SuperCharger vs. TurboCharger

The intention with this article is to shed some light on the theories behind turbocharging the modern engine, and the wonderful advantages it gives us in both speed and basic engine design issues. This post is also to try and give people a better understanding of supercharger systems and how they work to make such a simple thing as an engine even simpler (speaking in terms of tuning). I personally am tired of seeing people bicker over the differences between the two basic designs, and would like everyone to note the vast similarities among them instead. So without further delay... I will start with this simple statement:

"It makes no sense to build a naturally aspirated engine if what you are looking for is good power output."

To understand why this is so obviously true, we need to break the concept of an engine down into its simplest parts. There are 4 main parts to an engine that concern airflow:
-the induction system
-the cylinder head(s)
-the exhaust system
-the camshaft(s)

Now if you think of the engine in terms of airflow and forget about fuel for a minute, it becomes a very simple matter really. What we want to do is best flow air through the cylinder head, from the induction system to the exhaust system and then out into the world again. This is best & most naturally accomplished by pressure variation, because as almost anyone with a high school education knows, air naturally flows from areas of higher concentration (pressure) to lower concentration (pressure). Now let's assume for a minute that we are talking about an engine at or near sea level, well we can just forgo the exact physics of things and say that at both the induction system's inlet & the exhaust's outlet we have equal pressure (just under 15psi absolute pressure). So in order to flow air into this system we must always be working a balancing act between the three fundamental sections of the engine, which are exposed to each other only through the camshaft's orchestration of the valves. So forget everything else you know about engines and start thinking of what's under your hood in this way for the rest of this post .

NA ENGINES (naturally aspirated)
These must work within a maximum pressure variation of 0psi (which is really hard to create without massive pumping losses) and 14.7psi (maximum atmospheric pressure @ sea level). To add to the basic problem of how to flow air into and out-of this system, both ends of the system start out at the same pressure, meaning air doesn't naturally want to go IN or OUT. This can be accurately termed as a “pain in the ass”. Now engineers and enthusiasts alike have long been fascinated with how to make power from this setup, but I am talking specifically about supercharged engines here, and as I already stated "It makes no sense to build a naturally aspirated engine if what you are looking for is good power output." So forget about how you can best accomplish this through piston movement and it's effects on cylinder pressures, and understand that it's just a whole lot easier to get an engine to work if it's supercharged.

FI ENGINES (forced induction)
From a pure engine design standpoint, it makes MUCH more sense to pressurize the intake system than to run NA. When only the intake system is running under pressure well above atmospheric, it becomes perfectly obvious that air is going to want to flow through the engine exactly the way we want it to, and both cam timing & exhaust sizing becomes much less important to getting the system to work right (as it was before in NA setup). The air will naturally want to flow into the cylinder head, and then after the very strong power stroke (thanks to all that air) it will naturally want to flow out into the lower pressure exhaust system afterwards. Everything in the engine will be working at pressure above atmospheric and the pressure differences will be greatest in the induction system, so all air will want to exit out the tail pipe quickly and efficiently. One other thing should be said here: turbos technically ARE superchargers. A supercharger is ANY device that pressurizes the intake to above atmospheric pressure, and turbos do this exactly like superchargers do. The only difference is in how a turbo gets the energy necessary to perform it's job, and also that the turbo contributes to supercharging the exhaust system (or more accurately a portion of it, the exhaust manifold).

THE CASE FOR SUPERCHARGING
Since a crank driven s/c (s/c = supercharger) is what people are normally talking about when they use the term supercharger, I will no longer say "crank driven" to make the distinction between it and a turbo. Now using a supercharger makes a ton of sense simply because it only has a direct effect in pressurizing the engine on the side we want it to, the induction side. Since pressures will always be higher here than in any other part of the system (except of course during the engine's power stroke, but that's always sealed off from the rest of the system so we can forget about that complexity), it's very easy to make this combination a powerful one. NA engines often use large amounts of valve overlap to get the whole system to work properly at higher RPM, which has obvious drawbacks in that it's possible for the intake system and exhaust systems to interact in a negative way (since they operate at similar pressures). It's sometimes just as easy to get air flowing backwards through the system as it is to go forwards in an NA setup, which is one reason camshaft choice is so important to where in the RPM band best power will be produced. And here is where the beauty of supercharging is; neither valve overlap amounts nor perfect exhaust system designs are completely essential to keep everything flowing in the right direction. No matter how long the exhaust is exposed to the intake system through valve overlap, air should NEVER pass backwards through the system unless the supercharger stops working.

THE EVIL OF SUPERCHARGING
The evil of supercharging is that some of the power we finally get from combusting the air/fuel mixture must go back into powering the supercharger. So here we have designed this whole system that works so well, yet we have to power it with some of our hard earned torque. This is not a good thing, but then again nothing so simple is ever going to come for free. Do superchargers work? Of course they do, which is why many racing engine uses the technology unless the rules prohibit it. The net result is more total power from the system, but a portion of this power must be sapped from our output to make it all work.

THE CASE FOR TURBOCHARGING
This section is easy to write, because it's exactly the same thing as the supercharger portion. We have all of the same advantages, except for one major benefit. That benefit is that turbocharging runs off what is largely wasted energy, so that darn drawback of needing to power the system with some of our hard earned torque is removed. In this way, a turbocharger addresses the one main drawback to using a supercharger, but as you will see in a second the supercharger addresses the one main drawback of turbocharging.

THE EVIL OF TURBOCHARGING
Hopefully you now understand why it makes so much sense to forgo designing engines for NA use and just supercharge the sucker instead, at least when we are talking about how to best make power. And if you have been following what I have said, you will also understand the bad effect turbos have on our little perfect world of pressure variation.
A turbo is an ingenious little design that harnesses the wasted kinetic energy we dump out through the exhaust system to actually force more air into the engine. This is good for the same reasons that supercharging is good, but it has one major drawback: it of course increases the pressure within a portion of the exhaust system. While turbocharging a motor increases the amount of air that can be flowed into it, it has a negative effect on how easily we can flow it back out again. This weakens our positive pressure difference between these two fundamental sides of the engine, and causes both cam timing & exhaust system design to again become extremely important to making good power. This is most certainly not a good thing, but can a turbo overcome this drawback with the other inherent good it possesses? It certainly seems so, because in most current forms of racing where the rules don’t probihit the use of tubos or slap restrictions on their use, the turbo reigns supreme in terms of engine power output.



Now I didn't post this to make a statement about which system will work better for your intended use, because the answer is (as usual) "it depends". Sorry, but if there was such a clear cut answer do you really think people would still be debating this topic? A long time ago someone would have proven everyone else wrong, and either turbos or superchargers would no longer exist. Remember, these systems were designed and in use on production vehicles long before most of us were born, so it's not like this is a new debate. The purpose here is to educate people on exactly why we would want to supercharge or turbocharge an engine in the first place. Also I wanted people to see, from a basic and theoretical perspective, how each system is different in its function and it's relative pros & cons. Hopefully this discussion of basic theory helped some of you come to a better understanding of FI engines, and that my leaving out any real world examples actually made it easier to understand. I have written an article concerning the technical differences of the common systems, giving examples of different supercharger designs and their advantages/disadvantages[quote]


Info courtesy of Automotive Articals

Ahh, the great forced induction debate, one of my favorite topics. Great info Fasel, thanks for sharing.

Yeah no problem I stumbled across this artical earlier this morning and thought I'd post to see what everybodies thoughts where!! I know you diesel guys are all for the turbo's!!!

Thats the great thing about forced induction, you can always find a setup to fit your needs. Diesels generally lend themselves towards turbochargers, but sometimes they need a supercharger, or both. One size does not fit all, and thats what makes them awesome.

Yeah I personally like Turbo's for street and strip... The turbo lag gives slower start but once it spools you have a sweet top end... However for mud trucks I go more for the supercharger basically because you have boost off idle throughout the RPM range.

BLOWER!!!!

I see 76 is more for the supercharger then!! haha!

If its Diesel, I would generally go Turbocharger, if its Gasoline, I would generally go Supercharger.

Well being that my knowledge of diesel engines is about as large as a lima bean I would naturally say turbo only because I've never seen a supercharged diesel... However like I said before street cars are my choice for turbos due to the turbo lag... Turbo lag give no boost in performance when your in the low RPM range for when your crawling whereas a supercharger boost even at idle so whether your running 8000 RPM or running 800 RPM you have boost... IMO

I do have to wonder, is it possible to indeed supercharge a 6.2 liter chevy diesel rather than turbo? I have heard you cannot, which doesn't make any sense

Again like I said before I have no knowledge of diesel engines but I'll use my resources to find out if your interested?

batty1214 wrote:

I do have to wonder, is it possible to indeed supercharge a 6.2 liter chevy diesel rather than turbo? I have heard you cannot, which doesn't make any sense

I have heard you could, but never seen a working setup. The problem with the 6.2L is once you exceed a certain cylinder pressure, they have a tendency to grenade, which is why they are rarely seen with forced induction.

FaselZ71 wrote:

Well being that my knowledge of diesel engines is about as large as a lima bean I would naturally say turbo only because I've never seen a supercharged diesel...

Oh yeah, they are supercharged. But mostly its the 2 strokes that have them. The instant boost from the supercharger helps tremendously with emissions at low rpms, and allows the fuel to burn completely, giving you more power.

HOLY crap!!! That's one bad butt looking diesel!!! Nice photo rob!

I only wish it was mine......

Hell yeah that would be one badass ride!!! I wonder if it came stock that way or if he put that on aftermarket?? Kinda like stumpuller454 from TF he's got the supercharged 454BB I watched his youtube video... That truck moves! I was impressed when I saw that!

Thats aftermarket. Very few companies make them, but they do exist. Maybe one day the old 6.2L will get a Supercharger....

That would be nice to see...

Yes it would. Provided the engine can hold together.

True... So what is the maximum amount of cyclinder pressure the 6.2 can handle? Because if it's one set amount the just tune the boost to a lower volume... Or don't they make domed pistons to lower the compression ratio to allow for higher pressures inside the cyclinders?

FaselZ71 wrote:

True... So what is the maximum amount of cyclinder pressure the 6.2 can handle? Because if it's one set amount the just tune the boost to a lower volume... Or don't they make domed pistons to lower the compression ratio to allow for higher pressures inside the cyclinders?

I have seen varying tests. Somewhere between 3500psi and 4500psi (depending on combustion temps) is where pistons start to melt and the block comes apart.

There are tons of aftermarket goodies for the 6.2L, but they are expensive, and money is a luxury I dont have right now...

Quote :

money is a luxury I dont have right now...

Amen to that brotha!!! haha So just a question of curiousity? How the hell do the blocks start coming apart?

The early (pre1988) 6.2Ls have a lower nickel content, making them weaker then the 89' and up 6.2L. Under high cylinder pressures, the pistons have been known to move sideways and out through the block. Very messy to say the least.....

No crap!!! Man that would be messy no doubt about it!! You would think they would address the problem of the pistons going through the side! Or is it just one of murphy's law things and just can't be fixed?

FaselZ71 wrote:

No crap!!! Man that would be messy no doubt about it!! You would think they would address the problem of the pistons going through the side!

The pistons, rods, and cranks are way over built is the problem. Thats why in 89' they upgraded the blocks (and a few other things) to be on par. An early 6.2L can live a long, healthy life, but not with high boost. Most kits do not suggest going above 8psi with a turbo or blower. If I ever buy the kit I want, I will set it at 7psi and hope for the best.

Here is a good picture of the typical damage when it happens;