THSCC Forums

The way I determine when & where to shift is to run the course once, then see where I'm hitting the rev limiter & for how long, and determine whether shifting will help... not just by how long I'm on the limiter, but also by seeing whether it will help to keep the run better controlled/smooth if I short shift and keep it in 3rd for a while.

If I kept it in 2nd in the 2.5RS for every autox course I had ever run, I would literally have been on the rev limiter like this:

nananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananananan

on at least 5 courses.



So when I design a course with this in mind, I try to have a looooong stretch where I can stay in 3rd & be ok, or no 3rd at all - but definitely no "in between" stuff. The Danville course last year was designed to put the RS right at the rev limit twice, and allow the momentum of the RS to make up for the HP of the STi's. Unfortunately, some bastard in an HS Mini stole my FTPAX

Google "horsepower vs torque"

Hit #1
http://vettenet.org/torquehp.html

The answer is "gearing" as Mike stated, however HP *is* just torque times gearing (rpm)

No need to apologize, you are making a very common mistake.
http://www.elephantracing.com/techtopic/hpvstorque.htm
Since your gear ratios do not change you can take it out of the equation entirely if you are comparing the engine acceleration rates and the question becomes: will a car equipped with an engine making 200 hp and 100 lb/ft accelerate faster than one making 100 hp and 200 lb/ft?

From the home of tech: Corner Carvers on HP vs. torque. See Mike, you aren't the only one who is wrong.

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Rich
http://www.v8mongrel.com/

Well, the assumption I am making is that we don't want force, we want work. I don't think that is one that I need to be careful of.

EDIT - which is EXACTLY what Scott's link shows.

Kevin, edit your NAs please. The lateral scrolling is really annoying.

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Rich
http://www.v8mongrel.com/

We're saying the same thing and arriving at it through different reasoning. The link you posted talks about optimizing Linear Force, which in my calcs is what I call Force at the ground. All I am saying is that it can be calculated using Torque and HP can be ignored. Or it can be calculated using HP. Since they are interdependent, it doesn't really matter which is used, but in my mind it's easier to make force calculations with TQ instead of having to back RPM out of the HP numbers.

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

I don't understand what you are saying. You don't have to back out the RPM numbers from HP. You simply do a before and after HP readout on the shift. When the two numbers are equal and as high as possible then you have the right shift point. Doing the gearing x torque can yield different results (I have done that way too) that are wrong. Work (horsepower) is the only relevent data.

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Rich
http://www.v8mongrel.com/

Woah. There are like some words, with some meaning in this thread.

I am confused.

Rich -- Thanks. I will start shifting at 6100

Bwhahahhaaha!!

- dow

OK, so I spent 15 minutes making a spreadsheet. Someone who can host it post your email and I will get it to you.

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Rich
http://www.v8mongrel.com/

I'm using whp readings and not crank. My original rpm selections did not hit the max hp as it falls between easily readable lines Since I'm modded I'm higher than stock so you may be pulling incorrect data. What's the max HP you're referencing?

I reran using max numbers from memory from a recent dyno pull (I don't have the sheet in front of me) versus the one I originally referenced from my laptop and my HP matching shift points are: 2-3--> 6204 rpms, 3-4--> 6084 rpms.
The difference this time being 20Hp at peak and including the rpms where I first hit my max number.

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Beauty is in the eye of the beer holder.

Just used google and that came up. I know nothing about the car so it could be wrong.

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Rich
http://www.v8mongrel.com/

Hmm, I've run on two different dyno's and both where higher. Even stock. I ran on a Mustang dyno and a Dynojet. I'll have to refer to the Mustang plot when I go home since it closely resembles those curves (4th gear pulls) whereas my tuning dyno pulls were in 3rd gear. The mustang peaks are +80ish (hp and tq) from your plot.

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Beauty is in the eye of the beer holder.

Cobb Tuning has stock Subaru dyno plots on their website, also. I'm not sure exactly what you guys are looking for, because I haven't really been paying close attention... just thought that might help.

http://cobbtuning.com/sti/images/ap-sti-stage1.gif

Mike and Rich are confusing me. I do have my own understanding. I hope it is correct and this is what it is....

You can pretty easily calculate the wheel torque (Mike's force at wheel) for a given RPM if you know the engine torque at that RPM (which is why you need the dyno print out). With the vehical weight and the wheel torque values for a given RPM, you can calculate the instantanous acceleration value. Which as Rich (and those online resources say) will show that you will get max acceleration at max torque. Now the actual max value will be different on a per gear basis, but will happen at the same RPM.

I can then take the speed and wheel torque (for a given RPM and gear) and place that on a graph. Vary the RPM for that one gear and you should have a graph for that given gear. Do that for all gears. Then overlay those graphs. I think that IF for example your second gear graph drops lower than your third gear graph at some point in your available RPM range, this intersection would mean that your wheel torque is the same in both gears for a given speed (MPH). That means as your 2nd gear wheel torque continues to drop, your 3rd gear wheel torque is probably climbing. That is your shift point. The question is, will this happen before redline in the lower gear?

In my car 1st to 2nd doesn't intersect and 2nd to 3rd don't intersect. So this means run it up to redline and then shift. Each shift will give me a drop in wheel torque as the previous gear is always giving me more wheel torque across the entire RPM range. 3rd to 4th and 4th to 5th both touch, but just right at about redline. This means again that I will still shift at redline, but when I shift from 3rd to 4th and 4th to 5th, the acceleration should feel pretty much unchanged (wheel torque is about the same) while the 1st to 2nd and 2nd to 3rd upshifts will have a noticable drop in acceleration each time the gear changes as the wheel torque for the higher gear is lower when I shift.

I found this image on corner carvers interesting....



...which shows that if you care about wheel torque (or force), that this shows how a high RPM low torque engine can equal a low RPM high torque engine. This done via gearing. So RPM ( or HP ) is what does it.

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Richard Casto
1972 Porsche 914
2013 Honda Fit Sport
2015 Honda Fit EX
http://motorsport.zyyz.com
Money can't buy happiness, but somehow it's more comfortable to cry in a Porsche than a Kia.

Or, as importantly and perhaps more germaine to our discussion here, that high RPM in a lower gear at a high horsepower can result in superior acceleration to low RPM but high torque. Which is why you shift by HP, not torque.

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Rich
http://www.v8mongrel.com/

i got the two extra credit questions right

that read killed my whole morning, doh