The theory of boost?

[Stealer Of Souls]

Okay. I'm no fluid engineer...

Making boost.

Lets say that I had two identical turbos. Each produces 0.5BAR (max 240cfm) on engine A. At full flow 240cfm makes 0.5BAR on this engine.
If i could drive both turbos in parallel (I use magic) and both would behave exactly the same and exactly as if they were the only turbo attached, would I get three times the boost pressure, ie 3BAR (480cfm)?
And by this reasoning to make 1BAR each turbo needs to flow 160CFM?

Okay. Phase two.
I now take one of those turbos and put it on an engine which normally inhales 180CFM at max power. I put this turbo on and magically it produces 240CFM at the same max power RPM as before. Does the boost gauge read 0.33BAR boost?

Right. Now the application.
I have my magical turbo application. It flows 240cfm (@0.5BAR) into engine A at max power. It's overtake time and I push my red button which magically fires up turbo two (without affecting turbo one) and sets turbo two to flow 80cfm (irrespective of current boost level). Do I get my 1BAR max boost?

[drftnmaz]

damn you make that sound completed...

i can't answer your questions but to put it simply just get a wastegate set a 1bar and you'll get 1bar...

[Punter]

damn you make that sound completed...

Yes very completed

I understood boost (as in number of psi) to be related to the restriction of the engine. (like on a gze levin when you crush the exhaust the boost goes up) Therefore the relationship between boost and CFM would not be linear.

I could be wrong though ***Waits for someone clever***

[Lanius]

Without being an engineer either, I would presume you would have big problems with air velocity when you open the magic second turbo.

Air being diverted into the second turbo would reduce the airflow through the first turbo, causing either a drop or at least a flat spot in boost, until the second turbo gets up some revs. Even then, the reduced flow might not be enough to spin both turbos to the point where you get as much boost as you wanted from them.

[Ako]

Why, that idea is just crazy talk. Just have turbo A run .7bar instead, problem solved.

IF turbo A is too small to provide said boost level, exchange turbo A for something more suitable (bigger is better kiddies).

Turbo B can spend its time relegated to doing something boring, like working on a 1.3L deisel... That'll learn it for being a measly little midget turbo like you have described.




To translate that - Twin sequential setups are a waste of time and effort, just run one properly sized turbo (or two in parallel).

[Silent Knight]

To translate that - Twin sequential setups are a waste of time and effort, just run one properly sized turbo (or two in parallel).

Didn't seem like that much of a waste of time in the 2J...

[fivebob]

IF turbo A is too small to provide said boost level

Turbos do not provide boost, they provide the flow, engine restriction provides the boost.

[Ako]

I meant in terms of - if the turbo physically can't provide the flow needed. Think TD04 on 2L engine and you'll see what I mean.


Uuurgh... This question is worded too confusingly for me!


And I still stand by the fact that sequential setups suck balls.

[CozmoNz]

And I still stand by the fact that sequential setups suck balls.

mind explaining why there is a blower on that engine then? true, it does have dual paralell turbos hanging off the side, however the sc will be there for one reason, the same thing a small turbo in a sequencial setup is for.

[fivebob]

I meant in terms of - if the turbo physically can't provide the flow needed. Think TD04 on 2L engine and you'll see what I mean.

I understood what you meant, it’s just that your terminology was wrong, as is most peoples understanding of boost and flow. Boost does not provide power, flow does, boost level is just an indication of restriction in the engine.

Uuurgh... This question is worded too confusingly for me!

And I still stand by the fact that sequential setups suck balls.

I think you will find that this is just a theory question, not related to the practicalities of running sequential turbos.

[Ako]

I meant in terms of - if the turbo physically can't provide the flow needed. Think TD04 on 2L engine and you'll see what I mean.

I understood what you meant, it’s just that your terminology was wrong, as is most peoples understanding of boost and flow. Boost does not provide power, flow does, boost level is just an indication of restriction in the engine.

Uuurgh... This question is worded too confusingly for me!

And I still stand by the fact that sequential setups suck balls.

I think you will find that this is just a theory question, not related to the practicalities of running sequential turbos.

I know what you mean bud. I should have worded things better - this is TS afterall.


Cozmo - thats not a sequential turbo setup, thats a twin-charged setup. Probably works just great, but bugger it, whats the point in trying to go fast at 3500rpm with an engine capable of 9500+?


Ok, probably openeing up a can of worms I can't be bothered with.

[Stealer Of Souls]

I think you will find that this is just a theory question, not related to the practicalities of running sequential turbos.

And besides... In the application example given the turbo was fine for everything, and then for some random reason I wanted an extra bit of power, and for some strange reason I chose to use a second turbo. Heck I don't know why. Just fitted the question.



So am I right? For a engine A 160CFM is max NA flow. So 320 CFM would be 1BAR and 480CFM is 2BAR. No matter how the flow is made (single biggie, many littlies), as long as the flow is there (160/320/480CFM) the boost level for engine A will be as said (0/1/2BAR).
(Oh and let's ignore the possibility of flow going backwards through one of the turbos, lets just say it always magically only goes forward to the engine)

[fivebob]

So am I right? For a engine A 160CFM is max NA flow. So 320 CFM would be 1BAR and 480CFM is 2BAR. No matter how the flow is made (single biggie, many littlies), as long as the flow is there (160/320/480CFM) the boost level for engine A will be as said (0/1/2BAR).

No, it's not a linear relationship. Theorectically New Flow Rate = Old Flow Rate x the square root of (New Pressure divided by Old Pressure), but that may not be what happens in real life as the infernal combustion engine is a bit more complicated than the usual "pressure vs flow" model.

[Si]

Didn't seem like that much of a waste of time in the 2J...

or subaru.

or the FD batmos.




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[Adamal]

Didn't seem like that much of a waste of time in the 2J...

or subaru.

Question, have you ever driven one of the older twin turbo Subaru's?

[Ako]

He's taking the piss bud - he referred to the valley of death at the bottom of his post

A single, decent sized turbo tends to give better driveablility than the stock twins on either the FD or the JZ anyway!

[Adamal]

I wouldn't be too sure on that. The FD gives good response from its twin turbo setup, its just that the way its setup is a bit finiky and known to crap out.

You could always look at other twin turbo setups like the 1JZ and 2JZ, as well as the RB26DETT in the GT-R's.

Do you think the GT-R's and Supra's perform so well because of the crappy twin turbo setup?

You know why they go for a twin turbo setup on those, don't you?

And I tend to switch off when I see silly net speak cliches

[Dell'Orto]

Supras and GT-R's aren't sequential (afaik) which is why they work better. The FD's setup is still pretty average, with the primary tailing off before the secondary is at full noise.
I drove a 00 model GT Legacy last week, and the VOD was unbelievable...virtually no boost between 4 and 5000rpm

[CozmoNz]

arnt late model leggys single turbo again?

supras are seq twin turbo (2jz only) 1jz are just "2 turbos" same with the rb26.

god, end of the day THEY design the cars, not us, moan all you want but they did it for a reason.

and ako, i stated its not sequencial, i said its just 2 turbos with a blower attached, but thats the point in the blower.... to act as a smaller turbo :-/ (obviously without taking exhaust from the bigger brothers sitting directly above it).

[Dell'Orto]

arnt late model leggys single turbo again?

Yep, but a 2000 model is now 6 years old not really a late model as such.