Run 60 get 28, run 65 get 30, run 70 get 29.
 

Mercedes 3.0 TD
Seems like the turbo diesel boost helps more than the aero hurts. My sample size is small but am going to continue driving 65 in my Mercedes for more data. I think I am comparing similar driving conditions. The engine is supposed to have peak torque at 2000 and 65 mph is 2800.

The car supposedly has a cd of .4 and is fairly small. yet pretty heavy for it's size. The engine is strictly old school IDI TD, no computer.

Comments?

MPG should be inversely proportional to MPH.

Don't buy it. Maybe if it had taller gearing. Keep us posted as you get more data.

Should probably include data points for 50 and 55.

Tell us how you did your testing.

Tony I won't argue as to the theory. But I drove it for 3000 miles at 55.

My VW gets better mileage at 55 (55 mpg). It gets only 2-3 psi at 60. The Merc gets 5 psi at 60 and 8-9 at 65. Trying to find a way to get more boost at less speed.

Drop a gear? :p

Nicky It is a 4 speed auto and 27 years old. Everything above 30 is in top gear. You might be right.

If everything above thirty is in top gear, then thirty five is probably where you can get best economy.

Of course, that's problematic on US highways where going thirty is just as good as standing still...

When I put a turbo on my diesel I picked up at least 2 to 3 mpg on the highway. And thats compairing driving 55-60 when N/A to +65MPH w/turbo.

Diesel engines are nice because the more air you can cram into the cylinders the better the fuel econony and power out put will be.
If you had a boost gauge you may see why there is a fuel economy jump at the higher speeds. At 60 the turbo maybe under such light load it may not be kicking out any boost, at 65 you could be developing 1 or 3 PSI and that small change can make a lot of difference.

Water injection

I have to agree, I clock about 5 psi at 60 and 9-10 at 70. I have the waste gate set a bit high at 14 and drive it at a max of 12 psi. It will pull 14 psi at half throttle. I need to get an EGT.

By contrast my vw TD gets 2-3 psi at 60.

Water injection can raise the boost? I thought it was to lower the combustion temp.

Water injection puts more air into the cylinders by cooling down the air. You want to get the most air into the cylinders with the least amount of energy expendture.
Water injection is a very cheap way to get more air into the cylinders with very minimal energy input.

Water injection can raise boost with pre-turbo spraying (which I don't really recomend), but that is usually done under heavy load.

Every diesel should have an EGT and boost gauge. Also fuel feed pressure wouldn't hurt (put a gauge between your last fuel filter and before the injector pump).

It sounds like your Benz has a fairly restrictive exhaust housing. There is no need for a car with a diesel engine that big to run 9PSI just to roll down the highway at 70mph.
That measns the turbo is doing a lot of work when it doesn't need to be.

The dodge guys claim that keeping your numbers below the 600-6 mark is the way to go. That means keep your EGT numbers under 600'F and boost below 6psi.

I run about 1psi of boost at 55mph and up to 3psi at 70mph. My EGT numbers tend to be a little over 600'F if I am rolling along at 70mph.

I'm just reporting the facts:). It would seem to me that a restrictive exhaust would lower flow, turbine spin and boost? and a high intake pressure would be a result of having lack of flow there letting the pressure build up.

The car when I purchased it, didn't have a boost gauge. When I put one on it would go to 9 with a normal range of 10-12. It would boost to 1 at 60 and 5 at 62. I tracked it down to the ARV.

It had an intake turbine bypass so that it would not run boost even with the turbo spinning good. It was called an ARV. It had a fairly complex system that would bipass the intake air until the car got to 65 and had a fairly high load, then the ARV would close, allowing the air to flow into the intake.

I figured that if the turbo is spinning, it should be boosting so I disconnected it and immediately got boost at any speed above 35-40 cruise.

You can never hear the turbine whine you can not hear the exhaust much. Still 30 mpg isn't that bad for winter driving with snow tires on all wheels.

No. Sure there is twice as much wind resistance at 30 mph than there is at 60mph - it's a square law. But MPG isn't based solely on wind resistance, another important consideration is how efficient the engine is working. I got my best tank going 75-85 on california freeways.

Inversely proportion isn't quite the right mathematical wording to describe the relationship because, again, it's a square law.

If you look at the bsfc map, the road load runs along the 300 contour, so engine efficiency should be nearly constant between 50 and 70 mph. Maybe the torque converter has something to do with it.

http://ecomodder.com/wiki/images/thu...rbo_diesel.jpg