Date:         Wed, 8 Jan 2003 03:46:07 EST
Reply-To:     FrankGRUN@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Frank Grunthaner <FrankGRUN@AOL.COM>
Subject:      Gearing Fuel Economy and Performance
Content-Type: text/plain; charset="UTF-8"

Just out of frustration, I offer the following repost of my earlier post on 
the transmission and performance issues (first posted 10/01): 

Even though I'm in complete agreement with Alistair Bell's latest post, this 
discussion about the obvious undesirability of the diesel transmission for I4 
engine conversions has still stimulated my latent hemorrhoids again. And 
while I suspect that the majority of the adherents to this pseudo-wisdom will 
brush aside engineering data as simply obscuring the real understanding, I 
will try again to offer a few terse (really!) comments.

There are several issues: 1. Power, 2. Longevity/reliability, 3. Fuel Economy 
and 4. Noise. I will define the issue to consist of differences in final 
drive ratios and tire diameters. For tire diameters, I have treated two cases 
– the stock 185SR14 at 821 revs per mile, and the largest size tire I 
personally know to be installable on the 2wd Vanagon – the 215/75x15 at 755 
revs per mile. The stock tire is Michelin and the 215/75 is a Yokohama (my 
number sources). For the transmission final drive, there are four treated 
possibilities: 5.86 (Diesel Westfalia), 5.43 (Diesel au regular), 4.57 (2.0 
AC and 1.9 WB) and 4.83 (1.9 and 2.1 WB). Lets do them in order:

1. Power. It was my intention to post more curves to show the following, but 
I’m away from my primary computer so this more specific summary will have to 
suffice for now. I have calculated from the torque curves and the gearing 
including wheel radius the maximum available torque available at the road for 
a number of powertrain combinations. For simplicity, I have normalized the 
results to the power level put out by the normally aspirated Vanagon diesel, 
and I am only quoting figures for a correct ground speed of 60 mph. Of 
course, your engine may have been tuned by Dr. Porsche personally, or it may 
be the infamous car built on Feb. 31, or your mods may be the ones that the 
VW R&D staff have been trying to duplicate for a decade to corner the market 
and therefore your Vanagon has more power, goes faster and uses 40% less 
fuel! So be it! I’m tired. Anyway, I list the torque available as a 
percentage of that from the diesel. The numbers are:

Diesel Westfalia – 100%
1.9L WB with 4.57 – 119%
1.9L WB with 4.83 – 126%
2.1L WB with 4.83 – 150%

1.9L WB with 4.57 and 215/75x15  – 109%
1.9L WB with 4.83 and 215/75x15  – 115%
2.1L WB with 4.83 and 215/75x15  – 138%

1.8L I4 with 5.86 – 188%
1.8L I4 with 4.57 – 130%
1.8L I4 with 4.83 – 138%
1.8L I4 with 5.43 – 174%

1.8L I4 with 5.86 and 215/75x15  – 173%
1.8L I4 with 4.57 and 215/75x15  – 120%
1.8L I4 with 4.83 and 215/75x15  – 127%
1.8L I4 with 5.43 and 215/75x15  – 160%

2.0L I4 with 5.86 – 185%
2.0L I4 with 4.57 – 139%
2.0L I4 with 4.83 – 146%
2.0L I4 with 5.43 – 171%

2.0L I4 with 5.86 and 215/75x15  – 170%
2.0L I4 with 4.57 and 215/75x15  – 127%
2.0L I4 with 4.83 and 215/75x15  – 134%
2.0L I4 with 5.43 and 215/75x15  – 157%

2.0L Turbo I4 with 5.86 – 196%
2.0L Turbo I4 with 4.57 – 205%

2.0L Turbo I4 with 5.86 and 215/75x15  – 198%
2.0L Turbo I4 with 4.57 and 215/75x15  – 205%

OK, you can study the table. But some simple observations. Put the 1.8L gas 
digifant in a 5.86 vehicle and you have more torque than any other Vanagon  
combination with the exception of the Audi 3A Turbo. Even with larger tires 
you still have more than 25% more torque than the 2.1L WB. Put the 1.8L in 
with a 4.57 trans (the intelligent and obvious choice) and you’ll have a 
vehicle with the performance of the 1.9L WB. Add Air and you are down to 1.9L 
WB (4.57) oomph levels. Note that the 4.57 transaxle actually out performs 
the 5.86 in the case of the 2.0L Turbo.

For you TIICO buffs, I gather that the target trans of choice is the 4.83 WB 
deluxe. With that 4.83, you will have 3% less torque at 60 mph than your 2.1L 
WB brethren. With the 4.57 trans, you will be 8% down. Not bad for a satchel 
of cash. Of course, with the 5.86, you would be spinning with 24% more torque 
than the 2.1L WB. Not exactly burning rubber, but more than enough power to 
shock a Mark IV Jetta 2.0L driver at the stoplight GrandPrix, or add some 
serious ripples to the pantyhose of the purple-haired over 50 matron dawdling 
in her trip from the super to the car! (Been there ...).

2. Longevity/Reliability. Well firstly its an I4, so reliability is not a 
question. Longevity is more anecdotal. The engine is designed for piston 
speeds consistent with long term operation above 6300 rpm. In Europe, the 
mean highway driving speeds exceed 4000 rpm and the engines spin for 250,000 
to 350,000 km. As I have said before, the longevity issue is first order in 
load. For the same velocity, the engine load is 28% higher for the engine in 
a 4.57 vehicle vs. a Diesel Westfalia trans. Clearly, the 4.57 vehicle has a 
shorter life expectancy in this application as compared to the 5.86.

3. Fuel Economy. I have also digitized the fuel consumption maps. Now these 
are not the final word because of the part throttle vacuum maps (I have those 
too, but later on that group), but they are a good first approximation for 
the problem. As I have said previously, the fuel consumption is related to 
the work required and the engine efficiency. Having said that (yeh, I know, 
repeatedly!) the fuel consumption at 60 mph can be directly compared with 
knowledge of the engine torque level, the rpm and the horsepower required to 
propel the Vanagon at 60 mph on a level road with no head or tailwind. The 
numbers (for the 1.8L gas Digifant engine) are:

For the 5.86 final drive and 185SR14 wheels – 10.0 kg/hr.
For the 5.43 final drive and 185SR14 wheels – 9.62 kg/hr.
For the 4.83 final drive and 185SR14 wheels – 9.43 kg/hr.
For the 4.57 final drive and 185SR14 wheels – 9.43 kg/hr.

For the 5.86 final drive and 215/75x15 wheels – 9.69 kg/hr.
For the 5.43 final drive and 215/75x15 wheels – 9.43 kg/hr.
For the 4.83 final drive and 215/75x15 wheels – 9.33 kg/hr.
For the 4.57 final drive and 215/75x15 wheels – 9.25 kg/hr.

So, 8% less fuel consumption with a 40% reduction in rpm. The choice is 
clear. Keep the diesel! Er, or gear it down so you have less torque than the 
1.9L WB in a 4.57 car! Or spin and enjoy it.

4. Noise. Well, muffle the sucker. Discussed this before. No need to do so 
again here. Vibration and noise are both subject to a modest fiscal solution. 
I recommend a 2500 watt 9 speaker stereo. On my way to Monterey, I passed a 
CHP cruiser with the Gotterdammerung wailing. Sweet.

Final observations. If you don’t trust the engineers, go reduce the 
longevity, minimize the performance of your conversion and gain almost half a 
mile per gallon by swapping in the 4.57. Otherwise, enjoy the performance of 
the 5.86. These observations are really due to the relatively unique design 
of VW’s I4 engine family. The torque curve is nearly flat (as compared to the 
highly peaked performance of American Iron and Japanese spinners). This 
indicates that the volumetric efficiency of the 8V engines is a very minor 
function of rpm. The 1.9L WB is much more peaked. Much of this comes from the 
cam and the intake runner design. The 16V VW engines also have strongly 
peaked torque curves.

The most efficient utilization of an engine is attained when its torque 
maximum is achieved at the most desirable operating condition. From my 
perspective, cruising in top gear at 60 mph is pretty close to a good optimal 
operating condition. So, for the 2.0L Air Cooled engine, the 4.57 trans is 
ideal with stock skins. For the 1.8L Digifant, the optimum would be the 5.43 
regular diesel trans with 215/75x15 tires. The 2.0L I4 would be best with the 
5.86 trans and stock tires, while the 2.0L Turbo is well matched to the 4.57 
transaxle with 215/75x15 wheels/tires. 

And a note to Mark over my recent search for the 4.57 transaxle. The SAAB APC 
engine management system I am using with the Garrett T3, the Bosch 
LH-Jetronic on the Audi 3A engine has been optimized for minimum fuel 
consumption with maximum turbo response at 3000 rpm. According to a Swedish 
engineer who worked worked on the APC project, they designed the turbo 
control system to keep the engine (at any part throttle or full throttle 
condition)  at the maximum boost, just shy of detonation for any load. The 
fuel metering is a function of air flow which in turn is a function of 
throttle opening. In short, the engine system will run at 10 psi, with stock 
10:1 compression ratio, with an amazing volumetric efficiency over the range 
of 2000 to 6000 rpm but a maximum efficiency at 3K. Since there is power to 
spare, I've been looking for a DK to try. If not to my liking, (90 in 3rd!) 
I'll swap the DZ back in!

I hope someone has fun with this. I'm trying to get all of this data together 
to post on Alistair's new web site, then everybody's slide rule can again 
come forth!

Frank Grunthaner