Newbie question

[DSC]

A question I get asked a good ammount (and never really have an answer for) is the reason for high revs. If you have a 2000rpm powerband that produces the same amount of power, why does it matter how many rpm's you get the power?

powerband of 8000-10,000
vs.
powerband of 4000-6000

Why would one be better than the other?

[96SEChick]

Hmmmm.....very interesting question. I tried to think of reasons for this, and couldn't come up with anything. I'm very curious to know about this as well.

[DamnedButDetermined]

Originally posted by DSC
A question I get asked a good ammount (and never really have an answer for) is the reason for high revs. If you have a 2000rpm powerband that produces the same amount of power, why does it matter how many rpm's you get the power?

powerband of 8000-10,000
vs.
powerband of 4000-6000

Why would one be better than the other?

I am sure someone else can answer this question alot better than I, but here i go!

When you have more revs to play with, you can have a higher top speed. If your horesepower is higher up in the powerband, you can conserve gas by keeping the revs low around town (but this obviously isn't relivent in this forum). Well that is all i can think of right now. Let me think about it a little more!

[240racer]

think about how many rpms your tires are spinning at any given rpm. Lets say we have 24" tires (we actually have 25", but it makes it easier) If we are going 88ft/sec (60mph) and the circumference of the tire is pi*D then it's 2*pi (in feet) which is 6.28ft. and we are going 88ft/sec so that's 14.01 rotations (or circumferences) per sec. Which equals 840.76 rot/min (rpm). that means in order to match 840 rpm with an engine rpm that's higher then that, you need gears. Gears do two things at the same time, change the speed (rpm) and change the torque. If the gears are lower then the speed is reduced and the torque is increased. If the gears are higher, the speed is increased and the torque is decreased. HP does not changed through gears (neglecting friction) remember HP=(T*RPM)/5252 if you imagine that torque increases and rpm's decrease, then you can see that they cancel each other out and HP stays the same.

Now picture having a power band from 8,000-10,000 at 60 mph you could use overall gears (including trans and rear end) of 10:1
which means that your speed is reduced tenfold and the torque is increased tenfold. So if you are making 200ft-lbs of torque at 8400 rpm with 10:1 gears, then you are making 2000ft-lbs of torque at the wheels and they are spinning at 840rpm.
Now if you have a power band from 4,000-6000 rpm, then you would need a overall gear ratio of about 6:1 which means the wheels spin 6 times slower and they have 6 times the torque of the engine. So that would still be 60mph at 5000 (engine)rpm and if you were making the same 200ft-lbs of torque at the crank, now you multiply that by 6 to get 1200 ft-lbs of torque at the wheels at the same 840rpm.
to convert to thrust, which is really all we care about, since that is what pushes the car down the road. you divide by the radius of the wheel, which I conviently chose as 1ft, so it's the same number. that means if you make 200ft-lbs of torque at the crank at 8400 rpm, then you will make 2000lbs of thrust at 60mph with 10:1 gears. Also, if you make 200ft-lbs of torque at the crank at 5000rpm, then you will make 1200lbs of thrust at 60mph with 6:1 gears.
Hopefully you can see that the higher thrust comes from the engine that is making more power. 200ft-lbs of torque at 8400 rpm is 320hp and 200ft-lbs of torque at 5000rpm is 190hp.
you can do the calculations at the wheels too, it works the same.
I said that the wheels spin at 840 rpms at 60mph. So then you enter in the torque at the wheels to the equation and you see that 2000ft-lbs at 840 rpm equals 320hp and 1200ft-lbs at 840 rpm equals 190hp.

[96SEChick]

OMG--I think my head's going to explode over this. Thank you very much for your time and knowledge, but I'll admit--I don't understand it

[bbp]

holy **** 240racer! I need a drink after that!

I don't have the scientific answer but... generally lower evving engines have a longer stroke, which in turn they have a little higher displacement, which in turn creates more torque.

High revving engines, like the s2k, have a shorter stroke and lack the torque. they develop torque through the high revs of the engine. the torques are low revs is very low, until they spool up a bit.

i don't know that one is better than the other, depends on application. Ever drive a diesel truck? Very low revs, monster torque. In sports cars it seems easier to drive with higher revs because it gives you flexibility with shifting and such.

high rev=short stroke
low rev=long stroke

i personally prefer the long stroke mmmmmmmmmmm stroke

[96SEChick]

[butthead]huhhhuhhhuhhhuhhh....He said stroke[/butthead]

Sorry.....really long day at work.

[wherezmytofu]

application i guess, in top speeding it would be a must so u can reach higher speeds...since gear wise top speed is dependent on how high the engine can rev, in track it is done because most are smal displacement enignes and to get the nessisary hp from them they need to rev quite high

[tnord]

i would think that the inability to reach high engine speeds does not necessarily equate with the inability for the car itself to reach high speeds, as low engine speeds can be compensated for with gearing.

i'm gonna go out on a limb and say that an engine with higher RPM capabilities have better throttle response. my reasoning being such......

objects in motion tend to stay in motion correct? well, assuming equal displacements (a big leap of faith i know), a low RPM engine is not moving air as quickly as one that has a much shorter combustion cycle (is that a proper term?). so when the high reving engine stabs the go pedal, it already has air moving at a very rapid pace, which is easier to get moving again. of course........this could all be relative and i'm really not making any sense, it's just an idea.

[SR20Fastback]

Originally posted by tnord
i would think that the inability to reach high engine speeds does not necessarily equate with the inability for the car itself to reach high speeds, as low engine speeds can be compensated for with gearing.

i'm gonna go out on a limb and say that an engine with higher RPM capabilities have better throttle response. my reasoning being such......

objects in motion tend to stay in motion correct? well, assuming equal displacements (a big leap of faith i know), a low RPM engine is not moving air as quickly as one that has a much shorter combustion cycle (is that a proper term?). so when the high reving engine stabs the go pedal, it already has air moving at a very rapid pace, which is easier to get moving again. of course........this could all be relative and i'm really not making any sense, it's just an idea.

I think you just may be right


Edit: I know I know, another comment from the peanut gallery.

[DamnedButDetermined]

Originally posted by tnord
i would think that the inability to reach high engine speeds does not necessarily equate with the inability for the car itself to reach high speeds, as low engine speeds can be compensated for with gearing.

Very true, but a high revving engine will always have a higher top speed than a low revving engine with the same gearset. That is assumming, that the engine has more power that then air resistance.

I didn't completely understand everything 240racer said, but he seems to know his shizot!

SPECIAL REQUEST: 240racer, Please explain your poste a little bit more.

thanks,
DbD


P.S. Very good question DSC

[240racer]

Originally posted by DamnedButDetermined
Very true, but a high revving engine will always have a higher top speed than a low revving engine with the same gearset. That is assumming, that the engine has more power that then air resistance.

I didn't completely understand everything 240racer said, but he seems to know his shizot!

SPECIAL REQUEST: 240racer, Please explain your poste a little bit more.

thanks,
DbD


P.S. Very good question DSC

There is one thing I forgot, I didn't answer the question correctly. DSC asked what the difference was if they had the same power, and I kinda went with the assumption that the two hypothetical engines had the same torque at the two different rpm ranges. So I'll explain my post in a minute but let me first try to answer his question assuming we had two engines making similar power at those rpms.
The main problem with having only a 2000rpm band at the higher rpm is that it is less percent difference, so it's actually a narrower powerband. This means that you would need a closer ratio transmission. The other thing comes down to cost. The lighter pistons that are required to run higher rpms with similar stroke cost more. You could lower the stroke, but that would cause a reduction at all rpms, not just lower ones. However, if you had a higher compression ratio (either through forced induction or static compression ratio change) then you would make more torque at any given rpm and you could lower the stroke without sacrificing torque loss. Then you could raise rpm's since there is less reciprocating inetria, and if you could keep the torque up, then you could make more power.

[nrcooled]

Uhhhhh...I like cars too

Great post! Learning on the internet is almost as fun as looking at pretty women on the internet ::almost::

[HippoSleek]

240racer - appreciated. I couldn't choke out an explanation even though I knew about the concept.

Travis- you theory has two holes: 1) gearing can compensate (as you noted for top speed) and 2) torque. Torque and hp equate to throttle response. Punch a high revver down low (2500) and you get nothing. Punch a KA down very low (2500) and you get very little. Punch a KA in the middle (4500) and it's squirt and go. Punch a veetech in the middle and it lugs (4500). Punch a KA up top (6500) - nothing. Vteq up top (6500) and it goes.

Basically, after the bore, stroke, and head are done defining the power band, the car will have response there - and mostly there. While gearing and tuning can lessen or heighten this, a car has a range where hp and tq allow it to have quick response to throttle applications. Outside the zone, it just sucks.

[uiuc240]

A question I get asked a good ammount (and never really have an answer for) is the reason for high revs. If you have a 2000rpm powerband that produces the same amount of power, why does it matter how many rpm's you get the power?

powerband of 8000-10,000
vs.
powerband of 4000-6000

Why would one be better than the other?

240racer said it: percentage. That's the simple answer. A powerband from 4000-6000 is *longer* than a powerband from 8000-10000.

However, I'd like to add some other things for argument's sake. The engine with the higher powerband is also going to experience MUCH higher piston speeds, therefore increasing wear on internal parts, and decreasing reliability (more friction, more heat, less time between power pulses, etc, etc.)

F1 engines rev REALLY high because that's the most effective way to make NA power if restricted by displacement (simple math...more rpm = more hp). But, because of this, the engines barely make one race (however, that's not saying much, since neither do NASCAR, Indy, etc. cars).

But to answer the *original* question....neither is "better." If you have the right gears, the shorter band (up high) is great...and good for controlling wheelspin and power delivery. If you have less gears, the wider, lower powerband is better. Point is, if you change the powerband, it's WISE to change the gears. Hence the reason I want a J30 3.916:1 rear end....

Eric