Optimal Turbo Boost for Fuel Economy
 

Is there an optimal boost pressure for fuel economy at 70 mph cruise?

Background:
1992 300D 177,000 miles on the odometer (602.962).
Averaged 32.8 mpg over the last 27,000 miles with a high of 35.6 and low of 28.6

I drive the car 180 miles a day (weekdays), 165 of those miles are on the highway at about 70 mph.

Doktor Bert is running his ’78 300SD with 10 psi boost at cruise speed and getting about 25 mpg (http://www.peachparts.com/shopforum/diesel-discussion/326392-real-world-617-950-fuel-economy-1-000-miles-week.html). After fixing an idle speed problem (http://www.peachparts.com/shopforum/diesel-discussion/324878-idle-adjust-602-a.html) I have gotten the stock turbo setup to work again on my car (http://www.peachparts.com/shopforum/3020979-post399.html). At cruise in the ’92 300D there is no boost, which GSXR pointed out is normal. I was talking to a mechanic friend of mine who put a boost controller into a Dodge/Cummins to set cruise boost at 7 psi and he claimed there was an improvement in fuel economy.

Does this make sense and if so does anyone have an idea about optimal boost at cruise for fuel economy?

I will probably eventually do the pressure actuated wastegate swap and collected the data myself but I was thinking some of you might have a good idea how it might turn out, and save 3 months of data collection. :D

Maybe I'm thinking of this wrong, but any boost equals more fuel which equals less MPG.

So with a gas engine, keep vacuum high. With our turbo diesels, keep boost low. Both of these should improve your driving style and mileage.

That is what I thought. I am just wondering why you would choose to run boost while at cruise speed and if there was any validity to the claim that boost increased the economy on a cummiins.

My comments that "boost=less MPG" are based on personal experience in situations where cruise control was not used. In my OM602 the position of the throttle is different when driving the same route with and without boost. Hills require more throttle without boost so in theory there is a magical point where boost achieves better MPG than without.

I lean towards what 300D-91 says - for the best mpg, cut the manifold signal the ALDA. As for the Cummins, maybe it was set to 7 psi from 12 psi.

I don't choose to run boost at cruise speed, it just happens. I see at least 5 psi at 70 mph. Did Dave/gsxr really say it's normal to see no boost at 70 mph or did he say it's normal to see no boost when there's no load on the engine? A W124 at 70 mph isn't slicing through air. That engine is working. I understand what I see with my mechanical system but EDS isn't necessarily as straightforward. It could be that EDS is tuned for economy and, pardon my use of the word, artificially reduces boost under steady load and rpm conditions. The beauty of EDS in place of a mechanical system is that it can dynamically switch from economy mode to power mode... when it works. Maybe Hermann Göring was referring to EDS when he made his comment about the state of German technology.

Which begs the question of those who've converted from EDS to mechanical wastegate control - what's the effect on mpg?

Sixto
87 300D

For me I've averaged 28 - 29 MPG for the 23K miles I've owned my car and that hasn't changed for the 1.1K accumulated since doing the wastegate mod. (However my smiles per mile have gone up :D).

Keep in mind my switch to mechanical vs EDS control of my wastegate was part of a larger engine 'tune up'.

My Cummins gets less mpg under boost. I have only to note what's happening via the overhead. Unloaded, light throttle, no boost, down hill with a tailwind is best for mpg.

Sixto,

Dave/gsxr suggested that 0 boost at constant speed might be normal (http://www.peachparts.com/shopforum/3020993-post400.html). I asked originally because if I run vacuum straight to the wastegate actuator I see about 5-7 psi boost at 70 mph. I do see boost going up hills which goes in line with 300D-91's idea that there might be situations where boost is more efficient than changing the throttle position.

If the EDS is cutting boost at constant speed for fuel economy then I should be gratefully for the EDS. But if the difference in fuel economy is negligible I could definitely use more smiles/mile, then again who couldn't.:D

I think Junkman has the best idea. I just need to get a sail!

Biosudds I'm curious about the fact that your seeing vacuum.

For the 1.1K since I installed a vacuum / boost gauge (in my 300D) it's never shown anything below zero that I know of.

If you're really concerned about economy, then less speed is better - there's a hinge point around 55 mph where more speed dramatically increases wind resistance and mileage suffers. Below that, wind resistance is a minor factor toward achieving better mpg. But who drives 55 right?

Maybe try 60 for a day, see if you're a more relaxed person for it.

Got me, but I know 1 guy who doesn't; Sammy Hagar :cool:

I drive 55 when the speed limit's 45 :)

55 or even 60 isn't relaxing when all I see in my mirror is Kenworth spelled backwards.

Sixto
87 300D

I am running 14.7 psi in second speed range at WOT and 10 psi pretty much all the time. I am getting 25-27 consistently....

Remember that with a normal wastegate turbo you can not limit part throttle boost... Just maximum boost.

It takes energy to make boost. On my TDIs the computer will control the vanes in the turbo to make less boost when cursing, saving fuel.


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Making boost requires pressure in the exhaust. That back pressure becomes pumping losses. The standard Holset turbo on my '03 Dodge would always make boost, even at idle. It's ratio of back pressure to boost was 2:1, ie, cruising at 65 with 5-7 psi of boost means 10-14 psi back pressure in the exhaust. I replaced the stocker with a BD Super B which has a 1:1 ratio, and only makes boost when you need it. 65 MPH now = 2 psi of boost and 2psi of back pressure. It also equals an almost 10% improvement in fuel economy.