Correct shift points for max performance

[ilyack]

Hi everyone

I have a question about finding the correct shift points for maximum performance (in case of straight line acceleration)
I have a Mustang V6, and the the engine specifications are as follows:

193 HP @ 5,500 RPM
225 TRQ @ 2,800 RPM

Is there any way to derive the correct gear shift points from that ?

I heard that in high-performance driving whenever I shift I should do it so that the RPM in the gear I shift into would be the peak-torque RPM. So every time I shift, the RPM should end up at 2800 after the shift.

Is that true ?

Or should it be at the peak-power RPM?

The problem is that every time I over-revv it, I spin the tires, especially shifting into 1st, and from 1st to 2nd gears. And every time I spin the tires - I loose traction and thus loose speed.

I guess I could go with wider rear wheels for more traction, but that's sort of outside the topic right now. I'm trying to work with the current setup and the standard 225/55R16 wheels.

I did some calculations, and this is what it comes down to:
If you want the RPM to be 2800 (i.e. the peak-torque RPM) after each shift, then these are the shift points:

1->2 4714 PRM or 53.0 km/h (32.9 mph)
2->3 4190 RPM or 79.4 km/h (49.3 mph)
3->4 3724 RPM or 105 km/h (65.6 mph)
4->5 4118 RPM or 155 km/h (96.4 mph)

These numbers seem fairly reasonable to me.
The middle RPM between the peak toque and the peak power is 4150 RPM (simply an average), which is near the above numbers.

Does this all make any sense ?

Thanks
Ilya.

 

[Tmoose]

Assuming the specs are right, When you are at the torque peak (225 lb-ft @ 2800) you are making 120 HP. Why would you want to start the acceleration process in each gear with a mere 120 HP engine?

I'd suggest forget fretting about peak torque, except >possibly< when bench racing. When you shift you want to develop as much HP as possible, unless there are traction issues. Usually this means revving slightly above the power peak, unless that means you will hit the rev-limiter or exceed red-line. Best results will be determined with you driving your car through a hundred carefully monitored tests runs and a G-analyst or at the drag-strip. Test each shift point separately. Shift points determined with Calculators, newgroups, lengthy bar conversations, even when mixed with beer will probably NOT produce the best times.

You may need to try a low rpm (3000 rpm) launch and a a "short" 1-2 shift 4500, then rev the other gears to

 

[ilyack]

Makes sense ...

Thanks,
Ilya.

 

[patprimmer]

What Tmoose is saying is essentially correct.

To get the most out of the engine, you need to keep the engine in the area where you keep the maximum area under the HP curve.

If you can get a graph of the power curve, with rev limiter, redline and valve train separation points, then if you study the gear ratios, you can determine what rpm to shift at to maximise the area under the power curve, while ensuring you don't over rev.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MaxRaceSoftware]

for a real simple example at the DragStrip
shifting approx 300 to 500 RPMs above the point of Peak HP RPM usually in 90+ PerCent of cases gives the best ET

that would at least give you a starting point
from there you could refine the exact shift points that would result in the lowest ET..and an onboard data recorder or PlayBack capability Tach would help greatly


Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[Tmoose]

Oh, and be sure to tell your buddies the reason you go so fast is because you use the ~4000 rpm shift points mentioned in your first post that plonk the engine revs right at peak torque.

 

[harrisj]

On all 'sporty' auto transmissions that I've driven, using kickdown redlines the engine in all gears to give maximum acceleration.

This should tell us something. It's torque at the wheels that moves the car, not engine torque. And a higher gear diminishes torque more than dropping back down the engine torque curve, in most engines. I guess I'm repeating what patprimmer was saying more elegantly than I can.

John

 

[NormPeterson]

I attempted a little curve-fitting with those torque and power data, and first assumed a 6000 rpm redline. In that case, you should be making all your shifts at 6000. Even in the closest jump in ratio, which comes in the 3-4 shift, the power at the RPM that you drop back to is less than that which is available at 6000. Hence you only lose by short-shifting. If your redline were 6250, the shifts would probably want to be at 6250, 6250, and 6000 (although I don't think you'll be using 4th in the 1/4 with only 193 BHP). If you could justify using 6500 RPM, then you'd use 6500, 6250, and 6000. With this curve-fit, anyway.

The significance of peak torque RPM is that that is the RPM at which maximum acceleration is attained in any particular gear (approximately so, considering nonlinear drag). This has no connection to the maximum possible acceleration at that particular road speed.

Norm