Power limiter and gearing

[Mopatop]

Hi all,

Firstly, sorry if this has been asked before, I did search but I couldn't find anything.

I was wondering if someone could confirm something for me. I believe that is you have a heavily power restricted car (with a flat power curve), it is unnecessary closely gear your transmission as long as you can stay within your power plateau. When I'm doing time trials, I run very long gear ratios with very power limited cars. As the RPM always stays within the peak power range, it doesn't matter what gear you're in, because you're always producing peak power. Because I'm shifting less, I go faster - I think.

Is that right?

 

[darkblade986]

Just realize that HP isn't everything. Torque also plays a factor, and that curve drops off hard when you're on a power limiter.

HP tends to dictate how fast your car can go.
Torque tends to dictate how fast it'll get to that speed.

Regardless, you're definitely going to want to put your gears as close as possible to an "optimal" for the track. There's no reason to only make it 50% through 6th gear at the fastest part of the track when you have the entire 7th gear left.

 

[Mopatop]

Sorry man, don't take this the wrong way but I don't think you understand the relationship between torque & power and how that results in gearboxes.

 

[Parnelli Bone]

Hi all,

Firstly, sorry if this has been asked before, I did search but I couldn't find anything.

I was wondering if someone could confirm something for me. I believe that is you have a heavily power restricted car (with a flat power curve), it is unnecessary closely gear your transmission as long as you can stay within your power plateau. When I'm doing time trials, I run very long gear ratios with very power limited cars. As the RPM always stays within the peak power range, it doesn't matter what gear you're in, because you're always producing peak power. Because I'm shifting less, I go faster - I think.

Is that right?

In a way you are right, depends on the situation. If a car has a flat horsepower curve caused by the restictor, it is not going to have typical power fluctuations while the tach needle is running thru the peak HP area. In effect, the engine will have an area where its speed is gaining consistently (rather than running faster and faster towards peak, but then slowing after peak has passed.

The only factors left that slow the car, then, are its gearing, tire resistance, and aerodynamics...at least in the game, anyways. Power/speed fluctuations caused by a spikey horsepower curve how now effectively been removed.

If you've entered your car in an overkill situation, where you're faster than the Ai, you now have a huge advantage over them. Power is restricted, true, but there's a larger area where peak power is being doled out. In effect, the engine is able to run at peak for a longer period of time. The engine is gathering speed at its fastest rate for a longer period of time, therefore.

If you've entered your car into a race but are trying to restrict its power for competitive reasons, the limiter does help, but again, there's still going to be this larger area where the engine is making peak power. Again, engine speed gathers up more consistently in this restricted area, but it is possible to use the restrictor with some success. I've used it quite a lot, actually.

 

[chuyler1]

The performance reduction option is another case where it only gets you so far. It typically lowers the rpm range of your power band. You sacrifice the best performance at redline for a wider band below redline. If you drive with an AT instead of MT over use of performance reduction will result in slower lap times. However if you use a MT you can make use of the wide powerband by running taller gears and shifting well before redline.

 

[Parnelli Bone]

The performance reduction option is another case where it only gets you so far. It typically lowers the rpm range of your power band.

Uh....lowers the RPM range? As in, lowering where the peak area starts? Okay, I'll agree with you there, if that's what you mean.

You sacrifice the best performance at redline for a wider band below redline. If you drive with an AT instead of MT over use of performance reduction will result in slower lap times. However if you use a MT you can make use of the wide powerband by running taller gears and shifting well before redline.

All of this assumes peak power actually happens before redline, though. Some cars, surprisingly enough, don't hit peak power until after redline, which in some cases are PD screw ups. Other cars make their peak long before redline occurs.

The limiter in some cases can flatten out peak power, though, so that the tach now starts passing over a much broader peak area. This may or may not wind up placing peak power on the redline itself. Depends on the engine and how much power is being limited I guess.

 

[niky]

Torque at the engine doesn't count for squat. It's torque at the wheels that does all the work. And torque at the wheels is directly related to gearing and the rpm range that torque is generated at.

A car that produces 300 Nm of torque at 2000 rpm, 300 Nm that tapers off at 4000 rpm, will accelerate much faster at low speeds than one that produces only 200 Nm at 4000 rpm... but once you get past the first two gears, no amount of gear-tweaking can make up for the fact that you will be stuck in 4th gear when the other car is still accelerating hard in 2nd.

-

All the power limiter does is lop off power at the top, in a manner similar to cutting the top off an ice cream scoop.

The problem is, PP is calculated on an average. If PP were calculated from peak power only, the power limiter would be a huge advantage, as it is, it's only a big one.

 

[chuyler1]

Uh....lowers the RPM range? As in, lowering where the peak area starts? Okay, I'll agree with you there, if that's what you mean.

yes, that's what I meant. As someone mentioned earlier, it lops off the top of the curve. However it also messes with torque. You may see your peak torque number increase by using the power limiter but you'll also see that the torque curve produces less torque in the rpm range that is power limited. You may end up with less torque at redline but because the graph is so small and not to scale it's hard to say how much less torque you will have at redline compared to not using the power limiter at all.

I'm not sure if this is the best approach but I tune so that I have a flat HP line from redline down to the lowest rpm i use on a track. For example, my RX-8 runs to 9,500-10,000 rpm and when I shift from 2nd to 3rd, the needle drops to 7,800rpm. I adjust mods and power limiter so that I hit peak power at exactly 7,800rpm and it stays flat to redline. It makes my car a little slow off the line but once I'm out of 1st gear it is very competitive at the 450pp level i race at.