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The Wrenchrat Twin-Turbo Kit - New Parts

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In hindsight, this experiment could have been a lot more scientific. For instance, we could have run this cool new turbo kit against the eBay turbos we ran in the Sept. '10 issue. Or, we could have run this cool new turbo kit against itself by swapping a set of cylinder heads and making more power for less boost. But all of that is going to come later. For now, we wanted to get a taste of the kind of WOT power a well-designed twin-turbo system has to offer. Our reward was 936 rwhp from a 355-inch small-block or about 2.64 hp per cubic inch. That's close to 1,100 hp at the flywheel. What else do we need to say?

The Mill
We've seen stock 350-inch engines take plenty of boost without tossing chunks, but we knew this setup was going to be too much for a set of cast slugs swinging on some stock rods. The solution was simple: Garrick at Kelly's Block Welding in West Los Angeles prepped a four-bolt block for a little forced air. Ted Toki at Westside Performance added a set of custom Ross forged pistons, RPM H-beam rods, and a 3.48-inch nitrided and polished steel GM crank. The cylinder heads are the 227cc Competition Package from AFR, and according to the company's specs, they flow a staggering 329 cfm at 0.750 lift and more than 300 cfm from 0.500 lift on.

Even though the parts seem exotic, they all had shelf part numbers. The only hands-on tweaking was performed by Eric Solomon at Westside Performance to port-match the Edelbrock Super Victor intake to the cavernous intake port in the AFR head. The cam was suited to both the heads and the turbo, with a good lift number to put the valve right in the fat part of the flow curve, and additional LSA to reduce overlap and build boost in the cylinder. The ignition was an MSD billet distributor with a 6AL box. The carb was the 850 Holley from the Sept. '10 issue.

The Kit
The turbo kit from Wrenchrat makes monster power. For the price of a good centrifugal supercharger you get twin stainless steel turbo headers, two turbochargers, stainless downpipes, wastegates, the cold-side charge pipes and merge, a blow-off valve, and all the little parts to put it together. If you have a well-running engine with less than 9.0:1 compression, you can install it in a weekend, and with a few simple carb modifications, run it on pump gas.

What makes the system different from anything else we've seen is the 4-into-2 turbo header design. The system reduces turbo lag by maintaining the energy of each exhaust pulse to keep velocity high and scavenge the neighboring cylinder. The management of pulse energy allows the use of larger turbos that respond like smaller ones.

The exhaust manifolds have T4 flanges that allow you to run any turbo from 60 to 76 mm on the compressor side and up to 0.81 A/R ratio on the turbine side. The base kit comes with a pair of turbos that can make 650 to 700 hp on a pump gas tune or 1,000 hp with race gas, but you can order whatever turbos you want, or get a recommendation from the guys at Wrenchrat.

The Turbos
For this engine, Joe Delgado from Comp Turbo selected a set of 67mm CT4 turbos with X-HF billet compressor wheels and 65mm P-trim turbines in 0.81 A/R housings. The selection was based on the size of the engine, compression ratio, cam timing, and cylinder head design. Even though each of these turbos is designed to deliver a maximum of 95 pounds of air per minute and support 950 hp, we were looking for instant boost delivery in a usable rpm range. The CT4 triple-ceramic ball bearings are perfectly smooth, lowering friction losses and temperature for more speed. The turbo's design, combined with the X-HF high-speed billet compressor wheel, allowed Toki's '55 to make usable boost (6 to 8 pounds) as low as 3,100 rpm and make more than 400 lb-ft of torque. In addition, we saw torque gains of 527 lb-ft from 3,100 rpm to the torque peak of 978 lb-ft at 4,314 rpm. That is a lot of usable power.

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