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E46 M3 (2001-2006)

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  #26  
Old 04-21-2006, 03:52 PM
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wag-zhp wag-zhp is offline
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The key to maximum acceleration is keeping the engine in the sweet spot of its power band. At initial launch from a standing stop, you have to mate the spinning crankshaft to the stationary tires. To do that you want the greatest torque multiplication that your tires can handle without breaking traction and spinning free , and without causing the engine to drop below the sweet spot of its power band (or bog down).

On the other end of the spectrum, or once the car is moving, you need maximum torque multipication for your desired top road speed, without exceeding your engines max rpm, or going too far outside the upper end of the power band.

In between these two extremes, initial launch and vmax, is where your transmission comes into play. The job of your transmission is to keep the engine in its power band at any given road speed. If all of the gear ratios are correct, the engine gets to go from the top of its power band in 1st to the bottom of its power band in second, and continue this progression with each upshift. Disassembling and changing the transmission gear ratios takes quite a bit more work than changing the final drive ratio, so you won't find many people doing that to a car that sees commuter use.

I've seen a gear ratio calculator somewhere (EDIT: Here is the thread that contains a lot of good info and a couple of calculators http://www.bimmerfest.com/forums/sho...tio+calculator) that will allow you to plug in a final drive ratio and max engine rpm, and then calculates max speed for each transmission gear. What one would have to do is determine their engines rpm power peak and then enter different final drive ratios to get them to their desired road speed.

Changing to this "ideal" final drive ratio would have a very detrimental effect on your fuel economy, but your car would be incredibly responsive all the way up to vmax. You could actually use all 6 gears and keep your engine in the peak of its power curve, while someone with a stock final drive ratio isn't even near the top of their power band in 5th gear. In an acceleration contest, that seems like a clear advantage to me.

The trick to changing your gearing is figuring out what you want and what you can live with. Sure, it would be great fun to slingshot from 0 to 140 every time you took off. But would you want to listen to your engine singing at 4k or 5k while cruising at 65mph, or have to fill the fuel tank every two days?

If I remember correctly from playing with that calculator, I figured that my car would be just about ideal for me with something like a 3.5 or a 3.73 final drive, along with a limited slip differential.
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Last edited by wag-zhp; 04-22-2006 at 01:34 AM.
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  #27  
Old 04-21-2006, 08:33 PM
Augenstein Augenstein is offline
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OK, it was unanimous before - and now, by God it's *still* unanimous. Gears are ET producers.

What a relief! I thought maybe somebody was going to disagree.

Quoting Bob -

"If you are hung up on horsepower as a measurement, give some thought to the fact that your hp peak is at a higher rpm than your torque peak, so gears that keep your revs higher also give you more hp to use."

Yeah. That's why geared cars often have slightly higher trap speeds in a quarter mile compared to their stock counterparts. By the way, I said this earlier in the string.

"Yeah, you can compare the low-geared car in 3rd gear to the high-geared car in 2nd gear and get strange results, but you can also compare apples to oranges."

The reason that geared cars tend to have only *slightly* improved (or even identical) trap speeds is that they lose out when they have to shift earlier, and...

Oh, never mind. Presumably you've read this string (including SilverState's pointer), and if you don't get it by now, you're never gonna get it.

Although I spend a lot more time on road courses nowadays than I do on drag strips, I can tell you I've spent a *lot* of time on drag strips, and as a student of the sport (including the physics), I'm letting you know that *none* of this is theoretical. Gears give you the launch, and downtrack, you may have a little edge - or maybe not.

Bruce
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  #28  
Old 04-22-2006, 02:43 PM
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Bob Clevenger Bob Clevenger is offline
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What's the deal with trap speeds? Trap speed in a drag race is meaningless. ET is what wins drag races, and if we are talking pure acceleration, we are takling drag races. I have seen many drag races won by the car with the slower trap speed.

The origonal post, however, was talking about top speed. Top speed will be limited by one of two things: final drive ratio, or wind and tire resistance --- whichever comes first. If your car is geared so low that you hit redline while still accelerating, then your gears are too low to reach the car's max speed. Your car's theoretical top speed is when wind and tire resistance equals the force being applied between the tyres and the pavement. In most street cars thet is reached long before redline (or hp peak) in top gear. To reach the highest speed possible, you want this to happen at your hp peak. So the OP is correct that changing to 4.10 gears from 3.62s will not reduce his top speed. With his S54 he is probably still hitting his top speed before redline (or hp peak). Of course fuel consumption is affected drastically by using such low gears.
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  #29  
Old 04-22-2006, 05:07 PM
SilverState SilverState is offline
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As for mileage, I can still make it from Las Vegas to Los Angeles on one tank. And this is cruising at an average speed of around 100mph, so I am not too concerned with the the difference in mpg.
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  #30  
Old 04-26-2006, 04:42 AM
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Pinecone Pinecone is offline
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Actually in setting up a road race car, that is what you do if you have unlimited gearing choices. You gear top gear and rear ratio for maximum speed you can reach on the straight, which gives the most acceleration to get to that top speed. You should reach redline in highest gear just as you lift for braking (or maybe not quite redline to allow for a draft during the race).

You pick the intermediate gears to first give a proper ratio change from gear to gear to maintain the engine in the power band, and then fine tune to get the exact right gear for each corner you can, starting with the most important corner (corner leading onto longest straight).

In this case you do gear for max speed at redline and NO MORE. But gas mileage is not a consideration (except maybe in endurance racing).

And actually Pro Stock cars do the same thing, they gear their 5 speed Lenco tranny for the proper spacing to maintain in the power band, then the rear ratio gives them redline in 5th as they cross the lights. Gives lowest ET.
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