A.Wayne

Yeah i know the typical web forum thing , i think it is called Collective Ignorance, A question is asked and if you agree with the collective answer then you are good if you disagree then you are bad. THE FACT that you dissagree with my statement does not make it incorrect ! enough said ! less move on.

johne

Let me give it a shot.

All that matters is the amount of hp being applied to your wheels over the rpm range being driven. Here is a picture that may help show that two cars with identically shaped HP curves with appropriate gearing will have the exact same hp under the curve. Assuming that the two cars have identical weights and have no traction issues they well accelerate exactly the same. This is in spite of the fact that their torque curves are very different. The reason for this is that car with the higher revving engine will be geared twice as low. Therefore as far as the wheels are concerned the two cars are always receiving the same power.





I am actually considering buying a honda s2000 or a bmw z3 2.8 liter. The s2000 has 240hp and 153 lbft of torque. The z3 has 190 hp and 205 lbft of torque. So I had to make the decision over which one is faster. Well I did some calculations and explained it to the cute girl in my avitar as such:

well it's quite tricky to explain, especially w/o pictures but I'll give it a shot.
basically, all that matters is the hp that your car is experiencing at any given instance. That is what produces acceleration and allows a car to maintain a speed. What makes it tricky is that a car's hp is not constant by any means in fact it is very un-constant. So what matters is your average hp within the RPM band that you are driving in any given situation. For example in city driving you usually drive between 2000 and 3500 rpm, you accelerate to 3500rpm then you shift and your RPM's fall back down to 2000 and then you accelerate up to 3500 rpm then shift again. Whereas on the racetrack, if your car redlines at 7000rpm, then you drive between 4500 and 7000 rpm. So what really matters is your average hp within the rpm band you drive in. Most people focus on peak hp, or peak torque. These only matter when you are at those specific rpm.

Here’s some rough figures that I calculated for a 2.8 liter z3, a 2.8 liter z3 after 1999, and a Honda s2000.
city driving driving at lower “cruising” rpm’s
sporty city driving within the middle rpm range of the car
sporty driving the car at higher rpms with no risk of hitting the rev limiter
track driving the car at the highest rpm’s possible without hitting the rev limiter.

Average horsepower z3 2.8 s2000
city driving 122 107.6
sporty city 141 125.3
sporty 184.5 197.3
track 189.6 224.7

As you can see the z3 has more power during both city driving and sporty city driving. This is because it produces more power at the lower rpm’s via the larger amount of torque. However the z3 does not produce as much horsepower higher in the rpm band. So during racing style driving (redlining every shift) it has lower average hp.

Hope this helps you guys some.

-John

Chris White

I still have my headache….but if you folks still want to take a shot at it - don’t forget about that little torque multiplier in the back of the car – that’s where the effective rpm range of the power band makes the difference.

Chris White

johne

Chis,
Yeah, that's part of the point of my post. That even if your hp curve goes over a completely different rpm range, the gearing of the car is an equalizer. So if the HP curve is spread out over double the rpm range, you simple gear the car half as tall. Then the two cars will have identically acceleration characteristics. Of course the one car will be at twice the RPM which some people might find a bit "buzzy." lol

-John

streckfu's

The original post had identical gearing as a given (as well as weight, etc).

johne

Streckfu's951,
Yes that sounds like it solves the problem, unfortunately it just oversimplifies it. If the gearing is ideal for one car, and not ideal of the other and they have similar HP curves (not torque curves). The car with the ideal gearing will dominate. If they have gearing that is half way between each car's ideal gear ratios then they will perform identically. But in reality you should always gear a car according to its powerband. Another way of saying this is that you should give a car gear ratio's that will allow it to stay in the high horsepower region of it's rpm range.

In my first example about that would mean that if you shift at 5k rpm then your next gear would be at 3k. For the second example when you shift at 10k your next gear would be at 6k.

To clarify in my above picture, if the first car is traveling at 20 mph in first gear and that equates to 5k rpm, in the second car traveling at 20 mpg in first gear you would be at 10k rpm.

TurboTommy

johne,
that's also a good explanation, what you have there.

Basically that's what I said all along:
Gearing is the essential ingredient to bringing to different hp curves to the same page.
One can keep saying that the gearing is not supposed to be part of this comparison. But, that's impossible, because two different torque curves will never have the same gearing!

Understanding how gearing works is actually the key. After that, everything else just falls into place.
Those of you that played with meccano when you were kids should know what I'm talking about! LOL

T. Wayne
"Yeah i know the typical web forum thing , i think it is called Collective Ignorance, A question is asked and if you agree with the collective answer then you are good if you disagree then you are bad"

Yep, totally agree with you on that!

But, that's not me!
If it was, I would have agreed with you on this, because you generally speak with confidence and have given interesting info in the past.
This, however, you didn't say quite right!. Simple as that.

A.Wayne

In a nut shell this is what i said and mean't , you do not get one with out another as it is trinomial . to get hp you need torque that is academic and no need to be a echo about it as everyone involved in the discussion should know that by now . Those in the know understands that 600 bhp @ 6000 RPM CANNOT IN ANY WAY OR FORM be as fast as 600 bhp @ 9000 RPM IT IS THE Reason why every race engineer in the world looks for more RPM , Hence my statement HP with big RPM, I'm not going to debate this , call Ferrari or Porsche, as this is the bases for all mighty Pumps to increase it's output.

The FIA has made restrictors mandatory at all levels and forms of engines in motorsport...they work mostly due to the fact, that they limit the RPM on the engines. Our FIA Prepared EVO's use a 52 mm restrictor in front of the turbo which limits us to 7800 rpm before the power falls, without the restrictor we made 50 bhp more and could spin to 9000 rpm , the difference in lap times is well ,big , it was , what they wanted and it does equalize the cars turbo vs N/A
So by capping the rpm more boost produces more torque but not any more Hp at the top so with less rev's , less HP , but more torque, slower lap times , less MPH for everybody .
THat is how they contain the speeds and it is how nascar contain the speeds by restricting RPM via Restrictors. SO if you decrease RPM and now have to increase the gearing to get back to 200 mph it takes you for ever to get there , Hence you can run to the fridge and grab a beer and get back to your seat long before a car leaving pit lane on a superspeedway can get back to race pace
BUTT Slow accell. tall gearing , restricted top end HP no rev's.

So Hp/Torque : My Answer again ! GIVE ME THE HP ALL THE TIME AND I WANT BIG REVS , ALOT OF IT F1 TYPE OF REVS , GIMME, GIMME ! PHEW OKAY TIRED NOW , Pheew! got to get some rest this is wearing

www.tpgproducts.com.

TurboTommy

See now,
if you would've said, "600 ftlb.of torque @ 6000 RPM CANNOT IN ANY WAY OR FORM be as fast as 600 ft lb. @ 9000 RPM",
then you would've said it right!
Check out the formula. Think for a moment!. Don't be so stubborn! LOL

Duke

I can't believe this thread is still going on

A.Wayne

Tommy , dude i don't know what to say !

lleroyb

A friend sent this to me after a similar discussion. I helped me a great deal.

LOU