Help Reading/Understanding Dyno chart

[Jim T]

How come some dyno runs are done going through the gears, as in this chart?

And, why would horsepower gains be different depending on the gear you are in? For example, on this chart, in first gear there does not appear to be any gain - with big gains in other gears.

I assume they redline each gear. If a modification provides, say, 10 hp at 3000 rpm, wouldn't that gain be the same no matter what gear you are in?

Which chart (the typical, one gear dyno run or the multigear run) more accurately measures the effectiveness of a modification?

(On the chart, the vertical axis is HP, the horizontal is Road Speed)

[island911]

Interesting.

Can you post a bigger pic?

my guess; Wheel HP is Torque x RPM. The tire becomes much less distorted and lossy as it spins faster (in the taller gears) as the torque applied to it drops. . . .lots more rpms w/ less torque.

It also shows how easy it is to show the added HP the cool-collar provides

[island911]

My gawd; I just re-read what I wrote. . . do I always ramble like that?

Allow me to rephrase:

Power is made of Torque x RPM. With the engine spinning at peak engine HP, the drive wheels will be delivered with either, more torque @ fewer rpms, or proportionately less torque @ more rpms.

The tires however will eat up more power when they have a heavier torque load. . . .the extreme being when they leave blackmarks and lots of smoke. .. . .and thats where the extra power goes.

[Jim T]

Here's a bigger pic. I still don't understand how a mod can create X extra hp at 3000 rpm in second gear, and X+1 at 3000 rpm in third gear, on dyno rollers!

[island911]

Thanks for the bigger pic. Much better.

I certainly understand that it seems rather whacked to have a delta peak of ~40HP (~22%) between 1st and 3rd.

Hmmm. . ..If my previous conjecture, about torque on the tires, accounted for 100% of the deltas, the 4th gear peaks would be the highest. Though there is some fairly significant noise in the top gear section, which leads me to not trust that info. Was the car bucking on the rollers?

Well. .. . here's more conjecture.
Instumentation error. Depending on the dyno, the sweet-spot range may be set up for the rollers to be spinning at say 50-90mph (surface speed). Low roller rpms could producing grossly nonlinear force measurements.

Hopefully one of the experience dyno guys can chime in here.

EDIT: What car was being tested here?

[Wayne 962]

Originally posted by island911
My gawd; I just re-read what I wrote. . . do I always ramble like that?

YES!

-Wayne

[CamB]

I get the feeling dynos are supposed to work on the basis of a 1:1 gear ratio (eg 4th on many 911 gearboxes).

Where are the experts here!

[Jim T]

Yeah, where are the experts? I'm not sure I've ever seen a tech question like this go unanswered here!

The car is some kind of MB - I think maybe a supercharged one like a SLK230 4 cylinder.

[island911]

Originally posted by Jim T
Yeah, where are the experts? I'm not sure I've ever seen a tech question like this go unanswered here!
. . ..

No kidding Jim. Usually when I post a half baked response it motivates someone to clear the air.

Sorry I can't help more. I can say, I know Wayne did a bunch of research on dynos, a while back.

Maybe you can just wait for him to respond to this thread. I'm sure once he see this thread he'll be able to supply a quality response.

[slammed1000]

dyno's are figuring horsepower and torque at a 1 to 1 gear ratio. that is why most dyno tuners will run them in forth gear. the only reason they do all gear runs is to check tuning most of the time(carburation). there will also be a correction factor that is uses because of the atmospheric conditions. in your run it does look like the car could be tied down a little tighter because of the spikes up and down. this will create false dyno readings. i dyno motorcycles all the time so it all applies to cars too.

[jluetjen]

A related tidbit for consideration...

Most "standard" gearboxes as mentioned have a 4th gear which is about 1:1.

Some of the WRC teams (I believe Ford and Peugot) have developed drivetrains where the gearboxes shafts are underdriven (spun with faster shaft velocities then "normal") reduce the torque on the gears and the differentials, especially the center diff. They then run drop-down gears or ring-gears which bring the ratios back to "Normal" by the time the drive gets to the drive shafts. Along with reducing the torque stresses on the gears, I guess the reduction in torque at the diff's makes them much more progressive.

An interesting idea...

[CRH911S]

Sometimes I go to howstuffworks to get oriented on certain things related to automobiles. Try the following link and see what you think:

http://www.howstuffworks.com/horsepower2.htm

Regards,
Clinton