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Date:         Mon, 25 Feb 2008 00:24:11 -0800
Reply-To:     sam mccarthy <sfcompost@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         sam mccarthy <sfcompost@YAHOO.COM>
Subject:      Re: vanagon Digest - 24 Feb 2008 - Special issue (#2008-238)
Content-Type: text/plain; charset=iso-8859-1

Automatic digest processor <LISTSERV@GERRY.VANAGON.COM> wrote: RE: A Comment on TDi Engine rpm vs. Longevity and Economy

Frank, thanks for the treatise on engine longevity and such. Certainly not what most of us feel or assume is the reality when it comes to taking care of an engine. My question though, is about oil temps. I realize that you have make serious upgrades to the oil cooling system on your van. From what I read and hear from others is that the tdi oil temps climb rapidly over 3200 rpm. Is this your experience as well, and if so, running the tdi at those high revs sounds a bit dicey. Oil coolers and DK trans here I come. Sam M.

Date: Sun, 24 Feb 2008 19:39:31 -0500 From: Dennis Haynes <d23haynes57@HOTMAIL.COM> Subject: Re: A Comment on TDi Engine rpm vs. Longevity and Economy

You forgot to mention that as you lower the engine speed increasing the torque load, chamber pressures, and combustion temps, you are also increasing the production of NOx. NOx is one of the pollutants making it difficult for new Diesels to get compliance.

Dennis.

-----Original Message----- From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Frank Grunthaner Sent: Sunday, February 24, 2008 7:25 AM To: vanagon@GERRY.VANAGON.COM Subject: A Comment on TDi Engine rpm vs. Longevity and Economy

For some time I have seen the traffic on Vanagon TDi related lists as individuals assume elevated rpms are deleterious to both engine longevity and effective fuel economy. New converters are advised to regear transmissions dropping 3rd and 4th to put rpms in the range of 3000 for 70 mph.

I have tried to address this issue in the past (see Vanagon archives), wherein I offered a series of tables and measurements (for gasoline powered engines) showing that several key points dominate the reality of this situation. For this note, let me review the key points as they apply to TDi engines:

1. For a given speed, on a level highway the power required to maintain that speed is a function of the mass, the aerodynamic drag and powertrain losses. Not a function of engine rpm. The thrust required to maintain that speed is a constant at the tire/road contact patch and is a function of engine rpm.

2. For a given mass and Vanagon type (Westy, passenger van, panel van, pickup or syncro) the responsiveness or ability to accelerate from a reference speed is directly related to the torque reserve available at the tire/road contact patch. Reserve means how much power is left after providing the thrust to maintain the vehicle at the starting velocity. This torque is a product of the net gear ratio from the transmission, through the final drive and finally through the lever arm operating from the axle centerline to the tire/road contact patch. Larger tire (fewer revs per mile) but same gear ratio and final drive requires more torque from the engine to maintain a given amount of thrust.

3. Stress on engine components is directly related to peak chamber pressures (which are in turn directly related to cylinder temperatures). The higher the load factor at any given rpm, the higher the pressure. The more torque required by the driver at any given rpm, the higher the load factor and the higher the chamber pressure. Direct measurements on the 1Z/AHU and the ALH 1.9L TDi engines by VW engineers show that for the same load (torque output) at 3000 rpm and 4500 rpm, the 4500 rpm case has a 1.1% higher chamber pressure. In other words, stress in the VW TDi 1.9L engine is only a function of torque output, not engine speed.

4. The real fuel consumption of the engine is given by the product of instantaneous power output required and the brake specific fuel consumption (efficiency) of the engine at the speed and load factor the driver demands. Figures are published for the BSFC for the stock engines at maximum load. The units are grams of fuel at 20C per kilowatt hour of power delivered. For the 1Z/AHU with traditional wastegated turbocharger, the numbers are: 203@2100, 220@3000, 222@3500, 235@4000 and 250@4500. For the 1Z/AHU with VNT turbo or the ALH engine, the numbers are: 196@1750, 214@3000, 217@3500, 225@4000 and 241@4500. Again these are the full load at any given rpm values. The consumption at less than full load is different than these numbers but never better than the optimal efficiency number (2100 for the 1Z and 1750 for the ALH) due to the fluid dynamic efficiency variations.

5. The so-called redline for these engines is a mechanical fluid flow limit related to the diesel fuel burn time. A given amount of injected fuel takes a well-known time to burn. If the burn has not completed by the time the exhaust valve opens, the fuel will complete its burn in the exhaust tract, quickly destroying the turbocharger. As engine speed increases the time available to complete the burn gets shorter. On the 1Z/AHU and ALH engines, the stock controller shuts the fuel delivery off at 5200 rpm. This is the ECU or mechanical pump enforced redline. These engines have an engine destruction onset redline (compare directly to gasoline engine redlines) of between 7000 and 7800 rpm. When TDi bottom ends with rods, similar composition pistons and rings are used in racing gasoline engines they typically break above 10,000 rpm. In other words, the bottom ends of these engines are impressive and highly overbuilt for the diesel application.

6. The torque curve on the stock engine begins a sharp decline around 4200 rpm with a horsepower falloff at 4400 rpm. This is largely a function of factory tuning parameters. With a readily available chip (see Rocketchip.com for example), these engines experience the torque drop-off at about 4500 rpm and the power drop-off at 4800 - 5000 rpm.

7. The modern VW diesel engine is designed for continuous high speed service. It is rated for maximum horsepower at 4000 rpm. In the design stage as well as quality assurance testing the engines are run for the equivalent of 200,000 miles with engine speeds randomly varying followed by continuous 20 hour steady speed cycles. These engines are continuously run at 4500 rpm.

So what does this all come down to? Well, in a list of points in no particular order:

a.) If you regear your transmission from 4000 rpm at your desired cruising speed to 3000, you will save between 5 and 6% of your fuel cost due that rpm change. You may see more, but that is strictly due to changed driving habits. If the engine has a lower torque reserve for acceleration or hill climbing you may well back-off to enjoy the scenery. You will increase the chamber pressure in the engine by 33% or more if thermal effects are included. This means higher bearing loads, thinner oil lubricating films, and higher piston head and exhaust valve temperatures. This means a reduced engine life compared to the 4000 rpm case. You will also piss away a significant number of bones on transmission work.

b.) Regearing from 4000 to 3500 is less of a fuel saving and less stress difference, but the direction of effects is the same.

c.) Regearing from a 4500 rpm cruising speed to a 3000 rpm value will give a 10 to 12% fuel savings. Now the stress factor has increased by 50%. All other comments of a.) still apply. The change from 4500 to 3500 is left as an exercise for the reader.

d.) It should be evident that a reasonably maintained engine in stock tune can be run at 4200 to 44 rpm all day with no longevity ramifications. With a VNT turbo and a chip, 4500 to 5000 are accessible Cruise up to 4500 and emergency use to 5000.

e.) Vibration and noise are a problem, but as I have indicated in the past, a snorkel to off-road air cleaner with air intake through the sealed driver side compartment, 2.5 inch exhaust and pneumatic motor mounts eliminate this problem. A 500+ watt stereo system with satellite and HD radio together with the ipod can contribute sufficient decibels of Mozart to eliminate the issue.

f.) In my install (1Z engine, VNT turbo, PD150 intake with no EGR, Rocketchip, PP520 nozzles, lift pump, windage tray and oil pan baffle, DK transmission with 4 spider carrier, SA splash plates and 215/75-15 tires), my over the ground speeds are 62 @3000, 72 @3500, 82@4000 and 92@4500 in 4th. In 3rd, the numbers are 43@3000, 50@3500, 57@4000 and 65@4500. The shift in LA traffic mergers is typically @4000 to 4500 for and easy pickup by 4th. Just a stock DK with strengthened pinion. I'm considering going back down to 27-8.50-14 tires which would give me a 75 mph cruising speed of 3750 rpm, and 90 mph @4500.

I rest assured that immediately following this post we will find the conventional wisdom dispensed to the effect that a regear to 3000 is essential for the TDi Vanagon combination. Humbug!

BTW, for those who are searching for a conversion that puts serious vim and vinegar in the manual trans Vanagon for modest investment, easy service and serious longevity without spending the cash on the TDi route, the pre-ABA engines including the Digifant RV and the Audi 3A tied up to the DZ transmission are an impressive combination. Highly recommended!

As always, my apologies for the length.

Frank Grunthaner

In a message dated 2/23/08 11:12:37 PM, ScottDaniel@turbovans.com writes:

> 4,000 rpm in a TD engine is 'thrashing' a bit more I think, that a short > stroke waterboxer is. > > 3,500 rpm in a diesel is as low as I would want for my highest cruising > speed that I would expect to do without undo stress. > > I wouldn't ask it to do 70 at 3,500....... I wouldn't ask it to do 70 > stressful on the engine >


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