Here's one for the Math Majors / Engineers out there

I was wondering the actual distance of the front straight at Lime Rock Park in Ct, from the bottom of the downhill to turn 1?

Basically, I am trying to figure out if I will run out of revs by changing the gearing in my diff. I am 600-700RPMs shy of max in 4th gear. I'd like to use more of that power band down the front straight.

As an English major in college, I am open to any quick conversion calculators out there on the web, or the formula that I should use to determine the distance covered, while accelerating in a particular gear. I know wheel circumference, car weight, tranny gear ratio, diff ratio, and approximately how fast i am going at the bottom of the downhill. Am I missing any values?


Thanks.

Dave

Short answer (assuming that I can guess your redline and torque curve, and that you meant redline by "max") is that it would not help.

These might help if you want to play with numbers:

(new RPM)=(new R&P)*(old RPM)/(old R&P)

or

(new RPM)=(old tire diam)*(old RPM)/(new tire diam)

I don't think using the F=ma formula would help without using the actual torque curve of the engine, if that was your question. But I guess it would look something like this if the acceleration (or torque curve) was assumed to be constant (which it most definitely is not):

t=2*dist/(Vf+V0), where V0 is assumed to be the same starting speed coming off the downhill, and Vf is the speed at the end of the straight.

Now, if you have the actual torque curve in digital form, it could be digitally integrated to arrive at the new Vf, whether higher or lower than the old, and the new t, whether longer or shorter duration than the old. Perhaps a G-Analyst could get close...

(new a)=(old a)(new R&P)/(old R&P), but only for exactly the same instantaneous engine speed.

Go check out the software available at www.davesport.com

K

4 speed or 5 speed? If it's a 4 speed I'd think that you wouldn't want to change the diff ratio, as you may not be able to get a small enough change and you could end up having to stop accelerating before the end of the straight because you ran out of revs. It'll also leave very little room in case you do some chassis/suspension tweaks that gain you some corner speed, then you'll be faster on both ends of the straight and could run out of revs at the end.

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4 speed or 5 speed? If it's a 4 speed I'd think that you wouldn't want to change the diff ratio, as you may not be able to get a small enough change and you could end up having to stop accelerating before the end of the straight because you ran out of revs. It'll also leave very little room in case you do some chassis/suspension tweaks that gain you some corner speed, then you'll be faster on both ends of the straight and could run out of revs at the end.
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Its a 5 speed, but 5th is overdrive (0.80:1), and the revs drop way to low to get the car to pull like I'd want it to. The car has a 4.10 rear diff now. I have a 4.27 ring and pinion that I want to try in it. And if I do run out of revs, I can always hit 5th for the last bit of straight that I'd need it for. We are taking the car to the test day with the 4.27 installed to try it out before the RunOffs.

Eric - thanks for the formulas. I don't have the torque curve.

Knestis - I'm heading over to davesport.com Thanks.

Let me guess,

Team Jaegermister, you're in an e-21 318 with the early 5 speed. Right? You should be able to find as low as a 4.44 with some automatic transmission e-30's. It'll take some really low gears to even make 5th useable. Then because the ratio is so large between 4th and 5th the rest of your gearing is off. I'd think that the 4.10 would be close to ideal as it's also a 2002 gear. I did my school in a 02 and everytime I hit 4th the motor bogged some. Actually for that course the corners were fast enough and the straights short that I left it in 3rd and only in the sprint race on Sunday did I start to shift down to 2nd and back to third to get the rear to bite. You're motor should have a good torque band, have you dyno'ed the car yet? That's the only true way to tell where you should shift and what kind of gearing you'll need. You may find that you can rev to 7000, but there's nothing to be gained by shifting after 6500, or you may have to wait untill above 6800 to hit the torque peak in the next gear. When I want to test different final drive ratio's I go to GT-4 on the Playstation 3, you can find a car that closely matches in suspension, hp, torque, weight and tires, purchase the customizable transmission, then adjust the gear ratio's to match what's in your car. My shift points at Laguna Seca were within 50 ft of where they were in the game, just remember it's only a game and you can't feel the contact patches like you can in a real car, so figuring out what the car is doing is tough, but it's much less expensive than going out for real to try something you havn't yet bought. Good luck,

James

I have created a spreadsheet that is useful for determining gear, shift points, rear end ratio's etc.

If you enter the actual torque curve of your engine, and your gear ratios, you can determine exactly where you should shift.

You can download it from

http://homepage.mac.com/ferg

Hewland Gear Selector.xls

(also useful for any manual transmission).

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Let me guess,

Team Jaegermister, you're in an e-21 318 with the early 5 speed. Right? You should be able to find as low as a 4.44 with some automatic transmission e-30's. . When I want to test different final drive ratio's I go to GT-4 on the Playstation 3,

James
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Hey James,

Team Jagermeister, Yes, but this is acually for our FP car which is a 320i (e21) with the reg 5-speed and 15inch slicks. Our e30 318 runs 14inch DOT's with a 4.10 and the 5-speed. Bob has a 4.27, but we haven't come across a 4.44 yet. He is set on running the 15's, so that is what we are trying to gear for. No, we haven't Dyno'ed either car yet.

Is there a definite word if the Dynapck is going to LRP for the RunOff's? If yes, I'll be trucking both the FP and ITB car down there for a few pulls to get some numbers. THEN, we can start to figure out where we need to go with these cars. It has always been, "change this, and lets see what it does...... OK, now lets try changing this..... Never with any hard engine numbers. We've been flying blind in that respect.


Does anyone know of a Dyno in the NY Capital District area?


thanks for the Grand Turismo idea on the PS. I wish they had some of the tracks in there that we run on. Is the Glen contractually tied only to the Nascar game?

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I have created a spreadsheet that is useful for determining gear, shift points, rear end ratio's etc.

If you enter the actual torque curve of your engine, and your gear ratios, you can determine exactly where you should shift.

You can download it from

http://homepage.mac.com/ferg

Hewland Gear Selector.xls

(also useful for any manual transmission).
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David,

Thanks for the useful link to your downloads.
I like your webpage. Under "S2 Now and Then", how did you do the video insert in lower left corner? I have aq G5 with iMovie and iDVD. That is a very cool "side by side" comparison of laps.

Dave Austin

Dave, see the NE section on the dyno question.