Engine/Transmission (1994 - 1998) What is Boost?

[CorneliusJ]

I'm new to diesel motors and don't really know what boost is or the importance of boost. For example, if I limit the boost on my truck (#10 plate and 3k gsk springs) to 20 psi is that a performance issue. I tow a fifth wheel, 17,000 GCW, some mountains. How is boost related to egt? Can anybody can recommend a good book that explains these concepts? My truck makes about 32-34 psi max, but I put the stock boost elbow back in which limits boost to about 22psi. I can't see a major difference in performance between 32 psi and 22 psi by just driving the truck. Thanks.



Jim

 

[Howard Durand]

Jim, You are asking some interesting questions and have made some good observations. I am no expert on these engines either and am wondering just how much boost is good for an engine when it is mostly used for towing and every day driving. As I understand it, boost is necessary in these engines because they can't get enough air for good combustion without it. But I am thinking I am getting too much when I get the "turbo bark" coming off the accelerator too quickly.

There are some really knowlegeable folks on this forum and maybe they will present us with some more information.

 

[Forrest Nearing]

boost is a measure of manifold pressure... the air pressure inside the manifold...



at sea level we have roughly 14. 7psi of pressure in the atmosphere... if you are running 14. 7psi of boost on the gauge, you are effectively doubling the amount of air (more importantly oxygen) going into the engine...



and it goes up from there

 

[AMink]

The boost the turbo provides gives the engine (a big air pump) more oxygen to do its thing. On the 12 valve stock turbos, 32-33 pounds of boost is all you want to go. The turbo begins to get inefficient and starts pumping hot air. With a 10 plate and 3k springs, I'd try to adjust it so that a) it's not dumping black smoke all the time, b) it touches 32 at WOT, and c) your EGT's are not through the roof when hauling heavy up a hill. This will require some plate moving and boost elbow adjustment. Probably your biggest adjustment is in the right foot. :-laf Your 10 plate will allow more fuel in the mid range, but cuts it down at the top. If you start running hot in the middle it might be hard to keep it cool on the top even with the defueling curve offered by the 10. Have fun! 12v's are fun to work on.

 

[pwerwagn]

Maximum boost, by corky bell. Awesome book. It written pretty well, easy to understand for a beginner and also has some good info for those more technically inclined.



The way forrest put it is pretty much the simplest way to describe.

Ill just add that more air (nice cool air, preferably, the denser the better) will help lower egts. The leaner it runs, the cooler.



--Jeff

 

[Partyat33]

Head gaskets have been know to go at modest boost.

 

[CorneliusJ]

Thanks for the info. , what I get is that boost leans the engine by adding air, which lowers egt's, until there's too much boost which then causes higher egt's. If I understand what's being said, keep egt's in a safe range with a stock or an adjustable boost elbow and don't go over 30-32 psi to avoid blowing out something, is that right? Also, I'm going to order "Maximum Boost"



Thanks,



Jim

 

[champspa]

you added a no 10 plate now add the elbow and get that boost to mix with the fuel and it won't be such a black cloud following your truck as it will make hp instead of smoke air fuel =hp

 

[CorneliusJ]

Actually, my truck doesn't smoke at all as far as I can tell, maybe just a puff when I start it. Does anybody have an adjustable boost that you can adjust while driving? My truck is a 97 so no electronics.



Jim

 

[pwerwagn]

until there's too much boost which then causes higher egt's

No, to much boost wont cause high egts. Its the fact that your stock turbo wont supply cool air at higher pressure ratios.

A larger turbo will provide cooler air at a higher pressure ratio. It will be laggier though, so there are trade offs.

 

[CorneliusJ]

Help me to understand, I thought air got hot when compressed. If boost is a measure of the pressure added to the air, doesn't boost alone heat air. You know more about this than me, I'm just trying to get a handle on what is safe boost and the relationship between rising boost and egt's.



Jim

 

[CMorrow]

Yes, anytime you compress a gas(air) you get heat. Thats why you run the air thru the intercooler to lower the charged air temp.

 

[PSchwering]

Actually, my truck doesn't smoke at all as far as I can tell, maybe just a puff when I start it. Does anybody have an adjustable boost that you can adjust while driving? My truck is a 97 so no electronics.



Jim

You have two basic controls that govern Boost pressure. First, You have a wastegate, about all it is good for is preventing overboost conditions. 2nd, is your accelerator pedel. In wastegate closed position, the turbo is dependant on several things, including inlet & outlet temperatures & pressures, on Intake & exhaust sides, turbo efficiency, But the only thing you really have control of is the Go pedal.

 

[pwerwagn]

Help me to understand, I thought air got hot when compressed. If boost is a measure of the pressure added to the air, doesn't boost alone heat air. You know more about this than me, I'm just trying to get a handle on what is safe boost and the relationship between rising boost and egt's.



Jim

Yes, but some turbos are more efficient than others at doing this. Its actually density you are shooting for. You are correct in thinking that too much boost is sometimes worse than less (out of the same turbo. . ).

By running a larger turbo, you can run more boost and not gain much heat (if any) if you run it in is efficiency range. Then intercooler also helps this. This is why twins are also appealing, because you can operate each turbo in its efficiency range and still see well over 50psi of cool air (around 100psi in some cases. . ). Thats because you are running the primary at a PR thats efficient, then taking that air and compounding its density and still running the secondary in its efficiency range.



Does that make sense at all??

Look at some turbo maps and plot the operating range of the engine on the graph. You will see how some turbos can run cooler at higher pressures than others.

Corky Bells book does a good job of explaing this too.



--Jeff

 

[CorneliusJ]

I've ordered Maximum Boost am looking forward to reading it. I think what is confusing me is that I'm thinking the turbo is compressing air in a closed space with a fixed volume, i. e the intake manifold. But what you're saying is that an efficient turbo puts colder air, which is more dense and therefore contains more oxygen, into that closed space which then lowers egt's by leaning the fuel mixture. Am I on the right track?



Jim

 

[dpuckett]

ACtually, you are partly right- an efficient turbo will push the ait into the manifold at a cooler temperature, but the manofold isnt a fixed volume- it flows air at each opening and closing of the intake valves.



The engine is good for 40+ psi, along with the head gasket- I did it in my 91 repeatedly with the new marine grade head gasket (stock on your 97), and had no problems with 320K on the clock (which had been broken a time or two through the years) when I sold it. The TURBO is another story, and is the limiting factor in your max boost. Once a certain boost is achieved, your turbo will spin too fast, potentially causing bearing and other amage to internals, hence the wastegate. The WG protects the turbo, not necessarily the engine. Like already said, different turbos have different efficiency ranges.



The exhaust housing will also make a difference in EGT vs boost. The stock housing could be restrictive in your case, as you have a small housing with (moderately) increased fuelling, which can make the exh housing a bottleneck for exhaust flow, which can inrease EGts as well. Some have gone to a 14cm housing, while others opt for the non wastegated 16- choice is yours.



Daniel

 

[CorneliusJ]

Thanks for all the replies to my questions about boost. These teaching discussions have help me understand the relationship between boost and egt's. If I understand turbo efficiency, it is not about moving as much air as possible, but about moving as much "cold" air as possible, while keeping the boost level below a psi that will blow head gaskets, intercooler boots, or damage your turbo, and for a 97 with a stock turbo I think I read that it was efficient to approx. 30-32 psi, does that sound correct?



Jim

 

[CMorrow]

I think everything has been covered except one real inportant item. Once you get the right balance of boost for your application, keep in mind just because you can spoolup the turbo and get all that great power and smoke really quick, always back out of your boost gradually. If you don't, you'll end up with a barking turbo and eventually it will hand grenade.

 

[pwerwagn]

97 with a stock turbo I think I read that it was efficient to approx. 30-32 psi, does that sound correct?

yeah, thats a good general rule. as you will learn though (and a lot of people dont factor this in), efficiency depends on altitude as well. A higher altitude will put the compressor of the turbo into a higher pressure ratio, therefore a lower efficiency range.

I dont recall if maximum boost covers that area or not. It does cover pressure ratio though.



simply put, higher altitude, the same turbo will have to spin more rpm to make the same boost as it would at a lower altitude. This will put the compressor into a different range on the map, usually a less efficient one.



You'll enjoy maximum boost. It is one of the main building blocks to understanding a turbo setup correctly.

I just wish there was more information available for the turbine side of turbos than there is. .



-Jeff