turbo/supercharger and gas consumption

i have been arguing with a guy on and off about whether or not you use more fuel when you put on a turbo or a supercharger. from what i understand if you are puttin that much more air in, then you have to put in more fuel. right? sorry if this is old, but i have to know the true answer. it just seems illogical that you can put on a turbo/super and not have more fuel added also. thanks.

you do use more fuel

Yep, you gotta feed everyone of those extra ponies more fuel, then add another 10%+ x-tra fuel on top of that- so your engine doesn't come unzipped (detonation) and you flatten your rear tires with your own crankshaft.
Cruise gas mileage will typically take a small hit. Usually not much.

thanks. i was sure you had to use more fuel. i imagine you could run it without extra fuel, but like you said it would cause detonation.

Ever hear of Air/Fuel ratios? 14:1-15:1 for cruising, usually about 11.5:1-12.5:1 for WOT? Well the number on the right is units of air; the number on the left is units of fuel. Fuel is a constant in this expression. So, you have more fuel in the WOT state than in cruising. Now, horsepower=airflow. The more air you can get in and out of your engine, the more power you make. Forced induction means you force air through your engine. Pretty soon, those eleven units of air become pretty large units. So the one unit of fuel must become larger as well.

Ever hear of Air/Fuel ratios? 14:1-15:1 for cruising, usually about 11.5:1-12.5:1 for WOT? Well the number on the right is units of air; the number on the left is units of fuel. Fuel is a constant in this expression. So, you have more fuel in the WOT state than in cruising. Now, horsepower=airflow. The more air you can get in and out of your engine, the more power you make. Forced induction means you force air through your engine. Pretty soon, those eleven units of air become pretty large units. So the one unit of fuel must become larger as well.

I'm sure you mean 14:1 ratio: "the number on the LEFT is for air; the number on the RIGHT is for fuel"

The stoichiometric ratio of air to gasoline is 14.7:1 (or thereabouts), that is, the ratio where the fuel is completely consumed by atmospheric oxygen. In reality, it varies from 14:1 to 16:1 but the closer you get to the magic number, the more efficient your combustion is.

Because the ratio is fixed, if you pump more air under pressure, you will need the additoinal corresponding amount of gasoline to maintain the ratio. So, yes, you do use more fuel in most cases.

As for "horsepower=airflow," that is true if you assume that the air:fuel ratio is maintained, thus, "horsepower=fuel" also holds true.

Again, the above is ture only for gasoline engines. Diesel engines are not constrained by a fixed ratio (can range from 15:1 to over 100:1) which can realize more horespower per unit of diesel fuel (besides the fact that diesel has a higher caloric content than gasoline).

The stoichiometric ratio of air to gasoline is 14.7:1 (or thereabouts), that is, the ratio where the fuel is completely consumed by atmospheric oxygen. In reality, it varies from 14:1 to 16:1 but the closer you get to the magic number, the more efficient your combustion is.

Because the ratio is fixed, if you pump more air under pressure, you will need the additoinal corresponding amount of gasoline to maintain the ratio. So, yes, you do use more fuel in most cases.


Unless you like replacing pistons, valves, etc., you'll certainly run F/A ratios richer than ~14.7:1 for an "un-naturally" aspirated engine. More like 11.5:1 or lower. I'd say 11.5:1 is probably the lean limit for most engines where longevity is important.

Yea I fucked that all up.lol IDK what the hell I was thinking.... IF you read down, I did type it correctly there.lol

Peak power is generally made at ~12.5:1 on gasoline engines. ~11.5:1 is a safe tune for forced induction engines but ~12.5:1 still makes peak power. ~10.5:1 is usually the rich limit before you lose considerable power.

Ultimately the best way to determine best AF ratio is on a dyno where you can add/subtract fuel and look at the torque curve.

a turbocharger shouldn't affect your gass mileage that much if you have good driving habits because the added power is related to your foot position. at wot it'll consume more gas but you can have only a slight increase in fuel consumption if you drive for fuel economy.

pardon me, but what does WOT mean?

Wide Open Throttle(WOT)

it's when your foot is all the way down to the floor and the throttle body or carburator is completely open.

pardon me, but what does WOT mean?
Means the same as WFO.

Yep, almost all ecu's when they detect WOT, they switch over to a totally different, richer, fuelmap. I know people are always saying 'don't run too lean or you'l blow your engine' but what physically happens when you run too lean? Does it overheat? Does something snap/break/melt? i'm t3h confu53d...

Yep, almost all ecu's when they detect WOT, they switch over to a totally different, richer, fuelmap. I know people are always saying 'don't run too lean or you'l blow your engine' but what physically happens when you run too lean? Does it overheat? Does something snap/break/melt? i'm t3h confu53d...

A couple things can happen. The lean mix can cause overheating piston tops, valves, etc and/or detonation.

As you point out, most ECU's use the 02 sensors to hold air/f at about 14:1 or so until the ECU detects something close to WOT. They then resort to a fuelmap that controls injector rate simply from the MAF or MAP. (Manifold Air Flow or Pressure.) That map typically sets the a/f into the 12:1 area. (Most manufacturers set it even a bit richer to be on the safe side.)

In regards to the use of fuel with a turbocharged engine:... NO, you will not use more fuel when you turbocharge an engine. (At least not significantly.) Since the turbo is not 100% effective there are some inherent losses, but as long as you drive in the same methods, (rate of acceleration, etc.) Fuel consumption will be about the same. However if you utilize the extra HP available due to the boost, you WILL use more fuel.

Jim
SR Racing Inc

i thought i knew about engines etc...until i entered the forced induction forums...with all the maf, map, lambda o2 sensors an their workings an stuff....crazy