Whats More Fuel Efficient?

[Modena]

Was just thinking, what is better for fuel conomy generally, a blower or a turbo? In theory the turbo is meant to be more conomical as it only gets used after a certain RPM, while the blower kicks in immediately and eases off at higher RPM?

[djr81]

A turbo simply because it is powered by what would otherwise be waste heat (exhaust gas). A supercharger needs to be powered by the engine making sufficient hp to drive it.

[Modena]

A turbo simply because it is powered by what would otherwise be waste heat (exhaust gas). A supercharger needs to be powered by the engine making sufficient hp to drive it.

If its "wasted gases" why does it guzzle fuel so much once you put your foot down?

[djr81]

If its "wasted gases" why does it guzzle fuel so much once you put your foot down?

1. Because the only way you make horsepower is to burn petrol.

2. Because most tuners run motors on AFR's much richer than stoichiometric on WOT to keep the EGT under control.

[trism]

a turbo/ supercharged car will always chew more fuel than a similar N/A motor. why? because you are forcing more air into the motor, so that you can put more fuel into it, and thus, make more power.

but what djr81 is trying to say is that a supercharger is run off the crankshaft. it takes power to run a supercharger.

heres an example. lets say my car has 200hp. if i just put a supercharger on it, but dont connect it, the car will make say 180hp. because you need to drive it.

if i did the same with a turbo, then the power wouldnt change, because its just the exhaust thats running it

[djr81]

a turbo/ supercharged car will always chew more fuel than a similar N/A motor. why? because you are forcing more air into the motor, so that you can put more fuel into it, and thus, make more power.

but what djr81 is trying to say is that a supercharger is run off the crankshaft. it takes power to run a supercharger.

heres an example. lets say my car has 200hp. if i just put a supercharger on it, but dont connect it, the car will make say 180hp. because you need to drive it.

if i did the same with a turbo, then the power wouldnt change, because its just the exhaust thats running it

Not that it was asked, but at full throttle there is no compelling reason for a turbo/supercharged motor to be any less efficient that its naturally aspirated counterpart. What is being discussed is not absolute power output, but specific fuel consumption, ie how much fuel you need to produce 1kW of power. With regard to part throttle a turbo motor will always be less fuel efficient than an atmo motor because its effective compression ratio is much less. As a rule the higher your compression ratio the better fuel economy you get.

What I was saying is that if your supercharger consumes say 20hp on full throttle the engine needs produce that 20hp to drive the thing. The power (20hp) would otherwise show up in rwhp. Put another way you waste 20hp worth of petrol just making the motor run.

With regard to the turbocharger being driven by exhaust gas it is not a free kick. There are pumping losses from the motor that result from the need to drive the turbine on the turbo. It can be likened to having a very restrictive exhaust. Nonetheless these are less than their equivalent for a belt driven compressor.

[sidewayssam]

it would be turbo

[infamous_t]

Superchargers are known as parasitic for a reason...

[VB-]

turbo is more effecient than supercharger, hence you will get better economy

[MissR34]

naturally aspirated hahahahahahaha

[Kinks]

Superchargers are known as parasitic for a reason...

turbos are parasitic too, they cause backpressure in the exhaust which the engine has to push against.

however, they are still more efficient than a supercharger under most circumstances.

[GTR-32U]

Ok I am gonna vote the supercharger for a few reasons.I have owned both at some stage...For overall power output the turbo wins hands down...However economy ...not so sure of...Yes the blower robs power however turbo cars drink juice in the sense that you get lag while building boost.My supercharged Commodore I modified would pull 9psi at idle if I floored it..So I only ever needed the smallest of throttle openings for a very short time to get it moving briskly..And once the foots off the blower was only recirculating air...But put the foot down and the bypass valve shuts and booom away it would go.Under maximum acceleration I would say the turbo would use less, but in the cut and thrust of normal driving...very hard to say...never right off supercharged cars, My one had throttle response as good or better than normally aspirated

[scathing]

a turbo/ supercharged car will always chew more fuel than a similar N/A motor. why? because you are forcing more air into the motor, so that you can put more fuel into it, and thus, make more power.

That is also a matter of perspective, not absolute reality.

Firstly, Saab went to a complete FI range (at least in Australia, not sure if they still sell NA Saabs overseas) for fuel economy reasons. If we're talking "similar to NA" in terms of power output, going FI allows Saab to run a physically smaller engine, which improves packaging and weight, while still maintaining performance.

If we're talking "similar to NA" in displacement, then once again its not necessarily the case. What's one of the major driveline differences between the NA and FI versions of cars that have both (like the Skylines for example)? The FI cars tend to have taller final drive ratios, which means they sit at a lower RPM for a certain speed in a certain gear. The extra midrange of the FI car allows the car to maintain good acceleration with this different FD, which improves economy.

Also, if we're talking about modern FI engines, with relatively high compression and low boost, the engine's internals don't cause the economy hit that old-school turbo cars have. So they're still tractable through the midrange but the little extra air + fuel is offset by the fact that the engine is doing lower RPM because they can carry a higher gear, even around town.

This also leads to the fact that if we're talking "similar in NA" to performance, then its definitely an advantage for FI. The FI guys can run smaller, lighter engines. However, they could also run different gearing. As anyone who's compared a GTS and a GTS-t knows, one of the big differences is the final drive in the diff. The turbos have a taller final drive, which means they can do the same speed at a lower RPM in the same gear. If you were to gear a FI car to accelerate as slowly as a NA car, it would be far more efficient as the engine wouldn't be turning as hard.

I have a few mates that have 350Zs with APS TT kits (which is only a bolt-on engine mod kit, no modifications to the internals or gearing). Their around-town economy improved after the installation. Having that much extra midrange torque meant they could drive it a gear higher than when NA, and since its boosting across a wide rev-band with stock NA internals that extra gain also happens at around-town engine speeds. I think that they were pulling around 12% (1/8th) better economy after the TT install.

The last time I went for a drive with a 350Z TT up Putty Rd, he used about half the fuel I did while maintaining a similar pace behind me. Where I was thrashing the car around in 3rd or 4th at WOT to keep it on the boil, he was in 5th and 6th at half to three quarter throttle. I can only imagine how far ahead he would have gotten from me had he used as much petrol as I did.

If you're driving the car "normally" (from an Average Joe perspective) with only light to medium throttle openings most of the time, then a modern FI car will generally be more fuel efficient. Of course, once you go wide open throttle the simple fact that more air goes into the engine means you need more fuel, and so economy suffers.

Edited January 18, 2008 by scathing

[Stinky Rooster]

^^ seconded. FI is slowly overtaking NA as the fuel efficiency king. Just have a look at the latest Volkswagon engine "twincharger" puts a turbo AND a supercharger on to a piddly little 1.4 4 cyl, they get decent power out (for a 1.4) and it returns about 30mpg city and 48mpg highway!