big dave

Is there a general rule in trying to determine where a car should be shifted for best ets? For instance, a car making its peak horsepower at 6500 rpms, should it be shifted at its peak, or slightly above its peak? What variables need to be taken into account when trying to figure this out? Does the type of transmission and rear gear ratio have to be considered? I see on these boards quite a few guys who rev their cars well past what would be considered peak power for their cam/setup. Are there any advantages to this? On a recent trip to the dyno I found that my combo peaked at a little over 6k, I've been shifting at 6200. I'm changing my valvespings this weekend and should be able to take the rpms a bit higher if necessary. Thanks for any help you guys can give.

96z

It takes around 300-400 RPM to complete the shift. The 2-3 is longer, 400-500 RPM. My car made peak power at 6200 and I had the car programmed to shift at 6100 for 1-2 and 6000 for 2-3. So it really shifted at 6400 or so. You always want to complete the shift after peak power. 200-300 after is what I was always told.

big dave

Thanks for the reply. Why would it take that long to complete a shift? My computer was programmed to shift at 6200 and this is where it shifted. I have the rev limit set at 6400 and it has never touched it...at least not that I can remember.

OldSStroker

Rule of thumb for shift points? Sure. A few thoughts:

1) Torque at the drive wheels accelerates the car. Well, actually thrust at the contact patch but that's just torque at the wheel divided by the rolling radius of the tire.

2) torque at the wheel (or axle) is flywheel torque multiplied by trans ratio multiplied by axle ratio (and some slight converter multiplication at high rpm on an auto) all corrected for driveline losses. Example: 400 lb-ft(max @ say 4500) x 3.06 trans x 3.42 axle x 85% efficiency (just a guess in 1st gear) or about 3558 ft-lb. Let's say this is at peak torque. Let's say you have 400 hp (flywheel) @ 6000. That's 350 ft-lb there. If power drops off to 360 @ 6500, that's 290 ft-lb.

3) So torque at the axles follows ths shape of the engine torque curve (multiplied by the ratios); it rises, peaks, then drops off.
At some mph, the axle torque in second gear will equal the axle torque in first gear (as the engine winds past it's peak). Theoretically this is the shift point.

4) you could plot axle torque vs mph for each gear, or you could use a program like Drag Race analyzer or Performance Analyzer to figure the shift points. They also figure in converter multiplication as well as engine and driveline inertia which is important in the lower gears.

5) If there is enough spread in the gear ratios (like 1st to second in the 4L60), max engine speed may not be high enough for second to have as much axle torque as first. Then you shift at max engine rpm.

6) For most NA street driven engines, optimum shift point works out to 300-600 rpm over power peak. Really it's not all that critical for 1-2 and 2-3, but 3-4 and 4-5(in an M6) if you race to high mph, get critical.

7) Some folks don't accept 1) above. It's based on "F=ma"

My $.02

JordonMusser

tis true, a rule of thumb is 300ish over peak, BUT.. for example, my 401ci motor was shifted 800-900rpm past peak.. because it didn't fall off hardly at all after peak.

the easiest way to look at thing is this.. you want to maximize the HP you can put to the ground. basically, do an integral over the HP curve, starting at the point the revs "drop to" after the shift, and going to the shift point. when this value is maxamized.. this is your shift point.

96z

Try a calculator like this one out. Do a search on a google and a couple pages should come up.

http://www.bgsoflex.com/shifter.html

big dave

Thanks for the replies. Is it correct to assume that the longer you stretch your rpms past peak hp, as long as power doesn't drop off considerably,(assuming only a 10-15 hp loss @ 600 rpms past peak) there is more mph and possibly quick ets to be gained?