Hello all,

I was wondering if there's any way to tell the optimum shift points for your car based on a rear wheel dyno sheet? Obviously you would want to run through the max torque but after that, horsepower keeps climbing while torque is dropping. I know that torque wins races, so would you want to shift before the torque drops off?

Thanks,

Russ

 

Usually want to shift at about 500 RPMs after the HP starts to drop. Also depends on type of cam.

 

So even though the torque drops off like a rock, you should hold out until the HP drops off?

 

I never have been to drag strip to determine best time based on shift points. I normally listen to the engine and when it sounds like it dropping off I shift usually around over 6000 RPM.

I should also mention that I have a automatic, but when I run fast I shift it manually.

 

My shift points are as 74VETTE indicated, but I did the math to get there. Here's how:

Using your gear ratios for the different gears you need to subtract the ratios, ie. subtract second gear ratio from the first gear ratio to get the difference in ratios. This difference is the percentage of much the shift will cause a drop in RPMs. For example a difference in gear ratios of .16 will result in a 16% (1/6) drop in RPMs. Thus, if you shift at 6,000RPM then the RPMs of the engine after the shift will be 5,000RPM. This is a typical ratio, and it's a good general rule to use 1,000RPM as the amount that will drop after a shift.

With this 1,000RPM drop in mind, you will take a look at the HP curve. For those who think Torque is more important let's look at the two measurements for a moment. Torque is the ability of an engine to accelerate the vehicle by making the wheel spin faster, and is independent of the weight of a vehicle. HP is the ability of an engine to accelerate the mass of a vehicle at a given speed and vehicle weight.

For example, if an engine has high torque, but not much HP (like our stock V8's at around 1500RPM), then popping the clutch will make the vehicle surge forward quickly, but then fall flat on it's face because the engine just can't move the weight of the vehicle. On the other hand if an engine has high HP, but low torque then popping the clutch will make the vehicle moderately accelerate, but keep accelerating since it has the power to move that mass.

OK, with that little info in our brains we realize that we need to look at the HP curve because that is where our vehicle is going to be moving. The best shift will be where the RPMs at the end of the shift will be the same as the RPMs at the beginning...it's just a matter of drawing a line across the HP curve so that the RPM span is 1,000RPM, and you will find the exact RPM where you need to shift.

 

I like that. I knew there was a way to use the HP/TQ curve to determine the shift points.

Thanks,

Russ

 

You know, it's always seemed to me that you want to maximise the area under the Hp curve.
I mean if you take the integral of the hp from the starting RPM for second gear to the Shift RPM for second gear and maximise that you will have optimised the average power available for second gear, right?

Or maybe I've been working with math majors too long...