davepl

It came up in a thread I didn't want to randomize so I am breaking it out as separate topic here.

This is a question about chassis dynos, which I don't operate nor have access to (other than as a customer). I have access to an engine dyno, but that's completely different. So I'm no chassis dyno expert by any stretch.

Anyway, I keep seeing people asserting that you need to dyno in the highest gear, or more often, 1:1 gear. That can be problematic with our cars because the rear ratio (2:41?) means you're REALLY spinning those rollers by max RPM.

I don't know what the dyno manufacturers say - that's the real answer even if they're wrong, because it's their equipment.

However, I believe it doesn't matter what gear you're in. The dyno computer knows these things:

1) The drum speed change (in rpm or mph) - how much it was accelerated
2) The amount of time it took to do it
3) The RPM of the engine at every point along the way

That's it. That's all you need. Doesn't matter what gear you're in.

Why? Maybe you're thinking doing a pull in 1st would be cheating, because you have so much mechanical advantage? But no, because the drum accelerates less.

If you did a dyno pull in first it'd take a lot less time but you'd also only get the drum up to about 50mph. If you did it in 6th, it'd take a lot longer but you'd get the drum up above 150mph, so a lot more work done. The math all works out.

That's my point - while 1:1 might be convenient or recommended, I see absolutely no reason it'd be required for a dyno pull.

rflow306

It matters very little on an Inertia dyno like the dynojet. You can dyno in fifth with an A8 and get reliable results imo. Fourth is to short for modded cars as it will spin tire on the rollers and cause lower readings. Our 1:1 gear on the A8 is to long to get consistent results. The dynojet uses the engine tach signal to calculate and display torque. Without a tach signal it will display horsepower over time.

fleming23

I was under the impression that on a dynojet, horsepower was mathematically calculated based on the rotating mass and acceleration of those big rollers. If you spun the rollers in a lower gear, they would accelerate faster causing a skewed result. On a load based dyno like a Mustang, it doesn't matter due to how the resistance is measured.

PRE-Z06

Only speaking from experience of working at a shop using a dynocom, but dynoed a stock manual 4.8 truck in 2nd as governor wouldn't allow 3rd. Then made a pull after it was removed before tuning as 4th (1:1) wasnt possible with stock driveshaft, the results where different. Same as when I swapped from 3.42 gears to 4.10 in the rearend of my c5. I didn't pickup power on the dyno from the car accelerating in 4th gear faster, rather lost power from parasitic loss though went faster at the track of course. Iirc a mustang dyno asks you to enter the rear gear ratio to counter it, hope that helps.

Higgs Boson

I haven't seen every dyno there is but ever one I have seen will output different numbers by using different gears. I have even seen altered tire pressures put down different numbers so......

Take your internet dynos with a grain of salt.

Dead Horse says, "Dynos are a tuning tool, Wilbur!"

davepl

Not to beat on chassis dynos, but I only use them to compare a car to itself. Ideally on the same day.

But if this is causing any concern, imagine you have your A8 Z06. As we know, 1:1 is 6th gear, and the rear end is a 2.41.

Let's say you dyno the car, then change the REAR end, and now it's 2.73. Well then it turns out you need to be in 7th gear to get the same overall ratio, but you wouldn't in real world no dyno in 7th. Why? Because gearing is irrelevant.

I'm talking idealized, though why it would matter if you were in 4th or 7th I don't know. I can see that 1st might be WAY too fast (and hence noisy on the graph) to measure much.

Imagine your dyno can only plot once per second... it'd be over in a hurry in 1st. So there's the frequency limit.

In the final summation, remember roller speed (tire speed) doesn't matter, just RPM and torque, and torque is inferred from a change in roller speed.

lt1z

I use 5th in an A8 and 4th in an M7

juanvaldez

All dynos measure torque. Torque is multiplied by gears. To get accurate measurements you have to take into consideration wheel/tire diameter, rear end ratio, transmission ratio.

davepl

No you do not... You are completely wrong. I won't repeat why because that's already above.

Disagree? Explain how a chassis dyno works any why gear multiplication matters. I say it does not.

Bill Dearborn

Any gear other than 1:1 picks up added frictional losses which will reduce the power being measured. However, dynojets are only accurate to +/- 1% so the difference might be buried in the tolerance. On a car dynoing 500 whp the actual power could be anywhere between 495 or 505 HP.

Bill

KnightDriveTV

It's pretty simple...you buy a dyno, they factory train you to operate it, the people who build/design the dyno and software say "use a gear that is closest to 1:1", and then you listen and that's all.

I operated/own a Land and Sea, there are function to "autocalculate" which compensate, but this is a software interface to a large mechanical object. Who am I to question...they say 1:1, I comply.

If it weren't necessary, or were irrelevant, then why specify? Answer that question before any further debate. That said, I've dyno'd vehicles in lesser gears and without commanding the software to recalculate, the numbers were skewed and the base torque numbers were higher.

davepl

Not me. I'd listen to the trainers, ask a TON of questions, and as soon as they leave I'd take it apart, figure out how it works, understnad it, probably improve it. Unless it's the guy that invented the thing, the trainer is just telling you what he was told, and probably doesn't know why either. Unless life or limb is at risk, I'm not a big follower of rules I don't understand.

With all due respect Bill, that's incorrect, because the dyno does not account for frictional losses; that's up to the bench racers. It would be correct to say that deeper gears have more frictional losses (but you didn't say that, you said any gear other than 1:1).

How would the dyno know if you're in 5th with 2.73s or 6th with 2.41s? They're equivalent (let's say) final drive. Which is 1:1? When there's a differential involved who cares what gear the transmission is in? You may wish to minimize frictional losses and direct-drive is likely the best choice for that, but since you're already dealing with 15-20% losses, it just means a different loss factor.

All dynos do is measure the acceleration of the drum, interpolate a torque value, multiple by RPM, and divide by 5252. That's it. Doesn't matter what gear you're in. Yes frictional losses will change but they're not in the calculation anyway, so there's no magical "right" value. Probably 1:1 in the trans nets the least losses, but by no means does it eliminate them.

You can even dyno a CVT so long as you can get it to sweep up through the RPM range under load. Now that's a brain teaser.

Dave

Higgs Boson

I always TUNE in 1:1 gear as much as possible because that's a gear I will be in longer to get more data but is also not an overdrive gear to put excessive load on the engine. 1:1 is also the gear that you want to end the 1/4 mile in if your gearing is right. Since dynos are a TUNING tool, I have a feeling this is a TUNING rule rather than a power measurement rule.

However, dynoing in or as close to a 1:1 ratio as possible will definitely help keep things consistent across the board for comparison purposes so maybe it is also somewhat of a control. Plenty of monkeying around with dynos is done....I love when someone is selling a part and the before dyno pull is done with the hood closed and the after is with the hood open. Look ma, 40 horsepower!!

Higgs Boson

before we have our own discussion let's read some others and see if anyone's opinion changes:

https://forums.nasioc.com/forums/sho....php?t=1478091

http://www.stangnet.com/mustang-foru...h-gear.589468/

http://www.bimmerforums.com/forum/sh...-4th-gear-pull

Ben@WeaponX

A8 6th gear on a Z06 is closest to 1:1 and on our dyno, that puts you over 200MPH! We do most of our tuning in 5th on those and then will do the validation pull in 6th to save from doing continuous 200+ MPH pulls when dialing in the PE. Dyno'ing in 5th will read less till we do the accurate pull FWIW.

0Anthony @ LGMotorsports

We use DynoJet dynos here and most certainly it effects dyno numbers...a lot of things effect dyno numbers.

Rotational weight
tire size
tire pressure
alignment
diff ratio
trans ratio.

Since we swapped diffs around on the race cars between races we saw almost a 12 rwhp loss on the dyno going from a 3.42 diff to a 4.10 on the World Challenge car. Nothing changed but the diff but the ratio.

I have seen going from a heavy set of chrome 20" wheels to a lightweight set of ForgeLine's change dyno numbers by 8-10 hp

As for the C7 specific question, like the others have said a 6th gear pull with that tall rear ratio can make the car go well over 200 mph on the dyno which might read 10-15 more HP but the pulls take a long time and can be a bit ragged on the readout. I think most of us all do them in 5th gear for more accurate pulls but gives a lower number on paper.

juanvaldez

The dyno knows the engine rpm and the drum rpm. I can figure the gear ratio. A dyno also only measures torque. Everything else is calculated. In the old days it was a mechanical brake (thus brake hp) that was used now it is a generator turned by the drum to provide a variable load. It has nothing to do with accelerating the drum or torque and hp would fall to zero at a steady state.

0Anthony @ LGMotorsports

http://http://www.hotrod.com/article...is-dyno-guide/

PRE-Z06

12rwhp was the same loss I got switching from 3.42 to 4.10 gears as seen below, which I chalked up to the deeper/higher numerically gears causing greater parasitic loss from more friction on the increased number of teeth. Saw ~ the same gains when switching from the 315/30r18x10.5 repros to 17x8.5 magnesium wheels for the track fwiw OP.
https://www.corvetteforum.com/forums...o-results.html

rflow306

Not true for an Inertia Dyno like the dyno jet. A dynojet measures hp and calculates torque based on an engine rpm pick-up. It only needs an rpm pick-up to calculate torque. Like i mentioned earlier if you dyno with-out using an rpm pick-up it will not display torque but will display horsepower over time.