Engine/Transmission (1998.5 - 2002) Need a little Turbo theory !!

[CaptNemo]

We had a technical discussion about Turbos and back pressure at the shop the other afternoon.



The discussion centered around: If you remove backpressure from the after turbo portion of the exaust system, what is the result in manifold Boost PSI: reduced pressure or no change.



The subject vehicle had a gasoline V-6 engine with an aftermarket performace turbo system regulated to 8 PSI of boost. The main focus of the discussion was on peak boost PSI numbers.



One point of view was: The engine will lose boost pressure in the intake manifold because there is a loss of exhaust pressure for the incoming air to build up against.



The other point of view was: Since the turbo is controlled by the wastegate, and the wastegate is controlled (basicly) by manifold pressure ( with ECM input, other control factors ), then the wastegate will simply open later when the boost pressure reaches the desired PSI.



Both viewpoints agreed that the turbo would spool faster with less backpressure, so that is a "Gimmie".



Since this was an aftermarket performance turbo system, we can safely assume that the turbo compressor was not maxed out at 8 PSI and could be dialed to a higher boost setting.



Let's hear some theory.



.

 

[Hohn]

It is faulty reasoning to believe that the manifold pressure builds up against the backpressure in the exhaust!



Exhaust manifold pressure (drive pressure) builds up against the TURBINE, NOT the exhaust system after the turbo.

The discussion centered around: If you remove backpressure from the after turbo portion of the exaust system, what is the result in manifold Boost PSI: reduced pressure or no change.

If you could completely eliminate all backpressure after the turbo, you would see either MORE boost, or no change. You would NOT see a decrease in boost pressure at all.



The only way you would see the same boost pressure is if the wastegate was opening at that pressure. The WG will open to keep boost at a constant limit (if it can). In that case, removing all backpressure will have no effect on boost pressure, BUT the engine will have more overall airflow through it, EVEN AT THE SAME BOOST PRESSURE.



Remember, that a turbocharger's turbine (which provides the energy to turn the compressor side) operates off of a pressure DIFFERENCE. If you plugged up the exhaust, the pressure on both sides of the turbine would be the same, and you'd have no boost at all.



So, as you get into the fuel pedal, pressure and temperature build up against the turbine (in the manifold). This is also known as DRIVE pressure. Now, we remember from school that in order for there to be any flow, there has to be a pressure differential, and that flow always goes from high pressure to low pressure. The bigger the difference between the two pressures, the higher the rate of flow.



So:

1) Turbine operates off pressure differential.

2) Increasing the difference (by lowering backpressure) provides more flow across the turbine.

3) More energy to the turbine means more power sent to the compressor (assuming no WG to relieve exhaust)

4) More compressor power means more boost





Hope this covers it. If not, ask away.



Justin

 

[PC12Driver]

As I understand it...



It takes Xpsi of drive pressure to run the turbo at 8psi. Any change in drive pressure will change the boost pressure. So the wastegate maintains a constant drive pressure (exhaust backpressure) to maintain a constant boost. So the pressure in the exhaust manifold will be the same (Xpsi) at 8psi of boost, regardless of what's going on after the turbo (unless the exhaust is creating more backpressure than needed drive pressure).

 

[BarryG]

Justin:

As you know I am having some issues. This topic is educating me as I know very little about turbos and the theory behind them. I do have a question or two(mainly because my last turbo upgrade mild as it was did not go as well as I thought it would and if I need to fix it I want more knowledge so I dont screw it up again). How does air/gas velocity come into play. I would think zero back pressure would create more lag and lower boost than with at least some BP. Blow into a garden hose and BP would be higher but so would velocity thereby spinning the turbine faster creating more boost. Blow into a firehose same amount of air/gases but its velocity is slower thereby not spinning the turbine as fast and creating less boost. Am I all wet? I am thinking that a larger exhaust housing while loosening BP up some would create more lag and less boost(at least until you have the power to overcome it). Possibly made up for with the compressor side being more efficient at a lower shaft speed (ie more, longer, different angle of the blades).



Now thinking about it even more less bp means engine is breathing easier allowing more air into the cylinders even though it may be at lower boost pressure. Still all wet? I barely understand what I am saying, ok barely may be an overstatement.



Confused, help edumacate me, wanting to learn.

 

[RustyJC]

The last post is quite astute as it points out two approaches to turbine design - impulse turbines (which take advantage of gas velocity) and expansion turbines (which extract energy from the flow across a pressure/temperature gradient). A turbine can combine characteristics of impulse and expansion turbine designs as well.



In conjunction with the differing turbo designs, exhaust manifolds can be either a pulse-type (quicker response to load change) or constant pressure (all cylinders exhausting into a large plenum).



Rusty

 

[JasonCzerak]

Justin:

As you know I am having some issues. This topic is educating me as I know very little about turbos and the theory behind them. I do have a question or two(mainly because my last turbo upgrade mild as it was did not go as well as I thought it would and if I need to fix it I want more knowledge so I dont screw it up again). How does air/gas velocity come into play. I would think zero back pressure would create more lag and lower boost than with at least some BP. Blow into a garden hose and BP would be higher but so would velocity thereby spinning the turbine faster creating more boost. Blow into a firehose same amount of air/gases but its velocity is slower thereby not spinning the turbine as fast and creating less boost. Am I all wet? I am thinking that a larger exhaust housing while loosening BP up some would create more lag and less boost(at least until you have the power to overcome it). Possibly made up for with the compressor side being more efficient at a lower shaft speed (ie more, longer, different angle of the blades).



Now thinking about it even more less bp means engine is breathing easier allowing more air into the cylinders even though it may be at lower boost pressure. Still all wet? I barely understand what I am saying, ok barely may be an overstatement.



Confused, help edumacate me, wanting to learn.

You are correct on your hose side analogy. But Picture something half way down the hose that you are spinning, like a turbo. As soon as you turn the device, do you really want to keep pushing that same air down that same small tube? once it's past the turbo, the air is pointless and has done it's work and is now just in the way, get it out! right? hence 5" exhuasts open, wide open.



So, if you have 6 feet of pipe. and the principle here is to turn a fan. You want to have directional air (exhaust manafold is not that big inside per cylinder. ) to blast DIRECTLY on the fins. Once it's turns the fin, you have to pipe that air out side right?



ok. So take a 6 foot host, blow in it, measure say 10psi. now cut it in half, you now have 5 psi of pressure to move air out of the hose. You have to work half as much to get the same amount of air moved. (assuming no down pipe on a turbo in this last part ). Now you would add hte firehose to the make up for the last 3 feet.

 

[JRagland]

But Picture something half way down the hose that you are spinning, like a turbo. As soon as you turn the device, do you really want to keep pushing that same air down that same small tube? once it's past the turbo, the air is pointless and has done it's work and is now just in the way, get it out! right? hence 5" exhuasts open, wide open.

Exactly right. Even before I owned a turbo engine I never understood people making the arguement that backpressure in the muffler/tailpipe system was nessesary. But it is heard everywhere.

 

[JasonCzerak]

Exactly right. Even before I owned a turbo engine I never understood people making the arguement that backpressure in the muffler/tailpipe system was nessesary. But it is heard everywhere.

People are stupid



I think I felt a LITTLE LESS low end power, like pre-2PSI of boost once I cut the cat and muffler off. But that's not a big deal. expecially with low boost fueling on 5

 

[Hohn]

Justin:

How does air/gas velocity come into play? I would think zero back pressure would create more lag and lower boost than with at least some BP. Blow into a garden hose and BP would be higher but so would velocity thereby spinning the turbine faster creating more boost. Blow into a firehose same amount of air/gases but its velocity is slower thereby not spinning the turbine as fast and creating less boost.





Now thinking about it even more less bp means engine is breathing easier allowing more air into the cylinders even though it may be at lower boost pressure. Still all wet? I barely understand what I am saying, ok barely may be an overstatement.



Confused, help edumacate me, wanting to learn.

Your garden vs fire hose analogy is accurate, BUT NOT FOR EXHAUST. It's accurate for the turbine housing on the turbocharger. This is why smaller housings spool faster yet choke on the top end. Likewise, larger housings spoolup slower.



However, this is NOT the case with the exhaust pipe. Yes, a smaller pipe will have higher velocity.



But since air has MASS, that VELOCITY comes at a price! Restriction!



This restriction causes the pressure differential across the turbine to be LOWER, which reduces both spoolup AND peak boost.





A larger exhaust will reduce the velocity of the flow AFTER the turbo, yes-- but this is not important, because it's simply a sign that the pressure drop across the turbine is optimized.



Now, you CAN have an exhaust that's too big.







Try it sometime and you will see that no exhaust at all (naked turbo discharge) is MUCH worse than a 4" or 5" straightpipe. Spoolup will suffer, and so will performance.

 

[JFPierce]

I've been reading the theorys on turbo's, all very interesting. I just installed a combined boost and egt guage on my '03. I was amazed that at a steady highway speed that the turbo showed a boost of about 10. What about the boost having to over come atmosphere pressure before showing boost? I know that diesels have very low vacuume, but they have to have some vacuume or the fuel-air would'nt flow to the cyls. I have a turbo Corvair that has a manifold pressure guage that also is a vacuume guage. The left side shows vacuume of about 10 at a steady speed and you need to be wound up real good in third gear to show much boost on the right side of the guage, it max's out around 15 on the plus side. Since atmosphere pressure is around 15psi, and the diesel has to have a little vacuume under light throttle, how can the boost guage show a plus at steady speed? Also the Corvair shows more boost after I insulated the exhaust manifold pre-turbo, never any thought of egt. Maybe a dumb question, but just wondering.





jfpierce '03 duelly

 

[RustyJC]

I know that diesels have very low vacuume, but they have to have some vacuume or the fuel-air would'nt flow to the cyls. I have a turbo Corvair that has a manifold pressure guage that also is a vacuume guage. The left side shows vacuume of about 10 at a steady speed and you need to be wound up real good in third gear to show much boost on the right side of the guage, it max's out around 15 on the plus side. Since atmosphere pressure is around 15psi, and the diesel has to have a little vacuume under light throttle, how can the boost guage show a plus at steady speed?

To oversimplify, diesels don't have intake manifold vacuum because they don't have throttle plates like an Otto cycle (gasoline) engine. In truth, at idle they will show a slight vacuum relative to atmospheric pressure due to intake flow restrictions, but that's negligable. Think about it - at idle the piston will draw air into the cylinder as it moves down on the intake stroke even if the intake manifold cover is removed. The piston creates a vacuum in the cylinder under these conditions even if there is no intake tract vacuum. With the air now in the cylinder, the fuel injection event begins near the top of the compression stroke - the fuel isn't drawn in with the air like a typical gasoline engine.



With no throttle plate, the exhaust can spin the turbo fast enough at steady cruise to lightly pressurize the intake system.



In this regard, diesels really aren't comparable to gassers. That's why diesel engine boost gauges don't have a "vacuum" side like your Corvair's.



Rusty

 

[JFPierce]

You explained the answer so even I could understand it. Thanks Rusty.



Jim

 

[TJany]

"I've been reading the theorys on turbo's, all very interesting. I just installed a combined boost and egt guage on my '03. I was amazed that at a steady highway speed that the turbo showed a boost of about 10. What about the boost having to over come atmosphere pressure before showing boost?"



You also have to think about something else, psia, and psig, pounds per square inch absolute, and pounds per square inch gauge. psig is relative to the surrounding atmospheric pressure. anotherwords if you have 10 psi on your gauge, and the atmospheric pressure is 14. 7 than the pressure in your intake manifold is actually 24. 7 psia. this means that your 10 psig on your gauge is above and beyond atmospheric pressure, and thus is actually "overcoming" the atmospheric pressure



nonw back to turbo theory...

it is my understanding that a turbo creates all the back pressure it needs to spool. anotherwords, whatever happens after the turbo doesn't matter as long as there is no restriction. in this case, i'm not seeing how dumping the exhaust immediately following the turbo would be detrimental, unless somehow it creates to much turbulence or something through the turbine housing. I know with steam, you always try to have a "perfect" vacuum in your condensor to "draw" the steam through the turbine, and if you lose that vacuum, your goin nowhere.



Hohn, i'm not trying to contradict you here, i respect your words of wisdom on the board, just trying to see if there is anything to back up that you need an exhaust system of some sort, as exhaust systems will inherently cause some back pressure, or if this is just one of those times that "in theory i'm right, in practice i'm wrong"



Tim