I'm starting to see your guys point. A junkyard throttle body is a whole bunch cheaper than a $1000+ fuel management solution. And if it works at 220 and you don't need a whole bunch more why not. A miata with anywhere near 200 whp is a ton of fun.

 

As I say there's a guy over here with just under 280 with dual TB's and it's perfect...

It's really not going to effect power much

 

Any chance you can post some pics or link to some of these projects? I'm curious to see how it looks.

 

Great dialog guys, thank you.

I'm running 95/65 pulleys on the MP62 so max boost is 5.5psi on my gauge - so probably, what 175-ish whp?. No intercooler at all - baby steps. There are guys here running more boost than me w/intercooler and fuel management and they have the same idle problems because of the long intake tract.

Once I can get the idle and throttle response back close to stock (ie not stalling when cold) then perhaps I'll be ready to up the boost, add intercooling and management. Or maybe not, my car tracks all day and sits in summer traffic with no heat problems - shaving a few seconds might not be worth those headaches to me.

Of course that might change when I drive Steelrat's car - the other guy from Toronto doing the DTB Mod - with more boost and management and IC.

 

There's a whole bunch on mx5nutz.com. Here's one of the threads. Dual Throttle Kit - MX5Nutz.

 

Seeking opinions on the vacuum side of this project.

The recirc valve is mounted to a BOV tee at the outlet of the blower. It needs a vacuum line.

Adding the second throttle body means the blower bypass valve vacuum line cannot be hooked up to the blower outlet any more.

There is only one unused vacuum port on the inlet manifold and it is smaller than either the recirc or the bypass ports.

So,
If I use this smaller inlet manifold port and run a line to both the recirc and bypass valves is their response time going to be greatly affected by the flow restriction of the smaller port?
or,
Do you see any problem adding a tee off from the brake booster hose? (on North American cars the brake servo is engine hotside)
or,
How about removing that inlet manifold port and drill/tap for a larger port - or leave that port and drill/tap elsewhere for a larger port?

Am I overthinking this? It's winter here and the car is in storage so I can't test until spring.

 

*caution, I have no idea what you're talking about*

I believe BEGI uses a tee in the brake booster line for something, their instructions may be available online.

 

Massively so.

Just get a plastic 'T' connector used in windscreen washer jets etc and use one of the sources on the intake manifold.

 

Pics from the install so far.
Picasa Web Albums - Wendinator - Dual Throttle...

Comments:
I wanted the Recirculation valve on the intake side to catch the start of the pressure pulse from the stock TB closing but it was a space/mount/plumbing mess hence the blower outlet mounting.

I want to keep the added volume in the vacuum system to a minimum so I'm thinking of plumbing into the brake servo line to keep the run short.

The Recirculation valve is not supported by any brackets - to allow more engine/frame relative movement - it's a fairly large unsupported weight which might oscillate, guess I'll know better when I fire it up.

 

You had to know I couldn't wait until spring.....vac lines connected to intake manifold, battery reconnected, started first crank and she purred like a kitten.

But, what a strange noise that recirculation valve makes. Maybe it's the 50/50 vent/recirc. I ordered FM's deal of the day (3 Feb) BOV which is a full recirc capable. Other than the BOV noise, all is good so far - at least sitting in my underground parking spot it is.

 

Though Doppelganger didn't run 2 TB's, he did have a valve to relieve the pressure between the SC and the IM's TB. I think he let the additional air bleed to atmosphere.

What is the benefit of having the recirculation valve blow the air back into the initial intake tract? The SC TB should already be closed if the recirc valve has kicked in. Theoretically the MAF has already read that air as incoming, but the split second it was already needed for fueling has come and gone. The only thing I see happening is you have a pressurized (albeit lightly) intake tube which may impact incoming MAF readings (possibly negligible, but you get the idea). Why not let it blow in the wind?

 

Forgot to say that it looks like a clean implementation. Fingers crossed for you on maiden voyage!

 

My understanding is not vast but my belief is:
1) SC TB cannot be fully closed since air for idle is required by the IAC in the stock position.
2) Dumping the previously metered air - for which fuel calculations were done at the ECU - means that new air the MAF sees will cause new calculations at the ECU resulting in a short term too rich fuel/air at idle and Black bumper syndrome .

Thank you. I'll report after the event.

 

Car is still in the parking garage but i just had to tinker - you know the feeling.

I fitted the FM Deal of the Day recirc BOV and I wanted to check for it being closed at idle. It was, but there is a massive leak from the outlet port to the vacuum line (enough that the supercharger bypass valve won't actuate unless I blocked the BOV outlet). I refitted the other FM 50/50 BOV and it does the same although to a lesser extent. This cannot be right, can it? Do I just have two bad valves?

 

Out of curiosity where are you getting the signal for the bypass? When I moved the throttle with my m45 i had to move this line pre-tb which i imagine you would have to do for a dual setup as well.

 

So, you moved the TB to the inlet of the supercharger, yes? Are you sure the by-pass pickup went pre-TB? That would place it in the almost ambient intake tract. If yours is indeed pre-TB then your supercharger bypass valve spends the majority of its time closed. Your supercharger is working way harder than it needs to be all the time. It should be open until vacuum drops (as you mash the pedal for example) at which point it closes and you go boosted.

The Bypass valve needs to see vacuum. Before the dual throttle install my pickup was in the outlet plenum of the MP62. After the dual install my pickup is off the inlet manifold. i.e. the bypass vacuum line should always be on the vacuum side of the last throttle.

 

Sorry brain fart. I meant post tb with the tb in the stock location. Sounds like you have it at the same port i use. My bov leaks a lot of air out at idle but since i have a ms i dont care. I have a 2 port greddy knock of adjusted to hold peak boost when needed but at other times vent any excess pressure. One thing i did find is the bypass on the super and the bov didnt like sharing a vaccum line with eachother or anything else.

 

I took the car out for a quick test drive in the 9C sunny weather today (before the snow comes back next week). I have to say I'm a little (a lot?) disappointed. The car is waaaay down on power and makes the most amazingly annoying noises that seem to come from the supercharger outlet. (Like a dog howling down a plastic drainpipe).

The only good thing is there's no idle dip.

I know there's not much info here to go with but any thoughts from the guys that have made this work?

 

I'm much happier now.

I bought a Forge recirc valve (MSM fitment) after finding lots of leaking with the others I have (lesson learned!). Put a check valve in the recirc line to stop intake vacuum through the Forge bypassing the blower at idle and overrun. Tuned the throttle body relative opening by camming the throttle wheel. Found an unused vacuum port under the intake (part of connection to the Vics) and used it for the blower bypass valve so bypass and recirc valves are independently connected to the vacuum plenum in the intake.

Connected a calibrated digital pressure gauge (0.5psi increment) which showed max boost 6psi (so more than 6, less than 6.5) - my old Autometer analog gauge which I loaned to a buddy used to show 5.5psi.

Driveability is excellent. Throttle response time is virtually stock and no idle dip.

I need a few more miles before I pronounce this a complete success but it looks good. Can't wait for AutoX season to start....

 

I just finished my dual throttle project today..

My problem was matching the wire travel between the two TBs.
I have a BBK BTB (big throttle body) on the S/C.

I measured wire travel on both TBs. Stock requires 3.5 cm from closed to fully open, and the big one takes 2.75 cm.
I found out that the stock pulley rotates 75 degrees. (One would assume the full motion would be 90 degrees..)

3.5 cm wire travel at 75 degrees needs a pulley which is exactly 42 mm in diameter (where the cable rests) mounted next to it to provide 2.75 cm of travel.

So I made some drawings based on my calculations, and had a machine shop turn a pulley for me with the correct diameter and offset.
I mounted this pulley on the stock TB, and had a "wire shop" build me a custom throttle cable.
The cable sheath is 95 cm. Has adjusters on both ends, and the cable itself is about 135 cm or so. Cost me 20 bucks to have it made.

The next step was to modify (extend) the existing cable bracket on the IM to allow for the offset on the new pulley.

I did not use a BOV or a recirculating valve.
The trick is to crack the SC butterfly open a bit. Imagine setting your idle at 2500 rpm. That much. (Stock TB has authority on idle, anyway.)

The big butterfly has a very aggressive opening/closing rate, which is a definite advantage.

Adjusted both cables so there is no slack, and made damn sure WOT on the stock TB is actually WOT on the other side, too.

Turned the key, the car started and immediately settled into factory idle. It took a couple of very minor adjustments on the SC butterfly to smooth things up.

Revs instantly, has great pick up, punching the throttle idling down the road in second gear smokes both tires forever.
I have a 130/65 pulley combination, BTW.

I took it out on a nice, long and SLOW test drive after dinner. Very civilized behavior overall, drops down to idle perfectly from all RPM levels, and has perfect throttle response. I got on the gas a few times, and no problems whatsoever.

I am very happy with the results. No idle related problems any more.
Should have done this a looong time ago.

The only concern I have is the stiffer throttle pedal.
I'll try it with one coilspring removed from the TB. See what happens.

I will also post pics when I have a chance to take pics during the day.