Richard Holdener
September 1, 2009
Is it possible to coax 1,000 hp from the lowly Two-Valve mod motor?

When talk turns to maximum performance mod motors, the 4.6-liter Two-Valve variant isn't usually part of the discussion. Truth be told, there is a reason the Two-Valve motors take a back seat to the more performance-oriented Three- and Four-Valve combinations. It is a simple matter of valve count, or more accurately, the airflow that accompanies that missing valve.

Both the latest Four- and Three-Valve heads offer exceptional flow compared to the older Two-Valve motors. Sure, Ford improved the situation with the Two-Valve motors by offering the so-called Power Improved version back in 1999, but even the improved version was a far cry from its Three- and Four-Valve cousins. With similar displacement and wild cam timing available for all the mod motors, the thing that differentiates the Two-Valve from the Three- and Four-Valve motors is basically head flow. With little to no aftermarket support (until Trick Flow offers up its new Two-Valve head), the choice is limited to porting the stock castings.

The 5.0-liter displacement came primarily from the 3.75-inch stroker crank (up from 3.54-inch) from Coast High Performance.

In stock trim there is no comparison, and this trend continues even once ported. Where a fully ported Two-Valve head may reach 245-250 cfm, a ported Four-Valve motor will be a minimum of 50-cfm, better and may actually offer as much as 100 additional cfm per cylinder. That is a big performance obstacle to overcome, and the main reason why the Three and Four-Valve motors will be chosen before the Two-Valve for serious power levels.

With all that buildup, you might assume this is a story on the superiority of the Three- and Four-Valve motors, but nothing could be further from the truth. Despite the valve deficiency, the lesser Two-Valve mod motors can be made to produce some serious power. Just how much you ask? How does an honest 1,000 hp sound? That is a big number in anyone's book, but a serious step for the lowly Two-Valve. Naturally such prodigious power will require the use of forced induction, something that will help us overcome the breathing inefficiency inherent in the Two-Valve motor.

The increased stroke length required a dedicated piston and rod combination. The 13ccc dish pistons were combined with forged connecting rods to produce a reciprocating assembly capable of withstanding the punishment of 1,000 hp.

Another limitation shared by all the mod motors is bore spacing. When you go looking to increase the power output of your modular motor, think displacement. Unfortunately for modern Blue Oval enthusiasts, the basic architecture (actually, bore spacing) of Ford's modular motor limits the available bore size. It is possible to resleeve the blocks, but you'll never see the 4.125-inch or even 4.00-inch bores run on the earlier pushrod motors. With limited bore spacing, additional cubic inches must be obtained through increased stroke length. You can, of course, opt to install a larger 5.4-liter block, but the taller deck height creates hood clearance issues, not to mention the additional weight of the larger motor.

The best route to a powerful turbo motor is to start with an efficient naturally aspirated combination. When it comes to making power, there really is no replacement for displacement. Given the limitations of the bore spacing on the mod motor, the route to increased displacement must come from additional stroke length. By combining an increase in stroke length from 3.54 inches to 3.75 inches with a 0.020 overbore, the result is an increase in displacement from 281 ci (4.6) to 300 ci-the magical 5.0-liter displacement.

Thanks to companies like Coast High Performance, installing the longer 3.75-inch stroker crank into the 4.6-liter block isn't any more difficult than picking up the phone and providing a credit card number. CHP offers 5.0-liter (and 5.1-liter with the maximum allowable bore size of 0.070 over) stoker kits complete with the 3.75-inch stroker crank, forged connecting rods, and forged aluminum pistons. This combination will drop right into your professionally machined 4.6-liter block (iron or aluminum Cobra version) or the kit can be ordered as a dedicated short-block.

The 5.0-liter stroker assembly was installed in the awaiting production iron (0.020 over) block.

The mod motor kits are available in a variety of different compression ratios, but we chose a set of dish-top pistons with valve reliefs to provide a boost-friendly compression ratio with sufficient piston-to-valve clearance for our medium-sized XE270AH Comp cams. The 13cc dish-top pistons combine with the additional stroke and the machined and polished combustion chambers in our CNC-ported 4.6 Power Improved cylinder heads to produce a static compression ratio of 10.05:1. Though a tad on the high side for street use with high boost, careful tuning and a reasonable boost level will allow the motor to run successfully with 91-octane premium unleaded pump gas. The 5.0-liter stroker short-block was finished off with a stock oil pump, pan, and pickup, though the pan was modified to accept the oil drains from the turbos.

The stock Power Improved heads were shipped to Total Engine Airflow for a complete CNC porting.

With the short-block taken care of, we turned our attention to the airflow portion of the equation. All those additional cubic inches are worthless (or at least less effective) without a decent set of cylinder heads. Knowing that the flow rate offered by the stock heads was by no means sufficient for our turbo stroker, we shipped off the factory '01 PI aluminum cylinder heads to Total Engine Airflow. The heads were given TEA's Stage 3 treatment, which includes full CNC porting of the intake and exhaust ports, new Hi-Chrome valve seats (plus bowl blending) and even combustion chamber polishing to minimize the threat of detonation. The chamber was further worked to unshroud the oversized (47.7mm) Manley intake valves and 36.8mm stainless steel exhaust valves.

The ported heads were finished off by mod-motor maniac John Mihovetz, who performed the necessary vacuum test and valve job to ensure perfect sealing. Given the dissimilar valve lengths, it was necessary to install a 0.063 shim under each intake valve lash adjuster. The shims ensured adequate preload on the hydraulic lash adjuster.

The attention to detail is evident in this shot of the intake port. After CNC porting, each port was hand finished to produce airflow numbers that exceeded 240 cfm.

The CNC-ported PI heads were topped off with a set of Xtreme Energy cams from the Comp Cams catalog. Given the displacement and compression of this 5.0-liter stroker and the fact that our newly ported heads flowed so much better than stock (the intake flow increased to 242 cfm at 0.550 lift), we selected a set of appropriate cam profiles. The XE270AH cams offer 0.550 lift (both intake and exhaust), a 234/238 duration split (at 0.050), and a 113-degree lobe separation angle. We knew that the 0.550 lift of the Comp cams would take full advantage of the airflow offered by the CNC-ported heads, and the duration figures would allow the motor to continue to make power past 6,000 rpm (with the proper intake manifold).

Finishing off the 5.0 stroker was a custom intake design dreamed up by the author. The dual-throttle-body induction system seemed ideal for use on the twin-turbo motor (symmetry is always important). Besides, the stock PI intake was by no means adequate for this application. Dyno tests on this motor run in normally aspirated trim demonstrated the intake was worth as much as 35 hp over the stock PI manifold. The dual-plenum intake was fed by a pair of 75mm (5.0) throttle bodies from Accufab. After dyno testing, the intake manifold was reconfigured as a cross-ram to lower the total height and make it more underhood friendly.

The exhaust port received a great deal of attention as well. What we liked about the job performed by Total Engine Airflow was that they resisted the temptation to make things simply bigger and instead went to great effort to make things better.

Now it was time for the turbos. HP Performance in Roswell, New Mexico, supplied one of its twin-turbo kits for the buildup. Before covering the components supplied with the turbo kit, it is important to understand the merits of a turbocharger and specifically a twin-turbo system, beyond the ability to say you own a twin-turbo Mustang.

The descriptive phrase "twin-turbo" applied to nearly anything elevates that vehicle to a status above your ordinary turbo. Not so many years ago, the hot Porsche to own was the turbo. After their success with the 959 Supercar, Porsche realized the benefit offered by a pair of smaller turbos, thus the twin-turbo Porsche was born.

Even the combustion chambers received attention, as there is flow and power to be had from unshrouding the valves. The chambers were also polished to minimize detonation. The combination of chamber work, and stroke and piston design, produced a static compression ratio right near 10.0:1.

Is the twin-turbo model twice as good as the single turbo? In reality, no-but in the marketing-induced minds of the public, the answer is much closer to yes. The same thing holds true for your average 4.6 Mustang. While almost any form of forced induction will greatly improve power, a turbo system tops the list.

Is a single turbo system half as powerful as a twin turbo system? The answer to that question is somewhat more complicated than a simple yes or no, but if you applied a pair of equally sized turbos to the same single-turbo motor, the answer would obviously be yes. Regardless of the reality, twin-turbo rolls off the tongue so much better than just turbo

While the perceived reality is obviously important, does a twin-turbo motor differ dramatically from its single-turbo brethren? Despite all the hoopla surrounding the twin-turbo description, the single- and twin-turbo systems produce essentially the same effect on the motor. The job of the turbocharger is to provide additional airflow. This is accomplished by using the exhaust gases to spin a turbine. The turbine is placed in the exhaust stream where exhaust pressure is used to increase the speed of the impeller. The turbine impeller is attached to a similar compressor impeller by way of a common shaft. Being connected, exhaust spinning the turbine wheel will cause the compressor wheel to accelerate at the same speed. The spinning compressor wheel draws in airflow and forces it into the engine through (in this case) the intercooler, and eventually the throttle body.

The CNC-ported heads from TEA also received a set of oversized Manley stainless steel valves.

Whether this is accomplished with a single turbo or twin turbos, the effect is ultimately the same. The exhaust energy is used to accelerate the compressor wheel to increase the amount of airflow supplied to the motor. The additional airflow is seen as boost pressure. Generally speaking, the greater the boost pressure, the greater the power output

For the vast majority of street and race applications, it is possible to achieve a given power level using a single turbo. Single turbos are available to produce anywhere from 100 to 2,000 hp, depending on the application. The challenge with a single turbo is that the physical size of the turbo must be increased to support more power. A single turbo capable of exceeding even 1,000 hp is huge. Naturally the physical size can limit placement in the engine compartment, especially in the tight confines of a late-model Mustang. This is where twin turbos can be beneficial, as running a pair of smaller turbos provides much better packaging.

Before and after each valve job, the intake and exhaust ports were vacuum checked for proper sealing. After John was finished with the Serdi, the valve job on all cylinders sealed perfectly.

For certain applications, like the 4.6 Two-Valve GT motor tested here, it was easier to find locations for two small turbos than one large turbo. The tight confines of the modular-motor engine compartment require the pair of turbos reside up in the inner fender well on either side of the bumper support. This position provides a shortened route from the turbos to the intercooler. Forget everything you have been told about positioning the turbos as close as possible to the cylinder head to aide in spooling up the turbos. The response rate of the turbos has much more to do with the relationship between the normally aspirated power output and the turbine size than the length of exhaust between the head and the turbo.

While turbo selection is the critical element in the success of a good turbo system, the gang at HP Performance decided it was necessary to sweat the small stuff on their twin-turbo kit for the 4.6 Mustang. This was evident in the design of the tubular exhaust manifolds. Looking all the world like a quality pair of long-tube headers (only in reverse), the exhaust manifolds offered a significant improvement in exhaust flow compared to the stock (cast-iron) exhaust manifolds.

Since we plan on running aftermarket performance cam profiles on the turbo stroker, we installed a set of Crower valve springs and titanium retainers. The springs were set up to provide nearly 100 psi of seat pressure. The change in valve height required use of spacers under each of the intake lash adjusters.

Like the remainder of the exhaust and inlet system tubing, the headers feature Jet Hot coating to help maximize the exhaust energy to the turbos. The Jet Hot coating should also help extend the life of the tubing by protecting it from rust and the elements. It also helps to keep underhood temperatures down by retaining the heat inside the exhaust tubing. And the Jet-Hot coated inlet tubing (from the compressor housings through the intercooler to the throttle body) will help brighten up even the dullest of engine compartments.

To eliminate the possibility of exhaust leaks, HP designed the exhaust system with V-band clamps to provide a leak-free seal. Exhaust leaks will definitely limit turbo performance. Along those lines, HP also saw fit to use heavy-wall tubing and extra thick exhaust flanges to improve both sealing and tubing life

The Two-Valve 4.6-liter GT kit from HP includes everything needed to install the twin-turbo system on a modern modular motor. There were a couple of changes made to the kit for our dyno needs, one of which was the deletion of the mass air meter. Our 4.6-liter test mule was set up to run using a FAST standalone management system. In the stock kit, the intercooler is set up as a dual inlet (one from each turbo) and single outlet.

Our dual-plenum intake necessitated modifications to the discharge side of the intercooler. The single outlet was replaced with a tank that featured a dual outlet, one for each throttle body. Since our power goal required running upward of 20 psi of boost, we opted for a set of T04E-57 turbos. HP Performance supplied a pair of precision Tial waste gates to properly control the boost pressure. Since turbochargers are capable of astronomical boost levels, the wastegates are employed to keep the boost pressure at a predetermined level.

On The DynoThough the dyno version looks a lot like an old Can Am motor from the '60s and early '70s, rest assured this twin-turbo 5.0-liter stroker will be dropped into the engine bay of a Mustang. The intake manifold will be reconfigured into a cross-ram setup to drop the total height, but the power numbers are certainly impressive. In normally aspirated trim, the 5.0 stroker produced 462 hp and 446 lb-ft of torque. After careful tuning and cranking up the boost to nearly 20 psi, we were rewarded with over 1,000 hp-pretty impressive stuff for a Two-Valve mod motor.

We already mentioned the modifications to the air-to-air intercooler, but a more thorough description is necessary here. The front-mounted core was designed to shed the unwanted heat associated with forced induction. It is a law of nature that compression causes heat. Turbos heat up the air less than other forms of forced induction, but all forms will increase the heat of the inlet air. The higher the boost pressure, the greater the increase in temperature. In addition to the loss in air density (fewer power producing oxygen molecules per volume), the elevated temperature can also increase the likelihood of harmful detonation. The intercooler greatly lowers the temperature of the inlet charge air by forcing it through the heat exchanger. The heat exchanger uses ambient airflow to help draw the heat out of the charge air. The front-mounted position provides plenty of airflow once a Mustang is at speed, which it will be quite quickly after the installation of this kit. The efficacy of the large intercooler core effectively increases with boost pressure. The intercooler core also features a pair of provisions for compressor bypass valves, used to eliminate surge that can occur when you lift off the throttle at full boost.

With additional cylinder pressure and the attendant power output supplied by the turbo system, it was necessary to upgrade both the ignition and fuel systems (on the dyno). The fuel system was taken care of with an Aeromotive EFI fuel pump capable of supporting 1,000 hp. The pump was augmented with a Kenne Bell Boost-a-Pump to increase the supply voltage to the pump and further improve the flow capacity. Naturally the stock injectors were not up to the task of feeding a turbo motor, so we replaced the 19-pounders with a set of FAST 65-lb/hr injectors in a Wilson billet fuel rail.

The plenums were designed to accept dual 75mm throttle bodies from Accufab. The 5.0 throttle bodies look right at home on the mod motor.

Ignition mods included replacing the stock coil packs and plug wires with high-energy units from MSD. Voltage into (and spark energy out of) the coil packs was increased with a Kenne Bell Boost-a-Spark. The final element on the list of ignition upgrades was a set of Denso Iridium IT31 spark plugs (gapped down at 0.025). The precious metal plugs and shortened plug gap help eliminate misfires associated with boosted applications

As always, we ran the 5.0 stroker combination in naturally aspirated form to establish a baseline. Running the motor naturally aspirated allows us to verify the power gains offered by the twin-turbo kit. The turbo-ready 5.0 produced 462 hp and 446 lb-ft of torque running an air/fuel ratio of 13.0:1 and total timing of 29 degrees. Installation of the twin-turbo kit on the dyno required a number of modifications. After the installation, we had the twin-turbo motor up and running in no time with the FAST engine management system. Running vacuum/boost reference lines directly to the dual Tial waste gates resulted in a maximum boost pressure of 9.8 psi. We had a rising boost curve that peaked at 9.8 psi.

Naturally the motor was run with race fuel for safety, though this boost level was certainly achievable on pump gas. We dropped the total timing from 30 degrees to just 21 degrees, and the air/fuel mixture from 13.0:1 to 11.8:1. Once we had the air/fuel and timing dialed in, the twin-turbo motor produced an impressive 771 hp and 683 lb-ft of torque. Remember that all this horsepower and torque is coming from a motor displacing just 281 ci!

Before increasing the boost pressure, we dropped total ignition timing from 21 degrees to just 19 degrees. Running 16 psi of boost brought 937 hp and 829 lb-ft of torque. Stepping things up to 19.4 psi, and ignition timing down to 17 degrees, brought the peak numbers up to 1,006 hp and 872 lb-ft of torque. Now that's what we call a Power-Improved Two-Valve.