Backroad Shift Points

[KAR120C]

I crossed my 1000 mile break-in so went out yesterday to play with higher RPM’s and Track mode. Quite a difference when you can keep the car in the power band. Plus the sound just gets better and better!

Today it’s rainy here, so I played around with some info to educate myself on the best approach to backroad shifting before it's sunny again.

I’ve looked at some of the dyno results for the Emira that are posted on-line. The engine hits peak torque and flattens out from around 4200-4500 RPM to redline. For acceleration, I’m more interested in the torque than hp.

I also found a nice Komotec graph that plots Emira gear selection/RPM/and speed. I shamelessly stole the basics of that graph, and converted KmH to MPH. I then added the max torque range on top of that graph in a yellow box.

In this first graph, I show the results of shifting at the 6800 RPM redline. The red hashed line shows where the next gear’s RPM will be at shift. You can see that a redline shift will usually drop you back well into the max torque range.

This is probably great for track, but I wondered if it was necessary on the road.

In this graph, I show the minimum shift points that will still drop you back into the max torque range. I found it very interesting.

What it tells me is that I don’t have to wring out the engine to get performance. The key takeaway is that if I always shift between 5500 and 6000 RPM, the engine will still stay in a very useful power band. That will mean that I can shift by sound or a general glance at the tach.

For those past their break-in mileage, what is your experience?

 

[Nova]

Max crank torque is meaningless. At any driving speed, the more HP your engine is generating, the more wheel torque you will get, even if the crank torque happens to be lower than some other point on the RPM curve.

The Emira's HP curve is fairly linear from low to high RPM. You just drive it based on how much of that HP you want to make use of. For spirited street driving, you are probably mostly going to be using the engine in the 200-350HP range, picking the gear to stay in this range. You are not looking to extract that last 50HP out of the engine.

Cars where this doesn't matter are those with a flattened HP curve, like the AMG M139 version of the Emira. With this engine, the HP output at 5500RPMs is nearly the same as near redline. So picking a gear running at 5500RPMs gets you nearly the same wheel torque as down shifting to get at near red line.

 

[KAR120C]

Max crank torque is meaningless. At any driving speed, the more HP your engine is generating, the more wheel torque you will get, even if the crank torque happens to be lower than some other point on the RPM curve.

The Emira's HP curve is fairly linear from low to high RPM. You just drive it based on how much of that HP you want to make use of. For spirited street driving, you are probably mostly going to be using the engine in the 200-350HP range, picking the gear to stay in this range. You are not looking to extract that last 50HP out of the engine.

Cars where this doesn't matter are those with a flattened HP curve, like the AMG M139 version of the Emira. With this engine, the HP output at 5500RPMs is nearly the same as near redline. So picking a gear running at 5500RPMs gets you nearly the same wheel torque as down shifting to get at near red line.

HP is actually a measurement derived from torque. Acceleration is based on torque (that's why I like to use it). HP is more about power produced over time thus important for top speed.

The Emira torque climbs rapidly then is flat as a pancake after about 4500, but HP increases simply as result of the calculation for HP including RPM.

So, if I'm interested in making sure I can produce my desired acceleration on demand (like tracking out of corner) I'm going to want to be in that yellow max torque range.

 

[Nova]

This brings back trauma from my early days on Honda forums...

I promise you, if you run the calculations, at any speed, more HP = more wheel torque. Your car's acceleration is dependent on the wheel torque, which is a function of the crank torque multiplied by the gearing ratio. Looking only at the crank torque ignores the effect of the gearing.

 

[KAR120C]

This brings back trauma from my early days on Honda forums...

I promise you, if you run the calculations, at any speed, more HP = more wheel torque. Your car's acceleration is dependent on the wheel torque, which is a function of the crank torque multiplied by the gearing ratio. Looking only at the crank torque ignores the effect of the gearing.

I'm not trying to disagree, that's why I said that on an Emira engine I'd take it red-line on track (max HP and torque). My point was that for road driving, keeping the crank torque in it's maximum band is completely sufficient for everything you need.

I think we are saying the same thing. Just differently.

 

[Eagle7]

Basically torque is what gets you moving, it's the force that moves things. Horsepower is what keep you moving. Horsepower is the power to overcome resistance, which in a vehicle equals speed. You need both to get moving and keep moving, but in a situation where your driving through the corners, torque gets you moving again after you've slowed down. Torque is better for the twisties because you aren't pushing towards high speeds, while horsepower rules on a race course where you're pushing for maximum speed all the time.

The Emira feels quite tractable, and the long throw of the gas pedal allows for precise, minute applications of power which is perfect for momentum driving.

 

[KAR120C]

I really like supercharged cars and have had several. Prior to the Emira, my Evora S was the last one I had. Then I had a string of turbo cars. Turbos are fun and like my M2C really can get up a head of steam. When I test drove the Emira back to back with my M2C, I felt like it might be taking a step back.

The other day after break-in I got on it a lot harder and went higher into the rev range. It still didn't feel like it had that turbo power....until I looked at the speedo. It has big power but no drama. Feels just like a powerful NA engine. Now I remember why I loved SC engines so much!!

 

[Nova]

I'm not trying to disagree, that's why I said that on an Emira engine I'd take it red-line on track (max HP and torque). My point was that for road driving, keeping the crank torque in it's maximum band is completely sufficient for everything you need.

I think we are saying the same thing. Just differently.

Yep, in that range is where the engine is running near optimal parameters for a balance of output vs stress. It's a lot less stressful than near redline.