Twin Turbo? (petrol)

[dpsubaru]

I dont know much about this so really need some help but I understand that larger turbos tend to kick in at a higher rev range, i.e above 3000rpm resulting in turbo lag before this (although they can be adjusted to start lower) - right so far? . If so, when heavily uprating an engine (am thinking to buy a second scooby 2.5ltr base and completely mod the engine) to around 450-500bhp is it worth getting a second smaller turbo that begins at say 1000rpm up to around 3500rpm so it overlaps with the second turbo eliminating all lag and giving continual boost. Waiting to learn so comments please (fyi: when this is done it will prob be on an ej207 scooby block). all comments appreciated,

 

[HDi fun]

It's not just revs dependent. Engine load and throttle opening all affect this. Big turbos will work at lower revs it's just that the inertia of the thing means it needs more time to spool up. Note that this is not quite the same thing as turbo lag.

 

[pgarner]

youd still have the lag from the 1st one, youve then got the problem of getting enough air through the wastgate of the 1st to feed the bigger one or the air going over the 1st could case it to overspeed

 

[dpsubaru]

noted thanks so in the case of twin turbos how does it work, they both kick in at the same time?

 

[charger69]

with "Twin Turbo" I understand when u take 2 turbos, and you connect them both one to other, and with the air intake and petrol pump?
so instead of havin pressure of 0.6 bar, you have pressure of 1.2 bar.

 

[dpsubaru]

c69: if thats the case then they would need to be the same size to stop potential overspeed? my theoretical idea would have both turbos on seperate systmes with an electronic valve that kicks in and stops the flow to the first one (to stop overspeeding) and opens up the flow to the second one (would use an overlap range of around 500rpm to ensure continuous flow). This is only theory that i'm exploring at the moment so in practise maybe impossible.

 

[pgarner]

another idea for you might be to use a varible vane turbo,
these are mainly used on diesels due to higher cost but the vane are, yup youve guessed it, varible. this means that they adjust for the required boost meaning there should be minimal lag.
have a look at the current 911 turbo, it runs on vvt.

your theroy above would work well if both turbos had electronic external wastegates.
the presurised air from the first going over the second to help spool. then as the pressure increases the wastegate opens and unpresurised air goes into the 2nd one which is already up to speed.
dont know how hard it would be to map this all togeather or even if it would work in reality

 

[obi_waynne]

The best twin turbos use a progressive controller to switch the second one in for peak power. 2 Small turbos would be as good at peak as 2 large turbos.

Think carefully about where you want the power band to be. Too much power low down means a lot of wheelspin. Plus if the car is a daily driver the lag period means you are using less fuel effectively giving you an ECONOMY mode below 3000rpm.

VVT is a good compromise. Another suggestion would be a supercharger creating a twin charged engine with the turbo cutting in higher up the RPM band.

 

[dpsubaru]

thanks all - will take a look at the vvt today or tom and revert. some interesting stuff on turbos on youtube as well that have been watching giving me some ideas on this and for the full upgrade

 

[charger69]

c69: if thats the case then they would need to be the same size to stop potential overspeed? my theoretical idea would have both turbos on seperate systmes with an electronic valve that kicks in and stops the flow to the first one (to stop overspeeding) and opens up the flow to the second one (would use an overlap range of around 500rpm to ensure continuous flow). This is only theory that i'm exploring at the moment so in practise maybe impossible.

when there are 2 turbo's separately, i think that's BiTurbo.

 

[pgarner]

bi turbo is anything with 2 turbos whether they are separate, 1 doing 1 bank. or sequential.

 

[MasterAuron]

I wouldn't bother tbh. Don't go overboard and get a mahoosive tubby. If you're going to do a proper build, some good head work will help to reduce lag, and run management to map out a lot of the lag. You want your exhaust system as free-flowing as possible too. Stick with a single turbo and lag shouldn't be too bad on that large an engine even when looking for 400bhp.

 

[dpsubaru]

Have emailed Borgwarner (the guys who did the vvt for the porsche) so see what they have available, waiting for their comments.
Understand that for twin turbos the Toyota Supra Mk IV had twin turbos working the the same way talked about above, the first coming on at the lower end and then the second kicking in once the pressure is up.

 

[MasterAuron]

Understand that for twin turbos the Toyota Supra Mk IV had twin turbos working the the same way talked about above, the first coming on at the lower end and then the second kicking in once the pressure is up.

Supras do have sequential turbo's yes. My old man has one. Very very good response on the standard turbos.