Twin Turbos

[Greg_S]

Can anyone explain how different twin turbo systems work. not ala RS4 and RS6 but more along line of runnign two turbos in line or together so you have 1 big and 1 small or 2 smaller turbos etc. and how the ecu and running gear would go about controlling it all?

Greg

[bettonracing]

Basic overview as complete reply would require a webpage in itself:

Two same size turbos: ideal for engines with enough cylinders (when divided) to spool up two turbos. eg. 6cyl, 8cyl. V or I.

Benefits: use mid size turbos, quicker spool up time(than single large), still large airflow numbers, good alternative for street car w 6+cylinders.

Drawbacks: space. & making sure turbos are somewhat balanced in terms of turbo to int manifold pipes/intercoolers/etc.

Control: relatively similar to single turbo with few exceptions (wastegate controls... can be independent or simult. indep = better. can be remedied with POV as bopost controller). Boost is read in the manifold where it should be the product of both turbos hence fuel req is calculated normally (after calibration for increased airflow).

Sequential turbos:
1 small, 1 big. Small turbo spools fast, big turbo = hp. Many ways to 'split' them. Mazda uses a whole bunch of air hoses (rat nest!) to measure/move parts. Switchover point is precalculated using compressor/efficiency maps and varies slightly from preset point if manufacturer decides to allow 'compensation'. Other manufacturers use electronics/ boost controllers to determine switchover point. Exh manifold setup is usually engine-to-small-turbo with bypass pipe to big turbo on demand. I've seen eng to small turbo to big turbo (no bypass) which I considered to be praiseworthy (being sarcastic).
Benefits: obvious full range boost (lo-rpm/sml turbo/quick spool; hi-rpm/big turbo/hp) however I still hear 'lag' complaints about sequential setups (can't please u humans!).

Drawbacks: Complexity. sometimes space. complexity. more potential for failure (more parts). Complexity. Did I mention complexity? well just incase I didn't: Complexity.

Controls: Switchover pnt can be boost or rpm dependent. Usually rpm (b'c small turbo can reach full boost and would prolly maintain full boost for a few h'dred rpms before switchover). Bypass pipe has 'flap' to switch flow direction. Flap controlled by actuator (pneumatic or electronic).

hope this helps. lemme know if u want more details.

Planning a project?

[Riz_RS4]

Wicked info [img]images/graemlins/thumbs.gif[/img]

Can you imagine a RS3 with DSG!

No drop in rpms hence no turbo spooling when changing gears??? so the turbo`s are constantly running....... question is......

a) do turbo`s wear faster if your always in and out of turbo range OR
b) do they wear out if your constantly running them???

Cheers [img]images/graemlins/s3addict.gif[/img] [img]images/graemlins/thumbs.gif[/img]

[Crafty]

No drop in rpms hence no turbo spooling when changing gears??? so the turbo`s are constantly running....... question is......

Flat shift will give you the same thing [img]images/graemlins/grin.gif[/img]

[Greg_S]

well the reason i ask is a friend of mine has a mitsi 3000GT which use's sequential turbos and wondered how it was all set up. Also heard a few people talking of sequential turbo project using same engine as in my S2 for building big power cars in the states.

Before i try a sequential setup i think i need to take a few steps back and look at a normally aspirated or single turbo setup which is less complex since it would be my first project in that respect.

Have you experience with building these setups?

[bettonracing]

Riz S3:
The turbo pressurizes the intake and therefore has to push against this pressure = gradual death. They've been built to last but will die one day (maybe longer than the lifetime of the car but who knows).
on boost always = quicker death.
BOV are good to keep the turbo spooled (to some extent) but driving style (aka flat foot as per 'Crafty'. [doesn't have to be flat. just enuff for boost]) is one upgrade and an antilag turbo program is the ultimate upgrade. But that's another topic in itself (READ: engine soup)

Greg:
I've done a few of these setups but I've found them to be a pain in the a** to build and to tell the truth I don't see the ultimate benefit of them. And I've seen big power seqTT cars lose to properly built (in some cases) single turbo cars on a regular basis. What advantage of sequential systems makes u want to go that way? Also they tend to have boost spiking (usually a dip but up spikes too) problems at the switchover point if u're not meticulous with the setup.

My personal opinion: SeqTT setup is good for the dyno sheet (flatter torque curve) but the advances in turbo tech are making them obsolete. There are other few 'tricks of the trade' (I hate those words) to get low end torque. Plus there's always N2O... [img]images/graemlins/burnout.gif[/img]

[Greg_S]

well it will be some time before i have the funds, time and car to carry a conversion out on. Im not looking at making a sequential turbo setup just was interested in the workings off it and reading back on the history of different systems that have been used in the past.

would like to do something a bit mad to a mk1 golf one day, but that will have to wait for now.

[bettonracing]

Long detailed explanation but pretty much EVERY seq setup is going to have a flat spot at the switchover. Minimizing the duration of the flat spot or compensating for it is where genius comes in. Ever notice that most people who modify their seq setups usually opt for larger single turbo (some go twin turbos)?
Seq setups are similar to capitalism. It eats itself. To really benefit from seq setups, u end up losing the added benefit of the smaller turbo (exponential equation in favor of big turbo). Think intake airflow, exhaust airflow, boost levels and the switchover point. Remember that the smaller turbo needs less airflow to spool whereas the larger needs more. Figure it out. (a little more rewarding than just spilling everything) I'll still help though. Just ask.