turbos, backpressures, & fuel economy

[jdmetcalf57]

I have been doing some more reading on diesel engines. What I read was that to increase cooling and fuel economy most turbocharged diesels run with higher boost pressure than back pressure (exhaust back pressure). They usually have some valve overlap so that the higher boost than back pressure will make sure and clear exhaust gases out. I have boost and back pressure gages on my truck and have found that for most driving conditions the back pressure is several psi greater than boost. Sometimes it is much higher. Some conditions it runs about the same pressures but these are not where I run most of the time.

Has anyone experimented with different turbos and monitored boost and back pressure?

Seems to me like this may be a way to get some more fuel economy. I am sure GM designed this turbo setup for minimum cost and drivability over the entire range. I doubt fuel economy was very high on the design priority list. With fuel prices the way they are I could sacrifice performance in certain ranges to increase fuel economy.

[grape]

it's physically impossible, in a properly designed system with all things being perfect you will be lucky to see only TWICE the amount of psi in the exhaust as in the intake. In anything less than perfect it can climb to 4 times as great as the inlet pressure.

[JohnC]

Yeah, you can get that kind of efficiency on big honkin' engines that run all day long at the same power level and RPM, but it's not practical in a vehicle driven on the road. I suppose you could optimize it for highway driving, but you'd have no boost at all taking off from a stoplight...

[DA BIG ONE]

Porting turbine housing helps some.

[jdmetcalf57]

So you probably couldn't get to the ideal situation but supposing you tuned it to a certain load and speed does anyone have any idea how much it would affect fuel economy. Has anyone tried a different turbo and seen any change.

I've been able to make mods and go from 11 mpg to 15mpg with my camper so I know GM didn't design it for max fuel economy for my driving.

What is involved in porting the turbo?

[DA BIG ONE]

Originally posted by jdmetcalf57:
So you probably couldn't get to the ideal situation but supposing you tuned it to a certain load and speed does anyone have any idea how much it would affect fuel economy. Has anyone tried a different turbo and seen any change.

I've been able to make mods and go from 11 mpg to 15mpg with my camper so I know GM didn't design it for max fuel economy for my driving.

What is involved in porting the turbo? Few have used different turbos with varied success.

Bigger is great for upper mid to high range, but leaves much to be desired at bottom, smaller is on the other end low to lower mid range good but above that it stinks.

I have the GM8 and it is ok for my type of everyday use, but a setup w/2 turbos, 1 sized for low/lower mid range, then another sized for upper mid to high range switched somehow would be ideal.

Then there is the variable vane turbo that shows some value but it is computer controlled if I remember right?

Porting (expand diameter) the turbine housing exhaust port where it starts run to exhaust then to where it attaches to exhaust down pipe increasing exhaust flow, this is area where highest backpressure is after freeflow exhaust has been added. Making this area too thin will make it weak and prone to failure.

[toyboxrv]

I had heard in the past of waste gates that are separate from the turbo. It may be a crazy idea or one that has merit, but give me some opinions on whether or not one could be installed on a 6.5 off of the crossover pipe and plumbed into a larger 3 1/2 or 4" exhaust thereby eliminating some of the backpressure issue with the small 3" connection and downpipe that we are limited to with a stock turbo?

[grape]

Originally posted by toyboxrv:
I had heard in the past of waste gates that are separate from the turbo. It may be a crazy idea or one that has merit, but give me some opinions on whether or not one could be installed on a 6.5 off of the crossover pipe and plumbed into a larger 3 1/2 or 4" exhaust thereby eliminating some of the backpressure issue with the small 3" connection and downpipe that we are limited to with a stock turbo? we have a bingo.......

[jdmetcalf57]

I have my backpressure gage plumbed into the crossover pipe. I have unhooked the vacuum line to see how much back pressure the exhaust pipe is giving me. Under this condition it is very very low. Barely registering on the gage. Before I eliminated the Cat and made a larger exhaust it was close to 2 psi.

Given this I am not sure the crossover pipe waste gate will do much but I've thought of this too. Sure wouldn't have much back pressure.