shift points

[grumpyvette]

got asked this??

"When you are Drag Racing do you shift at peak torque, peak HP, or another point?"

 

look at this,

http://www.geocities.com/z28esser/ratio.html

 

now ideally you want to shift a a point that will drop your rpm level so that you drop back onto your torque peak, lets take the GM Muncie (M-21, M-22) 2.20 1.64 1.28 1.00 as an example and an engine that has its peak torque at 4000rpm and its peak hp at 5000rpm and a 6500rpm red line, thats a 34% drop from first to second and a 28% drop from 2nd to third and 3rd to fourth gear, so you want to shift from 1st to second as you pass 5400rpm (34%over peak torque)then from 2nd to third and 3rd to fourth as you pass about 5200rpm, this always puts you just past peak hp as you shift but more importantly it drops you on the torque peak each time so you spend almost all the time at above peak torque and near peak hp.

BTW in most cases if your more than 1000rpm past your hp peak when you shift you will be slower , the idea here is to maximize the time you engine spends at the area in your engines torque and hp curves that will provide the most average usefull torque per second.

[Guest kc6wfs]

More GREAT stuff from Grump!!!!!

I got to say I sure like all the info you put up here!!!

 

Thanks Grump!!!!!!!

 

Dave

[Guest Anonymous]

Mr. Grump ,you give this website class, with your posts

[Guest Anonymous]

Agreed, having Grumpy here is like a unending subscription to a great magazine for free, how good a value is that! Thanks for all the great links and info Grumpy.

 

,

 

Lone

[Wagz]

Grumpy, to say that you are a fountain of information is a gross understatement! You're more like Niagra Falls...

 

You need to pull all that vast knowledge you have together and write a book (or two). I'd buy 'em for my garage related library.

 

How 'bout a new section on this site called "Grumpy's knowledge bank and web links"

[Guest Anonymous]

Cool Grumpy!

[utvolman99]

Well, this doesnt bode too well for all of us using or planning to use the 700R4 trans does it? My Desk Top Dyno Numbers tell me I should be making peak Torque somewhere between 4500 - 5000 RPM with the engine Im building. Lets just say 4500 RPM. Using the above thinking this would be my shift points.

 

1st - 2nd 8500 RPM

2nd - 3ed 7290 RPM

3ed - 4th 6429 RPM

 

I would venture to say that after shift #1 I wouldnt have to worry about the rest!

 

For a tamer engine making peak torque at 4000K it would look like this

 

1st - 2nd 7556 RPM

2nd - 3ed 6480 RPM

3ed - 4th 5714 RPM

 

Once again, I dont think you would make it past 1st.

[utvolman99]

Hey Grumpy dont you think you would get better performance if you shifted at a point that would put you before your peak torque RPM? Using the engine Im building on DTD2000 and the GM Muncie (M-21, M-22) as an example. I would think you would want to look at your torque curve and shift at of point that would insure the highest average torque for the whole gear? Here is my torque curve with some interpolated values added.

 

RPM Torque (lb-ft)

2000 313

2500 327

3000 346

3500 373

4000 397

4500 409

5000 411

5500 396

6000 378

6400 351

6500 344

6700 328

7000 304

 

According to the formula above the shift points would be

 

1st-2nd 6700 RPM

2nd-3ed 6400 RPM

3ed-4th 6400 RPM

 

Average torque for each gear

1st = aprox 363 (when launching at 2000 RPM)

2nd = aprox 382

3ed = aprox 382

 

Average torque for each gear when shifting at 6000 RPM

1st = aprox 372

2nd = aprox 398

3ed = aprox 398

 

Dont get me wrong grumpy, Im not trying to call you out or anything. You probably know more about hot rodding than anyone Ive ever talked to. Im just trying to get some good discussion going.

[grumpyvette]

utvolman99 (yes there are exceptions, I knew that when I posted this)

I can see where you could get confused and Im sorry I did not make that clearer,most people let auto transmissions shift themselves so I was not talking about them,I should have removed them from the chart but if you have a manual overide control on your auto trans, normally shifting when your about 500-700rpm past your hp peak will get you in the ballpark, and of course gearing and your engine combo will have some effect here but the main concept is that if your engine spends almost all of its time in an rpm range with the engines torque peak at its low end and the engines hp peak at about 2/3rds of the way up to your shift point you should get the almost ideal torque curve overlaping your transmissions operateing rpm range and yes thats also assumeing a normal engine torque curve where the peak hp is about 1000-1500 higher than the torque peak and the hp peak is someware in the 5500-6500rpm range like your average high performance sbc over the years Ive seen way to many people that ALWAYS take the engine to just under the redline rpm on every shift while racing and in almost every case that slows the car down!

 

utvolman99 thanks for pointing that out as most people don,t take the time to think everything through and Im sure that your bringing that up will help someone.