Question on Dynor runs

[Kyle F]

OK, I havbe been wondering this for a long time. I understand how a dyno works and all, but here is my problem. If you hook an engine up to a dyno and it measures torque at the flywheel and then horsepower can be calculated off that. Fine makes enough sense. Now for a chasis dyno... why does gearing not impact the reading. I mean the whole resoan a transmission has gears is so that mechanical advantage is achieved and you can accelerate faster in yourt lower gears and then conserve gas at highway speeds.
No I know in most aut trannys 3rd is a 1:1, but then you have to take into account the rear end gear. So in effect if I took my car and ran it with a 2.73:1 rearend, then came back next week with a 3.73:1 why wouldnt the dyno read more torque? I mean a 3.73:1 geared car would accelerate faster than the 2.73 geared car would so why does the dyno not read this as more power? Someone explain this to me, it is really bugging the piss out of me

 

[Turbo 6 Justin]

generally people don't lie and there is a correction factor for the gearing. basically they plug in the rear end ratio, the tranny ratio (generally this is 1:1 as most autos run the dyno in 3rd) and spin a known mass drum at a certain acceleration and from that they calculate the TQ and the HP. there is also a spark sensor (not unlike a timing light) that determines RPM so the graph is complete. This is just what I have seen since I have never 'wasted' my money on a dyno, I would rather spend the $$$ on the next mod I want.

 

[Kyle F]

Well that makes since I guess, I wonder what would happen if say you put in thaty you had a 2.73 gear ans were running a 3.73 gear?

 

[Two Lane]

You'd just be wasting your dyno-dollars for some worthless data.

Do you know someone who did this?

 

[Kyle F]

I agree, but if someone wanted to make their car look like it made more power than it does, like magazine articles that want to publish big HP numbers, or like the GNs and TTAs dynoed to say that they have like a 4.11 gear and only had like a 3.27 so the dyno would show lwss hp? Maybe this is how GM got thier HP data for our cars!

 

[Chuck Leeper]

Chassis dyno runz

A chassis dyno uses the known wt of the drums and the diameter is also in the mix. The gear ratios of the car are not entered into the equation.
The reason most dyno operators run the pulls in 3rd gear is to get the MPH up enough that the trans doesn't downshift. A down shift will screw up the readings.

As for getting a dyno sheet for "BS rights",, that's most likely a waste of time and money. The dyno should be used to verify tuning changes, establish baseline no's for future changes, etc.

We use it to do just that, seeing if what we're doing is going in the right direction. These changes can be recorded in short order, and alot of pulls made in a day vs spending hours and hours at the track, where weather conditions change, track surfaces change. All these make for a difficult task of comparing apples to apples...

 

[Kyle F]

Re: Chassis dyno runz

Originally posted by Chuck Leeper
A chassis dyno uses the known wt of the drums and the diameter is also in the mix. The gear ratios of the car are not entered into the equation.

Ok, so then how does it account for the Torque multiplication due to gear ratios. Back to the original question... If I had one car and changed the gears how would the dyno not see more power if the car would accelerate the drums faster with the lower gear?

 

[Chuck Leeper]

Dyno??'s

There are different types of absorbers on the different mfgrs dynos.
1. Dyno-Jet uses the drums and actually records the time required to accelerate them to a given speed. [Shutdown of the pull] The computer then uses the HP figures to calculate the torque.
Gears play a part in this to the extent that a set of 3.42's will spin the drum to the shutdown point quicker than a set of 4.10's will, given the same RPM. Thus the 3.42's will show a slight advantage over the 4.10' on that type of dyno. There is no input of gear ratios into the system prior to the pulls, only weather info is used to create a correction factor. This dyno is a "max HP, ballz to the wall," type unit.

2. The Superflo dyno is an eddy current dyno, with drums, that can do steady state, part throttle, and max speed calculations. It has the ability to apply varied loads to the engine, whereas the Dynojet cannot.
You are correct in the statement that gears play a part.. we see minor differences, as we use the Dynojet unit. What we look for is a comparative set of data to see if what was done is working..
Sometimes yep, , sometimes nope,

Bottom line is: it depends on the dyno design as to what you will be able to get for your money.