bi turbo operation

[biturbo hero]

can anyone please explain the operation of the S4 bi turbo system?
and how does this differ from other multiple turbo setups. i have been asked many times and have given too many insufficient answers.

[blacka4]

um I am not sure what you are asking here. there is 1 turbo for each bank of cylinders. thus the 2.7T being a V6 there is 3 clyinders per head and one turbo per 3 cylinders, thus 2 turbos. then all you see that bipipe on top it feeds the turbos air then the piping is intercooled via 2 intercoolers (one per turbo) and then fed into one throttle body at the front of the engine under the bipipe and cover. hope that helps. someone will post a pic soom

[biturbo hero]

yeah i got that.
i guess what i'm asking is if both turbo's spool up at the same time, or one at a time.
i always thought that both turbos spool up at the same time feeding the throttle body in equal amounts, but i always wondered how this differed from twin turbo setup. if there is a difference between twin turbo and bi turbo.....

[ROUGHEDGES]

you also have to think about a twin turbo setup on a straight six opposed to a v6 engine.

[biturbo hero]

can you explain the difference between 2 turbos on a str8 6 versus bi turbo on a V6??

[pythaguras]

both turbos spool up simultaneously. i believe if one spooled, then the other it would be called a twin turbo, not a bi-turbo. someone correct me if im wrong

[blacka4]

ok i know they both produce boost at the same time on the S4. as for a straight 6 i believe that one starts them the other starts later in the rev pattern this is why most ppl who have TT supra's opd and do a single turbo upgrade to reduce a lot of lag

[Ben@SpeedBarn]

I don't theink "twin" or "bi" have anything to do with the setup. I think Bi is just used by audi. It would be a parallel or series setup that would differ.

[blacka4]

that too ben

[EricPA]

Twin turbo and biturbo are terms that are not consistent among car makers. However, there are two different ways to use two turbos. Two equal size turbos that work independently of each other (B5 S4) or one smaller turbo that spools up to a larger turbo.

[twinsnailsS4]

Our car's bi-turbos (or twin turbos) run parallel and work simultaneously producing boost. This differs from other twin turbos, like EricPa said, in that they are non-sequential.

An example of sequential set up is the FD RX-7. There is a primary turbo (the bigger one) that spools in lower rpm's and then once a higher range is reached, the secondary turbo kicks in and provides more boost (a healthy RX7 would go 8-10-8 lbs if I remember correctly).

The advantage of a twin set up is less turbo lag as opposed to a single turbo, which is usually a huge turbo and won't begin to spool until a lot higher in the rev range. Of course, once it does, hold on! Hope this helped.

[aussie_s4]

Originally posted by twinsnailsS4
Our car's bi-turbos (or twin turbos) run parallel and work simultaneously producing boost. This differs from other twin turbos, like EricPa said, in that they are non-sequential.

An example of sequential set up is the FD RX-7. There is a primary turbo (the bigger one) that spools in lower rpm's and then once a higher range is reached, the secondary turbo kicks in and provides more boost (a healthy RX7 would go 8-10-8 lbs if I remember correctly).

The advantage of a twin set up is less turbo lag as opposed to a single turbo, which is usually a huge turbo and won't begin to spool until a lot higher in the rev range. Of course, once it does, hold on! Hope this helped.

Not that I know anything about RX-7's but you'd think the smaller turbo would spool at lower rpm's, and the larger turbo would kick in at larger rpm's to give much more boost. A bigger turbo would have more inertia, so it wouldn't be the one used at low rpm's when there's less exhaust gas to spin the turbo.

[Jooce]

Originally posted by aussie_s4
Not that I know anything about RX-7's but you'd think the smaller turbo would spool at lower rpm's, and the larger turbo would kick in at larger rpm's to give much more boost. A bigger turbo would have more inertia, so it wouldn't be the one used at low rpm's when there's less exhaust gas to spin the turbo.

^^^ What he said...

[7trees]

You are 1/2 way correct, but it is just the opposite - they work like the gears on a bicycle. You need to use the biggest rear cog in the back of the bicycle to spin the wheel, once it gets going, you shift to the smaller cog to easily maintain the speed, or increase it. A bigger turbo is needed 1st to pull more air in quickly to create the inertia(this is where u are on the money). Then switch to the smaller for higher rpm's which the bigger turbo can't reach.

[aussie_s4]

Originally posted by 7trees
You are 1/2 way correct, but it is just the opposite - they work like the gears on a bicycle. You need to use the biggest rear cog in the back of the bicycle to spin the wheel, once it gets going, you shift to the smaller cog to easily maintain the speed, or increase it. A bigger turbo is needed 1st to pull more air in quickly to create the inertia(this is where u are on the money). Then switch to the smaller for higher rpm's which the bigger turbo can't reach.

Perhaps on your car, but in the real world, big fan--more air, little fan--less air.

Your logic seems confused on exhaust (turbine) and compressor sides of the turbo. The exhaust side is heat resistant (typically) heavy nickel alloy and is always smaller than the compressor side (the gear logic). This leverages the bigger but lighter aluminum compressor side. If they were both the same size there would be power loss due to inefficiencies of mechanics.

Small turbos are less thermally efficient and overspin creating turbulence hence more heated air. The bigger the turbo, the more airflow and less heat. A larger turbo at the same boost will make more power due to more airflow and cooler air. The engine needs more air at higher rpm's. High airflow makes power, boost is a product of the high airflow.

Go search on Google for compressor flow maps, and see the comparison of airflow to pressure @ turbine rpms. Small turbos spin faster sooner, creating more low end boost and less lag. After that they create more heat and loose boost at high rpm as they cannot keep up with airflow at the high boost. Stock cars use them so the lag is less...so is the power. You dont see small turbos on Lotus or Porsche for that reason.

Or just search on Google for Sequential turbochargers

[twinsnailsS4]

Aussie S4 is very astute. And I, suffer of dyslexia. I always confused the two (primary and secondary and bigger and smaller) since it is almost counter-intuitive that the bigger one isn't the primary one. I also reversed the boost, a healthy RX7 will show 10-8-10, no 8-10-8. But, I have been laboring under a mistake unfortunately.

A smaller turbo will begin to spool earlier and at a lower RPM than a larger turbo because less exhaust is needed to make it run. But it doesn't matter in the RX7 context because the RX7 has the same sized turbos [Twin Hitachi HT12 turbos w/51mm turbine & 57mm compressor, except the primary has a difference in its compressor housing]. Despite the same size, (and this is the part I am trying to find out...I will post tomorrow when I get my RX7 book out of the garage), the secondary turbo won't kick in until around 4500. I will post a good write up on it tomorrow. It has been a long time since I have been in the RX7 stuff.

I am glad there was good discussion, too bad it was prompted by a mistake (worst is it was mine). Sorry boys

[indica]

yeah i have seen a FD on a dyno and it has a very interesting power curve. You can actually see where the promary turbo cuts out and switches to the secondary turbo quite clearly on the dyno chart. it dips down right around 4500 RPM and then spikes back up. great explanation of the twin VS bi btw!!

[Agents4]

Hmmm...I always thought the Bi-Turbos went swung both ways?!?