Target afr's for na engines

[dandan]

Guys,

Just after some numbers from you regarding desired air/fuel ratios in normally aspirated machines.

I am currently monitoring mixture in a well modified 4 cyl 8 valve A Series engine and modifying carb adjustments for better mixture readings after gas flowing head, intake, adding decent exh and manifold.

Car has 270 and 280 degrees of duration on inlet and exhaust respectively and pulls clean from 2200 to 6500rpm. This is with an HIF44 carburettor and distributor based ingition.

AFR's as follows:

Full throttle:- Ranges from 11.0 to 13.6 as revs climb to 6500rpm.

Part throttle/Cruise:- Hard to quantify but typically 16:1

How do these numbers sound? I've got my Supra TT hitting 11.0 to 11.5 afr under boost but figured we could go a little leaner on an NA engine.

Under full throttle we're aiming for approx 12.8:1 across the engine speed range, is this a comparable figure to anyone else's NA engine tuning?

Thanks for any input chaps...

Dan

[Martin F]

This is purely a theoretical opinion but i think you'd be safer aiming for 13.6:1 on an NA engine. The reason we tend to be a little richer on TT engines is because there are so many variables, but on an NA engine i think things will be less changeable.

Best to wait for an experienced opinion though.

[alex h]

Moved as it's nowt to do with Supra's http://www.mkivsupra.co.uk/iB_html/non-cgi/emoticons/smile.gif

I can't be more helpful than that I'm afraid http://www.mkivsupra.co.uk/iB_html/non-cgi/emoticons/smile.gif

[Syed Shah]

From what I have read,

Tuners like to get AFRs to 11.5 or so on a turbo car and WOT, and around 13.5 on a NA engine. The leaner it gets = more power, but less of a safety margin, and less cylinder cooling from a rich mix.

Hopefully CW can give you some input on this.

[dandan]

Thanks chaps,

Anyone else care to offer some input?

[Martin F]

I just happened to be reading a book this morning that answered this question. To quote :

'In a naturally aspirated engine this varies depending on engine load and rpm, with most engines on full throttle producing best power in the upper rev range at an air/fuel ratio of around 13:1 to 13.2:1 '

[dandan]

Thank you Martin,

This has taken a back seat at the moment. Got a few jobs to do on the Supra and the Mini owner is away on holiday. I'll report back how we get on once we're all sorted.

(Some old technology bouncing around on the board now....SU carbs, Webers etc) Excellent...real tuning ?http://www.mkivsupra.co.uk/iB_html/non-cgi/emoticons/wink.gif

Dan

[Martin F]

Today mkivsupra.co.uk

Tomorrow Antiques Roadshow

http://www.mkivsupra.co.uk/iB_html/non-cgi/emoticons/killingme.gif