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Discussion Starter #1
All,

Please note that twin-scroll technology is by no means new. The Toyota 3S-GTE motor (Celica, MR2, IS200, and Corolla WRC) has been using twin-scroll (called twin entry) for about 15 years now, and the technology has been on diesel turbos for even longer than that. It's not really that dramatic of a difference anyway. Many 3S-GTE tuners remove the webbing that separates the two exhaust paths as it causes problems when the turbo is pushed to higher boost levels. It does indeed help with low-end response, but not nearly as much as a 25% bump in displacement. (2.0L to 2.5L).

We got the better deal here, fellas. A year from now all the hot STi tuners will be grinding the twin-scroll webbing away to make 400HP streetable cars. Don't cry for the twin scroll. Rejoice in the possibilities of two and a half liters!

-ch
 

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I too agree that the 2.5l is a great deal, not to step on any toes here, but I would rather have better displacement over staying with tradition. I don't really plan on doing any racing that would require a 2.0L, but I do plan on modding the car, and 2.5L's does make that easier.
 

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Discussion Starter #5
chawklit,

I understand what you're saying, but my point is that if you had twin-scroll and 2.5 liters, you probably wouldn't have 300HP. At least not at the same boost levels.

The reason for this is that the twin-entry configuration is designed to isolate exhaust gas pulses that encourage cylinder scavenging and to eliminate cylinder recirculation. However, at higher velocities (RPMs) these problems are no longer an issue. In fact, the separated exhaust manifolds actually increase the pump work of the motor and create turbulence at the tubine as the two flows are recombined. This translates to less peak horsepower, probably by some small factor (say 5-10hp, but that's just a guess). This is why no competition turbo has this feature. Twin-entry is something used on small-displacement production cars that cannot run high compression ratios due to low-octane pump gas.

So if you wanted 300HP and twin-scroll, you'd probably have to up the boost or do something else to gain back what you loose in exhaust inefficiency at high RPM. The torque peak might not even change, although you would certainly gain torque lower in the rev range. But believe me: the extra displacement makes up for the torque loss.

Now, if you're going to leave your car absolutely bone stock, I would agree that the twin-scroll might be nice. But as soon as you do anything significant, I think it will be mostly useless. I know that magazines have made a big deal about the twin-scroll feature and its impact on driveability, but honestly I bet more of these changes are due to engine tuning than anything else. An agressive throttle-position-based mapping can do wonders for even the smallest turbocharged motors. Just look at how responsive the WRC motors are--but but of course they also use higher compression ratios and anti-lag systems that fry those little turbos pretty quickly.

-ch
 
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