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Why exactly do you guys run the exhaust from the small turbo into the larger turbo? That will reduce the big turbos exhaust energy. It also results in backpressure. If you have separate exhausts for the small and large turbo you will have the same exhaust response but with less backpressure??
 

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If you run the exhaust housings in parallel and compressors in serial then whenever the large turbo exhaust port opens, all the exhaust energy will bypass the small turbo entirely. You won't get any compounding force from the small turbo and will be simply running a sequential setup.
 

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Completely unrelated question: what pressure sensors do you guys use on high boost motors? I am looking at compressor outlet temps at 4:1 and most pressure sensors are not rated for operation at the resulting temps. Ideally I would use a 4 or 5:1 Bosch Tmap to reduce the number of bungs, but these are not rated to 300F. Is it Honeywell PX3 for the win?


None of them seem rated for higher temps. I'm guessing most people put a separate temp sensor on the boost pipe and then run a separate bung for the boost and some tubing to a remote pressure sensor to separate it from the heat source, but I cannot figure out why there would be a 4 bar Tmap sensor with a rated operating temp of only 125C. I guess I can use the Tmap after the intercooler and separate pressure and temp sensors before the cooler.

Also, does anyone have a cheaper wastegate position sensor solution for a 38mm MVS wastegate than the Tial product? I bought a nice MVR position sensor for about $100 but the Tial sensor/cap is $300.

Thanks.
 

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If you run the exhaust housings in parallel and compressors in serial then whenever the large turbo exhaust port opens, all the exhaust energy will bypass the small turbo entirely. You won't get any compounding force from the small turbo and will be simply running a sequential setup.
It would do the same thing if you have the exhaust going to the big turbo or not. In fact there is data that shows the small turbo speed drops when the big turbo comes on. I would like to see a back to back test on this. Plus don't forget the exhaust energy coming out of the small turbo is basically just deluding the hot exhaust energy coming to the big turbo from the bypass.
 

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Am I'm misunderstanding your original question, though. You're essentially asking about a y off the header collector with a butterfly valve to the LP turbine exhaust, right?

On a normal compound set up, the exhaust is focused on HP, then bypassed around the hp in to the LP. Are what you referring to focusing all the exhaust energy at the LP, but 'bypassing' the hp to feed the LP?
 

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On a normal compound set up, the exhaust is focused on HP, then bypassed around the hp in to the LP. Are what you referring to focusing all the exhaust energy at the LP, but 'bypassing' the hp to feed the LP?
I'm curious on his question as well.
Although setup correctly, I feel it may be misleading to refer to them as High Pressure and Low Pressure. But more so Secondary (manifold charger) and Primary Charger (Atmosphere Charger). Setup up correctly, (I'll use a diesel as an example because of the internal gates on the s300's) the manifold charger would have the wastegate crack pressure set to around 25-30psi, and on average we see about 40-45psi out of the Primary turbo and around 75-80psi total boost.
 

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I refer to them by the small turbo and the big turbo. You can't screw it up this way. On the exhaust side it goes engine feeding little turbo feeding big turbo. On the intake side it goes big turbo feeding little turbo feeding engine.
 

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It would do the same thing if you have the exhaust going to the big turbo or not. In fact there is data that shows the small turbo speed drops when the big turbo comes on. I would like to see a back to back test on this. Plus don't forget the exhaust energy coming out of the small turbo is basically just deluding the hot exhaust energy coming to the big turbo from the bypass.
It appears that you're one of MANY people that mistakenly think that the heat energy itself actually does something in terms of making the turbine spin. This is not true. All we're trying to do with "keeping the heat up" is making sure that the exhaust is trying to expand as much as possible, keeping the velocity at the highest level possible. This is because it's the inertia of the exhaust molecules slamming into and pushing off of the turbine wheel that cause it to spin. That is the ONLY thing that makes it spin. And since KE=12mv^2 (note that velocity is squared) we get maximum benefit (energy transfer) from keeping the velocity high at (almost) all costs.
This is why diesels run comparatively smaller turbines, after accounting for displacement and compressor flow, than a similar power gas engine. The diesel exhaust gas is cooler and slower, so you need a smaller AR to get the exhaust velocity up as it's entering the nozzle of the turbine housing.
Also, once the small turbine and housing is up to operating temp there is very little heat lost, especially if everything is wrapped. Pretty much no different than if it was just a straight piece of pipe.

Running the turbines in parallel would absolutely destroy the velocity, as all exhaust would simply take the path of least resistance. The small turbo would not spool anywhere close to as fast as in a series setup, and that fast spool (lower rpm boost threshold) is most of the reason we're doing this on gassers in the first place. For example, Kevin's car being able to get up on the converter without needing spray.

Yes, series ends up with a higher backpressure ratio than parallel, but not so much that it actually creates a problem. When you run both compressors in an effecient range, the required shaft energy drops a ton. This is how Kevin's car is able to run a 1.5:1 overall even though the exhaust is passing through two turbines.
The small turbine may have 1.5xBoost feeding it, but it also has (guessing for easy math) 1xBoost on the outlet, which means it's operating as if it were only an 0.5:1 PR (in terms of the amount of KE stolen from the exhaust)
Then you have the big turbine being fed 1xBoost and roughly zero psig in the downpipe, so it's operating as if it were simply 1:1
This is drastically over-simplified, as there are tons of other variables, but I wanted to make it easy to understand at it's most basic level. Really need to be working with absolute numbers for it to be completely accurate.

The bottom line is that both turbos are corking up the exhaust JUST enough to get the compressor wheels to do the required amount of work, and running them in series allows you to manipulate how much exhaust goes to each turbine throughout the powerband. On the bottom end the small turbo spools like it's by itself, which wouldn't be possible with a parallel turbine configuration.
 

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Exhaust velocity is dictated by pressure differential. The hotter the temp the higher the pressure = the higher the velocity
 

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Right. Ideal gas law and all. PV=NRT

Chuck, can you draw a picture or make a quick sketch of how you're thinking of laying the exhaust out? Because from the quote below, it sounds just like how a normal compound setup?

[/QUOTE]
On the exhaust side it goes engine feeding little turbo feeding big turbo. On the intake side it goes big turbo feeding little turbo feeding engine.
If it's something worth testing, I could put together a rig to test with. I would need help with whatever telemetry information you wanted, but could built the rig and do the dyno work.
 

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Well it would be in parallel so the little turbo would be fed right off the manifold and have a wastegate. Feeding the large turbo would be a butterfly valve or big ass wastegate fed directly off the manifold. Once the little turbo hits its boost target and the little turbo is wastegating then the big turbo butterfly valve opens and starts feeding the big turbo. When the butterfly valve opens the small turbo wastegate closes since the wastegated exhaust is now going through the butterfly valve to the big turbo.
The intake side would be 100% compound. Big turbo feeding the little turbo.
 

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So like this:


It could be done without the small gate bypassing the small turbo using the butterfly to act as the small turbo gate. Run like this, you're losing all of the exhaust energy passing through the small turbo to spool up the big turbo so spool time is going to suck. There might not even be enough exhaust energy bypassing the small turbo to even bring the big turbo online. There might be enough energy to bring up a turbo similar in size to the small turbo, but you're not going to have enough compressor mass flow to compound the cold side.

Setups ran with the turbine side in series need to have a valve that flows enough that having a butterfly valve vs large bypass wastegate wouldn't make much of a difference. This is usually dual 44mms or a single 60mm on high performance motors.
 

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Yes but the enthalpy of the exhaust gas coming out of the little turbo is GREATLY reduced. Why would you route it back to the big turbo as it would be doing very little work? The temp is like 500F instead of 1300F. I know the arguments from both but would love to see a back to back. I am sure that will never happen as it is a plumbing nightmare and like a million hours of welding.
 

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Yes but the enthalpy of the exhaust gas coming out of the little turbo is GREATLY reduced. Why would you route it back to the big turbo as it would be doing very little work? The temp is like 500F instead of 1300F. I know the arguments from both but would love to see a back to back. I am sure that will never happen as it is a plumbing nightmare and like a million hours of welding.
Where is this heat energy magically disappearing to? It's all still there. The only thing that I can think of that explains your jump in this direction is a severe error in your estimation of how much exhaust gas is actually being bypassed. Yes we want hot exhaust, but you have to do a weighted average here. You can't just throw away everything that goes through the small turbine.

And why are you using theoretical words like "could" and "might" when this has already been done and tested? I know it's a long thread, but if you'd have read through the whole thing, these questions have already been answered. Kevin records a ton of data and shares it freely. Many others in this thread, myself included, have mirrored his results and added even more information.
Compounding has been around for several decades. One would think that if running the turbos in parallel for our specific goals was better, then that's what we'd be doing.

It seems what you're actually doing is trying to run a sequential setup but with the intake side plumbed in series.
 

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Armcclure, you're mistaken. Heat does work. And in thermodynamics, heat is pressure. It's all the same thing. By exacting work from the exhaust gas to spin the turbine, you reduce heat and pressure. What doesn't get reduced is mass flow. We need both to bring the big turbo online. We're asking 2l motors to spool 90mm turbos.

Even though the gasses coming out of the small turbo are low in temperature, they're still high enough extract more work out of. And we don't want to completely bypass the small turbo when the big one comes on as we're still asking it to do work.

Another issue is complexity in control strategy. It's much easier to control the different ramp-up rates and part throttle responses using a simplified system. The more complex the system, the more unpredictable it becomes in mixed driving. It doesn't matter so much for a max effort drag car, but for a street car, it's really important.

What may be interesting to test would be a 3-way valve on small turbos exhaust, but I can't think of anything that's commercially available. On low boost, the small turbo's exhaust would be routed to the large turbo and when the large turbo comes on, the valve closes the connection between the two turbos and dumps the small turbo exhaust to atmosphere. I don't think this could properly be done with standard wastagates. We would need a 3-way solenoid on steroids. Maybe

Like I said, chuck, if you want to lend some sensors I can put a sled together. I have a vw 2.1L put together I was waiting for a purpose to use. AWP block, AEB head, billet mains, ALH crank, JE pistons, IE rods (I think?) revolver cams, etc etc. What I don't have is a spare ECU and the multitude of temperature and pressure sensors to do proper datalogging.

something like this
 

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80% of a turbos energy comes from the expansion of the hot gases which drop in temperature accordingly. 20% is due to kinetic energy. It's a thermal machine predominantly. These are approximate of course...
 

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Armcclure, you're mistaken. Heat does work. And in thermodynamics, heat is pressure. It's all the same thing. By exacting work from the exhaust gas to spin the turbine, you reduce heat and pressure. What doesn't get reduced is mass flow. We need both to bring the big turbo online. We're asking 2l motors to spool 90mm turbos.

Even though the gasses coming out of the small turbo are low in temperature, they're still high enough extract more work out of. And we don't want to completely bypass the small turbo when the big one comes on as we're still asking it to do work.

Another issue is complexity in control strategy. It's much easier to control the different ramp-up rates and part throttle responses using a simplified system. The more complex the system, the more unpredictable it becomes in mixed driving. It doesn't matter so much for a max effort drag car, but for a street car, it's really important.

What may be interesting to test would be a 3-way valve on small turbos exhaust, but I can't think of anything that's commercially available. On low boost, the small turbo's exhaust would be routed to the large turbo and when the large turbo comes on, the valve closes the connection between the two turbos and dumps the small turbo exhaust to atmosphere. I don't think this could properly be done with standard wastagates. We would need a 3-way solenoid on steroids. Maybe

Like I said, chuck, if you want to lend some sensors I can put a sled together. I have a vw 2.1L put together I was waiting for a purpose to use. AWP block, AEB head, billet mains, ALH crank, JE pistons, IE rods (I think?) revolver cams, etc etc. What I don't have is a spare ECU and the multitude of temperature and pressure sensors to do proper datalogging.

something like this
Come on man... "work" does not mean making the shaft spin. Go drop your turbo in an oven or furnace and let me know how fast it spins from the heat alone.
For some reason you are trying to make this way too difficult.

In your drawing and description, with a 3-way routing the small turbo exit to the big turbo inlet, the small turbo would have effectively zero force being applied to the shaft. Path of least resistance.. both turbo inlets are on the same manifold...
 

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80% of a turbos energy comes from the expansion of the hot gases which drop in temperature accordingly. 20% is due to kinetic energy. It's a thermal machine predominantly. These are approximate of course...
what exactly do you think that expansion does?

I'll give you a hint. 100% (not 20) of the shaft rotation is caused by kinetic energy transfer.
 
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