Topic: The most accurate compression gauge...

[fendersrule]

As many of you know, I am on a quest on finding the most accurate compression gauge (http://forums.sohc4.net/index.php?topic=131952.0).

There isn't much information on the web about this topic of gauge comparisons, even though I know that most gauges that we use are not giving us the numbers seen in the Honda manuals. Here is what I do know:

1) Not all compression gauges are equal. Different fittings, hose volume, hose length, quality of gauge, and weather or not it has a Schrader valve in the fitting are all huge variables.

2) The Actron CP7828 so far is the most accurate gauge I have tested out of 3 generic "automotive" compression gauges and a Matco gauge. When I say accurate, I mean that the compression is around 15 PSI higher which is closer to spec from the manual, but still off. I've tested freshly rebuilt engines with valve jobs as well as older engines with little miles and I'm not getting close to what Honda calls for in the shop manual, so I know it's the tool.

3) The Snap-On motorcycle compression gauge is said to be one of the most accurate, though it has a long 18" hose which I find questionable. For a "true" motorcycle gauge, the hose volume volume needs to be 3cc, as most are 10cc. This technical information is not available from Snap-On, so at $220 this gauge I feel is a gamble when you don't "know" the technical specs of it.

4) Honda does not make, nor list a compression gauge. Kawasaki does, however.

My goal here is to have the same readout that Honda gets in the users manuals. I'm tired of having to factor in hose volume to give me my "true" compression. Steve on the Honda twins forums has fabricated a gauge that only has a 3" hose that barely clears the fins on the head, and he consistently gets 195-210+ PSI on fresh 450s.

Yes, I know that the goal of compression gauges in general is so that you can test whether or not there is variance between cylinders. However, it's really nice to also be able to have a accurate numbers, but with most gauges you will not get an accurate number. I want to buy or build a gauge that does.

So here are my options:

1) Get the Snap-On Motorcycle compression gauge. EEPV503. This has a huge 18"hose, but it's said that it's the most accurate because it has a Schrader valve at the tip. I just feel that this is a gamble for the money that may/may not be a "true" reading.

2) Get the Kawasaki part no. 57001-221 (gauge) and 57001-1159 (hose w/ 14mm adapter). The hose has been confirmed to have only 3cc of volume, which is really good. Read more about the Kawasaki vs Snap-On gauge here, in which the kawasaki produces significantly higher readings: http://www.klemmvintage.com/squish&comp.htm

The question is whether or not I can use this gauge/adaptor on my 350/450 engines, as well as my 750 engine. The 350/450 are M12 x 1.25 for the spark plug threads.

There are other compression gauges that Kawasaki makes. The trick is to find one that will work on Honda twins and fours.

3) Other option?

[dave500]

the length of the hose only takes more pumps to reach max pressure,like if you fit a larger tank to an air compressor it just takes it longer to reach max,get one with a valve at the beginning of the hose,some people split hairs with this concept,mines this long.

and i get this much.

[Markcb750]

the length of the hose only takes more pumps to reach max pressure,like if you fit a larger tank to an air compressor it just takes it longer to reach max,get one with a valve at the beginning of the hose,some people split hairs with this concept,mines this long.

and i get this much.

I believe this is only true if the check/relief valve is close to the threaded end.  If the check/relief valve is close the the gauge, the volume in the hose will be included in the volume of the combustion chamber.

Also the pressure loss accross the check valve must be added to the gauge reading. should be less than 5psi butthoonose?

Mark

[dave500]

thats the hair splitting i mentioned.

[Markcb750]

Butthoonose eh?

[fendersrule]

Good information so far from both of you. I've bought in to both theories before, but I tend to side with the one that the Schrader valve needs to be near the plug fitting otherwise the hose will add to the compression volume. That makes sense to me with my limited physics understanding, anyhow. It makes sense to me that regardless, the hose needs to be as small as possible.

I'm VERY tempted to blow some money on a Kawasaki NOS gauge and fitting, but I'm not sure which one to get. If it comes from Kawasaki, you know it's what they use and what they can vouch for. I just need help finding the right part nos.

The compression hose/fitting in the CB750 service manual looks the EXACT SAME as the one that Kawasaki offers:



Here's some pictures of the Kawasaki 57001-123:

http://www.flickriver.com/photos/kla6/sets/72157629138734211/

Dave, you're getting pretty good numbers using that gauge. What is it?

[kghost]

If you really want to split hairs........

Why not do a differential compression test?

It will actually give you a true idea of what the cylinder is doing and why.

As you mentioned the purpose of the "standard" compression test is to check for variations.....

The other purpose is check compression.

Obviously you're aware of the variables.......I'm just curious why you find it important to have the Honda numbers?

[dave500]

Butthoonose eh?

the pressure release near the guage on mine is a standard type schrader valve,the one at the inlet end is a special one with i guess a lighter spring and the guage came with two spares of this one.

[kghost]

While a compression test is a dynamic test (engine moving), a cylinder leakdown test (C.L.T.) is a static test (engine at rest). The compression test measures how much pressure the engine can produce while cranking; in contrast to the C.L.T., which measures how much pressure is lost in the engine. In a C.L.T. the engine is placed on TDC of the cylinder in question and using a similar type of connector as the compression test, we fill the cylinder with pressure. The tester then measures the volume of air needed to maintain a predetermined pressure in the cylinder. This reading is expressed in a percentage. Good cylinder leakdown readings should be below 5-8%.

The great thing about C.L.T. is that it deals with how well the cylinder is sealing and nothing else. The readings are not affected by carbon deposits, cam timing, or even engine cranking speed.

Another great feature of the C.L.T. is the fact that you can hear where the air is leaking out of the cylinder. When a cylinder has high percentage of leakage, first check the oil filler cap. Do you hear a hissing sound? If so, you may have pressure leaking by the rings. Is there air escaping out the exhaust? Is it escaping out the intake system? Then a burned valve may be the problem. If two adjoining cylinders have similar low readings and you hear leakage out the other cylinder, then a failed head gasket may be the problem.

Being able to pinpoint the exact source of the compression loss will tell you where the problem is; and not just that you have one. 


As for the Honda numbers:

One has to assume you are doing your test at or near operating temps?

WOT during the test with ALL the spark plugs removed?

[fendersrule]

kg- I run compression the same on all bikes. Spark plugs removed. Sometimes the carbs come off, but not always. Throttle wide open. Engine hot. Choke OFF. Battery charger on bike to assure full cranking power. Sometimes I kick to compare, but not always.

For curiousity sake, I found this image in the CB450 service manual:



Holy crap on the hose. I guess this is more evidence to support the hose shouldn't matter if it's equipped with both valves but more importantly, a valve near the bottom of the spark plug fitting!

Fittings also make a big deal. We tested JUST the fittings from a Matco set vs the Actron set and found a 20 PSI difference alone..

[kghost]

Why is the difference important to you?

[fendersrule]

Why is the difference important to you?

The same way an accurate health thermometer is important. It's the difference between going to the hospital and being fine.  Variance of around 35 PSI (which is very common with compression gauges) is the same way troublesome. Obviously, a leak down test is meant for pinpointing the compression loss to find what needs to be explored further, but you shouldn't do a leak down until you run an initial compression test. That initial compression check should be accurate using an accurate gauge IMO so you're not wasting time wondering "how" healthy your engine is.

With some people, they're fine with whatever the number is (knowing that it isn't "accurate") as long as it's consistent for every cylinder. But people like me who are perfectionists are wanting to get something that is as close to "actual" as possible...why not? It would make for a great tool in the tool chest.

This thread is seeking to find the most accurate gauge. What is an accurate gauge? The common thought is to use something straight from the bike manufacture themselves which is why I'm very interested in the Kawa gauge. I think it's out of production, however.

My rebuilt CB450 (valves lapped, new rings, lightly honed) produces 155 PSI both cyls with my Actron gauge, dry assembled. The other gauges I've compared with show 20 PSI less. The spec according to Honda is 185 PSI. Talking to my father on this, "there is no possible way that your engine is running any less than factory compression. Everything has been machined by Honda and checked to tolerance." Obviously, the engine isn't fully broken in yet so I do expect these numbers to go up.

That same gauge I used on my other CB450 (5,000 miles) produces 165/170 PSI.

Testing my SL350 (11,000 miles) grants me 170/163 PSI.

Testing my CB750 (15,000 miles) grants me 142/123/123/143 PSI.

Maybe the Actron that I have is a GREAT gauge and as "decent" accuracy, but maybe there's something that's MORE accurate. I'm trying to find out.

btw, I also set the valves, cam chain tension, and other general tune up items that affect compression before running a compression test.

[dave500]

if you have 35psi variance between guages you have some #$%* guages man!

[fendersrule]

Yes, those has been thrown away!

but I borrowed my friend's Matco gauge...which I think is a general automative gauge...probably a $60-70 gauge, and it produced 20 PSI lower than the $40 Actron. The fitting I believe is what we found for such variance...

Btw, I think "Sun Pro" is now "Actron."

[dave500]

hook them all into an air compressor and see how they match?

[kghost]

Takes a while for everything to seat it on a new engine.........

Your point on the new engine vs other 450 sort of makes the point:

Its relative. As long as you know what the gauge is doing. A disparity of that nature between 2 bikes with the same gauge is what's telling......not the accuracy of the gauge itself.

A factory calibrated gauge will certainly show the same difference between the 2 bikes.

Unless you know how Honda derived their numbers any attempt to replicate them will have an error.

Unless you use the exact same gauge the manufacturer used when they came up with the numbers. That exact same gauge must be calibrated.

If the mailman drops the box containing your shiny new gauge.....all bets are off.

A year after using it and having it rattle around the tool box / bench.......calibration will be off.

The most accurate gauge will not be a pointer/dial type.

Every pressure gauge even one from a motorcycle manufacturer will have a tolerance. More than likely the motorcycle manufacturer will have outsourced it anyway.

Your example referring to a thermometer is flawed logic. (Happy to prove why if needed and no I'm not making an attempt to get you fired up....it is just not the same).

The type of compression test you are performing is a RELATIVE indication of what the engine is doing. The test itself was never intended to be pin point accurate. Nor was it ever intended to be exact. its a base line reference only.

What you are attempting to do is turn the test...by virtue of an extremely accurate gauge....into something it is not.

[Bailgang]

Why is the difference important to you?

 Obviously, a leak down test is meant for pinpointing the compression loss to find what needs to be explored further, but you shouldn't do a leak down until you run an initial compression test. That initial compression check should be accurate using an accurate gauge IMO so you're not wasting time wondering "how" healthy your engine is.

You're loosing me here somewhere. How is someone wasting their time using a leak down tester wondering how healthy their engine is? Isn't that the reason why another person would use a compression gauge in the first place?  A leak down tester and compression gauge are telling you the same thing except in a different manner, a comp gauge is giving you a pressure # while the leak down tester is giving you a percentage. The key difference is that the leak down tester isn't influenced by carbon buildup, cam timing or the length of the hose leading to the gauge. My brother bought a snap-on leak down tester many moons ago a pretty much stopped using a compression gauge from then on. The only disadvantage I can see about a leak down tester is that an air compressor is required.

[fendersrule]

Found a nice article from Kawasaki. Reason I like this is because it details the part numbers and the thread fittings.

[kghost]

Why is the difference important to you?

 Obviously, a leak down test is meant for pinpointing the compression loss to find what needs to be explored further, but you shouldn't do a leak down until you run an initial compression test. That initial compression check should be accurate using an accurate gauge IMO so you're not wasting time wondering "how" healthy your engine is.

Hell if you want the most accurate get one of these: http://rlrowan.com/bmep-engine-balancer

You're loosing me here somewhere. How is someone wasting their time using a leak down tester wondering how healthy their engine is? Isn't that the reason why another person would use a compression gauge in the first place?  A leak down tester and compression gauge are telling you the same thing except in a different manner, a comp gauge is giving you a pressure # while the leak down tester is giving you a percentage. The key difference is that the leak down tester isn't influenced by carbon buildup, cam timing or the length of the hose leading to the gauge. My brother bought a snap-on leak down tester many moons ago a pretty much stopped using a compression gauge from then on. The only disadvantage I can see about a leak down tester is that an air compressor is required.

I didn't say that someone is wasting their time.

"How" is also a relative term. A compression test is a great way to see what the cylinder pressures are.

A marked difference between cylinders is cause for investigation.

Follow this:

You do a compression test on your 4 cylinder Honda.

The readings are as follows 1. 150 2. 148 3. 155 4. 70

Now obviously the reading for No.4 shows a marked difference between 1-3.

Certainly cause for investigation but the importance is the relative difference between the cylinders not the accuracy of the gauge.

Fenders example of various readings on various bikes shows this. The importance is both readings compared to other cylinders and other bikes. The accuracy of the gauge is not really a consideration

Point I am trying to make is that a compression test with a pressure gauge and cranking the engine is a RELATIVE test.

A differential compression test is not a relative test. Yes it does require a compressor.

It will tell you where the loss of compression is. Intake, exhaust, or rings.

It takes a set volume of air and compares it to what the cylinder is actually retaining.

It is not the same as a compression test. Cranking the engine with a gauge attached just tells you how much air the engine is pumping. Not how much leakage there is or why.

By al means do a compression test....and follow it with a leak down or differential compression test to diagnose. Its just that one is relative the other is not.

Fenders is concerned about the accuracy of his gauge.....I've stated it doesn't matter (within reason)
as long as you know what the gauge is doing.

Its splitting hairs.....and he's welcome to do that if its important to him.

Its a public forum.....and sure as little green apples someone will do a search and think they need the most accurate gauge possible. You simply don't for that kind of test.
 

[fendersrule]

I'm thinking about ordering Kawasaki part #57001-221 (gauge) and 57001-1159 (14mm short hose) for testing my twins.

Can someone remind me of the thread size for a CB750...I believe it's 12mm but it's good to make sure. I would bet that the CB750 would need Kawasaki's short adapter to clear the frame up front...but buying this kit for a CB750 would be a gamble but I know I have the room to use it on my twins. Here's a good picture of a 12mm with a "short" adaptor, but I doubt that this will fit on a CB750. Any thoughts? http://www.ebay.com/itm/Kawasaki-KL600-A-Special-Tool-Compression-Gauge-Adapter-57001-1183-NOS-/271256612971?pt=Motorcycles_Parts_Accessories&hash=item3f2827706b&vxp=mtr

kg, I don't disagree with anything you've said. I'm in the market for a compression gauge of my own, and I want the best.

[kghost]

Quality tools last a lifetime.

No disagreement there.

[Bailgang]

Why is the difference important to you?

 Obviously, a leak down test is meant for pinpointing the compression loss to find what needs to be explored further, but you shouldn't do a leak down until you run an initial compression test. That initial compression check should be accurate using an accurate gauge IMO so you're not wasting time wondering "how" healthy your engine is.

Hell if you want the most accurate get one of these: http://rlrowan.com/bmep-engine-balancer

You're loosing me here somewhere. How is someone wasting their time using a leak down tester wondering how healthy their engine is? Isn't that the reason why another person would use a compression gauge in the first place?  A leak down tester and compression gauge are telling you the same thing except in a different manner, a comp gauge is giving you a pressure # while the leak down tester is giving you a percentage. The key difference is that the leak down tester isn't influenced by carbon buildup, cam timing or the length of the hose leading to the gauge. My brother bought a snap-on leak down tester many moons ago a pretty much stopped using a compression gauge from then on. The only disadvantage I can see about a leak down tester is that an air compressor is required.

I didn't say that someone is wasting their time.

I know you didn't say that, I was referring to Fenders reply. I think he's splitting hairs too but all the power to him. 

[fendersrule]

I ordered a new Kawasaki gauge ($100) and a short 12mm adaptor fitting ($50). I chose this over the mighty Snap-on. I will need to order a 14mm adaptor fitting so I can test my twins and then I will be all set.

I will certainly be able to measure #1 and #4 on my CB750. The fitting may work for #2 and #3. Either way, just testing #1 or #4 would be good enough to see if the numbers are higher/lower than the Actron.

Basically, I am betting that the Kawa gauge, the Snap-On, or perhaps a really nice Mac gauge are probably the "best" you can get. I choose to go with the Kawa.

I'll post some results if you guys care to hear em.

[dave500]

you guys splitting hairs are lucky,i aint got no hairs to split!

[LesterPiglet]

Here's something else for you Dave.
What elevation are you at? Does it make a difference?