Cylinder heads are a major contributing factor to power production on the 4.6L modular Motor. Since the 2-valve heads flow nowhere near as well as the 4-valve Cobras, it’s not surprising that the 4-valve motors produced a great deal more power. The new 3-valve heads seem to be positioned right between the two in terms of airflow, though the variable cam timing certainly provides a benefit not realized by either the 2-valve or 4-valve motors. Since the modular motor (like every engine) is nothing more than a giant air pump, the flow rate of the cylinder heads is one of a number of factors that will determine the overall flow rate (we see as power) of the motor. The more air the motor can process (in through the induction system and out the exhaust), the more power it will ultimately produce. As with the intake manifold and exhaust system, bigger doesn’t necessarily mean better when it comes to ports. In many cases, increasing the port volume can increase absolute airflow, but (as always) there’s much more to the power equation than maximum flow. Were maximum flow the key variable, we would hog out the port to the maximum available dimension and watch the power grow. If only life were that easy!
This Tech Tip is From the Full Book, BUILDING 4.6/5.4L FORD HORSEPOWER ON THE DYNO. For a comprehensive guide on this entire subject you can visit this link:
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While peak-lift flow (measured at the maximum valve lift offered by the cams) is important, the reality is that the valve spends more time running up to and away from maximum lift than it does at maximum lift. Therefore, the flow rates throughout the valve lift curve are equally important. The low-lift numbers are even more important on overhead cam motors, as the architecture generally does not allow for high-lift values. This is especially true on the 4-valve motors, as the lift values even for performance cams generally do not exceed .500 inch. The 2-valve motors attempt to make up for their lackluster port design with higher lift (.550 to .600 inch), but when you only have .500 lift to work with, you better make every effort to maximize the flow rates at all the lift values below that point. After all, it’s the average airflow achieved throughout the lift range that produces the best power curve. Lucky for us, the 4-valve configuration lends itself to impressive low-lift flow numbers. While down on maximum available lift, the 4-valve heads also flow more at .500-inch lift than the 2-valve heads do at .550-inch lift (or even .600). In fact, a ported set of 4-valve heads might outflow a set of ported 2-valve heads by 80 to 100 cfm. All the extra lift in the world won’t make up that kind of deficit.
While most of the attention is paid to the flow rate through the ports, the reality is that the port flow rate is only part of the power potential offered by the head. In Chapter 4, I’ll explain that the bank-to-bank cam timing can be off dramatically. A related problem that can further skew the power output (even on a stock motor) is the lifter (or lash adjuster) preload. Due to production tolerances in the components, castings, and machining, the lifter preload can vary from .025 to .100 inch (or more). Excessive lifter preload can actually push the valve off the seat, greatly reducing or eliminating valve sealing in that cylinder. The reduced dynamic Compression naturally causes a drop in power. In addition to the lifter issue, the actual valve sealing from the production valve job can also hurt cylinder pressure. Since both the valve job and valve length ultimately affect the installed height, which in turn affects lifter preload, all of these variables are interrelated. Miss the valve job, valvestem length, or lifter preload, and the power will suffer. Since a valve job (and possibly new valves) is mandatory when performing head porting, care must be taken when it comes time to reassemble the new components.
The 4.6L 2-valve motors really respond to ported PI heads. We installed these Stage 2 CNC-ported heads from Total Engine Airflow (TEA) on a 1998 (non-PI) short block and were rewarded with nearly 90 hp.
In addition to cylinder head testing, this chapter also includes results on the effect of changes in compression. The compression ratio will affect power, with higher compression offering more power, but how much does the drop in compression hurt? Suppose you are in the market for a new short block and want to add a blower down the line, or maybe you already have one on your existing combination and you want to build (or buy) a dedicated forged short block to withstand the rigors of boost. With your current 4.6L, the static compression is around 9.2:1 for a 2-valve or 10.0:1 for a 4-valve. Dropping the compression by a full point will result in a sizable change in power. Many enthusiasts have built or bought a forged low-compression short block thinking that it will make more power than their stock setup, but run at the same boost pressure, the low-compression motor will most certainly make less power than before. Of course, the drop in compression may be necessary to allow you to run safely on pump gas.
The guys at Accufab put the effect of compression to the ultimate test on an assembled 5.4L 4-valve motor. The benefit to the increased compression is actually greater when you add boost into the equation, as the boost pressure becomes a multiplier. On the 5.4L motor, the increase in static compression from 8.2:1 to 11.5:1 resulted in a gain of over 60 hp. Testing on the supercharged versions of the same motors resulted in a gain of over 200 hp. Thus any gains produced by the increase in compression ratio were multiplied by the pressure ratio supplied by the blower. Therefore, every effort should be made to improve the power output of a naturally aspirated motor, including increasing the compression ratio. This is especially true if you are building a motor for a specific drag racing class that limits the size and/or speed of the blower or turbo. If the blower or turbo is limited, you must do everything you can to maximize the power output at the predetermined boost/impeller speed limit. Reducing the boost pressure at any given impeller speed can increase the flow rate of the blower. This is accomplished by improving the power output of the naturally aspirated combination.
It’s too bad the aftermarket has not embraced the 4.6L 2-valve motor, as a set of TFS Twisted Wedge heads for the 4.6L 2-valve would sell like hot cakes. The only option for 2-valve owners is to have the stock heads ported like these Stage 3 heads from TEA.
Most enthusiasts opt to lower the static compression when building a motor specifically for supercharging or turbocharging. The drop in compression will allow higher boost levels on pump gas, but it will decrease off-boost efficiency, mileage, and absolute power.
Production 4-valve heads easily outflow even a set of ported 2-valve heads, but these F500 heads from Ford Racing can really wake up a 4-valve Cobra motor.
Test 1: Early 2-Valve GT vs. PI 2-Valve GT
When first introduced to replace the venerable 5.0L (I’m only just starting to warm up to the impressive 4-valve stuff), the 2-valve 4.6L mod motor was rated at 215 horsepower (upped to 225 hp in 1998). In reality, even the 225 hp version never seemed to match the horsepower output of the similarly rated 5.0L, and it certainly never equaled the torque. In 1999, Ford introduced a revised version of the 2-valve 4.6L GT motor that offered 260 hp. Improvements included additional static compression, a revised cylinder head and intake manifold package, and higher-lift cam profiles. These later Power Improved (PI) heads differed from their early non-PI counterparts primarily in combustion chamber size. The PI heads offered a much smaller combustion chamber. The smaller chamber was offset by a piston design with a larger dish to maintain a reasonable compression ratio, but the combination produced an overall increase in static compression compared to the early 4.6L motors. The PI heads flowed slightly better than the early heads, too.
The PI motors also received a dedicated intake manifold to mate to the revised intake ports. A significant portion of the power gains offered by the PI motors can be attributed to the PI intake manifold. In fact, adapters are available through the aftermarket to install the later PI intake on your early non-PI cylinder heads. This allows early motors to take advantage of the superior breathing potential and power production of the PI intake. This allows early 4.6L owners to port their existing cylinder heads, which can be made to nearly equal the flow of the ported PI heads, and nearly match the power of a PI head swap. Of course, you won’t have the jump in compression ratio offered by the PI head swap, but nor will you have the expense either. The early non-PI motors also had cams with less lift. The later PI heads were designed to run cams with .550-inch lift, while the early heads maxed out at just .500. The early non-PI cams can be run in PI heads (see Chapter 4) but the reverse is not true without modifications to the valvetrain to accept the higher lift.
How does the non-PI engine stack up against the later PI version? By testing these engines on the dyno, we can find out what the power difference really is.
As luck would have it, I was able to run both the early non-PI and PI motors on the engine dyno in the exact same configuration. Both motors were equipped with all factory components but were run with an electric water pump (no accessories), a set of Hooker Super Comp long-tube headers, and an open inlet system (no MAF or intake tubing). The F.A.S.T. management system was used to dial in the air/fuel and timing. Run in this configuration, the early (1998) non-PI motor produced 260 hp at 4,900 rpm and 342 ft-lbs of torque at 3,500 rpm. Note the early power peaks; the 1998 4.6L was designed with torque and throttle response in mind, rather than ultimate power. As expected, the later PI motor exceeded those power numbers significantly. Run in the same configuration, the PI motor produced 293 hp at 5,000 rpm and the same 342 ft-lbs of torque at 4,100 rpm. The peak torque values were identical; the PI motor just produced it later in the rev range, which led to more peak horsepower. Shifting the torque curve did result in a slight loss in low-speed power, as the early non-PI motor actually out-powered the PI up to 3,800 rpm.
The engines were set up with no accessories or intake tract. They also benefited from a set of Hooker Super Comp headers.
Early 2-Valve GT vs. PI 2-Valve GT (Horsepower)
It should really come as no surprise that the PI motor out-performed the earlier non-PI version, but the early motor was on top until 3,800 rpm. The revised cam timing, improved cylinder heads and intake, and higher compression ultimately won out, allowing the PI version to produce nearly 40 more horsepower.
Early 2-Valve GT vs. PI 2-Valve GT (Torque)
This test was very illuminating, as the early 4.6L motors always felt better down low than the PI motors, despite the difference in advertised power. The torque curve illustrates why the early 4.6L Mustang motors felt like they belonged in a truck. Who builds a performance motor for a pony car that produces peak torque at just 3,500 rpm? No wonder they designed the PI motor and later 3-valve 4.6L.
Test 2: Early 2-Valve GT Heads vs. TEA CNC-Ported PI 2-Valve GT Heads
After subjecting an early 4.6L GT to a number of basic bolt-ons, I decided it was time to get a little more serious. This meant replacing the restrictive non-PI heads and intake. Installing the PI heads onto an early non-PI short block adds minor power through additional head flow but primarily through the increased compression. The higher-lift PI cams also offer some extra ponies. The only thing to watch out for when running the PI heads on a non-PI motor is that piston-to-valve clearance will be diminished. It’s not a problem with stock cams, but with higher-lift and longer-duration aftermarket cams, clearance should always be checked.
Though the stock non-PI heads were certainly restricting the power, the early intake was partially to blame as well. The early and late 4.6L intakes share very long runner lengths, but the cross section on the non-PI version was designed to optimize power production in a much lower rev range. The intake is a major contributing factor in why the early motors make peak torque so much lower than the PI. Unfortunately, the early and late intake manifolds are not interchangeable. (I wanted to compare them directly but could not without running an adapter.) The intake port openings on the cylinder heads differ in shape, and naturally, so do the corresponding intake manifolds. The best method is naturally to swap over all the PI components, including the heads, cams, and intake manifold. The additional flow combined with the smaller combustion chamber makes the PI head swap quite desirable to owners of early non-PI-headed motors, especially considering the lack of performance heads available for the 4.6L Ford. While the additional airflow and compression offered by a PI head swap definitely help, I wanted all the performance I could get. I shipped a set of PI heads to Total Engine Airflow (TEA) for CNC porting. TEA increased the flow rate of the PI heads from 177 to 225 cfm on the intake side, while the exhaust increased from 126 to 208 cfm. There were gains registered throughout the lift range, from .050 to .600 inch.
These CNC-ported heads from TEA should give us even more power than a regular PI swap. The CNC program included improved ports and revised, though somewhat larger, combustion chambers.
The one downside of the CNC porting from TEA was that the combustion chamber was included in the procedure. The CNC machining was designed to improve flow by reshaping the chamber, but in doing so, the chamber volume was also increased. I didn’t mind the slight loss in compression since I was planning on running a blower at some stage on this test motor.
The early motor had previously been upgraded with Comp Xtreme Energy XE274H cams, a set of Hooker headers, and an Accufab throttle body. Equipped with the non-PI components, the 4.6L produced 301 hp at 5,100 rpm and 346 ft-lbs of torque at 4,100 rpm. After installing the TEA CNC-ported PI heads and matching PI intake (we reused the same XE274H non-PI cams), the peak power jumped from 301 hp to 399 hp at 6,000 rpm. The peak torque was up as well, from 346 ft-lbs at 4,100 rpm to 390 ft-lbs at a slightly higher 4,700 rpm. The ported heads and PI intake bettered the early components by over 100 hp and 95 ft-lbs of torque. The gains would be even more impressive had I run the early non-PI motor out to 6,000 rpm, but the power was falling off rapidly.
To make sure we got the full benefit of the ported PI heads, we also added a PI intake manifold. The same cams, Comp Xtreme Energy XE274H cams, were used with each setup.
Early 2-Valve GT Heads vs. TEA-Ported PI Heads (Horsepower)
When you make a major modification to your 4.6L, this is the kind of power gain you want to see. Note that one curve tops out at 300 hp and the other reaches nearly 400 hp. Note also that the later PI motor not only made more peak power but also extended the usable RPM range, something that will further improve acceleration over the non-PI motor. This would make one impressive early GT. I tested 4.6L 4-valve Cobra motors that didn’t make this much horsepower, and all it took was a set of TEA-ported heads, Comp cams, and a PI intake on an early stock short block.
Early 2-Valve GT Heads vs. TEA-Ported PI Heads (Torque)
The torque gains looks like you just added a centrifugal supercharger to the 4.6L, rather than a set of ported heads and intake. The gang at Total Engine Airflow really did their homework on these 4.6L PI heads. Never known for their torque production, this 4.6L (just 281 cubic inches) produced nearly 400 ft-lbs and bettered the early non-PI torque production by nearly 50 ft-lbs.
Test 3: Early 2-Valve GT vs. FPS Early 2-Valve GT Heads, Cams, & PI Intake
In Test 2 we showed that installing ported PI heads onto your early 4.6L was effective; that extra power doesn’t come cheap. The alternative to installing a set of late-model PI heads is to simply have your early heads CNC ported. According to the gang at Ford Performance Solutions (FPS), a set of ported early GT heads will flow just as well as a set of ported PI heads. Plus, porting your existing heads means you don’t have to spend the extra green on a set of PI core heads. The downside to porting the early heads (besides the cost of the porting) is that the non-PI intake manifold will limit the ultimate power output. Topping off your CNC-ported early 4.6L heads with a stock early intake would be like asking a marathon runner to compete while breathing through a drinking straw. Unfortunately, the late-model PI intake cannot be used with the early heads due to the revisions in the intake port entry and the water passages. Ford Racing offers a trick aluminum intake for the early heads, but it’s pretty pricey. The ideal situation would be to be able to run the cheap PI intake on the non-PI heads – enter FPS.
FPS found a way to adapt the early GT heads to the PI intake manifold, thus providing ported PI-power minus the additional compression. The key to successfully mating the non-PI heads with the PI intake is the custom CNC program performed by FPS. In addition to improving the airflow of the heads considerably, the intake port shapes are modified to resemble the PI heads. Phase two of the transformation includes welding and reshaping the water passages on the early head to mimic those on the PI heads. The welding is performed to reshape the water passages and eliminate any potential water leaks. After the welding and reshaping, the intake mating flange is resurfaced (lightly) to ensure a tight seal using the stock PI intake gaskets. Having already exceeded 400 hp using TEA-ported heads and the stock PI manifold, I was excited to see what these FPS-ported non-PI heads could do.
The FPS heads are CNC ported and otherwise modified to mate up with a stock PI intake manifold. This keeps the cost of the upgrade down by allowing you to use your non-PI heads with the cheap PI intake.
These FPS CNC-ported heads feature higher-flowing ports that line up with the PI intake manifold, but the non-PI combustion chambers don’t get any smaller. As a result, the compression ratio isn’t bumped like it would be with a PI head swap.
The stock non-PI motor was first run with Hooker Super Comp long-tube headers, an electric water pump, and F.A.S.T. management (13.0:1 air/fuel ratio and 30 degrees of total timing). In otherwise stock trim, the 1998 4.6L 2-valve motor produced 260 hp at 5,000 rpm and 342 ft-lbs of torque at 3,500 rpm. The motor was then torn down to the short block and reassembled with the FPS-ported non-PI heads, PI intake, and a set of Comp XE262AH (PI-specific) cams. The cams were installed in an effort to take advantage of the added breathing capabilities of the ported heads. The PI-spec Comp cams offered .550 inches of lift and 226/230-degrees duration split with a 113-degree lobe separation angle. Equipped with the FPS components, the power jumped to 347 hp and 350 ft-lbs of torque. Remember that this swap did not include the additional compression offered by the PI heads, but the FPS components added 91 hp and 90 ft-lbs. The slightly wilder cam timing dropped some power below 3,800 rpm, but picked things up big time at the top end.
Early 2-Valve GT vs. FPS Top End (Horsepower)
These early 4.6L motors really respond to ported heads, cams, and a decent intake manifold. Check out the gains offered by the FPS ported non-PI heads, PI intake, and mild Comp cams. Where the stock motor fell on its face after 5,000 rpm, the FPS motor just kept pulling all the way to 6,000 rpm. The porting and cam timing were probably most responsible for the drop in power below 3,800 rpm.
Early 2-Valve GT vs. FPS Top End (Torque)
Here’s a perfect example of what usually happens when you shift the torque curve higher in the rev range. Shifting the torque peak from 3,500 rpm to 4,900 rpm costs some torque down low. It should be noted that the FPS motor actually produced a dual torque peak, producing within 1 ft-lbs at both 4,200 rpm and 4,900 rpm. The drop in torque at 3,000 rpm would be missed but only until you allow the motor to come on the cams and pull strong to 6,000 rpm.
Test 4: PI 2-Valve GT: Effect of Compression Ratio
Between all the different heads (and combustion chambers) I was testing for this book, I ended up with quite a few different compression ratios. I was curious to see just how big a difference the compression ratio could make. To make a long story short, Sean Hyland built us a 4.6L short block featuring a steel Cobra crank, forged connecting rods, and a set of dished forged pistons. Common 4.6L piston designs include flat-tops and dish volumes of 11 cc (stock early 4.6L), 17 cc, and 23 cc. Combining the 23-cc dish pistons with a stock 42-cc PI chamber results in a compression ratio of 8.95:1. For reasons of another test, we wanted to further lower the compression ratio, so Sean Hyland obliged and built a set of custom pistons with a massive 28-cc dish. Combined with a stock PI head, the large dish dropped the compression ratio to 8.44:1. Installing our TEA-ported heads (with 45-cc chambers) dropped the final compression ratio to just 8.1:1.
To properly test the effect of compression, I installed the very same components on each of the two short blocks. The cams, intake, heads, and all other components were removed from the high-compression ’98 GT motor and then installed on the Sean Hyland short block. This is very time consuming (and ultimately expensive in terms of dyno time), but I wanted to ensure that every aspect of the comparison was absolutely identical. Only then would I know the results of the change in compression. With all of the compression ratio calculations behind me, I ran the two motors on the dyno. First up was the high-compression combination. The 4.6L GT motor was equipped with the TEA-ported PI heads, a set of Comp Xtreme Energy XE274H cams, and the PI manifold. Additional goodies included 19-lb injectors and Keith Wilson fuel rail, a set of 1⅝-inch Hooker headers, and an Accufab 70-mm throttle body and matching elbow. Run with an MSD coil pack and Meziere electric water pump, the 10.1:1 4.6L produced 401 hp at 6,100 rpm and 393 ft-lbs at 4,800 rpm. The 10.1:1 motor was impressive, never dropping below 320 ft-lbs from 2,900 rpm to 6,200 rpm. Obviously, the Xtreme Energy cams and CNC-ported PI heads were well matched.
Next came the mad thrash to swap everything to the low-compression short block. After swapping on all the components from the high-compression motor, we were ready to run once again. As expected, dropping the compression ratio by two full points (10.1:1 to 8.1:1) resulted in a dramatic drop in power. The peak numbers on the low-compression motor checked in at 365 hp at 5,900 and 368 ft-lbs at 4,700 rpm.
This Tech Tip is From the Full Book, BUILDING 4.6/5.4L FORD HORSEPOWER ON THE DYNO. For a comprehensive guide on this entire subject you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
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Note that not only were both numbers significantly lower than the high-compression motor, but they also occurred 200 rpm lower in the rev range. Looking at the power curves, it’s clear that the combination of ported heads and aggressive cam timing worked best with the higher compression. The drop in compression reduced the power curve from 3,000 rpm all the way to redline. It looks like the old adage that each point of compression is worth roughly 4 percent in power is pretty accurate.
Sean Hyland machined these custom pistons with a 28-cc dish. This dropped our compression ratio with our TEA CNC-ported heads (with 45-cc combustion chambers) to just 8.1:1.
The other short block in the test featured a much higher 10.1:1 compression ratio. We built the 8.1:1 engine for forced induction, but we were curious to see how much power the reduced compression robbed before we installed the blower.
PI 2-Valve GT: 8.1:1 vs. 10.1:1 Compression (Horsepower)
The great thing about adding compression is that the gains will be available throughout the rev range. Of course, the drop in compression was performed in anticipation of forced induction (see Test 5), but the drop in power should make you think twice about dropping the compression down to 8.1:1. Believe me, you will be able to feel 45 hp, especially since the power is down from 3,000 rpm all the way to redline.
PI 2-Valve GT: 8.1:1 vs. 10.1:1 Compression (Torque)
A drop of two full points of compression feels like someone just pulled a plug wire or two. Where once the motor felt snappy and responsive, it now feels like it wants to stay in bed. Actually, the 8.1:1 motor still produced nearly 370 ft-lbs, thanks to a good set of ported heads, Comp cams, and the PI intake, but 20 extra ft-lbs through most of the curve and an extra 35 to 40 ft-lbs out near redline is hard to pass up.
Test 5: Kenne Bell Supercharged 2-Valve GT: Effect of Compression Ratio
Many engine suppliers offer low-compression short blocks designed specifically for boosted street applications. In all instances, a motor with higher compression will make more power, but the combination of high compression and elevated boost pressure will severely limit performance on a street motor due to the resulting detonation. The ideal choice for pump gas on a daily driver is moderate compression (9.0:1) and moderate boost, but it’s possible to improve on-boost performance by reducing compression and increasing boost. The tradeoff in detonation threshold is skewed in favor of less static compression and more boost pressure, but off-boost response (and cruise fuel mileage) will definitely suffer with a drop in compression. This is especially true with centrifugal superchargers, as the centrifugal blowers tend to run best near the top of the rev range where they make maximum boost pressure. Due to their climbing boost curve and inherent increased efficiency (defined here as hp per pound of boost), centrifugal superchargers can get away with and will respond better to higher static compression ratios. The instantaneous boost response of a twin-screw and roots blower will not tolerate as much static compression (or timing), but the efficiency of the positive displacement blowers diminishes with elevated boost levels (more so on the roots design than the twin screw).
Our low-compression short block made for a compression ratio of just 8.1:1. The lower compression engine made less horsepower for a given boost level, but you can run more boost on a given octane level or detonation level.
The other short block made for a 10:1 compression ratio. Though this is a great compression ratio for a naturally aspirated engine, this is less true for high boost. At this reasonable boost level, the 10:1 engine made as much as 64 more horsepower and 54 more ft-lbs of torque.
To see how the high- and low-compression motors respond to boost, we ran one of each with the 1.7L twin-screw Kenne Bell blower. The pulleys (6.5-inch crank and 2.75-inch blower) were not changed between the two motors. We went with 65 lbs/hr injectors (we wanted plenty of injector for higher boost levels later) and a 75-mm Accufab throttle body. The total timing was 24 degrees and the air/fuel ratio was 11.5:1 (for both compression ratios). To keep detonation in check, we ran 100-octane race fuel – necessary because of the relatively high compression.
Run on the high-compression (10:1) short block, the Kenne Bell supercharger pumped out 600 hp at 6,300 rpm and 539 ft-lbs of torque at 4,400 rpm. The pulley ratio produced a peak boost pressure of 10.2 psi at 3,800 rpm and a final boost pressure of 9.3 psi at 6,300 rpm. The boost pressure changed from one motor to the next, despite identical pulley ratios. We attribute this to the fact that more flow was needed to fill the volume in the low-compression motor.
The Kenne Bell 1.7L blower assembly was then applied to the low-compression (8.1:1) 4.6L with equally impressive results. The boosted low-compression setup made 533 hp at 6,300 rpm and 500 ft-lbs at 4,400 rpm. Compared to the high-compression supercharged motor, the peak power was off by 67 hp, while peak torque suffered just 39 ft-lbs. Note that the change in compression reduced the power output across the board, from 3,000 rpm to 6,300 rpm. It’s interesting to note that the boost pressure was slightly lower on the low-compression motor than the high-compression version. The peak boost registered on the low-compression motor was 9.3 psi at 3,800 rpm, while the boost finalized at 8.7 psi at 6,300 rpm. Remember, we ran the same pulley ratios on the two motors, so the compression was the only variable responsible for the loss in power and boost pressure. The other side of this test is that the lower-compression motor would allow you to run more boost for a given octane before you experienced detonation.
Supercharged PI 2-Valve GT: 8.1:1 vs. 10:1 Compression (Horsepower)
The additional compression ratio actually had more of an effect on the supercharged combination than the naturally aspirated versions. The reason for this is that the boost pressure multiplies the difference in power. While it’s possible to run a higher boost level with lower compression (at a given octane or detonation level), there will always be a loss in power when dropping down from a compression ratio of 10:1 to 8.1:1. For a serious race motor (like the one in Tests 6 and 7), you have to combine the high compression and high boost to maximize power, but for a street application, the best bet is probably a reasonable (9.0:1 or less) compression and a reasonable amount of boost.
Supercharged PI 2-Valve GT: 8.1:1 vs. 10:1 Compression (Torque)
The drop in compression from 10:1 to 8.1:1 resulted in a consistent drop in torque production. The largest drop was 54 ft-lbs, but the loss was consistently 40 to 45 ft-lbs. This is the equivalent of a good 2 to 3 psi of boost pressure.
Test 6: 4-Valve 5.4L: Effect of Compression Ratio
This Sullivan intake for the 5.4L gives you the option of running a carburetor or fuel injection. As you can see, we’re running multiport fuel injection and a throttle-body adaptor.
Sometimes luck is with me while gathering information for a book. More often than not it’s me building and testing the motor or components that is in question, but once in a great while I will run across an opportunity that can provide the necessary information. One such opportunity was when Mod motor guru John Mihovitz was running a test on a pair of 5.4L motors. He planned to compare high-and low-compression short blocks using the very same external components. The tests run on this 5.4L 4-valve motor precisely mirrored those run on our own 4.6L GTs. I guess great minds think alike (though it can be added that fools seldom differ). The 5.4L motors were assembled with drag racing in mind, featuring all the right hardware including forged steel cranks, forged rods (aluminum on the low-compression version), and forged pistons. The change in compression ratio came from a change in piston design, as both motors relied on the very same heads, cams (including cam timing), and induction system.
The 5.4L 4-valve motors were built with high horsepower in mind. The 4-valve heads came from a Navigator, but don’t let the humble soccer-mom beginnings fool you. The Navigator heads feature large intake ports that were ported to unleash an additional 50 cfm per runner. Naturally, the ported heads required something other than a long-runner Navigator intake manifold. Knowing the motor would require substantial intake flow, the 5.4L was topped off with a Sullivan intake casting. Designed to accept a conventional carburetor, the short-runner aluminum intake also featured provisions for fuel injectors. A 90-degree inlet elbow was used to mate a 90-mm Accufab throttle body to the carburetor flange. The 90-mm throttle body ensured adequate airflow to the high-RPM 5.4L motor, while 150-lb/hr injectors and an Aeromotive A1000 fuel pump (with Kenne Bell Boost-A-Pump) ensured adequate fuel delivery. The heads received a quartet of Sean Hyland 4-valve cams. The (eventually) supercharged motor was equipped with Stage 2 intake cams (.452 inches of lift and 225 degrees of duration) and Stage 3 exhaust cams (.474 inches of lift and 235 degrees of duration). The cams were installed at 107 degrees.
The 5.4L 4-valve motor was also set up with a set of custom 1¾-to-2-inch step headers, an MSD coil pack, and a F.A.S.T. engine management system. A number of different timing and fuel curves were tested to optimize each combination. The low-compression motor produced 478 hp at 7,300 rpm and 395 ft-lbs at 5,500 rpm. Running the very same components on the high-compression short block upped the power output to 543 hp and 439 ft-lbs. The additional 3.5 points of compression upped the power output by 65 hp. According to the old rule of thumb that every point in compression is worth roughly 4 percent in power, we can calculate that increasing the compression by 3.5 points should yield a gain of roughly 14 percent. If we multiply 1.14 × 478 hp, we get 545 hp, which is very close to our actual peak power of 543 hp. I guess the compression formula is pretty accurate, although it should be mentioned that the gains in power are actually greater in the 8.0:1 to 11.0:1 range and tend to diminish slightly thereafter (upping the compression ratio from 8.0:1 to 9.0:1 will likely yield greater gains than going from 12.0:1 to 13.0:1).
By going with a flat-top forged piston, we’re able to raise the compression ratio quite a bit. This piston features valve reliefs to hopefully prevent any piston-to-valve contact.
4-Valve 5.4L: Low Compression vs. High Compression (Horsepower)
The nice thing about increasing the compression ratio is that it adds power throughout the curve. Jumping from 8.2:1 to 11.7:1 resulted in a gain of 65 hp (from 478 hp to 543 hp).
4-Valve 5.4L: Low Compression vs. High Compression (Torque)
Note that the shape of the torque curves remained consistent despite a change in static compression of 3.5 points. The additional squeeze simply elevated the entire curve.
Test 7: ATI Supercharged 4-Valve 5.4L: Effect of Compression Ratio
Back in Test 5, I took a hard look at the effect of changes in compression ratio on a 2-valve 4.6L GT motor. The idea behind the test was to illustrate the change in both power and the boost curves caused by a drop in compression ratio. Obviously this was an important (if time consuming) series of tests, as in almost all instances, a drop in static compression is recommended when adding forced induction. This is especially the case when we are talking about street motors, as the limiting factor in terms of power is almost always the octane rating of the pump gas. Sure, you can toss in a tank of 100-octane race fuel and go a bit wilder with boost, compression, or total timing, but running 91 octane will definitely
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