This may be obvious to others, but for maximum acceleration, when is the best time to shift gears? I am most interested in 3rd to 4th and 4th to 5th. The reason I ask is that I am looking at my torque curve and it peaks at 4,500 RPMs (630 at the wheels) and tails off to 425 by 6,500 RPMs. HP peaks at 5,000 (at 560) and starts to tail off a little bit from 5,500 to redline where it sits at 530hp

So, when is the best time to shift? I am thinking I want to maximize torque all the time, so shift after 5,500 when the declining torque drops below what the next gears starting torque will be at that speed. Does that make sense?

This seems like a science, so there must be some math behind this.

 

Your method sounds exactly right. You want the car to have the most power at all times for the best acceleration.

Your car seems to peak on power very low in the RPM range and tail off quite a bit. Is that deliberate because you wanted more power lower down or a result of the best tune your tuner could find for those mods?

 

You want to hold the lower gear until redline, because you put more power to the ground in the lower gear.

 

And keeping it in the same gear up to redline will keep the power going continuously without the 300 millisecond cut in power during an up shift.

 

Torque gets you moving from the stand still and it horsepower that keeps you moving once you leave the line..
Rule of thumb,basic 101,you usually shift at an RPM about 10% higher then where your peak HP is..A dyno sheet and knowing the gear ratio spread would show you more exact..This is due to gear recovery during shifts to keep it in the HP zone..
It is faster to accelerate past peak HP,redline established, then pulling up to the HP range zone.That is called gear recovery.
I am talking about drag racing type driving here..Roll ons and intial hits have more to do with your TQ range..

 

It's all about torque and gear multiplication. You want to be where you're putting the most torque to the ground taking into account the gearing. It's almost always redline in 1st through 3rd and also 4th and higher, but not always.

I have a spreadsheet with all the calculations, to bad it's at work.

Later, Steve

 

HI,Sorry to say BUT I think you might be mistaken,unless you are talking about diesel motors or work truck motors.
Its all about the HP and RPMs to get the quickest acceleration once you launch off the line and are moving..If you are talking about getting the best power acceleration, like in a 1/4 drag race or longer.
You need to get the most HP in the RPM area/curve that your motor spends its time in during the gear..Gear Recovery is also why you shift past PEAK HP.
TQ does not win accleration drag contests,unless you stay in 1 and 2 nd gear..lol..
You can read any high performance motor building book about what I am informing you on..But everyone has their right to their own opinion...And I dont know everything..
I have built many motors and drag raced in my life..I am just sharing my info here...We all learn by sharing.

 

I agree with this especially for 1->2, 2->3, shifts. 3->4, probably a tad under redline would be optimal but this is set-up dependent, i.e. big turbos-shift @ redline, small peaky turbos-shift a little earlier. Basically when you shift you want the rpm to land in the power zone and that's dependent on the gear ratio.

 

We're saying basically the same thing. Hp is just a factor of torque, but you are correct, maintaining your hp through the top of the rev range will give you more torque to accelerate the car in the rpm range where you have the gear multiplication advantage.

Give me a little time and I will post a chart.

Later, Steve

 

A quick way to measure this theory and to see how quick your car feels by seat of the pants>>Trying going thru the gears and shifting at peak TQ each time ,, Then try that shifting your car at around a 10% higher RPM then your peak HP...

 

I'm not saying you shift at peak torque. Basically you don't shift until one of the following.
1. Your going to be making more torque in the next gear (factoring gear multiplication)
2. You hit redline

As you can see, both will be well above your torque peak.

Later, Steve

 

HI Steve,,I think it should be the word HorsePower instead of Torque when you go into the next gear after your take off..Maximum torque is made lower then maximum horsepower,as the redline dictates that..
Sorry we are going back and forth with this..
I believe and have proven at the drag strip thats its HP that accelerates and wins races once you are off the line..
Their are many articles in many high performance driving books that explain this on how to get maximum acceleration..
Later,John

 

Hopefully the chart below will help.

HP is made with RPM (at a very basic level), so the higher your hp in maintained, the longer your car will maintain torque and the more gear you can run. This is why you are correct that your goal is HP.

The blue areas of the chart below show what gear you should be in at different rpm's and MPH. You can also use it to determine your shift points.

I used an online "stock" dyno chart, corrected for crank hp (the torque numbers were high, but it appeared this was due to a boost spike in the lower rpm range. Either way it shouldn't effect the caluclated shift points.

Discussions are always good, and this should definitely generate some discussion.

According to the chart, you should shift at redline in 1st, 2nd and 3rd. In 4th and 5th, you should shift at 6,000rpm.

 

OK,

I've done a little research and believe that since HP is a function of Torque and RPMs (i.e., HP = Torque * RPMs/5252), the goal is to keep the car in the max HP range as much as possible.

That said, for my car, and most 996TTs the peak HP comes between 5,000 and 5,500 RPMs (562 at the wheels). This was true before my turbo upgrade/tune and after as well, though it is worth noting that after the tune the peak HP curve is flatter (longer) at the top, which is a good thing. After 5,500 RPMs my peak HP starts to tail off as Torque fades reducing to 530 by 6,000 RPMs and it holds pretty steady from there to redline. Conveniently as you will see in a moment, my HP is 530 at 4,500 RPMs.

So, with that background, ideally I want to manage my shifts so that my upshift does not take my next gear RPMs minimize the HP drop above 6,000(530hp). Since HP at 4,500 is conveniently 530, I want to shift when the upshift will result in next gear RPMs above 4,500.

My transmission gear ratios now come into play to determine what the RPM drop will be for each upshift. My X50 tranny is stock, so from the owners manual I get:

1st = 3.82
2nd = 2.05 (53.7% of first)
3rd = 1.41 (68.8% of 2nd)
4th = 1.12 (79.4% of 3rd)
5th = 0.82 (73.2% of 4th)
6th = 0.75 (91.5% of 5th)

Playing this out for shifts between 5,500 and 7,000 for each gear, I get the following useful information:

Shift 1st to 2nd at 7,000 and end up at ~3,757 RPMs, lower than you want, but at redline

Shift from 2nd to 3rd at 7,000 and end up at ~4,815 RPMs

Short shift from 3rd to 4th at ~ 5,700 RPMs (peaked at 530 HP) and end up at ~ 4,525 RPMs just above 530HP

Short shift from 4th to 5th at 5,500 RPMS (peaked at 530 HP) and end up at 4,510 RPMS just above 530hp

Short shift from 5th to 6th at 6,000 RPMs (peaked at 530 HP) and end up at 4,500 RPMs at 530 HP

All of this assumes want to continue to accelerate for at least a few more seconds, i.e., ignores the lost acceloration of the shift time.

I am a bit surprised by the short shifts to 4th, 5th and 6th, but that's what my math says. Does anyone see flaws in this math? It seems to be a product of flat peak HP between 5,000-5,500 with drops on both sides of that range.

I am happy to be told this is wrong if you have math or other facts that make the case.

Thanks all

 

tq vs. pwr!
one of the most discussed topics in car forums since the digital revolution

to accelerate optimal you need to keep in the best area under the power curve.

IIRC there is an extra thread here in 6speed discussing exactly that.