heinrich

Brendan I WANT IT. My little girl is into Pooh big-time. Also she's grown rather fond of Gamera and company lately ... oh dear, halloween will be difficult. Imagine us cobbling THAT costume together. And Dad is supposed to be Monster Zero aka King Gidorah.

mark kibort

You guys are a funny bunch!

Im just waiting for the lights to come on!

maybe over the beers someday!!

mk

Chazz

Chris, according to my Owners Manual, the best specific fuel consumption occurs at about 3000 engine rpm, so I would not expect my car to have better steady state - 80 mph - fuel consumption as a result of changing to a lower, numerically, rear end. Your car may be tuned differently to produce better specific fuel consumption at lower engine speeds but you really can't generalize on that.

blau928

Chris,

I have to say that a lower RPM point does not equate to better fuel economy. That is entirely related to the state of the engine, its tune, and the amount of load on the engine, aerodynamics, and other factors.

I have noticed, that my S4 gets the best mileage at about 3,000RPM... I had another car that got better mileage at 4000 RPM (It was a GTI 16v) than 2,000 RPM.... All in top gear, cruising on the Hwy....

And as far as MK's numbers go, I also have to agree..... As always, the real answer is "It Depends," on the goal set for the desired applications, and the expected outcome/s.. Science guys, not seat of the pants.... And no, it does not always only relate to road racing.....

Guess I better get into my Nomex Quick...!

HTH

mark kibort

Thanks Richard.
Guys, i dont post that stuff to argue, i post it to help, as the torque/HP/ gearing discussion always is challenging for some intuitively. I do have a lot of exprience on the topic, using it in my work on a day to day basis, and this is why i post the information . (as well as training other in the field). heck if i was "wrong" I would think some much smarter-than-I vp of engineering would have had me fired/ re-educated long ago.

If any of the information is wrong, i would l love to hear about it. REALLY!
Its not about being right, its about all of us having the correct information.

The bottomline, is that gearing changes only the efficiency to which the HP/torque is appliced to the ground. the goal of taking a mass (car) to a speed (mph) over a time, is HP by definintion. Hp is the rate of doing work (Hp= W x time). work is torque over a distance (W=Force X D)and then finally, force (or loosely said as torque, and better said NET force) is mass x acceleration.

so, in looking at what gear box will get us to 100mph quickest, you have to look at the average torque , through the gears over time to that speed. (actually the area under the cumulative torque curves or HP curve) One gear box may favor 100mph as a target speed, and another may favor 115mph. this is just plain fact (physics), one may have a higher or lower numerical final drive and one may have all of its gears shifted lower than the other. Because of the trade offs, no one gear set can be good for all speed targets or ranges.

Ive tried as hard as I can to post all the information i can on the subject, but some will refuse to accept the obvious due to pre-concieved ideas. the best post was vinhue's post of the computer simulatiuon that showed NO difference of the two gear boxes as far as acceleration times for a particular speed range. (GT vs S4, having a 15% gear for gear ratio difference up to 155mph and 4th gear) this was muddled by a Gtech post of a 2:1 reduction on a truck. gear closeness changes the equation. the truck was using 4 gears to get to 45mph or so, while the truck in stock gears had to use only 2. gear closeness also effects efficiency on how the HP/torque is put to the ground.

Gearing doesnt buy you hp and it doesnt assure you get better 1/4mile times if you go up numerically. Hp does this, and more accurately, efficiency of the hp you put to the ground is dictated by the gear ratios and their spacing. Otherwise, we would never even have this discussion, as infinitely variable gear boxes would have the engine run up to max HP and stay there! Right???

and since street driving (by definition ) has to do with infinitely variable speeds and ranges of speeds, its impossible for anyone to say " yes, this GT gear box ratio is the best!") Because, you need to look no farther than 0-60 vs 0-80 or 0-100 vs 0-115 or 0-125 vs 0-155. Both a S4 and a GT gear box will have clear advantages as many times as they have disadvantages.

Ok, onward

MK

blau928

Mark,

You're Welcome..

This reminds me of the other 9 zillion post argument of Centrifugal VS Twin Screw superchargers.. or Air to Air vs Air to Watre Intercoolers...

I think many of the R'Listers and others have preconcieved notions, and forget, or do not realize that the entire thing is about managing a set of variables to a desired or expected outcome.

The airflow, pressure, spark, heat, fuel, stroke, bore, valve size, area, and all the other things are just variables in a very complex equation.... However, all of the variables are solvable, as the science is not new... Material technology and application may be new, but the entire set of variables for an internal combustion engine has been deciphered...

The most overlooked part in my opinion, is the "setting of the Expected outcome" or the design parameters based on intended use, using the 928 platform as a starting point....

Now, I'm getting 2 Nomex 3 suits, 'cause it's really gonna fly....!

heinrich

Mark what is your job? You make some grave and very simple errors below:

HORSEPOWER:
horsepower = (work done per unit of time)
horsepower = (force times distance **divided by** time).
horsepower IS NOT = (W x time).

TORQUE:
torque = (force needed to produce rotational motion) usually measured in foot-pounds
torque = applied force X ((distance from the pivot point to the point where the force is applied).
force IS NOT also known as torque
torque IS NOT also known as "net force"
torque IS NOT (m X a)

WORK:
work = (force times distance).
work IS NOT torque over distance
work IS NOT (T X D)

mark kibort

acutally, its more closely related to the "lighter flywheel and lighter wheels give more rear wheel hp on the dyno!" Notice, i said on the dyno, where acceleration times are usually 7-10 seconds, thus negating most all intended "HP" gains on a dyno. the difference of this analogy, is that there is ALWAYS a gain in HP, its just so small that a dyno wont be able to detect. however, quickly reving the engine can show these gains (ie , sub 1 second raps of the engine from 1000rpm to 5000+rpm) and of course, 1st gear can be effected, but usually, with our cars 1st is just a gear that will spin at any rpm so its just the driver job to keep the wheels from spinning unti he grabs 2nd.
so, in the lighter flywheel, wheel, tire catagory, it also " Depends" it depends on the acceleration rate for which the gains will be realized. 5lbs a tire, 10 lbs for both tires could be 10hp or .0001 hp as a savings. it clearly DEPENDS on how you are using the HP.

mk

heinrich

Are the lights on yet Mark?

BC

Looks like CD creator time on the laptop. Let me see if I can.

heinrich

Thanks Brendan

mark kibort

Semantics,
why are you arguing the obvious.

HP: yes, HP is the rate of doing work. also can be stated, HP=Time X work. (force over distance x time) . definition of HP was derived from 550lbs lifted 1 foot over 1 second. Or WEIGHT X Distance X Time. Your point?
Power can be given in terms of an applied force by : P=F x v or by P= torque x w (angular velocity).

Torque: yes, its a force in a rotational plane. your point. torque can result in a force. you started making the error of not using the correct units. (again semantics that i didnt point out) it would be poundals, slugs, etc for mass)
I used the Term, "net force" due to that would be the force avaible for acceleration. a force without a change in position is just a force with no acceleration. (sitting in a chair, there are two forces acting on each other, with no acceleration, thus 0 net force for acceleration) we all know the force to the rear wheels is greater in 1st than it is in 2nd and so on. so, its the average force that we have to look at over the speed range and distance traveled. It can easily be worked backward to torque. (its like you are talking several differnce languages at once!)
you tell me what you can understand, and we will discuss in the terms you can understand.

so, what is your point?

mk

Now you are trying to read into my statements, made simple for most to understand? I was saying things like " torque over distance" as not to mean torque /distance but torque over a distance (ie T x d)

Really, what do you disagree with when we speak of gearing and the torque /hp to the wheels relationship (and rates of acceleration overall)
0-100mph times, etc?

heinrich

Riiiiiight .... semantics. Sure. So, math according to Kibort:

(time x work) = hp = (work/time)

heinrich

kinda like

(totally misguided) = kibort = (a great physics educator)

heinrich

IS THE LIGHT COMING ON MARK