CapitanCombo

I was about to remove the double throttle assembly yesterday and during the work I made some considerations about the flow thru the UIM.
I was hoping to get a small increase in flow, removing the double assembly, but mainly I did it because I don't like this stupid device.
I noticed that the DT butterflies have a diameter of 52mm, a bit more that the throttle body upper butterflies (50mm) and I thought, oh well, at least Mazda engineers did the right thing, increasing diameter to compensate charge loss.
But then I noticed another thing.
As you know, lower butterfly (primary) feeds the two smaller intake pipes of the UIM (inner two), these have a diameter of 30mm on the LIM connection side, the primary butterfly has a diameter of 45mm.
Doing some quick area calculation pointed me to get that primary butterfly area is 12.5 % bigger than the two pipe.
Nice, I thought, maybe, if they did it the right way, the pipes are also tapered, they have, I mean, some degree of conicity in the direction of the flow, this helps against the detachment of the boundary layer near the pipe wall, and also, as in the DT example, it helps with charge loss because of butterfly shaft, screws and so on.
But the strange thing came when getting to the upper (secondary) butterflies: these feed the two outer pipes, that have a diameter of ONLY 38mm!
And also the outer pipes have not the same lenght, the one closer to the firewall is longer, I think this is done to get the TB horizontal to the floor (maybe the engine is not??).
Getting to the area again, the butterflies are 73.1% bigger than the pipes!
This is why, nobody feels improvement in removing DT, and maybe also why the upper butterflies do not open fully.
I noticed that, in all of the TB I could see, the upper ones never got fully open on WOT, they always stayed some degrees down.
I tried to change the leverage of the TB to get them fully open, and I was successful but this got me worse with the 3K hesitation, having not the possibility to coordinate secondary butterflies opening with secondary injectors power up, apart from getting no improvement at all, in flow.
Has anyone clues about why Mazda engineers could have done this?

RX7Wishing

holy crap. my head hurts.

DamonB

You're making this way too complicated. As long as the butterfly itself is not the prime restriction to the intake then it is big enough in any circumstance. The runners in the UIM and LIM themselves provide most of the losses. No sense in making the butterflies bigger unless you address the manifolds.

CapitanCombo

I don't want to make them bigger, even if I saw a big TB kit sold somewhere
I was just wondering why are they SO big comparing to the runners
It has no sense for me

Ciretose

They are most likely larger tan the runners so that _they_ will not be the restriction. The reason the butterflies don't open completely has to do with air flow. I'm not an aerodynamicist, but I will try to explain it as best I can. I think it has something to do with laminar flow or something like that, anyway, there is more and smoother air flow over the throttle bodies if they are a few degrees off of fully open. If you look at other cars, theirs don't open fully either.

just my .02, take it or leave it.

CapitanCombo

I don't agree with that
I have seen a lot of TBs in my life
and all of them always opened fully, both in carburettor and injection systems.
Only in the '7 I saw this fact.
Maybe this has something to do with the presence of the DT after them, I mean, I think is a matter of aerodynamic flow, as you say.
I agree on what you say about being them bigger, just non to cause restriction, if you read my post I measured the primary to be 12.5% bigger, but the secondaries are over 73% bigger than their runners.
This is too much for me.

ORX705

the 2nd gen rx7 have the same. some owners mod it to open fully.

What are the benefits in removing the 2ndary butterflies? How does it effect drivability on a stock 3rd gen?