Why do performance upgrades compromise fuel efficiency?

[jplacson]

I was talking to a gear head friend of mine (Audi dude) and I was wondering about something... if you bump up the HP of your engine (without increasing displacement), why does it consume more fuel?

I mean, it seems that forced induction systems burn more fuel... I would have thought that since your getting more air to fuel, you'd be burning less fuel to get to the same speed you did without the induction.

I would assume that with more HP/torque, your engine would have an easier time getting up to speed and cruising, since theoretically your foot wouldn't be as close to the floor as without the mods.

Or is it just cuz once you get mods, you end up with heavier feet? hehehe.

[MarkD]

I was talking to a gear head friend of mine (Audi dude) and I was wondering about something... if you bump up the HP of your engine (without increasing displacement), why does it consume more fuel?

I mean, it seems that forced induction systems burn more fuel... I would have thought that since your getting more air to fuel, you'd be burning less fuel to get to the same speed you did without the induction.

Power produced by a motor is proportional to the amount of FUEL used by it. The air/fuel ratio of an internal combustion engine using gasoline as a fuel is fixed in a narrow range, (typically between 12.6:1 to as lean as around 15:1 ). You have to envision that you are really adding fuel, and the air is added to keep the a/f ratio in the correct range. I'm explaining it this way to clarify what you seem to think about adding more air and making the system more efficient. If you add air, you have to add fuel to maintain the proper a/f ratio. If you don't maintain the a/f ratio in this range, the internal temperatures and exhaust gas temperatures will rise, causing detonation and possible meltdown. (and higher amounts of unwanted emissions). If the a/f ratio gets below 12:1 or so, the motor will start losing power (and efficiency as all the fuel will not be completely burnt).

When a motor with EFI is cruising at part throttle (PT), the O2 sensor attempts to keep the a/f ratio around 14.7:1. (stochiometric fuel ratio).

When the transition is made to wide open throtte (WOT), the a/f ratio gets closer to 12.6:1 to cool the pistons and help prevent detonation.

If you increase boost in a forced induction motor to 0.5 bars (~7 lbs) you are forcing in ~50% more air mass than it had when NA, so will have to add 50% more fuel to maintain the a/f ratio. This should also increease the power output by close to 50%.

Just remember you can't get power without burning fuel. In a properly tuned modern motor it's already burning at close to optimimum efficiency.

Mark

[jplacson]

oh!!! ok... lemme see if I got it right... for forced induction systems... the air that's added is basically NEEDED to keep up with increased fuel input via larger injectors or regulators? So basically, all power upgrades are there to allow you to burn MORE fuel, and in the process increased air intake is needed? if so... that makes a whole lot more sense now.

[MarkD]

oh!!! ok... lemme see if I got it right... for forced induction systems... the air that's added is basically NEEDED to keep up with increased fuel input via larger injectors or regulators? So basically, all power upgrades are there to allow you to burn MORE fuel, and in the process increased air intake is needed? if so... that makes a whole lot more sense now.

That's correct. I noticed a few mistakes in my original post but they are corrected now, you may want to take another look.

The biggest change was to this paragraph, the others were typos:

If you increase boost in a forced induction motor to 0.5 bars (~7 lbs) you are forcing in ~50% more air mass than it had when NA, so will have to add 50% more fuel to maintain the a/f ratio. This should also increase the power output by close to 50%.

Glad to help.

Mark

[Jon K]

Ok mark maybe you can explain this. I am babysitting an 99 Eclipse GSX with 360 dyno horse power. He's got forged internals, lowered compression, larger valves, P&P, etc. We're running @ 22psi. What you mentioned about PT and WOT seems related to this. My a/f gauge will read 1 tick into rich when at PT. When at idle it flops erratically unable to read. When I go to full wide, the engine is on the most rich tick on the digital gauge. I often backfire LOUDLY. The engine isnt flooding or anything, its running amazing, but I am trying to get it to lean out some because my friend in an M3 said I was blowing a lot of black smoke. I've used the computer to calibrate a "lean" setting but I think we're going to need another regulator... We went through more than 1/2 tank of gas in only 68 miles... granted we were smoking an ML55. Maybe you can give some insight, I know other DSM owners get better mileage than this.

I know its better to be too rich than too lean, should we just accept the **** mileage and clean the white cars rear bumper off about every day or can this be significantly bettered?

[MarkD]

Ok mark maybe you can explain this. I am babysitting an 99 Eclipse GSX with 360 dyno horse power. He's got forged internals, lowered compression, larger valves, P&P, etc. We're running @ 22psi. What you mentioned about PT and WOT seems related to this. My a/f gauge will read 1 tick into rich when at PT. When at idle it flops erratically unable to read. When I go to full wide, the engine is on the most rich tick on the digital gauge. I often backfire LOUDLY. The engine isnt flooding or anything, its running amazing, but I am trying to get it to lean out some because my friend in an M3 said I was blowing a lot of black smoke. I've used the computer to calibrate a "lean" setting but I think we're going to need another regulator... We went through more than 1/2 tank of gas in only 68 miles... granted we were smoking an ML55. Maybe you can give some insight, I know other DSM owners get better mileage than this.

I know its better to be too rich than too lean, should we just accept the **** mileage and clean the white cars rear bumper off about every day or can this be significantly bettered?

It's really hard to give you any useful advice on this. The reason is I don't know exactly what all the mods are. Your running 22 psi boost, what was the original boost and hp? Have you changed the fuel pressure? I'm assuming you have raised it. Did you change injectors? Also, you need a wideband a/f ratio meter to properly monitor what's going on, not a narrow band meter which is what you propably have.

If you raised the fuel pressure and maybe also changed injectors, the mixture may be too rich even when you decellerate, causing the backfire. I don't believe in raising fp to increase fueling. If you want to inject more fuel, you should get bigger injectors and re-calibrate the fuel maps.

Mark

[Jon K]

We do have narrow band, we have upgraded injectors and i believe F/P also... thanks for input

[AllanS]

Are you using a rising rate fuel pressure regulator? Does it allow you to set the onset point?

Also, Mark is right- get a wideband meter. You can get nice ones for under 400$ that include an o2 sensor as well.