RB25 DET idle speed control on heavily modded engine

[Chris Wilson]

I am heavily modding a Nissan Skyline R33 GTS-t RB25 DET engine. Big

external wastegate turbo, 280 duration cams, steel internals, Motec ECU, blah blah.

My question is idle speed control related. Why did Nissan have 3

separate means of idle speed control? It has a bi-metallic strip cold

start idle up device under the plenum, a solenoid for A/C on and PAS

load idle up, and a stepper motor device for normal idle speed control

(and a throttle butterfly and accelerator pedal of course... :-)) My

goal is a track day car, perfect idle control isn't too high on the

agenda, A/C is ditched, and with the cams a fairly high idle is needed

anyway (1200 RPM or so). Emissions aren't too much of a worry either. As

such should i just run a conventional stepper motor idle control and

blank off the other 2?

I am confused as to why Nissan didn't just use the stepper motor to

control the idle though, it seems unnecessarily complex.

Thanks.

[MonstaS4]

The Nissan AAC valve is not a stepper motor. It is a solenoid valve which is fed a PWM signal (constant frequency - duty cycle varies from 0 - 100%) from the ECU. My guess is this valve alone dosn't flow enough air to keep the idle high enough when under extreme load. i.e engine still cold, a/c on, power steering pump under heavy load. If they made it flow higher, it probably wouldn't control accurately.

I'm running a Haltech E11 ECU on my RB30DET. I still have the cold air bypass valve hoses connected (but no power to the valve). The a/c idle-up solenoid is disconnected and the haltech drives the AAC valve. Cold idle is set to 1200rpm and warm idle is set to 850rpm and it controls well, although the idle can drop to 700 if the a/c is on. Hope that helps.