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.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.
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.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.
Just because you've done it 20 times doesn't mean that it is the recommended practice.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?
Before answering look at all the related factors: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?
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.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.
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.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
Not so much the grades as it is the length of the grade or the frequency of the grades .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.
FALSE!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.
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.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
DBFrom 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