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If I run my car at cruising 50 mph and running 12.4 AFR and then lean the carb so it runs 14.4 AFR
how much less fuel does the engine use at the same speed 50 mph.
the difference between 12.4 and 14.4 AFR is about 16%, does it mean that I got 16% better mpg at 50 mph when I run 14.4?
I'm not certain where your coming up with 16 percent. Most VE tables top out at 127.
but still, I've tuned enough fuel injected engines on the dyno to know that 16 percent sounds like a lot more than the difference between 14.4 and say 12.5
however, if your looking for fuel economy, why not run it 14.6-14-7. Just at cruise RPM and throttle position. Higher throttle settings set around 13.8 -14.2.
13.0 -13.3 wide open.
Any need to set it fatter than that???
Last edited by 4-vettes; Jul 26, 2020 at 07:34 AM.
I'm not certain where your coming up with 16 percent. Most VE tables top out at 127.
but still, I've tuned enough fuel injected engines on the dyno to know that 16 percent sounds like a lot more than the difference between 14.4 and say 12.5
however, if your looking for fuel economy, why not run it 14.6-14-7. Just at cruise RPM and throttle position. Higher throttle settings set around 13.8 -14.2.
13.0 -13.3 wide open.
Any need to set it fatter than that???
Im talking about carburator tuning, not efi
Im just curious how much difference in fuelconsumption it is to run for this exempel 12.4 vs 14.4
Drag out the general energy equation and........no, sorry. Too many years ago.
Just theoretical....the thermal efficency of converting fuel into work is optimal at the stoichiometric ratio. If you move away from that (richer or leaner), thermal efficency gets worse. You will be moving closer to stoichiometric so the thermal efficency should go up. Problem is, all the other factors that get affected when the air fuel ratio is changed.
The math gets a lot longer when you need a better answer.
Whether fuel injection or carburetor is providing the fuel, the above answer is still correct. Lean it a bit when at cruise conditions and fatten it when at WOT. The difference is in HOW you accomplish that with FI or carb.
you may save more than the straight percentage. it is more efficient at 14.6. so the 16% more fuel not only goes out the exhaust, it makes the other 84% not burn as well. just remember, 14.7 don't work that well at full throttle. on paper it should, but... and if you get a little leaner, it will start burning aluminum since there isn't enough gasoline in there.
I habe change from a vacuum carb to a double pumper, and cant get it running above 12.4 on cruising, idle is about 13.4, wot about 12.5.
It doesnt help going Down on main jets, I think air bleed jets may help, but my carb dont have changable air bleeds...
I was 12.ish yesterday today I'm at 13.4ish cruising. The heat wave here coupled with High humidity is causing A point change. If your experiencing similar dont sweat it. As far a MPG set your carb to your engines best performance. A narrower than stock lsa will like your AFR readings.
I habe change from a vacuum carb to a double pumper, and cant get it running above 12.4 on cruising, idle is about 13.4, wot about 12.5.
It doesn't help going Down on main jets, I think air bleed jets may help, but my carb don't have changeable air bleeds...
You need to tune the "Idle Feed Restriction". This part of the carb feeds fuel to the idle circuit as well as the transfer slots. When at cruise, (around 2K rpm or below) you are NOT feeding fuel from the main jets, so jet changes do nothing. Mostly using the transfer slots.
I have a R4779 750 DP, Holley. With no idle feed restrictions in place, at cruise as you stated, I get black spark plugs. With .021" restrictors in the idle feed, I get white plugs at cruise, and lean ping at times.
Currently I'm running .018 restrictors and now have light tan color plugs. Seems to be the correct restrictions for my particular carb.
With all the experimenting with restrictors, fuel mileage didn't seem to change that much.
What did change drastically, was with the restrictors in place the engine did not "load up" like it did without the restrictors. "Load up" = after a long cruise, a few WOT passes thru the gears was required to clean out the crap.
You need to tune the "Idle Feed Restriction". This part of the carb feeds fuel to the idle circuit as well as the transfer slots. When at cruise, (around 2K rpm or below) you are NOT feeding fuel from the main jets, so jet changes do nothing. Mostly using the transfer slots.
I have a R4779 750 DP, Holley. With no idle feed restrictions in place, at cruise as you stated, I get black spark plugs. With .021" restrictors in the idle feed, I get white plugs at cruise, and lean ping at times.
Currently I'm running .018 restrictors and now have light tan color plugs. Seems to be the correct restrictions for my particular carb.
With all the experimenting with restrictors, fuel mileage didn't seem to change that much.
What did change drastically, was with the restrictors in place the engine did not "load up" like it did without the restrictors. "Load up" = after a long cruise, a few WOT passes thru the gears was required to clean out the crap.
Idle feed restrictor # 25
thanks, there are some aftermarket metering blocks on the market, can you recommend some?
ive seen some of them has jets for emulsion, What is that for ?
Do I get same result at cruising with bigger idle air bleeds (on a carb with changable bleeds) vs restrict IFR on a carb without changable air bleeds?
Last edited by 70rs/ss396; Jul 26, 2020 at 02:01 PM.
thanks, there are some aftermarket metering blocks on the market, can you recommend some?
ive seen some of them has jets for emulsion, What is that for ?
Do I get same result at cruising with bigger idle air bleeds (on a carb with changable bleeds) vs restrict IFR on a carb without changable air bleeds?
Not sure why you are asking about aftermarket metering blocks ? The ones that came with the carb are likely the best for your carb. You just need to do some tuning.
I did increase my Idle air bleeds in the beginning, however it showed no improvement in the rich cruise condition.
I then went for the fuel restriction which gave me the results I wanted.
I'm no expert on air bleeds, yes i would like to have changeable air bleeds, just to do some experimenting. However what I have now works great, so am leaving the air bleeds alone.
I would do some testing with your idle feed circuit.
It's quite simple, just use a piece of non corrosive wire and place it in the idle feed hole. About 1/2 to 5/8" long. Just use a piece long enough to bottom out in the hole and is not protruding past the face of the metering block.
I think my .018 wires was from a leg of a transistor. Source can be wires from resisters, leds, etc.
You should have a digital Vernier caliper to keep track of different size wires (restrictors)
Last edited by Barry's70LT1; Jul 26, 2020 at 02:29 PM.
Not sure why you are asking about aftermarket metering blocks ? The ones that came with the carb are likely the best for your carb. You just need to do some tuning.
I did increase my Idle air bleeds in the beginning, however it showed no improvement in the rich cruise condition.
I then went for the fuel restriction which gave me the results I wanted.
I'm no expert on air bleeds, yes i would like to have changeable air bleeds, just to do some experimenting. However what I have now works great, so am leaving the air bleeds alone.
I would do some testing with your idle feed circuit.
It's quite simple, just use a piece of non corrosive wire and place it in the idle feed hole. About 1/2 to 5/8" long. Just use a piece long enough to bottom out in the hole and is not protruding past the face of the metering block.
I think my .018 wires was from a leg of a transistor. Source can be wires from resisters, leds, etc.
You should have a digital Vernier caliper to keep track of different size wires (restrictors)
There is metering blocks with changable IFR jets, so you dont have to use wires....
you may save more than the straight percentage. it is more efficient at 14.6. so the 16% more fuel not only goes out the exhaust, it makes the other 84% not burn as well. just remember, 14.7 don't work that well at full throttle. on paper it should, but... and if you get a little leaner, it will start burning aluminum since there isn't enough gasoline in there.
This answer makes a lot of sense. The excess fuel has to go out the exhaust since there is no more available oxygen for combustion, but the ability to combust all the air that enters the engine is also a factor.
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by Barry's70LT1
It's quite simple, just use a piece of non corrosive wire and place it in the idle feed hole. About 1/2 to 5/8" long. Just use a piece long enough to bottom out in the hole and is not protruding past the face of the metering block.I think my .018 wires was from a leg of a transistor. Source can be wires from resisters, leds, etc.
You are correct in stating that you can fine-tune the AFR at light throttle cruise by using a restrictor wire in the idle feed restrictor orifice. What I have found works perfectly in many cases is a simple staple out of an office stapler: Pop 2 staples out of a stapler and straighten out one of the bends so you have an "L" shaped piece of wire. Stick the short leg of the "L" into the IFR, and sandwich the long leg under the metering block gasket for retention. This will quite often get the cruise mixture up from mid-12's into the mid-14's, and it will also make your idle mixture screws more finely tuneable (will change idle mixture screw settings from 3/4 turn out to 2 turns out, as an example).
I habe change from a vacuum carb to a double pumper, and cant get it running above 12.4 on cruising, idle is about 13.4, wot about 12.5.
It doesnt help going Down on main jets, I think air bleed jets may help, but my carb dont have changable air bleeds...
You are correct in stating that you can fine-tune the AFR at light throttle cruise by using a restrictor wire in the idle feed restrictor orifice. What I have found works perfectly in many cases is a simple staple out of an office stapler: Pop 2 staples out of a stapler and straighten out one of the bends so you have an "L" shaped piece of wire. Stick the short leg of the "L" into the IFR, and sandwich the long leg under the metering block gasket for retention. This will quite often get the cruise mixture up from mid-12's into the mid-14's, and it will also make your idle mixture screws more finely tuneable (will change idle mixture screw settings from 3/4 turn out to 2 turns out, as an example).
Lars
Do I get the same result changing to bigger idle air bleeds, instead of restrict fuel, there Will be more air via a bigger idle bleed jet?
fuel consumption vs afr ratio
