I recently came across a deal for an ITB setup for my turbo RX7. I always wanted to try them out since my old Rabbit days, and I am trying to get some good throttle response (95% street car, will see occasional track day and autox). Trying to find information about plenum design and volume is proving difficult, and a lot of what I am finding requires a lot of flowbench experimentation, which I do not have. The setup is:

1.3L 2 rotor engine
Turbonetics T66, but with the newer F1 turbine wheel
stock ports for now but eyeballing a half bridgeport with the eventual rebuild
looking for a powerband around 4-8k RPM, redline is 8.5k
intake manifold is IDA-style
TBs are 50mm diameter
I know I could gain response with direct turbo to plenum piping, but I am running a front mount intercooler.

I think that is all the pertinent information but can expand anything needed. It has velocity stacks but I haven't measured them yet as some designs I have seen use them and some just have runners from the TBs into a cylinder.

I have seen a few places that tout the benefits of a dual plenum design, a cylinder with a cone attached side by side with a certain size slot in between, but I'm looking for some actual information, not just "this is best."

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IR Manifolds for 6 cylinder or V-8 engines, typically have a very very sharp pulse before that cylinder injests fuel and air into its bore.
The signal is so sharp that the wave actually travels backward up the runner/throttle/velocity stack and raw fuel and air suspend over the bellmouth.

At some point the rpm of the engine is high enough that the reversion cloud goes away and then the engine runs much closer to the proper air/fuel ratio and
becomes very responsive.

Finally, at higher speeds the runner (feeding only one cylinder) becomes a restriction to power based on only one dedicated fuel source/injector.

So when you make the IR intake into a Plenum intake you now have a less sharp signal to the atmosphere, the engine runs smoother at a lower rpm, and the plenum
allows air and fuel from two throttles to be consumed by each cylinder as it goes thru the firing order.

I am first off not a Wankel expert. I do understand the 3 lobe architecture and the dual rotor configuration.
So I see the engine tuning like a 6 cylinder engine with two very small plenums (basically the throttle and runner area).

Adding a communication plenum would allow the "6 cylinder engine" to tune like a high performance tunnel ram drag engine.

With Boost now you have the same basic physics being applied but with more charge density due to the higher pressure/mass flow per combustion event.

Richard Holdener has shown on several dyno tests the advantage of a tuned intake system even with boost.

So the question is 'What are you really trying to do?'

Tom V.

 

Thanks Tom, sorry I was away for a while. What I am really shooting for is the most responsive system I can get with what I have, keeping in mind this is primarily a street car. That is also the reason I went with a smaller 66mm turbo instead of a massive one. Top end speed is fun but not practical on the street.

In regards to the restriction due to single injector per runner, I actually have 4 total (2 per rotor, 720cc primaries to handle idle/low load, 1680cc secondaries for high). I have ports for 6 injectors but am just going to run dummy plugs in the extra 2.

I am not looking at a plenum as an intentional means to communicate between runners (though I do plan to have a joint connection for MAP sensor purposes), but rather as a necessity to get the boost from one turbo into two intake runners.

I see plenty of designs similar to this, though I would need the inlet in the other direction:
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I am just unsure of any volume necessities (I have seen multiple sources citing 1.5X engine displacement but there are many arguments as to the true displacement of the rotary), or if the intercooler and piping would effectively count as plenum volume. Maybe the plenum volume won't play a significant role in my application, assuming it is in boost most of the time?

 

I just found out that the company that makes my ITBs also makes a plenum specifically for my application. Kind of pricey for what it is but it will just bolt on and I can be done with it and back driving, so I think I'll give it a shot.

It looks like this, maybe I was just overthinking the volume thing:
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If the company is selling the thing they must have some time invested into the design.
Can you post a link to the company so that the board can get a idea of what the design looks like?

Tom V.

 

Sure, I failed at linking pictures again. The link for the site is: https://www.efihardware.com/products/1648/IDA-turbo-plenum-for-50-mm-throttle-body-bores. Very basic design but if it works that's what matters to me. Still have to inquire about shipping from Australia.

 

The air comes in, smashes against the wall, disperses to the two runners somewhat evenly and with little velocity and makes great power.

The Ford GT-40 Mustang manifold basically follows the same idea for air distribution. If it works why change it.

Tom V.

 

Will the advantages of the individual throttle be diminished with the plenum shown in the link? I realize there is a difference in having volume before and after the throttle plate to some degree but I would consider myself a novice in intake manifold design.

Thx!

 

The wrap around intake works well on a naturally aspirated production engine and most of the time ok on a low boost application since the intake is a plastic piece.
I have seen the plastic manifolds rupture at a weak point on higher boost applications vs aluminum.

Tom V.