Measuring compression

[a10 gf]

The pistons I got are 7.5 according to srm, I do not have the old 6.5 to compare with, and you are right, they look like 6.5.

You got me hooked 

Keeping with the xt ref. numbers, the "std" = (14.7 x 9), + 15% to get to 156psi.
Applying this einsteinian theory, the A 6.5 "std" would be (14.7 x 6.5) +15% = aprrox 112psi. A 7.5 would be around 130psi. And does the cc, air humidity and intensity of kicking interfere in any way  

Back to practical reality, I am happy with the reading above 110 (supposing my gauge is showing passably correct values), and beeing quite even between cylinders. If anybody else could take a test on their engine, would be interesting to compare, and maybe come up with what may be some normal average. As the xt350 numbers suggests, there is quite a lot of room.

[bsa-bill]

I think I could keep up with this if I really really tried, however I do find it interesting and get the general grist of it.
So it gives me great pleasure       to be able to contribute a smidgin - 6.5 piston are indeed dished, I have a pair in the shed +60 if anybody is looking for some

All the best - Bill

[a10 gf]

Couldn't let it go. Did a test on the xt350, around 145psi. Perfect according to specs would be 156psi. 9-1 ratio.

If the cc etc has nothing to say, this all makes sense. By adding 10 to 15% (the adiabatic heating factor ?) to the basic atm 14.7 x compression ratio, this should result in a good psi value, indicating a healthy engine.

And an atm x ratio with smaller or without any added %'s would indicate an engine on the decline  +  ????

e

[snowbeard]

I just picked up a compression tester at an estate sale, so I have to admit it could be off.  but the fellow was an mechanical engineer for NCAR here in Boulder, and had soldered the free spinning end of the hose even, so I assume he knew what he was shooting for.  still in the original package, tho well used.

I also don't know what my compression ratio is supposed to be, or what it actually is.  I have a 57 spitfire scrambler by the factory records for engine and frame numbers, but it could have had lord knows what done to it. I've thought a lot of things were in really bad shape, but they turned out to only need minor attention for the most part.  I think that someone who ran this bike really knew what they were doing.  (that wouldn't be me)

but I tested the old gal tonite, the altitude is 5300 or so, and it was raining pretty hard on a new storm just started.  what that might mean to the atmospheric pressure is beyond me?

but anyway, it tested at 120 psi.  took four to five good kicks to get it up there, and the engine was cold, but both sides were spot on at 120/119.


I have to say I was pleased after having been told of the smoke on my right cylinder recently.

[tombeau]

 I tested mine recently and got a reading of 170psi on both cylinders.
Initially id id it with the throttle shut getting a reading of 130 but was told I should have the throttle open whilst testing compression.
Cheers,
Iain

[snowbeard]

what are you running Tombeau?  any mods or stock A10?





btw, thanks for the posts helping me decide about using the lysette style seat, I am fond of it for now!

[tombeau]

Hi Snowbeard!
No problem for the help on the lycette seat.Have you got any pictures with it fitted?
My motor is a Road Rocket, its fitted with a Spitfire cam, but last rebore I couldn't get high compression pistons for it, so its running standard ones. It was infuriating at the time as I did notice a decrease in grunt, but considering that it used to pink a bit on the leaded fuel we got then, its probably for the best now.
I must admit the difference in figures copared to everyone else here makes me wonder if the gauge I used was a little optimistic, at the time I was just worried that compression be the same on both cylinders.
Cheers,
Iain

[snowbeard]

ok, everybody heard him ask me for pics 

this is the most recent, with the red plug wires from that fellow on BB that was selling sets for $25
att 1
http://img.photobucket.com/albums/v632/snowbeard/my%20bikes/082508.jpg

this is a little earlier, but still the same seat mount
att 2
http://img.photobucket.com/albums/v632/snowbeard/my%20bikes/rightside0408.gif

att 3
http://img.photobucket.com/albums/v632/snowbeard/my%20bikes/june08.jpg

I made two plate brackets for the front since it didn't come with the standard yoke, they aren't attached to eachother, just bolted to the frame mount and each side of the seat frame, but they seem to be holding solid.  the rear is mounted with two bolts thru round pipe spacers to raise it a couple inches to level, since the springs I got with it were the 4" style.  with the swingarm shocks and the springs that are in the seat, it makes for a pretty decent ride.  I might try to set it back just a few inches someday, but it's fine for now as well.