When should I shift?

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jl1101

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My S2000 produces about 350hp at 9500rpm. When I use the power limiter to lower the power to 280hp, it says it peaks to 280hp at 7400rpm. So the question is, with my power limiter on, should I shift at 7400rpm or 9500rpm? The graph in the car setting page says the torque declines from 7400rpm to 9500rpm but I don't quite feel it.
 
I would shift at the point, where the torque is the highest, and since it's naturally fairly high in the rev range I'd say somewhere in between 7400rpm and 9500rpm? :D
 
hard to say without a engine power curve graph, take a photo of the graph and I'd probably be able to answer
 
Here you go. Thanks!
 

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jl1101
Here you go. Thanks!
Click to expand...
Yeah, in that car you should shift at rev limit.:)
A general rule of thumb is that you should shift just before where the power drop, not torque. Depending on how peaky it is of course. but this one in not peaky, it got a wide powerband, and most importantly, it doesn't drop at high rpm.;)
 
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Here's a graph I used in my Garage to illustrate when to shift...

Power Graph.jpg


Each car will have a certain drop in revs when shifting. If in this car for example the drop is 1200rpm, it's best to shift at 4800 rpm and stay at peak power when you upshift, when it will drop to 3600 rpm.
 
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Johnnypenso
Here's a graph I used to illustrate when to shift in my garage..
Click to expand...
That's not very useful if he's driving on track though :sly:


Johnnypenso
Each car will have a certain drop in revs when shifting. If in this car for example the drop is 1200rpm, it's best to shift at 4800 rpm and stay at peak power when you upshift, when it will drop to 3600 rpm.
Click to expand...
It's actually ideal to go a little bit* past peak power. I see that curve drops a little bit before 4800 rpm. If it were completely flat the ideal shifting point would be at 4900-5000. Wheel horsepower stays higher in a low gear, even past peak.

*Dependant on power decline and gear spread.
 
Torque is what gives you acceleration, however the effect of that torque is reduced when up shifting by the ratio of that up shift. And the change in RPM of course.

Power is what gives you speed, and is only affected by change in RPM.

So it depends on what your need is; maintenance of high speed or increasing speed (acceleration).

You do need to look at your gear ratios to see what RPM difference there will be at each shift point, and work out where the benefit of a lower gear's higher mechanical advantage is offset by the rate of drop-off of torque.

In the peculiar case of having a flat segment of power curve, you have an artificially fast torque decline at higher revs, so you can expect that earlier shifting will be advantageous.

To figure this all out, you need to plot the effective torque against road speed in each of the gears, and see where the lines cross. By "effective" torque, I mean taking into account that higher numbered gears reduce the torque available at the back wheels. So this is completely dependent on your gear ratios.

There are track-dependent considerations too. Gear changing takes time, and usually there is a loss of torque and power transmission during that time. If the shape of the track is such that target gear would only be used for a brief period, it may be better to avoid the changes (both of them!). Also, changing gears when cornering hard is not usually a great plan, since the delicate friction balance can be upset.
 
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If the car is naturally aspirated, take it all the way to the redline.

If it's turbocharged, if the speed starts dropping off at 8700 RPM for example, shift just before to prevent any loss of speed.

A good example of this comes from when I raced the hateful AEM S2000 in a league event. It revs pretty high, but the power drops off after 8500 RPM. It was best to shift then to gain some speed rather than shifting at redline.
 
jl1101
My S2000 produces about 350hp at 9500rpm. When I use the power limiter to lower the power to 280hp, it says it peaks to 280hp at 7400rpm. So the question is, with my power limiter on, should I shift at 7400rpm or 9500rpm? The graph in the car setting page says the torque declines from 7400rpm to 9500rpm but I don't quite feel it.
Click to expand...

I always thought it was really hard to understand the difference between torque and power (BHP). Do the engine produce two different types of forces? At the same time? How does that work?

Then I read a great article online that explained it: Torque is the force of the engine, while power is the work per unit of time. I.e. the torque says how strong the engine is at a certain RPM, while power (BHP) says how much work it's doing.

Think of it as having two different persons:

Person A is strong (much torque)
Person B is weak (little torque)

They're both having a job to do. They're going to move a pile of sand with the weight of 1000kg from one end of a room to the other end. Person A can carry 100 kg per round (because he's strong), while person B can carry 10 kg per round (because he's weak).

However, person A worked slow and could only do 1 rounds per minute, while person B could do 10 rounds per minute.

For person A this gives: 1*100=100kg per minute of work.
For person B this gives: 10*10=100kg per minute of work.

Thus the weak person did just as much work as the strong person throughout this period of time.

I don't know if it made things any clearer, but a low torque engine working at high RPM can produce just as much power (BHP) as a high torque engine working at low RPM, because RPM is telling you the speed of their work (the work per unit time).

In racing we want as much power (BHP) as possible, so what's interesting to us is mainly the BHP output of the engine. However, the torque curve is important as well, because it tells you where the engine is the most effective (most force per revolution) which is good to know if you want to save fuel during an endurance, for instance.

If all you want is power, you only need to look at the BHP though.
 
This forum has a nice posting on when to change gears, including a great chart.

The colored lines show the torque at the wheels, and the gear change points are where they cross over.

Changing to a higher numbered gear reduces torque at the wheels, so you don't do it until the engine torque has dropped off too much in the lower gear.

I really like this chart, especially because it's nicely annotated. I was about to create one of these in Excel, but this is easier. A little googling will probably locate a when-to-change-gear calculator if you're interested. If GT6 were to present power and torque data in a more useful way, that'd be a good tool to have.

image.jpg
 
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Some cars should be short-shifted several thousand rpm shy of redline for maximum acceleration. Some should be shifted deep into the redline the moment before bouncing off the limiter. For others, right at the redline is best. Spend a lap experimenting. First find the maximum rpm the car can do, and then watch when your acceleration increase begins to slow down (if it slows down at all. This usually is just for turbo cars.)
 

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