Real world cylinder compression?

[redglbx]

Thanks Cliff ! I don’t 100% agree with Mike (great asset here) as I’ve been doing this as well for over 50yrs but he does make sense and I certainly don’t totally disagree. Thinking about it, by increasing the valve clearances effectively reduces the cam timing duration so it will increase the compression, should also show a slight power loss on a dyno. advancing the cams like I did also gives a pretty big bump in compression And a low end torque bump with really no loss of top end due to the 75-77 cams being fairly radical and their power peak really happens above redline so advancing them brings it within the redline.

But all this still doesn’t fit with when my Wing-was new in 76 and I checked the compression within the first 100-200mi and it was at 135-140psi brand new. It stayed that way until I milled the.040in off the heads and advanced the cams 4deg. Advancing them +6deg was really a bit to much because the motor would really fall off above 7500-7800rpm.

Anyway thanks for replying and I hope my rant wasn’t too boring or annoying, thx

[gltriker]

Thanks Cliff ! I don’t 100% agree with Mike (great asset here) as I’ve been doing this as well for over 50yrs but he does make sense and I certainly don’t totally disagree. Thinking about it, by increasing the valve clearances effectively reduces the cam timing duration so it will increase the compression, should also show a slight power loss on a dyno. advancing the cams like I did also gives a pretty big bump in compression And a low end torque bump with really no loss of top end due to the 75-77 cams being fairly radical and their power peak really happens above redline so advancing them brings it within the redline.

But all this still doesn’t fit with when my Wing-was new in 76 and I checked the compression within the first 100-200mi and it was at 135-140psi brand new. It stayed that way until I milled the.040in off the heads and advanced the cams 4deg. Advancing them +6deg was really a bit to much because the motor would really fall off above 7500-7800rpm.

Anyway thanks for replying and I hope my rant wasn’t too boring or annoying, thx Nope

Cam,
Read all 3 pages of Mike Nixon's, "Five must-haves for a good-running GL1000" topic.
I have and discovered there was a Lot More meaningful discussions and technical data presented in those 3 pages than I remembered.

question: how did you advance the cam timing? an offset pulley indexing key or some such modification in that area ?

[redglbx]

Cliff, I had a local guy EDM key ways in at 2,4 & 6deg. If I was to do it all over again, I’d just have it done at +4deg on the crank pulley instead of the cam pulley’s. But I was experimenting and dyno’d it after the changes , plus I wanted to see if there was anything in being able to advance or retard one side vs the other, nothing there. Compression is the real indicator of the results.

I’ll add that the +4 gave me a very flat power curve that only varied from peak torque 2 ft/lb from 2200-6800rpm. Horsepower was also pretty flat but did loose 2hp off of the peak. It really doesn’t feel faster because there’s no peak/surge/on the cam feel, it just pulls.

Iplay with the cams on everything anymore, just to much to be gained, on the double overhead cam bikes (cbx’s) advancing the intakes but just putting the exhaust straight up gives a really nice flat curve with a really nice power bump on both torque and hpwer, but the wings are just single cams. If I had unlimited funding I would have custom cams ground with just the intakes advanced but the exhaust just straight up. I’ve modeled the GW engine on a computer simulation program to find improvements and found really not much in the cams past the advancing them. I feel good about the modeling as the actual dyno work repeated the predicted results nearly exact. I feel good about the results.

[gltriker]

* Post#19

For truest testing results, utilize a 12mm threads compression testing adaptor that screws directly into the spark plug hole.
20191213_204517_1637796073345_resized (1).jpg

https://www.ngwclub.com/forum/viewtopic ... 93#p803393 <<<<< READ THIS

regarding a Best Practice set-up, 14mm threads-to-12mm threads compression testing reducers are not X recommended

20200112_113223_1637982760730_resized (2).jpg
by removing its check valve, this cylinder compression testing adaptor converts to a cylinder leakdown testing adaptor, too.

The example of a 12mm compression testing adaptor hose assembly I had presented in my Post# 13 is a Snap-On product. part number EEPV5G12L 5/17/2025 MSRP is $69
I unexpectedly snagged it (stole it, actually), from a seller on eBay around 2 years ago. anyways.........

A far less expensive alternative is a 12mm testing hose assembly which is available from Motion Pro. part number 08-126Y
Its product description is: MOTION PRO #08-126Y – 4-STROKE LEAK DOWN TESTER REPLACEMENT 12MM HOSE

Edit May 18th, 2025
The likelihood of locating this Motion Pro 12mm hose assembly on-line is ...

20211221_220306_resized_1.jpg

Yes, it is described as a Leak Down Tester Hose, but it also has internal threads already machined into the 12mm end to accept the installation of a Schrader style check valve assembly, too. A valve stem core, if you will. Motion Pro calls it a Snifter Valve.
Motion Pro will sell you a 'Snifter Valve' . NOTE: Seems there just might be a different set of operational pressure specifications for a "Snifter Valve" core, versus a typical automotive applications valve stem core.
edit on 1/02/2022 ! Turns out a proper compression tester valve core employs a very low opening pressure. A very weak spring pulls the plunger closed.
* sources?
-Snap-On sells the cores. MT24DB1 ??

-Milton #1251-2, box of 5.

-Lisle® 20100 - Valve Core

20211221_214224_resized_1.jpg

*

ADDED April 21, 2025

It looks like you’re using a long tube/hose from cylinder to gauge. I think Cliff had some info on this causing numbers to be lower than spec, but actually might be close to spec? Anyone?

A proper 12mm compression test adaptor will have its check valve positioned as close into the cylinder head's combustion chamber cavity as possible.
Visualize where the spark plug ground strap is positioned in the spark plug hole when the spark plug is fully installed and torqued into place. That's the place the check valve belongs.
If a typical generic 14mm to 12mm reducer adaptor is utilized, the all-important check valve is always positioned farther away from the spark plug hole opening into the cylinder head chamber.
In essence, that extra unchecked area in the spark plug hole and 14mm to 12mm adaptor is added as if the tested volume of the cylinder head combustion chamber has been increased; resulting in a lower calculated static compression ratio number, witnessed as a lesser pressure result on the cranking compression tester gauge face.

viewtopic.php?p=803393#p803393

Always continue cranking the engine over during the compression test session until the needle climbs no higher on the gauge face.
Forget the ages old advisement to count to 10 compression strokes. Why? I've observed multiple times during the compression testing sessions with my '75 GL1000 engine, as the individual cylinder pressures are reaching higher numbers, the gauge needle will twitch back forth a few psi (5-10 ish) until the needle finally stops climbing on the gauge face. Then I release the starter button.

@ericheath
I've longed to perform the testing procedure to measure cc's of the GL1000 cylinder head combustion chamber with an NGK D8EA spark plug, fully installed.

Then, after removing the NGK D8EA spark plug, perform another cylinder head cc's test with a proper checked 14mm compression tester hose adapted down to 12mm spark plug hole threads diameter to compare the difference in liquid testing fluid displacement volume (cc's) between the correct and incorrect spark plug hole compression testing adaptors.

I have an old GL1000 cylinder that sorely needs to be prepped for this project but haven't invested in a clear square of chamber covering acrylic sheet. Also need a graduated burette for the testing liquid.
They are expensive...



Done

[kerryb]

Just two compression examples to add, my '83 with 120k miles and a serious rod knock ran smooth and strong with 90+ psi compression ( good power, just a nasty noise). A '77 I'm working on now measured 150 on all 4 after 10 minutes of run time after a long sleep. Apparently it had been well cared for during it's 48k miles.