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      02-21-2014, 04:21 PM   #1
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Can we get a longer 7th gear?

I really wish 7th gear was an overdrive to keep the rpms low on the highway.
I wouldn't want to mess with the differential ratios, but I do wish the car could be at 2,000 rpms rather than 3,000 rpm's at 80 mph.
Think its possible to put in a longer 7th hear without modifying anything else?
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      02-21-2014, 04:39 PM   #2
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Quote:
Originally Posted by roastbeef
I really wish 7th gear was an overdrive to keep the rpms low on the highway.
I wouldn't want to mess with the differential ratios, but I do wish the car could be at 2,000 rpms rather than 3,000 rpm's at 80 mph.
Think its possible to put in a longer 7th hear without modifying anything else?
Here's something no one really understands about ratios and highway revs. At 2,000 RPM at 80mph your fuel economy wouldn't be any better than it is at 3,000. The thing is the engine struggles to move the car at such low rpms with that kind of drag at 80. If you're worried about fuel economy that is. Also changing the ratio the make that change would be a bit of work.
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      02-21-2014, 04:41 PM   #3
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      02-21-2014, 04:47 PM   #4
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Quote:
Originally Posted by Eddien123
Quote:
Originally Posted by roastbeef
I really wish 7th gear was an overdrive to keep the rpms low on the highway.
I wouldn't want to mess with the differential ratios, but I do wish the car could be at 2,000 rpms rather than 3,000 rpm's at 80 mph.
Think its possible to put in a longer 7th hear without modifying anything else?
Here's something no one really understands about ratios and highway revs. At 2,000 RPM at 80mph your fuel economy wouldn't be any better than it is at 3,000. The thing is the engine struggles to move the car at such low rpms with that kind of drag at 80. If you're worried about fuel economy that is. Also changing the ratio the make that change would be a bit of work.
I do understand the concept of an engine working harder at a lower rpm, but I feel this engine does have enough power to move the car without too much more fuel consumption.
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      02-21-2014, 05:25 PM   #5
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Quote:
Originally Posted by Eddien123 View Post
Here's something no one really understands about ratios and highway revs. At 2,000 RPM at 80mph your fuel economy wouldn't be any better than it is at 3,000. The thing is the engine struggles to move the car at such low rpms with that kind of drag at 80. If you're worried about fuel economy that is. Also changing the ratio the make that change would be a bit of work.
This is incorrect.

If you changed the gearing to where the rpm dropped to 2000 rpm at 80 mph, you'd garner substantially improved fuel economy. The lesser reason is that you'd have reduced overall internal friction at 2000 rpm compared to 3000, and the greater reason is that you'd have to use more throttle at 2000 to maintain 80 mph (due to reduced engine power at 2000 compared to 3000), thus reducing pumping losses.

And no, the engine is not struggling at 2000 rpm. If it were, though, you'd get even better mpg. Best mileage would be garnered by having to use full throttle to maintain 80.

The "easiest" way to change highway rpm is to drop the final drive gearing. The penalty for that would be comparative laziness off the line, and possibly more clutch wear. Once well under way, there'd be little difference between the stock and geared cars, acceleration-wise.

Bruce

Last edited by bruce.augenstein@comcast.; 02-21-2014 at 05:35 PM.
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      02-21-2014, 08:45 PM   #6
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Quote:
Originally Posted by bruce.augenstein@comcast.
Quote:
Originally Posted by Eddien123 View Post
Here's something no one really understands about ratios and highway revs. At 2,000 RPM at 80mph your fuel economy wouldn't be any better than it is at 3,000. The thing is the engine struggles to move the car at such low rpms with that kind of drag at 80. If you're worried about fuel economy that is. Also changing the ratio the make that change would be a bit of work.
This is incorrect.

If you changed the gearing to where the rpm dropped to 2000 rpm at 80 mph, you'd garner substantially improved fuel economy. The lesser reason is that you'd have reduced overall internal friction at 2000 rpm compared to 3000, and the greater reason is that you'd have to use more throttle at 2000 to maintain 80 mph (due to reduced engine power at 2000 compared to 3000), thus reducing pumping losses.

And no, the engine is not struggling at 2000 rpm. If it were, though, you'd get even better mpg. Best mileage would be garnered by having to use full throttle to maintain 80.

The "easiest" way to change highway rpm is to drop the final drive gearing. The penalty for that would be comparative laziness off the line, and possibly more clutch wear. Once well under way, there'd be little difference between the stock and geared cars, acceleration-wise.

Bruce
Lower revs does not always mean better mpg. If the engine can't turn the gear without dumping more fuel into the cylinder then that doesn't help your economy. So it comes down to less fuel with more combustions (at 3,000 RPM) or more fuel and less combustions (at 2,000 RPM) it would essentially even out. That is why economy cars like accords don't have extremely low revs. The engine just struggles and uses more fuel.
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      02-21-2014, 08:59 PM   #7
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Yea, I get it, but the power is there with the m3.
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      02-21-2014, 09:33 PM   #8
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GM performance cars have had 6 speed manual trannies since the late 80s. 4th gear is 1to 1, 5th is od (.74), and 6th is very deep (.50) allowing very low cruising rpm and very good fuel consumption on the highway. A nice added benefit is quiet engine operation. Well over 10 yrs ago when I was still in school (where does the time go?) I had a Firebird Formula with the LT1 V8 and t56 trans. The car was a rocket for back then (mid 13s 1/4), and I could easily get almost 30 mpg on the road.

Not an M3, but I put a slightly shorter rear end in my car (from 3.15 to 3.38) but of course 6th gear is not at all tall for an overdrive. At 75 the car spins 2850 rpm, still gets good fuel econ, but if it was spinning 2200, the car would be obviously quieter, and of course get slightly better econ.

I certainly like the ratio spacing on my bmw (1-5), but I wish 6th gear was taller. 6th is only useful for cruising above 45-50 mph anyways, so why not get more out of it?

I am planning on getting an e90 m3 one day, but revving at a relatively high rpm on the highway is annoying, esp since it has enough power to do so at a lower rpm, and since they are such pigs on gas.

I know, I know... nobody buys them for fuel economy, but for the output they really are pigs. I'll still have an I6 in the driveway for road trips that don't involve going to the track
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      02-22-2014, 12:36 AM   #9
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Quote:
Originally Posted by Eddien123 View Post
Here's something no one really understands about ratios and highway revs. At 2,000 RPM at 80mph your fuel economy wouldn't be any better than it is at 3,000. The thing is the engine struggles to move the car at such low rpms with that kind of drag at 80. If you're worried about fuel economy that is. Also changing the ratio the make that change would be a bit of work.
There are plenty of 4-cylinder cars that run lower rpm than the m3 on the highway. It has nothing to do with economy and everything with autobahn/high speed performance.
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      02-22-2014, 08:13 AM   #10
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General rule behind the efficiency of internal combustion engines is that the efficiency is greatest with higher throttle and lower rpm's. General efficiency charts of an engine will show this.

All these contribute to friction which is tied into higher rpm/lower throttle application:
- friction from pistons, bearings, cams, chains, belts, and any other rotational part pre trans. Higher rpm = higher frictional losses
- Pump/air flow losses due to higher volume needing to be pumped thru the engine combined with partial throttle blade restricting airflow and causing turbulence with the blockage.
- fuel usage to throttle application isn't 1:1. You may put more gas into each combustion cycle dropping the RPM, but the number of cycles/injections is less. From 2k to 3k rpm, you do have 4 sparks / injections for every revolution in the 4 stroke system. You have essentially have 50% more injections at 3K rpm vs 2k.

This doesn't necessarily mean you should put it in 7th going 30 mph, as you also need to be within the engines effective power band. If the torque at 2k is very similar to 3k rpm, normally the engine will be more efficient at 2K.

Only real way to analyze is to see the efficiency charts of this engine, but all this crap was in my I.C.E. book during BSME courses.
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      02-22-2014, 11:06 AM   #11
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At least you have a 7th gear. We manual guys are stuck with six.
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      02-22-2014, 12:01 PM   #12
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Quote:
Originally Posted by Eddien123 View Post
Lower revs does not always mean better mpg. If the engine can't turn the gear without dumping more fuel into the cylinder then that doesn't help your economy. So it comes down to less fuel with more combustions (at 3,000 RPM) or more fuel and less combustions (at 2,000 RPM) it would essentially even out. That is why economy cars like accords don't have extremely low revs. The engine just struggles and uses more fuel.
Look. Without rancor, you need to do some more studying on the topic.

First, lower revs (meaning lower revs at cruise) results in reduced fuel consumption - for the indisputable reasons I've already mentioned.

Second, of course you'll need more fuel per power pulse in order to make the same power to maintain 80, but this in no way addresses the topics of friction and pumping losses. Do you dispute my claim that best mpg will be at full throttle at whatever cruise speed? If you do, you are disagreeing with any engineering tome on the topic, written in any language.

Do you disagree that one of the two* key reasons that diesels get better mpg than gas engines is because they effectively run at full throttle all the time, with power regulated only by fuel flow? If you do, you again need to do more reading.

*The other reason is that diesel fuel contains more energy per gallon than gasoline.

Lastly, do a little reading on pumping losses, would you? On the intake stroke, each piston has to pull down against a significant vacuum at cruise (because the throttle is nearly closed), and that effort robs power from the cylinder that's firing at the time. These losses may account for a 10-15 horsepower loss at cruise (out of the 50-60 you'll need to cruise at 80), meaning you're using fuel to do nothing more than power the next piston on its intake stroke. At full throttle, pumping losses are minimized (perhaps two or three HP), so you're effectively having to feed fewer HP to maintain cruise, thus maximizing fuel economy.

BMW obliquely addressed this topic more than 30 years ago, when they said that best MPG would be obtained by accelerating at full throttle and shifting at absolute minimum rpm - meaning actual acceleration would be leisurely, but most effective, mpg-wise.

Bruce
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      02-22-2014, 12:10 PM   #13
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Quote:
Originally Posted by Petros View Post
At least you have a 7th gear. We manual guys are stuck with six.
ya but the DCT 7 gearing is short and aggressive.
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      02-22-2014, 12:49 PM   #14
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Quote:
Originally Posted by Ezio
Quote:
Originally Posted by Petros View Post
At least you have a 7th gear. We manual guys are stuck with six.
ya but the DCT 7 gearing is short and aggressive.
Yeah but I think the DCT guys have best of both world, shorter gearing for better accel. and taller 7th gear for highway cruising! The only thing they don't have is the fun to drive!
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      02-22-2014, 02:53 PM   #15
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Quote:
Originally Posted by bruce.augenstein@comcast.
Quote:
Originally Posted by Eddien123 View Post
Lower revs does not always mean better mpg. If the engine can't turn the gear without dumping more fuel into the cylinder then that doesn't help your economy. So it comes down to less fuel with more combustions (at 3,000 RPM) or more fuel and less combustions (at 2,000 RPM) it would essentially even out. That is why economy cars like accords don't have extremely low revs. The engine just struggles and uses more fuel.
Look. Without rancor, you need to do some more studying on the topic.

First, lower revs (meaning lower revs at cruise) results in reduced fuel consumption - for the indisputable reasons I've already mentioned.

Second, of course you'll need more fuel per power pulse in order to make the same power to maintain 80, but this in no way addresses the topics of friction and pumping losses. Do you dispute my claim that best mpg will be at full throttle at whatever cruise speed? If you do, you are disagreeing with any engineering tome on the topic, written in any language.

Do you disagree that one of the two* key reasons that diesels get better mpg than gas engines is because they effectively run at full throttle all the time, with power regulated only by fuel flow? If you do, you again need to do more reading.

*The other reason is that diesel fuel contains more energy per gallon than gasoline.

Lastly, do a little reading on pumping losses, would you? On the intake stroke, each piston has to pull down against a significant vacuum at cruise (because the throttle is nearly closed), and that effort robs power from the cylinder that's firing at the time. These losses may account for a 10-15 horsepower loss at cruise (out of the 50-60 you'll need to cruise at 80), meaning you're using fuel to do nothing more than power the next piston on its intake stroke. At full throttle, pumping losses are minimized (perhaps two or three HP), so you're effectively having to feed fewer HP to maintain cruise, thus maximizing fuel economy.

BMW obliquely addressed this topic more than 30 years ago, when they said that best MPG would be obtained by accelerating at full throttle and shifting at absolute minimum rpm - meaning actual acceleration would be leisurely, but most effective, mpg-wise.

Bruce
I wasn't saying a drop in RPM from 3,000 to 2,500 would be bad for economy. But most people assume getting a highway cruise at 1500 will do the trick and it will not. Plus this car isn't even designed to be an economy car so M has no reason to drop the highway revs.
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      02-22-2014, 05:30 PM   #16
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      02-22-2014, 08:18 PM   #17
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Quote:
Originally Posted by Alex07M3 View Post
Yeah but I think the DCT guys have best of both world, shorter gearing for better accel. and taller 7th gear for highway cruising! The only thing they don't have is the fun to drive!
think about it this way. DCT can be viewed as a performance option where 6 speed fun option.
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      02-23-2014, 12:54 AM   #18
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Quote:
Originally Posted by Eddien123 View Post
I wasn't saying a drop in RPM from 3,000 to 2,500 would be bad for economy. But most people assume getting a highway cruise at 1500 will do the trick and it will not. Plus this car isn't even designed to be an economy car so M has no reason to drop the highway revs.
If I assume "the trick" is significantly better mpg, then dropping the gearing enough to obtain 1500 rpm at cruise (from 3000) will absolutely do the trick. The car most assuredly will be a comparative pig out on the highway with that gearing, but will also most assuredly be much better on fuel, for the reasons already documented.

Your opinion on this is to the contrary, so I'd like you to provide additional proof points to bolster your position, since I've already addressed your point about equal fuel being used at a lower rpm due to the need for more fuel per power stroke. Again, that point doesn't address either friction issues at higher rpm or increased pumping losses.

Bruce

PS - Porsche has addressed the economy issue with a tall seventh gear on the 991 911, so why wouldn't BMW do that.
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      02-23-2014, 09:46 AM   #19
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      02-23-2014, 12:54 PM   #20
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Quote:
Originally Posted by ///Mangler View Post
Valvetronic. No throttle plate(s) at all.

T
I have to seriously declare that I just don't get this. I believe BMW has declared that Valvetronic reduces pumping losses, but I don't see how. Can you explain this?

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      02-23-2014, 02:04 PM   #21
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What don't you get? Just like ITB decreases pumping loss over single traditional TB.
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      02-23-2014, 02:24 PM   #22
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Quote:
Originally Posted by bruce.augenstein@comcast.
Quote:
Originally Posted by Eddien123 View Post
I wasn't saying a drop in RPM from 3,000 to 2,500 would be bad for economy. But most people assume getting a highway cruise at 1500 will do the trick and it will not. Plus this car isn't even designed to be an economy car so M has no reason to drop the highway revs.
If I assume "the trick" is significantly better mpg, then dropping the gearing enough to obtain 1500 rpm at cruise (from 3000) will absolutely do the trick. The car most assuredly will be a comparative pig out on the highway with that gearing, but will also most assuredly be much better on fuel, for the reasons already documented.

Your opinion on this is to the contrary, so I'd like you to provide additional proof points to bolster your position, since I've already addressed your point about equal fuel being used at a lower rpm due to the need for more fuel per power stroke. Again, that point doesn't address either friction issues at higher rpm or increased pumping losses.

Bruce

PS - Porsche has addressed the economy issue with a tall seventh gear on the 991 911, so why wouldn't BMW do that.
You may also correct me if I'm wrong, but I believe the cars with a top gear like that are the Panamera and the Cayenne which are not performance cars (unless you get the turbo models.) Point is, they start as family 4 doors and get reverted to performance cars. My other point was the M3 was a performance car and BMW didn't need to give it good fuel economy they just needed it to be fast.
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