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Dyno Testing CXRacing's Twin-Turbo System - PSIchotic Stroker
Makin' over 900 hp with a carb'd twin-turbo 347
What constitutes a cool engine build? High-boost, twin turbos, sick power—check, check, check. You can add big-inch, stroker short-blocks, wild cams, high-rise intakes and free-flowing cylinder heads, too. Lesser magazines might be satisfied with any one of these, but that's not how we do it at MM&FF.
With wretched excess in mind, we decided to combine all three elements to create nothing less than a PSIchotic stroker. In truth, we wanted to test a new twin-turbo system offered by CXRacing, but why stop at just a simple turbo test when the dyno session can be so much more?
Since fuel-injected 5.0L's get all the love, why not toss a bone to the carb contingent and ditch the injectors and electronic management system? That's right fellow Ford fanatics, the twin-turbo kit fed all that wonderful boost through a simple carburetor.
Like most of the cool forced-induction on the market, the twin kit from CXRacing is actually designed for the fuel-injected 5.0L Ford. But as much as we love the fuelie Fords, not every small-block receives fuel through electronic injection.
Carburetors not only work well, they can also be combined with blowers, nitrous, and even turbos with tremendous success. In fact, carburetors actually offer a few advantages over injection, including cost savings, simplicity, and additional charge cooling. It might be common to see a supercharged or turbocharged 5.0L Ford, but when was the last time you saw one blowing through a carburetor? Though designed for the side-mounted throttle body on the 5.0L, the tubing on the twin-turbo kit provided by CXRacing was easily altered to feed the carburetor.
The turbo kit includes exhaust manifolds, turbos, wastegates, stainless exhaust tubing, aluminum intake tubing, and all the clamps and couplers to connect the system. The kit also features all the oil feed and drain fittings, braided oil feed (but not rubber drain) lines, and air-to-air intercooler. The exhaust manifolds were designed to accept T3 turbine housings, meaning they will accept any size T3 turbo, but our kit came with GT35-style turbos said to be capable of supporting over 500 hp each.
Given the ease with which we exceeded 900 hp, we feel certain the rating was accurate. Before running in anger, it was necessary to orient the compressor and turbine housings to line up the oil feed and drain fittings, the compressor discharge, and then mount the turbine housing to the exhaust manifold. The wastegates were also mounted to the provided wastegate flange on the manifold.
The wastegates were used to regulate boost, but a small turbine A/R sizing resulted in a rising boost curve. CXRacing offered a larger A/R housing to cure the issue. It was necessary to drill holes in the oil pan to serve as drain back from the turbos. This was made easier with the provided bulkhead fittings that eliminated welding by simply bolting to the pan (with pan removed).
To test the merits, we built a 347 stroker with forged internals that included a 4340 forged crank and rods combined with forged flat-top pistons from JE. With sufficient strength and displacement, we added the necessary power producers, including heads, cam and intake. Air Flow Research supplied a set of new CNC-ported 205 Renegade heads, which were combined with a Crane hydraulic-roller cam and single-plane, Funnel Web intake. Effective for both normally aspirated and boosted combinations, the Crane cam offered a 0.579/0.595 lift split, a 236/244 duration split and 114 LSA.
Crane also supplied the necessary lifters, double-roller timing chain, and hardened pushrods. The AFR heads offered flow rates exceeding 330 cfm, or enough to support over 660 (normally aspirated) hp. They were more than enough for our 347. Ensuring plenty of high-rpm power, the single-plane, high-rise Funnel Web intake was fed by a Holley 950 Ultra HP carburetor and fired by an MSD billet distributor. In normally aspirated trim, the 10.8:1, carbureted 347 stroker produced 519 hp at 6,900 rpm and 434 lb-ft at 5,400 rpm.
After our baseline runs, we swapped on the turbos. Installation was easy and we were pleased that the exhaust manifolds offered plenty of clearance for the factory plug wires. The thick flanges sealed perfectly using Fel Pro exhaust gaskets and ARP header bolts.
For boosted use, we replaced the Holley Ultra HP carb with a conventional Holley HP series, modified by Carb Solutions Unlimited (CSU). Designed specifically for blow-through applications, the CSU carb featured an adjustable boost-reference power valve. CSU also supplied the necessary carb bonnet.
The use of the carburetor instead of the 5.0L throttle body required that we change the discharge tubing from the intercooler. The remainder of the kit remained as-supplied for the 5.0L. The fix was an easy one, as CXRacing also offered all manner of turbo components for the do-it-yourselfer, including the required aluminum tubing. A U-bend and a couple of 90-degree bends were all it took to attach the intercooler to the carb bonnet. The kit also included a blow-off valve to eliminate the pressure surge that occurs when you lift off the throttle at high rpm/boost.
Here are a couple of things we noticed when installing the kit. The driver's side wastegate required use of a short oil filter—available at any auto parts store. We ran the 3-inch downpipes as up-pipes on the dyno as they fit better. A crow's foot wrench made installation of the exhaust manifolds much easier—as Ford header bolts are always a pain.
We installed a carb spacer equipped with a variety of different vacuum fittings to run the wastegates, blow-off valve, and provide a vacuum/boost signal to the fuel- pressure regulators and dyno readout. To ensure adequate fuel delivery to the carb, it was critical to boost reference the fuel pressure regulator.
With everything hooked up, we dropped the static timing from 35 degrees down to 23 degrees and eventually revved the motor high enough to reach 919 hp and 747 lb-ft of torque at just over 15 psi.
The power and boost were climbing rapidly (1 psi and 45 hp per 100 rpm), but we shut it down at 6,600 rpm. The twin-turbo kit was plenty powerful, the carb worked flawlessly (dropping charge temps even lower than the intercooler alone) and (best of all) the PSIchotic stroker was alive and ready for more action.