question about the dyno

[drew416ex]

I will be getting my 400 dynoed pretty soon and was just wondering out of curiosity, will my gearing affect my hp numbers? Im running 15/38 and I am waiting on my 36 tooth rear to come in but I wont have it in time for the run.

[rideracelivemx7]

uhmmm im really not sure. thats a really good question. i dont htink it will i just think, just your acceleration is all. but let me know. say, how much is dynoing anyway?

[drew416ex]

Around here its $85 for the dyno, and then a aditional $75 for them to jet it using the sniffer on the dyno. I figured it shouldnt affect anything too much but I was just curious.

[rideracelivemx7]

not bad, i really wana see how much mines pumping with all the mods now. i dont think i have anyone local though. its a hike from where i am

[GPracer2500]

I've never personally done any back to back dyno runs with different gearing. So maybe there's something about it I don't understand and I'd be surprised by the results.

BUT, I don't think a gearing change is going to impact the numbers on a Dynojet dyno. I say that with a caveat. There are certainly things that can change results. And some of them might be a side effect of gearing. Rolling resistance for example. The taller your gearing, the faster the tires are going to be spinning for a given RPM. The faster they're turning the greater the rolling resistance = lower HP numbers (particularly at high speeds). Unless your talking about really fast speeds or maybe a really big swing in gearing, I wouldn't expect the difference to amount to much.

Traction could be another difference maker. If shorter/taller gearing effects the available traction during a WOT run, that could monkey with the results. But again, a normal change in final drive gearing probably isn't going to change traction much.

Also, I've seen multi gear runs similar to these showing that traditional two-shaft motorcycle transmissions (basically, any manual tranny ATV or motorcycle that's not a Harley) will dyno practically the same once out of the low gears. I believe any difference one might see will really be caused by things like rolling resistance, traction, and maybe other such things. If you're getting substantially the same results out of 3rd or 5th, it's hard to figure how a modest final drive change could be significant.

This is an old ZX1100.
http://www.rbartick.com/pics/run5.gif

This is a KTM supermoto with a 4spd.
http://www.richardwardracing.co.uk/news_files/dyno_thru_gears800.jpg



Do you know what tires your going to use? Although the "gearing" change caused by a slightly different size tire shouldn't matter, a high traction dyno tire won't rob anything like a slipping knobby can.


Have fun at the dyno! Just remember, some dynos read high, some lower. Use it as a tool for tuning--whatever the results--and you can't go wrong. I've made the mistake of arriving at the dyno with the wrong expectations and walked a way not feeling great.

$0.02

[rideracelivemx7]

he's exactly right makes sence. gearing won't change you top hp it will affect how much hp you have at each rpm. lets say you run 18s and shot gearing for torque. you'll have 30hp at lets say 6grand. or run 20 inche tires with long gearing for speed and you might have 30hp at 9 grand.

[drew416ex]

The main reason Im doing it is to check out my jetting. There are a few flat spots that I want to try to correct, but have a hard time figuring out exactly what throttle position the problem is at. I race mx but have a set of old hoosier flat track tires im gonna put on for the dyno to try to get some traction.