[wakesupremo]

I have fitted bosses in my exhausts for o2 sensors to set up my carb and try and improve my fuel economy. What is ideal AFR at idle and high revs under load?

[62posbonny]

Every engine combo will want something a little different. That said, mine likes 13.8 idle, about 12.6 at WOT. This is on a 455 with an Ultradybe 280/288 cam.

[Formulajones]

That's a loaded question. Every engine will be a bit different and timing curve will play a roll in what afr's the engine will like to see.

I find timing with a good vacuum advance curve plays a much bigger roll in mpg than AFR.

I make sure the engine has a solid 44-46 degrees at cruise with vacuum advance all in first.

From there Ive gotten as much as 21 mpg with these old cars with an afr only in the 13's at cruise. I generally dont run them any leaner than about 14:1 anyway as I've found no real return in mpg beyond that. Going too far just has diminishing returns.

It's interesting to play with, just don't get too hung up on a certain number and beat yourself up over it.
Drivability is the main goal and the AFR gauge is more of a tool to see where it lands for future reference.

[ta6point6]

I try to hit 14.1 for emissions and fuel economy. It is interesting to watch the numbers but they do change with outside tempature, time of day and highway driving. This chart kind of shows it all

[Lee]

"just don't get too hung up on a certain number and beat yourself up over it."

TOTALLY agree. The numbers are reference points, not magic. Your engine could like an AFR that is a bit different than other motors. Your wideband reading could also be off a bit for some reason. 13.8 to 14.0 usually works great for most motors - but find what value works BEST for your motor. 12.8 seems to work well for most naturally aspirated pump gas motors that I've worked on, but sometimes a motor will run better at 12.9 or 13.0, or maybe even 12.7.

Experiment. Take notes of what you try and the results. Re-test, to make sure the data was accuate. Then, come back and share what you found :-)

[TCSGTO]

I agree don’t chase AFR numbers. Just remember the AFR number is an average of 4 cylinders on one bank, each one of the 8 cylinders will be at different AFR’s. Give it what it wants.
It’s not uncommon for these carbureted engines to have ratio spreads like 10-16 from one cylinder to another and average 13 even though one hole is pig rich and one extremely lean.

[AG]

For reference only, my street car cruises with an AFR of 15.5 and WOT is at 12.7. My race car has the best ETs with a 12.8 AFR and I've tested between 11.5-13.2, a lot of jet changes at the track. I use a dual sensor FAST wideband.

[Half-Inch Stud]

Still Carb'd here. Here's a clue: the atomization extend matters for power. So the O2 sensor may indicate the residual oxygen in the exhust, so what to do?

1. Increase fuel QTY.
2. Change FI fuel atomization to more vaporous.
3. Change FI fuel atomization to more dropletous.

Review 2 & 3 to attain the least oxygen.

[Brentco]

Originally Posted by ta6point6

I try to hit 14.1 for emissions and fuel economy. It is interesting to watch the numbers but they do change with outside tempature, time of day and highway driving. This chart kind of shows it all

That’s a good graph. But the line showing best fuel economy at 16 is a bit misleading, IMO. That might work in some instances with modern computer controlled fuel injection cars. But with carbed cars, going any leaner than stoich (14.7) generally gives you no better, or even worse, fuel economy, as Formulajones said. You wouldn’t want to go for 16 anyway, because that’s right on the knife edge and normal fluctuations could bring you up into engine damaging lean territory unless you manage your tune like a hawk.

[wakesupremo]

Thanks again for all the answers. In my head I was aiming for 13.7 ish so wasn't far out.

[Formulabruce]

Originally Posted by Formulajones

That's a loaded question. Every engine will be a bit different and timing curve will play a roll in what afr's the engine will like to see.

I find timing with a good vacuum advance curve plays a much bigger roll in mpg than AFR.

I make sure the engine has a solid 44-46 degrees at cruise with vacuum advance all in first.

From there Ive gotten as much as 21 mpg with these old cars with an afr only in the 13's at cruise. I generally dont run them any leaner than about 14:1 anyway as I've found no real return in mpg beyond that. Going too far just has diminishing returns.

It's interesting to play with, just don't get too hung up on a certain number and beat yourself up over it.
Drivability is the main goal and the AFR gauge is more of a tool to see where it lands for future reference.

THIS can save you time and frustration. Every gasoline has different amount of alcohol, and most want richer mixture than the AFR which were really derived for fuel injection cars, Not Carb engine.

[72projectbird]

If you're focusing on a target number for various driving scenarios, you'll drive yourself insane attempting to nail down those numbers.

Example: I have an AEM wideband in my 71, and sitting at idle it'll bounce between 13.9 to 14.8 as the ecu is constantly moving timing, fueling and idle air around. Plus the overspeed/underspeed table uses ignition timing to make fueling and idle speed corrections. If I tried targeting a specific number at idle, I'd probably rip the gauge out in frustration lol

Under WOT I like to see it between 10.0 to 11.4 at the most with the timing I run. You'll notice when screwing around with a wideband, the numbers become much more stable in reading while doing partial throttle cruising and WOT pulls.

Like others have said, dialing in your timing is a great place to start. If you're chasing fuel numbers, but the timing is bouncing around or becoming unpredictable under load, targeting AFRs will be a nightmare.

[wakesupremo]

I fully understand what everyone is saying about not chasing the numbers. I really want to find out how far out mine is because 1 1/2 mpg is no joke. I know that ive got a fast car and, at the track, its great but it has taken away the joy of street driving on a nice sunny day.

[Formulajones]

I completely understand the mpg thing. I've always paid attention to mpg and tuned accordingly. I would not or could not enjoy these cars on a daily basis if they were gas hogs.

The AFR gauge is useful in the fact that it'll tell you if you're way out in left field or in the ball park.

I as well as others would be interested if you report back what you find.

[johnta1]

If you're wanting better fuel mileage, use a vacuum gauge. It would help more probably?


Any use under heavy load will not be good mpg.

[ta man]

Originally Posted by wakesupremo

I fully understand what everyone is saying about not chasing the numbers. I really want to find out how far out mine is because 1 1/2 mpg is no joke. I know that ive got a fast car and, at the track, its great but it has taken away the joy of street driving on a nice sunny day.

What is your engine combo and drivetrain?

[1980 TA]

I'll second using a vacuum gauge. When I'm tuning my Fitechs, I always pay attention to the MAP readings. I'd recommend trying to find a test loop that you can drive consistently and get a feel for your vacuum readings under fairly consistent road conditions.

For me, it is running to town and back. I get speeds from 25 to 70, small and medium hills. After driving it as many times a week as I can, I've got a feel for the MAP readings or vacuum in your case. As I've been adjusting things, I'm trying to get the lowest MAP readings (highest vacuum). For my Lemans going from 14.2 at idle to 13.8 , picked up 3 inches of vacuum. It seems to like 14.2 for general cruising.

My TA likes 14.5 at idle and 14.1 cruising.

As for MPG on the same back and forth loop, my 2018 silverado gets 13, the Lemans 8.5 to 10.5 and the TA 10.5 to 13. I'm hoping the OD swap for the Lemans I'm working will boost the mileage some. With the Pontiacs, the biggest obstacle is not enjoying the power as much as it really plays havoc with the MPG. Both Pontiacs are 455's with FITECH.

[ta6point6]

My AEM is on my 79 350 th400 carbed truck. I do not take on highway to often but when I do I see 15's, yes I drive slow 75 stay alive. When truck is warm and coming back from junkyard after a 30 minute ride it hits 14.5 and leaner at times usually at 55-60 mph. My truck is stock with most of the emissions vacuum stuff unplugged. Last time I checked my truck gets about 12 mpg which I think is pretty good for trucks called squares. I have vacuum connected to distributor, transmission module and that is all. All the other TVS switches are disconnected on truck. I know 16 is mainly for fuel injected vehicles and fuel economy.

[wakesupremo]

Originally Posted by ta man

What is your engine combo and drivetrain?

Its a 0.060 over 1968 400 with a 4.25" stroke. Fully forged, 6.8" H beam rods Lunati 4/7 swap cam and Johnson 1.65 roller rockers ,victor junior manifold with Holley 850 DP. All MSD ignition. It made 600HP at 5700 RPM and 600 ft/lb torque at 4700 RPM. Has run 11.2 @ 122 mph best but regularly runs 11.4 @ 120mph.
Its a 1969 firebird convertible thats great to go for a sunny evening drive but the MPG is ruining that experience.

[ta man]

Originally Posted by wakesupremo

Its a 0.060 over 1968 400 with a 4.25" stroke. Fully forged, 6.8" H beam rods Lunati 4/7 swap cam and Johnson 1.65 roller rockers ,victor junior manifold with Holley 850 DP. All MSD ignition. It made 600HP at 5700 RPM and 600 ft/lb torque at 4700 RPM. Has run 11.2 @ 122 mph best but regularly runs 11.4 @ 120mph.
Its a 1969 firebird convertible thats great to go for a sunny evening drive but the MPG is ruining that experience.

What heads and gearing? My car gets 16 to 18 mpg and a buddies will get close to 20mpg on the highway. Both have tight convertors..gearing from 308's to 373's we have both been in the same mpg range. Both cars run in the 10's at 125mph plus. We both use SD KRE heads and OF 2.0 cam.