Maximizing Vacuum on ITB setups

[wgknestrick]

I just solved what I thought was (is) a concern when trying to tune individual throttle body setups with only a MAP / Speed Density ECU with no alpha N. ITBs setups have notoriously been difficult to tune this way due to very low vacuum levels at idle and above. This greatly reduces the resolution on the ECU.

I was forced into a speed density, stand alone, ECU based on my application and the fact that my setup is turbo on top of the ITBs. (I require MAP compensation). To run a speed density ECU on this type of setup, you just make a vacuum manifold with connections to all of your ports below the TBs and then connect your MAP sensor (this is fairly standard). The theorized problem I always had was that for every 1 cylinder drawing vacuum at a time, I always had 3 others loosing it through the open TBs. With this theory, you would always loose vac faster than you could accumulate it.

I have found that adding (# cylinders - 1) check valves in these port tubes, greatly increased the measurable vacuum in the accumulator. These 3 diaphram check valves caused vacuum to go from -2 in-HG at idle to -15in-Hg. I am running 3 check vavles to ensure that 3 cylinders can contribute vacuum ONLY to the accumulator, while the last allows the accumulator to see the + pressure of the turbo. This brought the # of open TBs leaking vacuum back down to 1 (like a single TB) and now my vacuum levels support this.

While I haven't progressed far enough yet with the car to see if I loss any response time (probably did a little though) in the MAP signal to negatively affect things, I just wanted to share a solution that could be helpful to others.

Bill

 

[murpia]

What happens if you connect the MAP sensor to only one of the throttle bodies? Can you not assume the others are experiencing the same conditions? Or is the variation in throttle flow too great?

Regards, Ian

 

[Deltona]

Ian: You get pulsing which confuses the map sensor as it needs a constant signal.

wgknestrick:

Thanks for the informative post and please keep us informed of how it all works out.

 

[ChristianK]

I have found it best to take all four pressure sources from under the throttle plate and plumb them to a vacuum box. This helps smooth the MAP signal and it makes the signal an average of sorts. If need be, you can install small brass restrictor pills to limit the pressure changes from throttle tip-in. You can also install those inline filters on the pressure source lines to help damp the signal.

Take care,
Christian.

 

[wgknestrick]

The check valves pretty much eliminate any need for restrictors (they are substituted) and should always make more vac at idle compared to them.