Let's say we have 2 engines of the same model from a manufacture, same size and HP rating (let's say 2.3L with 160hp)

One is turbocharged

the other is Normally Aspirated

It seems that under normal driving conditions (just cruising on the highway) the turbocharged engine would be better since it does not have to have as aggressive camshafts or compression as the N/A motor

at WOT, both should be sucking down the same amount of fuel...or possibly the turbo would be using more fuel to enrichen the mixture since a lean condition would be more fatal????

my brother had a twin turbo 300zx always complained about gas mileage

One engine is like a E production engine? where its 2.0 liters and is tuned for 200hp but at much sacrifice to drivability and economy...

Whereas the other is a turbo, and can be tuned for driving without the turbo....


Seems simple to me... The turbo will have better gas milage.

Whenever we drive the turbo's, we get around 28mpg.. maybe more... If we get on the boost, it gets pretty crappy.. but its producing a buch of hp... so thats to be expected....


but if the engine was one of those honda twin cams.....

why would the turbo have the same hp rating? or is that hp rating before the turbo kicks in?

the turbo should have more hp and mpg... I would think anyway

QUOTE (Andyrew @ Dec 20 2005, 02:34 PM)

One engine is like a E production engine? where its 2.0 liters and is tuned for 200hp but at much sacrifice to drivability and economy... Whereas the other is a turbo, and can be tuned for driving without the turbo.... Seems simple to me... The turbo will have better gas milage. Whenever we drive the turbo's, we get around 28mpg.. maybe more... If we get on the boost, it gets pretty crappy.. but its producing a buch of hp... so thats to be expected.... but if the engine was one of those honda twin cams.....

yea, but what if the n/a motor is not high strung and the hp level is well within reason for a streetable engine?

something like your V8....1 motor has a stock cam and small turbo to put out 250 hp

other one is n/a with just a bigger cam and head work to put out 250hp

neither motor is really stressed however it does seem the turbomotor might be able to achieve the hp at a lower rpm???

QUOTE (tat2dphreak @ Dec 20 2005, 02:50 PM)

why would the turbo have the same hp rating? or is that hp rating before the turbo kicks in? the turbo should have more hp and mpg... I would think anyway

let's say your limited to 160hp, period

As much as I don't like my friends cars, they have Talon's from the early 90's. Both turbo's, bot get like 28mpg avg if they keep out of the turbo most of the time. I would say tuned properly, a turbo engine will yield better mpg than a na engine if driven properly.

But if you had to really hammer the engines, the turbo ones really suck the juice so it would flip. The na under load uses less juice than a turbo under load provided the turbo engine was a little peaky and the na engine was a torque monster.

It would be better to back up and ask the question "Which has a better BSFC?"

From Edelbrock

Brake Specific Fuel Consumption is the ratio of fuel consumed (in lbs. per hour) to horsepower produced. This ratio is a direct indicator of how efficiently the engine converts fuel into power. Most factory gasoline type engines run approximately a .50 to .55 Brake Specific Fuel consumption (BSFC) range while a highly efficient normally aspirated race engine operates at approximately a .40-.45 BSFC.
Most common turbocharged or supercharged engine configurations run in the .55 to .60 BSFC range.

But that doesn't tell all the story either, because the BFSC numbers given above are probably generated at peak power, and not at 'normal driving conditions'.

Even then, I'd have to go with the forced induction having better BFSC across a RPM and power range because I guess that issues of volumetric efficiency dependencies upon induction geometry and cam/valve timing are reduced.

Compression ratios complicate the issue too. Most turbos have a lower ratio, hence less efficiency while running without boost. Get on the boost and the effective cr (and BSFC) goes up. Most turbos however, are set up to run rich under boost, further muddying the waters. On paper, a turbo is more efficient because wasted energy (exhaust heat) is utilized to produce power....but the real world is not so simple. I think in the aviation world, where rpm's and loads are more constant, the turbo engine will be more efficient overall. With land vehicles, it would depend on the set-up and type of driving. Having said all that, I admit I really don't know the answer.

Go check out http://www.fueleconomy.gov/
It's a great data base of all the Manufacturers' EPA specs on fuel economy. I compared an 87 Saab 900 turbo, to it's naturaly aspirated brother, and according to the site they both get the same mileage 21/27
Ofcourse these numbers are based on figures that Manufacturers' probably pull out of thin air. But for conversations sake. A couple of summers back we averaged 29mpg on a 7k mile trip out to cali and back in my girlfriends Saab 900 turbo. And that was with regular speeds between 80-95mph(Western TX)
I fell turbos on average have the same mpg/dispacement ratio as natualy aspirated, but with an incress in peak HP. my .02

Samson

Ultimately, it's not the right question to ask. What you really want to know is: for two engines of a given level of power, will the turbo or the NA engine be more efficient? The answer is still "it depends". It's possible to build a turbo 1.5 that has the same power and torque as a 3.0, but the 1.5 could still get better mileage, simply because it's smaller.

Higher compression ratios IMPROVE gas mileage, right up to the point where detonation happens (then mileage goes into the toilet). The only reason we're not all driving gasoline spark-ignition engines with compression ratios of 14:1 is that current fuels won't handle it at peak loads, but they would handle it (in many cases) at cruise loads. There have been some interesting experiments done over the years with variable compression ratio engines, but so far, none of them have provide viable.