[herbz]

I see this going on so much on these forums, and I feel it deserves its own thread. I'm always seeing people say "well car X has a better HP/Weight ratio, so it's clearly faster than car Y." WRONG.

What seemingly everyone forgets about is Gearing and gear ratios most importantly. When combined with other factors the difference is even greater.

Read this site: http://www.welltall.com/ymc/discovery/car/gear.html

Conclusion
So there is a trade-off between maximum speed and acceleration when we are talking about gearing. If you double the gear ratio, you doubled the acceleration but you also halved the maximum speed.

Basically, when comparing two different cars 0-60 or 1/4 times, Horsepower to Weight doesn't mean jack shit when considered alone!

Hopefully this will stop on these forums but I doubt most people have the mental capacitance to understand this concept but oh well.

[lawdude]

I doubt most people have the mental capacitance to understand this concept but oh well

Mental capacitance, hmmmm. Maybe that can be accomplished via a Vulcan mind-meld.

[ynotony]

I am pretty sure most people know this.

[musc]

That's pretty basic, but its incorrect he's incorrect to say if you double the gear ratio, you double the acceleration. Remember that acceleration is change in velocity usually measured in meters per second per second. Velocity is speed with direction. Say I was recording a 400 meter time in my M3 with a final velocity of 106 mph in 13.4 seconds. If I were to double the gear ratio, I would probably have an equal, if not probably lower end velocity with roughly the same or slightly quicker time. If anything, my acceleration would be equal, not doubled.

I can see the point you are trying to make though... gearing has a lot to do with the equation. But the majority of cars have fairly optimal gearing for there engine, particularly when you are talking about performance cars. Most cars could stand a little more gearing if you are trying to optimize the 1/4 mile time.

[teknochild]

where?.... i have not seen people preaching hp/weight ratios on this forum...

thell mention it because most production cars have very similar gearing, but thats all really

[bnj]

Variation in HP/Weight across cars is much more than variation in gearings. Thus it's fair to say that generally in acceleration the hp/weight has a more significant effect than gearing.

That's very true, however, that hp/weight alone does not explain all. There are a lot of other factors like gearing, traction, drag, weight distribution etc. Gearing needs to be matched to the engine power but all manufacturers do this pretty ok. Cars can be kept quite well in the high power band all the time. Wide powerband gets the car to accelerate faster since the average hp-figure during the acceleration might well be higher than that of a car that has higher peak power, but peaky power curve.

The referred site was not the most informative. For example, it did not take into account that it takes some time to shift. There are rally cars that can apply a gearing reducing the top speed even to around 100 mph. However, we do not want to shift our civil trannies the way they shift their heavy duty dog-boxes, real fast click, without clutch.

[Top_Gear]

Different gearing changes the torque-rpm curve but NOT the power. By changing your gear ratio you can increase your torque but will decrease the amount of time you can maintain that torque (i.e., you'll hit engine's redline sooner). So you can get your car out of the blocks sooner than the other guy -assuming no problems with traction- but he will catch up to you when you run out of RPMs and have to upshift sooner than he does.

There is no free meal in gearing (unless you racing somebody over a very short distance.)

[lawdude]

I'm still trying to figure out what "mental capacitance" is.

[SROC3]

its when you place a capacitor inside a person's brain and then try and measure the person's intelligence level - hence, "mental capacitance." This is very common on planet Glornak 5.

[herbz]

Originally Posted by bnj

Variation in HP/Weight across cars is much more than variation in gearings. Thus it's fair to say that generally in acceleration the hp/weight has a more significant effect than gearing.

That's very true, however, that hp/weight alone does not explain all. There are a lot of other factors like gearing, traction, drag, weight distribution etc. Gearing needs to be matched to the engine power but all manufacturers do this pretty ok. Cars can be kept quite well in the high power band all the time. Wide powerband gets the car to accelerate faster since the average hp-figure during the acceleration might well be higher than that of a car that has higher peak power, but peaky power curve.

The referred site was not the most informative. For example, it did not take into account that it takes some time to shift. There are rally cars that can apply a gearing reducing the top speed even to around 100 mph. However, we do not want to shift our civil trannies the way they shift their heavy duty dog-boxes, real fast click, without clutch.

:rocks:

[herbz]

Originally Posted by catm3

That's pretty basic, but its incorrect he's incorrect to say if you double the gear ratio, you double the acceleration. Remember that acceleration is change in velocity usually measured in meters per second per second. Velocity is speed with direction. Say I was recording a 400 meter time in my M3 with a final velocity of 106 mph in 13.4 seconds. If I were to double the gear ratio, I would probably have an equal, if not probably lower end velocity with roughly the same or slightly quicker time. If anything, my acceleration would be equal, not doubled.

I can see the point you are trying to make though... gearing has a lot to do with the equation. But the majority of cars have fairly optimal gearing for there engine, particularly when you are talking about performance cars. Most cars could stand a little more gearing if you are trying to optimize the 1/4 mile time.

true... it's all taken in context... but when you consider other factors like Engine Mods (which can significantly alter the power band), traction (stick tires), and down time between shifts, it can mean up to .5 - 1second time of "variance" between a predicted amount using JUST Hp/Weight.

i agree with your point just expanding on it

[teknochild]

Originally Posted by Top_Gear

Different gearing changes the torque-rpm curve but NOT the power. By changing your gear ratio you can increase your torque but will decrease the amount of time you can maintain that torque (i.e., you'll hit engine's redline sooner). So you can get your car out of the blocks sooner than the other guy -assuming no problems with traction- but he will catch up to you when you run out of RPMs and have to upshift sooner than he does.

There is no free meal in gearing (unless you racing somebody over a very short distance.)

top gear GO AWAY you have already proven to know absolutly nothing about car engines in the other thread, please shoot yourself

[e36jakeo]

weight to power ratio is especially important at lower speeds. At higher speeds drag becomes increasingly important.

Case in point: A Lotus Elise has an impressive 10 lbs/HP ratio, and accelerates like mad from 0-60 (4.4 secs). But given this stellar weight to power ratio, it really slows down above 100 MPH because the drag to power ratio winds up being very high. On a fast race track this becomes very apparent when cars with higher lb/HP ratios catch up and pass on long straights.

The most extreme example of this is motorcycles. Bikes with 4 lbs/HP ratios do 140 MPH in the quarter mile and then slow down and top out at 170-180 due to massive drag for their size and weight. A car with that lb/HP would go 240 MPH with the right gearing (McLaren F1).

[teknochild]

true e36 when you get to high speed weight isnt as much of a factor as air resistance, the lotus engine is a tiny 4 cylinder that doesnt produce enough power to overcome it

kinda like adding another 2000 pounds to the car, it isnt going to effect a huge v8s ratio to much cuz its already heavy but that would half the elise's

[Jonn335]

Torqe at the wheels

As many an old hand will tell you its torque which produces acceleration, not power. power is just figure which is calculated by measuring the torque the engine can produce, and really just tells you how much energy per second is required to maintain the torque which makes the car go faster.
I produced this graph, by multiplying the engine torque by the overall gear ratio in the gear the car will be in at a given speed. I was interested to see how the torque at the wheels compared between the 335i and the E46 M3. The cars are very diferent in terms of their power delivery yet seem to be well matched in terms of performance. It turns out the torque to the wheels from 0 to 100MPH (as on the graph) is very similar.
The M3 and 335i both have fairly flat torque curves, but the M3 torque (and hence power) stay well below the 335i untill 6500 rpm. BUT the lower gearing, coupled with the extra 1000 RPM the M3 is capable off allows the wheel torque to be the same at any given speed. So basicaly you get the same torque to the wheels, but at a higher engine speed.
Looking at the torque and power produced by the M3 engine, and comparing it with the turbo N54 it looks to me like there is huge potential for power output increases at high RPM, from the 335i, i dont think 450WHP is unrealistic with just bolt on mods, and a lot of messing with the managment (cam timing etc). H

[musc]

Very interesting, John335. The Xede allows a sizeable increase in torque without changing gearing. I know some guys with the E46 M3 have moved up to 3.91 and 4.10 gears, and have felt a big difference in the 'feel' of the car, in that it feels like it has more torque.

I'd much rather increase the torque through engine output as opposed to adding more gearing, as there is a point of diminishing returns when adding too much gear. Thanks for posting the graph.

[Jonn335]

I have a 335i on order (will March never come?) and was looking into fitting lower gears myself as 3.08 seemed to be a bit tall. There is so much torque at lower RPM, i don't think i will bother now, unless someone comes in with a rev limiter increase to at least 7500 it isnt worth it

[Jonn335]

this is the same graph, but added is the 335 fitted with 3.64 final drive, same as the E46M3, as someone elses said you dont get something for nothing, it won't actualy make the car quicker overall, just at certain speeds

[BayernMuenchen]

Traction

[Top_Gear]

Originally Posted by Jonn335

as someone elses said you dont get something for nothing, it won't actualy make the car quicker overall, just at certain speeds

Very good set of plots. BTW I would change the above statement a little by saying that the new gear ratio will make the car QUICKER at some speeds but SLOWER at other speeds. It's a good way to shape the torque curve but the net gain is zero.

[teknochild]

Originally Posted by Jonn335

Torqe at the wheels

As many an old hand will tell you its torque which produces acceleration, not power. power is just figure which is calculated by measuring the torque the engine can produce, and really just tells you how much energy per second is required to maintain the torque which makes the car go faster.

im reading the underlined as in order for the car to generate the same amount of torque the engine has to run faster, im sure thats not what u meant though so would you mind rephrasing this?

[JEllis]

Yes, I agree that most people understand there is more to acceleration than HP and weight.

However, I still believe that, for example, the main reason why the 500HP Shelby GT500 is slower than the 500HP Z06 because of weight despite equal amounts of power.

Jason