Ok. I would suggest that it is VERY important that your engine builder understand that you don't want to rev past 6500 rpm. There are good points to keeping the rpm down anyways...better driveability being chief among them.

Again - good luck to you. Sounds like you have a really neat project going.

 

Thanks a lot, Im sure when I call up Joe to buy the cam he and I will have a very lengthy discussion about all this kind of stuff!

 

Are you going to be dynoing the motor without the car? or have a rear end to use in the meantime? just wondering cuz otherwise you couldnt wait till it was done in order to find peak power bc u;ll need the rear already. im in the same boat, tryin to pick the right gear the first time b4 the motor is done.


I think i might be able to explain why you go past peak rpm for shift points, but typing it out make it sound more difficult to understand than it is, so unless you still arent sure, ill save ya from readin that post, let me know. haha

 

No, im not worried about the rear till mid next year, unless it calls upon me sooner , so i will be dynoing the chassis, not the motor. Just want to get ideas beforehand. If you feel like explaining indepth go for it, I will understand it, I was just always told to shift before peak, or maybe im thinking of redline on more stock motors? Aside from that fact, I believe now that you do shift above, but why not fill me in why?

 

ok, from the top:

1) the goal of this application is to minimize E.T.
2) to minimize E.T. you'll need to MAXimize acceleration
3) Accelleration = Force / Mass; so you'll want to MAximize the net force on the car at ALL times, and minimize the mass (obviously).

4) To maximize new force, you need to examine the rear-wheel horsepower generated throughout the run (and w'll ignore negative forces like air friction that are somewhat untouchable beyond redesigning the body, lowering the car, or even have a significant impact at our speeds).

Otto-cycle engines being what they are, you'll need to understand that force will not be constant, but will be affected mostly by RPM. That said, we want to maximize the hp AVERAGE (by time/duration) in the rpm range the engine will see.

During your run, you will experiment with launch RPM's to reduce your 60' times, and your'll want to choose a rear gear that will help you maximize the use of full gearing (use the full gear without hitting red-line, or running at rpms too low). Obviously for engine building you're looking at little impact on launch rpms, and a redline can be anything you choose... just adjust the rear gear latter.

This leaves one important factor in determining the target "power band" when building an engine: Shift Points

Shift Points are a result of tranmission gear ratios, and should be primarily determined by the engine's dyno numbers. Rear gear has no affect here. The best way to develop goals for your "power band" is to determine the transmission you'll be using, and obvious limits to the cost of the engine (i.e. redline).

With a 1994 to 2004 T56 (same ratios as used in all the vipers and non-Z06 C5's), and a 6500 rpm shift point we find that the engine will decrease to the following rpms during an upshift:

1st gear (2.66): N/A
2nd gear (1.78): 4350
3rd gear (1.30): 4747
4th gear (1.00): 5000
5th gear (0.74): 4810
6th gear (0.50): 4392

If you use a '93 T56 (same as the Cobra R and the Z06 through 4th gear) you'll get these:

1st gear (2.97): N/A
2nd gear (2.07): 4530
3rd gear (1.43): 4490
4th gear (1.00): 4545
5th gear (0.80): 5200
6th gear (0.62): 5038

Eitherway, you're looking at a powerband of 4500 to 6500 for the entire run.

5) Look at a dyno graph some time and look at the hp difference between peak hp, and 2000 rpms below it. The lost rpms after a shift is the reason the 4th gen fbody has a topspeed tested in 5th gear. Once you're going 160mph+, a powerloss by shifting to 4400 rpms makes it impossible to accelerate further.

so... look for a hp peak around 5500 to 5800, depending on how the curve starts to decrease after peak hp. Personally I'd shoot for a 6000 rpm peak and dyno the engine to determine the best shift points for each gear (6500 rpms really is arbitrary, you'd be better off running a bit deeper in 1st gear, and a bit less in 3rd to get a more consistant power band).



obviously the BEST way to get your shiftpoints down is to have the engine in hand, built to maximize power over 2000 rpms. pick a target hp, then adjust shift points so that the hp average is higher on average (typically meaning the peak hp is about 1/2 to 2/3 into the power band... but definitely not at the end )

So there is... average hp during the run, choosing shift points by looking at your dyno sheet. There's tweaking to do after that, but that's the bulk of it.

 

Ooh. I couldn't have explained it any better myself. If you use that tranny chart but put the 700R4 gears in there, you can see why it's a bad tranny choice for drag racing.

On the other hand I use a 1.76 first gear powerglide and shift at 7000 rpm. It's like launching that T56 in second gear then shifting to fourth around the 1/8 mile mark and finishing the run in that gear.

 

Dang man, that is awesome, didnt expect that much of an explanation, I wont even have to bring this up again when the engine is done, just dig this up and do the calcs myself, then get some second opinions.

That sounds ideal for a street car to shoot for a 6000rpm peak power point, since most of the power then will be lower, and will pick up stronger at lower Rs. Then I will be able to shift at or around 6500 it looks like.

Anyway, thanks for the opinions guys.

 

yeah thats a long explanation Steve in Seattle , basically what my post was gona look like, 2 which is why i stopped b4 i got a hand cramp haha


in case there is any more doubt, its basically just maximizing your time in the best possible average powerband.

say your peak power is at 6000rpms, but it doesnt drop off much up to 6500. now say you lose 2000rpms per shift ( to make it simple). between 6000 and 6500 you may lose some power, but if the span between 4000 and 4500rpms is a bigger power difference than that seen between 6000 and 6500, then on average you put more power to the ground throughout the run by shifting higher and dropping down to a higher rpm for the next gear.


ex.
4000rpm 400hp
4500rpm 450hp
.....
6000rpm 500hp
6500rpm 475hp

by revving from 6000 to 6500 you gave up 25hp, but once you shift you will be at 450hp instread of 400, a gain of 50. so in the end, it was better to rev higher in this case.

now its just a matter of finding out where your powerband is, what mph and gear you expect to finish the race near, and matching up the rear gearing to help make that possible. no sense in havng a gear too big or too small just to have it, if it doesnt give you the best run (good launch + right gear + max time in powerband)

 

What they said (and great explanation, Steve).