Carburetor Air Fuel Ratio

[Timothy B.]

My question is about tuning a carburetor for best lean cruise A/F ratio 35-45-55-65 MPH, and the main jets are reduced to where the engine surges then up one size at a time until it's steady with no surge. At this point is the ratio considered 14.7 or could it be leaner than that.

In reading, this is the correct procedure but not sure how to tell if it's leaner than stoichiometric. After determining this, a 25% increase in sqin. area for power enrichment would equal 50% of the main jet diameter which should yield 12.5 A/F ratio for best power. This would compute to the PVCR size, so if the main jet ended up at .065 (#65), the PVCR (power valve channel restriction) would be approx .033 (50% of the main jet size) compared to the .039 that is in my 3810 holley.

It's my understanding the holley carbs are rich at cruise and power enrichment..

Leaning main jet cruise this way so the engine is telling you how much fuel it wants could also be double checked with a O2 sensor if a bung is installed in the header pipe and read with a DVOM showing the voltage at cruise. I read 450MV (millivolt) is stoichiometric so we would not want to be leaner than that at cruise conditions.

All thoughts are appreciated..

[Clem Z.]

Re: Carburetor Air Fuel Ratio

My question is about tuning a carburetor for best lean cruise A/F ratio 35-45-55-65 MPH, and the main jets are reduced to where the engine surges then up one size at a time until it's steady with no surge. At this point is the ratio considered 14.7 or could it be leaner than that.

In reading, this is the correct procedure but not sure how to tell if it's leaner than stoichiometric. After determining this, a 25% increase in sqin. area for power enrichment would equal 50% of the main jet diameter which should yield 12.5 A/F ratio for best power. This would compute to the PVCR size, so if the main jet ended up at .065 (#65), the PVCR (power valve channel restriction) would be approx .033 (50% of the main jet size) compared to the .039 that is in my 3810 holley.

It's my understanding the holley carbs are rich at cruise and power enrichment..

Leaning main jet cruise this way so the engine is telling you how much fuel it wants could also be double checked with a O2 sensor if a bung is installed in the header pipe and read with a DVOM showing the voltage at cruise. I read 450MV (millivolt) is stoichiometric so we would not want to be leaner than that at cruise conditions.

All thoughts are appreciated..

fuel distribution in stock manifolds is not that good so i would not lean too much for WOT. MPFI engines can be leaned out because the have equal fuel distribution to each cylinder

[Duke W.]

Re: Carburetor Air Fuel Ratio

See the Fall 2010 Corvette Restorer - article by Williams et al.

Duke

[Bruce B.]

Re: Carburetor Air Fuel Ratio

Tim,
If you are interested in a good wideband O2 sensor look at ones made by Inovate Motorsports.
I have a MTX-L (similar to thier LC-1) in my 62 Corvette. It comes as a kit which includes the sensor, a weld in bung, the controller and a gauge along with softwear. It is plug and play.
It is a nice unit and in addition it the gauge it provides data to the ECU I am using.
The output is 0 volts =7.35 AFR up to 5 volts = 22.39 AFR.
A very handy tuning tool.

[Gerard F.]

Re: Carburetor Air Fuel Ratio

Tim,

When I first restored my replacement 3810 on my 67/327/300 I left the .062 main jets on, which came with it, figured it would be great for economy. On a cruise, especially uphill, it kept overheating.

Not sure it was the jets, but when I changed the jets back to .065, the problem went away. Maybe it was a vacuum leak or something else, but the change to the stock jets made a difference. I still do 17.5 mpg on a trip.

I might be all wet, but that's my experience.

[Duke W.]

Re: Carburetor Air Fuel Ratio

Rather than messing around with installing wideband O2 sensors on your car, you are probably better off finding a Dynojet chassis dyno with wide band O2 sensors that can be inserted into the tailpipes.

You can get both WOT A/F data (should be in the range of 13.5-12.5:1 at high revs) and part throttle data by lightly accelerating from 1500-4000 (keep manifold vacuum less that what is required to open the power valve) like John McRae did with his Special 300 HP engine. Typical behavior is that went starting a WOT pull from low revs the A/F is overrich at low revs but leans out as revs increase, but as long as it doesn't go leaner than 13:5:1 at peak revs, the WOT mixture is okay.

The article also explains why you usually can't get down to a stoichiometric cruise A/F ratio with a carburetor/manifold induction system before you hit lean surge.

In addition to the A/F data, making WOT pulls from 1000 to the redline will yield the full range performance curves of your engine.

Rather than making changes to the fixed primary powervalve channels (from a practical standpoint, you can only make them bigger by drilling them out) make any adjustment to the WOT A/F ratio by changing the secondary main jet as required.

If your engine is not significantly modified, you will probably find that the OE jetting is pretty close as long as you don't live at high altitude and you do the testing in mild temperatures. And remember that carburetors are not very precise fuel metering devices, and actual A/F will vary with air density (inlet air temperature and pressure), so all you can really do is get the A/F ratio into a reasonable ballpark range.

The need for very precise fuel metering to control emissions is why carburetors ultimately disappeared in the eighties.

Duke

[Timothy B.]

Re: Carburetor Air Fuel Ratio

Jerry,

I am curious if the car with .062 jets had a slight surge accelerating uphill during cruise. IMO, I don't think the leaner jetting at cruise would make the engine overheat. How did the engine run with the smaller jets?

Duke,

I will read that article again, I would rather have the engine a little rich than above stoichiometric. I also agree that on our stock engines the factory jetting is probably close to the mark. I am not sure I agree about reducing secondary jet size and leaving the PVCR as doing that would unbalance the carburetor. If you are driving and not using the secondaries the primary barrels would be rich, secondaries barrels would be lean.

If the .065 jet size for the 585-600cfm holley is close for the stock engine, increasing the sqin. area 25% to get to 12.5-13/1 WOT air/fuel ratio would only require a PVCR size of .033. The factory size of .039 shows how rich these carburetors are built. I understand the WOT ratio has to be taken into consideration at least checked when reducing the power valve channel restriction on the primary side.

I think you answered my real question when you state that on carbureted cars with stock manifolds the lean best cruise A/F ratio is still going to be below stoichiometric (14.7). I enjoy tinkering and will not be taking my 3810 apart but I think there is some fuel milage and performance to be gained here and it's nice to understand carburetor tuning.

[Joe C.]

Re: Carburetor Air Fuel Ratio

My question is about tuning a carburetor for best lean cruise A/F ratio 35-45-55-65 MPH, and the main jets are reduced to where the engine surges then up one size at a time until it's steady with no surge. At this point is the ratio considered 14.7 or could it be leaner than that.

In reading, this is the correct procedure but not sure how to tell if it's leaner than stoichiometric. After determining this, a 25% increase in sqin. area for power enrichment would equal 50% of the main jet diameter which should yield 12.5 A/F ratio for best power. This would compute to the PVCR size, so if the main jet ended up at .065 (#65), the PVCR (power valve channel restriction) would be approx .033 (50% of the main jet size) compared to the .039 that is in my 3810 holley.

It's my understanding the holley carbs are rich at cruise and power enrichment..

Leaning main jet cruise this way so the engine is telling you how much fuel it wants could also be double checked with a O2 sensor if a bung is installed in the header pipe and read with a DVOM showing the voltage at cruise. I read 450MV (millivolt) is stoichiometric so we would not want to be leaner than that at cruise conditions.

All thoughts are appreciated..

Wideband O2 sensors use the value "lambda" as a ratio of actual air:fuel ratio to stoichiometric. A value less than "1" indicates richer than 14.7:1. A value greater than "1" indicates leaner.

Pre emissions controlled engines should not be expected to cruise with ratios any leaner than about 14.5:1. Of course, the high duration SHP cams will be comfortable cruising at richer ratios because they won't atomize the fuel to the same extent as engines with short duration cams having less overlap. There is also the issue of reversion, which takes place to a proportionally greater extent with higher overlap cams.

Richer mixtures cool the chambers, keep the engine out of detonation longer and burn slower. Before dialing in air:fuel ratios it is necessary to have your spark advance optimized, and the sweet spot for old designed Chevy smallblock burn characteristics is between 48 and 52 degrees spark advance at SS cruise. Once that is done, then shoot for a cruise mixture of about 14.2 - 14.5:1 with the 250/300 HP cam and 13.7 - 14.0:1 with the SHP cams.

Here's a screenshot of some readout from my Zeitronix ZT2 showing steady state cruise of a Corvette with 327/365 engine:

[Clem Z.]

Re: Carburetor Air Fuel Ratio

Jerry,

I am curious if the car with .062 jets had a slight surge accelerating uphill during cruise. IMO, I don't think the leaner jetting at cruise would make the engine overheat. How did the engine run with the smaller jets?

Duke,

I will read that article again, I would rather have the engine a little rich than below stoichiometric. I also agree that on our stock engines the factory jetting is probably close to the mark. I am not sure I agree about reducing secondary jet size and leaving the PVCR as doing that would unbalance the carburetor. If you are driving and not using the secondaries the primary barrels would be rich, secondaries barrels would be lean.

If the .065 jet size for the 585-600cfm holley is close for the stock engine, increasing the sqin. area 25% to get to 12.5-13/1 WOT air/fuel ratio would only require a PVCR size of .033. The factory size of .039 shows how rich these carburetors are built. I understand the WOT ratio has to be taken into consideration at least checked when reducing the power valve channel restriction on the primary side.

I think you answered my real question when you state that on carbureted cars with stock manifolds the lean best cruise A/F ratio is still going to be below stoichiometric (14.7). I enjoy tinkering and will not be taking my 3810 apart but I think there is some fuel milage and performance to be gained here and it's nice to understand carburetor tuning.

carbs from the factory are jetted for sea level and you can reduce the area of the jet 2% for every 1500 feet you are above sea level and still be OK

[Duke W.]

Re: Carburetor Air Fuel Ratio

Don't overlook the spark advance map if you are looking to optimize fuel ecomony. You didn't say whether you have a 300 or 350 HP engine, but both have the same lazy centrifugal curve.

Get a spring kit and get the centrifugal curve as aggressive as possible without detonation. The OE VACs are okay, but the 350 HP version is more aggressive than necessary, so it could cause some transient detonation with an aggressive centrifugal curve when you open the throttle. I recommend the 12" B20 or B26 rather than the 8" B28 OE equivalent for the L-79. For the ten bucks part cost and easy R&I, it's well worth it.

When John McRae first road tested his Special 300 HP engine it showed some lean surge at cruise, which was probably the result of low rev reversion due to the Special 300 HP camshaft's very late phased inlet valve event. One size up from OE on the primary jets cured the lean surge, but that size was clearly the lean limit.

Then it was on to tuning the spark advance map, and we got it a lot more aggressive without detonation, despite a true CR of close to 10.5:1. It probably could have tolerated lighter springs than the lightest in the spring kit, which were the final choice.

Once the SOTP road testing was done, it was off to the dyno and the numbers indicate the "tune" was pretty well optimized within the tolerance range of the crude (compared to modern cars) engine control system in the form of a carburetor and centrifugal/vacuum spark advance control. If you understand the technical issues and have the time, you can get pretty close with SOTP testing. Then you can do a final check and fine tuning, if necessary, on a dyno.

Unfortunately John didn't drive his new Special 300 HP engine enough to get any fuel economy data.

You will probably find that under part throttle acceleration - enough to open the power valve, but not the secondary throttle - the mixture will be somewhat overrich, but the somewhat oversize power valve restrictions along with accelerator pump injection is what gives instant throttle response. If you go too lean on either, the engine may hesistate. In the overall scheme of things, this transient overrichness under part throttle acceleration doesn't have a huge impact on fuel ecomony, and if the engine doesn't respond well to light throttle modulation, it will be unpleasant to drive.

WOT mixture changes - as long as you don't have to go more than 2-3 jets sizes either way of OE are best handled by changing the easily accessable secondary jets. Given the turbulence in the manifold plenum, the additional fuel added to the secondaries probably gets distributed fairly evenly, but mixture variation between the leanest and richest cylinder can be as much as 20 percent at some engine speed/load conditions although most of the time it's probably no more than 10 percent.

Duke

[Gerard F.]

Re: Carburetor Air Fuel Ratio

Jerry,

I am curious if the car with .062 jets had a slight surge accelerating uphill during cruise. IMO, I don't think the leaner jetting at cruise would make the engine overheat. How did the engine run with the smaller jets

Tim, as I remember, the car with the .062 jets just ran hot on an uphill cruise (at elevation 2000 or so). This was with a 3810 with a new throttle base.

Don't know if it was a vacuum leak, a timing issue, or the jets. But when I went to the .065 jets, the overheating problem went away.

Just can't figure how leaning the jets, would cause overheating

[Timothy B.]

Re: Carburetor Air Fuel Ratio

Jerry,

That sounds more like a radiator issue than anything else, with no surge I believe you would be OK with the leaner jetting.

If your 3743 metering plate has a .039 PVCR like mine, there would be no need to worry about lean A/F conditions at part throttle load where power enrichment is needed. Mathematically, the PVCR would only need to be .031 to achieve the desired 12.5 A/F ratio for power based on a .062 main jet.

Have you ever pulled a few plugs to see how they look, just curious. I think the 3810 is jetted rich from the IFR to jets and PVCR probably because it's also used for the 350hp engine.

How is your hardtop restoration coming along?

[Duke W.]

Re: Carburetor Air Fuel Ratio

An overly lean cruise mixture that causes perceptible "lean surge" can definitely cause overheating if the lean surge persists such as climbing a long highway grade.

What is happening is that cylinders are misfiring - the spark fails to ignite the mixture in the cylinder, but the mixture finally ignites when it flows past the hot exhaust valve, which means it's burning in the port and exhaust manifold.

This will considerably increase the amount of heat transfered to the cooling jacket from the exhaust port. And it might also cause the manifolds to glow cherry red or even hotter orange.

Duke

[Gerard F.]

Re: Carburetor Air Fuel Ratio

An overly lean cruise mixture that causes perceptible "lean surge" can definitely cause overheating if the lean surge persists such as climbing a long highway grade.

What is happening is that cylinders are misfiring - the spark fails to ignite the mixture in the cylinder, but the mixture finally ignites when it flows past the hot exhaust valve, which means it's burning in the port and exhaust manifold.

This will considerably increase the amount of heat transfered to the cooling jacket from the exhaust port. And it might also cause the manifolds to glow cherry red or even hotter orange.

Duke

Duke, Tim,

I think that was exactly what was happening to me with the .062 jets on the 3810. My test run was on a 5 mile stretch of highway with a moderate uphill grade. Cruising at 55 -65, by the time I got to the end of the run, I'd be near overheating. It would then cool off on the way back down.

When I changed to the .065 jets the problem went away.