A lot of people think that the 20B exhaust sleeves are restrictive for noise reduction. This is not true. They were designed this way for specifically for spool and low end torque. Mazde specifically engineered them to have a narrow opening in order to work with the narrow exhaust manifold:



This helped Mazda achieve their targets for boost response and low-end torque:



That doesn't mean you shouldn't swap sleeves. It just means that Mazda used the narrow sleeves for a specific performance-oriented reason: they wanted a torque curve that could compete with the V8's (or twin turbo 6 cylinders) of the time.

 

This is downright AWESOME. Have you thought about compiling all your notes into a single 20B-REW FAQ thread? This is the kind of info that new owners need to see!!!

 

Excellent info as usual. Thanks!

 

Ack! Can't edit.

I'm curious if this translates to use on a modern tubular exhaust system to the same degree. Also, I'm curious if it matters where you shave the volume.

Since in a tubular manifold, we are relying on the gentle bends to guide the exhaust, I think the volume is the only variable. Supposing a 20b manifold with equal length, 50mm, 18" runners, we could expect a volume, excluding the collector, of ((pi*25mm^2) * 475.2mm * 3) = 2797740mm^3

Shrinking to 44mm runners ((pi*22mm^2) * 475.2mm * 3) = 2797740mm^3 = 2166569.85mm^3

33% reduction as opposed to Mazda's 40%, hmm. I'm not sure people are seeing gains like Mazda.

Also, if volume reduction has such a significant effect, why are people against quick spool valves when we (20b users) can't get a fully divided manifold? Aren't we effectively reducing the volume of the exhaust system (inside the turbo pre-turbine, albeit with leaks) by half for that portion? Is this section less effective per unit of volume reduced than say the sleeve or manifold area?

 

yes. the design specs the internal dimensions. what or how you make it doesn't matter at all, as long as its made to follow the specs. the spec in the article is a 36mm pipe, but the production part is cast iron

it probably does matter where you shave the volume, but its pretty obvious what and why mazda did theirs

 

I can't speak for others. You know the 87-88 turbo engines had a quick spool valve from the factory?





Later Mazda switched to the divided manifold (ditched the quick spool valve) because it maximized the pulsation effects on a 2 rotor.

 

I wouldn't consider myself a 20B expert, but I have read the literature.

 

I'm quite familiar. As far as I know, there is no 3 way divided turbine housing to allow a 20b to run a true, fully divided, fully separated manifold all the way to the turbine blades. By having 3 runners, we are forced to run collected or semi-divided. The divided workaround I've seen is to split the center exhaust runner, but I'm curious how much energy is lost having exhaust expand into both chambers vs. closing off volume to make up for the collection.

 

They exist. I know ABB has used them on specialized 5 cylinder diesels for marine applications:


Coden, "Controlled Pulse turbocharging of medium speed 5-cylinder diesel engines," 2005

 

That...is....awwwesome....

 

Don't mind me, I have more research to do now, lol

 

Here's how the triple-scroll turbo worked. On a 5 cylinder you've got an oddball firing order and weird pulsation effects that can cause backpressure problems. This is due to the fact that a 5 cylinder has a 144 crank degree firing interval. So on the three scrolls it worked like this

scroll 1) single cylinder
scroll 2) two cylinders
scroll 3) two cylinders

At low rpm/load, they are all divided up among the three scrolls like that. At high rpm, two valves open to connect scroll #2 and #3 with the single cylinder scroll #1. That effectively turns it into an undivided manifold with less pulsation in order to improve turbine efficiency. The whole point of this configuration was to ultimately reduce smoke emissions on marine diesels (as part of an overall optimzation program). That sums up the 22 page paper.

 

Specs on that triple-scroll turbine housing? Availability?

 

Unfortunately I have no specs on it and I'm not sure if it was ever mass produced. The research paper above is the only place I've ever seen reference to one.

 

All of this is made obsolete by VG turbos, which basically have an infinitely adjustable scroll point instead of distinct scrolls with a switch. I have considered adapting one to a rotary, the control scheme can be made pretty simple.

VG turbos are not super popular right now because the diesel manufacturers pioneered them, but the advantages are less obvious on a diesel where low end power is abundant anyway, so many are electing to ditch the VG turbos in favor of a simplified setup, or, interestingly, sequential twins....

 

The point of a VGT turbo is to completely eliminate the wastegate. It would not be easy to size an off-the-shelf model appropriately for doing this on a rotary. Most recent VGT turbos rely on oil pressure or a fully electric actuator to control the position of the vanes. Only the early 1st generation ones are pneumatic. I have some specs on Cummins and Garrett VGT turbos for diesel applications if you are interested.

The variable turbos are used as part of a control strategy to reduce NOx emissions. Using a variable vane turbo allows more flexibility for the PCM to adjust EGR valve (usually a butterfly valve on new diesels) position simultaneously with the position of the vanes. See Hawley, "Variable Geometry Turbocharging for Lower Emissions and Improved Torque Characteristics," 1999. I have it if you're interested.

I think Porsche is the only OEM using VG turbos on mass produced gasoline engines for passenger cars. They're certainly expensive. Diesel engines for trucks are expensive already so buyers expect to pay a price premium.

 

This is a actual twin scroll turbo. Unlike how everyone always claims a divided turbine housing is a twin scroll.