Twincharging- Effects of turbine wheel/housing size- FAO Warspeed!

[StavFC]

Hi guys. Long time lurker, but never posted until now, but I think you guys (esp Warpspeed as he has practical experience on this) may be able to help.

I have a 1.6ltr supercharged engine (~200bhp currently, using a 1.2ltr rootes style blower) that I've almost completed a twincharged conversion on.
While 300bhp is MORE than enough for this car (id be fine with 250tbh, its the response and powerband I'm more interested in), turbo choice I can't decide on due to one of the big advantages of twincharging (well, the 'proper' way, ie turbo blowing in to supercharger and with no on-throttle s/c bypasses) which is much higher boost pressure than pre-turbo backpressure.

My current setup (as it was more of a test on how effective twincharging is at spooling turbos than anything else) is using a fairly large turbo I had handy, a 500bhp+ capable one that I know hits full boost on 2ltr (that produced ~140bhp in N/A form) at around 4500rpm.
I was confident this would spool quickly due to having at least 50bhp more (before adding the turbo) than on the 2ltr, and it would certainly give very low backpressure compared to boost pressure, but it's capable of twice the power that I'm aiming for, and more to the point, I'm less confident of how it will spool now than I was previously. Does anyone have any experience to guesstimate it?

My other option, is a much smaller turbo (which I have handy, and can be fitted if I modify the manifold and downpipe) which is good for ~300bhp, and while I know, especially in conjunction with the s/c, will spool instantly no matter what, I'm wondering if this will be TOO small to be worth while in twincharged form? I'm worried it's too small considering the much smaller turbine and housing, and the ideal situation of more boost pressure than backpressure.

I remember talking to Warpspeed many years ago (not on this forum) about it, as he had done something similar, but my mind is blank to what the result was then...

Any help really appreciated

 

[TexasAceTA]

How much boost is the supercharger making right now?

You can do some crude calculations based on the boost in Apsi to get an idea of what the turbo will spool like with the supercharger.

The supercharger will make the engine look bigger to the turbo. If you are making say 8psi on a 2L motor then the turbo will think it is mounted to a 3.1L motor.

8 psi = 1.55 Apsi

2.0L x 1.55 = 3.1

It is crude but should get you in the ballpark.

You can Call me TA

I carry a gun cause a cop is too heavy.

If guns kill people, then Spoons make people fat.

 

[StavFC]

~16psi.
What is Apsi? Either way, luckily with it being double what you said, that would be 3.1 Apsi, and 3.1 x 1.6 is 4.96.
If that's accurate, I won't have any problems in the slightest, but the smaller turbo I mentioned would be a big restriction.

Now my confidence is returning, lol

 

[TexasAceTA]

Well, technically is it called PSIA, got the A in the wrong place lol.

It is basically using the BAR scale except adding 1 to whatever the BAR number is. You are adding in the ~14.7psi of atmospheric pressure that is on everything basically.

In the case of 16psi.

16 psi = 2.1 PSIA

2.0 x 2.1 = 4.2l



You can Call me TA

I carry a gun cause a cop is too heavy.

If guns kill people, then Spoons make people fat.

 

[StavFC]

Ah, absolute, but in bar, thats fine.

So yeah, 2.1bar absolute, if the calc is correct on my 1.6 that is 3.36, so even if that's remotely accurate, it will spool no prob.

I'll stick with the current one then.

 

[TexasAceTA]

Oh, yeah missed you had a 1.6l, Then your math is right there.

And yes you should have no problem spooling the turbo, the real problem will be making use of the turbo on pump gas. It will hardly be making any boost when you are well into the 20psi range.

You can Call me TA

I carry a gun cause a cop is too heavy.

If guns kill people, then Spoons make people fat.