Potential MAP Reference Point

[Radical_Goodspeed]

So I'm in the final stages of assembling my single turbo RB30/26 and had a question regarding MAP reference points. 

I've seen several recommendations such as tapping the cylinder 2/3 ITB, tapping the intake manifold at cylinder 2/3, or using a point on IAC.

First two are doable but require permanently modify part and the third is "out" as I plan to delete the IAC. 

All that to say my question is can I used the "bleeder" in the center of the ballance tube as a MAP Reference?

I'm running a catch can so I don't need it for the PCV system. My thought process is it "pulls" from all 6 cylinder, and it's between the ITB and the cylinders making it ideal for MAP reference according to what I can find.

Thoughts?

[GTSBoy]

Getting a decent signal from all 6 throats is a challenge. I don't know for sure, but I suspect that the stock balance tube is not ideal for it.

I have done it on an ALFA 4 cylinder (about 35 years ago, so don't ask for too many details). We drilled 4x holes in the manifold runners, put in some fittings and ran hoses to a decent sized (I think it was about 20mm diameter) pipe that ran the length of the inlet manifold. So, it was quite a decent volume. There is a "tuning" balance to be found between the volume of the common plenum on such a thing and the diameter of the pipes running from it to the runners. You need the volume to be large enough to damp out the sharp spikes in pressure signal you get as each runner gets sucked on by its cylinder, but not so large that it becomes too slow to respond to actual changes in MAP. And you need the hoses to be small enough to transmit the signal quickly, but not so small that they delay the signal.

You might have to have more than one go at it, if there isn't any actual success based wisdom to be had here. Hopefully there is.

Anyway, I would not do it on only a couple of cylinders.

I would also not care about "permanently modifying a part". Just bloody drill holes and make stuff better. There is nothing sacred about any GTR unless it is a genuine museum piece that you shouldn't be modifying at all anyway.

[joshuaho96]

  On 30/03/2025 at 11:18 PM, Radical_Goodspeed said:

So I'm in the final stages of assembling my single turbo RB30/26 and had a question regarding MAP reference points. 

I've seen several recommendations such as tapping the cylinder 2/3 ITB, tapping the intake manifold at cylinder 2/3, or using a point on IAC.

First two are doable but require permanently modify part and the third is "out" as I plan to delete the IAC. 

All that to say my question is can I used the "bleeder" in the center of the ballance tube as a MAP Reference?

I'm running a catch can so I don't need it for the PCV system. My thought process is it "pulls" from all 6 cylinder, and it's between the ITB and the cylinders making it ideal for MAP reference according to what I can find.

Thoughts?

Expand  

Isn't there a fitting on the back of the balance tube? That's what the OEM boost gauge uses.

[GTSBoy]

  On 31/03/2025 at 1:40 AM, joshuaho96 said:

That's what the OEM boost gauge uses.

Expand  

That's just a gauge, with a certain amount of electronic damping, and no-one cares if it is timely-accurate. I'd hesitate to use it for an ECU MAP input without knowing that it was a nice signal. Responsive but smooth. Not laggy. Not lumpy.

[joshuaho96]

  On 31/03/2025 at 3:53 AM, GTSBoy said:

That's just a gauge, with a certain amount of electronic damping, and no-one cares if it is timely-accurate. I'd hesitate to use it for an ECU MAP input without knowing that it was a nice signal. Responsive but smooth. Not laggy. Not lumpy.

Expand  

AFAIK I've seen other people recommend using the back of the balance tube as well. Andre Simon from HPA at least. I can't imagine it being that far off from ideal even if it was only used to drive a boost gauge from the factory.

[GTSBoy]

It might be fine. I was just saying that I wouldn't trust it without testing it.

[robbo_rb180]

Where ever the fuel reg gets it reference is what I use to T into for map reference been a very long time so where it goes to plenum/runners I have no idea but never had issues.