1.9 AAZ torque curve relative to efficiency

[Gizmoman]

In an effort to "optimize" the 1.9 IDI TD in a Vanagon, what would be my ideal RPM target at 70 MPH?

Stock torque & HP curves are "Nominal power is 51 kW or 70 HP at 4500 rpm. Maximum torque is 133 Nm at 2600 rpm"
Here is the link: http://www.4crawler.com/Diesel/SAE/vwtdsae.shtml#4-CYLINDER_1.

Based on that, is 2600 my ideal RPM? Currently I am running 27" diameter tires (16" rims) and a stock 4 speed gasser tranny, the tach reads about 3500 @ 70 which I'd rather lower a bit.

My engine is no longer stock but I have no dyno numbers. Just using the stock numbers as a baseline. Just to be clear, is max torque my target or max HP, or is the ideal target somewhere in between?

[theman53]

I personally think right under to at the peak torque is where you see the best mileage. If you could run 2,400-2,600 rpm I think you will see the best mileage overall. YRMV and I have no link to someone else saying it, but it is what I have found in my driving.

[8v-of-fury!]

A little off on your power and torque numbers.

55 kilowatts (75 PS; 74 bhp) @ 4,200 rpm; 150 newton metres (111 lbf·ft) @ 2,000 rpm

Furthermore, this is my 1.6TD;

51 kilowatts (69 PS; 68 bhp) @ 4,500 rpm; 133 newton metres (98 lbf·ft) @ 2,500 rpm

I get the best mileage (51.32 MPG or 4.58 L/100km) out of this car @ 75mph and 3300 rpm. Stock and modified.

[Gizmoman]

A little off on your power and torque numbers.

55 kilowatts (75 PS; 74 bhp) @ 4,200 rpm; 150 newton metres (111 lbf·ft) @ 2,000 rpm

Furthermore, this is my 1.6TD;

51 kilowatts (69 PS; 68 bhp) @ 4,500 rpm; 133 newton metres (98 lbf·ft) @ 2,500 rpm

I get the best mileage (51.32 MPG or 4.58 L/100km) out of this car @ 75mph and 3300 rpm. Stock and modified.

Thanks for the feedback. They weren't my numbers, I found them in the link I posted.
I'm planning for a transmission rebuild (looking for a spare to work on at my leisure) and may change a few ratios while I'm at it. The current 4 speed I have nets higher RPMs than the engine likes I believe.

I personally think right under to at the peak torque is where you see the best mileage. If you could run 2,400-2,600 rpm I think you will see the best mileage overall. YRMV and I have no link to someone else saying it, but it is what I have found in my driving.

I'd love to have a five speed that put it at 2,600 at 70-75. If that's where the torque is, It should be able to push the brick through the wind at that RPM. I guess that's what I'm trying to find out. Using the word "efficiency" was probably a bad choice.

[DogDiesel]

I don't often respond to such, but here I have tried before and learned.

I geared my caddy, first by going 3:67 from 3:94, then adding the 0.71 fifth, and still loose on RPM.  
Now that I was outside the conventional gears, I increased my tire size.  
Finally, she was 2450 at 55mph.  I am editing this, because I went back and re-ready your posting.
70MPH!??  2450.  Anemic.

I changed engines and went from a 1.9TD with a 1.6TD turbo, to the anemic 1.9TD turbo.  
A difference you could tell.  Not anemic, but if I was pulling a Vanagon body. Anemic.
Installed a Giles pump, and oyeouuuhhh!  Will install the other Turbo when I aftercool.
Point is the turbo must be spooling and not droop, or the taller gears will make you downshift.

I took one of the factors out, when you go to a larger engine, because I went to a double row, race radiator.  Suggest you consider this issue also.

With my caddy body weight, I have it just right.  My gut says with the Vanagon weight, you will be anemic at 2450, and 2700 or 2800 is more right.
Rule of thumb is you should still be able to accellerate in top gear up a 7% grade with moderate throttle.  If you live in flat land, gear taller, if you live in hills and mountains, then your gears must be lower for more RPM.  
I am pulling 2590 at 70.  A vanagon might be guttless at this gearing.
Wayne

[8v-of-fury!]

What does a Vanagon weigh? Pulling my ATV trailer with me and my brother in the car and our gear we probably gross 3500-3600lbs. Much more than that?

I am on 185/60/14's and run a 3.67 FD with the .75 5th. I can motor that load without too much trouble at 2000. Granted, mine is a TDI with a little more fuel than stock.. but the stock turbo at near stock boost when cruising. It is completely effortless at 2400-2600. Even in 5th gear, up steep grades.

[the caveman]

I wish I could remember more details, but when I had my transporter, which only had 4 seats, and carpets, weighed in at 4000 lbs. 1.9 NA with a Giles pump,big intake and exhaust; probably close to same hp but a bit more TQ then a 1.6 turbo. 4.86 rear .816 5th and with 215/16's it would run real nice all day at 110-113; 30mpg, but mileage did go way up if I ran at 95-100.

[theman53]

The other thing I just remembered is your intake is short runner. So that may push the TQ curve up some and you may have to run it higher in the RPM range...I guess what I am saying is it will probably be all trial and error for you.

[the caveman]

A pal of mine has been running an AAZ intercooled with an aircooled transmission-possibly 4.57 R&P. He says he runs at 125 and will get pretty good mileage at that speed. again, not a camper, but loaded up with tools.

[Gizmoman]

The other thing I just remembered is your intake is short runner. So that may push the TQ curve up some and you may have to run it higher in the RPM range...I guess what I am saying is it will probably be all trial and error for you.

Well, my current tranny is getting worked and I think if I rebuild another to have ready, I just need to get the taller R&P.
Wish I could find a used 5 speed so I'd have a low 1st as well.