The V8 Owners Forum -

Hi Guys, I need some help as to what what AFR's and ignition retard to try and run at when under boost?

Engine: 4.6RV8 with 9.4:1cr, (standard 4.6 pistons, top hat liner block) piper V846BP 270 cam. Using megasquirt II extra with coil packs, plugs 1 grade cooler, WB02 sensor, procharger centrifugal supercharger, intercooler, in a Dax Rush (700kg).

I need to fill in the AFR look up table to help in calibrating the ECU, I'm confidant with AFR for idle, cruise, low load etc but need some help as to AFR to aim for when at 1psi, 2psi, 3psi .... 10psi.
I don't know how much boost it will make, I'm hoping for 10psi but it all depends on my SC if its man enough?

Also how much ignition retard per psi of boost?
I've read threads on engines running roots blowers sugesting 1deg/1psi but maybe I could get away with less? more efficient supercharger+intercooler+light car?

All help much appreciated
Tom.

Hi Tom!
I'm out this weekend testing my charger setup. I'll let you know how the ign timing goes as I'm starting with 1deg retard per pound of boost.

A safe setup would be 1 deg / psi. And 12:1 afr
Then work your way back from there. Personally, i only got back to about 12.5 ish and stayed there - my rationale was my wideband could be at least .5 out, so worse case i'm only 13:1

Just scored another wideband this week, which will be my "roving" wideband for diagnostics and tuning other cars. I'm going to try making an exhaust clamp for it.

I wouldnt run a boosted engine as lean as 12.0.

Some do and get away with it. But I wouldnt take the risk.

Perhaps if was only 5psi or so without too much hot air it might be ok. But if you do get 10psi, and have no intercooler etc...

I'd be playing safe into the 11's Certainly at the higher boost levels

Thanks for the comments.

That's given me a starting point. I still have a lot of unknowns so will start very rich and retarded and gradually take fuel and spark retard out keeping an eye on plugs and trying out my home made microphone/clip/amplifier/headphone knock detection device!

Tom.

Dont go too rich or retarded though, as both of those can cause problems.

Aim for say 11-11.5 and you will be safe. And as for timing, perhaps using the 1deg per 1psi boost over base n/a timing may give a safe starting point. But there is no guarantee. Just have to try it and see