De-catting and AFM settings

[chassis 33]

Who can advise on this, it's been asked several times I think but I'm yet to hear a definative answer. Just de-catted the Chimaera and while there are different tune resistors for cat and de-cat, (i havent swapped btw) there's also different settings for the AFM. From the RPi site...

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

“Air Mass meter setting (non cat). The voltage reading on the CO value here should be between 1.0 and 1.5.

Air Mass meter setting (cat). The best setting for the CO value here is 1.8 volts.”

Do I keep it at 1.8 volts or knock it back to between 1 and 1.5?

Regards
Iain

[Cider Andy]

This is in your racer, Iain? You probably know as much as, if not more than, I do about this. But here's my take on it:

If you still have the original 14CUX chip, now you've removed the cats you should be able to select the non-cat map by fitting the green tune resistor. If the car runs ok with that resistor, check the AFM voltage as per the RPi website. But what's actually important isn't the voltage itself, it's the exhaust emissions once the AFM CO screw is adjusted, as would be measured at the MoT station.

[shpub]

Personally I wouldn't touch the AFM meter unless I had a gas analyser attached as this is needed to calibrate the AFM. With a decatted car, it is virtually impossible to meet the emissions anyway so if this is for your racer then I would take a different tack. De cat and then get the ECU chipped. This will allow the tuner to not worry about the emissions so much in terms of power and driveability. Playing around with the AFM could cause more problems than it solves.

[Griffter]

The AFM 'trim' adjuster seems to control a fixed offset voltage applied to the AFM signal. It has an effect on CO at idle, but the AFM output dwarfs it as the air flow rises and so the trim has a negligible effect as the engine revs/load rise. I paid for some rolling road time to find this out, but really it needs to be set up on the RR with a CO meter attached to get it right, then left alone.
I think it's best regarded as an idle CO adjustment. Problem is a chip or timing change will change the idle CO, so strictly speaking you could end up adjusting it quite a lot. I think the RPi values are a guide, and if it needs tweaking to get the idle emissions right for the MOT then so be it.

[chassis 33]

Thanks for the advice, it's not for the racer, that was decatted and RR setup ages ago with various bits and bobs tweaked.

I've swapped in the dustbin from the racer so that I can swap back the original come MoT time, the problem is neither car have the original chips, so I doubt the settings from the racer will be the same as the road car, likewise I don't know if the "aftermarket" chips come with several layers of map to suit the different tune resistors.

I guess next step is to consult the powers that be to see what they say. I'll report back.

[spend]

If you want to swap the cat back in for MOT, I would have thought leaving the AFM be would be the best bet. By all means plug in a resistor (2p) for decat (but remove & plug the lambda sensors - forgot 1st time).

I have run mine with no cat without changing anything, it runs OK - but its too loud for anything but short bursts....

Dave

Edited by spend on Thursday 28th December 19:09

[GreenV8S]

If you've just swapped out the Y piece then presumably you still have the lambda sensors in the manifolds, in which case I think your best bet is to keep the whole ECU in the original 'cat' configuration and hope the cat removal doesn't throw the fuelling requirements too far out.