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

Re: 5th gear upgrade. .70 ratio!

I think they're significant, but how will we know if they fit? I don't have the extra cash to lay out for gears that may not fit. Is there any source that gives the dimensions of those gears so we can match the sizes to a gear set that is known to work?

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.

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correct. I'll post up the graph I have later on, it's saved in my anchient vag-com laptop which is in the back of the car.

Re: 5th gear upgrade. .70 ratio!

Lug-Nut, I'm pretty sure we are on the same page.

Lug Nut: You then came out with the "peak torque output RPM" implying 1900 rpm.

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Me: ...more efficient production of any given amount of HP due to operating closer to the peak torque output RPM.

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I had meant the "any given amount" to imply any throttle setting, just at 1900 RPM. I had not considered less than 1900 RPM earlier in this discussion as it did not seem relevant regarding highway speeds.

Re: 5th gear upgrade. .70 ratio!

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.

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If you look at the BSFC chart posted earlier in the thread, you see that the ENGINE is most efficient at about 95% load at about 1750 rpm. It is burning 197grams of fuel for every kWH produced.

It is not practical to operate the engine in this region for normal cruising... just because you are turning fuel into power at the most efficient levels doesn't mean that you're turning that power into motion most efficiently!

So, assuming constant hp to maintain a constant speed, the goal is to gear your car so that you can move as far to the left(and up) along the HP curves(red dashed lines) on the graph as practical.

moondawg

Re: 5th gear upgrade. .70 ratio!

MANY years ago I was rebuilding a 36 hp motor for my '56 Bug.

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Boy do I wish I still had my 56 Bug. My dad bought it new and then gave it to me in 1971 with 248k original miles on it. In 1975 the crank broke in half so I put a 40hp engine in it.

I haven’t looked inside the tranny. But dry ice on a shaft will have a similar affect to heating the gears. Have removed many gears and bearing races that way.

Re: 5th gear upgrade. .70 ratio!

If you look at the BSFC chart posted earlier in the thread, you see that the ENGINE is most efficient at about 95% load at about 1750 rpm. It is burning 197grams of fuel for every kWH produced.

It is not practical to operate the engine in this region for normal cruising... just because you are turning fuel into power at the most efficient levels doesn't mean that you're turning that power into motion most efficiently!

moondawg

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I had thought that chart was not accurate and was drawn up by somebody. Plus I don't see how on a vnt turbo engine its most effective at 95% load. That would require max boost and very high EGT's. The turbo alone would eat up alot of heat/energy and be a major drag. but thats just my $.02

Now big diesel and some of them uses vnt's turbo work best at lower RPM's and higher loads. So I might be wrong.

Re: 5th gear upgrade. .70 ratio!

The graph simply shows that at engine speeds greater than 2000 RPM's it takes less fuel to produce a a given amount of power at lower RPM's than it does to produce the same amount of power at higher RPM's.

This is why a 5th gear upgarde is benefitial to those of us who drive in 5th gear with engine speeds greater than 2000 RPM's.

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: <font color="red">14.1 </font> horsepower at 70mph. we'll just guess it takes another 20hp for drivetrain loss. total est is 34hp.

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.

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as I look at the chart a 250rpm drop at the 35hp section 3000rpm down to 2750 would account for a gain in 4-6% of BMEP. From 320grams of fuel down to estimated 300 grams. or about 3mpg increase.

Re: 5th gear upgrade. .70 ratio!

The graph simply shows that at engine speeds greater than 2000 RPM's it takes less fuel to produce a a given amount of power at lower RPM's than it does to produce the same amount of power at higher RPM's.

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Why stop at 2000 RPM? Doesn't the same statement apply all the way down to the limit of the chart - 1000RPM?

Rik

Re: 5th gear upgrade. .70 ratio!

Under 2000 RPM's things get tricky. Just look at dyno plots of the cars and you will see that torque falls off as the RPMs get lower. At lower speeds you have issues with how much boost the turbo is producing.

At 1000 RPMs you would have to produce twice as much torque to maintain the same speed with the engine spinning at 2000 RPM's. And down that low the engine would really really struggle to produce that kind of torque.

Re: 5th gear upgrade. .70 ratio!

My second sedan with the 1Z engine produced 7 hp at 750 rpm on a dynamometer. This was achieved by spinning the rollers at idle in second and quickly shifting into third. The engine didn't stall. The D-B-W took control since the engine speed was now less than normal idle and added maximum fuel to get things back up to 950 rpm. The roller rotation rate increased at a rate equal to 7 hp from that start of 750 rpm.
Plugging the numbers for HP and into the equasion [(750 rpm times lb*ft)/5252 = 7 hp] makes the torque about 49 lb*ft.
Think about that: fully 1/3 of the peak maximum torque of 149 lb*ft is available at 200 rpm LESS than idle rpm.
The concensus appears that about 15 hp is required to maintain highway road speeds. The factory gearing is such that about 2000 rpm, or very near to that, is the engine speed at highway road speed. The maximum torque possible at that engine speed is about 150 lb*ft for a potential of 54 hp. If 15 hp are actually needed, the torque needed to maintain highway speed is about 15/54 or 28% of the maximum possible.
Highway road speed would therefore require 41.6 lb*ft.
The engine makes 49 lb*ft at 750 rpm, a surplus of 15%.
Unfortunately the D-B-W system won't allow operation at this low an engine rpm. It WOULD be necessary to use the parking brake to keep speed below 55 mph.

p.s. Someone please check my math.

Re: 5th gear upgrade. .70 ratio!

49 lb.ft at 750 rpm is (49 x 750 / 5252) = 7 horsepower. Highway road speed (assuming the above assumptions) requires 41.6 lb.ft - but AT 2000 RPM, if it's geared taller (for lower revs) than it needs more torque.

There is lots of misunderstanding happening here. The red lines on the posted BSFC chart are lines of (roughly) constant horsepower (I can spot errors in these lines - the BMEP and RPM should multiply to a constant, and they don't, but they are close enough for the following illustration).

If you want to find out the most efficient engine RPM under specified horsepower output conditions, you figure out the appropriate constant-horsepower line and plot it on the BSFC chart, then you find where along that line gives the lowest BSFC. There is no single universal number, it will be different for each power output. If your actual RPM under those conditions is at a different engine speed, you can look up the BSFC under that condition and compare it to the optimum.

It will generally be found that the best BSFC is obtained by running the engine at or near full load at the lowest possible speed that is consistent with getting the required power output.

If you gear the engine purely to be able to achieve best BSFC and you don't consider anything else, you are probably going to get something which is very unpleasant to live with, has to be downshifted on every acceleration or hill, and it would not surprise me if the low oil pressure and high load conditions eventually lead to pounded-out main bearings (and blown-up VNT15 turbochargers).

You basically need to draw another line on that chart, which starts at 903 rpm and zero BMEP (idle), and extends upward through 14 bar BMEP and 2000 rpm. Anything to the left of that line is basically a "no-go" zone due to lugging, regardless of how attractive the BSFC's are over there ...

Re: 5th gear upgrade. .70 ratio!

torque = force
horsepower = work = (force x distance) = (torque x RPMs)

You could have all the torque in the world but if its not spinning you are getting no work done.

So the faster a shaft is spinning the less torque you need to accomplish the X amount of work. The slower the shaft spins the more torque that is required to perform X amount of work.

Re: 5th gear upgrade. .70 ratio!

Under 2000 RPM's things get tricky. Just look at dyno plots of the cars and you will see that torque falls off as the RPMs get lower.

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I don't have to look at a dyno plot: the chart already shows it! After all, BMEP converts directly to torque...

If you had a really tall 6th gear and were running at highway speeds, it would be more fuel-efficient to run at 1,500 RPM than it would be to run a lower gear at 2,000 RPM. That is, of course, assuming the road was flat, no head wind, etc., so that you didn't need to lug the engine... and you'd need to downshift to accelerate. And, as pointed out, this doesn't take into account any problems running at low RPMs may cause the engine, turbo, etc.

Maximum fuel efficiency measured in miles per gallon (regardless of speed) is obtained in highest gear close to the lowest sustainable RPM. When I've mapped it, it's something like (very approximate) 35 mph in 5th gear at 1,100 rpm or so.

Rik

Re: 5th gear upgrade. .70 ratio!

1-Does anyone have (or know where one is posted) a 'how to' for changing out the 5th gear?

2-Rammstein: Any luck on finding out if those audi gears fit?

Re: 5th gear upgrade. .70 ratio!

1-Does anyone have (or know where one is posted) a 'how to' for changing out the 5th gear?

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Same for final drive if anyone knows or has it

Re: 5th gear upgrade. .70 ratio!

2-Rammstein: Any luck on finding out if those audi gears fit?

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No clue. Gotta have to find the tranny it's in, at the junk yard, and take the gear out.

Re: 5th gear upgrade. .70 ratio!

Rammstein, Or you could just have someone look them up in ETKA and from that you can determine what type of tranny it is. Thats the easy way to eliminate possibilities.

Re: 5th gear upgrade. .70 ratio!

Rammstein, I wish you'd share some specific info like part #s. Don't be selfish, help us conserve some diesel, prices are gonna be real high this winter!

In regards to the rpm discussion, I certainly agree with the conclusion of lower rpms being better as long as the car can maintain speed relatively efficiently. My questions would be regarding some theoretical purely conjecture for discussion limits, namely:

1) At say 50-60mph, how much hp/torque is needed to simply maintain speed on a flat?
2) What is the lowest rpm that can provide this hp/torque?
3) Would idle injection fuel quantity provide this sufficient hp/torque in 5th gear? If it does then the most efficient cruising rpm would be the ecu programmed in-gear low limit, somewhere around 1000rpm. Considering the TDI's compression this may well be the case.
4) What would the mimimum possible fuel consumption be in 5th gear just above idle? Idle fuel consumption is something less than 1 Liter/hour if memory serves, but that is out of gear, I'd guess the increased gearbox/drivetrain friction and wind resistance would double that, so perhaps 2 L/hour is possible? Assuming 50mph that works out to a theoretical low limit of 100mpg.
5) What would the 5th gear ratio have to be for 50mph to be equal to 1000-1100rpm in 5th? I understand this would be quite impractical but no less interesting for the sake of discussion. Looking at the table I think it is about .5?

Re: 5th gear upgrade. .70 ratio!

In regards to the rpm discussion, I certainly agree with the conclusion of lower rpms being better as long as the car can maintain speed relatively efficiently. My questions would be regarding some theoretical purely conjecture for discussion limits, namely:

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You're in luck! I was just working up a spreadsheet for my 2005 Passat GLS TDI Wagon (5-speed automatic (Tiptronic)), so I have the numbers for that. I've assumed standard air density, temperature, sea level, 0% grade, etc. Your results may vary.

1) At say 50-60mph, how much hp/torque is needed to simply maintain speed on a flat?

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Needed horsepower at a given speed is a function of rolling resistance, air resistance, gradient (if any), and transmission/other losses. Here's what I calculated for the Passat:

35 mph - 7.2 bhp
50 mph - 13.9 bhp
60 mph - 20.7 bhp
70 mph - 29.7 bhp (more than 2x required power for 50 mph, and more than 4x required power for 35 mph)

2) What is the lowest rpm that can provide this hp/torque?

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RPM is dependent on road speed, wheel/tire size and gearing only. For the gearing on the Passat (5th gear - 1:0.74; and final drive - 1:3.10 = 1:2.29 gearing in 5th), and Continental ContiTouring Contact CH95 205/55R16 tires (listed at 834 revs/mile at tirerack.com), I calculated the following:

35 mph - 1,116 rpm
50 mph - 1,594 rpm
60 mph - 1,913 rpm
70 mph - 2,232 rpm

3) Would idle injection fuel quantity provide this sufficient hp/torque in 5th gear? If it does then the most efficient cruising rpm would be the ecu programmed in-gear low limit, somewhere around 1000rpm. Considering the TDI's compression this may well be the case.

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Yes sufficient torque is available. Torque for a given horsepower and speed has to take into account rpms. Here are the torque values and equivalent BMEP values based on the power and RPM figures above (Passat TDI has a 1,968 cm3 displacement = ~2.0L). I also show an estimate of the % of maximum available torque used.

35 mph - 34 ft.-lbs./ 2.9 bar (24%)
50 mph - 46 ft.-lbs./ 4.0 bar (21%)
60 mph - 57 ft.-lbs./ 4.9 bar (23%)
70 mph - 70 ft.-lbs./ 6.0 bar (29%)

The estimated specific fuel consumption (SFC) as measured in grams/kilowatt-hours (I used the fuel map published on page 5 in the second of 2 reports in this EPA document: www.epa.gov/OMSWWW/reports/adv-tech/420r04002.pdf and very slightly adjusted it to match the torque curve for the 2.0L TDI) for the given BMEPs and RPMs shown above is as follows:

35 mph - 291 g/kW-h
50 mph - 289 g/kW-h
60 mph - 270 g/kW-h
70 mph - 246 g/kW-h

The engine is running more efficiently at 70 mph. But it also has to perform a lot more work. Multiplying the SFC by the power needed gives the following fuel consumption rates in grams/second and liters/hour (based on a specific density for diesel fuel of 856 grams/liter):

35 mph - 0.435 g/sec; 1.83 l/hr
50 mph - 0.834 g/sec; 3.51 l/hr
60 mph - 1.154 g/sec; 4.85 l/hr
70 mph - 1.516 g/sec; 6.38 l/hr

This is equivalent to the following fuel efficiency figures:
35 mph - 72.5 mpg; 3.25 l/100km
50 mph - 54.0 mpg; 4.36 l/100km
60 mph - 46.8 mpg; 5.03 l/100km
70 mph - 41.6 mpg; 5.66 l/100km

As a note: considering that the car is rated by the EPA at 38 mpg highway and the EPA automatically deducts 22% from the tested figure (= 48.7 mpg tested figure) to arrive at the published figure, these calculations look to be pretty reasonable.

4) What would the mimimum possible fuel consumption be in 5th gear just above idle? Idle fuel consumption is something less than 1 Liter/hour if memory serves, but that is out of gear, I'd guess the increased gearbox/drivetrain friction and wind resistance would double that, so perhaps 2 L/hour is possible? Assuming 50mph that works out to a theoretical low limit of 100mpg.

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See above. At 35 mph (~1,100 rpm) in 5th gear, fuel consumption is 1.83 l/hr = 72.5 mpg = 3.25 l/100km

5) What would the 5th gear ratio have to be for 50mph to be equal to 1000-1100rpm in 5th? I understand this would be quite impractical but no less interesting for the sake of discussion. Looking at the table I think it is about .5?

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Depends on specific gearing. Given the final drive ratio on the Passat of 3.10:1, a 5th gear of 0.50:1 would get you 1,077 rpms at 50mph.

Rik

Re: 5th gear upgrade. .70 ratio!

So what would the fuel consumption be at 50mph using the .50 gear?

Re: 5th gear upgrade. .70 ratio!

So what would the fuel consumption be at 50mph using the .50 gear?

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237 gm/bkW-hr
0.683 gm/sec
65.9 mpg
3.57 l/100 km

Re: 5th gear upgrade. .70 ratio!

Thanks Brian,
I realized my mistake, but not until about three hours after you had posted.

Highway road speed would therefore require 41.6 lb*ft.
The engine makes 49 lb*ft at 750 rpm, a surplus of 15%.

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Highway road speed was defined as 55 mph. The 41.6 torque was calculated from an estimate of 15 hp required for 55 mph. My mistake was in considering torque only and claiming that the torque possible at 750 rpm was sufficient for 55 mph. It isn't.
There isn't enough rotational speed to multiply that torque into the power needed. I had already said so in the post. Fifteen hp was the agreed amount needed for 55 mph, 7 was all that was possible at 750 rpm. I'm short by 50%. I'd need 50 lb*ft at double the rpm, or 100 lb*ft at 750 rpm, or some other combination of lb*ft and rpm that multiplied to 7500 in order to obtain the needed 15 hp.
Increasing the TDI's rpm to about 1200 also has the effect of increasing the torque. The two combine to make about 20 hp. Gearing the car to allow 1200 rpm operation at 55 mph will result in operation at about 75% of that rpm's full load capacity.
Seven-fifty rpm just isn't enough for 55 mph.

Re: 5th gear upgrade. .70 ratio!

The concensus appears that about 15 hp is required to maintain highway road speeds.
p.s. Someone please check my math.

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it take 14hp just to overcome air drag. you still need to account for tires and drivetrain losses.

Re: 5th gear upgrade. .70 ratio!

torque = force
horsepower = work = (force x distance) = (torque x RPMs)

You could have all the torque in the world but if its not spinning you are getting no work done.

So the faster a shaft is spinning the less torque you need to accomplish the X amount of work. The slower the shaft spins the more torque that is required to perform X amount of work.

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true, which is why if you lower the rpms by 250 (3000 down to 2750) with a gear change and keep all other items the same (speed etc). The engine must produce 3 more ft/lbs of torque. and the best thing is the TDI works better at lower RPMS thus that increase in 3ft/lbs does not come at the cost of increase fuel injected. It actually lowers the amount of fuel injected.

Less fuel, less RPMs, less engine wear, less heat, less dirty oil. can't beat it. However the TDI has been proven to go over 300,000 miles without any mods. So the maintance issue is kinda pointless. Just how many miles would you put on one car before a drunk driver, old person, illegal spanish person etc etc wrecks into your car.

Re: 5th gear upgrade. .70 ratio!

Ok, so here they are. I don't know if they fit, but they are the tallest gears made by Audi.

From a 1984 Audi 4000:

.684=013 311 159 B from 2M tranny
.641=093 311 159 C from 3Z tranny
.60=013 311 159 C from 2M/3M tranny
.537=093 311 159 A from QF tranny

Re: 5th gear upgrade. .70 ratio!

ok all this talk about fuel consumption vs engine rpm vs road speed is all pretty interesting and I think I understand most of it.. From what I can tell, it will take quite a while to get back the initial investment on fuel savings alone. Even assuming I can sell my stock 5th gearset(providing I don't destroy it), it'll still take me almost two years worth of driving to make up for the cost.. To me that's not worth it for the fuel savings alone.

What makes me want to instal the .681 5th is simple... Top Speed... The .681 5th gear with the stock final drive would give me a theoretical top speed of about 145@ 4700RPM (would increase to 155mph with 3.156 final drive) if I can get the engine to produce enough power... That's 15 more MPH than it is now at redline.. Right now, my TDI(stock) can pull about 4400rpm in 5th gear, which is a touch over 120 mph..

I think with 50% more engine power, which isn't unreasonable from some of the dyno charts I've seen on here, I shouldn't have too much of a problem getting up near redline with the .681 5th gear.. Maybe not with the 3.156 final drive, but with the stock final drive it should do it.. I think it would be pretty neat to say "I bet you $100 my TDI can go faster than your IROC!". Hey I might even be able to recover the cost of the gears from a few of the kids around town who think their civics are fast...

I may be a little off on my rpm vs road speed numbers but the point is the same...
I used this page to come up with my figures: www.csgnetwork.com/multirpmcalc.html

Re: 5th gear upgrade. .70 ratio!

120mph = Calculated HP loss due to aero drag is: 66.1 horsepower.

145mph = Calculated HP loss due to aero drag is: 116.6 horsepower.

155mph= Calculated HP loss due to aero drag is: 142.5 horsepower

this is just for aero drag, must still account for drivetrain losses.

you would need alot of torque near the end of redline. plus a intercooler and oilcooler to control EGT's. It would take minutes of Full Throotle to go from 120-155mph.

Re: 5th gear upgrade. .70 ratio!

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.

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IIRC, the total gearing for 1st-5th was nearly identical (within a couple percent) to the A4 5-speed. They basically just "added a 6th gear on top".

Too bad the VW 6 speed is heavy, complicated, expensive, and rare this side of the pond.

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Hear hear...IMHO, that would be the "right way" to do it. (Not that there's anything wrong with the way you did it, or other people who changed the final drive and/or 5th gear...just that it maintains stock gearing and adds another overdrive, which I feel the A4 needs anyway.)

Mike

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.

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My calculations are all based upon the usual 3mph speedometer error present in A4 TDIs (probably gassers, too). The figure "2150rpm @ 60mph" is the basis for all the other calculations, and seems to agree with my car (stock gearing and tires), given that 3mph error. (In other words, 2150rpm occurs at about 63mph in my car, which translates to 60mph actual.) DISCLAIMER: I have not tested my speedometer yet to prove that the 3mph error applies, but it's been quoted so widely by so many who know what they're talking about, I'm accepting it for now.

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).

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Nothing so complex or physics-based. I'm calculating all based on the ratio of current speed, gearing, etc. based on the figure "2150rpm @ 60mph". In other words:

(RPM_at_50MPH) = (50/60) * (RPM_at_60MPH)

or

(RPM_at_60_with_3.156) = (3.156/3.389) * (RPM_at_60_with_3.389)

Simple math...no tweaks for what "should be" happening included. Just the assumptions that for a given gear, the relationship between RPM and road speed is linear, and that gearing is equally linear.

PM me if you'd like a copy of the actual spreadsheet.

Mike