Turbo Boost vs MPG - PeachParts Mercedes-Benz Forum

JamesDean · #1 10-15-2012, 12:13 PM

Hello everyone,

I had a thought the other day.

When I first drove my 300D 2.5 home from Columbus. It got 39MPG.
This number is high from what I've heard for a 300D 2.5.

As I worked on the car, I am now realizing the turbo is not actually doing anything. No boost. The car is still drive-able, just slow. 0-60 is 20 seconds. Even slower when the governor kicked out and I was stuck driving in second gear. A 240D would have beat me!

This got me thinking.

If my 300D can get HIGH 30s with no boost, and (presumably from everyone else's numbers LOW 30s with boost..

I only really need boost at certain times. Maybe I can get the best of both worlds.. I could employ some type of electronic wastegate controller...

What do you guys think? Or is my reasoning here flawed? Was my 39MPG a fluke, now given that the turbo was NA?

Simpler=Better · #2 10-15-2012, 12:45 PM

If the boost wasn't kicking in, then your ALDA was limiting fuel. By limiting fuel, no matter how hard you mashed that pedal your injection pump was being limited.

If you had an operational turbo and the fuel to match (POWAA!), would you take it easy on the pedal, limiting yourself to a 20second 0-60?

On my 300D (different engine, I know, I know) I had it setup to relax the turbo on the highway at the flip of a switch. When cruising I ran 8psi, when flooring it I ran 18psi.

A little extra boost can help your efficiency even if you're not completely matching the fuel quantity. You will have to monitor your intake pressure vs exhaust pressure to find the sweet spot.

JamesDean · #3 10-15-2012, 01:04 PM

Quote:

Originally Posted by Simpler=Better

If the boost wasn't kicking in, then your ALDA was limiting fuel. By limiting fuel, no matter how hard you mashed that pedal your injection pump was being limited.

If you had an operational turbo and the fuel to match (POWAA!), would you take it easy on the pedal, limiting yourself to a 20second 0-60?

On my 300D (different engine, I know, I know) I had it setup to relax the turbo on the highway at the flip of a switch. When cruising I ran 8psi, when flooring it I ran 18psi.

A little extra boost can help your efficiency even if you're not completely matching the fuel quantity. You will have to monitor your intake pressure vs exhaust pressure to find the sweet spot.

Hmm.

I like this switch idea. What controller did you use? 90% of my driving is highway at speed, no real need for the turbo to be dumping boost. If i can switch it to a better mpg setting all the better.

Simpler=Better · #4 10-15-2012, 01:38 PM

I was running a VNT with a homebuilt regulator. http://www.peachparts.com/shopforum/2185307-post52.html

For the stock turbo you could splice in two boost controllers, and remote controlled valves(think electronic vacuum switches from the climate control or a gas car's EGR) to enable/disable each one. One at 12psi and one at #psi for cruising.

Brian Carlton · #5 10-15-2012, 01:46 PM

Quote:

Originally Posted by Simpler=Better

On my 300D (different engine, I know, I know) I had it setup to relax the turbo on the highway at the flip of a switch. When cruising I ran 8psi, when flooring it I ran 18psi.

A little extra boost can help your efficiency even if you're not completely matching the fuel quantity. You will have to monitor your intake pressure vs exhaust pressure to find the sweet spot.

I'm not understanding the need to "relax the turbo" on the highway. The road load demand on the highway is only about 15 hp, so the boost would naturally be below 8 psi unless the turbo was exceptionally small and the vanes were closed. I don't believe this would be possible with the typical OE Garrett setup.

scottmcphee · #6 10-15-2012, 02:00 PM

I'm just wondering what you'd label the switch on the dash for the active position, maybe "Suffer", and for the normal position, "Relief" ?!

Simpler=Better · #7 10-15-2012, 02:15 PM

Quote:

Originally Posted by Brian Carlton

I'm not understanding the need to "relax the turbo" on the highway. The road load demand on the highway is only about 15 hp, so the boost would naturally be below 8 psi unless the turbo was exceptionally small and the vanes were closed. I don't believe this would be possible with the typical OE Garrett setup.

It might be different with the stock setup, I never messed with it.

With the 2056V I noticed a drop in temps while cruising with the vanes open vs closed. I don't remember the actual amount.

Brian Carlton · #8 10-15-2012, 02:22 PM

Quote:

Originally Posted by Simpler=Better

It might be different with the stock setup, I never messed with it.

With the 2056V I noticed a drop in temps while cruising with the vanes open vs closed. I don't remember the actual amount.

That makes some sense with the variable vanes. If you ask for more boost, the charge air temperature will climb and the temperature will likely rise a bit to compensate.

I'd be curious if the variable vanes could be opened sufficiently to decrease the boost close to zero. At that point, you've got a naturally aspirated engine. The real question is how would it fare in fuel economy with no boost whatsoever?

The answer might be valuable to those with stock setups if it offered some fuel economy benefits.

BillGrissom · #9 10-15-2012, 02:30 PM

Wow. How are you measuring 39 mpg? The current EPA figures for my 1985 300D is 19 mpg hwy, though I get ~22 mpg. I hope you aren't naively going by the markings on the fuel gage for consumption and the odometer for distance. Use the calibrated pump at fuel stations by measuring from fill-up to fill-up. For distance, use highway mileage posts or a GPS.

The turbocharger recovers exhaust energy, so disabling should make the engine less efficient. Diesels run WOT all the time, so a turbocharger is an ideal match. Some people who have added a turbo to 60's gasoline engines claim it improves mileage slightly. That is hard to believe since boosting the inlet pressure only to drop it across the throttle plate is a waste. Might be a very minor improvement from adding a little energy (heat) to the inlet air.

Simpler=Better · #10 10-15-2012, 02:31 PM

Theoretically you can, but as I've been told the 2056V is on the small side. An external wastegate could always be added to bypass the turbo.

JamesDean · #11 10-15-2012, 02:48 PM

Quote:

Originally Posted by BillGrissom

Wow. How are you measuring 39 mpg? The current EPA figures for my 1985 300D is 19 mpg hwy, though I get ~22 mpg. I hope you aren't naively going by the markings on the fuel gage for consumption and the odometer for distance. Use the calibrated pump at fuel stations by measuring from fill-up to fill-up. For distance, use highway mileage posts or a GPS.

The turbocharger recovers exhaust energy, so disabling should make the engine less efficient. Diesels run WOT all the time, so a turbocharger is an ideal match. Some people who have added a turbo to 60's gasoline engines claim it improves mileage slightly. That is hard to believe since boosting the inlet pressure only to drop it across the throttle plate is a waste. Might be a very minor improvement from adding a little energy (heat) to the inlet air.

I filled up the tank at the station in Columbus, reset the trip, got home, filled it back up. Click to click on pumps. Calculated to 39.4 mpg. Verified the trip accuracy with my phones GPS.

tjts1 · #12 10-15-2012, 03:06 PM

Quote:

Originally Posted by BillGrissom

Wow. How are you measuring 39 mpg? The current EPA figures for my 1985 300D is 19 mpg hwy, though I get ~22 mpg. I hope you aren't naively going by the markings on the fuel gage for consumption and the odometer for distance. Use the calibrated pump at fuel stations by measuring from fill-up to fill-up. For distance, use highway mileage posts or a GPS.

The turbocharger recovers exhaust energy, so disabling should make the engine less efficient. Diesels run WOT all the time, so a turbocharger is an ideal match. Some people who have added a turbo to 60's gasoline engines claim it improves mileage slightly. That is hard to believe since boosting the inlet pressure only to drop it across the throttle plate is a waste. Might be a very minor improvement from adding a little energy (heat) to the inlet air.

The turbocharger is not a magical free energy device. It creates a huge exhaust restriction in order to spin up the turbine. If the engine can breathe enough air without boost for a low load situation (aka cruising on the highway) theres no reason to maintain that exhaust restriction. Keep the waste gate wide open until you put your foot down. Lets say >30% throttle angle for example.

I'm thinking about reusing an old Megasquirt 1 controller to do a few things on my diesel. Not fuel injection obviously but if I can get RPM, throttle angle and manifold pressure signals to the MS box, it can easily control the waste gate any way I want to, PWM water meth injection proportional to engine load and RPM, turn on/off the electric cooling fans at any temp I want... etc. It can also take inputs from an EGT sensor, wide band O2 sensor (Bosch LSU 4.9 designed for diesel), coolant and intake temp sensors etc. Its on the back burner after I get the 5 spd swap and #17 head sorter. I spent too much time messing with EFI. Not having all sorts of multi color wires and zip ties all over the engine bay just doesn't feel right.

JamesDean · #13 10-15-2012, 03:27 PM

Quote:

Originally Posted by BillGrissom

Wow. How are you measuring 39 mpg? The current EPA figures for my 1985 300D is 19 mpg hwy, though I get ~22 mpg. I hope you aren't naively going by the markings on the fuel gage for consumption and the odometer for distance. Use the calibrated pump at fuel stations by measuring from fill-up to fill-up. For distance, use highway mileage posts or a GPS.

The turbocharger recovers exhaust energy, so disabling should make the engine less efficient. Diesels run WOT all the time, so a turbocharger is an ideal match. Some people who have added a turbo to 60's gasoline engines claim it improves mileage slightly. That is hard to believe since boosting the inlet pressure only to drop it across the throttle plate is a waste. Might be a very minor improvement from adding a little energy (heat) to the inlet air.

My dads 300SD averages 25-26 on the freeway. Its been a while since that number has been checked but last time it was around there. His normal is about 23-24 for his commute to work and back

cullennewsom · #14 10-15-2012, 04:22 PM

Quote:

Originally Posted by JamesDean

Maybe I can get the best of both worlds.. I could employ some type of electronic wastegate controller...

Let's do it.

What do you want, modes?

Ebeneezer Scrooge mode, only allow boost if there is a cement truck bearing down on us.
Mosquito Season mode - AKA Super Speed (well, as super as a 617 gets)
Peacetime mode - for when there is no war in the Middle East (cheap Diesel!) we can sort this one out last, I think we have the time.

JamesDean · #15 10-15-2012, 06:41 PM

Quote:

Originally Posted by cullennewsom

Let's do it.

What do you want, modes?

Ebeneezer Scrooge mode, only allow boost if there is a cement truck bearing down on us.
Mosquito Season mode - AKA Super Speed (well, as super as a 617 gets)
Peacetime mode - for when there is no war in the Middle East (cheap Diesel!) we can sort this one out last, I think we have the time.

LOL.

I bought a cheap manual kit for the time being. Hell, it was cheap enough maybe I can rig up some kind of simple valve and run two kits! Time will tell.

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