Drag Racer question (shift-points)

[Anonymous]

How do you determine where the optimal shift-point is in your RPM range?

At the Peak Torque or at Redline? Combination of both?

It seems that if one were to shift their 300 I-6 at it's optimal shift point using torque level (low RPM's), you would have to shift so soon that,depending on rear end gearing the vehicle would barely be moving...

 

[Anonymous]

i'm not sure totally but i know it can be easily tested on a dyno...but i think that u should be able to just give 'er in a gear and see how high the rpms go smoothly before the needle starts to slow...so i guess it would be the highest point that u can rev to before it feels like its lagging...i find that flooring the gas and keeping it in first really gets the car accelerating good but starts to slow down after about 55-60 MPH which is just over 4,000 RPM in first...so i would guess that my optimum shift point would be somewhere around 3500-4000 RPM from first gear...never tried the other gears though...how would u determine the redline of a vehicle? peak torque i dont thing would be the optimal shift point...although peak torque at low RPMs is good i think for a good strong launch...but after that u dont really need too much torque, just good breathing and fuel feeding abilities to keep it going strong...my peak torque is at 1,600 RPM but i know that it would be very bad to shift at that point...even when i shift from fist to second at 50 MPH the acceleration ist as good so the highest RPM possible before power drops is what i need for racing...knocks a few secs off the 1/4 mile as apposed to just keeping it in drive.

 

[Asa]

from all i've heard, you shift a few hundred RPMs above your peak horsepower, that way when you shift down, you are hitting it right as the power peaks

 

[Thad]

This is an over simplification, but
Torque is the force to move an object.
Shift point should be such that the RPM drop after gear change is on the front side of the torque curve before it peaks so the generated force is increasing.

If shift is later and RPM drops to a point on the backside of the torque curve then there the force being generated is diminishing.

This is where gearing gets important, to place the run with in the parameters of the torque curve and the car will be still "pulling" in the lights and not having peaked before then.

 

[xctasy]

Thad is on to it!

Read off the maximum revs at maximum power for your year of car. Then go 15 to 20% over that speed on a stock six. On a hot six, the shift point is about 5 to 10% over the maximum power rev range. The power peaks, and then drops off sharply. There are a few optimum shift sites on the web.

Bowling and Grippo, on the Tech pages, is easiest to use.

For the 300 I6, the power curve drops sharply because its so restricted by the stock carb. If maximum power is 120 hp net at 3500 rpm, then change up at 4200 rpm. (15-20% over the maximum power band)

If there is a 2 or 4-bbl carb and offy intake, the power band may move up just a smidgen to 150 hp at 3800 rpm. The change-up point then stays the same at 4200 rpm cause the power band doesn't drop off like Mount Blanc.

It's a bit long to go into it, but StrangeRanger covered the loss in torque at each gear change in posts in the Hardcore session. 54Ford showed a link to one racers site a while back, which shows the details in graph form on a Formula Ford racer with 116 hp. I covered the details on torque loss or gain per each gearchange on a six cylinder 1984 EFI Falcon about a year ago. I confused it a bit with too much detail. The calculations are very basic rimpull figures used by Catipiller on there earthmovers in the early 60's. You Americans have done all the ground work on this, and I've just summurised it.

If you do the math, you'll find hot cammed cars like 5 to 10% over the power band shifts, stockers with very mild cams have to be wound out well 15% to get sufficent torque gains on each unpshift.

 

[MandarinaRacing]

Here's a calculator I like to play around with, it's practical, but the only way of knowing for sure it's with dyno run to get accurate Hp and torque readings.
http://www.slowgt.com/Calc.htm

Alex

 

[rbohm]

8) if you dont know, or cant afford time on a dyno to find out, you can experiment with shift points. you will need a tach for this. you want to find the point when the engine just starts to nose over during acceleration, this is usually just past your peak hp point, like about 100 rpm. you then have a starting point to work with. start by making consistant runs shifting at that rpm. once you have established a baseline, 5 runs should do nicely, you then vary your shift point, make another 5 runs but add say 500 rpm to your shift point. then make another 5 runs and subtract 500 rpm from your baseline shift point. which ever has the quickest average, is the direction you want to go in. you then have your new baseline, and you raise or lower the shift points by 100-200 rpm at a time to fine tune your shift point.