Generic engines question

The Original Tom · Post by **The Original Tom** » Sat Aug 25, 2007 8:49 pm

The answer may be obvious, but I have a question.
Which is has better fuel consumption: a 2.0l engine producing 300bhp, or a 5L engine producing 300bhp? (not nececarily those engine sizes, but just for example i.e a Chevy V8 and a Subaru flat-4)

It may seem obvious, but even though more cubes = more fuel, it can produce all that power at about 5000rpm, however the small engine will be revving it's little nuts off at about 9000rpm with a turbocharger and radical cam.

I know it's difficult to say, and if torque comes into it then it'll over-complicate things more, but I was just interested really.

ian.stewart · Post by **ian.stewart** » Sat Aug 25, 2007 9:03 pm

Easy, Chevy not force induced and 53 years old
and subaru turboed and modern computer designed,

But get a late model LS1 and stick a turbo or supercharger on it at lowish boost and 800hp is not totally out of the question, is that not so Stevie Turbo

The Original Tom · Post by **The Original Tom** » Sat Aug 25, 2007 9:06 pm

Ok, maybe my choice of engines to compare was poor

.
So which would be more efficient? The scoob at 300 or the LS1 at 300?

Lewis · Post by **Lewis** » Sat Aug 25, 2007 9:23 pm

Depends what driving you're doing

Alley Kat · Post by **Alley Kat** » Sat Aug 25, 2007 9:31 pm

Impreza's pretty thirsty, low 20's. Think they run fat to help avoid detonation. Some of the injected Chevy guys reckon 25mpg on a run, in fact I think my mate gets 30 from his TPI which is better than our 2.8 V6 Quattro ... annoyingly!

bill shurvinton · Post by **bill shurvinton** » Sat Aug 25, 2007 10:48 pm

The short answer is it depends on a lot of variables. In engine research Brake Specific Fuel Consumption or BSFC is one of the key parameters. Now this varies at different points in the operating curve (which is why some big engines get very good motorway mileage), but in general at a particular HP a turbo engine will be more efficient than a normally aspirated.

This is because of expansion ratio. for every HP of explosion in the engine about 1/3 goes into the crank, 1/3 goes into heating up the oil and water and 1/3 goes down the exhaust, wasted in a natasp, but in the turbo it does useful work spinning up the turbine.

However there are then about 3 pages of gotchas. For example the std scooby has a pissy small intercooler on top of the engine, which doesn't cool that well, so the engine needs to be run richer than ideal under boost, blowing away the efficiency gain. (I could get onto my water injection hobby horse at this point).

Post by **katanaman** » Sat Aug 25, 2007 10:52 pm

At a guess I would say they will both be better than each other in different situations. A V8 to a four would have more friction losses but is a torque monster so doesn't have to be buzzed to go anywhere. On the other hand a scooby in town off boost will return much better than the V8. The less pistons is more efficient has been proven in F1 where they went from 12-10-8 and every time they used less fuel. Its also why there aren't many V12 engines anymore because the fuel costs were horrendous. Jag V12 for example making 300bhp was a shocker on fuel. They are coming back a bit again now they have good fuel systems but they are still bad on fuel. Wait a couple of years for the new F1 regs to come in limiting engines to 1600cc with turbo and you will get your answer for sure.