing a vac synch....you are aiming to get them all equal to one another.
If I measure 130 on 3 cylinders and 105 on the other...there is a problem. If I measure 93, 91, 90, and say 89 it shouldn't be a problem. Even though the numbers are low...they are close to being equal to each other. If the low readings were a problem it would hardly run if at all.

[fastbroshi]

I don't worry about any of these because unless you can get your hands on one of the genuine Honda compression guages that they used when developing these, you will never know exactly how far you are off of spec.  I think that's why the prevailed thinking has been to disregard readings that are a bit low and give even readings more priority.   

FYI I there is a check valve in the adapter #7 style compression set.  It's right at the end of it; you can see the needle easily.  I have one very similar to that and when I've used it I can see the pressure reading increasing in a stepped manner.  No heaving up and down.  It does not bleed off pressure when an exhaust valve opens.