Wednesday, May 25, 2005

visualization of gearing/shiftpoints

NEW DOWNLOAD, version 2.3, now with graphing! :)

So while my spreadsheet is cool, not enough people have precise numbers and dynos in different gears and such to really take full advantage of its capabilities.

Thus, I'm gonna combat the stupid myth of 'if you peak at 6k then just shift there' that you hear so often on LS1tech on a realistic example.

Pictures are worth a thousand words, so I grabbed some data from Trackbird's stockish 2002 camaro, as he's the only guy I know that has dynos in different gears. Oh, it's good to be a crewchief ;) I used 3rd gear data for 1st and 2nd, as 3rd and 4th were the only data I had.

I graphed 4 curves, picturing the relationship between horsepower at different speeds, in different gears. Some of them overlap of course, as gears do.
The blue dots represent the horsepower curves for the entire range in each gear. Pink dots are the HP which would be used to accelerate, with given parameters (shift points, gearing, tire size).

Now for the cool part:
T56 is a good trans, well spaced for LS1 powerband. 4L60E however is a different story. I used my old setup (3.23 final gear, 4L60E trans, 315-35/17 tires) with Trackbird's dyno graphs (since it's stockish enough) for the purpose of this demonstration.

Look at the 1-2 shift: 6000rpms drops to 3100rpm, which brings it down from 290hp to only about 180hp. That's sad, becuase it kills the whole myth of big american V8's with a lot of down low grunt. Sorry guys, but it's just not there anymore. 180hp while your car weighs 3500lbs will yield a lousy acceleration.

2-3 shift is not that much better. This time we're going from 6000rpm to 3700rpm which yields us a drop of about 70hp. So this time not only you end up with lousy 220hp, but you're already in 3rd gear, which is 1:1 (3.23 effective) ratio.

Now let's look at the modded scenario (3.73s, 6400/6300/6200rpm shifts):

1-2: 6400rpm drops to 3400rpm 'landing' on lousy 195hp again. It's better than before, but still lousy. The good part is that for that few miles an hour, it wasn't going from 180 to 195hp, it was dropping from 280hp to 270hp. That's the trick about shifting past the peak power. You might be losing power, but you're not losing it nowwhere near (almost 100hp difference!) as badly as if you were to shift to a higher gear earlier. So the gain is twofold, you don't lose lower gear, and you gain on the higher gear--nice!
2-3: 6300rpm drops to 3900rpm 'landing' on much more pleasant 225hp. The logic is the same as with 1-2 shift: you don't lose in 2nd anywhere near as much as you'd lose by shifting to 3rd earlier. Then as a added bonus, you start off in 3rd on more power.

So how can we quantify this?

I set up a scenario where we'd use 1-2-3-4 gears on both auto and manual transmissions. 30-140mph run, should demonstrate how higher shiftpoints combined with shorter final gear, yield a better acceleration.

I added up all the hp cells 'participating' in this simulation and added them up. Also, I counted how many of them there were, giving us all we need to see the average horsepower rating throughout the whole accelerating process.

case 1 (6000rpm shift, 3.23s): 263.89HP
case 2 (6400/6300/6200, 3.73s): 270.65HP
That's a 2.5% increase, but more importantly it's throughout the whole range! Not only that, but it's also rearwheel horsepower rating, this is exactly what causes acceleration (well, if we ignore the air resistance, but it's not like you can cheat out of this one). So practically speaking, it's power for free, but in that effective sort of way, not some theoretical instantenous peak rating that marketing guys love so much.

Also, this is on a stockish aplication. In cases where power does not drop off 'till much higher rpms, proper shifting will reward you even more.

If you're stuck with the A4 transmission, you can learn where it hurts the most, so you can spray there. Right after the shift in 2nd would be NICE, but remember that a to make about 400hp (let's say ~200shot) at 3100rpm means you're making 677lb/ft of torque, so while it will help tremendously, it might not be the best thing to do as far as reliability is concerned.

I hope that demonstrates a bunch of points:
1. Various applications: where to spray for drag racers/where to shift for road racers/what speed would be the best to start off with for a highway race
2. Common sense sucks as far as numerical integration is concerned. Some things you just gotta calculate for yourself, you can't just rely on myths and popular opinions to.
3. Peaky powerbands (yes, that's you, LS1 people!) do need properly matched transmissions to go alongside with them. There are reasons why little Hondas with peaky VTEC powerbands rev beyond 8000rpm and come with 4.3+ final gears. While they don't have the sheer stupid power, they always stay on it, and in combination with lower weight (which ultimately is the other part of the acceleration equation) they yield similar results as far as acceleration is concerned.

So in conclusion: don't just concern yourself with making power. Learn to configure your setup so you actually get to use it.


At 8:57 PM, July 09, 2005, Blogger 405HP_Z06 said...

Nice work Marcin!

At 9:39 AM, November 20, 2005, Anonymous jjnada said...

wow. this is really interesting. thanks for posting your research - i'll be taking this into account for my next tune.

once again, great info. thanks so much.


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