i8acobra

Here, you're incorrect. Turbos aren't spooled by exhaust velocity. It's all about heat and pressure differentials. Huge amounts of backpressure in front of the turbo and no backpressure after it causes the turbine to spin. Garrett is correct. Corky Bell's book explains this in detail. There also is no need for scavenging on a forced induction motor because the intake is under pressure, not vacuum.

Whatnot

Here, you're incorrect.....I read Corky's book and am familiar with its prenciples. Velocity and pressure are directly correlated to volume. Heat is just a factor in that with high heat, means higher pressure, so heat is a indirect factor leading to pressure, which is the direct force and velocity. Are you familiar with Http://en.wikipedia.org/wiki/Bernoulli's_principle and fluid dynamics? If you have a fixed volume, if you have a certain pressure, the velocity will be a certain velocity. If you increase that pressure, the velocity increases. If you decrease the volume, the presusre increases as does velocity, if you increase the volume, the pressure decrease as does velocity.



You have a fixed volume at when the piston is at the bottom of travel, and as it moves up, the volume is quickly decreasing, increasing the pressure, the velocity of the air increases too. The air will try to escape to a lower presusre area (the exhaust piping) and the best route is the exhaust port, the tiny hole can't release air as fast as the piston is moving it, and you have a very high pressure and gas velocity out of the exhaust port. You have a smaller exhaust port, higher velocity, larger, lower velocity. Though changing exhasut port size would be expensive and too difficult, so they do this on the turbine of a turbo....the A/R. The smaller the A/R the smaller the nozzle, the much higher pressure and velocity causing the turbine to spool very fast. If you have a larger A/R, your gas doesn't need to escape to the lower pressure area as quickly and you have a lower pressure and a lower velocity. and it spools slower. But with a large A/R, when you flow a ton of air, you get opitmal gas pressure/velocity to spool the turbine and resulting compressor to give a ton of bosot. With a small A/R, when you are high rpm (a ton of air) you can only create so much pressure, and your will now cause your piston to really have to push, robbing a buch of tq from the crank, and this is 'choking out' your turbo. Which is why expensive turbo's have a way a of changinb the A/R, giving you small fast spooling A/Ra t low rpm, then it has higher AR/R at high rpm to not chocke it out.



Though with FI, you need the piston to push the exhaust up and out of the cylinder, as that high pressure is needed pre-turbine as you can only get so low of a pressure post-turbine. That is what I meant by 'purposeful backpressure'. But if you have a nice turbular manifold, you can still get a slight bit of scavenging from the engine and that momentum of the exhaust gas can still have a high enough pressure to spool the turbo. So the ex man is helping reduce tq on the crank, and you have ex man scavenging, and the tbe is helping reduce the pressure on the turbine outlet, helping spool faster (debateble aparently). So with a turbo, you have two forms of scavenging.





Though I don't have enough money to test or mathimatical/physics degree to prove it, which is why I merely said, I disagree with what Garett says about turbines having no scavenging effect post turbo. As you said even, the lower the pressure behind the turbo, the easier it will spool. So you said Garett is correct, but you then agreed with me, saying a turbo is driven off of pressure diffirential. So I am not sure which one you think??? It makes sense to me that a a turbine would merge the exhaust pulses making it een more effective at scavenging.

wheel_of_steel

I like your way of breaking it down whatnot. The difference between FI and NA intake is of a different nature than what you listed. Or rather, your torque requirement theory sounds fine, but like you said, it probably only makes a negligble difference. The fundamental difference between the two intakes is that NA engines have a relatively undisturbed intake manifold, whereas turbo engines are just blustered full of air. So in an NA motor, there is a chance for some elegant harmonic activity in the air.



This is because of the cyclical nature of a four stroke engine. The intake charge has inertia, so when the intake valve closes, this fluid actually piles up against the back of the valve - then bounces back up the manifold runner. Then it hits the top of the intake manifold end tank, and bounces back down the runner again. Much like exhaust tuning, by changing the intake manifold characteristics, you can change the revs at which the engine makes use of this lovely free HP. Thus we see systems like VRIS on Mazda's KL-DE engines - in fact almost every manufacturer has some sort of short/long runner or tuned intake plenum in their lineup.



Turbo cars tend to ignore this, as far as I can tell. Maybe an OEM with advanced modelling could still manage to tune this, but the effect is far less noticeable with turbocharged cars. I can't comment on supercharged cars, but FWIW I don't see much discussion of intake manifold theory surrounding supercharged performance cars. My best guess would be that the very high air pressure in the intake manifold would make the air far more stagnant and resistant to quickly darting about the manifold.



I think that garret means that there isn't post-turbo scavenging in the traditional sense of the word. A large pressure differential is desirable for spool, but there's no actual 'pulse tuning' to bother with in this case.

Whatnot

Hmm, never thought what Garett say like that. Maybe I did read into that wrong to what he was getting at. Though I never actually heard/read him saying it, just someone else saying what he said. It was in regards to a 5" tbe. And I was told that Garett said the largest diameter or open turbine outlet was optimal for power/spool as there is no scavenging past that.



Though now that I think about it, perhapse the turbine takes away so much energy from the exhaust gases that the velocity is too low to scavenge? So it just causes turbulence and backpressure and the larger dia. the less turbulence and back pressure?

i8acobra

There is no "sucking" with forced induction. When the intake valve opens, air is forced into the cylinder. This also negates the need for scavenging since even forced induction motors have some overlap. The pressurized intake air pushes out any exhaust still in the cylinder.



I'm really not even a little bit interested in debating principals that are already written in stone.

187sks

^That.



Correct post is correct.

Whatnot

I never said there is sucking in FI, If you read the quote you quoted me I clearly say "So with NA". The next paragraph I wrote (and deleted as it was unecessary) about FI, but again, I never said anything about sucking.

faithofadragon

anyone else ever notice that this guys posts are ALWAYS a f*cking novel?



TLR any of it

Whatnot

But I am sure everyone noticed this guys posts are USUALLY long. If you don't read them, that is fine. You aren't the one who A, asked for help, or B questioned a simple concept requiring a more in depth explanation and/or examples. Being that it is online and open source, you are absolutly free to read it if you wish, as is anyone else who choses to, but if this was a conversation, you weren't in it, and it wasn't aimed at you, so it isn't realy necassary for you to read it.



Though your post was probably meant to be rhetorical in an insulting manner, I decided to answer it with the obvious answer, and attempt to draw out a question that doesn't need an answer *as a rhetorical question doesn't) and upt more length on it, making this relativly short post of mine actually rediculously long in that it requires no answer at all and I am still able to get a few paragraphs out of it lol. Turning an insult and laughing at me, to me laughing and finding humor in myself....and possibly annoying you even more lol

i8acobra

Ummm... every single 4 stroke engine on the planet... intake, compression, power, intake, etc... firing order has nothing to do with it. As the piston nears the top of the exhaust stroke, the intake valve starts to open, so the intake and exhaust valves are open at the same time. For the record, your post makes it clear that you don't know what "overlap" is. "Overlap" the the time when both the intake and exhaust valves are open. You do know that happens, right?



Again... set in stone.