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- - Shift Points for Maximum Acceleration (http://986forum.com/forums/boxster-general-discussions/6447-shift-points-maximum-acceleration.html)

Quote:

Originally Posted by 986Jim

This is simply NOT TRUE!!!! Spend some time at the rack track. Your Porsche can make 250whp between 4500rpm and 6500rpm range steady but a car that makes 200whp starting at 4500rpm and makes 300whp at 6500rpm will be faster.

jim, you're missing a lot of points. when we say average horsepower at the WHEELS, we mean after GEARING is factored in. when you run a car on a chassis dyno, gear ratios are entered in order to calculate the RWHP that the ENGINE is generating. this takes the transmission out of the equation. in order to calculate a car's expected acceleration, we CANNOT ignore the transmission and the mechanical advantage it provides.

an example:

current torque at flywheel: 200 ft-lb
gear ratio: 2.00
rear end: 3.00
wheel diameter: 13.00 inches

the car is making 1108 ft-lb of torque at the wheels. now, let's say the car weights 3000 lb and is travelling at 60mph (88 feet / second). the car is performing 3000 * 88 = 264,000 ft-lb / sec of work. this is equal to 480 HP. a chassis dyno would have factored in the gearing and given us an answer of 200 ft-lb less drivetrain losses (~170ft-lb).

the car that is putting out the most average power at the wheels (including mech. advantage of tranny) over a set distance will get there the fastest, period. if you win a race, you HAVE DONE THIS. you had a set amount of work to do; you did it the fastest. you have maximized average power. in most cases, this does occur if one shifts at or near redline, but this is not ALWAYS the case.

imagine a car that's horribly tuned whose torque SHARPLY falls off at 4000 rpm. what if it redlines at 8000rpm? the car would drag ass from 4000 - 8000. using this RPM range would be a waste; the driver would want to sandwich the motor's sweet spot (4000 rpm) in the center of his shifts.

that having been said, engineers / tuners want to take advantage of the power that RPMs provide, particularly with sports cars. they will try to maximize this advantage by tuning the car to produce more torque in the high rev range, translating into more power. because most sports cars are well tuned by the manufacturers, it makes sense that your experience tells you to shift at / near redline. i'm just saying that this isn't always true.

Quote:

Originally Posted by 986Jim

Here's the thing about the camero. Even tho it lost steam up top, it still will accelerate faster if you take it to redline. Making the car accelerate fast has more to do with being where the engine can accelerate than where it makes the most power over the logest part of the curve. Changing the intake manifold would have just made it more efficient and could beat the NSX but he should still shift at redline.

Lets say the Camero has a redline of 6000rpm. But makes peak power at 4500rpm which is probablly pretty close to the truth. Lets also assume the gears are long which they are so switching to the next gear will induce a drop of 2000rpm from whatever point you are at. Based on the curve of that engine it would probablly be best to shift at 5500rpm and land at 3500rpm where your in the greatest part of the curve. The problem is the engine will not accelerate as fast even though your in the strongest part of the curve.

Shifting at 6000rpm even though your way over the curve is better. The engine accelerates faster at 6000rpm with less hp than it does at 4500rpm with a lot more, thats what I'm trying to say. So shifting at 6000rpm and landing at 4000rpm is in a point where the engine will accelerate faster even though your not in the maximum part of the torque curve. It's all about how freely the engine can accelerate.

It's for the same reason many companies say you can't spray nitrous on a motor at less than 3000rpm. This is true most of the time but not all the time. The engine car freely rev in 1st and 2nd gear in most cars so you can spray them right at 1000rpm outta the hole in drag racing. You can't do this in 3rd onwards however because the engine ability to accelerate is lower so the stress on the motor with all that power is too high and will melt a piston.

This is the same way the engine works when accelerating at the top. It will accelerate more freely between 5500-6000rpm even tho it's making less power so acceleration will be greater even with less hp. I know this doesn't sound right, but a quick trip to the strip with that Camero would prove this, short shifting the engine into a zone down low where it doesn't accelerate is worse.

There is a trade off. You have 2000rpm you must accelerate in no matter what. What will accelerate faster? The zone from 3500-4000rpm or the zone from 5500-6000rpm? Look at this graph and you will see what im talking about. The Green zone is better because it will accelerate in the last 500rpm faster than the first 500rpm in the red zone. This is only true because its up higher in the rpm band where it's easier for the engine to accelerate. Even though it's outta the curve, the last 500rpm (5500-6000) is just a faster spot for the engine than 4000-4500rpm spot at the beginning of the shift.

What we are really debating is which 500rpm are better? The 500rpm frpm 4000-4500 or the 500rpm from 5500-6000. The rest of the rpm in the middle we are both in regardless.

Hope that helps,

You're ignoring torque at the wheels here by just looking at the engine curve and talking about rpm levels. You can't tell where acceleration is optimal without looking at torque made at the wheels. Insite is trying to tell you the same thing.

Mtch was talking about accel runs that he ran himself where he found short shifting to be better than running to redline on an engine who's torque peaks early drops quickly thereafter.

No response, 986Jim must have been beaten into submission.

Quote:

Originally Posted by NickCats

Just got back from a 2 hour romp. Man, I love this car !

Question, does anyone know ( or know where I can find ) the shift points for a 99 Boxster ?

Example ( fake numbers ) :

1st Gear : redlines at 30 mph
2nd Gear : redlines at 60 mph
3rd gear : redlines at 90 mph
4th gear : redlines at 120 mph
5th gear : redlines at 150 mph

Thanks !

Nick

If you're asking about max acceleration rpm, download the spreadsheet and put in the correct information. It will tell you where to shift. The torque curve and gear ratios have been conveniently provided in your owner's manual. I've deducted 14% of the torque at the wheels for driveline losses in the calculation so you don't need to account for that.

Quote:

Originally Posted by blue2000s

Mtch was talking about accel runs that he ran himself where he found short shifting to be better than running to redline on an engine who's torque peaks early drops quickly thereafter.

This is the point I was trying to make. I initially ran to redline, but found better results short shifting.

I knew I could have made minor modifications to the Z-28 to open it up, but had plans on super-charging, so I wanted save up and do it all at once. However, once I'd saved the money, I couldn't see spending it on a Z-28

Quote:

Originally Posted by I love Bourbon

hey 2000s,

pull your head out of your butt

if you could actully read your engineering or whatever books

you would understand that torque is force, while horsepower in force/time or work

your best shift point would certainly not cut out your point where you are making the most power.

you shift based on power not torque.

maybe you should invest in a better education.

(I actually registered to this forum to post, as you are too ignorant to let slide)

I can't belive you people are giving this moron as much thought as you have

a little information in the hands of the ignorant is a dangerous thing

Let me guess, you're a kid who hopes to be an engineer someday and you're almost sure what you've learned in high school is enough to make your argument. It's not.

I'm not going to waste my effort trying to educate you, kid.

[QUOTE=I love Bourbon]

"hey 2000s,

pull your head out of your butt

if you could actully read your engineering or whatever books

you would understand that. . ."

You could make the same point without being nasty and insulting. You would look better too.

I think the Owners' Manual has a chart with them in the back of the book.

[QUOTE=Steve Fine]

Quote:

Originally Posted by I love Bourbon

"hey 2000s,

pull your head out of your butt

if you could actully read your engineering or whatever books

you would understand that. . ."

You could make the same point without being nasty and insulting. You would look better too.

The only point the kid has to make is "hey, I'm overflowing with more testosterone than intelligence"

Quote:

Originally Posted by Steve Fine

I think the Owners' Manual has a chart with them in the back of the book.

The owners manual does show a plot of vehicle speed vs engine speed for each gear, but it's not much use in telling the best shift points.

Quote:

Originally Posted by blue2000s

No response, 986Jim must have been beaten into submission.

Nah, it's not that. I'm just getting tired of the bickering back and forth. Drive your car however you like really. For me, I know the fastest times I can ever get from my car will depend on my driving style, and for me it's pretty clear how I plan to do that.

This whole thread is bench racing which is pretty pointless.

Quote:

Originally Posted by 986Jim

Fair enough, but if you get the chance to drive a car where redline didn't net the best times, you'll now better understand why.

Quote:

Originally Posted by blue2000s

Fair enough, but if you get the chance to drive a car where redline didn't net the best times, you'll now better understand why.

Here we go again...

Just out of curiosity what car have you driven and raced that gave better times in any form of racing where you didn't shift at redline?

I have Dragged Race blown Domestic's on Alcohol, Imports on Race Gas and Pump gas, Drag raced Motorcycles as well and road course and full blown track street bikes. I have done auto-x (where there is no shifting really depending on the course) and 3 day track events, done driver for a day with a whole host of Porsche cars, Formula 2000 test days in the open wheel cars, mud buggies, and tried my hand at dirt track even. Hell I even did a weekend in full Donzi race boats with twin 1000hp motors. None of these forms of racing in all the different cars etc. I have driven was what your saying ever true. If I find any examples of this I will definitely let you know. In September this year I'm going to Mosport with Pfaff Porsche/Audi/VW to drive about 30 different models from Diesel Turbo V10's to Twin Turbo Porsches, if I find anything I'll be the first to let everybody know. Hell even one of my collegues from another magazines I used to work with did a NASCAR test track day in a restrictor plate car and even with those they were told to redline the cars right out, in that big ass V8 with a restrictor plate on it.

If you have an example for me please, I'm all ears...

Just a reminder that these threads CAN proceed without any insults at all. I already deleted one in the spirit of keeping the dialog on the up and up.

Play nice gentlemen.

It is ALL opinion anyway, as well, this IS the internet.

Thanks

Quote:

Originally Posted by 986Jim

You've driven alot of cars but they all sound like their dedicated racing machines. If that's all you've got experience with, then I'm not surprised with your beliefs. You may be looking at a bit too narrow of a view, the world's not all race cars.

Throwing out mtch's statements, have you ever tried different shift points in a Camaro, or any other cars with big engines that are made for low end torque for throttle response rather than being made for racing or high performace street driving? How about a GMC or Dodge diesel 4x4 with a towing package?

Quote:

Originally Posted by blue2000s

Throwing out mtch's statements, have you ever tried different shift points in a Camaro, or any other cars with big engines that are made for low end torque for throttle response rather than being made for racing or high performace street driving? How about a GMC or Dodge diesel 4x4 with a towing package?

A diesel? Wow, ok well enough of this thread. I'm done.

How about some exit music?

:ah:

Quote:

Originally Posted by 986Jim

A diesel? Wow, ok well enough of this thread. I'm done.

How about some exit music?

:ah:

Yup the typical torque output of a diesel engine is an example of where shifting before redline will maximize torque production at the wheels.

You have to consider more than just race and sports machines if you want to make an all-encompassing statement like you have above. If you look at a narrow range of cases to make a decision, you're bound to miss something.

By the way, you're still ignoring mtch's statement.