My mpg, what do you think?

[Woolfi]

Hello David !
1. A AFR of 14,7 is Lambda 1, which is stochiometric perfect.
For max power under full load lambda of 0,85 is best.
If you are driving with AFR 12,1 under heavy load you have a lambda of 0,823, which is a litle bit to fat. The max power is slightly lower
AND the mileage is also slightly worse than with Lambda 0,85. Therefore a "loose-loose situation".
The problem with carburettors is, that if the average lambda-reading of one carburettor is lets say 14, you have a spread of lambda up to 25% between the cylinders. Therefor 12,1 can be 15,1 or less than ten. Also elder EFI-systems have this spread between different cylinders.
2. You motor is not original, has more power than the original motor and you like to have this power. For best mileage a AFR in light cruising should be between 16 and up to 18. If you would have a thicker needle, you mileage would be higher. Maybe you would reach also 22 up to 25 mpg with your car. If you prefere full power under all conditions,the cost of this is a poor mileage.
David Vizards wrote in his famous book "Tuning the A-Series Engine" that the AFR for best mileage should be higher up to AFR 20/22.
If you would have needles , which are thicker (leaner) between 0mm and lets say 8 to 10 mm high of needle, your AFR in light load would be higher than your 14:1. With this higher AFR your car would have a better mileage. Roger Bywater told me, that the V12 HE-motor is running well with a cruising-AFR of roundabout 18 to 17: 1 and was able to run with a AFR up to 22.
The problem with the thicker (leaner) needle is, that if you press the pedal to the floor at less than 2000 rpm, the AFR would stay weak for some seconds, until it is becomming rich at lets say AFR 13.
This weakness is causuing a roundabout 4 - 8% lower torque. If you want max torque at every rpm when suddenly pressing the peda to the floor, you must have a needle which is slightly fat also in light cruising. With a simple constant depression carburettor it is not possible to have a weak light load and also a fat mixture, if you suddenly press the pedal to the floor.
There have been SU-carburettos like the HIF38 in the Austin Metro, which had a weakening device, to have a weak mixture at cruising and also a fat mixture, when suddenly accelerating strong. David Vizard has described this device in his book.
I hope you can understand my words, because my english is very "weak" (AFR 25).


Regards Wolfgang Gatza

[PeterCrespin]

Very good advice Wolfgang thanks.

My issue with David's A/F readings are that he has his probe in the tailpipe IIRC. I'm no expert (and on a given car it may make no difference) but the AF ratio read at the tailpipe may not be exactly the same as at the manifold. There can be small air leaks along the system at the joints and as I understand it any leak will tend to make the A/F look weaker than it is.

So David *might* still be running a little rich even if his tailpipe sensor told him he was at stochiometric, which it doesn't - it says slightly rich. In fact it is pretty rich for cruise, given it ought to be well under stochiometric under those conditions.