is dct faster than manual transmission

hey whats up ... i feel kinda stupid not knowing the answer to this question but i would like to know anyway being that the dct m3 and manual transmission m3 both have the same engines does the dct have a higher top speed than the regular transmission being that the dct has 7 gears and the manual only has six?

DCT has numerically lower (higher top speed) ratio in top gear.

DCT 7th gear is 1:1, and diff is 3.154 = 3.154
6MT 6th gear is 0.872 and diff is 3.846 = 3.353

The DCT has ~ 6% higher CALCULATED top speed advantage. No way does either of them pull off > 200mph on an actual road though.

I don't understand why the DCT doesn't use lower ratios throughout, and build it with a calculated top speed around 175.

thanks for the info but i am not the brightest crayon in the box when it comes to gear info.. everything you wrote might of well been in chinese as i did not understand it... so if ya dont mind can you put it in m3 info for dummies so i could understand it a lil eaiser... i think i got one part that the dct is 7 percent faster? but has low gear speeds...

Originally Posted by iluvmymthree

thanks for the info but i am not the brightest crayon in the box when it comes to gear info.. everything you wrote might of well been in chinese as i did not understand it... so if ya dont mind can you put it in m3 info for dummies so i could understand it a lil eaiser... i think i got one part that the dct is 7 percent faster? but has low gear speeds...

OK, here's the deal.

Using the gear ratios from BMWUSA, and the tire diameter for the OE Competition Pkg tires from Tirerack, the CALCULATED top speed for the M3 is:

DCT: 199.55mph
6MT: 189.51mph

So, ASSUMING THE FACTORY ELECTRONIC SPEED LIMITATION WAS REMOVED, my GUESS is that the 6MT can pull to redline in 6th, and go ~190mph. Knowing the BMW speedos are often about 4% OVERindicating, I'd expect you could see a speedo reading of ~ 197mph.

I doubt the DCT can pull to redline in 7th gear - I just don't think 414hp is enough to go 200mph in that car. I don't know exactly where from the 166mph shift into 7th, to the 199mph calculated (207mph indicated on speedo) top speed that the DCT would top out, but if I had to guess, I'd say the low 190s. You could likely hit an INDICATED 200mph (192mph actual speed) ...

As far as what all the numbers mumbo jumbo means ...

All of the gear ratios in discussion, require the engine to spin the number of times listed in the gear ratio, to turn the output side of that gear ONCE.

So for example, if you're in first gear, and your M-DCT gear ratio is 4.78, then for each 4.78 times that the engine completes a single revolution, the OUTPUT shaft of the transmission will spin ONCE. However, since that output shaft is the INPUT to the differential, we also have to consider the differential's 3.185 ratio. To do this, we can MULTIPLY the two ratios. So, to spin the rear tire ONCE, the engine must spin 4.78 x 3.185 (15.22) times.

So, about 15 engine revolutions = 1 tire revolution for an M3 with M-DCT in 1st gear. How does that get us to top speed in this (or any) gear?

The engine can spin up to the redline of 8400rpm. Speed is usually described in mph, so we need to match our units and determine how many revolutions per HOUR the engine can spin. 8400rpm x 60min/hr = 504000revs/hour.

Now, if we can spin the engine at 504,000 revs/hour, and it takes ~15 revs (15.22 is the actual number) to spin the tire ONCE, then 504,000/15.22 (33,114) is the max number of TIRE revolutions/hour.

Now all we need to know is the tire diameter, or the number of tire revolutions/mile, to determine the max speed in 1st gear.

In this case, the ZCP rear tire spins ~793 revs/mile. We know we can spin this tire at 33,114 revs/hour, so lets find out how many mph that is ...

33,114 / 793 = 41.75mph.

Since we're talking about top speed in TOP gear, let's calculate THAT:

DCT:

7th gear ratio = 1:1, so each spin of the input (engine) = 1 spin of the output
differential ratio = 3.185, so 3.185 spins of the driveshaft (transmission output/ diff input) = 1 spin of the tire

If our engine is spinning at redline, we have the same 504,000 revs/hour as above.

To calculate our total final ratio, we multiply the transmission ratio (1) by our differential ratio of 3.185, and get 3.185.

So, our rear tire at redline in top gear is spinning 504,000 / 3.185, or 158241.75 revs / hour.

If we divide that by the number of revs required to go one mile (793), we get the calculated top speed of 199.54mph. This answer differs from what I stated at the beginning by 0.01mph, because I rounded a bit in this example to keep the text clean.

I'll leave the 6MT calculation to you, but the answer is ~189mph, as stated above.

Damn. Check out the big brain on Kurt...... (what movie?)

To the OP:

It's been said that the DCT has consistently recorded faster 0-60 times if this helps your cause at all.

Originally Posted by poleposition

Damn. Check out the big brain on Kurt...... (what movie?)

A GREAT movie, and quote machine, you bad M.F. !

To the OP:

It's been said that the DCT has consistently recorded faster 0-60 times if this helps your cause at all.

0-60 and top speed are likely a percent or two faster for dct. But the decision to go dct is more about style than speed.

I prefer the technology, convenience and speed of dct, but I totally get the driver involvement advantage of manual.

Sent from my DROID BIONIC using Bimmer

Originally Posted by Kurt_OH

0-60 and top speed are likely a percent or two faster for dct. But the decision to go dct is more about style than speed.

I prefer the technology, convenience and speed of dct, but I totally get the driver involvement advantage of manual.

Agreed!

I've had my car a while and dct shifting still feels weird and slow. I know it makes sense as a convenience and daily driver, but I miss popping a clutch and dropping it into 2nd.

I feel dct lag.

I have not experienced that. My DCT changes gears in milliseconds. What is your transmission set on when you feel this lag? Certainly not S5 or 6. I could see feeling a delay if youre in D1 or S1.

Originally Posted by Kurt_OH

DCT has numerically lower (higher top speed) ratio in top gear.

DCT 7th gear is 1:1, and diff is 3.154 = 3.154
6MT 6th gear is 0.872 and diff is 3.846 = 3.353

The DCT has ~ 6% higher CALCULATED top speed advantage. No way does either of them pull off > 200mph on an actual road though.

I don't understand why the DCT doesn't use lower ratios throughout, and build it with a calculated top speed around 175.

The gear ratios is designed around the torque curve and 0-60 time. Top speed in most cars with 7 spds usually is in 6th. gear. In a Porsche the automanual shifts faster than you can shift a manual trans, quicker to 60 mph, but the top speed is slower than a manual.

Originally Posted by Dave 330i

The gear ratios is designed around the torque curve and 0-60 time.

Yup. They can't have you make a 2-3 shift before 60; that's a key selling point for cars and they can't mess it up.

Top speed in most cars with 7 spds usually is in 6th. gear.

Surely (Shirley?) you're not suggesting the M3 DCT can't exceed 166 unlimited? That's simply not the case. What's your point?

In a Porsche the automanual shifts faster than you can shift a manual trans, quicker to 60 mph, but the top speed is slower than a manual.

ALL about gearing.