dj55b

iTrader: (2)

I'm just trying to learn all this stuff, from the given dyno below of my car (84 GSL) how do I know at what given rpm each gear is my power range? The run in the picture was done in 4th gear so i'm guessing for that particular gear those would my numbers.

Eagle7

I'm not sure your dyno run went high enough to give the "right" answer to this question. You need to know your transmission gear ratios and calculate the correlating RPM change when you shift from one gear to the next. As you upshift from one gear to the next, you will drop from RPM Rx to Ry. Draw vertical lines on the dyno chart at Rx and Ry, and see how much area is under the HP curve between the vertical lines. Choose a different Rx, calculate the corresponding Ry, and compare the area under the HP curve. Choose the Rx that gives the highest area under the curve. The Rx for one gear becomes the Ry for the next gear. You may end up compromising one gear's shift point to improve the lower portion of the next gear.

dj55b

iTrader: (2)

alright ... still lost on what was mentioned above ... as for the run not going high enough ... it starts dropping after around that point so it was no need to keep on revving it that high

tims

basicly what he is saying. Is to keep the engine operating at the max torque and HP all the time. so if you shift a 5000rpm how many rpm will the engine lose when shifted to the next higher gear? and what is the area under the curve and if I move the shift point up 500 rpm what is the new area. best number wins. with this graph I would start at 7000.

dj55b

iTrader: (2)

so I'm should be looking at the graph and go at lets say 100rpm interval and add up my vertical line for HP and vertical line for TQ and them both up for that particular rpm, start from 7000 and work my way down, the highest numbers in there are my shift points,

dj55b

iTrader: (2)

One more thing though where does the gear ratio fit in there? I found out the gear ratios

1 - 3.674
2 - 2.217
3 - 1.432
4 - 1.000
5 - 0.825

That should be about correct.

tims

with the tire size calculate the rpm in each gear and plot it on the dyno graph and see what looks best.

dj55b

iTrader: (2)

Tire size? How does that play in there? and how am I calculation that? Am I taking the tire circumfrance size?

Eagle7

Not torque, just HP.

Suppose you shift from 2nd to 3rd at 7000 RPM. That would drop you to 4521 RPM (7000*1.432/2.217). Your HP dropped from 147 to about 105.

Now suppose your dyno plot had run out to 8000 RPM at 130 HP. You shift from 2nd to 3rd at 8000 RPM. That would drop you to 5167 RPM. Your HP dropped from 130 to about 120, and you've got a big fat peak in the middle. You went past the HP peak to pick up more area under the curve.

Repeat the exercise for all your gears, remembering that the shift point for one gear determines the starting point for the next gear.

dj55b

iTrader: (2)

ahh ... thanks alot ... thats all i needed to see one equation

anewconvert

iTrader: (1)

tires size can change your effective gearing. Smaller wheel/tires cover less distance in a given rotation, so they result in lower speed at a givien RPM than a larger wheel/tire combination. You have to know your wheel tire size to know your true rpm in a given gear. that being said its really only important if you switch tires or wheels because otherwise its just multiplying by 1 when comparing different RPMs.


BC

dj55b

iTrader: (2)

well my rims and tire combo basically add up to the same as stock with the exception that it is 6" wide instead of 5.5 but i don't see width taking effect on rpm as circumfrance would

Eagle7

Tire size and diff gearing has nothing to the change in RPM when shifting, only what the resulting speeds are. At any particular track, a change in effective gearing may move your optimal shift points to more desirable positions on the track, or eliminate some shifts.

anewconvert

iTrader: (1)

exactly


BC

wrankin

Umm, did you get that backwards? Torque is what will accelerate the car, not HP. Essentially you want to compute the point where your shift will take your RPMs back to a point on the graph where you have as much, if not more torque than before the shift. From a casual glance at the dyno chart, it looks like a shift point around 6500 should be good enough. Torque is starting to roll off there.

-bill

Eagle7

No, your engine torque gets multiplied by the gearing to produce wheel torque. When you upshift you're picking a smaller multiplier. HP is what you want. And you want to go past the HP peak so that when you upshift it doesn't drop off so much. Maximize the area under the curve.

P.S. Carroll Smith's "Drive To Win" (highly recommended) - "For maximum performance you always have to shift past the peak of the horsepower curve." pg. 2-45.

wrankin

Ahh thanks, Marty. I knew that I was forgetting something in there.

-b

speedturn

iTrader: (1)

The torque numbers are all you need to look at.

Engine torque x trans gear ratio = torque delivered to propel the car forward

for each potential up shift point, compare the torque delivered out the trans

lets look at the 3 to 4 shift with your wide ratio tranny
at 7000 rpm in 3rd, you have 110 ft-lb x 1.432 = 157 ft-lbf torque out of tranny
you shift to 4th you drop to 4900 rpm
at 4900 rpm in 4th, you have 123 ft-lb x 1.00 ratio = 132 ft-lbf torque out of tranny

your car will accelerate slower after you do the upshift, because there is less torque propelling the car forward. Like eagle7 said, you should have made your dyno run out to higher rpms, since your engine will not be hurt by rpm until you exceed 8000 rpm. If you get more torque info at higher rpms, you will probably see that you can shift at even higher rpms and maintain more forward acceleration.

This is why race cars run very close ratio transmissions, and why they always operate on the flat or declining side of the torque curve.

KevinK2

You can look at either hp, or torque out the trans, for shift points, as others have said.

The posted curve would have you shift at the max rpm on the curve, 7300, for all gears.

You should have run it out to near redline. ASSUME you did and got 124 hp at 7670, and 107 at 8000. New shift points:

1-2, 7700
2-3, 7900
3-4, 7700
4-5, 7300

Gains would not be great, except for 3rd to 4th where there is some time for payback. 3rd at 7300 rpm has 15% more wheel torque than 4th, so you hold on for 8% more torque at 7500, and shift at the equal-torque point ( hp or wheel-tq curves crossing ) at 7700 rpm.