Afr

[1795]

Greetings everyone,

I thought that I would start a discussion about what an appropriate AFR is for a car based upon it's use and look for opinions (of which I image there will be plenty )

I had my race car on a chassis dyno recently tuning the carbs in. The person running the dyno was knew Weber's and we were able to get them balanced and worked on the jetting. He ended up tuning the carbs to run in the 12.7-13.1 range at WOT for maximum HP. A lot of the work that they do is for drag racers and high performance street cars.

When I discussed the AFR numbers with my engine builder later, he said that those numbers are fine for a drag race car, but not for a road race car. He wanted to see a high of 12.4 for an AFR to avoid being lean. I tended to agree with him, but wanted more advice.

I sought advice from John Garuti, who is in charge of the engine shop at Cobra Automotive and asked him what he tended to shoot for. John had some nice advice. He said that an engine dyno or chassis dyno operate in optimum situations that do not exist in driving situations. AFR's in the high 12's-low 13's will produce maximum HP, but in a road race situation they could lead to engine failure. One of the greatest contributing factors is the g force that the car is encountering. In high g corners the fuel will be pushed to one side causing that side to be rich and the other side lean. If you are running at the edge with being lean that could lead to a significant lean condition. When they are tuning at the track they change spark plugs after every session to check for whether the car is running lean or rich. In his opinion, you cannot get a good read on plugs after they have been used once.

John further suggested that to truly operate at the safe end of the lean vs rich AFR range for maximum performance that people usually sacrifice a couple of engines to find that point. If you are not at the price point to be able to afford that, then tuning to be in the low 12's is a safer bet.

Thoughts? I will be increasing my jetting from where it was just set to, back to where it was running slightly rich. Does this concept apply to EFI as well, or just carbureted engines? I would think that EFI would be less affected by g force, but I could be wrong. How often do we consider the way the car will be driven outside of the controlled environment of the dyno session?

Jim

[Igofastr]

I would assume EFI would be immune to G forces...assuming you have adiquate fuel pickup/pressure.

[spdbrake]

Although I think port injection would be a immune to G-effects I also highly doubt the 1.0 g that a Cobra road race car pulls would have any real effect on fuel vapor mixture entering the cylinders as the intake valve opens and the piston retracts, sucking the fuel air mix in. It just doesn't stay in suspension more than a nanosecond before a cylinder grabs it from the intake.
Example of an FIA on track: https://www.youtube.com/watch?v=xhsGzD3vNX8

[Igofastr]

I think with carbed motors, the position of the floats can actually make a significant difference in a racing application (center-hung vs side-hung). Not so sure that it would specifically effect one side more than another, but that could largely depend on the style of intake you're using. Can't say that I have any practical experience here...just postulating. Maybe one of our builders will chime in.

[1795]

I also wonder to what extent that it would be influenced by the type of carb and intake manifold. I think that a Weber system with an independent runner system would behave more like an EFI than the intake on a four barrel.

I do know that if you do not have two pickups in the fuel cell that even with baffling on hard corners you take the risk of a pickup sucking air instead of fuel as you get low on fuel.

I also think that the length of the race or how long you are running your engine is a contributing factor as well to a safe AFR number. In a short sprint you are probably doing less damage running on the lean side than in a longer race, such as a road course where that heat will build for longer periods of time.

[Sycraft]

mine is at 12.09 to 12.49 pending rpm and load Dyno tuned

[Snake2998]

Sorry for late reply - couldn't get logged in until now.

My comment is that all following this advice must be losing huge power due to overly rich fueling and most probably retarded timing because of a problem most probably confined to cars running in very specific conditions, not normally experienced on the road or during track days i.e. with a dual plane manifold (or side-draughts) and very high cornering G's.

In normal conditions you would not be coming hard onto the throttle until the cornering G's are significantly lowered (and hence fuel starvation reduced) ) unless you are in "drifting" mode so your AF ratio should depend largely on your specific driving situation. It is normal that AF ratios below 12.5 : 1 are confined to turbocharged cars with high compression ratios combined with retarded ignition to reduce detonation but this does cost significant power.

If I were in the situation where this could be a problem I would certainly fit broadband AFR sensors in each bank to see if there is a genuine problem in my situation and use profile rather than accept the cost of fuel and power loss due to some "generic' opinions based on experience in unusual (for me) driving situations and natural and reasonable reputational preservation by engine builders.

[1795]

You are correct in that most people driving on the road will not really have to worry about it, and unless you are driving your car on a track day like you would a race car you probably may not experience that potential problem either. There are racers, including myself, who do start getting back on the throttle before hitting the apex on most corners.

I would suggest that if someone were to do a track day that they should consider making sure that they do not fall much below half a tank of gas, as most gas tanks have a single pickup and the fuel will slosh from side to side (baffles and foam will mitigate it somewhat, but will not stop it completely) and the fuel pump could suck air instead of fuel.

The intent of the thread was to stimulate a conversation. There are no universal rules and so much comes into play and is engine and driving style specific.

[Morris]

Jim

Here’s a couple of photos of the Fuel Cell set up in KMP259.....we have the main and reserve lines in the Container within the fuel cell.....but then we installed additional fuel containers in each corner of the fuel cell....to capture the fuel in the rear corners of the cell and pump into the main container ....this ended our fuel starvation coming off of the long corners like Ovals & Road America....

[Morris]

[1795]

Morris,

Thanks. I will have to see if I can fabricate up something similar. Right now I just have to two pickups under the foam sitting in the rear of each corner.

Hope that all is well with you.

Jim

[Morris]

Jim

The container within a container prevents the major sloshing of fuel across the tank....

On some of my race cars....we used an external sump for the fuel.....so an additional tank that the fuel pumps from the cell pumped into....then, two more fuel pumps pumped the gas forward to the Carb.....and if you really wanted to get tricky.....put the additional tank inside of your fuel cell and fill it first.....then when you fill the fuel cell they can only pump in the 22 gallons you are suppose to have......(not counting the extra 1 or 2 gallons hidden inside the fuel cell.......).....Unfair Advantage

[1795]

Morris,

My Mom always said, "Cheaters never prosper." Of course, that does not count in racing, where they seem to prosper the most....

We do not have a limit on how much fuel you can have for a race in vintage racing, but preventing sloshing is something that I would like to incorporate. I have not been to Road America yet, but it is one of the places that I plan on making it to.

As always, thanks for the advice. Working on some setup changes for Watkins Glen in a couple of weeks.

Jim

[Morris]

That’s why Mark Donahue said “it is the Unfair Advantage that counts”.....out smart everybody else.....much like Smokey Yunick did....

[patrickt]

It's only a fair advantage when it's mine. If it's the other guy's advantage, then it's unfair.

[Lioninstreet]

Originally Posted by 1795

... He ended up tuning the carbs to run in the 12.7-13.1 range at WOT for maximum HP. A lot of the work that they do is for drag racers and high performance street cars.

When I discussed the AFR numbers with my engine builder later, he said that those numbers are fine for a drag race car, but not for a road race car. He wanted to see a high of 12.4 for an AFR to avoid being lean. I tended to agree with him, but wanted more advice..

One of the several factors influencing the answer to this question is air density. ie. Are you racing at the same altitude and weather conditions your dyno was tuning you at? If not, and for example you drop in altitude when you go racing, you just leaned yourself out and vice versa.

Also what kind of fuel are you typically running and is it always constant? ex. Pump gas these days (unless specifically a constant brand of 100% gasoline) will also effect AFR as the ethanol blends we get stuck with vary in consistency. I remember when running in a limited modification, pump gas carbureted class that we dynoed muled something like 5 different gasoline brands and trust me, they don't all burn the same, produce the same kinds of power, and (fun fun) they vary from batch to batch.

As for the higher octane race fuels, we surprisingly found that the lowest octane we could run without deto, the higher the BMEP was. This allowed for less ignition advance because of a more violent, faster flame front.

Just don't run down to your local Citgo with your high revving small block and tank up with their 87 octane pump gas because I told you so...

I know this doesn't directly answer your question, but it should cause some pause when tuning down to the last 2-3% to make a HP graph look better look. IMHO, for road racing (or even the aggressive weekend joy rides), you'd prolly be better served by honing driving skills, swapping stuff out looking for better tires & brakes, suspension tweaks, etc. This instead of working the AFR/HP game. For the 1320, it's a bit different of course.

[hauss]

I can not speak to cars but, years of motocross racing, hard hits and lean angles. I would say float height would be the most critical, that is what I would be playing with once jetted on dyno .Also I have seen sponges in tanks, to control sloshing. Can"t remember who was making them, it was back in the 70s. Let us know more what carburetor intake and engine you use.

[spdbrake]

I am curious if anyone has tried the Holley Hydramat. Although it will not help in baffling / fuel shift it does appear to work quite well in preventing fuel starvation. More of a drop-in solution for sure.

https://www.holley.com/products/fuel_systems/hydramat/
https://www.jegs.com/p/Holley/Holley...aAv2EEALw_wcB#

[Jim Vander Wal]

I have a Hydromat but haven't used it yet. It will cover the bottom of the main chamber of my tank. I believe that it will help as opposed to a standard pick up.

When I spoke to Holley tech support the main concern is if the outer surface is damaged as that degrades the performance.

Jim

[eschaider]

Jim (1795 Jim),

Soliciting opinions about where someone else set their particular combination of parts AFR-wise to operate reliably is a waste of time. Most of those people never measured the actual AFR their car was using other than maybe a tailpipe sniffer which is notoriously inaccurate.

Buy some dyno time with a competent tuner and let him tune your car. It is money well spent and parts well protected. The idea that AFR's change based on equipment used could not be farther from the truth. AFR's are exclusively based on the stoichiometric chemistry of the fuel being used and only the fuel being used.

The dyno equipped tuner will be able to optimize both your engine's ignition requirements and the engine's AFR to obtain a metric they call MBT which stands for Maximum Brake Torque timing. It is a three dimensional metric where the dimensions are, ignition advance, AFR and maximum brake torque.

Once your tuner finds MBT for your engine you will not be able to improve upon power without a power adder or a different engine build. Here is a chart from some emissions work that fuel system calibration engineers used in the early 90's when they were attempting to reduce the oxides of Nitrogen to meet the every changing EPA requirements.



Note that peak HP occurred between 12 and 12.5:1 AFR for n/a gasoline engines. This coincides with a lambda of 0.82 and 0.85 when using pure gasoline w/o any ethanol content.

When you add ethanol to create E-10, the Stoichiometric Point for the fuel moves from 14.7 to 14.1. This change in Stoich Point will result in a change in target AFR for max power. Additionally because of the introduction of ethanol into the fuel, the target lambda will be enriched by a few percentage points to 0.81/0.83. That moves your target AFR for max power to somewhere between 11.4 to 11.7.

If you are serious about attempting to optimize your engine's fuel delivery don't rely on urban legend and so-in-so did whatever he did but there was full moon so your mileage may vary.

Fuel system calibration is not black magic, it is simple and basic but, and it is a big but, you need a competent tuner, with a dyno and with all those little air bleed and fuel metering gizmos used in what ever carburetor you are using. Although it should go without saying, the tuner not only needs to have all the toys, he needs to know how to use them. Anything less and you are basically at the far end of the room, someone has blindfolded you, spun you around three times, put a dart in your hand and told you to throw it at the wall. Wherever it lands, that is your AFR!

We all know better but sometimes we can easily get sucked into the urban legend solutions. Don't do that to yourself — or your engine! Go to a competent tuner who has the tools, the metering components for your carb and most importantly the knowledge to do the job correctly. BTW one of those absolutely essential tools is the dyno.


Ed