Why use dual throttle body? (supercharger)

Mr nanotech · Postby **Mr nanotech** » Wed Mar 13, 2013 3:19 pm

This is a little embarrassing but I'm a little desperate. I've collected all the gear to supercharge my car (but sadly have zero time to do any of it :evil:

) and I've been trying to get my head around the 'dual throttle body' setup I've read so much about.
There are countless threads on miata.net, clubroadster, miataturbo etc... But nomatter how much I read, I feel I have more questions.

From what I gather it is to stop idle drops and rpm dives between gearing off throttle etc by altering the pressure in in the crosspipe/intake tubing. So one tb stays in the stock position and the other before the supercharger and they are piggy backed together.
The part I don't understand (mostly all of it) is how this would make a difference, if both TB are closing at the same time since they are piggy backed together, then doesn't that defeat the purpose of having two in the first place as you may as well just have one and it would basically do the same thing? Then if they went at different times, I can't see that being anything other than detrimental as well since it would mostly likely negatively affect the air pressure as well wouldn't it?

I've talked to a lot of mechanics now and none of them can see any logic to this but apparently we are all missing something as the majority of SC setups I've seen use dual TB.

Can someone here explain this to me in very simple terms a 9 year old could understand?

gslender · Postby **gslender** » Wed Mar 13, 2013 3:30 pm

Less volume means better control. With one TB placed before the SC you have a LOT of volume, and the idle control valve struggles to control/regulate it.

When you have two TB, the smaller volume is easier (and is what the OEM intended) for the 1st TB to control/regulate that volume by the idle control valve.

The reason for the 2nd throttle body is purely for the SC.

I'd recommend you try both - as there is 1 downside to the two TB which is noise. It whines more on half throttle due to reflections and pressure changes. Single TB doesn't have that problem, but the idle dips and surges are more prevalent.

G

hks_kansei · Postby **hks_kansei** » Wed Mar 13, 2013 3:37 pm

gslender wrote:Less volume means better control. With one TB placed before the SC you have a LOT of volume, and the idle control valve struggles to control/regulate it.

Volume meaning Volume of air in the crosspipe I presume? (ie: OEM air is in there at ambient air pressure, charger = pressurised and forcing in more air for the same space?)

Mr nanotech · Postby **Mr nanotech** » Wed Mar 13, 2013 3:58 pm

Thats the part that confuses me, Nik.

By the way I bit of bg on my setup just for any who ask.

-Eaton m45 with resirc valve and 9psi pulley
-TB in stock position as well as another before the S/C
-BA Falcon xr6 turbo intercooler with stainless 2.25" piping (2" for the tight sections with reducers)
-GM iat sensor, lc1 wideband O2 sensor and Mx6 variable tps -connected to DIYPNP
-550cc injectors

Think thats everything?

hks_kansei · Postby **hks_kansei** » Wed Mar 13, 2013 4:14 pm

Here's a thought. (prob a Q more for Gslender)

If the issue is because of a pressure differential between the OEM throttle and the intake piping, why not instead of using a second throttle just have a valve that opens on idle to vent the pressure (ie: a BOV)

Either venting to the air, or pipe it back to the pre-charge intake piping to equalise the pressure?

I'm just thinking it would probably be a lot simpler than working out multiple throttle cables and linkages.

It would just be a matter of working out a way to trigger the BOV only when the car is at idle? maybe some sort of electrical signal to open a valve?

Mr nanotech · Postby **Mr nanotech** » Wed Mar 13, 2013 4:21 pm

Already exists dude. It's called a recirculation valve. It's controlled by vacuum. It works basically the same as a BOV but it recirculates it back into the system through a bypass instead of into the atmosphere.

That's my point. If you have a recirc or even a blow off valve, then why do you need a second throttle body? In theory this would be basically doing the same thing already.

Postby **NitroDann** » Wed Mar 13, 2013 5:19 pm

The issue is that you need the normal throttle body to get the normal intake volume to quickly and effectively control rpm, and to shut the engine off of air quickly to let revs drop etc, the TB in front of the supercharger stops it from getting air, which it can just force past the manifolds almost closed TB, using boost.

When you back off the throttle at 5000rpm because superchargers build boost with rpm not load its forcing 15psi or more of air into the manifold TB, because its cracked open the boost gets through and the revs drop very slowly.

Easy,
Dann

Mr nanotech · Postby **Mr nanotech** » Wed Mar 13, 2013 5:45 pm

THAT makes sense! Thanks man. I guess I've been over thinking it a lot.

gslender · Postby **gslender** » Wed Mar 13, 2013 8:16 pm

All SC have a TB before the SC.

As Dann points out, it is needed to control the SC as it will just keep on producing boost.

The 2nd TB is the one added back in stock location, before the SC. It is only used to help with idle, which with the SC choked with the 1st pre-SC TB, you just have a sh*t load of extra volume to cope with.... Just imagine where the idle valve would be and when you have SC, pipes, IC etc it adds to volume that the idle valve can't control.

Just remember the original and only TB was the one moved to before the SC. The system works fine like that if the idle valve was made to cope with that much air volume. The stock Mx5 OEM idle valve isn't. This is why the 2nd TB is added back at the stock location.

Ok?