The Effect of Dynoing in Different Gears
 

Hey folks,

After reviewing all of the dynos people have been nice enough to share with me (thanks!) I have made an interesting, but not really surprising discovery:

The gear you dyno in affects the torque output --A LOT.

See here for reference http://www.the370z.com/forced-induct...tml#post733065

Note the different torque values --a range from 226 (what I would consider typical on a dynojet) to as high as 240 (way high on a dynojet)!!!

I know the 7AT should be dynoed in 4th; converting from RPM to MPH, everyone is around 115-125 MPH, and that appears to be consistent after reviewing all the drfs I have.

Thus, I find very consistent torque data for the 7AT's (BTW, at worst, about 1% lower than the 6MT, often equal, so no clear evidence of significant differences in drivetrain losses).

However, for the 6MT, some show speeds reaching 115-125 while others are over 140 MPH! Clearly different gears!

Everyone exceeding 140 MPH also shows as much as 5-15 more lbft of torque (again, see link above for reference)!

I think what we need is offical consensus on what gear is appropriate so that comparisons are consistent across dynos -- without this it will be even more difficult to figure out what is going on from dyno to dyno even using the same dynamometer!

Thoughts?

I say whatever gear takes you to no faster than 115-120 on the 6MT is the correct one to use -- is that 4th???

Also, it takes several pulls in the same gear, activating the same load cells in the ECU maps to get the ECU to adjust to changes from mods or tune -- if you vary the gear from run to run, this will also obfuscate the impact of a given change.

You want to dyno in 5th gear for the 6 speed and 7 speed. But the 7 speed will hit the speed limiter unless it's been removed with a flash. So most are done in 4th.

I was under the impression that you want to do dyno runs in the gear that is closest to 1:1 ...

What gear will limit your top speed to about 125 MPH? I see about the same torque and speed values for the 7AT and 6MT when this is the top speed.

BTW, the gearing ratios for the two trans are:

Gear M/T A/T
1st 3.794 4.924
2nd 2.324 3.194
3rd 1.624 2.043
4th 1.271 1.412
5th 1.000 1.000
6th 0.794 0.862
7th - 0.771

Yes, but apaprently you can't in an untuned 7AT and people seem to randomly dyno in 4th or 5th on the 6MT... hence the problem.

However, when top speeds are approximately equal, so are measured torque values between the two transmissions.

Wonder how the final gear factors into this too...

They are: MT = 3.692 AT = 3.357

Hmmm. Now I want to dyno in 5th and see what happens (I'm tuned).

One other thought:

If we assume the final gear is also the final multiplier (is that logical? Math-types chime in) then the difference in effective gearing between the 7AT and 6MT is less when both are dynoed in 4th than when dynoed in 5th, given the higher numerical final gearing of the 6MT.

Thus I think everyone should actually just dyno in 4th, both 7AT and 6MT, in order to make comparisons more comparable.

Yes? No? :confused:

Transmission gearing and "multipliers" are very complicated; the final gear has little to do mathematically with the rest of the gears. The final drive has more to do with the rotation after it's been through the transmission. If we're in 5th gear, the ratio is 1:1 in the motor/transmission connection... then if the final gear is 5 and the diff is 1, that means that for every one revolution of the engine, that's one revolution of the tranny gear, which is five revolutions of the final gear and the diff gear is 1, so that means the wheel turns a total of five times for every single turn of the engine.

Sorry, just building up my own logic there.

Of course, with the data you've given me, assuming that both the AT and MT cars have the same diff and diameter of tire, we can say that the AT car will produce less torque because it has a longer final drive, if RPM and gear (5th for 1:1).

TL;DR

AT has less torque when RPM, 5th gear, same diff and tires than an MT.

I'm probably wrong but at least we're thinking now.

Then, this would suggest that at least to keep everything scaled similarly we would want to dyno both in 4th.

Likewise, in terms of performance, both cars would be in 4th gear up to 125 MPH. By 5th gear, the differences in final gear ratio make their performance too dissimilar, and in any case due to wind resistance and power limitations, neither will achieve the top speeds possible based on gearing.

Put another way, the MT will have a big mechanical advantage in 5th gear over the AT in either 4th or 5th, whereas they will be more similar mechanicaly in 4th.

Thus, I think 4th gear might be preferable for both cars.

Im curious to see what the AT puts down in 5th gear. Power values won't be affected as much, at least not on a dynojet, as power is computed based on the rate at which the drum is acellerated.

However, a mechanical advantage due to gearing will probably affect that to some extent, even if the greatest difference is in measured peak torque rather than peak power.

Mechanical advantage makes a huge difference. I can now understand where you're coming from when you say the MT's should dyno in 4th. The only issue now is that everyone has to have equivalent values, including the SAE error adjustment, whether or not you're using a Mustang or Dynojet (or some other type of dyno), whether or not it's loaded, braked, etc.

You also have to factor in that anyone who is running larger wheels or heavier wheels may appear to have less torque because of the rotational inertia or rolling resistance and what not.

Overall, the gear suggestion is a good place to start, but dyno numbers are always all over the place because of the variance in testing. Temperature makes a massive difference, especially when people use CO2 extinguishers, ice bags on their turbos or superchargers. Sure, that's forced induction, but even an NA car will experience mildly better numbers in colder weather.

Also, did a bit of math:

-----RPM--Gear--Gear Ratio--Final Drive

AT--1500--5th----1.000------3.357 = 5035.5 RPM drive shaft
MT--1500--4th----1.271------3.692 = 7038.798 RPM drive shaft

Even without the diff ratio and the wheel circumferences considered, we're looking at a 2000 RPM difference in the driveshaft. It's irrelevant to power and torque but clearly there is a significant difference.

What are the RPM's for both in 4th gear then? They should be much closer, meaning a more fair comparison.

Actually, I kind of am only speaking about Dynojets and SAE correction factors for just that reason -- to keep everything on the same scale when making comparisons from vehicle to vehicle and in order to more accurately determine the impact of mods.

I guess I personally care less about what people do on other dynamometers as you can't easily compare from one to the other anyway.

See further here Turbo Magazine's Dyno Dash - Tech Review - Turbo Magazine

However, for dynojet users, we should be able to get stable values across run conditons, vehicles, and transmission types.

Or... if MT drivers feel like they aren't getting to take advantage of their mechancial advantage in 5th, it's important that they take note of the gear because it's going to greatly affect how we interpret the impact of mods.

This may also explain why even on dynojets, claims about stock power and torque values are surprisingly variable.

I see consistency in obtained values across vehicles and transmissions when everyone is gear limited to about 125 MPH and has approximately the same mechanical advantage in acelerating the drum (i.e., 4th gear for both AT and MT).

Anyway, no matter what gear is used, it's important to use the same gear across runs or sessions in order to give the ECU a chance to see the same load cells in the fuel/spark/vvel maps in order to apply adjustments.

In other words, if you dyno in 4th and then later in 5th, you're going to get very different results from run to run.

EDIT:
in 4th gear using 1500 = AT drive shaft = 7110, so yes, they are much closer in drive shaft rotations when both dynoed in 4th.

That mechancial advantage of 2000 RPMs clearly translates into significantly higher torque values measured at the wheels (and slightly higher power) for the MT on the dynojet when dynoed in 5th over 4th.

What you say is true. The only matter now is getting other people on the same page.

Heh. Well, that's why I post these threads :)

6MT at 1500rpm in 4th = 319.66 wheel rpm
7AT at 1500rpm in 4th = 316.45 wheel rpm

370z purist, your thinking about final drives and multiplying the numbers all wrong, the final drive is the differential gearing so it's after the driveshaft.

Here is a link to stick to your favorites Top speed calculator

Okay, my math was ***-backwards too.. :owned:

If we divide not mulitply to get those values, then...

AT @1500 in 5th = 446.83
MT @ 1500 in 5th = 406.28

But... the same conclusion holds -- in 4th, both transmissions have about the same mechanical advantage, but if we dyno the MT in 5th and the AT in 4th it will be an apples-oranges comparison due to a greater mechanical advantage for the MT.

So then do you agree everyone should just dyno in 4th?

That seems to be the most commonly selected gear anyway... I see very few runs done in 5th, but when I do, those always show higher (i.e., outlier) torque values as I had noted above.

EDIT: Actually, the 1% better wheel RPM due to gearing for the MT over the AT in 4th corroborates with the occasional evidence I see of about 1% more peak torque for the MT over the AT too... hmm.

Maybe I'm thinking the same thing that you guys are talking about here, but why not figure out which gears give you the most similar roll out between transmissions? Lets go hypothetical here. Lets pretend fifth gear in the mt gives you a roll out of 20 ft and 4th gear only gets you 15 ft. Then the AT gives you a roll out of 17 feet in fifth and 14 ft in fourth. It would be a much closer comparison if you used 5th gear in the MT and 4th in the AT in this scenario. Granted in order to figure out roll out you would have to know the circumference of the tire. Also if you change tire sizes it would require a recalculation. It would be awesome if you figure out a way to make this work.

Honestly though I look at dyno graphs to see what percentage they gained. The logic I apply is that if they gained 10% hp or 10% torque, then I should see something pretty close to 10% on my car regardless of what dyno or gear I use as long as I keep those two variables constant.

Well, up to 125ish MPH, both the 6 and 7 speeds are in 4th gear, so I think that makes them more comparable than 5th to 5th.

Your point on the tire size and roll out distance is well taken too, but so long as everyone uses the OEM diameters, the difference will be negligible. Also, even if they are using different sizes, the effect will be further minimized in 4rth than 5th given the closer startng points.

In terms of determining changes to an individual car, it doesn't matter so long as you use the same gear each time, but I've now seen a few dynos where baselines were taken in 5th and post-modification dynos were in 4th, which led to all sorts of confusion.

Also, I was trying to figure out why all the data I saw pointed to a baseline wtq on a dynojet of around 225 whereas some people claimed a higher baseline of about 240 -- the answer, it seems, is that it depends on the gear used on the dyno.

Someone also recently started a thread asking about typical baseline torque, so clearly it's not just me/us who find this issue somewhat important :p

At minimum, it means people need to indicate the gear they used at the dyno when posting numbers (which some members have been good enough to do) in order to interpret their results.

For the 6MT I have 4th gear good for about 25MPH to 125MPH and for the 7AT I have the same.

Maybe the problem is with the Dynojet's compensation for the torque factors of different gears? Or maybe the Dynojet isn't taking the different between the 6MT and 7AT into account when computing torque?

It seems it should get the same answer in every gear if it was doing the math/model correctly.

BTW, I attached the PDF of the iPad spreadsheet I use to compute speed/torque in gears.

It is based on hand interpolating from a large dyno scan and uses the same values for auto and manual (which should be right, I think) but some day I will update with some more precise figures. It was originally done to show why you should shift at redline.

^^^ Cool -- I'll check it out :tup:

I think it isn't compensated on the dynojet, which might be where the use a 1:1 ratio idea began...

But, yes, it is clearly an artifact of the gearing... that said, I now want to dyno in 5th to compare and contrast...

And here I was thinking I had accomplished something great. :owned:

Eh right idea, wrong direction, same conclusion :D

I had it backwards too :o

Yeah, I could see how doing before and after dynos would skewer the results and make for bad comparisons. I guess i just assumed people would be smart enough to use the same gear for before and after. But yeah, I think the easiest thing to do would be to just post which gear the car was dyno'd in. I do have one assumption I figure you would be able to clear up since you have a lot of info on this. I'm assuming that for any given tranny, that the dynos done in the lower gear are producing more torque due to transmission ratios correct? Maybe not a huge difference but still noticeable right?

I could be totally wrong here, but i think the 1:1 rule was started for a few reasons.

First, cars have all sorts of gear ratios, but what we are after is engine output. It's not feasible to put an engine on an engine dyno so we go after the reading that is closest to that, and that would be a 1:1 ratio output from the engine to the gearbox. Final drive is mostly irrelevant, as it won't necessarily affect peak power, but where you make peak power.

Second, disregarding overdrive, the 1:1 gear is usually towards the top of the gearseat without being in overdrive, so it takes a lot longer to go through that gear, which equals a longer amount of time for the dyno to take readings. Imagine trying to count a stack of money thats being put down bill by bill every second, then imagine trying to count a stack of money that someone just throws down all at once.

That would have been my guess, but it seems that there's a mechanical advantage in terms of wheel RPM (which is how dynojets compute power -- the speed the known weight of the drum is rotated), which means a higher gear will produce higher numbers (at least to a point -- not sure what would happen if ratio was < 1:1 ).

Anyway, so far in every run I've seen, 5th gear runs always nets higher torque and power values than 4th or 3rd.

Here's what I found on these topics...

From Chassis Dyno Guide - Transmissions And Final Drive Ratios - Hot Rod Magazine


And this...

From Is Your Dyno Lying? - Tech Stuff - Auto Reviews - Car and Driver

Finally, an interesting discussion on NASIOC on this...
Dyno runs in a 1:1 gear ratio? - NASIOC

Not sure if that matters for the 370Z as the autos have full lock up after 2nd gear -- i.e., a hard connection rather than strict fluid coupling (evidenced by the limited evidence of appreciable difference in drivetrain losses).