Compression Guage Comparison

[leonardhcross]

A Member had suggested that using thread adapters on compression testers might give a false reading when compared to using testers that came with the proper 12 mm threads installed. We tried a number of variations and came away concluding that, indeed, using the 12 mm tester gave a 10 psi higher reading. It would be good to hear whether others have found the same thing. The video shows our methods. Mostly, we had fun.https://www.youtube.com/watch?v=BDqhUE18X-Y

[redglbx]

Very interesting guy’s ! I will add that a non-instrument grade gauge (not used for calibration) has a accuracy requirement of +/- 10% of full scale , a calibration grade grade (according to NIST,) is 1% of full scale and a lot more $$$.. So both gauges being 300psi full scale have an accuracy rating of +/- 30psi ,, but most are better like yours..

So with that said your switching the hose between the gauges a being able to repeat the results just by switching the hose is very good ! You did great ! I don’t understand why having the schrader valve directly in the combustion chamber would give you a 10psi boost but your experiment proves it does,, that’s a true head scratcher…

Now with that said I would ask what difference would 150psi vs 160psi give you ? Great test though !

[redglbx]

Cliff, you & Len did a great job on this, but it still really doesn’t explain why having the schrader valve at the end of the hose vs higher up the hose makes any difference,,, pressure is pressure no matter where it’s sampled so long as it’s a common source. Your testing showed that it is a real “thing” I’d just like to understand why…

Also, just a comment on the 10psi increase.. it’s not something that you’d feel or really notice w/o some good equipment, imho it’s more important that all the cylinders are equal or close to equal to get the best performance out of the engine… 10psi lower would probably give you a bit lower fuel economy which you could maybe see,, maybe.. it probably would lower the engine output some but you’d need a dyno to see it and the emissions would be affected to some degree but again you’d need some pretty specific testing equipment to measure it.

Finally, please keep in mind that I am not criticizing you and Len’s work here, simply wondering out loud.. What you guy’s did here is very interesting and I’m just trying to process & understand it… Again, imho,, my opinion is that the compression matters more that it is the same or close to the same across all the cylinders.

As a side note Honda has used the same 171 +/-2psi number across the board on most of their engines but I have only seen a few oe engines that actually meet that number, most like my 76 actually are quite a bit lower,, my 76 had 135-145psi from new and stayed that way until I milled .040in off the heads & advanced the cams +4 degrees. Cam timing, and restrictions in the intake tract like the valves & air cleaner all affect that compression number which is why they want you to hold the throttle full open…

Anyway, just my thoughts and a great discussion..

[pidjones]

Seems to me, increasing the chamber volume by moving the Schrader further away would decrease the compression ratio.

[redglbx]

PID, that’s as good of an explanation as I could think of ! The schrader valve has got to equal a couple of cc’s of volume ,,,, makes perfect sense .. and adding the adapter ahead of it adds even more volume ,, that would explain it ! Great thread !

So with that explaining things , it says that more equal numbers across all the cylinders is more important to the health and performance of the engine…. Right ???

[Sidecar Bob]

I was about to say the same thing: The gauge should be calibrated for the fitting that came with it and adding the adapter increases the volume being compressed so the reading will be lower.

[redglbx]

Cliff,have you by chance measured the cc’s of displacement of that adapter ? That would be added to the combustion chamber volume cc’s .

[redglbx]

Cliff, really nothing to see, I’ve been using the same compression gauge (RAC Brand) since I was 16, I’m 71 now,,, when I bought my 76 in October if 1976 I made an adapter like so many do using an old spark plug so yes the schrader valve is up inside the old plug so it will be a little lower. I will check the adapters volume at some point basically for grins and giggles.

The main thing I learned here is that yes using any adapter that adds volume in front of the Schrader valve will lower the compression recorded.. That being said it really doesn’t make any difference in how the motor runs,, it is more important that there is minimal difference recorded cylinder to cylinder.. my .02

[CYBORG]

[redglbx]

Pay no attention to the man behind the curtain !!! if you either make a filler or fill the adapter with epoxy or similar to eliminate the added displacement you’ll get the exact same result,,, but you will need to drill a small communication hole,, I made a filler plug for mine and seen the same increase…. But again, so long as all the cylinders are the same it doesn’t make a rats(?) ass difference in how the motor will run ! Out !

[CYBORG]

Pay no attention to the man behind the curtain !!! if you either make a filler or fill the adapter with epoxy or similar to eliminate the added displacement you’ll get the exact same result,,, but you will need to drill a small communication hole,, I made a filler plug for mine and seen the same increase…. But again, so long as all the cylinders are the same it doesn’t make a rats(?) ass difference in how the motor will run ! Out !

I agree. I haven't taken a compression check on any of my bikes in years. They run great. So I don't care. Plenty of other things to worry about. Carb sync on the other hand.... valve Clerence.... oil changes..... tire pressure... brake fluid....clutch adjustment..... and I always check the horn. Never know when some distracted cage driver might to try and take my lane space.

[redglbx]

Cyborg,,, ,,, I will say that while doing all my compression testing for the .004 vs .006in valve clearances in the “other” thread I did find that the #3 exhaust valve was excessively loose somehow , not sure how that happened but I reset it to the oe .004in and then ran out of back,, so maybe today I’ll get up there and put the plugs in it and fire it up for the first time in a year or two,, hopefully

Then the plan is that I’m going to change the spark plug wires out from the current 7mm solid core wires to 8mm spiral wound suppressor wires.. I know that the EMI from the solid core wires really raises havoc with electronic stuff like the Dyna ignition, and may have some effect on killing the modules,, I have no evidence if any of that but know that the automotive companies have gone to great lengths to minimize any EMI,, and good spiral wound wires will minimize EMI with only a very slight bump in resistance, win-win I think,but again no evidence of any of that damage but it won’t hurt.. if I still have the chuff in the exhaust I’ll move to the coils from there ,, the Dyna coils seem to be responsible for a lot of these kinds of weird things, we’ll see.. wish we had other sources for electronic ignition stuff !

[CYBORG]

Cyborg,,, ,,, I will say that while doing all my compression testing for the .004 vs .006in valve clearances in the “other” thread I did find that the #3 exhaust valve was excessively loose somehow , not sure how that happened but I reset it to the oe .004in and then ran out of back,, so maybe today I’ll get up there and put the plugs in it and fire it up for the first time in a year or two,, hopefully

Then the plan is that I’m going to change the spark plug wires out from the current 7mm solid core wires to 8mm spiral wound suppressor wires.. I know that the EMI from the solid core wires really raises havoc with electronic stuff like the Dyna ignition, and may have some effect on killing the modules,, I have no evidence if any of that but know that the automotive companies have gone to great lengths to minimize any EMI,, and good spiral wound wires will minimize EMI with only a very slight bump in resistance, win-win I think,but again no evidence of any of that damage but it won’t hurt.. if I still have the chuff in the exhaust I’ll move to the coils from there ,, the Dyna coils seem to be responsible for a lot of these kinds of weird things, we’ll see.. wish we had other sources for electronic ignition stuff !

I sing the phrase Of the C5 for point conversions but I'm fine with the factory stuff on the 1200's

[Rat]

Let’s try to get along guys …

Gord

[redglbx]

Cliff, I guess I’ve been a little surprised when you just went off on me,, as I originally said it was all just some random thoughts and maybe discussions between what I thought were just friends,, I’ve been fortunate to have had many differing experiences and was hoping that some might benefit from them.. insulting you was never my intent.. I will leave it at that, and just do what you please at this point…..