Post by
Q45tech »
https://forums.nicoclub.com/q45tech-u112.html
Wed Jul 24, 2002 2:29 pm
The 90-96 Q throttle body is ~ 3" [77mm], from the pictures of the Maxima unit it appears significantly different [not in function ] but lay out. 3" [7 sq. "] is suffucient to flow 350Hp of air.
Commetary:Nissan always uses a larger than normal TB vs air flow requirements [unlike domestic designers] which makes the programming of ecu kind of critical..the closed vs WOT MAF voltage and how this relates to engine demand [load]. Notice that even at 60 mph the throttle is barely opened.
The point is: even 5% more area will upset drivablity [ability to make fine linear increments with the accelerator pedal].Generally the whole intake tract is a balanced system of components and each component is allowed only so much restriction [the Nissan limit for air horn through filter, MAF, connecting hose and throttle body is around 21" WC in total] with the TB flowing around 7" of the total restriction at the rev limit [roughly the same as the MAF] and the air box, filter, and input as a system [7,7,7].
On NA engines [like Q, G20, J30] changing the TB has resulted in some power increase [usually only 1-1.5% ......because the runners and plenum are also a tuned system [volume, diameter, and length to match the cylinder] at the expensive of drivability!
Measuring the plenum vacuum at WOT tells you the restriction. Unfortunately the typical vacuum gauge is designed for idle [18-22" HG = 9.8 negative PSI] and 28" WC. is only 1 psi] so you need to measure [with accuracy] the range between 0 and 1" HG which requires the Dwyer Manometer or taking the TB to a flow bench [which Nissan spent time on to pick the one you have].
Before you go changing, measure your differential vacuum [of the existing TB].A cheap trick is to smooth the throttle plate center support and the screws which hold the plate on. This curve bump in the middle causes 20% of the restriction. Careful as you don't want to remove too much of the mechanical strength.
Doing any mods to the TB forward is an exercise in minute improvements, much like working on the cylinder heads [port matching] as hundreds of engineering hours have been spent already to squeeze as much as they can out.
Stillen use to sell a massaged TB for the Q it yielded 3-4 HP at best and had funky linearity except at WOT.
I really don't think dropping the [TB] restriction by 10% or 0.7-1.0" WC is worth the effort unless you are supercharging or running NOX
If a mod doesn't create 10 HP [5%] it is probably unnoticeable especially if it only occurs at the peak of the rev band.