Davejordan

Boyan

jim lockwood

It depends on how even your engine's mixture is from cylinder to cylinder. It also depends on whether you are running gasoline or some ethanol enhanced fuel

IMHO, you wouldn't want you leanest cylinder to be any more lean than 15.5:1 at cruise if you are using gasoline. If ethanol enhanced fuel, that maximum lean mixture would be 14.9:1.

68hemi

It also depends on your elevation sea level vs. 7000-8000 ft. will effect it since the air is thinner the higher the elevation.

jim lockwood

Um, yes and no. Yes, the mixture will change as elevation changes if you do nothing to compensate. But the target air/fuel ratio remains the same as what I wrote.

SWCDuke

...depends on operating condition. Best idle is usually in the range of 13.5 to 12.5:1. At WOT it should be no leaner than 13.5 at peak revs. Cruise - as lean as possible without "lean surge", which is probably about 14.5:1.

Duke

jim lockwood

What fuel are you assuming?

66coupe427

Please explain "lean surge". Thx

SWCDuke

Modern E10, but it won't be much different with straight gasoline as the E10 O2 content is only 3 percent by mass.

In my "Tale of Two Camshafts" article the first cam design for a '67 327/300 resulted in some lean surge during road testing, so one size larger main jet was installed in the OE Holley, which eliminated the problem, and a cruise A/F test during the dyno session yielded 14.5:1 on E10.


The first design cam (Special 300 HP cam) had a later closing inlet valve than the second design (McCaw Special) cam, and I attributed the lean surge to higher reversion at low revs due to the later IVC.

Lean surge is a slight roughness at steady state cruise or hesitation on light acceleration that is usually caused because the leanest cylinder(s) are misfiring intermittently due to a lean condition.

Fuel injection engines can run at lean as about 15.5:1, which usually delivers best fuel economy, but modern engines are set up to run at stoichiometric, because stoic. creates the least "engine out" emissions and maximizes the catalyst efficiency.

The straight gasoline stoic. ratio is usually about 14.7:1 for most blends, but since E10 has a small amount of O2 the actual A/F ratio is slightly richer in order to consume the extra O2.

Duke

cbernhardt

Here is Holley's target AFR table for a 350 cu. in. engine running gasoline.

AZDoug

I am going to guess they are basing that on a lumpy cam, (lets cover all bases), in the motor that will have reversion and dilution at intermediate throttle.

I am running about 15.5 on my motor in the intermediate RPM and MAP ranges. Of course it was chassis dyno tuned.
Doug

cbernhardt

Yes, this is just their base tune and they expect each user will tune it to suit their individual needs.
I think they make it somewhat rich on purpose.
Charles

jim lockwood

That 14.5:1 must have been an average of all the cylinders in a bank; it's hard to believe it represents the most lean cylinder. On an FI engine with relatively better mixture distribution, 14.9:1 is easily achievable on E10 without lean surge.

Dad's '66 427

Forgive me for the question, as this is out of my realm, but how are you determining A/F ratio at each cylinder? Sensors in the intake or exhaust at each cylinder?

jim lockwood

One way would be to monitor exhaust gas temperatures at each exhaust port since air/fuel ratio can be correlated to exhaust temps.

I don't have that capability. I use a wide band air/fuel gauge which samples an entire bank of cylinders. I get away with doing that because I'm monitoring air/fuel on FI engine and all cylinders will be very, very close to the same.

Carbureted engines..... not so much.

tbarb

With the carbureted engine fuel distribution is the limiting factor so it will be richer by nature.

Jim, I bet the fuel injected engine runs very nice at 14.9 - mid 15's A/F. When you can tune them lean the left over hot oxygen in the exhaust gets sucked back in with valve overlap to aid the combustion of the next cycle.

jim lockwood

GM's target AFR for Rochester FI at cruise was 15.5:1. I adjust FI units for about 15.3:1 to allow for the possibility of measurement error.

SWCDuke

Of course. On a chassis dyno test there is usually one wide-band O2 sensor, so it's measuring an average on one bank. Due to uneven mixture distribution in carb/manifold systems you have to run an average richer A/F ratio than fuel injection systems, which, if properly designed and manufactured have much more even fuel distribution and can tolerate leaner mixtures.

Since it took one size larger jet on that Special 300 HP engine to eliminate the lean surge, we knew that the cruise ratio was as lean as possible and the 14.5 average from the O2 sensor is probably a ballpark maximum lean number for typical carbureted engines.

Duke

Dad's '66 427

Thank you for the explanation.

When I was tuning our blown alcohol dragster, we had two port nozzles with jets that we would use to tune. During idle, the fuel going through the top of the blower kept the motor running, but during the run it was mainly port nozzles. One set of nozzles was used for Stage RPM, we called them 'dribblers', that only flowed fuel until the barrel valve was opened fully and we cracked a check valve to flow fuel to the other nozzles that we called the 'tuners', for WOT and the remaining of the run. I would make adjustments to the nozzles based on exhaust temps, taken at the header just out of the exhaust ports, and based on what the motor was telling us with squished rod bearings. With the blower, even with a diverter plate, not all cylinders got the same air, so as you described, we made the correction with the port nozzles. The jets in the nozzles were all over the place, balancing the cylinders. I was just wondering how you were coming up with the A/F ratio at the leanest cylinder, which I now understand you did with the whole bank.

Looking forward to more discussion like this, figuring out how to make these cars run better!!!

Davejordan

I Dyno my Stock L79 66 Vette. The reason I was asking about the A/F mixture was the Vette made 170 RWHP SAE. The Dyno Guys said the engine was running way to lean at 15.5 for this engine and the spark plugs were white/gray. They estimated the RWHP loss to be 50hp or more. I took the Vette over to a Chevy Tuner recommended by the Dyno guys. He said the Holley Float Bowls were too low, the timing was off, and the ethanol 93 gas was a serious problem for a 11:1 Compression Engine. So he reset the Timing, raised the floats, and put in one gallon of Leaded Racing fuel 110 octane to 10 gallons if 93 ethanol gas. I haven’t got to the Dyno yet but I feel an improvement in power and the engine seems happier. You think this is just in my head?