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      07-13-2011, 12:04 AM   #1
em koop
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Powerband of S54

I recently completed my first track day in my 2008 Z4M coupe. (not my first track day, but first in this car) Tons of fun, no surprise there, but an instructor who rode with me suggested that while my lines were good, I was often in too high of a gear.

Before I went, I read up on the powerband of the engine, and to my surprise, the stock engine appears to make more torque at 4500rpm than it does at say 7000, despite its reputation for being a rev-happy car.

So, unlike a Honda Vtec engine where you really need to stay close to the redline to make max torque, it would seem to me that the car would pull harder in 4th at 5grand than it would in 3rd at 7500, for example.

Or, at least the engine makes more torque at a lower rpm, but perhaps because of the gearing maybe the car would pull harder in 3rd in the example above, even though its making a bit less torque ?

My butt dyno did not feel like it was pulling much harder at 7K than say 5K, but I realize that could be misleading. (unlike the Honda vtec which really does feel more alive right near the top, but it is more "peaky")

Any comments/suggestions from those more familiar with this ? Should I always strive to keep the revs as high as possible, even tho it doesnt feel like it's pulling harder ?
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      07-13-2011, 12:07 AM   #2
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You are correct on where the peak torque is.

For autocross I never get it to redline in first gear. I always shift early and get it into second gear at peak torque so I'm ready for turning and having the torque at it's max.

I hope that makes sense.
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      07-13-2011, 12:24 AM   #3
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I know there's a time/speed benefit to having an engine rev very high even if the torque isn't all there, keeping you in gear longer, as opposed to say a much higher torque diesel engine only revving to 4500rpm
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      07-13-2011, 12:38 AM   #4
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As most engine builders will tell you "RPMs kill engines.". That's not to say the S54 can't handle high revs, however an engine held at 8200rpm vs. one held at 3500rpm will not last nearly as long. Strictly principle at this point since these engines were build to go over a hundred thousand miles, but physics are physics.

Holding at a higher RPM probably isn't the greatest idea, that being said the benefit of using a higher reving motor that still has a linear powerband is to get the most out of your torque curve. While the peake may be at 4500rpm, if you were to shift at 4500pm at the track, you'd be way out of the "sweet spot" in the next gear. By reving the engine out a tad you're able to maintain that peak in the next lower gear. Look at it this way, those areas where you were at 7500rpm and holding, if you where to shift prior you would have been just slightly above the peake torque in the next gear while maximizing acceleration.
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      07-13-2011, 03:06 AM   #5
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Yes, I do understand that to max your acceleration that you dont want to upshift too early, because as you say, the revs will be below 4500 by the time you complete the upshift, so it wont pull as hard as it could. If you want the best 1/4 mile time, shifting closer to the redline is in order.

Road course lapping is bit different I think... at times my instructor was basically suggesting that I should be milking the 7900 redline for all it's worth, and hold lower gears longer instead upshifting/downshifting.

I guess the real question is, I know the S54 makes less torque at 7500 than 5000, but if I'm in 3rd instead of 4th, does the extra acceleration due to the ratio of the lower gear more than compensate for the lower torque due to higher revs?

I guess I was hoping that some hardcore track rat with data acquisiton gear could provide proof that 7500 in 3rd pulls harder than 5000 in 4th (or prove it doesnt...)

Of course, engine life and $$$ is a concern, so I dont drive every single lap as though I'm going for a formula 1 pole position, but it's interesting theory to me.

Thanks !
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      07-13-2011, 04:31 AM   #6
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Horsepower generally leads to faster top speeds, but the part of your powerband that you actually feel is the torque- that's to say when you get thrown back in your seat, you're feeling torque. Torque is the force from the engine that actually makes the change in speed happen. However, in petrol powered engines, more horsepower means more torque. Diesel engines generally have more torque than petrol engine per liter by design.

Torque is pulling power, and it’s best demonstrated as the grunt that gets you moving, while horsepower is what keeps you going.

The horsepower of an engine is how much work the ENGINE can do over time.
Torque, on the other hand, is a measure of how much a force acting on an object causes that object to rotate. It's the pulling power of the engine translated through the transmission.

So both are important. Generally speaking though, if you want more pulling capacity, torque is going to be important. But you aren't going to have much torque without much horsepower. And again, this will all depend on the type of transmission that you have. Hopefully this shed a bit of light on the situation for you.

^ The above was taken from sources online.

With that said, I feel that BMW knows what the eff they're doing, so I'm going to shift around redline if I want best acceleration

In any case, I want a turbo

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      07-13-2011, 09:32 AM   #7
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There is one additional consideration, the mechnical gearing / accelleration
advantage of running in a lower gear. (eg your car will accellerate much faster
in 1st gear vs. 6th gear)

With all high reving N/A motors, I am aware of, this you will be close to or at
readline before the torque to the wheels will exceed the next higher gear or
at redline. To max. acceleration with an S54 you will need to shift at the
highest RPM you are comfortable with. As mentioned earlier higher "piston
speeds" put significant stress on the motor.

Last edited by BookerM3; 07-13-2011 at 07:56 PM.
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      07-13-2011, 09:52 AM   #8
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Quote:
Originally Posted by em koop View Post

I guess the real question is, I know the S54 makes less torque at 7500 than 5000, but if I'm in 3rd instead of 4th, does the extra acceleration due to the ratio of the lower gear more than compensate for the lower torque due to higher revs?

I guess I was hoping that some hardcore track rat with data acquisiton gear could provide proof that 7500 in 3rd pulls harder than 5000 in 4th (or prove it doesnt...)
What you have to take into consideration is the time it takes to shift vs. the difference in torque between 7500rpm and 5000pm, in addition to where you will be downshifting for the next corner, it an additional downshift is worth it. While these cars doesn't have a torque curve like say a Z06, the difference in torque drop off isn't as drastic as say an S2000 where you have to stay above a certain threshold every second to worry too much on the top end. That being said, if I'm coming into a corner and only have to leg it out for a second or two at high RPM to get through it, I'll do that over shift. There's a track here in the South that going 4-5th I cross the start finish line just as I am nearing redline. I'll normally leg it out till right before the limiter hits so I can know consistently that I'm crossing the line before that shift and getting every bit out of each lap.

Being it was your first track day, there is so much more speed to be found, so instead of thinking about it too much and taking a F1 data approach, just get back out there and drive and see what works best.
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      07-13-2011, 01:22 PM   #9
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Quote:
Originally Posted by onelove View Post
What you have to take into consideration is the time it takes to shift vs. the difference in torque between 7500rpm and 5000pm, in addition to where you will be downshifting for the next corner, it an additional downshift is worth it. While these cars doesn't have a torque curve like say a Z06, the difference in torque drop off isn't as drastic as say an S2000 where you have to stay above a certain threshold every second to worry too much on the top end. That being said, if I'm coming into a corner and only have to leg it out for a second or two at high RPM to get through it, I'll do that over shift. There's a track here in the South that going 4-5th I cross the start finish line just as I am nearing redline. I'll normally leg it out till right before the limiter hits so I can know consistently that I'm crossing the line before that shift and getting every bit out of each lap.

Being it was your first track day, there is so much more speed to be found, so instead of thinking about it too much and taking a F1 data approach, just get back out there and drive and see what works best.
This is very good advice. As a novice driver myself I was overly occupied with finding "max power" on a couple of tricky corners at the track I frequent (a crested blind corner and a 90 degree left into some esses). My instuctor told me to leave it in 3rd and focus on improving my braking, line, and throttle rather than finding a gear (2nd) that landed me in the top of th rev band since, just like you said, the car has plenty of power even a bit farther down in the revs range.

I actually found leaving it in 3rd works just fine with these corners as the last thing I needed to worry about was "optimal" revs. Braking, line, smooth out on the throttle--much better than trying to add in yet another variable at this point for me. Maybe at some point in the future there's a couple of seconds to pick up with hitting second, but it's a PITA anyway on those corners and there's a lot more time to be lost on messing up the basics.

Now blasting down a straight is another story and the only consideration there is how much you're willing to beat on the car lap after lap.
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      07-13-2011, 01:40 PM   #10
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Quote:
Originally Posted by BookerM3 View Post
There is one additional consideration, the mechnical gearing / accelleration
advantage of running in a lower gear. (eg your car will accellerate much faster
in 1st gear vs. 6th gear)
This is exactly why I'm asking, the gear can make a big difference. Riding my bike is a good example; I know that my skinny legs can apply the same (tiny) amount of torque against the crank no matter what gear I'm in (until I get tired ). But, if I'm in a lower gear, the bike will accelerate much faster than a higher gear. Cars are more complicated because both the torque and gearing mechanical advantage change in each gear; one goes up and the other goes down. So, at what point is it a net loss or gain ?

Since this not an S2000, and the torque weakens north of 7K, I'm not convinced I really want to keep it up there, for both longevity and performance reasons. No disrespect intended towards my instructor, the rest of the advice he gave me made lots of sense.

It's not my first track day, just the first in this car. Still, I know that being in a marginally sub-optimal gear for a few seconds is not my biggest impediment to my lap times at this point in my skill development, but being a very analytical person, I find this more interesting than the average sane person would... to be honest, I'm not looking for excuses to shift even more as I am finding it hard to heel-toe in this car with the stock pedals.

Last edited by em koop; 07-13-2011 at 01:46 PM.
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      07-13-2011, 02:29 PM   #11
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I was at Watkins Glen on Monday and Tuesday, first time with the Z4MC. I was experimenting with the gear selection for the first few runs. I was all over the place - going to fifth on the front and back straights, jumping down to third on the entry to the Bus Stop, etc. I eventually figured out that you really only needed third and fourth on this track. The rev range was kept between 4500 and 7000; 4500 was about as low as I could go and still get some good punch out of a corner.
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      07-13-2011, 04:51 PM   #12
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Quote:
Originally Posted by Claymore View Post
I was at Watkins Glen on Monday and Tuesday, first time with the Z4MC. I was experimenting with the gear selection for the first few runs. I was all over the place - going to fifth on the front and back straights, jumping down to third on the entry to the Bus Stop, etc. I eventually figured out that you really only needed third and fourth on this track. The rev range was kept between 4500 and 7000; 4500 was about as low as I could go and still get some good punch out of a corner.
True story. The Glen is my favorite track and where I have most of my experience. I think I'm coming up on 30 days, actually!

One thing about gears up there. As you get faster through The Esses, you will need to shift to 5th as you approach the Bus Stop's braking zone. But unlike some, I find a shifting to 4th is ideal for the Bus Stop, Outer Loop combination. Going down to 3rd makes the car a bit skittish and you have to be very smooth with your throttle or you'll upset the balance and swap ends in the Outer Loop.

I ride 4th all the way to the Toe where I go to 3rd. Short shift it into 4th AFTER the car settles/crests the hill and stay in 4th all the way around. Occasionally, I'll jump down to 3rd for Turn 11 if I want to get around someone in the front straight.

I'll be up there in September. You going again?
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      07-13-2011, 05:03 PM   #13
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Quote:
Originally Posted by JCz04Bimmer View Post
True story. The Glen is my favorite track and where I have most of my experience. I think I'm coming up on 30 days, actually!

One thing about gears up there. As you get faster through The Esses, you will need to shift to 5th as you approach the Bus Stop's braking zone. But unlike some, I find a shifting to 4th is ideal for the Bus Stop, Outer Loop combination. Going down to 3rd makes the car a bit skittish and you have to be very smooth with your throttle or you'll upset the balance and swap ends in the Outer Loop.

I ride 4th all the way to the Toe where I go to 3rd. Short shift it into 4th AFTER the car settles/crests the hill and stay in 4th all the way around. Occasionally, I'll jump down to 3rd for Turn 11 if I want to get around someone in the front straight.

I'll be up there in September. You going again?
I think the one thing that's great to learn is that your limits increase as your driving time increases. What works for your first 4-6 track events, will seem like nothing by your 20-30th. As you exit the previous corner quicker, you now enter the next corner quicker and what was once fine, no longer is. That's one of the things that make track driving such a blast, pushing the limits and then relearning them again and again.
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      07-13-2011, 05:24 PM   #14
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Quote:
Originally Posted by JCz04Bimmer View Post
True story. The Glen is my favorite track and where I have most of my experience. I think I'm coming up on 30 days, actually!

One thing about gears up there. As you get faster through The Esses, you will need to shift to 5th as you approach the Bus Stop's braking zone. But unlike some, I find a shifting to 4th is ideal for the Bus Stop, Outer Loop combination. Going down to 3rd makes the car a bit skittish and you have to be very smooth with your throttle or you'll upset the balance and swap ends in the Outer Loop.

I ride 4th all the way to the Toe where I go to 3rd. Short shift it into 4th AFTER the car settles/crests the hill and stay in 4th all the way around. Occasionally, I'll jump down to 3rd for Turn 12 if I want to get around someone in the front straight.

I'll be up there in September. You going again?
Probably won't be back until next year. I'll be running at VIR and possibly NJMP the rest of the season.

With regards to gear selection, I have it in fourth all they way up the esses and into the Bus Stop (back straight speed about 125 until the braking zone, then about 75 at the entry). I use third at the turn-in in the Chute, then up to fourth briefly until the Toe, then third until the track-out there. Back into fourth running up to turn 9, then third through the apex and up the hill into turn 10, fourth coming across the bridge and back on to the straight. Sometimes I'll drop it to third on turn 11 so I can get some power into the straight.
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      07-13-2011, 08:42 PM   #15
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Quote:
Originally Posted by Claymore View Post
I was at Watkins Glen on Monday and Tuesday, first time with the Z4MC. I was experimenting with the gear selection for the first few runs. I was all over the place - going to fifth on the front and back straights, jumping down to third on the entry to the Bus Stop, etc. I eventually figured out that you really only needed third and fourth on this track. The rev range was kept between 4500 and 7000; 4500 was about as low as I could go and still get some good punch out of a corner.
What color was your car? I passed an M when I was leaving the lake Sunday going home. Was it you? I have a boat in Watkins and spend a lot of time there.
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      07-13-2011, 09:22 PM   #16
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What color was your car? I passed an M when I was leaving the lake Sunday going home. Was it you? I have a boat in Watkins and spend a lot of time there.
Black (black sapphire metallic).
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