Supercharger - technical questions...

[diss7]

Hi all,

***edit, this post requires an attention span greater than a labrotory mouse to read through, if reading is hard for you, there are plenty of pictures on the Internet for you to look at.

I've decided to explore the S/C option for my 86, purely because I feel it fixes what the car is lacking, low end punch.
Now, I realise that there are a lot of fan boys and hard parkers on here whose actual knowledge consists of regurgitating information they have read on other forums, passing it off to be first hand experience. If this sounds like you, please control yourself.

I'm not looking for a turbo/supercharger debate.

I understand turbo systems fairly well. I've spent the last 10 years owning and modifying JDM turbo cars. What I've learnt over that time, is that 'generally' high power turbo cars, do not make good street cars. Now, obviously, that last comment is pretty subjective and opinionated, but hey I'm the OP.

I have a few cars at the moment, and the one that gets driven least is the most powerful. I have a KE20 corolla with a 250rwkw SR20DET in it. I wouldn't describe it as overly laggy, the turbo is only slightly larger than a disco potato. (Tomei M8270) I find the car too impractical for the street, and it really only now serves as a 'shits and giggles' street car, or a drag car. And to focus it more as a drag car, will require me to tub it, build the motor, etc etc.

I don't want my new 86 to turn into this. My 86 is to be my daily, and I just want to finish it to a level that I wish it was presented from factory. Some forged lightweight 17" wheels, some slight suspension tweaking, and some more power. I want the power to be quiet, yet instant. I don't want to wait until 3000rpm, and actually have less power/response below this.

Now, this thread was going to be about discussing the differences in the twin screw type supercharger, and the centrifugal type supercharger. But, I've realised that I don't fully understand the supercharging route in the first place.

I've read a lot of articles and watched a few videos. Yes, this all makes sense in principle. But these explanations are revolve around WOT examples. This is entirely what I am trying to avoid, making a decision and setting a DD up based on WOT, at the (unknown) sacrifice of the other aspects that make for a great DD.

Before I get into some examples, I should point out that I am aware that my difficulty in completely understanding these systems probably revolves around my understanding of pressure and volume. FYI, my understanding of this is that pressure/volume are directly related. If one container is compressed to 1 Bar, it effectively contains twice the volume of air than it did at atmosphere. I realise that my understanding here maybe flawed, as I have seen discussions on similar turbo systems running the same PSI but one flowing more air than the other. This has never made sense to me.

Anyway, lets look at an example.
I am boosting along in my FI 86. At 5000rpm and WOT it is boosting 10psi and honking along pretty well. I am coming up to a corner that I cannot take at WOT, so I immediately pull back on the accelerator to say 25%.

Now, this is how I understand how a turbocharged system reacts.... As soon as I pull back on the accelerator, the throttle body closes. The air in the intake manifold is then consumed by the engine creating a pressure difference before and after the throttle body. This pressure difference triggers the BOV to open and exhaust the **EDIT- pre ** throttle body pressure. Also, because the motor is now making significantly less power, the exhaust gas volume is significantly less, so the boost that was being used at WOT is no longer generated.

Same situation in a supercharged vehicle. When the throttle body closes, where does this excess pressure/air go? One thing that appears universal in supercharger systems, is that they do not require a BOV. Now, a turbocharged system with no BOV would suffer compressor stalling to relieve the pressure, but I have also read that a supercharger does not do this either. (I would assume this would be very bad for something connected to the crank to want to do something like this) So first question is, where does this initial surplus air go? Is it forced into the engine despite the fact I don't want the power? Surely not. This then leads onto another example.

Lets say that I am cruising along at 5000rpm is 3rd gear on the highway. Now a turbocharged system at cruising throttle would not be producing boost. If I put my foot down, the turbo would spool fairly quickly and I would then be boosting. Yet a supercharged system would be boosting, even though the system effectively doesn't need it.

One way I have tried to understand supercharging, is that it is effectively an NA motor that is always running in a pressurised atmosphere. Because its 'atmosphere' always contains more volume of air, it makes more power, or needs less throttle to achieve cruise. But again, what I am struggling to get my head around is that the supercharger always 'pumps' the same volume of air per engine revolution, regardless of everything else. Now obviously this volume of air is enough for the engine to consume the air at WOT while the system can maintain a certain pressure. What happens when the engine is running at 10% throttle, where is all this extra air going?

Am I looking at it the wrong way? Is the supercharger is somehow only able to attain a certain pressure, and the volume of air that it pumps is directly relative to the air being consumed by the engine? So, that if the engine was say somehow able to be rotated by outside forces, the supercharger would be pressurising the intake manifold to a certain pressure, despite the system not taking any air in? I realise this is probably what is happening, but I still see a turbocharger/supercharger as a pump, and the engine as a balloon.

I'm sure someone will be able to explain this. I'm also sure that I'm not the only one who is struggling with this.

Assuming that I understand that, I'm back to my original question. Twin screw vs centrifugal system. From what I can tell, twin screw is superior. It is simpler, provides more boost at lower rpm etc etc. I am comparing the HKS and Bullet systems btw.
I also like how the twin screw system has an extremely short path from the 'pump' to the motor.
The HKS system goes through a lot of piping (FMIC) where as the bullet system has W2A which I am assuming is some sort of plate setup post supercharger, on top of the motor. I would then comment that because of this much shorter 'path' the bullet system would be significantly more responsive. However, that comment is entirely based on my understanding of a turbo system. Would this distance between the 'pump' and the engine actually make a difference in response?
The second point about the two systems is, if the twin-screw is indeed more responsive and able to provided more boost at low RPM, then why would a company like HKS with all their R&D go with a centrifugal type S/C. At the moment, all I can see that this type of system offers is less whine.

[RYU]

I would love to answer whatever questions you have but I got a quarter of the way thru your novel and stopped.

If you have straight forward questions, ask them and we'll comment.

[serialk11r]

In short, superchargers come with a clutch, bypass valve, or both. They will blow off pressure when you lift, but they will build the pressure faster than a turbo because the turbo not only blows off its pressure, its turbine speed also drops. When you get back on the throttle now you need to wait for the turbine to spin back up as well as for the intake tract to pressurize, so the transient response is poorer. A supercharger in theory should be pretty much like NA especially if the supercharger is downstream of the throttle.

Internally wastegated turbo increase backpressure at state cruise because turbos are designed to create some extra backpressure (borrowing engine power like a supercharger does, except less efficiently) to spool faster. Electronically wastegated turbos have less of this problem. This costs a little bit of fuel economy off boost. The bigger the turbo the less boost you have at low rpm so the less you'll notice.

Superchargers are also sure to decrease your fuel economy at steady state cruise, but how much depends on the design. If it's got a clutch that's decoupled, then you don't lose anything. Next best is Roots supercharger with a bypass valve, the power consumption is on the order of a power steering pump or alternator. A spinning centrifugal supercharger will eat up some power, and a spinning twin screw supercharger will eat a huge amount of power.

If you have a positive displacement supercharger (roots or twin screw) there's also the ability to mess with the valve timing a bit for various different effects, that would affect a turbo's spool time.

IMO the biggest problem with superchargers is that there is no widely available efficient positive displacement design. Eaton TVS (the clear winner when it comes to positive displacement SC) can only be found on very few cars, all with rather large engines.

[subatoy]

I got tired of reading half way through that!
Im off to bed.

[diss7]

Originally Posted by RYU

I would love to answer whatever questions you have but I got a quarter of the way thru your novel and stopped.

If you have straight forward questions, ask them and we'll comment.

Thanks for your feedback. I have edited my original post for the benefit of others like yourself. FYI I don't believe my questions were straight forward and I think that is better to ask a better explained question so that the answers and moments received are relative; for the benefit of all.

Sorry, I realise I have exceeded a ten word response. If you're not yet over loaded, please pm with any future issues so that this thread doesn't turn into another flame war.

[gmookher]

wait till sema, buy vortech like I plan to

[TouchMyHonda]

I would be all about a 350-400whp capable sc setup.

[Ravenlokk]

tldr, turbo.

[EvoFanatic]

Originally Posted by diss7

I'm not looking for a turbo/supercharger debate.




Now, this thread was going to be about discussing the differences in the twin screw type supercharger, and the centrifugal type supercharger. But, I've realised that I don't fully understand the supercharging route in the first place.

Before I get into some examples, I should point out that I am aware that my difficulty in completely understanding these systems probably revolves around my understanding of pressure and volume. FYI, my understanding of this is that pressure/volume are directly related. If one container is compressed to 1 Bar, it effectively contains twice the volume of air than it did at atmosphere. I realise that my understanding here maybe flawed, as I have seen discussions on similar turbo systems running the same PSI but one flowing more air than the other. This has never made sense to me.

Anyway, lets look at an example.
I am boosting along in my FI 86. At 5000rpm and WOT it is boosting 10psi and honking along pretty well. I am coming up to a corner that I cannot take at WOT, so I immediately pull back on the accelerator to say 25%.

Now, this is how I understand how a turbocharged system reacts.... As soon as I pull back on the accelerator, the throttle body closes. The air in the intake manifold is then consumed by the engine creating a pressure difference before and after the throttle body. This pressure difference triggers the BOV to open and exhaust the **EDIT- pre ** throttle body pressure. Also, because the motor is now making significantly less power, the exhaust gas volume is significantly less, so the boost that was being used at WOT is no longer generated.

Same situation in a supercharged vehicle. When the throttle body closes, where does this excess pressure/air go? One thing that appears universal in supercharger systems, is that they do not require a BOV. Now, a turbocharged system with no BOV would suffer compressor stalling to relieve the pressure, but I have also read that a supercharger does not do this either. (I would assume this would be very bad for something connected to the crank to want to do something like this) So first question is, where does this initial surplus air go? Is it forced into the engine despite the fact I don't want the power? Surely not. This then leads onto another example.

Lets say that I am cruising along at 5000rpm is 3rd gear on the highway. Now a turbocharged system at cruising throttle would not be producing boost. If I put my foot down, the turbo would spool fairly quickly and I would then be boosting. Yet a supercharged system would be boosting, even though the system effectively doesn't need it.

One way I have tried to understand supercharging, is that it is effectively an NA motor that is always running in a pressurised atmosphere. Because its 'atmosphere' always contains more volume of air, it makes more power, or needs less throttle to achieve cruise. But again, what I am struggling to get my head around is that the supercharger always 'pumps' the same volume of air per engine revolution, regardless of everything else. Now obviously this volume of air is enough for the engine to consume the air at WOT while the system can maintain a certain pressure. What happens when the engine is running at 10% throttle, where is all this extra air going?

Am I looking at it the wrong way? Is the supercharger is somehow only able to attain a certain pressure, and the volume of air that it pumps is directly relative to the air being consumed by the engine? So, that if the engine was say somehow able to be rotated by outside forces, the supercharger would be pressurising the intake manifold to a certain pressure, despite the system not taking any air in? I realise this is probably what is happening, but I still see a turbocharger/supercharger as a pump, and the engine as a balloon.

Assuming that I understand that, I'm back to my original question. Twin screw vs centrifugal system. From what I can tell, twin screw is superior. It is simpler, provides more boost at lower rpm etc etc. I am comparing the HKS and Bullet systems btw.
I also like how the twin screw system has an extremely short path from the 'pump' to the motor.

Okay I will try and answer your questions in a while, I just wanted to highlight what I thought your main points are so I don't forget and have to read your novel again.

[ForReal-Someday]

I believe, what you need to think of, is that the SC compresses air behind the throttle. So the amount of air it compresses is variable depending on throttle position.

It's not like a turbo where the entire intake tract is pressurized. It's only pressurized from the SC outlet through the intake manifold, and then the cylinder.

Hope that helps.

[GTerran]

You really need to find a car builder to be able to properly answer these questions. Most people on forums like this are just going to call it a novel and do nothing for you :P

Ill take a stab at it, but Im coming from a physics and eng background and are newer to cars. so I could be talking out my ass without knowing it.

to answer your final question (the one Im most likely to not lead you wrong).

the difference between cent and screw is that cent is (as the name suggests) a Centrifugal pump, it develops all its compressed air by spinning the ever loving f*ck out of it and momentum carries it out to the eng. These pumps allow for blow-back? which is excessive air pressure down stream pushing back through the turbine. this will cause damage over time and is why these systems on both turbos and cent SC have blow off valves for any mod/high pressure system.

one should note that the differences between a cent SC and a turbo is the operating speed. Both have basically the same compressor, the difference is SC will spin at anywhere between 15-35k RPMs making boost while a turbo will spin well above 100k RPM which is why turbos can make a lot more pressure.


now for the screws. screw type also runs off of crank or accessory pulley just like cent, but it the shape of the turbine or turbines in this case that is different and what give it its different properties. the twin screws create what is called a POSITIVE DISPLACEMENT PUMP as best as I can tell. this means that any air compressed by the pump will not be allowed back trough it on anything resembling normal operating pressure. THEORETICALLY it could easily compress air to hundreds of PSI before exploding if enough force/speed is put to it. now, normal or race driving will never see that, you would have to attach a new drive motor to create that situation. i'm speaking all theoretic s, the SC is not designed for those applications and will destroy itself long before hand.

Now, at idle, SC actually run at a slight vacuum. I believe this is because of the design characteristics but it prob best for the engine to not be under pressure when its just sitting their.

Superchargers Still have to deal with what is called 'Heat of Compression' which is the air being 'heated' by putting more of it in a given volume. Its this property of gasses and liquids that will throw off your 1bar=2x vol rational. most SC dont compress the air enough to have to worry about excessive heat gain over ambient temperature, def so on DD applications.

it also makes a very lovely sound when the engine is running :B



Downside of SC in street applications.

The biggest is going to be vampiric losses, It takes work (HP) to drive the pulley on a supercharger, this is compensated in most applications when the engine is under load by its ability to compress the air. the one time its actually hurting you is in DD applications, this is because it doesn't actually take much power to keep you at the same speed on the highway; therefore, your Mile Per Gal decreases by a measurable percent. example: say it takes 25 hp to maintain 70mph on the highway, the SC you attach has a 5 hp draw on it. now you are using 30 hp to maintain the same speed.

As long as you stay to a small application of SC, that's about all you have to suffer really.



/rant

[madfast]

Originally Posted by diss7

Before I get into some examples, I should point out that I am aware that my difficulty in completely understanding these systems probably revolves around my understanding of pressure and volume. FYI, my understanding of this is that pressure/volume are directly related. If one container is compressed to 1 Bar, it effectively contains twice the volume of air than it did at atmosphere. I realise that my understanding here maybe flawed, as I have seen discussions on similar turbo systems running the same PSI but one flowing more air than the other. This has never made sense to me.

ideal gas law: PV=nRT so pressure and volume are inversely proportional. if you have a fixed volume and you pressurize it, you will have LESS volume. think of squeezing a balloon, it will make it smaller.

however pressure in and of itself is NOT the same as boost. you must think of boost as people filling up a room. the people are air molecules and the room is the intake manifold for instance. boost only occurs if the amount of people coming in the room is more than the amount leaving the room. what happens is there is a "back up" of air molecules. and since the volume is fixed, there is a rise in pressure.

Same situation in a supercharged vehicle. When the throttle body closes, where does this excess pressure/air go?

One thing that appears universal in supercharger systems, is that they do not require a BOV.

on a positive displacement SC there is a bypass valve that recirculates the excess air. also dont forget that the supercharger is connected to the crank. so if you lift off, the engine rpm goes down and so the sc rpm goes down, thus less air. on a turbo, the compressor wheel will continue to spin independent of the engine rpm, thus the need for a bov.

[ame="http://www.youtube.com/watch?v=gKViNCZhIqc"]Eaton TVS Supercharger - YouTube[/ame]

Yet a supercharged system would be boosting, even though the system effectively doesn't need it.

why would the SC be boosting? the SC spins in relation to the engine rpm. so if the engine is at low rpm, the SC is also spinning slowly. is it still moving air? yes. what happens if that air is still too much for the engine? bypass valve.

But again, what I am struggling to get my head around is that the supercharger always 'pumps' the same volume of air per engine revolution, regardless of everything else. Now obviously this volume of air is enough for the engine to consume the air at WOT while the system can maintain a certain pressure. What happens when the engine is running at 10% throttle, where is all this extra air going?

with a PD SC because it is connected to the crank, the amount of air pushed by the SC is proportional to the amount of air used by the engine.... theoretically... ideally....

Is the supercharger is somehow only able to attain a certain pressure, and the volume of air that it pumps is directly relative to the air being consumed by the engine?So, that if the engine was say somehow able to be rotated by outside forces, the supercharger would be pressurising the intake manifold to a certain pressure, despite the system not taking any air in? I realise this is probably what is happening, but I still see a turbocharger/supercharger as a pump, and the engine as a balloon.

that's exactly what is happening. thats also why people say a SC makes max boost at idle. that "conventional wisdom" is probably rooted in the old school SC V8 muscle cars of the past. most likely back then SC tech wasnt good enough such that a properly sized SC would not push more air than the engine consumed. thus boost wont rise more than that at idle. remember that boost is backed up air molecules. you have to push more air than the engine can use to make boost. so if you cant push more air than the engine uses, you wont make more boost

The second point about the two systems is, if the twin-screw is indeed more responsive and able to provided more boost at low RPM, then why would a company like HKS with all their R&D go with a centrifugal type S/C. At the moment, all I can see that this type of system offers is less whine.

probably cuz cent SC is way easier to make. with a PD SC a proper setup would involve a bespoke cast SC housing/manifold with integrated A-W IC...


now lets examine what happens in a low displacement 4 cyl... http://www.hotrod.com/techarticles/e...s/viewall.html


this is the classic "flat" boost curve (and subsequent tq curve) that we all associate with PD SCs...

now what happens if we add some cams?

boost actually goes down! we are not changing how much air we push, but we changed how much air the engine uses. thus less back up of air molecules, thus less boost. boost curve is still pretty flat though...

now lets add a bigger SC


look at that boost curve. what happened to max boost at idle? turns out if you push hella amounts of air into a 2.0L 4 cylinder, the air gets backed up and thus we get more and more boost... take note that the TVS SC is almost as large as the engine itself...

[carbonBLUE]

Originally Posted by madfast

ideal gas law: PV=nRT so pressure and volume are inversely proportional. if you have a fixed volume and you pressurize it, you will have LESS volume. think of squeezing a balloon, it will make it smaller.

however pressure in and of itself is NOT the same as boost. you must think of boost as people filling up a room. the people are air molecules and the room is the intake manifold for instance. boost only occurs if the amount of people coming in the room is more than the amount leaving the room. what happens is there is a "back up" of air molecules. and since the volume is fixed, there is a rise in pressure.





on a positive displacement SC there is a bypass valve that recirculates the excess air. also dont forget that the supercharger is connected to the crank. so if you lift off, the engine rpm goes down and so the sc rpm goes down, thus less air. on a turbo, the compressor wheel will continue to spin independent of the engine rpm, thus the need for a bov.

why would the SC be boosting? the SC spins in relation to the engine rpm. so if the engine is at low rpm, the SC is also spinning slowly. is it still moving air? yes. what happens if that air is still too much for the engine? bypass valve.



with a PD SC because it is connected to the crank, the amount of air pushed by the SC is proportional to the amount of air used by the engine.... theoretically... ideally....




that's exactly what is happening. thats also why people say a SC makes max boost at idle. that "conventional wisdom" is probably rooted in the old school SC V8 muscle cars of the past. most likely back then SC tech wasnt good enough such that a properly sized SC would not push more air than the engine consumed. thus boost wont rise more than that at idle. remember that boost is backed up air molecules. you have to push more air than the engine can use to make boost. so if you cant push more air than the engine uses, you wont make more boost




probably cuz cent SC is way easier to make. with a PD SC a proper setup would involve a bespoke cast SC housing/manifold with integrated A-W IC...


now lets examine what happens in a low displacement 4 cyl... http://www.hotrod.com/techarticles/e...s/viewall.html


this is the classic "flat" boost curve (and subsequent tq curve) that we all associate with PD SCs...

now what happens if we add some cams?

boost actually goes down! we are not changing how much air we push, but we changed how much air the engine uses. thus less back up of air molecules, thus less boost. boost curve is still pretty flat though...

now lets add a bigger SC


look at that boost curve. what happened to max boost at idle? turns out if you push hella amounts of air into a 2.0L 4 cylinder, the air gets backed up and thus we get more and more boost... take note that the TVS SC is almost as large as the engine itself...

it takes a long time to come up with a response like that! glad people do their research first!

[Mechazawa]

Here is my $0.02. The OP has a lot to learn about forced induction systems, he is going to get a lot of information on forums that is not necessarily totally correct (and this is not a comment on any previous post).

I recommend. [ame="http://www.amazon.com/Maximum-Boost-Turbocharger-Engineering-Performance/dp/0837601606/ref=sr_1_1?s=books&ie=UTF8&qid=1350053594&sr=1-1&keywords=maximum+boost"]Maximum Boost: Designing,Testing,and Installing Turbocharger Systems (Engineering and Performance): Corky Bell: 9780837601601: Amazon.com: Books[/ame]

and [ame="http://www.amazon.com/Supercharged-Testing-Installation-Supercharger-Systems/dp/0837601681/ref=sr_1_2?s=books&ie=UTF8&qid=1350053669&sr=1-2&keywords=bell+supercharging"]Supercharged! Design,Testing and Installation of Supercharger Systems: Corky Bell: 9780837601687: Amazon.com: Books[/ame]

I come from the Miata world where Corky is pretty revered, If his Miata kits are any indication, there shouldn't be any doubt that the guy knows his stuff. Read these and you will know more than most of us.

I have built a pair of good functioning turbo systems from scratch over the years, so let me cover bypass valves and wastegates quickly.

Turbo systems have two pressure relief valves, a wastegate that relives exhaust manifold pressure, and a blow off/bypass valve that releases pressure from the intake system between the turbo compressor and the throttle valve.

It's a little counter intuitive, but the wastegate regulates boost. It allows some exhaust gas to bypass the turbine/exhaust side of the turbo. It is triggered to do this by intake manifold pressure. So what happens when boost gets too high is the wastegate opens and less exhaust gas goes trough the turbine, some of it will instead be diverted downstream of the turbine or to the atmosphere depending on the specific implementation. With reduced exhaust flow through the turbine comes reduced torque on the turbo shaft which causes it to turn more slowly against the pressure it is generating on the intake side, this reduces gas flow through the intake side of the turbo, and therefore intake manifold pressure (AKA boost).

Now the bypass/blow off valve is used to relieve pressure in the pipe (part of which may be an intercooler) connecting the compressor/intake side of the turbo to the throttle body. This is only needed when you suddenly reduce the throttle opening, the sudden reduction of gas flow into the engine will cause a pressure spike in the previously mentioned pipe. If it's bad enough the turbo assembly can actually turn backwards, in any case it will slow down, it can cause damage to the turbo, but it will also result in slower throttle response when you open the throttle valve back up (because the turbo has a greater rpm differential to overcome). The bypass/blow off valve releases this pressure when the spike occurs and prevents damage to the turbo while improving throttle response.

Corky explains that a lot better than I do though.