I have heard that you should do a dyno pull in the gear closest to a 1:1 ratio. That said, following are the gear ratios for the 996 TT:

1st gear = 3.82:1
2nd gear = 2.05:1
3rd gear = 1.41:1
4th gear = 1.12:1
5th gear = 0.92:1
6th gear = 0.75:1

Now, normal reasoning would dictate the use of 5th instead of 4th since it is only 8 points off as compared to 12. The only argument I see to use 4th would be the possibility of a greater torque output.

Can I get some opinions on this?
Has anyone done a back to back dyno in 4th and then 5th to see the difference?
This sounds like a question for Chad. BTW, where has Chad been lately?

 

Ken,

Dyno pulls are always down in fourth gear.

 

Okay. Why, when 5th is closer to a 1:1 ratio?

 

I don't want to simply use 4th because that is what was done, just like you cook your turkey at 400 degrees because that is what your mother did!! I want to question convention. I want reasons why 4th would be preferred over 5th or vice versa.

 

Ken - AWE is now going to be a sponsor - (early welcome guys) so please take video and detailed pictures of the event if you can

 

Darren,
You can count on it!

Although you guys have probably already seen it before, here is a link to a video shot at AWE last spring.

DynoVideo

 

Dyno for GT3 should be done in 5th gear...it's exactly a 1:1 ratio

 

Hi.
The gear to use for a dyno run is chosen for these reasons:
(IMHO)

1 - high enough to avoid high torque because it makes it hard to
keep the tires from slipping.
2 - high enough to make the pull long enough for a larger number
of data samplings.
3 - Hoping for the most mechanically efficient gear (usually one of
the higher gears that the car is designed to spend most of it's
time. This is not strictly necessary, but if you get 596hp to the
wheels in third, and you get 600hp to the wheels in fourth, you
typically want to hear the higher number.

Joe Weinstein

 

This excellent writeup was given by Todd, the President of AWE:

Issues to consider:

No matter what gear set you dyno in, it is ultimately multiplied by the final drive.

Considering that the 996TT final drive is 3.44:1, you would have to have a .29:1 gear set to totally eliminate the gear multiplication issue.

Thus, in essence, the 1:1 ratio becomes a myth when it comes to chassis dyno testing.

However, gear set selection is important nonetheless:

A gear should be selected that allows a long enough pull to gather meaningful data, and produces a big enough load on the engine to produce realistic results. The lower the gear, the less load on the engine. Load is essential to mimic on street/track driving.

Yet, the gear selected should not be so tall that the ending speed is far outpacing the cooling capacity of the dyno test cell.

In reality, there are extremely few chassis dyno test facilities that have even barely sufficient cooling systems, and the far majority of them have cooling systems acceptable for just a 20mph dyno pull. Considering that 4th gear on the 996TT gets you north of 120mph, a subpar fan system will skew data big time.

We have found that we are able to test in 4th gear with reliable data, due to the special fans that we installed. Testing in 5th would create excessive heat and unrealistic data.



The bottom line is that while gear set does affect torque multiplication, it is essentially impossible to create a true 1:1 gearing effect without installing a special gear set/final drive, and even then the fan system of the dyno cell becomes the limiting factor.

As long as all the cars being tested are being tested under the same circumstances, the data becomes valid when cross referencing.


__________________
Todd Sager
President
A.W.E. Tuning
1-888-565-2257
www.awe-tuning.com

 

I actually disagee with Todd (at my peril), in the area of 1:1.
Some people seek 1:1 as some sort of magical goal without
understanding why it was ever interesting, and Tod is correct
that the final drive always multiplies torque.
The real reason folks originally sought the 1:1 wasn't for
the overall ratio, it was because the usual gearbox's top gear
used to be 1:1, *and it was designed as a straight simple
path from input to output, producing the least internal power
loss of any of the gears*.
The final drive imposes it's own drag, which is not alterable,
but the gearbox drag is alterable, based on the gear chosen,
in that some are more efficient than others. The 1:1 was just
a shorthand for choosiing the most efficient gear, which we should
always do nowadays, even if no gear is an actual 1:1.

 

Hi Joe,

Are you sure that 1:1 ratio actually results in less driveline loss? The studies I've perused in the past do not show any quantitative data of efficiency vs ratio.

My understanding is that frictional losses are dependent upon load and speed, hence a gearbox spinning at a faster given speed will have more frictional losses vs a gearbox spinning at a slower given speed, and the same with a gearbox under acceleration vs coast down.

Therefore, if minimizing frictional losses were the focus of choosing an appropriate gear for dyno testing, then perhaps the *lower* gears would be better, since lower internal gearbox speeds are seen in these gears. For a given input (engine) speed of say 5000 rpms, the pinion gear and final drive gear will be moving *a lot* faster in 4th gear vs 2nd gear, and the overall losses will be higher. A 1:1 ratio is not "transparent" in this case.

Thus, I would say that the torque multiplication factor has a more tangible effect on the wheel power produced, and thus it *should* be considered as a variable more than the potential frictional losses.

Given that, I believe the myth of the 1:1 ratio stems more from the obvious effect gear multiplication has on measurable wheel power rather than on the more esoteric issue of frictional losses.

 

Hi Todd! No, I'm not saying that 1:1 provides the least loss
just because it's 1:1. I believe that typically, and maybe moreso
in the past, the top gear in a transmission was most easily and
simply designed to be most efficient as a 1:1 ratio. If a box
has a 1:1 ratio, but it goes through more jackshafts than another
gear, then that other gear is likely to be more efficient.
Frictional losses are dependent on load, and speed, and
mechanical efficiency. The load is at max power, so that part is a
constant. The speed is always the same at the tranny input, and
at the speed at the output is lowest with the lowest gear, as you
said. What remains is the mechanical efficiency of each gear,
which differs according to the design of the box. Typically the
tranny designer will try to make the usual cruising gear or top
speed gear the most efficient, whether they want fuel efficiency
or maximum speed (for most cars this will be the same gear).
That used to be the 1:1 ratio. After a while, fancier cars began to
offer an optional 'overdrive', which were new fancy ratios to get a
higher ratio than 1:1.
So I think there is a tradeoff between the lower friction of the
output end of a tranny in the lower gears, vs. the usual overall
design efficiency biassed toward the freeway gear. Actually, the
differential gear and wheel bearings etc will also provide more
resistence at higher gear ratios.
Aside from this, I am not sure what the torque-multiplication
effect is, because (ignoring for the moment the differing
friction losses described for the different gears), the same
*power* is being delivered to the wheels, whatever gear is
chosen. A lower gear delivers more torque, and fewer RPMs,
and a higher gear does the reverse. As long as the dyno can
measure both accurately in either case, it can equally multiply
the two to get the (same) power. I do think that it is practically
more hard to measure the power accurately in lower gears
because the duration of a pull is shorter, and tire slippage is
harder to avoid. Thanks in advance for your thoughts,
Joe