Good compression tester?

Hey folks - I'm trying to find a replacement to my not so great harbor freight compression tester, but the market is SATURATED with cheap ones.

I'd rather pay more and get a good, consistent unit - does anyone have experience or opinions on that? My use case is before and after race weekends - so it'll see some milage.

Thanks!

The Snap-on universal unit is only 80 buckaroos:
https://shop.snapon.com/product/supplemental/Universal-Compression-Tester/SPDTU3PB

Eastwood is 133:
https://www.eastwood.com/compression-tester-kit.html

Matco 111:
https://www.matcotools.com/catalog/product/ct66k/compression-tester-set

There are a million in the 15-30 range.

The longer the hose, the less accurate it is.  FYI.

In reply to Pete. (l33t FS) :

I've heard that, but most of the "universal" ones seem to have fairly long hoses - I'm still browsing....

Though I guess it's also worth noting that for my use case I'm most concerned about consistent results to gauge health over time. Though maybe I need to be looking more at leakdown testers....

The Matco one looks identical to the snap-on one I used to have. I wasn't very pleased with it. It did not survive the move.

I bought a Dayuan universal one from Amazon and I've been very happy with it for the last 9 months or so. https://www.amazon.ca/gp/product/B07BKWPPDR/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

I've found that a compression tester is really only useful to confirm what I already know.

In reply to accordionfolder :

I have a HF unit and I suspect it reads very low. I'm getting 100lbs on healthy motors.

I would also pay more for one that is accurate.

In reply to accordionfolder :

Consistency IS key, but my Mac gauge has a really long hose, and it would consistently be 20psi or so low compared to a co worker's Snap On gauge.

So I would have rotaries that would start and run just fine with 40-50psi on the gauge.

Really, the coming thing is to get a pressure transducer that you measure with an oscilloscope.  Or even just scoping an amp clamp on the battery cable while you crank the engine - that and a way to get a trigger signal from the #1 coil can get you a really accurate relative compression without having to turn a wrench.  Important when so many cars have spark plugs inaccessible without major disassembly.

But with a pressure transducer, you can analyze the pressure curve vs. TDC and find things like which camshaft is off time or the like.

Pete. (l33t FS) said:

In reply to accordionfolder :

Consistency IS key, but my Mac gauge has a really long hose, and it would consistently be 20psi or so low compared to a co worker's Snap On gauge.

So I would have rotaries that would start and run just fine with 40-50psi on the gauge.

 

Really, the coming thing is to get a pressure transducer that you measure with an oscilloscope.  Or even just scoping an amp clamp on the battery cable while you crank the engine - that and a way to get a trigger signal from the #1 coil can get you a really accurate relative compression without having to turn a wrench.  Important when so many cars have spark plugs inaccessible without major disassembly.

But with a pressure transducer, you can analyze the pressure curve vs. TDC and find things like which camshaft is off time or the like.

I like the idea of the relative draw + trigger point - is there a DIY or off the shelf set of information on that? Maybe a science project for me to try out and compare the data with what the compression + leakdown says? An arduino project with some sensors might prove the point. Neat thought - and makes good sense.

Neat thoughts, but I still suspect I'll need to bandaid this with the 'ol fashioned way for a while. I saw a few compression testers with braided hoses - not sure if they help appreciably or not. Funny I'm not finding much online for comparing compression testers other than clearly AI written comparisons of the topological information.

More context, I'm starting to rent some spec miatas out and I'm trying to gauge relative health. A reliable source I've talked to that's been doing this a long time indicates that head refreshes by a quality builder and a good bottom ends can get you a lot of the way there - but right now I just have seat of the pant information.

ShawnG said:

The Matco one looks identical to the snap-on one I used to have. I wasn't very pleased with it. It did not survive the move.

I bought a Dayuan universal one from Amazon and I've been very happy with it for the last 9 months or so. https://www.amazon.ca/gp/product/B07BKWPPDR/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

I've found that a compression tester is really only useful to confirm what I already know.

Bummer, I was hoping the snap-on - which looks generic - would at least be tough and generic, lol.

I've had this one, apparently since 2015. Had HF before that. Only comparison I can give you is that my numbers for 2 holes on a Subaru motor were relatively close to the numbers a "race" shop eventually got. Assume he has a better gauge than me. I've never gotten a reading that was shockingly far off of what I was expecting. 

Regarding accuracy,  I've never seen a compression tester that has been calibrated.  I've also never heard of anyone having theirs calibrated. 

Snap-on doesn't make a lot of their own stuff when it comes to something like this. They just source one they're happy with and charge you enough to cover the "free" parts you'll be needing. 

OTC probably makes them for other tool companies.  That one looks like my Mac in different colors.

Generally speaking, you can't go wrong with OTC unless it's something really weird, like you need a ball joint press that has more height than their offering.

In reply to Pete. (l33t FS) :

Good to hear, I found OTC in a few places - I guess I'll gow ith them!

In reply to ShawnG :

Right, I don't think the absolute number (for the most part) is really that important, but a consistent number is essential from my understanding - though it'd be a nice bonus to be in the ballpark, lol.

FWIW, our old scope at the shop would give you a compression reading based on the engine data you fed it, along with current draw from an amp clamp and tach input.

This unit was old enough that it used a 386 PC.

It didn't give real world numbers but it gave relative numbers for comparison like Pete was saying. 

I'm sure there would be a way to recreate something like that with a modern laptop or phone.

Couldn't you just hook one up to a compressor tank with a known pressure to test them for accuracy?

Assuming the tank's gauge is accurate of course.

Since you want consistency not accuracy just take a compression reading on one cylinder a few times, see how much it changes.  Variance can be due to a couple things, air leaks and a bad gage would be my guesses.  The Oring on the end could also be leaking, also easy to test.  The gage can be compared to others, maybe a regulator.  2 gages in parallel on an air tank for an hour would also be interesting.  

You might also consider what sort of numbers will trigger a reaction.  Less than 5 psi for before/after a weekend would make me interested but not react.  Another 5 psi on the next weekend would show a possible actionable situation.  A 20 psi drop over a weekend would say do a leakdown and find the problem.  Just my opinion, but evaluate your HF tester and see how good/bad it is.  

Know your compression gauge.  Back in the olden days, my Mac unit was used on a daily basis, back when a guy did a compression test with every 20,000km service.  It was a different number than my coworkers Snap on, which was a different number than...

If the cylinders are even, and fall into the range you expect from your gauge, thumbs up.  If those numbers don't match some online specification, well,  tough.

I bought the otc unit posted above, and it seems nice.  My old one, which just had a rubber tip, and was held in by hand, works on about .03% of modern cars...plus, I can't really see through the lens anymore.

But it's been well loved.

aircooled said:

Couldn't you just hook one up to a compressor tank with a known pressure to test them for accuracy?

Assuming the tank's gauge is accurate of course.

That will calibrate the gauge, yes, to whatever level of accuracy you feel is important before getting to a national standard calibration (thinking of things like the piece of metal that defines the Official US Gram).  

The issue is that since we are adding the internal volume of the hose to the cylinder's volume, now we have to take into account that volume, which is why we do four or five compression strokes and there is a check valve at the end of the hose, to pump up the hose to what pressure the cylinder can generate. And now we're also relying on the accuracy of when that check valve is opened and released with pressure difference.

Lots of little things that add up to all different gauges reading slightly differently.

This is a large reason why they don't really tell you absolute numbers, just look for relative differences.  And why a pressure transducer that eliminates all of that is the technically best solution, even before you get into being able to analyze the pressure curve on a running engine, where you can actually see when the intake and exhaust valves open and close, and you can actually see the other cylinders while those valves are open...

It's a bit much for simply checking engine degradation over time, a regular ol' gauge is fine for that, but great when you get a sick engine and would like to try to find a direction to look for why it's sick.

In reply to Pete. (l33t FS) :

I still think as a "science project" that'd be cool.

Thanks for all the input folks! I'm still digging through the offerings, hoping to find one that stands out.

I had the same issue with a HF gauge, read way low on a fresh engine - took it back and got another one - that one was spot on with my old tester. The reason I got the HF unit was to get the adapters for the holes these tiny spark plugs used in modern engines.

The main goal is just to compare to the other cylinders of course, but I would like it to at least be relatively accurate.