When to Shift Calculator

[Thumper]

Here is a website that explains why shifting early can actually be better than rapping out each gear. It also had a nifty calculator as well.

 

http://www.datsuns.com/Tech/whentoshift.htm

[Pop N Wood]

meaning if your torque peak is at 3000rpm, that's the RPM your car accelerates the hardest

 

Too bad it is based on a wrong assumption.

[JMortensen]

Still looks like he has the right idea, which is riding the hp peak.

[Sideways]

Too bad it is based on a wrong assumption.

 

Care to elaborate a bit on this?

[Dragonfly]

Looks to me like he is doing it correctly, what IMHO he is not infasizing enough though is that you NEED to know what your torque values are at the rear wheels. I think he is using the formula from the "Automotive Math Hand Book" to make his calculations.

 

I have an excel spreadsheet that was written by a guy for the BMW M5 but was modified by me for my Z which allows you to input transmission ratio, diff ratio, tire size, weight of car, drive line effiency % and your rear wheel torque values at certain RPM's and it will calculate your acceleration in G force assuming 100% traction, the spreadsheet also gives you the thrust at the rear wheels. This is a great program if you know all the information and it makes it real easy to determine the optimum shift points for acceleration. One other very cool feature of this spreadsheet is that it allows you to put in a second rear diff with a different ratio and do a direct comparison between them.

 

Any one who would like a copy of my modified version of this spreadsheet drom me a PM with your email in it and I will email it to you. I do not have the unmodified version anymore and I don't remember exactly where I found it, but the orriginal author said anyone who wants to use it is weclome to use it.

 

Dragonfly

[Challenger]

So the point is the shift so when you get into the next ratio your around the peak power?

[Dragonfly]

So the point is the shift so when you get into the next ratio your around the peak power?

 

Yes.

 

Like everything else there is always a compromise and the biggest compromise with this is how fast you can shift... the faster you shift the less RPM's you drop and most of the time the less RPM's you drop the more power you have available to you.

 

Dragonfly

[datsunscom]

That spreadsheet was done a long time ago and I should redo it. What I need to emphasize is you need to know the torque at the rear wheel across a wide spread of RPM. The trick is to shift when value the torque at the wheels *after* the shift is the same at torque value before the shift. Sometimes this is not possible. Attached is a .GIF for demonstration...bear with me

Crankhshaft torque x gear ratio x axle ratio x tire size = torque at the wheel. Given that the wheel size is constant (for the purposes of this topic) and the axle ratio doesn't change, all we care about is torque x gear ratio.

 

In my example, the VG30ET here is making 290ft-lbs of torque@4700RPM. multiply it by 3.1:1 1st gear and we get a value of 899ft-lbs. Shifting at 4700rpm drops this engine to 3049RPM in 2nd (far right column). Torque in 2nd @ 3049RPM is 623ft-lbs (310ft-lbs at crank x 2.011 gear). Better not shift here, even though you're past your torque peak. working up the chart, at 6300rpm we get 629ft-lbs in 1st, shifting to 2nd puts us at 4100rpm and 627ft-lbs..you have your shift point - FOR THAT GEAR. If 3rd is 1.3:1 and 4th is 1:1, the shift points move lower by about 700RPM...so depending on the torque curve and trans ratios you will have different shift points for different gears.

 

If the image isn't clear enough and/or you want the actual sheet or data I just worked from, let me know.

 

NOTE: It gets confusing when you know that in any one gear, your car will accelerate the hardest at its torque peak (barring aerodynamics at higher speeds) ...some might think this means shift when you will end up at your torque peak once you've shifted, but this rarely works out.

 

Bottom line: Torque at the wheel after the shift should be equal to torque at wheel before the shift.

[Thumper]

That spreadsheet was done a long time ago and I should redo it. What I need to emphasize is you need to know the torque at the rear wheel across a wide spread of RPM. The trick is to shift when value the torque at the wheels *after* the shift is the same at torque value before the shift. Sometimes this is not possible. Attached is a .GIF for demonstration...bear with me

Crankhshaft torque x gear ratio x axle ratio x tire size = torque at the wheel. Given that the wheel size is constant (for the purposes of this topic) and the axle ratio doesn't change, all we care about is torque x gear ratio.

 

In my example, the VG30ET here is making 290ft-lbs of torque@4700RPM. multiply it by 3.1:1 1st gear and we get a value of 899ft-lbs. Shifting at 4700rpm drops this engine to 3049RPM in 2nd (far right column). Torque in 2nd @ 3049RPM is 623ft-lbs (310ft-lbs at crank x 2.011 gear). Better not shift here, even though you're past your torque peak. working up the chart, at 6300rpm we get 629ft-lbs in 1st, shifting to 2nd puts us at 4100rpm and 627ft-lbs..you have your shift point - FOR THAT GEAR. If 3rd is 1.3:1 and 4th is 1:1, the shift points move lower by about 700RPM...so depending on the torque curve and trans ratios you will have different shift points for different gears.

 

If the image isn't clear enough and/or you want the actual sheet or data I just worked from, let me know.

 

NOTE: It gets confusing when you know that in any one gear, your car will accelerate the hardest at its torque peak (barring aerodynamics at higher speeds) ...some might think this means shift when you will end up at your torque peak once you've shifted, but this rarely works out.

 

Bottom line: Torque at the wheel after the shift should be equal to torque at wheel before the shift.

 

Ahh you made that thanks. I've used that or a similar website for years.

[Pop N Wood]

NOTE: It gets confusing when you know that in any one gear, your car will accelerate the hardest at its torque peak (barring aerodynamics at higher speeds)

 

But why are you limiting yourself to one gear? If you do the math you will find the car will accelerate the fastest when you shift into a whatever gear puts the engine at it’s horsepower peak.

 

It is the twisting force at the rear wheels that determines how fast a car accelerates. Rear wheel torque is simply the engine torque times the overall gearing ratio. Higher engine torque means more rear wheel torque. But so does lower gearing. You need to strike the proper balance between the two quantities.

 

So if the car is cruising in say 3rd gear with the engine turning at the torque peak RPM and you floor it, the car will accelerate at the highest possible G’s for 3rd gear. But lets say you shift into a lower gear first and then floor it, what happens? Going to a lower gear will move the engine RPMs off the torque peak (bad for acceleration), but will also give you a higher gear ratio (good for acceleration). At what point does the drop in engine torque exceed the benefit of lower gearing? It isn’t all that obvious but it turns out to be whatever gear puts the engine at the horsepower peak. This is gear/engine RPM combination that makes the car "accelerate the hardest". I can prove this mathematically.

 

Bottom line: Torque at the wheel after the shift should be equal to torque at wheel before the shift.

 

This is the same as saying the engine horsepower before the shift should be the same the engine horsepower after the shift. I feel this is a more simple and intuitive way to describe what is happening.

 

I also say forget engine torque curves. They only confuse the issue. Just look at the HP curve. Arrange your shift points so the engine HP before and after shift is the same. That will give you the maximum theoretical acceleration. Your own analysis confirms this statement.

[Sideways]

But why are you limiting yourself to one gear? If you do the math you will find the car will accelerate the fastest when you shift into a whatever gear puts the engine at it’s horsepower peak.

 

I'd really like to know this math- sorry for the skeptisism but everything ive ever read about ars and come to know about physics has led me to understand that a car will accelerate its hardest at its peak torque- and its acceleration rates match its torque curve exactly. I dont mean to be rude- its just if this information if wrong i would seriously like to know the truth.

[BobbyZ]

The car will accelerate fastest at its peak torque to the wheels. Lets say the peak torque at the crankshaft is 200ftlbs at 3000RPM. Does that mean you want the engine to stay around 3000RPM? The engine may be making its peak torque but you are not using the other thing you have going for you. Torque multiplication!

 

Putting 200ftlbs through the transmission at a ratio of 2:1 means you are actually getting 400ftlbs out to the driveshaft. Lets say that shifting down to second reduces the engine torque to 150 ftlbs but the gear ratio is now 3:1. The output torque at the driveshaft is now 150*3 = 450ftlbs. We have increased the output torque using the torque multiplication of the transmission.

[Pop N Wood]

Jim Beam is preventing me from repying right now.

 

I may hit up my old buddy Jose Cuervo next.

 

I will get back to you tomorrow because I am fading fast.

[datsunscom]

 

It is the twisting force at the rear wheels that determines how fast a car accelerates. Rear wheel torque is simply the engine torque times the overall gearing ratio. Higher engine torque means more rear wheel torque. But so does lower gearing. You need to strike the proper balance between the two quantities.

 

I also say forget engine torque curves. They only confuse the issue. Just look at the HP curve. Arrange your shift points so the engine HP before and after shift is the same. That will give you the maximum theoretical acceleration. Your own analysis confirms this statement.

 

I hate to tell you this, but you and I are largely saying the same thing: "Higher engine torque means more rear wheel torque. But so does lower gearing. You need to strike the proper balance between the two quantities."

Exactly...and shifting when torque at the wheel AT THE WHEEL is the same value after the shift as before.

 

"It is the twisting force at the rear wheels that determines how fast a car accelerates. " CORRECT - you just defined torque at the wheel. Now...calculate that with a horsepower number. You can't, you have to convert it to a torque value first. Related, but not the same.

 

"Arrange your shift points so the engine HP before and after shift is the same. " EXACTLY! You will note when you do this math, the torque at the wheel is exactly the same! Same with HP! I ran into this years ago doing the math and first I was shocked, then I thought about it and went "DUH".

 

"If you do the math you will find the car will accelerate the fastest when you shift into a whatever gear puts the engine at it’s horsepower peak."

 

You are saying somthing different here. Again, do the math and you'll find the "engine HP before and after shift is the same" statement of yours is the correct one.

[datsunscom]

I'd really like to know this math- sorry for the skeptisism but everything ive ever read about ars and come to know about physics has led me to understand that a car will accelerate its hardest at its peak torque- and its acceleration rates match its torque curve exactly.

 

You are correct in both counts, as long as you mean "a car will accelerate its hardest at its peak torque to the wheel". Until you're going fast enough for aerodynamics to start screwing things up, plotting your accleration curve will also plot your torque curve. This is how G-Tech and the like can give you a torque curve by using accelerometer.

 

The chart below is from a VG30E. Note that this car will accelerate harder from 30MPH when it's NOT at it's *enigne's* torque peak. Gearing MUST come into play when calculating shift points and acceleration curves.

 

RPM / TQ - gear - "TQ at wheel"

3500: 167 - 1.3 - 217ft-lbs [3rd gear about 35MPH]

4000: 160

4500: 160

5000: 155

5100: 153 - 1.9 - 290ft-lbs [2nd gear, same road speed as 3500 in 3rd]

5500: 145

6000: 135

 

When I wrote my article on my website, I was using "horsepower to the wheel" (confusing in retrospect) but came up with the same result.

[Pop N Wood]

OK. Jim Beam seems to have left the building.

 

After looking at your stuff more closely, you are right. We are, for the most part, saying the same thing.

 

But, you started out by talking engine torque and made the statement that a car accelerates fastest when the engine is at it's torque peak. That statement is the one I took exception to. Without the “in any one gear†disclaimer it is untrue and misleading.

 

Just dump any talk about torque and only look at engine HP. In my opinion it will simplify the article and make it less subject to misinterpretation.

 

Now...calculate that with a horsepower number. You can't, you have to convert it to a torque value first. Related, but not the same.

 

Well, yes you can. HP is torque times RPM. If you know the HP and the RPM, you can compute the torque.

 

In fact, HP is the one constant in the system. Torque can go up and down, RPM can change, but HP is conserved (neither created nor destroyed as the thermodynamic law says). In your lossless analysis HP to the wheels is the same thing as engine HP. Why add the qualifier?

 

Electrical engineers use this constant power method all the time when evaluating RF circuits. Exact same thing when evaluating mechanical power transmission systems. Power in equals power out. A lossy system just has some of the power being dissipated as heat.

 

Power is the rate at which you can do work. It is the fundamental entity governing how fast the car can accelerate. Throwing torque into the mix and then having to distinguish between engine torque or rear wheel torque just confuses things. My opinion. Other people may come to the same understanding differently.

 

Gear the car so the engine spends as much time as possible at the highest HP possible. Seems pretty damn simple to me.

 

For anyone with an engineering background it is a straightforward exercise to derive an equation for maximum theoretical acceleration vs. engine output. I did it in a thread here some years ago. Pretty much killed the thread too. Later I found the same equation in a text about automotive engineering. When you take into account gearing you will find the maximum, traction unlimited, acceleration capability of a car is inversely proportion to the speed of the car and directly proportional to the engine HP.

 

Acceleration being a function of speed is significant too. Knowing that helped me understand the dilemma of max acceleration in one gear occurring at the torque peak vs. maximum overall acceleration capability at that speed. Comparing acceleration in different gears only has meaning if you use the same vehicle speed in both places.

[datsunscom]

Acceleration being a function of speed is significant too. Knowing that helped me understand the dilemma of max acceleration in one gear occurring at the torque peak vs. maximum overall acceleration capability at that speed. Comparing acceleration in different gears only has meaning if you use the same vehicle speed in both places.

 

Maximum overall acceleration capability. Many people forget to account for that one!

http://www.datsuns.com/projects/fv-s/design_notes2.htm

 

The article talked about gearing taller to compensate, but the limit of available traction is discussed too

 

Bottom of the page has charts that displays that dilemma. Just because you add 100ft-lbs of torque to the equation doesn't mean the car will accelerate any harder in 1st. If you're already traction limited, more power won't help until you get to a speed / gear that can get that extra power to the ground.

 

Traction is why a 500hp & 700hp Z-car can have identical 0-60 times. If their available traction is identical (and roughly stock) the 700HP car will be faster only at higher speeds. 0-60 might be the same, but 60-100 won't.