Cranking compersion figures

[49er]

When the 850 was last opened to replace a dogey base gasket (the Vesrah woes), I lightly honed the bores before re-assembly. Some of the thatching from the re-bore, 10K kms earlier was still present, but I decided to very lightly rehone anyway.

3K kms after the rebore, the compressions ranged from 170-180 psi. After the rehone and another 3k kms, I've just found the compressions now range 180-190 psi.

The engine did seem a little more responsive, but I was surprised at the 10 psi increase between compression checks.

Has anyone fitted Wiseco 10.25-1 kits and recorded their compression numbers. I have checked the achieves, but can only find figures for stock engines or figures on sick engines.

At 10.5-1, I've been able to run 91 RON, I think equivalent to the American 87 MON, without heating or detonation issues. Is this also the case with the Wisco kits?

[posplayr]

No expert on Ron v.s. WON but when I first got my Weisco 1166 kit together the bikes started and ran on Regular (87 here), but underload seems to have issues. Since then I always run premium (91 here) and any pinging/burbling under load disappears.

I have done a mix while on the trip with Bill and no apparent issues (Bill did of course have issues). But I generally go full fill 91 when I can. I have not checked the compression numbers.

My head only had a light skim (0.002-0.003") and the cylinders none.

[49er]

No expert on Ron v.s. WON but when I first got my Weisco 1166 kit together the bikes started and ran on Regular (87 here), but underload seems to have issues. Since then I always run premium (91 here) and any pinging/burbling under load disappears.

I have done a mix while on the trip with Bill and no apparent issues (Bill did of course have issues). But I generally go full fill 91 when I can. I have not checked the compression numbers.

My head only had a light skim (0.002-0.003") and the cylinders none.

Thanks Jim. Looks like most folks here do as you have. Fit the kit and just run it.
It would be nice to do a comparison, especially the Wiseco kits for the 2 valve engines, such as the 1000's. Any feedback on timing limitations and fuels used would be good.

[chef1366]

I ran an average of 140-150 compression. This is using pounds. Yours does seem a little high for a 10.25-1 piston.

[49er]

I ran an average of 140-150 compression. This is using pounds. Yours does seem a little high for a 10.25-1 piston.

Thanks Bill. I am quoting psi too. JE I0.5-1 pistons, but still running stock cleaned up cams and ports.

[SqDancerLynn1]

Those #s are not out of line... My 9 to 1 850 runs around 130 psi. Years ago As I recall 165 psi was ideal number. Your choice of camshaft will have a drastic effect on the compression. I built a 360 smog motor years ago with a zero deck clearance, cut .050 off the block and another .030 off the heads. Made it very interesting to fit the intake

[jimb12]

chef1366, you list having the .340 Web cams, is that the #223 grind? If so, they have 10 degrees more duration than a stock cam, which will lower the cranking psi a little.

[49er]

chef1366, you list having the .340 Web cams, is that the #223 grind? If so, they have 10 degrees more duration than a stock cam, which will lower the cranking psi a little.

Yes, extra duration and overlap will affect lower cranking speed compression figures. That's the trade off people make when looking for maximum HP. Great for the track, but affects bottom end performance for road use.

Lynn, thanks for those figures. I also did some radical chopping on an early pushrod rally engine that required the slotting of side cover mount holes and shimming of the rocker towers to compensate for the amount of metal removal on the block and head. Great learning projects, eh!

[posplayr]

Yes, extra duration and overlap will affect lower cranking speed compression figures. That's the trade off people make when looking for maximum HP. Great for the track, but affects bottom end performance for road use.

Bill and my motors were very close save for possible differences in the head and the degreeing of the cams. On the last round he had higher compression due to skimming the head as well.

With the 0.340's at 105/107 as per Webcam Reco; if there is any loss it mid range it don't feel that way.

With Bill's he was degreed 110/110 and so it was a bit of a light switch at 6K RPM.

The 4:2:1 seems to have further contributed to my broad mid range.

Bill and I were going to take our bikes together down to a local SB dyno for comparison. Now he went and blew his up and will probably end up with some 1230 kit or something so the opportunity has slipped away.

[posplayr]

I have come to the conclusion that high compression numbers don't really mean alot. Before doing anything to my ED, I had compression numbers in the 185-195 psi range. I was really excited as that seemed to indicate I had a high compression big bore kit

As it turned out it was largely due to carbon on top of the STOCK pistons and motor

[49er]

Bill and my motors were very close save for possible differences in the head and the degreeing of the cams. On the last round he had higher compression due to skimming the head as well.

With the 0.340's at 105/107 as per Webcam Reco; if there is any loss it mid range it don't feel that way.

With Bill's he was degreed 110/110 and so it was a bit of a light switch at 6K RPM.

The 4:2:1 seems to have further contributed to my broad mid range.

Bill and I were going to take our bikes together down to a local SB dyno for comparison. Now he went and blew his up and will probably end up with some 1230 kit or something so the opportunity has slipped away.

Indeed, a shame that you didn't get to do that comparison.

Regarding your next post, I'm surprised at those figures. That's a lot of carbon buildup on a stock engine to get figures that high. It would need to be thick on the cambers as well. It may be that a PO had skimmed something off the head and/or block surfaces. For that much carbon, the owner would have needed to baby it (no regular blasts up to redline) during a long period, while running poor quality fuels. The other alternative is that the oil rings were stuffed for a long period, but that would cause lower psi numbers through bore washing and worn compression rings and you would have noticed blue smoke between accel/decel, while riding.

Why did you decide to go big bore after finding high compression numbers like that? The word avarice comes to mind.

[posplayr]

Indeed, a shame that you didn't get to do that comparison.

Regarding your next post, I'm surprised at those figures. That's a lot of carbon buildup on a stock engine to get figures that high. It would need to be thick on the cambers as well. It may be that a PO had skimmed something off the head and/or block surfaces. For that much carbon, the owner would have needed to baby it (no regular blasts up to redline) during a long period, while running poor quality fuels. The other alternative is that the oil rings were stuffed for a long period, but that would cause lower psi numbers through bore washing and worn compression rings and you would have noticed blue smoke between accel/decel, while riding.

Why did you decide to go big bore after finding high compression numbers like that? The word avarice comes to mind.

I had to dig back into the archives a bit.

http://www.thegsresources.com/_forum/showpost.php?p=752209&postcount=9

I think ultimately, it was a combination of Rapidray and the fear of oil leaks coulped with the 49K miles that finally swayed me to pull the head/cylinders.

[posplayr]

Another rice cooker photo. These used to posted somewhere here i though but have not been able to find them. The good news is it appears as if the PO was gentle on the engine and so I have not bothered the lower end of mine.

Cant get to it at the moment, to check head height but the stocks were stock 72mm with no evidence the head had ever been off.

[chef1366]

I don't think that's enough carbon buildup to cause that much jump in compression numbers. Are you sure your guage is any good?

[posplayr]

I don't think that's enough carbon buildup to cause that much jump in compression numbers. Are you sure your guage is any good?

same one that measured the compression on the bananna at 135 psi; U still getting wheelies?

we can calibrate it on Tuesday

Also used on the GS750; You seemed to think it was pretty fast on Camino Cielo

http://www.google.com/imgres?imgurl=...ed=0CBAQ9QEwAw

This is significantly higher than my 1981 GS750 which ranged from 145 to 165 and is supposed to be at max about 175 psi