Engine/Transmission (1998.5 - 2002) OK, what exactly is Boost?

[WCrawford]

Can someone explain to me in layman's terms exactly what boost is. I have a '00 2500 with several mods (chip, injectors, valve body, exhaust, torque converter) and just recently added x-monitor. What range should I be looking for? Boost reading rises to @15 when accelerating but drops off quickly to @5 at 70mph, and 2-3 at 45mph. Reading drops to almost 0 when decelerating and above idle. Does this sound normal? Max is set at 19 with alarm at 30. Don't think I'll reach either of those.

 

[Turbo Tim 1]

What type of performance box and injectors do you have? A stock 24 valve should make up to 22 psi when really getting into it or pulling a big hill. My truck with just the Edge comp will make 34-36 psi of boost max and cruising at 73 mph will run between 8-14 psi depending on if I've got a tail wind or head wind and up or down a hill, 10 or 11 psi would probably be the average number at that speed. As for what boost is, it's compressed air, the turbo is pushing air into the engine, the more it pushes the higher the boost, the more air you feed the engine the more fuel it can burn and the cooler the exhaust will be. The combustion chambers of your engine are a certain size and hold a certain volume of air the more you push air in the higher the pressure in there will be. Hope this helps, by the way your boost numbers look low.

 

[rrausch]

Boost is a measurement of air pressure, in P. S. I. inside the tubing which runs from the turbo, through the intercooler and on to the intake manifold and up to the intake valves. If you push the gas pedal down suddenly, a lot of fuel will flow into the cylinder and burn. As the burned gasses exit the cylinder they force the turbocharger wheel to spin faster, this in turn makes the other side of the turbo act like a small air compressor and forces more air through the intercooler and toward the intake valves, which raises the pressure. This in turn forces air through the (open) intake valves into the cylinder.



Without the turbo the engine would have to suck air. Diesels use a lot more air than gas engines, and it takes considerable power to suck a lot of air. In the old days before turbo's I remember my Pop driving behind smoky, slow old Diesel trucks which would accellerate SO slowly. We always breathed a sigh of relief when we could pass them. With a turbo there is positive air pressure on the intake side of the head, instead of negative and the positive air pressure helps force more air into the cylinder. The more air you can get into the cylinder, the more fuel you can burn. Hence the diesel engine doesn't need more cubic inches to increase power--just more boost pressure and more fuel.

 

[bmoeller]

Originally posted by rrausch

Boost is a measurement of air pressure

Boost is the pressure above atmospheric pressure that the turbo makes.



Great turbo info- http://www.dodgeram.org/tech/dsl/FAQ/turbo_faq.htm

 

[WCrawford]

Thanks for the replies. I now have a better understanding of what I'm looking at. By the way, I have a BD PNP box. I'm all BD. I've read pro's and con's, but I'm o. k. with the performance I've gotten thus far. Besides, what I have is already paid for and I don't think the boss(MRS) would like for me to start replacing a lot of expensive parts that I convinced her I had to have. I tow a 26' TT. Sounds like I may have a bit of a air leakage problem (in or out?). Any suggestions on common causes or solutions if things get worse? If these readings are pretty normal, then I will rest at ease. We head for the Rockies in about a month and I surely don't want any problems there.

 

[rrausch]

Thanks bmoeller that is a good article.



But ALL air pressure as measured with any sort of gauge is a measurement above local atmospheric pressure isn't it?

 

[NPloysa]

You may be leaking air. Check all your boots and piping. Start at the compressor outlet and tighten all band clamps going into the intercooler, after intercooler up to the intake horn. Also check for holes in the boots or cuts or slices that may only open up if its pressurized (making boost).

 

[bmoeller]

Originally posted by rrausch

But ALL air pressure as measured with any sort of gauge is a measurement above local atmospheric pressure isn't it?

Originally posted by bmoeller

Boost is the pressure above atmospheric pressure that the turbo makes.

 

[jerrethomas]

i got a qustion also, if exhaust gas drives the exhaust turbine which drives the intake side impeller. if it just the exhaust gases that drive it, why can't you reach full boost sitting still in neutral and floor it. the reason i ask is i was told that it was the heat of the exhaust gas that drove the impeller. which would make sense that when you give it more fuel on the peddle, egt goes up and boost increases. just a qustion i've been meaning to ask.

 

[UN-S1L3NT-24V]

Originally posted by jerrethomas

i got a qustion also, if exhaust gas drives the exhaust turbine which drives the intake side impeller. if it just the exhaust gases that drive it, why can't you reach full boost sitting still in neutral and floor it. the reason i ask is i was told that it was the heat of the exhaust gas that drove the impeller. which would make sense that when you give it more fuel on the peddle, egt goes up and boost increases. just a qustion i've been meaning to ask.

I too was wondering about that...

 

[rrausch]

It's not the heat of the exhaust gasses, it is the pressure which drives the turbo blades.



The short answer to 'no boost in Neutral' is the RPM's eat the boost all up.



You only develop boost when the volume of exhaust gasses exiting is much greater than the volume of the intake flow of air. If the difference is only somewhat greater there is not much boost. If there is no difference; if the system is equal, there is no boost. You can't build a lot of boost in Neutral because you have to take your foot off of the pedal too soon. The system equalizes very quickly with the pedal down, in Neutral, and as it equalizes boost goes down.



So at speed in Drive, at say, 1200 RPM, if I mash the pedal a sudden load of newly ignited fuel suddenly upsets the equation and all that fuel sends a LOT of exhaust gasses out through the turbo as the engine RPM slowly catches up.



But if I were in Neutral and mashed the pedal there is nothing to keep the engine RPM's from catching up quickly, the increased RPM's use up the boost quickly, and the system equalizes quickly. And you are back to a no or low boost situation.

 

[jwilliams3]

Good one Robert!

 

[Hohn]

Dang, I got into this post too late!



RRausch is right on. Boost is a measure of RESISTANCE to intake flow versus the air that's TRYING To flow.



For example, if the turbo compressor is pumping 300cfm (at atmospheric pressure) into the engine, but the engine is taking in all 300cfm, you won't see any boost.



But if if the turbo's putting out 600cfm, and the engine can only handle 300cfm, then the pressure goes up. So instead of 300cfm @14. 7psia, you'll have 300cfm @ 29. 4psia, or about 15psi of "boost".



Now if you increase the amount of air that the intake system can flow via head porting, cam, or the like, you'll see a DECREASE in boost levels even though the engine is flowing more air.



So think of boost as a ratio (like Robert said) of air TRYING to flow into the engine (from the turbo) vs airflow that's ACTUALLY flowing into the engine.



If the engine can only flow x amount of cfm, then we can stuff more air into the engine at that given cfm by raising the pressure.



There's also a certain loss of efficiency as pressure is raised. If you double the pressure, you don't *quite* get double the airflow. They higher you raise boost, the less efficient it becomes.



I second the comments about a boost leak-- you should have more than 20psi easily, with a box and injectors.



I usually check for leaks by pressurizing with an air compressor, then using soapy water to see if there are leaks.



Justin