Ok so one of the first things that an owner wants to do is fool with the turbo for more power. Lets take a stock 24 valve and ask a couple of questions about the turbo. Ok number one is what benefit does a larger turbo or higher boost accomplish? It seems that the higher boost and the larger turbo is only to control EGT's, is this true?

I can see where higher inlet pressure will achieve somewhat higher compression pressure, but this does not seem the need or purpose. Now what I am getting at is to find if there is any benefit to higher boost pressures if your EGT's are well in control.

There must be some hp benefit to higher compression pressure, if so, at what pressure does that play out? The factory seems to wastegate at 20 pounds or so. Big trucks like to lower this to 16 at cruise with a non wastegate housing. The question comes up than as to what are the benefits of running higher pressure other than EGT's or using up horsepower.

I am starting to think that boost should only be high enough to control EGT's. That being in control you would be better off with lower pressure which would mean less resistance to the exhaust flow.

Any experts out there, I am really interested in this....

 

You are thinking along the right lines, Bill. For maximum efficiency you want only enough boost to burn the fuel completely. Boost only makes HP because it allows us to add more fuel. You can think of boost as backpressure on the compression stroke. The less boost you can run at cruise and still maintain acceptable EGT, the better your mileage will be.

If Banks programmed his VGT turbo controller right it will help fuel economy tremendously. There's no reason an empty truck needs to be pushing 10 psi of boost cruising on the highway.

 

Intake boost increases the overall pressure ratio of the diesel cycle substantially. An increase in the pressure ratio of any thermal cycle (Rankine, Otto, Brayton, Diesel) equates to an increase in efficiency. It is NOT only about more air to burn the fuel, it is about recovering wasted exhaust heat and using that heat to drive a turbine which then drives a compressor which thus increases the pressure ratio of the cycle thereby increasing the efficiency. Boost pressures at any load contribute to high efficiency.


Kevin

 

I think you're leaving out the backpressure the turbine causes, as well as the energy used to push the piston up against the excess boost.

The 3rd gen trucks would get much better mileage if they were only pushing 3 - 5 psi at empty cruise instead of 10 psi.

 

I run about 10 psi boost at 70mph cruising. Is that ideal or is my fuel milage suffering?

 

Less boost would help your fuel economy. The question is, are you willing to give up quick turbo spooling to gain 1, maybe 2 mpg on the highway? With a manual trans you gotta spool the turbo 4 times to get to cruising speed.

 

The backpressure the turbine creates is a pumping loss for the engine, however when this energy is used to compress the intake charge the net gain on the cycle is positive.

The energy used to push the piston up against the excess boost gets returned right back to the crank the next cycle down. You don’t just compress it and forever loose that energy... moments later you expand it and get all the energy back. Remember a diesel engine is the machine which operates on the Diesel Cycle. It is so named after its inventor and the thermodynamic cycle he invented, not the fuel.

All thermal cycles see improved efficiency with increased pressure and temperature ratios. This is one of the undeniable facts of the world. This is why increased compression ratios increase fuel efficiency. Boost increases the dynamic compression ratio.

Grab a book on thermodynamics, it will explain it all very well.

You will not get better fuel economy with less boost. The only exception would be if you were running a boost which was so high it was out of the efficiency map of the turbo. Then the pumping losses from the increased exhaust pressure could exceed the gain in efficiency from the added boost. But when you are talking about the cruise boosts we all see, being out of the map is obviously not the case.


Kevin

 

OK, but I'm not gonna be the one to tell Bruce Mallinson he's doing it all wrong.

http://www.dieselinjection.net/

What he does works, can you explain why?

 

I know all about his shop but I am sorry, I won't comment on his work.


Kevin

 

I went through this same conversation with the folks at Scheid Diesel, because after reading everything on here I wanted a Bigger turbo than my stock. He explained all that latin stuff to me and when he was done I asked for his recommendation on what to use. He sugessted just keeping what I have because it's the best compromise between spooling speeds and keeping the the psi's low enough for fuel economy. He also said that as long as I stayed below the 150hp injectors a new turbo would not lower egt's enough to even notice a differance. I was trying to hand this guy money and these were his recommendations.

 

In the past years, I have read dozens of articles, some of which I have printed out. The same questions came up in reference to our trucks but many said it did not work that way on the small diesel. My observations and any knowledge came from past information of great number. I posted the thread to get other opinions before getting into it with money. I had the impression that the larger turbo housing gave better economy with more lag on pulling situations only. Since I have posted this thread to see if as I thought it works for all purposes of engines. Unless you need the extra boost to cool the egt's because of more fueling, the extra boost would cut horsepower and economy. Right now economy is a very important thing to most of us due to fuel prices. I thought it would be a good exercise for many on this board to see the capabilities of a larger housing with the disadvantage of slower lag times. Also not having done it to a B engine, the responses would be good to hear. I will be trying the larger housing and I beleive that I have no good reason to go larger than four inch exhaust (stacks)

A couple on here have commented that the mileage that I claim and a couple other claim are exxagerations. This is not true, the mileage can be brought up and I am hoping for like 14 working the truck. That would save me a lot of money per year. I drive the small truck doing mid sized work due to the lower cost of operations. Around here I have most of my competition beat by 2 to 3 mpg. Generally they do not believe 13 doing this work.

On the same subject, I have tried something new to me although suggested on the board before, I have been driving by the boost gage. On hills letting it back down to keep the boost low, it seems to work great. Of course I know that is registering less fuel being burned, but a good gage of it.

 

As many have stated before, I too find better mileage driving by the boost gauge. Keeping it under 10 psi on interstate trips gives me best results...adds 1/2 to 1 mph at gross wt. Hard to get use to driving that way!

RJ

 

Yes, harder at 29,000 or 30,000

 

As usual, kevin is right.

One way to think of overall efficiency is something called Carnot efficiency. Basically, the bigger the difference between peak temperatures, the more efficient a cycle is.

For the most part, you ignore pressures, because of the relation of pressure and temp, so higher pressure MEANS higher temps, and vice versa.

Thus, if you have a HOTTER peak combustion temp, or a COOLER temp at BDC, then *either* is more efficient. This is why higher mechanical (static) compression ratios are more efficient than lower ones.

For max efficiency, you would want BOTH a hotter peak combustion temp AND a lower EGT.

Every turbo has a "sweet spot" where it produces the most boost for a given amount of turbine power. These are the islands on the efficiency maps.

Our stock turbo just happens to be running in its sweet range at the stock limit of 20.6psi.

So what happens when we add boost to the combustion process? Well, adding boost increases combustion temps two ways: 1) increasing peak pressure @ TDC, 2) increasing the RATE of combustion due to the presence of more oxygen. Consider a woodstove. When you close the damper, the rate of combustion slows, but doesn't stop. This is because of the reduction of oxygen. Conversely, if you have a hot fire and blow compressed air on it, the combustion rate speeds up, and the fire gets much hotter. Taken to the extreme, some LOX would make combustion happen to fast that your wood (and the stove containing it) would instantly be a pile of ash, albeit after a HUGE release of heat.

So boost, affects not just the quantity of fuel that can be burned, but the rate at which is will burn. The RATE is very important.

Note that this causes higher pressure/temp at TDC, *AND* lowers EGTs by making more oxygen available! Since we are increasing the gap between peak temps-- BINGO, greater efficiency.

But there's more to MPG than just engine efficiency. Efficiency only helps you to better use a set quantity of fuel. MOre than this, MPG is affected by the quantity of fuel in general.

In other words, using 150hp when you only need 100hp KILLS fuel economy far more than can be offset by the fact that you are using that 150hp more efficiently.

So, if you want max fuel economy, drive as slow as you possibly can and switch to an HY35-9. As long as you don't "need" more than 25psi or so to pull a hill, the HY should deliver better MPG, at least on paper.

jlh

 

I know on my 93 when I went to a 12cm housing my economy dropped about 1.5mpg over the 16cm housing I had. After I re adjusted the pump (especially the afc) I got about one mpg back. My goal of the 12 was to get boost quicker to reduce low speed smoke. It has helped but it dosn't breathe free like the 16 did when getting near the shift points. I was thinking of converting my h1c to a hx35 hybrid to see if that helps. I'm not sure what housing I should run with it when I get it done.