Best Shift Points

[beepbeep]

graph and gearing numbers... and some Matlab quality-time.

[svandamme]

Originally posted by beepbeep
Well I'm still claiming that optimal accelleration trough gears will only be achieved by choosing shift points that maximize sum of swept areas under power curve for particular run trough the gears.

Prove me wrong.

i'm not going to argue with that, but let's be honest
do you calculate it everytime you get yourself a new car???
do you take it to the dyno everytime you change something on your car, which might affect the power/torque curve???
do you really think it gives you a huge advantave over the regular butt guess shifting folks???

[beepbeep]

Originally posted by svandamme
i'm not going to argue with that, but let's be honest
do you calculate it everytime you get yourself a new car???
do you take it to the dyno everytime you change something on your car, which might affect the power/torque curve???
do you really think it gives you a huge advantave over the regular butt guess shifting folks???

No, but I was a math geek long time ago and I still like elegant predications ;-)

Now bring turbocharger rotational inertia and tyre-slip at launch into equation and it starts to get interesting...

[svandamme]

lol

but i think you'll agree, that it's mostly a matter of sensing when to shift, and that's the most real advice to give to somebody who's new at shifting and specifically shifting in a sports car...

[beepbeep]

Well rule of the thumb would be to shift a little bit north of max power point (especially in 930), but there are no thumbs in mathematics...

[Embs]

I usually shift at about 7K right when the valves start to float!!


Todd

[porschenut]

Aw, let's just shut up and drive....

[masraum]

The real question is does it matter if the transmission is in a Club Sport, a regular 3.2L car or a turbo-look?? And would it make one of those faster on the track!!

[Bill Verburg]

Goran, you are, of course, correct. A graph of engine power is needed to get the best shift points for accelleration.

Something like this?

[D Hanson]

Originally posted by beepbeep
Well I'm still claiming that optimal accelleration trough gears will only be achieved by choosing shift points that maximize sum of swept areas under power curve for particular run trough the gears.

Prove me wrong.

That is what I have always heard. My car also seems to run better with a little more spirited driving. When I putt around, it seems to bog down a little or feel like it is loading up. Don't know who to explain it, but a good blast to the red line keeps the revs feeling real snappy and responsive.

On my 964:

In running 0-60s on my G-Tech and after I did my dyno, I found it best to shift at about 6,300 to 6,500 instead of my 7,000 red line. I stay in the torque better, the back tires lets out a scream and and I run just as quick if not quicker at the 6.5k +or- a few.

On both 996s:
I pretty much take it to red line to get best results on G-Tech.

[thabaer]

Bill, can you recreate that graph showing rear wheel torque ?

[Bill Verburg]

Bill, can you recreate that graph using torque ?

All you need are the torque #s from a dyno, gear ratios(final and individual) and tire heights or revs/mi?


Thats my engine w/ current and proposed tires.

[thabaer]

I shocked ! You don't have that data on hand ?

[Bill Verburg]

I have all the pertinent information for my car. Why would I need more

[Bill Verburg]

Charlie, You might be interested in this one I did comparing my C3 w/ stock engine, 915/44, 225/50x16 tires and the same car w/ 3.8RS, 915/67, 275/40x17 tires

[kbrown]

I'd like to hear what Walker thinks, I know he has had plenty of engines opened, under every driving condition imaginable.

[Thack]

"ever heard an F1 car?
then imagine sittin in one, with earplugs with a radio ...and a helmet on top ...you can't hear the differences in your engine noise well enough to use it for shifting..."

That's not entirely true. I raced Grand Prix bikes (two strokes) and you can hear the motor and feel it but it revs so fast the needle just swings over to 12K-14K like you are in neutral. The power comes on so fast you have to check the tach so you don't bounce off the rev limiter. It pulls really hard then falls flat, it can catch you off guard on a long straight. The power can be in such a small band (like 10K to 12.5K, and it's geared to stay in that band after every shift) that the tach helps you learn when to shift so you don't trust your ears or seat. Also you look at it if your power falls off so you can tell the mechanic where the hiccup is. Or use it as a reference to note how the engine performs.
All the same reasons to note for a street car, it just happens more slowly.

[GSpreeman]

I took my brother in-law out last weekend and we cruised around for a few minutes in the 5-6K range. Figured I took some miles off the engine but man did it sound wonderful!

[Paul Franssen]

How many times have I -over here- heard the remark, "yes, no rust, but watch it, the car was imported from the U.S. and the engine is full of muck cause the engine hasn't seen any speed".

I guess that's just hearsay, of course.

My guess is that, yes of course, "the overlappong torque areas" give you the best speed and, in general, very average shifting ranges will put least stress on the machine. It's just so that, on our dry-sump aircooled flat-6 engines, "average" is not to be exaggerated, meaning ...anything between 3000 and 4000 is pretty cool, not to worry in the least. And I repeat, in 5th gear, 4000 rpm gives my ROW exactly 100 mph, which I have done all day long in the past over an 800 km distance (before I got scared of traffic tickets).

I think it was Nostatic had a point: "stay in 2nd to be able to react quick". Sounds reasonable to me. For reasons personal to my car, my policy is different, I use 2nd to get out of 1st (I NEVER stay in 1st for any length of time, gosh is that ever unpleasant!), use 2nd for as long as I remain accellerating or -for a short period- higher up in the revs (I don't cruise in 2nd), and -generally- cruise in 3rd (or 4th but then I should be out of city streets already...). Wanta know why I avoid staying in 2nd? Because when really "slowpoking" in 2nd, she tends to jump out of that gear, which doesn't happen in 3rd... grin grin...

Somehow, I still think most people in Europe would exhibit a slightly different "driving profile" from the U.S. I think the type of road infrastructure, and the directional shape and physical size of city-streets, suburban-streets etcetera... also play a large role in that. I sure do not wish to "patronize" our U.S. Porsche friends, but my feeling is that the "average" U.S. driver (1) doesn't have a manual box and (2) if he/she does, shifts at far lower revs. If I had a 5.4 liter V-8, I probably would do likewise... (I don't really know this first hand, though).

[masraum]

Belgik, you're probably right in a lot of ways, thank goodness that I'm not the "average" US driver.

How's that bumper sticker go? "Drive it like you stole it"