Brain teaser

[Wine Slut]

Here's the story... I'm riding home from a freinds house on my rz streetbike and I'm very low on gas. No gas station for 20 mi. and I'm not sure I can make it. Speed limit is 55 and, due to recent encounters with the law, I'm keeping it under 60. If I use 6th gear, I'm off the pipe and have to use more throttle to maintain speed. If I use 5th gear, the rpm's come up enough that I'm on the pipe(just barely) and I can use a much smaller throttle setting. Wich gets the better fuel economy? Higher rpm & less gas or lower rpm & bowaaaaaaahhhhhh???

[ysr612]

On the pipe in this case. My bike is just on the pipe at 60.

[This message has been edited by ysr612 (edited 08-29-2001).]

[Mongo]

Since I'm trying to avoid going back to the office to load up for Memphis I'll take a shot at this....

I was originally thinking lower revs because the piston will suck the same volume of air/fuel every stroke but then I thought about it as I typed and if the needle is in further the fuel is less even though the air is the same. So, if you are running lower revs it sucks the air/fuel mix in less times per minute of riding but uses more fuel to do so (hence the need for more throttle). If you are on the pipe then it sucks the mix in more times per minute but because it is more efficient use of the fuel it needs less. I think now that you'd have to try both and see since without knowing the actual amount of fuel used for a given amount of throttle you can't compare the more fuel/less times versus less fuel but more times, at least not accurately.

Of course more info might help - if the revs were twice and the throttle position was approximately half, I'd guess it was a wash - the fuel usage equaled out. If being on the pipe used more than half the throttle needed for the lower revs then odds are good that lower revs were more fuel efficient overall, if it was less than half the throttle needed for lower revs that being on the pipe was more fuel efficient. Of course you have to adjust my "halfway" analogy by the amount of difference in revs as well to get a true feel for it. Judging by your comment that it was a much lower throttle setting to be on the pipe (and assuming it was not a huge amount higher in revs) I'd have to guess that piping was better in this case.

But the real question is - did you ride home or push home?

[Wine Slut]

It was hard to do, but I kept it in 5th and made it home.(Thank you!)
Just like my race-bike, there is no tach, but after years of riding these things I'll take a guess at rpms.
5th gr/60mph/5850rpm/30-35% throttle
6th gr/60mph/5600rpm/45-50% throttle.
Of course, I used to just cruise at 65-70 mph and this was never an issue. Now that I try to behave more like a concerned citizen, maybe I'll have to change gearing for better legal cruising. Of course, shortening the gearing is going to hampper the bikes already pathetic 125 top speed but, well, you know what I'm saying....

[50Joe]

I'm no IC engine expert, but my gut instinct is that more throttle at the same speed would dump more fuel into the cylinders that is not being put to use to increase speed, so it ends up as unburned fuel in the pipes. Seems like I should drag out my Thermodynamics books, but that is a painful thought all it's own.

[Wine Slut]

The factor that complicates eveything is that a 2-stroke depends on the back pulse of the expasion chamber to keep the fuel mixture in the cylinder. At over rev, the pulse hits to late, and a portion of your fuel goes out the pipe. When your on the pipe, evrything is right, the pulse hits right at the same time the fuel would spill out the exhaust port, keeping it in. but when you're to off the bottom of the pipe, the pulse hits before the fuel is completely in the cylinder, not only decreasing the amount of fuel in the cylinder, but also pushing a small portion back into the carbs. This "double carberation" leads to a rich(sluggish) running condition. Instead of changing gearing, maybe I should look into a less radical(lower powerband) exhaust?

[50Joe]

But that's what I thought the whole YPVS system was supposed to help.

[werase643]

I'm shure they were wired wide open years ago.....

[TZ350G]

You should have driven home at your engines torque peak. The torque peak is were your engine would have the best (for this case most power for the least fuel) BSFC (brake specific fuel consumption) So use the transmittion to set your engine RPM to it's torque peak for the speed (MPH) you want to travel at and then use the throttle to keep it there. Of course you may need a dyno chart to find the torque peak. http://www.wera.com/ubb/smile.gif http://www.wera.com/ubb/biggrin.gif

[Wine Slut]

Originally posted by TZ350G:
You should have driven home at your engines torque peak. The torque peak is were your engine would have the best (for this case most power for the least fuel) BSFC (brake specific fuel consumption) So use the transmittion to set your engine RPM to it's torque peak for the speed (MPH) you want to travel at and then use the throttle to keep it there. Of course you may need a dyno chart to find the torque peak. http://www.wera.com/ubb/smile.gif http://www.wera.com/ubb/biggrin.gif
I am now a wiser man.....