Hill Climbing Efficiency

BoboBear

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I go up this 2 mile long 3 - 4% grade hill every day at 45 mph. For best fuel efficiency should I use 5th gear at 1500 rpm, or 4th gear at 2000 rpm?
 

scooperhsd

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In that case - take it in 4th. Your engine will like you better, and if you need to speed up - you can without a downshift.
 

oEo

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yeah, take it in 4th.
Hills are awesome. My PD cruises. The Pa Turnpike was finally a joy (if you want to call it that). Locally, going up hills is so easy. It's amazing the difference between my TDi :D and the crappy 2.0 :eek: I had a few years ago.
 

tditom

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can any of you explain how taking the hill in 4th would net better fuel economy than taking it in 5th? that was the O.P.'s question.
 

Fix_Until_Broke

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5th gear - no question about it (and I even have the taller 5th gear, and I'd still take it in 5th) - Also, my reference is with a stock powered car (RC2 and PP502's are very recent additions).

Basically - the higher the engine load (for a given RPM) the greater the thermal efficiency. To climb the hill it takes XX horsepower, at 2K in 4th you are operating at (lets say) 50% of what the engine can produce at 2000 RPM, at 1500 in 5th you are operating at (lets say again) 75% of what the engine can produce at 1500 RPM. Less parastitic (water pump, power steering pump, alternator, etc) losses are also contributing factors to improved vehicle efficiency at lower engine RPM's
 

Bob_Fout

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tditom said:
can any of you explain how taking the hill in 4th would net better fuel economy than taking it in 5th? that was the O.P.'s question.
While the OP asked about fuel econ, he may not realize the possible dangers of taking a hill that steep at low RPM in high gear. That's where the collective knowledge of TDIClub can help.

Also. Peak torque is around 2000 RPM. How that impacts this situation, I'm unsure.

EDIT: To the OP: 2 miles of hill...fuel econ is the LEAST of your worries. 4th gear over 5th. 1500 RPM in flat land might be OK in 5th, but up a 4% grade.... give your motor a break and downshift to 4th)
 

Bob_Fout

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The chance of grenading the turbo and lugging the motor. 1500 in 5th is fine on flat land, but not up a decent grade.

A 4% grade at 1500 RPM in highest gear....come on... we don't have V6 or V8 torque at 1500 RPM.
 

tditom

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I don't think you'd damage the turbo if you don't stomp it while at low rpms and in 5th.

I've never tried it myself- have you?
 
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scooperhsd

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Bob's reasoning is exactly why I would do it in 4th. Heck, the 4 speed Auto would do that in 3rd, to achieve the "climb the hill at or above 2000 RPM".
 

Brock_from_WI

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If this were me I would likely run it in 5th. As long as I still have room under the go pedal I would stay in 5th gear. If I had to speed up / slow down a lot I might go to 4th. The way I see it is if you’re loading the engine to 75%, what the difference between running 75% at 1500rpm at 45mph or 75% running 85mph on flat open land, same demand on the engine right? Or am I missing something?
 

vw4life

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for a comparison my passat TDI with tiptronic has the instant fuel economy reading. Regardless of its absolute accuracy it is a good comparison.

I used to to find out that I can get the same fuel efficiency going up a hill in my area in 5th gear at 90 km/h as 80 km/h in 4th gear. So I go faster in fifth so the RPMs are a little higher and there is less chance of lugging- not to mention I get up the hill faster so in effect I use less fuel.

I suggest that doing 50 mph (or slightly more) up your hill in 5th might be a reasonable compromise between fuel efficiency and turbo longevity (accept for the radar cameras).
 

rotarykid

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In a car without any mods you should be fine .

But if you have a taller 5th , taller tires or remapped ECU which can give you higher boost use care .

The strain of prolonged pushing of a modified TDI engine up a steep grade could/can cause the head gasket to let go . Also in higher altitudes ( above 5,000' or 1,600 m ) under sustained loads you could have a turbocharger compressor stall making the turbocharger go boom :eek: !!
 

03_01_TDI

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1500 RPMS will require massive amounts of torque (IE TURBO work - which is heat)

IMO 4th gear is much safer and might be plus or minus 1mpg of what 5th would get.

--I used to own a Honda Insight and could achive the same MPG on hills in 3rd gear as opposed to 5th gear (very tall gear). But the Honda gas engine was very effecient at higher RPMs.
 

scooperhsd

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Honda = revver :) - our 1988 Acura Integra doesn't really start making good power until 3000 RPM - and it's very happy at 7000 RPM (getting close to redline).
 

Dunno513

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for the same reason that a smaller engine will actually get worse mileage than a larger engine when faced with the same extra workload, go with the higher rpm's and lower gear selection. Or just buy an auto that does this kind of calculation for you.. ;)
 

scooperhsd

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As everyone likes to keep pointing out - our TDIs are not Class 8 heavyduty engines with a rev limit of 2200 RPM - lugging at 1500 RPM is not very good for them. Let it rev and run where it can handle the load most comfortably.

1500 RPM, load like that is likely to have your car blowing all kinds of soot out the tailpipe, not to mention the possible danger to your turbo. @ 2000 - 2200 RPM, you are just over the peak torque RPM of 1900 and the engine will be best able to pull you up.
 

AZGolf

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Bob_Fout said:
The chance of grenading the turbo and lugging the motor. 1500 in 5th is fine on flat land, but not up a decent grade.

A 4% grade at 1500 RPM in highest gear....come on... we don't have V6 or V8 torque at 1500 RPM.
Bogus assumption there Bob. I climbed out of Camp Verde in AZ on I-17 in fifth gear at less than 50 mph with no problem. The road is a 6% grade so if it is a holiday then you will have to tail gate all the RVs doing the mountain crawl. My turbo is still in one piece after no less than 20 climbs.

The 6 cylinder gasoline variant of the Passat has a 172 HP 2.8L engine
that develops it's peak torque of 173 ft-lbs at 4200 RPMs. The 1.9L TDI develops it's peak torque of 149 ft-lbs at 1900 RPM. So, which one do you think has more torque at 1500 RPM?????

Bob, do you really own a TDI?
 

scooperhsd

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AZGolf said:
Bogus assumption there Bob. I climbed out of Camp Verde in AZ on I-17 in fifth gear at less than 50 mph with no problem. The road is a 6% grade so if it is a holiday then you will have to tail gate all the RVs doing the mountain crawl. My turbo is still in one piece after no less than 20 climbs.

The 6 cylinder gasoline variant of the Passat has a 172 HP 2.8L engine
that develops it's peak torque of 173 ft-lbs at 4200 RPMs. The 1.9L TDI develops it's peak torque of 149 ft-lbs at 1900 RPM. So, which one do you think has more torque at 1500 RPM?????

Bob, do you really own a TDI?
Just because you've done it 20 times doesn't mean that it is the recommended practice.

And yes - I do - and I KNOW the 4speed auto would kick back to 3rd (if not 2nd) in this situation.
 

Drivbiwire

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BoboBear said:
I go up this 2 mile long 3 - 4% grade hill every day at 45 mph. For best fuel efficiency should I use 5th gear at 1500 rpm, or 4th gear at 2000 rpm?
Before answering look at all the related factors:

-Do you have a head wind
-Tire pressure
-Weight of the car
-Beetle or B4 Passat (Cd)
-Grade/slope of hill factoring in distance
-SPEED that you are going...ie load

The TDI is per VW more efficient at 2000 rpms than it is at 1500 rpms...But how much really?



Of course you need to determine the amount of fuel consumed for each hp produced when all the variables are factored in...




It could be argued that you would use slightly less fuel at 1500 rpm However load the slower engine rpm will dictate just slightly more fuel per hp will be consumed.

A bigger isssue is ENGINE WEAR. Diesels ESPECIALLY the TDI have the HIGHEST ENGINE WEAR at HIGH LOAD & LOW RPMS. Based on engine wear alone this would negate the benefits of running at 1500 vs 2000 rpms.

And this also takes us back to the slope of the road, if the load difference between on gear and the other is only 5% (based on throttle input) 1500rpm may be the best option, if you have a difference of (pulling out of thin air) 20% then the lower rpms are not the best thing for your engines health.

Yes diesels make a lot of torque in the lower rpm range but the engine itself for lowest wear needs to be in the 2000rpm+ range for optimal wear performance.

With all that, Steep slope = 2000 rpm, shallow slope = 1500 rpm.

DB
 

EddyKilowatt

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BoboBear, I think it's safe to say your mileage will be better in 5th, but there'll be less stress on your engine in 4th.

How much less stress? That's an "it depends" question. How heavily loaded is the car? Is there a headwind? Is it hot and/or high altitude? Are you running a light thin oil for fuel economy (or other) purposes?

The real question is how hard are you making the engine (and turbo) work to climb at 1500 in 5th. If you're only halfway into the pedal and the engine is purring happily, I myself don't think you're abusing the engine. In their paper bragging about the VNT TDI when it came out, the VW engineers were proud of the engine's ability to make torque at 1500 or even 1200 rpm, thanks to the turbo's flexibility.

But if your foot is close to the floor, and/or you think you can feel every cylinder firing... that's too much load. You're certainly not going to lose much econmy shifting down to 4th.

I myself use my boost gauge when this question comes up... somewhere on here I got the idea that one of the boundaries of the surge region is 6 psi boost at 1500 rpm (n.b.: at sea level). So if I find that I need more than that to get up a hill (or to pick up speed), I'll downshift.

If anyone reading this thread has an automatic AND a boost gauge, they could sure shed some light on what VW's factory programming asks the engine to do in situations like this. What sort of boost and RPM combinations are associated with kickdown shifts?

Ditto if anyone has a ScanGauge... it has a "% Load" readout function, which I'm pretty sure is the signal used by automatics to trigger kickdown.

Regardless of what engine speed you decide to use on this hill, remember to put your foot into it occasionally and accelerate to 2.5 - 3 krpm, in order to cycle the VNT mechanism and vanes.

Eddy
 
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AZGolf

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scooperhsd said:
Just because you've done it 20 times doesn't mean that it is the recommended practice.

And yes - I do - and I KNOW the 4speed auto would kick back to 3rd (if not 2nd) in this situation.
I have done it over 20 times with no damage so why is it not a recommended practice? If you abuse your engine or tow a trailer then all bets are off.

I just checked my owner manual and there is nothing in there that said I should not use 5th gear at 1500rpm while going up hill at 45mph.
 
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IndigoBlueWagon

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Your first post says a 4% grade. That's not very steep: the interstate limit is, IIRC, 6%. Unless the car's heavily loaded I'm betting it will climb it in 5th without a lot of effort. I like the idea of doing it at 50 in 5th, but then again I don't drive for economy.
 

Bob_Fout

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Peter it's steep enough to show the grade though.. or that just common in mountain states?
 

AZGolf

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Drivbiwire said:
...
A bigger isssue is ENGINE WEAR. Diesels ESPECIALLY the TDI have the HIGHEST ENGINE WEAR at HIGH LOAD & LOW RPMS. Based on engine wear alone this would negate the benefits of running at 1500 vs 2000 rpms.

Yes diesels make a lot of torque in the lower rpm range but the engine itself for lowest wear needs to be in the 2000rpm+ range for optimal wear performance.
....
DB
Actually, the TDI have the highest engine wear at high load and high RPMs because it is capable of higher RPMs than most diesel. There is no evidence pointing to less engine wear going from 1500 to 2000 rpms in a TDI. Whereas in standard engine design, reduced RPM can result in improved durability if the engine is also built from heavy components so that metal fatigue is not a problem. Best engine wear rate is not directly related to the optimal engine performance range.
 

rotarykid

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IndigoBlueWagon said:
Your first post says a 4% grade. That's not very steep: the interstate limit is, IIRC, 6%. Unless the car's heavily loaded I'm betting it will climb it in 5th without a lot of effort. I like the idea of doing it at 50 in 5th, but then again I don't drive for economy.
Not so much the grades as it is the length of the grade or the frequency of the grades .

For instance ,



In Missouri & Iowa they have many steep short grades with no breaks in between on I-70 & I-80 . With the high speeds that are normal on these stretches of highways the steep grades without breaks in between are hard on an over tuned engine ( timing jacked up to really early BTDC & larger tires are installed ) .

Because the hills are just long enough to cause a heat build up & close enough together not to allow the engine to cool off a problem arises . What you end up with is an extreme heat cascade in the turbocharger & the head which can lead to a blown head gasket & cooked turbine shaft in an overtuned & overgeared drivetrain/engine .

In an over tuned engine with an over geared drivetrain the cruise slams the power to the engine causing an extreme heat build up under these conditions .

In an engine/drivetrain with really early BTDC timing & oversized tires like mine has a boost spikes when the cruise slams the power on in the 20+ psi gauge range . The result is that it really pours the heat into the head stressing the head bolts & gasket . The boost power in the engine is so hard that you can hear the engine pounding out the exhaust .

At the top of the hill it then lets up for just a few seconds then it slams the boost back in making max power back again . This translates to the engine really pouring on the fire on each successive hill as you start back up & down the hills . The small break you get at the top & bottom of each hill do very little to cool the engine . So with the little breaks then it's wide open again back up the grade in high gear @ high speeds you really cook the engine .

After about 4 to 5 hours of this the time it takes to cross this terrain in Missouri & Iowa the head bolts & gasket & turbocharger have had quite a work out . If you haven't blown the water out you are lucky , this time :D .
 

Drivbiwire

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AZGolf said:
Actually, the TDI have the highest engine wear at high load and high RPMs because it is capable of higher RPMs than most diesel. There is no evidence pointing to less engine wear going from 1500 to 2000 rpms in a TDI. Whereas in standard engine design, reduced RPM can result in improved durability if the engine is also built from heavy components so that metal fatigue is not a problem. Best engine wear rate is not directly related to the optimal engine performance range.
FALSE!

Where an engine makes its power has NOTHING to do with engine wear. The wear is due to high pressure applied by the rings and slow relative motion over the cylinder walls. A TDI is a short stroke diesel approaching a square bore ratio, This results in lower relative piston speed requiring higher engine rpms to permit the lowest wear rates...Thus the term High Speed Direct Injection diesel.

Conversely a long stroke diesel (CAT) has a higher relative piston velocity due to the longer stroke and distance the piston travels, so the optimal wear occurs at a much lower overall rpm range. Stroke has more to do with the wear on the rings than power or capable rpm range.

The engineers strongly disagree with you and your understanding of this is purely a myth!

From the
[FONT=TimesNewRoman,Bold]"ISSUE of WEAR [/FONT][FONT=TimesNewRoman,Bold]280. WE Heraeus Seminar"
Integrating Friction and Wear Research
May 27 – 29, 2002
Ilmenau, Germany

Besides the optimization of technical tribosystems, the analysis of wear maps also facilitates the optimization of endurance tests. For example, it is not well known that in diesel engines of modern passenger cars or trucks, maximum wear of the radial surface of piston rings occurs at low speed and full load

ref. A. Gervé, B. Kehrwald, Anwendung der Radionuklidtechnik (RNT) für die Entwicklung von Verbrennungsmotoren, Tagung Einsatz neuer eßtechniken bei der Untersuchung von Verbrennungsmotoren zur Unterstützung bei Neuentwicklungen, Haus der Technik e.V.,
10.-11. März 1993, Essen, (1993), Nr. E-30-321-056-04
And this is but one example of how HSDI engines operated at high load and low rpms results in significantly higher wear and damage to the piston rings.

This research is applied to the pre-programmed test cell run-in that is performed at the factory. The engines precise wear is calculated so to achieve a near perfect wear in before installation in the vehicle.

From Mercedes regarding break-in

"Drive your vehicle during the first 1000 miles (1500 km) at varying but moderate vehicle and engine speeds."​

ref. pg 302 of the CDI owners manual
DB



[/FONT]
 
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