## 50v on a dynojet dyno

ive got a pal with a dyno. he said he usually puts bikes into third gear for dyno runs.

dunno why?

so should i adjust my gearing a bit lower to simulate "not in top gear" ish

i usually run 13 54 but had 12 54 on for the last run

ive made a couple of pipes and want to try them out so should keep same gearing for best comparison but what gearing should i put on for an accurate dyno reading?

has anyone done this?

## Re: 50v on a dynojet dyno

Ask him why? It might have something to do with the accuracy of his system as a function of rpm?

## Re: 50v on a dynojet dyno

On my syno there's a single setting for gear ratio. That's used to calculate things like engine rpm as a function of roller rpm. Granted my dyno is homemade and not a dynojet. My best guess is that he's got the gear ratio set to what's typical for third gear in a vehicle. it shouldn't really matter

## Re: 50v on a dynojet dyno

ive just read you put into whichever gear is closest to 1:1 ratio and that is usually 3rd or 4th gear.

## Re: 50v on a dynojet dyno

From what i understand the idea is to have the roller change rpm in a manner that most closely mimics the acceleration of the vehicle so the engine is loaded the same way it would be during acceleration.

In other words, the mass of the roller is going to be sized for the vehicle its meant to test, there are certain assumptions being made by the software, and those assumptions are only valid if the roller is being accelerated similar to the way that the vehicle would accelerate. If 300 hp gets your car from 15 mph to 100 mph in 10 seconds, you will want the roller to speed up at the same approximate rate.

Since your bike is variated, and the roller will be way too heavy, those assumptions will just be slightly off and you won't be getting a 'true' hp number, but you can still use the relativistic HP for tuning purposes.

The other thing about the dynojet, is that once its up to speed it can just keep going unless it has a load brake. You could get your moped up to 100 mph indicated, but your rod would likely come hanging out. The operator will probably tell you this, but you really don't want to pull much past the RPM /speed you see on the street. Its meaningless numbers at that point and your risk going home with your connecting rod hanging out the bottom of your engine.

## Re: 50v on a dynojet dyno

Ideally you should lock your variator or run super heavy weights to make it act like a single speed and run a low rpm clutch stall. this way you'll get a good sweep of your powerband and will be able to use that to tune the drivetrain later on. A pull lasting 3-6 seconds is the goal to get good data. If it's an inertia dyno that's made for 100+hp bikes you may need to change the gearing so it doesn't take you 20 seconds to spin it past your peak.

## Re: 50v on a dynojet dyno

> Joe Schuitema wrote:

> -------------------------------------------------------

> Ideally you should lock your variator or run super heavy weights to make

> it act like a single speed and run a low rpm clutch stall.

yeah forgot to mention that, i've seen plenty of dyno charts where they don't do that just because they are only looking for peak performance numbers.

i'm curious about the 3-6 seconds thing, that would make sense for a race car or motorcycle that can go 20-100 in 6 seconds, but would that be the same for a moped accelerating from 10-50 mph in like 10 seconds? I guess i don't totally know the science behind that. It makes sense you would want your time-rate-of-change of the rpm of the roller to be within measuring accuracy.

## Re: 50v on a dynojet dyno

The 3-6 seconds gives the computer enough data points and should put a reasonable amount of stress on the engine. Speed doesn't matter.

## Re: 50v on a dynojet dyno

Yeah typically you see peak power/torque in the first few seconds

## Re: 50v on a dynojet dyno

hmm that doesn't make sense to me but i've only run on inertia dynos a few times and i don't really know much about them. I have seen operators put it in a lower or higher gear depending on how quickly it runs up to speed. When we dyno'd my buddy's 150 hp civic (b16, thats a good number!) at a shop that does like 1500 hp turbo LS's, he ran in 2nd. When i ran my CB160 at my buddy's shop that does 200 hp literbike race engines, he ran it in 3rd and we just sat there for like 20 seconds to get it up to speed.

I get that speed - like absolute wheel speed- doesn't matter, but you do need to get through a full rpm pull. No moped is going to accelerate a superbike-sized dyno through full rpm range with normal gearing in 3-6 seconds, like i said the 18 hp cb160 motor took like 20 seconds.

The one i'm working on is hydraulic for my own purposes and at the motor co we always used eddy/inverter brakes, so thats really the only formal experience i have. Maybe i'm overthinking it?

## Re: 50v on a dynojet dyno

No you are not overthink it…..these are issues I talked about with my friend that has a dyno with lots of experience tuning high end euro bikes. Even talked about having to use the battery powered “starter” option on the dyno to get the heavy ass drum to spin with the lower hp. I have wanted to try it for a while but haven’t had the courage to break a perfectly running moped to see 8-12hp.

## Re: 50v on a dynojet dyno

We do this with our Puch. Longer gearing so the dyno run will be longer. The engine will get more load, so more realistic power output.

I've had some times where the gearing was way too short, the engine couldn't even go to its peak power.

## Re: 50v on a dynojet dyno

interesting stuff.

what the dyno has been invaluable in is testing the pipes ive been making.

i made a doppler streetcup mod with longer length but same diameters and that gave more power earlier and peaked at 8k rpm and only 0.6hp down off the std doppler that was mismatched for my relatively low durations 176 126. i then made adjustments to the pipe but found they made it worse, each time we were able to jet it using the gas analyser.

i do shut off once i get past 11k. it will rev past that on the road.

so for comparisons it is good.

ive never been a fan of numbers. hp to me means means nothing, and one dyno to the next can be totally different.

only top speed and cruising speed on the road interests me.

## Re: 50v on a dynojet dyno

> Sander Mulder wrote:

> -------------------------------------------------------

> We do this with our Puch. Longer gearing so the dyno run will be longer.

> The engine will get more load, so more realistic power output.

>

> I've had some times where the gearing was way too short, the engine

> couldn't even go to its peak power.

This:

One time I dyno ran my race bike with a Gila that went to 11.4 hp and followed it with a Ciao. The ciao has every high end malossi bit fitted to it you can get. Stock 12:1 ratio tranny wouldn’t even move the rollers. Bike topped out at 3.8 hp.

\$2000+ worth of bullshit for 3.8 hp

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