Dynamometers explained?

[gregajo]

Hey guys, a question? The local Harley dealer had a free dyno day yesterday. Couldn't resist. So, can someone explain what one is looking for in a graph of torque and horsepower? I am wondering how one uses this info for tuning ? Need a dyno for dummies intro to this stuff. Thanks, Greg

 

[biker_reject]

What'd you run? Generally, you do a baseline run, usually stock. Then add your performance upgrades and do another run. If they're tuning for you, they do several runs. The chart will show your power and torque curve compared to the modified bike's power and torque. You should see an increase in hp and lb./ft and smoothed out curves. It has been my experience that HD dyno operators are only good at tuning HD's. I'll never forget the look on the operator and so-called bikers standing around when a dude brought in a ZX12, new on the scene at the time. They thought something was wrong with the dyno when it recorded 162hp. Priceless... HD shops won't (at least not around here) dyno tune a "metric" bike.

 

[gregajo]

I took in my sportster since my cafe project bike isn't finished yet. The previous owner had put a Vance and Hines 2into1 scramble exhaust on it so I don't have a stock base line for this bike. I am betting that the PO did not make any carb adjustments after changing out the exhaust. Not sure but I think the bike is running too rich and has actually lost some HP with these pipes? Also being a quick freebee I am guessing that the operator wasn't putting much effort into the run?

 

[MR915]

I think the dyno is a better tool at giving A/F readings than straight up HP/torque. I'm sure they didn't record the A/F on your free run.

 

[crazypj]

Yep, without the gas analyser connected up it isn't worth much, that's the thing that will give you tuning information

 

[gregajo]

Well they didn't give me numbers that make any sense to me after doing a little bit of reading. The bike runs very strong and has plenty of get up and go. But the print they gave me indicated a max HP of 45 and max Torque of 58 (which was a little spike near 80 mph while the rest of the curve was closer to 48). The motor was definitely not warmed up for the run as they were moving the bikes through quickly for an hour or so.

 

[crazypj]

That's about right for an 883 Sporty, 1200 will be around 62bhp.
You should be getting a bit more with exhaust though so probably need to go back and get a run with EGA

 

[gregajo]

Thanks PJ, I appreciate the info. I like the way the bike runs now and it scoots real well for my purpose of running around town and such. I don't have the money to go crazy chasing a little more power. But it still seems I could do a little tweaking to make what I have pull a little quicker. I need to do some more reading about this and learn how to play with carbs.

 

[Cuba Libre]

Hey guys, a question? The local Harley dealer had a free dyno day yesterday. Couldn't resist. So, can someone explain what one is looking for in a graph of torque and horsepower? I am wondering how one uses this info for tuning ? Need a dyno for dummies intro to this stuff. Thanks, Greg

I am a little confused. Are you asking how dynamometers work? or how to use the information provided by a dynograph?

M.

 

[gregajo]

Yes M, how to use the info is what I am after. The sportster I have looks like the previous owner spent a lot customizing but it appears he spent his money on show rather than go. So now it will be up to me to make it as fast as it looks (at least sometime when I have some extra cash). It tis point still trying to learn about this stuff and especially about "the art of tuning". Greg

 

[Cuba Libre]

Ok Greg, gotcha.

Then the dynamometer will provide you with a set of base line curves for power and torque, "base line" meaning a description of what you have now to be conpared to whatever you achieve later when you start tweaking your engine.

A few things to consider:
- In all probability, and based on my experience, the "run what you brung" free dyno days are 90% show and 10% essence - especially if no gas analyser is used. So treat this as something fun rather than as something serious.
- If you are seriously... serious about tuning or modifying your engine, make sure to have it run on the same dynamometer, under the same ambient conditions (if possible) and (again if possible) by the same man. "Corrections" exist to compensate for these differences but it will be to your benefit to try and eliminate as many external variables as possible. The reason is that dynamometers are notoriously inconsisent in their results. If you take the same bike on the same day in two different shops in the same area you'll probably get two different sets of curves - even from dynos of the same brand/model. Add to that the "correction factors" applied (usually at will) and you can get very confused very quickly.
- A dyno run (not just the graph) can provide you with quite some information, provided the man who runs it knows how to use it
- There will always be a difference (most times a considerable one) between the bike being run on the dyno and the same bike running on the road. So, again, the dyno run is not meant to be taken as a simuation of riding the bike at full song but as a test run under "nominal" conditions.

I could probably talk more about dynos but believe it would be besides the point. Bottom line: find a shop near you. Run the bike as it is now (with the gas analyser). Then start playing with the power plant (and the power train - there are hidden horses there too). Compare with what you started with. Adjust/correct/pay as necessary and repeat cycle until you burn a hole in your pocket (I should know). Simple.


Enjoy and keep us posted!

M.

 

[gregajo]

Thanks, I have much to learn but love learning about this stuff.

 

[crazypj]

Greg, you need something more like this, separate box underneath with fuel air ratio.
This is an 'all gears' run as I wanted some idea how fuel was doing during acceleration on my first 378cc motor(but got a bunch of clutch slip with stock clutch springs : )

 

[gregajo]

Greg, you need something more like this, separate box underneath with fuel air ratio.

Ok, so looking at the Air/Fuel curve I am assuming one wants a fairly steady line at the ideal mixture ratio?

 

[crazypj]

If possible.
The dotted line is 'ideal', I'm rich almost everywhere as it's while I was modifying carbs for K&N filters
At a fixed throttle, (while cruising) you actually want things to lean out slightly to give better gas mileage

 

[gregajo]

Thanks, I get it. And it would have been helpful to get this print with a run. I was guessing my bike was running a bit rich due to a lot of popping on deceleration. But the bike wasn't really warmed up for the quick pull they did.