[Subscribe to Daily Digest] |
 | [Main General Bulletin Board | BBFAQ |
Prev by Date | Next by Date | Post Followup ]
Member Login / Signup - Members see fewer ads. - Latest Member Gallery Photos
Do you know how to shift a turbo? (Looooong) Posted by PGAero [Email]  (#1143) [Profile/Gallery]
(more from PGAero) on Fri, 17 Apr 2009 13:11:48
In Reply to: Does Anyone Know How to Shift a Turbo?, moeman, Thu, 16 Apr 2009 21:09:23 Members do not see ads below this line. - Help Keep This Site Online - Signup |
As you mentioned, the Saab turbo engine typically has a very broad, flat torque curve. Aero'ed gave a good example of why it's good to drive through the rev range, even in a turbo.
Let's assume, for a moment, that the torque curve is perfectly flat from 2000 RPM to 5000 RPM (It's not, but it's pretty close, right?).
Now, since torque is the measure of how much force can be transmitted to the ground by the (front) tires (Torque = Force*perpendicular distance... and that perpendicular distance is the rolling Radius of the wheel/tire combo), then the force causing acceleration in, say 3rd gear, is constant through the rev range of 2000-5000 RPM. Now, since F=m*a, and since the force remains constant, as does the mass of the car, then the acceleration remains constant in 3rd gear. Now, the tricky part. The transmission modifies the torque created by the engine based on the gear ratio. First gear actually increases the torque of the engine through gearing. Second gear increases the torque, but by a lesser amount than second gear, so the force accelerating the car is less in second gear than in first, and likewise through the gears.
During a 0-60 run, you want to maximize the acceleration of the car. Since you can't reduce the weight (mass) of the car (after a certain point), you want to maximize the force applied to the car (through the pavement). If you know that running in first gear results in a greater force than does 2nd, than does 2nd, than does 3rd, etc..., then doesn't it make sense to keep the car in 1st as long as possible?
Here's some numbers on the 9000 Aero (5 spd):
Torque 258 lb*ft at 1950 rpm to about 5000 RPM
Final Drive Ratio: 3.61:1
First Gear: 3.38
2nd: 1.76
3rd: 1.18
4th: 0.89
5th: 0.70
So, the car can produce the following amounts of torque (force) in each gear, between the indicated RPMs (Force is calculated by dividing torque by the +/-12" rolling radius of the 205/55, which is 1 foot, which means that the values of torque and force will be the same with that tire):
1st: 258*3.61*3.38 = 3148 lb*ft (3148lbft/(1ft)=3148 lb)
2nd: 258*3.61*1.76 = 1639 lb*ft (1639 lbs)
3rd: 1099 (1099)
4th: 828 (828)
5th: 651 (651)
Remember that the force is transmitted through not one, but two tires, and some frictional losses reduced the torque at the wheels anyway, so each tire is asked to transmit something less than half of the force calculated above. What would happen if you put huge tires on your car?
So, if I want to accelerate fast, then would I be better off shifting in to 3rd gear as sooner or later? You got it, it's best to shift later.
Since car engines do not produce perfectly flat torque curves, there comes a point where the torque at the higher RPMs begins to drop off, which means the force accelerating the car begins to drop off. You want to shift when the next highest gear, at the correspondingly lower RPM, provides as much force as the one you just shifted out of. In all cars, this point is beyond the RPM that torque values begin to drop, since the subsequent gear ratio reduces the torque to the wheels.
Now, on to horse power... Power is the (dot) product of the force and the distance covered. Since a car moves faster at 5000 RPM than at 2000 RPM (in the same gear), it covers more distance at higher RPM, so the power expended must also be greater. That's why an engine that has a flat torque curve has a steeply rising HP curve. And an engine with a rising torque curve has an even steeper HP curve. Really, you measure torque, and you calculate power. HP = torque x rpm / 5252 (As Aero'd said earlier).
I feel like this kinda jumps around. If you want me to clarify, I'll be glad to, but I might want a bit of time to organize my thoughts.
A couple of notes:
*I know that weight and mass are different quantities, but on the surface of the earth, they are linearly related by "g", so don't give me any guff ;o)
*Certainly friction and air resistance play a role in this, but they do not affect the conclusion. You'll just have to trust me on this one.
posted by 69.230.108...
_______________________________________ Current: '03 9-5 Aero Wagon, 5spd, Polar/Black Past: '06 9-5 Combi, AT, Polar/Black '04 9-5 Aero Wagon, AT, Nocturne/Granite '03 9-5 Aero Wagon, AT, Steel/Charcoal '00 9-3 Viggen, 5D, Silver/Black '93 9000 Aero, 5MT, Cirrus/Black (Owned this one twice) '86 900 SPG, 5MT, Edwardian/Buffalo Grey
, Fri, 17 Apr 2009 13:11:48 <-- Viewing This Message, Mon, 20 Apr 2009 10:48:05, Mon, 20 Apr 2009 14:03:39, Mon, 20 Apr 2009 10:54:38
, Mon, 20 Apr 2009 09:40:52, Fri, 17 Apr 2009 15:25:50, Wed, 22 Apr 2009 10:10:36, Fri, 17 Apr 2009 12:14:27
, Fri, 17 Apr 2009 06:03:59, Sun, 19 Apr 2009 04:28:17, Sun, 19 Apr 2009 08:58:37, Sat, 18 Apr 2009 17:25:46
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
