Twin Scroll , Yes or No ? (Part I)

This test was done on two separate cars but with the same mods minus the header and turbo kit they were running. So while not apples to apples its a really close comparison.

Both cars had Perrin FMIC, 3" Turbo back exhaust, 850cc injectors, fuel rails, with stock heads and cams.

Both cars tested were also run on Shell 93oct and boost for both runs was kept the same at 21psi.

Run #41 was with Full-Race's full twin scroll turbo kit. Running the HTA35r, t4 turbine 1.06ar housing . This twin scroll kit does seem to spool up a bit faster but on the street the differences are less noticeable. We also noticed that with the FR kit the power started to fall off pretty good above 6k rpms. Our best guess is that the power loss up top is caused by the small primaries in the full race header.

Overall we were expecting huge things from this kit but feel a little let down. Power aside the fitment of this turbo kit required major work to get it to clear both the firewall and frame rails.

Run #09 was a standard GT35 rotated turbo kit with a tial V-band .63ar turbine housing. It shows to spool a tad bit slower but there is no power loss in the upper rpms. This turbo kit was a one off built for a customer as he wanted an exact copy of what we have run all year in our time attack car.

AMS however has just released their rotated turbo kits running the tial v-band housings. We are hoping to now use the AMS kit instead of building the one offs we had in the past. I would expect AMS's new turbo kit to perform very similar as the ones we built, but with that rock hard AMS reliability and warranty. Here shortly I will have an AMS turbo kit for testing and will be installing it on my personal 08sti. I will start a new thread with pictures and dyno results when I do so.


Douglas Wilks
TopSpeed Inc.

Here are the plots of boost vs Rpm for both turbo kits.

Blue line is Full-Race Twin Scroll HTA35r

Red is STD GT35r with Tial turbine housing.

Claims by Full Race

About Twin Scroll:Twin-scroll turbo system design addresses many of the shortcomings of single scroll turbo systems by separating those cylinders whose exhaust gas pulses interfere with each other. Similar in concept to pairing cylinders on race headers for N/A engines, twin scroll design pairs cylinders to one side of the turbine inlet so that the kinetic energy from the exhaust gases is recovered more efficiently by the turbine. For example, if a four-cylinder engine’s firing sequence is 1-3-4-2, cylinder 1 is ending its expansion stroke and opening its exhaust valves while cylinder 2 still has its exhaust valves open (while in its overlap period, where both the intake and exhaust valves are partially open at the same time). In a single scroll AKA undivided manifold, the exhaust gas pressure pulse from cylinder 1 is therefore going to interfere with cylinder 2’s ability to expel its exhaust gases, rather than delivering it undisturbed to the turbo’s turbine the way a twin scroll system allows.

The result of the superior scavenging effect from a twin scroll design is better pressure distribution in the exhaust ports and more efficient delivery of exhaust gas energy to the turbocharger’s turbine. This in turn allows greater valve overlap, resulting in an improved quality and quantity of the air charge entering each cylinder. In fact, with more valve overlap, the scavenging effect of the exhaust flow can literally draw more air in on the intake side while drawing out the last of the low-pressure exhaust gases, helping pack each cylinder with a denser and purer air charge. As we all know, a denser and purer air charge means stronger combustion and more power... but the benefits of twin scroll design don’t end there. With its greater volumetric efficiency and stronger scavenging effect, higher ignition delay can be used, which helps keep peak combustion temperature in the cylinders down. Since cooler cylinder temperatures and lower exhaust gas temperatures allows for a leaner air/fuel ratio, twin-scroll turbo design has been shown to increase turbine efficiency by 7-8 percent (faster spool, quicker response) and result in fuel efficiency improvements as high as 5 percent. It is wise to size the turbine housing A/R larger than the single scroll turbine A/R typically used!