I've read different opinions on the actual numbers for the JE, so I bought a compression tester and took some readings. After warming up to operating temp, I came up with 215psi every time. I don't know how to convert the PSI number to an actual compression ratio. I'm hoping you guys can help.

P.S. It's stock bore using Web 109 cams if that matters. Thanks.

 

When I entered the data the compression is 12.1 to one.
I hope you have the decompress removed or deactivated for test.
If not deactivated, the compression is 13.5 estimate.

 

Thanks so much OMR, could you tell me how you figured that? The decomp arm was removed on the 109's and I didn't bother putting it back on. I was told that it wasn't necessary to have it.

 

I think the only reliable way is the cc the engine with the piston at BDC and TCD and measure the difference. I may be all wet, but I feel that cranking speed, rod length, cam timing and if the carb is open or closed while cranking can put a lot of variables into converting cranking pressure into a reliable static compression ratio. But it will give you a good estimated dynamic CR.

 

jsimonh,

Because you said stage 2 you have 45tm and you did open the throttle when cranking.

Also what is your altitude?

 

Yes, the TM45 was in the WOT position, the rest is full stage 2. I'm in SW Oklahoma which is 1300 feet. If it matters it was about 75 degrees outside yesterday. I did the test 5 times, cranking until compression stoped going up.

 

<div class="FTQUOTE"><begin quote>Originally posted by: OMR

jsimonh,



Because you said stage 2 you have 45tm and you did open the throttle when cranking.



Also what is your altitude?</end quote></div>

OMR do you know the rod length for the DS650? For my calculators I need it and do not have it.

 

The compression for altitude brings the compression up to 12.25 to one

You are getting close to the edge for pump fuel, what are you running for gas?


Stir: The rod length is 144mm or 5.669 inches.

OMR

 

Thanks OMR.

 

<div class="FTQUOTE"><begin quote>Originally posted by: OMR

The compression for altitude brings the compression up to 12.25 to one



You are getting close to the edge for pump fuel, what are you running for gas?





Stir: The rod length is 144mm or 5.669 inches.



OMR</end quote></div>

AVgas, that was kinda the reasoning behind the question. It runs real strong with a direct mount UNI and a 145 main, but I can't get the plug to look right. I went to a #9 plug and I got the heat line on the strap right where it curves, but my plugs are still white. I can jet up to the point it bogs down, but the plugs still look white. I was wanting to know the true compression to make sure the AVgas was even needed. There isn't a readily available source for race fuel around here or I would just use it. AVgas seems to run good, but there isn't any solid jetting guidelines for it. At 12.25 to 1 it looks like I'll have to keep running the AVgas or buy some thicker gaskets to run pump again. Is there any guidelines on what octane to use? I could mix the AVgas with pump if needed.