Effect on power of rich AFR
 

Question for the knowledgable tuners please:

Is there a rule of thumb to follow to establish how much power is lost by having too rich AFR?
For example - Rover 1.8 K-series on throttle bodies etc.. 190bhp@7400rpm; AFR 11.6

Now if the AFR was nearer 12.6 like it should be, how much power may be gained??

i dont know of a rule of thumb mate

You may not gain power. It depends if you adjust the ignition map to suit the leaner, faster burning charge. I doubt you will loose power on a modern head design like a K and you will certainly gain MPG.

yeah, sure, ignition timing needs to be optimised for maximum benefit, but there WILL be a power loss due to an over rich AFR like that??

**edit**
.. interesting read here - http://www.innovatemotorsports.com/resources/rich.php

You cant say there will be for certain as to keep the EGT's safe at a leaner AFR may not be easy on certain engines.

But yes on an N/A motor on TBs with a modern design etc i would expect to see some gain in power.

Somewhere between 13 and 12:1 AFR will give peak power on most engines. Eitehr sid of these figures the timing will need advancing to achieve similar power. This is due to the burn speed of the fuel mixture being fastest around the 12.6:1 region. If you optimise your map for 12.6:1 and then richen up you will lose some power. If you optimise for say, 11:1 and then lean off, you will likely detonate the engine, so no direct comparison can be made really as it depends how the map was optimised with regards ignition advance in teh first place.

The other problem then of course though is if you map PURELY for the max power output, you could end up with so much heat that you dont get that power for long, hence we dont all run cossie's at 12.5:1 even though there would almost certainly be horsepower gains in doing so (hence you see that figure so often on a dyno and so rarely on the road!)