Air Fuel Ratio For Optimal Performance

You need to get in touch with some of the Northwest owners on this site, who regularly dynotest their cars. In fact, "navygakman" (aka Stewart Chung) has/had the .pdf copy of his test which listed the A/F ratio at each point of power development. His carb car is modified, done by Matt "Kermit" Morgan. The numbers you list are in the ballpark of his.

(I have this .pdf copy, but don't want to dispense it out without his prior permission...)

 

Philip
I recently dynoed my 308 to re-jet to suit the new exhaust - A/F at max power was 14:1.
Don't have data for any other revs as it was an old dyno that doesn't produce data printouts
Hope this helps a little
BTW, Dyno time was the best investment I made
John

 

On a dynojet dyno on 3rd gear, my QV likes to be around 13.0-13.2. 1st gear liked about 12.5, 5 gear about 14. The "right" A/F ratio depends on the rate the rpm is changing and how long you want the engine to run. I've seen drag racers jet up and go faster until they got to about 10, then more didn't help. I've also seen road racers on a long fast track (Daytona) go jet lean and pick up mph at the end on the front straight until they saw specks of aluminum on the plugs. The private guy then go back up one jet size, the factories leave it as is and replace pistons at the end on the race. I run my street cars, including the QV at about 13 because it's a good balance, it doesn't foul plugs on melt pistons. It's basically impossible to keep a carb car at one A/F ratio across the whole rpm band, carbs just can't control the mixture that accurately, I'd try to keep it between 12-14 in warm weather, if you're tuning on a cold day, stay to the rich end, maybe 11-13, it changes a lot with temp and air pressure.

 

Hi Philip,

Air/fuel ratios are considered in either steady state, or transient conditions. Transient also includes warm up, in addition to acceleration.

At idle, about 12.25:1

Steady state: 13.2:1 (you will have peak BMEP here)

Start and warmup: very rich to begin with 11.0:1 to 10.3:1, more fuel required as ambient temperature drops

Acceleration:
This is dependent on throttle velocity (change in opening position rate). Faster opening requires richer mixture. At normal operating temperatures, no richer than 12.7:1

Deceleration:
Lean to control backfires, though no leaner than 17.1:1

(Credit Joe Pettitt's book on engine building for most of the above information).


You can tune on a dyno, though a Dynojet is an inertia dynamometer, and cannot hold steady state. It's fine for full-power pulls. Tuning is best done on an eddy current dyno.

Philip, is your wide band advertised as a true UEGO? Bosch makes a nice compromise 'almost wide band' Lambda sensor for about $150, and reading voltages directly gives the same information.


Best regards,

Rob Schermerhorn

 

Here's another thought.

If you are not octane limited, the 13.2 nuber is probably right, it matches what I got on the dyno.

If you are trying to run pump gas with high compression or a lot of boost, it seems to work out better to run rich. The rich mixture lets you run more timing without detonation and pulling the timing back robs more hp than the rich mixture does. I never got a chance to play with this on the ferrari because I broke a fuel injector the day I was at the dyno, alls I got was the mixture data. My friend the the drag car running 10 A/F has a 12.5 CR chevy and runs it on 93 octane.

I think if your engine is stock, the factory timing specs are probably right and you shouldn't have to worry about detonation, so shoot 13.2.

 

I used lamda readings when setting up my motec. .87 for full throttle and 1 for cruise. How does that relate to a/f ratio? I note that I'm not getting as good fuel consumption as I should, so I'm not convinced that lamda 1 at cruise is correct.

 

This site has a lambda-to-AF ratio calculator (scroll down the page a bit):
http://www.rbracing-rsr.com/rsrgauge.htm
For benzina, I think it's as simple as:
lambda value*14.7 = air/fuel ratio
(so for lambda =1, the AF ratio is 14.7:1, and for lambda = 0.87, AF = 12.8:1)

 

First, optimal AFR can only be determined experimentally

On a normally aspirated vehicle, start with the fuel you are going
to run and then tune for 12.5:1 afr. This can be done on the dyno
or on the street using some form of UEGO sensor.

From there, to the dyno you go and slowly lean the vehicle out
until you have both max HP and max Torque. Which, I can assure
you will occur at both different AFRs and different Spark Timings!

(With EFI, this is a matter of keystrokes.. you carb guys better have
a full jet kit and a few days handy )

The key is to either hit peak numbers (not "knock-limited) and leave it,
or to hit knock (knock-limited) back it off a bit (make it richer) and then
leave it.

If you hit 13.5:1 afr you are pushing the envelope extremely.

The actual AFR depends upon engine design, compression ratio,
carb/EFI atomization efficiency, and about 100 other factors which
all relate to BMEP.

With a forced induction vehicle, depending on the fuel and boost you
can start as rich as 10.0:1 and lean from there.

Also, be very wary of "bargain UEGO" sensors. Some work well, and
others don't. A bad UEGO is worse than NO UEGO.

Jim Conforti