RB26 ITB conversion question (regarding vacuum port size)

[godzilla_753]

Hi all,
I decided to undertake the foolish challenge to try and adapt the RB26 throttles to my RB25 neo. Well ive gotten the hard part done but now im plagued with what looks to be a large boost leak. At best Im reading .4 bar of vacuum on my gauge which is not going to work. The car runs sluggish and smell like hell.

now Ive literally gone over the piping a dozen times, Ive pumped up the entire system full of air with a compressed air hose and sprayed soapy water and checked for bubbles. I had numerous small leaks at the various fitting and such but nothing major. After fixing all these leaks Im still sitting at .4 bar of vacuum at idle. Now im thinking 2 things:

A) another boost leak yet unfound
B) I noticed that the stock 26 inner intake manifold has a balancing beam that has 6 ports that run into it from each throttle, this allows all the vacuum to be balanced between the cylinders and gives a common spot for all vacuum to be taken from. I noticed that the ports on the manifold look to be about a 1/4 inch in diameter. My ports that I drilled and tapped into my own runners is about 2-3mm across. I realize now after having all these problems with vacuum that this might be one of the reasons Im not seeing full vacuum at idle. Do any of you guys concur ? Could the size of my fitting impact how much boost/vacuum i get due to the restriction in size? I figure that the guys at nissan knew what they were doing when they built the engine, maybe I should drill em out and put in bigger fittings..

What do you guys think?

and Ill snap some pics for you guys to look over my setup. ill post them up tonight/tmr

[Krazykouki]

The ITB's themselves are ALL supposed to be tweaked individually using 4 vacuum gauges. 1 gauge for each throttle body and 1 control. This is how you adjust the throttle linkage. Please post the proper vacuum as well as .4 bar is something around 5/6 psi, vacuum is read in Hg or inches. Your RB25 should have a minimum of 15 or more HG/inchs of vacuum AFTER the throttle bodies. If the car is running like s***, bogging etc..etc..it is highly likely that they are not properly dialed in and some cylinders are getting way more air than the others. You need to also make sure you have the TPS calibrated correctly, the IACV is working properly and the idle adjustment screw is NOT all the way out. Having the idle screw all the way out will result in a weird vacuum reading as well while trying to dial the ITB's in.

I've adjusted MANY RB26's to run properly do to installation error from other shops that don't know what they are doing and can say that it can be a pain in the a**.

[godzilla_753]

here are some pics:

[godzilla_753]

The ITB's themselves are ALL supposed to be tweaked individually using 4 vacuum gauges. 1 gauge for each throttle body and 1 control. This is how you adjust the throttle linkage. Please post the proper vacuum as well as .4 bar is something around 5/6 psi, vacuum is read in Hg or inches. Your RB25 should have a minimum of 15 or more HG/inchs of vacuum AFTER the throttle bodies. If the car is running like s***, bogging etc..etc..it is highly likely that they are not properly dialed in and some cylinders are getting way more air than the others. You need to also make sure you have the TPS calibrated correctly, the IACV is working properly and the idle adjustment screw is NOT all the way out. Having the idle screw all the way out will result in a weird vacuum reading as well while trying to dial the ITB's in.

I've adjusted MANY RB26's to run properly do to installation error from other shops that don't know what they are doing and can say that it can be a pain in the a**.

I see what your saying. In regards to my setup I thought the throttles had to be closed fully during idle, I bought the throttles and made sure to not mess with the settings when I installed them thinking that whatever they were at must have worked for the past owner. Now I AM taking vacuum readings after the throttles, as in between the throttle plate and head. In regards to the IACV valve I chose to block it off for the moment due to the problems I was having, I tried to minimize my vacuum sinks to help with diagnosing the vacuum problem. With the TPS- can having a misadjusted TPS cause vacuum problems?

I think I understand what your are trying to say though, I think my problem is two fold, I might have to adjust the throttles to gain better response, and I also need to obtain proper vacuum. but do the two go hand in hand?

[Krazykouki]

Having the TPS adjusted wrong can trigger the sensor cause the rpm's to raise slightly but it should not severely hinder anything but remember, raising the rpm changes the vacuum.

The RB26 base idle is quite higher than the RB25/20. First, make sure your base idle is as close to spec as you can get it for the RB25 and set the timing at the RB25 base idle. Then, raise the idle to the RB26's spec and this should give you better readings. You should adjust the itb's with the RB26 idle as you should be reading the vac spec for the RB26 instead of the RB25. Also, having your timing to far advanced or retarded could cause big differences in vacuum pressure as well.

If the problem is still not corrected, I would pressure test the intake manifold while it is installed but you will have to remove the intake cam so that all the valves are closed to do a proper check.

Can you post up what the vacuum is at base idle in Hg or inches? It would help.

[godzilla_753]

sorry for the super late reply, I ended up figuring out the problem.
Buddy of mine brought over a smoke machine that they use to find leaks, it takes baby oil and has about 5-6 psi of pressure, I plugged up everything i could and closed the throttles, we pumepd up the system with smoke and found a few small leaks here and there. even after sealing it all up tight we were still running at about 9 psi of vacuum.

turns out the rb26dett engines only make about 9 psi of vacuum from factory, after swtiching to the ITB setup I guess the vacuum value changes due to the way the vacuum is gotten from this new setup. car runs great, boosts fine, there is a little bit of a high idle issue, but I did have the IACV unplugged and the TPS is adjusted correctly.

Im picking up a boost controler tonight, im gonan turn up the boost to 13 and see what it can do,
Ill post up some pics for your guys,

thanks for the help.