Supercharger/Increased MPG?

[JanGeo]

The pressure on the exhaust impeller by the exhaust gasses restricts the flow and increases the pressure on the piston so that it does not pump out the gasses as easily so it does end up loading the engine down a bit more than a free flowing exhaust then combine that with the intake pressure against the throttle plate when you are not running wot and you end up putting more strain on the engine all the time. With an electric supercharger you activate it when you need it off battery power and bypass it in normal use. Nitrous is often a better solution with less modifications and little or no impact on the oil system and no intercooler needed either.

[GasSavers_bobski]

Originally Posted by BEEF

not sure if it is right or not but turbos usually have a much larger diameter exhaust as far as I have seen anyway.

Exhaust sizing is much more important with normally aspirated engines which rely on cylinder scavenging during the overlap period between the exhaust and intake strokes. If you go too large, the exhaust pulse leaving the cylinder expands into the big cavity that is the piping and doesn't carry any momentum behind it to pull air/fuel mix through the dome of the combustion chamber at TDC. Going too small obviously results in restriction as engine output increases, potentially resulting in flow reversion if the back pressure gets severe enough.

The only critical exhaust part of a turbo system is the manifold between the engine and the turbine. The turbine gets a lot of it's energy out of the exhaust by taking advantage of the pressure pulses that occur at each exhaust valve opening event. Keeping those pulses coherent lets the turbine extract maximum energy by letting the pulses expand as they pass through the turbine. Once the exhaust has passed that point, the less restriction the better, thus the large pipe.

[theholycow]

Originally Posted by bobski

Exhaust sizing is much more important with normally aspirated engines which rely on cylinder scavenging during the overlap period between the exhaust and intake strokes.

...and this is what you lose when you go turbo (in addition to the extra load on the pistons).

[GasSavers_JoeBob]

I don't think anyone mentioned the number 1 reason not to add a supercharger...it'd be too tempting to use it for its intended purpose...to go faster, quicker!

[Snax]

The whole issue of exhaust sizing and tuning cannot be fairly compared between consumer turbo and non-turbo vehicles, as the manufacturers end up making huge compromises in the designs of both systems. I agree that turbo power is not 'free', however the requirements of a normally aspirated system for tuning the power curve and noise suppression have a price too. That cost can often be offset by using a larger less restrictive exhaust with a turbo, as the turbo itself acts to muffle allot of the noise. The same exhaust modification on a non-turbo car may make noise levels unacceptable, whereas many turbocharged motors do not even need mufflers to meet noise limits.

[GasSavers_bobski]

Originally Posted by JoeBob

it'd be too tempting to use it for its intended purpose...to go faster, quicker!

Yah, well.
That in itself isn't a bad thing. It's that with this particular tech, it means burning additional fuel, which is kind of frowned upon around here, as the site's name implies.

Originally Posted by theholycow

...and this is what you lose when you go turbo (in addition to the extra load on the pistons).

That depends greatly upon the manifold design and turbo sizing. Both N/A and Turbo application manifolds focus on maintaining the coherence of exhaust pulses. One can get scavenging effects out of turbo manifolds by putting long (ideally equal length) smooth flowing runners between the engine and turbo. Given the space constraints in many engine bays, doing so often results in some fairly wild looking manifolds:


As for piston pressure, have you ever looked at a turbine? It's not reverse-pump with a bunch of closed chambers the exhaust gasses pile in to, it's a series of identical spiraling passages attached to a rotor. Yes, I suppose there will be some restriction compared to a straight pipe due to the directional changes in flow as the exhaust gasses pass through the turbocharger housing, but those changes in direction have been engineered to be as smooth and efficient as possible - you don't see any square turbocharger housings.

[fowljesse]

I wish I could remember where to site it from, but it was proven that small turbos increased FE. I think it was on BMWs, or Mercedes.

[theholycow]

Originally Posted by bobski

As for piston pressure, have you ever looked at a turbine? It's not reverse-pump with a bunch of closed chambers the exhaust gasses pile in to, it's a series of identical spiraling passages attached to a rotor. Yes, I suppose there will be some restriction compared to a straight pipe due to the directional changes in flow as the exhaust gasses pass through the turbocharger housing, but those changes in direction have been engineered to be as smooth and efficient as possible - you don't see any square turbocharger housings.

Sure, it's made to harvest the energy as efficiently as possible without wasting any; but you can't get around the fact that you're using energy from the engine to pressurize your intake air. Add it to an existing car+engine combo and all you gain is power.

[bowtieguy]

Originally Posted by bobski

Given the space constraints in many engine bays, doing so often results in some fairly wild looking manifolds...

i once watched the guys on Horsepower TV install a twin turbo on a corvette...at the rear of the vehicle.

http://www.powerblocktv.com/site3/in...15&ep_sea=0602

[GasSavers_bobski]

Originally Posted by theholycow

Add it to an existing car+engine combo and all you gain is power.

You're harvesting waste energy and feeding it back into the engine in one form or other. The power and torque curves change significantly... Additional torque can mean upshifting earlier, and lower engine RPMs. Whether the engine is more efficient under those conditions would depend on the specific setup and tuning, so it's unwise to generalize. All I've been really getting at in this thread is the relative inefficiency of super vs. turbochargers. I think it's safe to say that bolting on a supercharger will result in reduced fuel economy, where it would depend on the specifics when it comes to a turbo.

Originally Posted by bowtieguy

i once watched the guys on Horsepower TV install a twin turbo on a corvette...at the rear of the vehicle.

Yeah, I've seen that before as well. It would work, but would take way longer to spool up than it should, probably result in greater exhaust back pressure (compared to a manifold-mount turbo) and take way longer to pressurize all the charge piping running from the rear end of the car back up to the engine.