5th gear upgrade. .681 success 11% RPM drop

Mike_M

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Black 2002 Jetta GLS
Re: 5th gear upgrade. .70 ratio!

final drive is 3.389
Thanks, David! Given that info and the discovery of the .681 5th, here's my updated spreadsheet:



Not quite as tall as a Euro 6-speed (that'll do 69mph at 2000rpm in 6th), but pretty darn effective, I'd say.

Mike
 

IndigoBlueWagon

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Re: 5th gear upgrade. .70 ratio!

So where does one source a 3.156 final drive and how hard is it to install?
 

G60ING

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Re: 5th gear upgrade. .70 ratio!

I've done the .717 5th gear and it does not drop the rpms enough to worry about lugging the engine. it drops the rpms by roughly 50-75rpms at 55mph and may be 125-150 rpms at 75mph.

I did this mainly because I was able to sell my .756 5th for $250 and bought the .717 for $330 so for $80 I was able to to the thoughts out of my mind. Would I do this gear ratio swap again...nope. Because the gain was not worth the time. Maybe a .68 but then I bet there would be a lugging of the motor at 50-60mph. oh well my 2 cents. I may not have the math to back it up but I have played with a couple gears sets before.
 

03_01_TDI

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Re: 5th gear upgrade. .70 ratio!

\Parasitic drag increases with the square of speed.

http://www.gtechprosupport.com/support/AeroDragCalc.htm}


Calculated HP loss due to aero drag is: 14.1 horsepower at 70mph.

Calculated HP loss due to aero drag is: 17.3 horsepower at 75mph.

With stock gearing the engine need to produce 30ft/lbs of torque just to make up for Parasitic drag @ 70mph

With the .681 gearing the RPMs will be lower at 70mph so you'll need an extra 3ft/lbs of torque. Just to account for the normal drag at 70mph.

So the question can the engine decrease its friction losses or operate at a higher BMEP from a 250rpm decrease and save more than 3ft/lbs of torque? if so, then the math says the gear change is worth the effort. Cost effective - depends on if we elect another oil guy as president again.
 

jackbombay

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Re: 5th gear upgrade. .70 ratio!

Parasitic drag increases with the square of speed.

http://www.gtechprosupport.com/support/AeroDragCalc.htm

Calculated HP loss due to aero drag is: 14.1 horsepower at 70mph.

Calculated HP loss due to aero drag is: 17.3 horsepower at 75mph.

With stock gearing the engine need to produce 30ft/lbs of torque just to make up for Parasitic drag @ 70mph

With the .681 gearing the RPMs will be lower at 70mph so you'll need an extra 3ft/lbs of torque. Just to account for the normal drag at 70mph.

So the question can the engine decrease its friction losses from a 250rpm decrease and save more than 3ft/lbs of torque? if so, then the math says the gear change is worth the effort. Cost effective - depends on if we elect another oil guy as president again.
The closer you are to 1900 RPM the less fuel required to produce a give amount of HP/torque.
 

IndigoBlueWagon

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Re: 5th gear upgrade. .70 ratio!

I've done the .717 5th gear and it does not drop the rpms enough to worry about lugging the engine. it drops the rpms by roughly 50-75rpms at 55mph and may be 125-150 rpms at 75mph.

I did this mainly because I was able to sell my .756 5th for $250 and bought the .717 for $330 so for $80 I was able to to the thoughts out of my mind. Would I do this gear ratio swap again...nope. Because the gain was not worth the time. Maybe a .68 but then I bet there would be a lugging of the motor at 50-60mph. oh well my 2 cents. I may not have the math to back it up but I have played with a couple gears sets before.
I think the 7.17 with the taller final drive would be a good combo. I think going with the .68 would be too big a gap between fourth and fifth. The 7.17 with the taller drive may give me about 8-10 more mph at a given RPM at highway speeds, which may be worth it. I don't think my car with RC3 will have any trouble pulling the higher ratio.
 

Lug_Nut

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Re: 5th gear upgrade. .70 ratio!

JB,
You slipped up in your math. 1900 RPMs x 150 ft-lbs = 54.3 hp.
Yes I did. Big time. I don't know how. The 54 hp figure is correct.
The closer you are to 1900 RPM the less fuel required to produce a give amount of HP/torque.
Not if the graph is to be believed. The red line furthest to the left shows less fuel is used at 1400 rpm (200~210 g) than at 1900 rpm (220~230) for the same power.
My intent isn't to produce X amount of power. My intent is to move S distance. A fourth concentric red dashed line added further to the left of the three shown could intersect the 210 g and 1100 rpm point. (Note that no torque above this point is possible, thus "full load"). The concentricity of a fourth red line would imply that the same power available at this 210g and 1100 rpm point would require 250 g at 1900 rpm. That is almost 20% more fuel for the same power.

btw. I have no college, had a D- in highschool physics and needed three years to pass two years of algebra.
I'm the LAST person that should be commenting about others.
I had also forgotten the problem of "hubris". I'll try not to forget again.
 

IndigoBlueWagon

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Re: 5th gear upgrade. .70 ratio!

I had also forgotten the problem of "hubris". I'll try not to forget again.
Jon, won't an intake cleaning take care of the hubris? Or perhaps some PowerService?
 

david_594

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Re: 5th gear upgrade. .70 ratio!

JB,
No college, but man do you fake it good. You must know something or you made one heck of a good guess. At the lower RPM's things get a little tricky and in fairness I have pretty much ignored them in my considerations. Most of my 5th gear runs are between 65 and 75 mph so I tend to stay above 2000 RPM's.

If you look at the differences between 2250 and 2000 RPM's for the same load you will see that at 2000 RPM's it will use roughly 5% less fuel.
 

watercop

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Re: 5th gear upgrade. .70 ratio!

I'm quite pleased with my 3.156 / 0.717 gearing. I think the 0.681 5th with 3.156 final would be worth a try as well. I wouldn't worry too much about the jump from 4th to 5th unless you operate on extremely mountainous rural roads. I must caution folks that changing the final from 3.38 to 3.156 is NOT cost effective. I'm glad I did it, but it is not cost effective.

Personally, I think a 6 speed is an even better fit. It should provide very low 1st and reverse ratios for maneuvering in very close quarters and extra torque for starts and turns on dirt or gravel. Too bad the VW 6 speed is heavy, complicated, expensive, and rare this side of the pond.

Great job, David, unearthing this. Just going from 0.756 5th to 0.681 is a good bit better than just going from 0.756 to 0.717 and should give a noticeable mpg bump and more satisfying operation at speed.

Please don't sell this to gdr703, though - with this he'd wax my butt much more often in the monthly mileage contest!
 

david_594

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Re: 5th gear upgrade. .70 ratio!

LOL, Is this going to be like the cheater's advantage for the mileage contest?

I paid for my set of gears tonight so hopefully I will have them on this side of the pond in like 2 weeks. (crosses fingers) Hopefully they will fit.... otherwise they are pricy paper weights


Watercop, honestly I think the 3.156 would be awesome but unless I pick up a tranny for cheap in the near future and I magicly learn how to rebuild one then it is defenitely not an option for me. Although at the right price I might just consider attempting it...
 

watercop

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Re: 5th gear upgrade. .70 ratio!

I hope it fits. The more folks that regear for mpg the better off we all are.

I'll be sorely tempted to try the 0.681 along with my 3.156 if it works for you...just what I need, another uneconomical project

America is overgeared!
 

jackbombay

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A4 Jetta
Re: 5th gear upgrade. .70 ratio!

The closer you are to 1900 RPM the less fuel required to produce a give amount of HP/torque.
Not if the graph is to be believed. The red line furthest to the left shows less fuel is used at 1400 rpm (200~210 g) than at 1900 rpm (220~230) for the same power.
I have a different graph that is easier to understand that is HP per fuel quantity per HP and 1900 is were the A3 TDI produces HP the most efficiently, so I used 1900. I also used 1900 because we were discussing figuers in the 2400-3000 range and as the engine gets closer to 1900 RPM (descending from 2000+ RPM)it produces HP more efficiently.
 

Drivbiwire

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Re: 5th gear upgrade. .70 ratio!

Pulling the gears is VERY easy! Don't sweat the puller instead you are supposed to heat the gear using an electric heat gun to 100C, the gear(s) slide right off!

If you try to pull the gear without heating it you will crack it nearly every time.

Same thing goes for reinstalling, heat it to 100C and it slide right onto the shaft.

The gears in the A3 are the same all the way through the 5 speed trannies in the 2004/5/6 model years. The changes were in the casting of the case and how it mounts in the A4 engine bay using the pendulum mounting.

DB
 

VelvetFoot

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Re: 5th gear upgrade. .70 ratio!

Hmmm... I didn't know about the heating. I don't think my Bentley said anything.

david: Do you have any more details you can provide, like cost and who you bought it from? Is the set new?
 

moondawg

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Re: 5th gear upgrade. .70 ratio!

Same thing goes for reinstalling, heat it to 100C and it slide right onto the shaft.
If you can find a drive-in freezer, you can also cool the transmission below freezing to help install the gear!


moondawg
 

ymz

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Re: 5th gear upgrade. .70 ratio!

>>If you can find a drive-in freezer, you can also cool the transmission below freezing to help install the gear!<<

Who needs a drive-in freezer? Just come up to Montreal in late January and park outdoors...

Yuri.
 

meganuke

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Re: 5th gear upgrade. .70 ratio!

For those of us without subzero temperatures (yet) and no drive in freezer or electric heat gun, could we put the gears in the oven? I'm assuming there's a good reason to not use a torch.

Put gears on greased cookie sheet. Place in preheated oven at 450 degrees for 45 minutes. Rotate once while cooking. Mix with flour, sugar, eggs, etc.
 

Fix_Until_Broke

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Re: 5th gear upgrade. .70 ratio!

Your speeds, I assume are calculated/actual MPH not indicated? Because my stock (AFAIK) Jetta runs about 2025 RPM at 60 indicated MPH which I assume is due to speedometer error. Need to get a GPS to verify.

Could you provide the equations you used to generate your chart: I use this
MPH=(RPM*TireDia)/(Trans*Final*336) which makes some assumptions regarding tire diameter and circumfrence which may be up for questioning (not that pi is up for question, but rather how well the diameter translates to circumfrence practically).
 

PDJetta

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Re: 5th gear upgrade. .70 ratio!

MANY years ago I was rebuilding a 36 hp motor for my '56 Bug. I messed up the (metal) timing gear on the crankshaft and had to replace it. I took the crankshaft and new timing gear to the shop that did my parent's automotive work. I think the old gear was removed with a press, but the new gear was placed in an oil bath in a common electric skillet and heated for half an hour or so and it was then just slipped right onto the crankshaft. Perhaps a similar method could be used for putting a gear onto a transmission shaft.

--Nate
 

watercop

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Re: 5th gear upgrade. .70 ratio!

I more or less concur with his speeds being calculated / actual rather than indicated. If your speedo is anything like mine (was) 60 mph indicated is about 57-58 actual. Before speedo correction and regearing I turned 2,000 rpm as indicated by the tach at 57 mph GPS.

I don't think swapping 5th for a different ratio impacts speedo indication. My R&P swap evidently did but the parts included a new speedo gear ring to compensate so I noticed no change

If Vag-Com can be believed, my tach appears accurate.

Regarding tires, you can get tire revolutions per mile at tire manufacturer websites or at tirerack.com

I have found that calculating diameter and then circumference and then revolutions per mile from the sidewall dimension numbers often yields a different (by up to 2%) value than the published revolutions per mile.

I believe my 64 mph (GPS) at 2,000 rpm indicated would rise to 67.3 mph (GPS) were I to install the 0.681 gearset...tempting, oh so tempting!
 

Lug_Nut

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Re: 5th gear upgrade. .70 ratio!

...1900 is w(h)ere the A3 TDI produces HP the most efficiently, so I used 1900.
This whole excercise of this thread in which we are guessing which ratios are the best for fuel economy are related to distance traveled per unit of fuel. I truly couldn't care less about how many hp per gallon. I care about how many miles per gallon, and 1900 is not where the TDI engine produces distance most efficiently.
If I wanted the engine to make all of its potential peak torque at 1900, I'd drive with my right foot on the floor and use the parking brake to regulate my road speed. That way I'd always be at full load, always making maximum torque, and apparently, I'd also be getting the best possible fuel economy.
 

Fix_Until_Broke

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Re: 5th gear upgrade. .70 ratio!

Where does one source a 3.156 final drive?
Looks like there are sources being discussed for the 0.681 5th

Someone should put all of this information together on gearing and what is required to perform the changes, options for sourcing parts, tips/hints etc.
 

GoFaster

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Re: 5th gear upgrade. .70 ratio!

3.156 final drive is standard on all the A3 and B4 cars. The O2A tranny in those cars appears to have entirely interchangeable innards with the O2J tranny in the later cars. The big trouble with changing the final drive is that since the pinion gear is part of the output shaft (as with practically any transaxle), you have to remove the trans from the car, split the tranny cases and disassemble everything on the output shaft in order to exchange output shafts ... Fifth gear can be exchanged with the trans still in the car.
 

jackbombay

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Re: 5th gear upgrade. .70 ratio!

...1900 is w(h)ere the A3 TDI produces HP the most efficiently, so I used 1900.
This whole excercise of this thread in which we are guessing which ratios are the best for fuel economy are related to distance traveled per unit of fuel.
Exactly! And we know that air resistance at 60 MPH is constant, lets say it requires 20 HP for an A4 Jetta to maintain 60 MPH. The engine in our cars burns less fuel at 1900 RPM to produce 20 HP that it burns at 2900 RPM to make 20 HP.


If I wanted the engine to make all of its potential peak torque at 1900, I'd drive with my right foot on the floor and use the parking brake to regulate my road speed.
When did I say anything at all what-so-ever about using the max torque of the engine at 1900??? You are misunderstanding my posts, go and re-read them before you continue to spout off with stupid acusations of WOT with the handbrake applied to maintain 1900 RPM.
 

QuickTD

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Re: 5th gear upgrade. .70 ratio!

For those of us without subzero temperatures (yet) and no drive in freezer or electric heat gun, could we put the gears in the oven? I'm assuming there's a good reason to not use a torch.
In the interest of domestic peace I have my own personal toaster oven for this very purpose. I use it to heat bearings and gears and the occasional shop lunch. Yard sales are a good source. It is the best $10 you're likely to spend, trust me. I once thought I'd bake some paint in the wardens oven, never, never, never again...
 

GoFaster

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Re: 5th gear upgrade. .70 ratio!

You made the mistake of baking paint in TOWMBO's kitchen oven?

I betcha that only happened ONCE, and you heard about it for a long time afterward!
 

03_01_TDI

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Re: 5th gear upgrade. .70 ratio!

"Exactly! And we know that air resistance at 60 MPH is constant, lets say it requires 20 HP for an A4 Jetta to maintain 60 MPH. The engine in our cars burns less fuel at 1900 RPM to produce 20 HP that it burns at 2900 RPM to make 20 HP."

Lets me see if I get this right. 20hp at 1900rpms = more torque than 20hp at 2900rpms. Its not MAX torque just more torque. The torque doesn't come at the cost of more fuel. The torque at 1900rpms is more than 2900rpms becuase of engine friction and flow of injectors and air all being at a more Effecent rpm area.

Like I posted in this thread the engine only needs a 3-9 more ft/lbs of torque at lower rpms to produce the same HP.

I guess the previous poster got tired/confused of all the guys saying and diesel works better at full load. And I agree full load on a TURBO engine can't be the most effective. That turbo cuases most of the parasitic loss when at full load.
 

IndigoBlueWagon

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Re: 5th gear upgrade. .70 ratio!

final drive is 3.389
Thanks, David! Given that info and the discovery of the .681 5th, here's my updated spreadsheet:

So if stock final drive is 3.156 can I use the table on the left to estimate the RPM drop? Looks correct, as it matches my engine speed (accounting for speedo error) at 60 MPH. If so, 2600 RPM at actual 80 MPH would be great.

I'm going to Portugal in two weeks: wonder if I could pick up a gear and bring it home in my luggage?
 
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