ITB question on the vacuum

[andygtt]

Aiming this question at those who have done ITB setups on cars that used to run a single throttle.

I am doing an ITB setup on my turbo car... Main reason is I want DBW so I can have downshift blips.

Sounds like a lot of work for something so simple, but it means (and has resulted in) some significant changes, for example my M800 ecu is max'd out on the pin count, so instead of throwing further money at it expanding it etc I decided to buy an M150, Im also changing the coils for more powerful items and have bought ID850 injectors (purely for a little extra safety margin as I do run both pretty close to what i consider the limit).
Additionally my very ugly intake plenum is on display under the rear clam due to the perspex engine cover and it would be great to have a decent looking setup.

So with all these other major changes I might as well go whole hog with ITB, tuned runner lengths and a carbon plenum.... I have done the research and made the plenum, trumpets and the ITB (massive amount of work), had the car put on a hub dyno on actuator pressure to get a full 'before' reading and I am now on the final stage making the vacuum take offs, brakes of the DBW actuators etc.

The car is a 3.0l V6, I run a rising rate fuel pressure and vacuum brakes along with a MAP sensor.

My plan has been to run 2 separate vacuum take offs, 1 that will feed the fuel pressure actuator and brakes, the other dedicated top MAP.... but given I am going to be setting the ECU to blip the throttle on down shifts, and on track run full throttle upshifts, will i actually have enough vacuum to provide assistance to the brakes or is it going to be marginal.

I know they have a 1 way valve so in theory you have spare capacity, but cars like my M5 have electric assisted brakes.... is this a hint that in practice its going to be an issue?

[andygtt]

Max_Torque said:

With the caveat that i have no idea why you needed to go ITB to get DBW

I didn't, its just if I am changing so many other things including the plenum etc then I might as well go ITB rather than stick with the single throttle... plus I do like a challenge and developing new things

Max_Torque said:

in my experience, you won't have sufficient vacuum source to ensure complete brake booster performance, and even if you do, the bleed air will almost certainly mess up your AFR distribution.

Even without any throttle blipping on downshifts you will have to cross link your ITBs to get a decent vac source, which kinda defeats the point to some degree

Move the brake booster to a mechanical vac pump (usually driven off cam shaft) i'd suggest!

I was hoping to get some real world experience from you on this... sounds like i can just have one then which feeds the fuel pressure and MAP, of cause it will need to be linked to give a balanced source

[andygtt]

stevieturbo said:

Given it isnt a heavy vehicle etc....maybe just dump the servo ?

That certainly removes that aspect of any problem.

Then for fuel pressure regulator...just take this reading from the plenum which would be more stable, and tune around the fact it will never see vacuum ?

My Ultima with very similar weight and brakes had no servo... brakes felt horrible on the road and I definitely don't want to go that route again. Lots of electric diesel pumps around, will probably fit one at the front of the car.

[andygtt]

Inline__engine said:

what makes you think you wont have enough vacuum for brakes? unless youve got cams with quite a bit more overlap than stock it should be fine. if not an electric vacuum pump (Hella UP30 or UP32), vacuum reservoir with vacuum switch works to supplement to whatever level of vacuum you need.

My concern over having enough vacuum is because I have 3 significant factors:
1. My current setup has Long runners with a plenum at the end so a lot of volume under vacuum, the new setup the runners are shorter plus only half of them can be under vacuum, plus the plenum isn't under vacuum.... So comparatively I will have a fraction of the volume of air available to take vacuum from.
2. On straights I can run full throttle shifts, so Zero vacuum on straights.
3. In the corners I will run a downshift blip to hold the revs while changing down.

All these factors reduce the vacuum available over the stock car... Cams are actually fairly mild.