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volkswagon

How does a turbo work?

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I wasn't sure where to post this but I'll do it here. I had a guy tell me that a turbo charged engine will make more horse power with a muffler than with a straight pipe because a turbo works on back pressure. Is that true? I always thought it was volumn of air flowing thru it so therefore a freer flowing exhaust system would make more power. Can anybody clear this up?

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I think there are 2 variables here. A while back I was at a stoplight and along came another cummins with his exhaust brake on, well the turbo was screaming. Have you ever put your hand over one side of a little fan and watched it speed up? Same principle, I think. So back pressure allows it to speed up because it just spins the same air around itself without having to chop up new air. Since the compressor is on the same shaft, it would be producing more boost. But that's where variable #2 comes in. What good is boost when you have back pressure so the flow isn't there? It just stops flowing and hence why it was braking the engine. So although some back pressure might spin the turbo faster, I am not sure it is really beneficial in the end, because of variable #2. :2cents:

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Well now I was taught that the more flow from the working engine under load is the factor.I have always assumed that the more fuel burning creates more exhaust flow and that spins the turbo up harder.I worked on big stationary engines for 20 years, and all of them only produce turbo pressure when the engine is under load working. None of them had any back pressure.You need exhaust flow that is my opinion. No Flow of gasses no turbo spin up.

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Now a little deeper. As everyone knows there is a HX35 and HY35 turbos. HX35 are 12cm2 at the exhaust opening and the HY35 are 9cm2 at the exhaust opening. HY35 allowed for the turbos to spool up faster for the automatics vs. the HX35 which was used on the manuals. Now I know there is a lot of poor man tuners out there and heard of several stories of taking the exhaust housing off and changing with a 16cm2 to make it breathe more. Which it will this will drop the drive pressure and EGTs but increase the amount of lag and smoke. So with a larger exhaust housing your going to need much more fuel to spin and the same amount of boost as previous boost level but now your boost limit is higher and EGT will climb slower. This is where AH64ID idea of use VG turbo on his 3rd gen going to be awesome. Be able to control the exhaust housing electronically in the cab. In the city be able to close it down to like a 9cm2 and have quick spool from light to light controlling smoke. But hit the interstate and open it up to like a 22cm2 and allow it to breathe. :drool:(Curious how that going???)

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Your right there. I can only get 5psi if I redline my truck in neutral. Under a load is a different story. You do need flow and you get flow from a lot of fuel being burned. I am not sure if the guy at the stoplight was building any boost, I just know it was screaming so it is something to think about.

I am not sure it is really back pressure that does it the more that I think about it. I think it is more the fact that he is just chopping the same air rather than the back pressure doing it. The turbine needs to spin as fast as possible to produce boost and I think back pressure would hinder this. So I'm going to say the muffler produces less boost.

--- Update to the previous post...

Found this on another site. I will break it up since they have it as one huge run on paragraph.

VOLUMETRIC EFFICIENCY AND BACKPRESSURE

Although the turbine recovers wasted exhaust gas energy from the expansion of the hot exhaust gas, the kinetic energy of the flowing exhaust gas and the acoustic energy of the exhaust gas, the working turbine also causes an increase in exhaust gas backpressure. This increase in backpressure can reduce the engine's volumetric efficiency.

A typical, streetable turbo system has more exhaust backpressure than boost pressure and the power gains from such systems are due to the increase in the density of the intake charger, not due to increases in volumetric efficiency. (Volumetric efficiency, if you don't remember, is the volume of intake charge inhaled during the intake stroke vs. the actual displacement of the cylinder. VE is expressed as a percentage; the larger the VE, the better.)

Backpressure is higher than boost pressure because the smaller turbine housings and turbine wheels used to ensure a quick spool-up time also, by nature, restrict the exhaust flow. We will explain the mechanics of this in more detail a little later. Racing turbos, the latest generation of medium-sized turbos and turbochargers for engines where throttle response is not much of an issue (like fixed industrial engines, long haul trucks and aircraft), have free-flowing turbines that have less exhaust pressure than intake pressure. Engines using these turbos often do have improved volumetric efficiency.

This condition, where boost is higher than backpressure, is called crossover and crossover is what ever turbo system designer strives for. In crossover, VE percentages as high as 110 percent are not unheard of. Unfortunately, some of the design features that can create a free-flowing turbo can also contribute to turbo lag, something that is not desirable in a street-driven car that needs a wide dynamic power band.

Excessive backpressure is hard to manage in a boosted four-stroke engine. Excess backpressure causes what is known as reversion. Reversion is when hot exhaust gas gets pumped backwards into the engine during the overlap period. Reversion can cause the engines internals to get excessively hot as cross flow of the cool intake charge during overlap is one of the ways an engine cools its self internally. Hot internal parts can trigger uncontrolled combustion and engine-destroying detonation. Because of this, it is sometimes not a good idea to really crank the boost on an engine that has a small, high-backpressure turbo - in other words, the kind of turbo that usually comes on a factory turbo car. This is a good reason not to go crazy with a boost controller on a factory-equipped turbo car. A little more boost, perhaps 4 to 5 psi might be tolerated, but trying for 20psi could be flirting with disaster.

On small turbo cars with a lot of backpressure, camshaft overlap should be kept to a minimum. This means that the stock cam usually will work best. To deal with the problems associated with backpressure and reversion, the engine's tuning must also be compromised with richer mixtures and more retarded timing than what would normally be optimal for the best power. Even on full race turbocharged cars with low backpressure turbos, camshaft overlap should be several degrees less with more lobe separation angle than on an equivalent naturally aspirated engine, unless physical measurements indicate that the engine is in crossover in the engine's operational range.

Because of backpressure and VE issues, the correct turbo size for the application is very important when designing a turbo system. A small, quick-spooling turbo can be restrictive, causing a great deal of backpressure and reducing VE at higher rpm. This means that small turbos should be limited to lower boost levels. A big, free-flowing turbo can be laggy and unresponsive, making it unpleasant for street driving but producing awesome power at higher rpm.

To combat high backpressure and possible reversion, the compromised tuning needed to prevent destruction with an overboosted small turbo will also reduce power. If a small turbocharger is running backpressure to boost ratio of more than about 1.8:1, a supercharger has a good chance of performing better. Fortunately, it is easy to design a reasonable responsive, powerful turbo system with a ratio of less than this.

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A little more boost, perhaps 4 to 5 psi might be tolerated, but trying for 20psi could be flirting with disaster. So in the light of that what happens when we put on boost foolers and boost elbows and get 15 to 20 psi more boost out of our stock turbos?

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Well the HX35 will go up to 35psi and still be fine, after that it becomes too small of turbo and just creates heat because it has to turn so fast to get anything over 35. The wastegate opens at ~20psi stock and the boost elbow keeps it from opening until whatever you set it at. The wastegate really is wasteful. It dumps pressure so you end up stopping at a lower boost pressure. If you were to plug it, you would get max efficiency out of the turbo until you went over 35psi. The wastegate dumps pressure out so it doesn't drive the turbine, which is wasteful. I made a really rigged up wastegate actuator that will not open the wastegate until it hits 35psi and my EGT's are lower and I have more power because I am building more boost sooner. Before, I would have to get to the wastegate opening pressure, then outflow the wastegate to go any higher. If I floored it I could get to 30 eventually. But with my wastegate actuator, I can hit 30 at 3/4 throttle because I am not having to work against the wastegate. That answer your question?

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It sure does answer my question but now I'm pretty intrigued with that waste gate actuator. How did you do that? Is it something I could do? And is it doing the same job as a boost elbow or something different yet?

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Have you ever put your hand over one side of a little fan and watched it speed up? Same principle, I think. So back pressure allows it to speed up because it just spins the same air around itself without having to chop up new air. :2cents:

That is how turbos explode themselves in an overspeed situation. I like to use the Vacuum cleaner as an example. When you put your hand over the hose and it speeds up is it working harder? No it is in a vacuum where there is no more load and it has actually speeded up. This is the same concept as when you are running with a plugged air filter and why it is very bad for a turbo as it actually can overspeed and dynomite itself. Drive pressure at/in/before the turbo is different than backpressure from a muffler. Don't confuse drive pressure with back pressure as they are 2 different things.

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It sure does answer my question but now I'm pretty intrigued with that waste gate actuator. How did you do that? Is it something I could do? And is it doing the same job as a boost elbow or something different yet?

I have never used an elbow so I don't know how it compares. I would just get an elbow as thats what the rest of the world has done lol. Mine is reallly overengineered and the parts are hard to find and I really don't know how much better/worse it is compared to an elbow, so I would stick with an elbow, they are cheap and simple. The difference with mine is that is keeps the wastegate shut until it hits 35, whereas if you set an elbow at 35, it would slowly be opening the wastegate up until that point. There is a tiny hole in the elbow and you adjust how much flows through that hole. So if you have your wastegate tube and lets say I am building 30psi, outflowing the wastegate of course, you can put a tiny hole in the wastegate line (with the elbow) and that will drop the pressure in the line after that point, so then the wastegate might only see 15psi or something depending on the amount of flow you have going through that tiny hole. When you hit 35psi, the wastegate might see 20psi, it will open, and that is how the elbow limits it, by making it see lower boost. Mine stays completely shut until 35 and at that point the wastegate slams open and drops me to the 30psi I saw before (outflowing the wastegate). So what I try to do is stay under 35psi, then I know I am getting all the flow since the wastegate hasn't opened yet. I also rigged up a light from a brake light switch (the one for your brake pedal) so I can see when the wastegate actually opens.

That is how turbos explode themselves in an overspeed situation. I like to use the Vacuum cleaner as an example. When you put your hand over the hose and it speeds up is it working harder? No it is in a vacuum where there is no more load and it has actually speeded up. This is the same concept as when you are running with a plugged air filter and why it is very bad for a turbo as it actually can overspeed and dynomite itself. Drive pressure at/in/before the turbo is different than backpressure from a muffler. Don't confuse drive pressure with back pressure as they are 2 different things.

Ah, that makes sense. I knew it speeded up anytime you blocked it off. I took one of my 80mm computer fans and I can put something over the side that it sucks air in and the weird thing is I can put something on the other side and it has a vacuum on both blocking plates. Maybe it is trying to pull in air but cant so it creates some sort of vacuum? Not sure what the deal is. The overspeeding thing makes perfect sense. From how I understand it, the dirty air filter will cause overspeeding but a muffler will slow the turbo down.

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I used to work on worhtington 5clh model engines. the pistons were 12 inch bore and each one weighed 500 pounds.The turbo weighed in at 1500 pounds. Maximum pressure was 6 psi on the manifold.Flywheel weighed 5 tons. Red line was 750 rpms and then it shut down for safety.Hows that for a turbo

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I used to work on worhtington 5clh model engines. the pistons were 12 inch bore and each one weighed 500 pounds. The turbo weighed in at 1500 pounds. Maximum pressure was 6 psi on the manifold. Flywheel weighed 5 tons. Red line was 750 rpms and then it shut down for safety. Hows that for a turbo

Sounds awesome! I've always been intrigued with huge engines and love to hear specs like these.

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