Optimum Shift Points

[ralphinnj]

We all know from looking at dyno graph's that maximum torque occurs at much lower RPM's than maximum HP. Something like 2000-2500 RPM for torque and as much as 1000 RPM higher for HP. Since torque is what provides propulsion, why do most programs, including stock, shift at RPM's more consistent with maximum HP? That is, wouldn't you want to run as close to maximum torque throughout a quarter mile run?

Ralph

[UNBROKEN]

IMHO...if you shift that low...where the TQ peak is...you'll never stay at that peak TQ point. You'll get under the turbo and it'll fall on it's face. It takes some RPM to keep things going in the right direction.

[ralphinnj]

I agree with not shifting at the peak, but one should shift "centered" around the peak. So if each gear costs you a total of 700RPM (just a guess), then you would shift at 350RPM above torque peak. I mentioned this to Gene about a year ago and he said they had experimented with lower shifting and got better times. Many people confuse lots of noise with more power. Lots of noise (other than being lots of noise) is also power being transformed into noise, and not traction. I don't know anyone who has done a methodical study of shift points.

Ralph

[UNBROKEN]

Me either Ralph...and I agree on the lowered shift points. But if I remember right they were talking about down to 3850 or so instead of 4100+.

[TKOPerformance]

This is drag racing 101 stuff. The gas guys figured this out a long time ago, and essentially it breaks down as discussed. Horsepower is meaningless, because torque is what moves the vehicle down the track. Horsepower is just a statistic that expresses in a single number the interplay between torque and RPM. Peak torque is the key. When the vehicle goes through a gearchange RPM is lost. The idea is to put the engine back at near peak torque RPM after the gearchange. This will allow for the best acceleration, because peak torque is where the engine is pulling the strongest. Where things can get really tricky is that it isn't typically optimum to shift at the same RPM for every gear change. The reason is that the RPM lost with each shift is a function of the gear ratios in the trans and I've never seen a production trans where the gears were so perfectly matched that the RPM loss was exactly the same with each shift. A lot of autos have a tall first gear and a much lower second gear, so a lot of RPM is lost between 1st and 2nd, and not as much is lost between 2nd and 3rd. Ideally you'd rev higher before the 1-2 shift to make up for this discrepancy, and then not have to rev as high on the 2-3 shift. This was tough before computer controls, because the only way to do it was to have a manual valvebody in the trans and shift it yourself. However, today with the electronic trans controls you may be able to program each shift RPM individually (someone who knows for sure please chime in here).

[ralphinnj]

There is no doubt that the new tuners (SCT at least) can optimize shift points gear by gear. I am sure the ratios vary, so your arguement about having to address each gear change uniquely is important. As far as shifting to the peak torque point, I would disagree. I would argue that you want to shift such that you straddle the peak torque, which means when you 1st engage the new gear, you are a little below the peak torque, and pass through it until the next shift. On the torque vs. RPM or time curve, this yields more area under the curve than starting at the torque peak. More area means more total energy delivered. Which means more acceleration.

One question I have is, how well does a dyno torque curve represent what really happens in a moving vehicle? Is it close enough to tune the shifting?

Ralph

[TKOPerformance]

Yes, you wouldn't want to hit the actual RPM where peak torque occurs, because as soon as you shift you'd be off the peak again. Peak torque only occurs for a microsecond, so you need to get below it, but still close to it after the shift, then run past it to the point where after the next shift you'll be back below, but near the torque peak again.

I would say that a chassis dyno should produce a viable torque graph from which you will be able to determine your optimum shift points. There's actually an equation for shift points, I'll see if I can dig out the book that has it...

[ralphinnj]

Thanks, good stuff.

[william_04_X]

very good TKO..with the torque graph you can see the engines powerband(the RPM's where the motor is making good torque)..you'll want to shift just before the torque starts to drop off and if your gearing is correct, you will bring the rpm's back down to where you are still in the powerband..

the goal for street engine power/building is usually a broad torque curve..on a graph, it will start steep and increase/decrease slightly through increased rpm's, until it gets to a point where it will fall quickly and lose efficiency..keeping the motor in the band will lead to the best performance..

i'm sure the tuner's take into account the motors torque curve and transmission gearing when selecting shift points..you don't want to upshift and drop too low as you'll feel a flat spot(diminished acceleration) after the change..waiting too long to shift and that flat spot is before you shift..if tranny gearing/shifting is correct relative to engine torque, then you will feel steady acceleration through all the gears..

[TKOPerformance]

I found it:

RPM after shift = ratio shift into/ratio shift from x RPM before the shift

You need to know your trans ratios to make this formula work for you. An example would be a typical 6.0 with a 5R110 auto trans. Let's look at the 1-2 shift, where 1st gear has a 3.09:1 ratio and 2nd gear has a 2.20:1 ratio. International rates the 6.0's peak torque at 2,000 RPM. I'm not sure if the peak torque RPM climbs with modifictions like a gas engine (anyone know that can chime in?), but for now we'll look at a stock truck. If you assume that the stock shift point is around 3,700 RPM then the equation looks like this:

2.2/3.9*3700 = 2087

This puts the motor right back at peak torque. However, the 2-3 shift at the same RPM yields the following:

1.538/2.2*3700 = 2587

This is a gain of 500 RPM and beyond peak torque, so there is probably an advantage to shifting 2-3 at a lower RPM. If you short shifted it at 3,000 you'd end up with this result:

1.538/2.2*3000 = 2097

Theoretically this seems like it would produce a better ET, but I certainly don't have all the available data. If Innovative could chime in here with their expereience it would be great. The optimum shift RPM is really determined by the RPM you want to see after the shift, which could be affected by several factors.

[ralphinnj]

Thats the kind of approach I am talking about. On the modded trucks I have seen, peak torque goes up to around 2500 RPM. So, yes, it does go up. Be interesting if someone applied this approach on a well-dynoed truck and then measured various shift strategies at the track (by getting ET's, etc.).

Ralph

[william_04_X]

everyone's mods will have slightly different results..to find your BEST setup, a dyno run could be in your best interest for a good baseline..talk to local racers for a reference...you want a good operater with good equipment..then get a G-tech for fine tuning..i've found mine to be an excellent tool for cheap $$$..then spend time at the track on test and tune day to optimize for track conditions..

[UNBROKEN]

I can tell y'all this...you drop your WOT shift points down to the 2500 range...you ain't goin' nowhere fast.

[ralphinnj]

I wonder why? If that is the peak, and you enter that gear at the peak, it should only be worse if you shift even higher (and come in at, say 2800-3000 for the next gear). I am not saying it is not true, but I am saying then that the dyno torque peak is not the only variable to be considered. Does anyone know what are the right variables? I would have thought torque was it (and obviously the gear ratios as discussed earlier). But Rich is saying in practice that does not work. So....what are we missing?

Ralph

[InnovativeDiesel]

Shifting a vehicle always comes down to the rate of acceleration. All of the newer processors have a calculated acceleration rate parameter that can be datalogged to determine the best shift points.

I've paraphrased some info here from a company who builds acceleration test software.

Most mistakinly believe that the fastest way from point A to point B is to keep the engine RPM at the torque peak (or as close as possible). The technique is usually stated as "shift just after the torque peak.

Unfortunately, *engine* torque does not tell you the full story. What matters is the torque to the rear wheels, including the effects of the transmission. We all know a truck doesn't accelerate as hard in third gear at peak torque RPM as it does in first gear. The transmission amplifies or multiplies the torque coming from the engine by a factor equal to the gear ratio. So to determine how much the car is accelerating at a particular instant, you have to know both the torque output of the engine as well as the gear ratio.

Torque and power are (almost) flip sides of the same coin. Increasing the torque of an engine at a particular RPM is the same as increasing the power output at the same RPM.

Power is just as useful and relevant in determining vehicle performance as is torque. In some situations it's more useful, because you may not have to play with gear ratios and a calculator to understand what's going on.

A truck accelerates hardest with gearing selected to stay as close as possible to the engine *power* peak, subject to the traction capability of the tires.

Not all trucks should be shifted at the redline for maximum performance. But it's true for many. You can determine optimal shift points by graphing horsepower vs. velocity or transmission torque vs. RPM. Engine torque alone will not determine shift points.