Supercharger Vs Turbo: Help me with your Google-Fu

Ongoing discussion, scratch that, arguement with a guy at work.

Same engine, same boost level, one is supercharged, one is turbocharged, which makes more power? I'm saying turbo, he argues they're the same.

My position is that the turbo is harnessing the energy otherwise wasted from the exhaust and extracting some of that (addidng a slight parasitic increase to the engine) to compress the intake air. Since the energy is otherwise wasted, it's inherently more efficient than using useful energy to drive the blower, hence, more power. Research reaffirms this. However, do you guys know of a back to back comparison done between the two? The key is the same boost level.

I know why there are both, and the relative benefits of both (my MR2 is blown and the no lag is quite nice...)

Show me your google-fu.

https://www.youtube.com/watch?v=eA5kj1lSMBo

Boost level doesn't really mean anything, so the answer isn't really readily available.

I will say turbocharging is a more efficient way to make power, but doesn't inherently mean it'll make more power without more information.

You'd also have to define how you're measuring "power." Peak HP? Peak TQ? Most area under the curve? Highest average HP?

I think it would also depend on where you measure the power. Crankshaft, wheels, etc

I always thought turbo had lag, supercharged was instant. But both were equally capable in the long run?

I don't know, but consider: The turbo isn't just recovering wasted energy from the exhaust. It also creates some nonzero backpressure which probably has some effect, although I am not sure how much. Granted, a Top Fuel car requires 600 hp or thereabouts to turn its giant and mostly old-tech blower, but most applications don't seem that extreme. It worked great in the ZR1, eh?

While a supercharger is usually more efficient at low RPM, a turbocharger is a lot more efficient at higher RPM. So at high RPM, where you produce max HP, a turbocharger will need less power to produce a same pressure and volume.

I realize this is an absurd simplification but with a turbo, and probably a supercharger, the cfm is just as important as the psi. You could make the same psi with a t25 and a hx60 but you will have wildly different cfm and thus wildly different performance.

The last direct comparison that I remember seeing was for a Rolls Royce Merlin engine(should tell you how old this study was) where they compared the efficiency of an exhaust driven turbo vs a mechanically driven supercharger. The paper found that the mechanical compressor used up 150 hp to produce 400 hp. I cant remember the exact numbers for the pumping losses attributed to the turbo but they were pretty small.

Too many variables to be able to assume how much power each is making, but it's pretty easy to make one good assumption:

Centrifugal compressors are more adiabatically efficient than positive-displacement pumps. That means the turbo will almost certainly be shooting out cooler and denser air at a given pressure ratio across the 'compressor' side of turbo or super.

But that's not the whole story. It probably IS possible to build a turbo setup so crappy that it makes less power than supercharger at the same boost level. One thing you said is that the turbocharger is harnessing otherwise wasted energy and only adding a slight parasitic drag to the engine, which isn't really true. Turbos are a lot closer to running off pressure than they are to running off heat. Now, the pressure is generated by raising the temperature of air in the engine, but if the engine weren't spinning you'd still have jack crap for pressure. Turbos work by using the engine to pressurize the turbine inlet, sometimes by a LOT. A lot of small turbos (relative to the motor) running high boost are putting 40psi of headwind against the engine while it's trying to pump its exhaust out, and that creates a LOT of pumping losses that would not otherwise be there.

So, while it's probably possible to build a turbo setup that really does mostly use 'wasted' energy and only adds a tiny pumping loss to the engine, that turbo would not do what we expect a turbo to do (spool at a reasonable rpm, double the engine's power, etc). The kind of turbo setups enthusiasts actually like take a lot of power from the engine in pumping losses, which is why upgrading only the turbine side of a turbo still shows power gains even though it doesnt affect airflow INTO the motor.

So anyways, given the wide range of possibilities of compressor efficiency and exhaust pumping losses at a given boost level (and thats just the turbo, the blower has a range of possibilities too), boost level is a pretty useless thing to compare apples to oranges with.

You want a direct comparison? Same car, same dyno, same engine management, same boost?

https://flyinmiata.com/tech/dyno_runs/M45_vs_VoodooII_1.6.pdf

Of course, the real answer is "it depends". It's all about specification of the parts.

Vigo wrote: Centrifugal compressors are more adiabatically efficient than positive-displacement pumps. That means the turbo will almost certainly be shooting out cooler and denser air at a given pressure ratio across the 'compressor' side of turbo or super.

This is not necessarily true, as centrifugal superchargers exist.

I really hate FM for not pushing through with the Uberchargers. I <3 twinscrews.

In theory, a turbocharger holds a small benefit in absolute efficiency, which is why forced induction purpose-built racecars almost always have turbos instead of belt-driven superchargers when the rules allow them to choose.

In practice the inherent efficiency difference is much smaller than the other variables in forced induction engine design. Proper turbo/blower selection is critical, but you can't just go out and design your own (not unless you have an F1-style budget!), so you have to pick from what's on the market. The realities of packaging are also important, the ideal solution may just not fit in the available space. Then there are the supporting systems that need to be designed properly -- exhaust, intake, headwork, etc. In an ideal world, you almost certainly want different cams for turbo vs blower, for example. Real world kits have to meet certain budget targets and are building on factory designs, so it may well be the case that for a Miata the turbo is a better choice, whereas for some other kind of vehicle the opposite is true.

You can find examples of good turbo systems beating out bad supercharger systems (Keith's Voodoo vs M45 example, say), and I'm sure you can find examples the other way too.

Correct me if I am wrong but I don't believe he asked about efficiency. Turbo wins there BTW.

Turbo will produce more power. The theory I have, which is correlated by the Flyin' Miata run, is the energy required to create the same amount of airflow in a turbo is less than the energy required to create the same amount of airflow in a supercharger.

Simply turning the supercharger takes more power from parasitic drag than from the power loss from back pressure from the turbo. Because the turbo is taking it's power not only from exhaust flow but from exhaust heat. Which the engine was going to expel anyway. The supercharger is only operating off of the crank which is directly pulling power from the engine, which if it wasn't there sucking some power it would use that power to propel the vehicle.

As said before, there are other circumstances involved in a real world applications

Beans, the Ubers had a problem. By the time you spun the compressor fast enough to get it into the right efficiency range, it would eat the bearings at redline. Very narrow window of operation. Add into that some very dodgy quality control from the supplier of some of the parts, and it wasn't a difficult decision to pull it from market. They sure made a sweet noise when wound up, though.

I'm having trouble thinking of a successful modern racer that uses a supercharger and goes around corners.

This is not necessarily true, as centrifugal superchargers exist.

Yes, and they have CENTRIFUGAL COMPRESSORS.

Simply turning the supercharger takes more power from parasitic drag than from the power loss from back pressure from the turbo.

The amount of power lost to pumping losses on a turbo setup is frequently underestimated in my opinion. In general i still think your statement is true.

ecause the turbo is taking it's power not only from exhaust flow but from exhaust heat.

Take a torch to a turbine housing and see if the turbo starts spinning. The engine is using the heat to expand the charge air (which is also the exhaust), but the turbo is mostly using the flow which mostly comes from the engine pumping air. You can spool a turbo with cold air but you cant spool a turbo with hot air that isnt being pumped into it.

Vigo wrote:

Simply turning the supercharger takes more power from parasitic drag than from the power loss from back pressure from the turbo.

The amount of power lost to pumping losses on a turbo setup is frequently underestimated in my opinion. In general i still think your statement is true.

ecause the turbo is taking it's power not only from exhaust flow but from exhaust heat.

Take a torch to a turbine housing and see if the turbo starts spinning. The engine is using the heat to expand the charge air (which is also the exhaust), but the turbo is mostly using the flow which mostly comes from the engine pumping air. You can spool a turbo with cold air but you cant spool a turbo with hot air that isnt being pumped into it.

Take the delta difference over the turbine wheel.

You will see the drop. Yes the air needs to move but that isn't where most of the energy comes from.

It's also why ceramic coating on the manifold will help spool. Keeps the heat inside.

Another factor to keep in mind is that a turbo, being in the inlet and exhaust, serves to quiet both. That can help a lot on some tracks with tight sound limits. "Honda kids" tend to bow more to NA (normally aspirated) engines as being somehow more manly than boosted ones, but they forget that NA engines make power with compression and rpm - which means noise, and a allot of it. With a 92 (or even 90) dB sound limit, NA engines have a real problem, and not just from the exhaust.

The days of high-end only peaky turbos is over if the designer uses sensible care in choosing the unit. I like to show this dyno chart below showing that a smaller turbo can do wonders for "area under the curve." This is a 2.4 liter Honda.

I also had a supercharged truck for a while and was somewhat unimpressed with it; it had great low end but sort of died off at higher rpm.

I'd say that "in general", if you want to make smaller power gains, NA is best (cheapest.) For moderate power, a supercharger is easier and probably cheaper. However, once over "some" level of power, it's a turbo all the way since boost can simply be increased to suit.

You will see the drop. Yes the air needs to move but that isn't where most of the energy comes from.

I disagree with "most". Having said that, i can acknowledge an unquantified importance to heat, but for people who don't know much about turbos, telling them they run on heat leads them to put the cart before the horse. If you look at truly high performance manifold design, you see heat retention taking a distant back seat to 'other factors' which happen to include velocity (speed and direction) into the turbine. If heat retention was the end-all there would be a lot less tubular header porn on the internet.

There are turbo systems out there that rely more on the exhaust pumped out of the engine more then the heat. Rear mount turbos don't take much advantage of the heat like manifold mounted turbos. They are more like crank driven supercharges, but use exhaust pressure instead of a belt. Compare such a system to a traditional turbo system on the same engine and you will see that they need to use smaller turbine housings and still can't keep up with a traditional set up.

Otto cycle or miller cycle? Superchargers have a distinct benefit there.

turbo for peak HP numbers ie... i'm a better human being because my car makes 800hp

supercharger for higher "under the curve" power makes better power in the rpm range that you actually drive around in.

twin charger is for real men.

Fueled by Caffeine wrote: Otto cycle or miller cycle? Superchargers have a distinct benefit there.

Because the extra compression is needed for the thermodynamic cycle to work.

In the end this conversation is the third week of thermodynamics 2.

Heat is the primary motivator of a turbo once it gets moving. And the heat is spent either way.

That is why the end-of-system turbos don't work as well.

Long tube headers are for balancing the exhaust pulses and to maximize exhaust flow rate. Because without using the exhaust for something you just need to get it away as fast as possible.

and before it is said about just running open exhaust, remember 2 stroke expansion chambers? Those acoustics and pressures help to charge the cylinder. That shows you how much energy that needs to be organized and handled to get a engine to run right.

ICEs are very interesting studies.