1g1yy

It is torque that accelerates a car. To determine which car with which set of gears will outpull the other you simply determine the overall gear ratio of each (diff. gear ratio times trans. gear ratio ), and the car with the lowest ( highest numerically ) ratio will have the highest torque multiplication, and outpull the other. This is given, of-course, that all else (such as the two engines torque band ) is equal. Also this is ONLY AS LONG AS THE TWO ENGINES ARE RUNNING AT THE SAME RPM. Obviously the car with the lowest overall ratio will run out of it's "powerband" more quickly, and need to downshift, at which point the higher geared car might catch it.

It doesn't matter which gear a car is in. All that matters is the overall ratio, and keeping the car in it's "powerband". This is why (car, power, etc. being equal) a car set up for the 1/8 mile will always outpull a car set up for the 1/4 mile. IN THE 1/8 MILE. Put the two on a 1/4 mile track and even though the 1/8 mile car will get the jump, the 1/4 mile car will still be accelerating at the end of the 1/8 mile where 1/8 mile car will be out of gear and RPM. :yesnod:

As for which set of gears is best, 4.10's or 3.42's, it depends on what you want to do with the car. An A4 with 4.10's has an overall first gear ratio of 12.546 to one, which is almost useless on the street with street tires. The M6 has a higher (lower numerically) first gear so it can use 4.10's to good advantage. My car is an A4 with 3.15's and doesn't get any traction in first or second on the original runcraps. But this problem is about to be solved with new tires and Z06 wheels. Then I will be saving for some 3.42's or 3.73's and a TC. :D :cheers:

Oh, and a chassis dyno does measure RWH reasonably accurately. The reason it is measuring less HP after a (higher numerical ratio) gear change is not because the engine is making less power, or because there is that much more friction, but because it is measuring RWH. With a higher (numerical) gear ratio the engine basically does less work over time, which is what a measure of HP is. Give your engine a break, and get some gears!! :yesnod:


[Modified by 1g1yy, 4:36 AM 1/21/2004]

Jbrady

Gears multiply torque.

The torque at the rear wheels is ALWAYS more than at the engine.

Dynos correct RWTQ to reflect engine RPM but the truth is the torque at the wheels is MUCH higher.

Example: 320 lbs/ft torque at the engine, transmission in 4th gear (1:1 ratio, no multiplication) stock 3.42:1 gears = 320 x 3.42 = 1094 lbs/tq, now subtract the drivetrain loss (est 15%) and you have 930 lbs/tq at the rear wheels.

Same as above but using 4.10 gears: 320 x 4.10 - 15% = 1115 kbs/tq

Now, if you add the transmission gears into the equation it gets really interesting and easy to see why some cars can lift the front of the car (weighing thousands of pounds) off the ground.

I do not have the stats for the T56 first gear but lets use 3.35 as a guesstimate (someone can correct me later)

320 torque at the engine x 3.35 first gear x 3.42 final drive - 15% loss = 3116 pounds of torque at the rear wheels in first gear in a STOCK car.

The 4.10s look like this: 320 x 3.35 x 4.10 - 15% = 3736 RWTQ

So, YES, the RWTQ is DEFINTELY effected by gearing.

Now, going from a 3.42 to a 4.10 is a 20% reduction. This equals a 20% increase in RWTQ. It also means that the engine will reach redline at 20% less MPH in EACH gear. So, in comparing identical vehicles except for the final gears, the 4.10 vehicle will have more RWTQ than the 3.42 vehicle until it must shift into second gear. The 3.42 vehicle will then have more RWTQ until it shifts to second at which point the 4.10 takes over, and so on.

NIGHTMARE...2001

:eek: Holy Crapoli!!! :eek: I'm impressed! :flag
(remind me to never play Chess with you!) :leaving:

BlueDragon

This is all really Kewl, but the cold hard fact is that a car with a 3:15 or :42 will pull a certain number on the dyno and when the same car has the rear end changed to a 3:90, :70, or 4:10 The dyno will show a drop in HP and torque, does this mean the dyno is wrong?

Actually the right answer is above in one of the replies! (1G1YY) The dyno is measuring the amount of work the engine is doing.

Confusing isn't it!:jester

Jer


[Modified by BlueDragon, 8:49 PM 1/20/2004]

1g1yy

Yep, you're off on the first gear ratio on the M6. I don't know what it is, but it is much higher (lower numerically) than the A4 first gear which is 3.06 to one. That's why 4.10's work better on an M6 car. :thumbs:

Jbrady

Ok, got off my lazy behind and did a search. Looks like the standard C5 6-speed ratios are:
1st- 2.66
2nd- 1.78
3rd- 1.30
4th- 1.00
5th- 0.75
6th- 0.50

And the Z06 has
1st- 2.97
2nd- 2.07
3rd- 1.43
4th- 1.00
5th- 0.84
6th- 0.56

So the stock C5 multiplies engine torque 9.10 times in first gear and the Z06 multiplies engine torque by 10.16 in first gear.

Midwayman

CJS, I dont think you have a clue what you're talking about when it come to dynos. If they didnt have a way to correct for gearing there is no way they could dyno diffent cars on them. In fact the latest dynojet dynos have automatic gear ratio calculation. In any case you're just proving my point by saying that observation shows the 4.10 losing hp. Basic basic physical dicates that the mechanical advantage of a 4.10 gear would show more power at every rpm than a 3.43. The fact that it doesnt means that the gear ratio is being compensated for and the lost power is something else.


Jbrady has the basic idea right.
higher gearing = more actual power at the wheels

Dyno's display compensated numbers though. (This is why you can compare autos and manuals, etc) They are trying to display Engine hp based on a wheel measurement. Typically they account for gearing, tempeture, barometer, and humidity. (IE SAE hp)

BlueDragon

Mmmm, not trying to be difficult here but I thought Dynos were to measure the RWHP and RWTQ. Not Engine, FWHP and FWTQ. Why would they care about the gearing? there is a certain amount of HP and TQ measured from the center of the wheel to where the tire meets the drum. That is what the dyno measures, nothing more nothing less. I would agree if we were going for FWHP or FWTQ but the dyno only measures at the point of contact (rear tires).

The question at hand is why does the dyno measure less as the gear ratio changes from say a 3:XX to a 4:XX and not more. Seems like it is going the wrong way.

Doesn't the dyno measure acceleration against a known load over a period of time to get the power figures? I think the Accel comes from the tach input, the known load is the drum.

Otherwise tire diameter would have to be figured in as well to arrive at the overall ratio and be correct, and I don't think they enter that.

CJS is right the tested HP always goes down. The question is why?

The only thing I can think of is that the dyno measures the drum rotation over a given amout of time and some how plots that against the tach input.

This means that with the lower ratios, the engine is showing more engine RPMs per drum RPM for a given amount of time (engine winds up faster but the drum turns slower) vers the taller gear set which would have fewer engine RPMs per drum rotation for the same period of time (engine winds up less fast but due to the ratio turns the drum faster)

Interesting delima!:conehead
Jer

CJS

Midwayman, I just know this fact: I had my car tuned and dyno tested at the Vette Doctors (Forum Vendor) and they told me my MEASURED rwhp would be LOWER because I have the 3.90 gears vs the 3.42 stockers.

Every single post I have ever read on this forum ( for the last 3 years) where the question has come up, the poster states that his MEASURED rwhp was less because of his gears being changed to a short gearing.

NeptuneBill, has reported on this consistantly and even worked out figures showing how much rwhp is lost depending on gear ratio. (by memory it is 10 less for 3.73. 15 less for 3.90 and 20 less for 4.10's)

I do not claim to be a dyno expert, I only know want dyno operators have told me, and what I have seen on this forum. And that is, that your rwhp will be measured as LOWER, with shorter gears, on all of the dynos that I have seen reported on this forun.

But I would like to learn more. Do you have any reports of a case of dyno measured rwhp going UP, with shorter gears? If so please post them.

BoostManiac

Gearing will definitely affect your dyno numbers, especially if you have FI, since it loads the turbo/supercharger more. I know of a Supra that picked up an extra 50 RWTQ peak by doing a pull in 5th as opposed to 4th gear, back to back! He only gained about 13 peak RWHP though. The only problem for dynoing in 5th gear for most cars is the drum speed, in his case 185 mph!!! Most dyno operators don't like such high speeds............

BlueDragon

Dyno dumped me at 190 in fourth (A4). Was the safety limit for the dyno.

Jer

Midwayman

CJS, Im not disputeing that you lose HP with gearing on a dyno. I just think you have the wrong idea of why. Its additional frictional loses mainly. (Instead of the driveshaft turning 3.42 times to rotate the wheels once, it now turns 4.10 times)

The fact that you dont see the HP going up is really my point. Bigger gears SHOULD show more torque and HP. Instead 20hp is about 6% loss on a z06 for a gearing change of about 20%(3.42 to 4.10) This means that the gearing is accounted for on the dyno and the 6% loss is other factors. (Much easier to explain than 26% loss, eh? )

1g1yy

:banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghea Some of you don't have a good understanding of what torque and horsepower is. If you had such an understanding you would understand that the questions you keep asking have already been answered. Without a good understanding of, especially, horsepower you cannot understand the answer, and so you keep asking the same question, not realizing it has already been answered. :rolleyes:

CJS

A couple of more points on this discussion, which I have gleaned from the Dynojet website, I suggest you read it yourself though, very interesting.
http://www.dynojet.com/gmhtpmag.shtml

"Once we have the work, we can find horsepower" "To simplify: we get horsepower by multiplying the mass, acceleration and the distance, then dividing that product by time multiplied by 550."( apparently they look for horsepower first, then calculate torque)

Torque can be figured by multiplying the horsepower by a constant, 5252, then dividing that product by the speed at which the thrust force was measured. ( This is how they calculate torque after they have found the Horsepower)

Generally with rear wheel numbers, axle ratio is not considered in the torque computation. For comparison purposes, this makes more sense. The computer factors out the axle ratio by using engine speed data in the torque derivation. (This explains why the dyno operator does not enter rear axle data, the dyno just looks at engine rpm and drum rpm, then calculates horsepower from this)

In the real world, the measurements and computations are not quite that simple, but the complex methods Dynojet Research uses to apply these laws of physics and their mathematics to accurate measurement of rear wheel power is a proprietary secret. (This explains why the rwhp figures are off, they just won't tell us why!)

While Dynojet can measure "coast down" power consumption by a vehicle's powertrain, they cannot accurately measure parasitic loss for the purpose of figuring flywheel power output from rear wheel output Differences in power losses during acceleration and deceleration prevent this. ( this statement says you can't accurately work backwards to find flywheel horsepower)

CJS

Quote: "Basic basic physical dicates that the mechanical advantage of a 4.10 gear would show more power at every rpm than a 3.43. The fact that it doesnt means that----- the gear ratio is being compensated for----- and the lost power is something else."
"Bigger gears SHOULD show more torque and HP. Instead 20hp is about 6% loss on a z06 for a gearing change of about 20% (3.42 to 4.10) This means that the----- gearing is accounted for on the dyno----- and the 6% loss is other factors"

Midwayman, I am trying to understand your point, so let me ask you this question:

1) are you saying that the dyno computer software sees that the gearing of a 4.10 geared car is different than a 3.42 geared car and so the computer takes away the 20% increased "power" ( I assume you mean torque here not horsepower) of the 4.10 geared car to make it more equal to the 3.42 geared car?

2) and then the increased driveline friction and rotational losses of the 4.10 geared car is what accounts for the 6% less rwhp still seen?


[Modified by CJS, 11:27 PM 1/21/2004]


[Modified by CJS, 6:39 AM 1/22/2004]

CCrane65

Ok, my head is spinning. I think I'll stick with computer networks, they're a lot simpler. HA!

JYC5

OK, you guys seem way over my pea brain to understand your calculations, however, if you wanted to determine the best possible gear for the most HP or Torque, wouldn't you think whatever gear you use, should be whatever gear pulls you thru the traps at redline. Obviously, you don't want to be shifting just before the traps, as your HP or Torque will be less than the car is capable of producing, at that time. Clear as mud?

:crazy:

knewblewkorvette

:leaving: :leaving: :leaving: :leaving: :leaving: :leaving:

NIGHTMARE...2001

While I do not have the engineering mind, like some on here :lol: ...I have to admit, I am intrigued with the discussion, and have enjoyed seeing the research information unfold. :yesnod:

Good thread gentlemen! :cheers:

BlueDragon

Zackly, this is the answer I was looking for. Sort of confirms my thoughts on the subject. One could say they measure the amount of work and back into the HP and TQ (at the rear tire) through the math.

jer