CO Trim values / RPI guide to setting up

[nig-gtv]

The RPI website suggests setting the CO trim values on the AFM using a voltage set to 1.8v (although there seem to be a few posts on the forum about people setting then significantly differently)…….

Does anyone know what effect changing the voltage has… does an increased voltage increase the CO and make the car run weaker? Or decrease the CO etc etc?

RPI guide:

http://www.rpiv8.com/carbs-2.htm#setting-up

Cheers

[griffter]

I spent sometime on the Rolling Road trying to figure exactly this out. Basically, increasing the trim voltage increases idle CO. Difference throughout the rev range under load is negligible. Some people claim that tweaking the CO trim can produce pops and bags, but my experience doesn't back this up.

Isn't over 1.5v for a Tornado chip? I thought standard chips generally needed 1.1-1.5v.

[blitzracing]

I did try setting the trim value against an air fuel ratio meter, but ended up with very erratic fueling once off tickover. Once I went back to the DC settings its fuel mapping is rock solid, so I would not recommend doing it any other way. Ive been told it has some influence up to 2500 rpm, which ties in with my experiance. The real problem, is there are no circuit diagrams or block diagrams to explain exactly whats going on, so the myths prevail

[nig-gtv]

griffter said:

I spent sometime on the Rolling Road trying to figure exactly this out. Basically, increasing the trim voltage increases idle CO. Difference throughout the rev range under load is negligible. Some people claim that tweaking the CO trim can produce pops and bags, but my experience doesn't back this up.

Isn't over 1.5v for a Tornado chip? I thought standard chips generally needed 1.1-1.5v.

Ok - cheers, so does increased CO mean less oxygen therefore a richer mixture?

From memory I think it says 1.8v for a standard catted v8.

[griffter]

Yes, increased CO = richer.
I think the trim adjust basically works by adding a bias voltage to the air flow meter output. So by increasing the trim, 3.0v become 3.2v for example. The ECU 'thinks' more air is flowing through so it increases the fuelling to suit. Thing is 0.2v as a proportion of 3.0v is negligible (roughly 7%). 0.2v as a proportion of 1.0v is more significant (20%), hence the greater effect at low air flow rates i.e. at idle.