LVDell

I've only answered this question for my track cars (GT3 and spec boxster) and it is VERY easy to figure out. Print out a copy of your hp/tq curve to see where the "meat" of the power is and then you can determine the range of rpm that is "best".

Then print out a copy of your gear sheet and determine where each shift point will land you in the next gear. Not as easy in the street boxes to hit it perfectly which is why I had the box in my GT3 regeared.

Here's a couple pics of what I am talking about (from my Boxster race car). It shows that the best part of the power band (the meat) is roughly from 4250-5800 rpm. The key, is to not only shift where it drops you in the meat of the band (or as close to it) in the next gear but also not using rpm that is on the decline (6000+ in this example where both hp and tq drop sharply).

Obviously, the optimal shift point is different in each gear but having a good number for all gears (6K in my example) works for all gears. Alternatively, since I have a shift light setup I also have each gear programmed for each own individual optimal shift point.

ivangene

hey hey hey - we'll have none of THAT on this board !!

Dave!

One thing about that is that you've got more power at 6400rpm than you do at 5000rpm, and a lot more than at 4250. I'm not a racer but wouldn't it make more sense to rev out to redline? The gearing is such that there's no point where power at redline is less than power at the rpm of the next gear.

Edit: That was in response to LVDell

More edit: Basically, why not use the rpm range where power is in decline if it's still higher than the power available in the next gear?

LVDell

It's not about hp that you need to take advantage of, it's the torque. And in my example, it was just approximates for sake of example (for all gears). Im my race car, the optimal shift point is actually just a tick under 6200rpm (not redline) if you want exact numbers.

**look at the torque value for 6400 versus 5000**

Dave!

Okay this is getting into a torque vs. power tangent but I was under the belief that peak power is far more important than peak torque. And I know that math doesn't equal the real world but Power = Force * Velocity, where Force includes all drag forces and acceleration.

Shifting at 6000 rpm based on that dyno doesn't even get you to the engine's peak power.

LVDell

It's not getting off on a tq v hp tangent. It's getting into an explanation of WHAT IS OPTIMAL.

Sounds like somebody needs to read up on what hp and tq is

FYI.....you need to re-read my last post. Just a tick under 6200 is the optimal shift point not 6K. 6K was a generalization (and technically the just under 6200 statement since it is specific for each gear). The whole point that should be taken away from this discussion is that to maximize acceleration you want to shift where you are able to take advantage of the available torque. Looking at where you will ned up NOT where you are is the key!

**if we want to get VERY specific (like has been done on my race cars) then you need to factor what the torque is at the entire rpm range for EVERY GEAR. This is a general discussion not specific so I don't think it is appropriate for this thread**

Dave!

I understand what torque and hp are and I know how they're related. But acceleration is directly linked to power and you'd be robbing yourself of power by shifting too early. Simply put the power is greater at 6500rpm than it is at the rpm where the shift would come. With closer gears that wouldn't be the case and you'd want to shift earlier.

LVDell

Apparently you don't understand. I give up.

Dave!

Well you're not exactly saying anything for me to understand. I'm asking why you would plan your shifts around maintaining optimal torque instead of planning shifts around maintaining optimal power when power is directly related to actual acceleration, not torque.

Edit: I completely understand that you plan shifts for where it will put you in the next gear, but I'm saying you should base that on the power curve, and I want to know why you say base it on the torque curve.

LVDell

Actually hp and torque are DIRECTLY related.

Here's a fact to help understand it:
You can accelerate as FAST (read "the same") at two different rpm's in the same gear where the TORQUE is the same. Remember, it's a curve not linear.

This is why HP can rise while torque remains relatively constant. So don't get caught up in the game of what your hp number is.

If we are both accelerating and I shift a few hundred rpm's before redline while you continue on while you lose both torque and hp rapidly, you are losing acceleration (and time) while I have shifted back into the meat of the power power. While you will land in the same range as I did (and dependent on gear you might have a slightly higher hp number, we will have the same torque value and even further, in some gears YOU will have a lower torque value).

I don't have time to sit down and put together a matrix of shifting at redline versus shifting at under 6200 (or whatever each gears optimal range is) right now as I have to finish writing a test to give in a couple hours.

I have done this extensively with my engineer for my race cars so trust me the values are optimal.


FYI....here's a formula to play with different numbers to see the relationship

HP = TQ*RPM/5252

GNR996

Made perfect sense here Dell.

the point I get is you when you shift into the next gear you want to be in the "meat" again so you don't have to wait for the curve to match speed again.

and btw, was a good explanation.

LVDell

Very welcome. It's VERY CLEAR if you have the time to put together a shift point matrix for various rpm's.

mavthenav

do we really care about this thread anymore apart from the redheads...... those pics woke me up.

Dave!

This is just not true. An ap1 S2000 has the same torque at 3500rpm as it does at 8500rpm. They will not be accelerating the same at 3500/8500, it will be accelerating much much harder at 8500rpm because the power is over twice as high at 8500 than it is at 3500.

Just look at an LS2's power curve, if you base shift points on torque alone you'll be shifting at something like 5000-5500 rpm, well below the 6000rpm power peak.

Hell, an LS2 has the same torque at 2300rpm as it does at 6000, definitely won't be pulling as hard at 2300.


Except I will have higher power in the last few hundred rpm than you after you shift, and when I do shift I will come into the next gear at higher rpm and power. That power will translate to more acceleration. This is a general statement since I don't know the specifics for your car, but if you were shifting from 4250-5800, the meat of your torque curve, and I was shifting based on the meat of the power curve I would be significantly faster (I know you're not shifting at 5800).

I'm not saying your shift points are wrong, I'm just asking about the reasoning.

Edit: I understand completely the torque/power relationship. Completely. I also understand that acceleration = power / (mass*velocity) (Not counting for drag forces there but it doesn't change the acceleration/power relationship, just the coefficients).

Edit 2: I also want to make it very clear that I know you're timing shifts for where you'll be in the next gear; you're just doing it based on torque and I'd do it based on power.

Sneaky Pete