Heads, Valvetrain, and Intake Manifolds
Heads
Ford is using and sticking to a unique head not found in most American cars. Ford has an overhead cam head instead of an overhead valve. Instead of having one cam, they have two or four cams; the cams are in the heads. The overhead cam design can rev much higher and easier than the standard overhead valve design. Ford has two basic versions of heads, the SOHC or single overhead cam heads and the DOHC or double overhead cam heads.
Ford has quite a few more 4V heads than 2V heads. The first type of heads are called “B” heads. These are found in the 1996-1998 Cobras and 1993-1998 Mark VIIIs. These heads are also known as swirl heads. The “B” heads have two intake valves as well as two exhaust valves. These heads flow 230 cfm at .500 inches of lift on the intake side and 145 cfm at .500'' lift on the exhaust side. There is a very unique item to the “B” style heads known as IMRCs or Intake Manifold Runner Controls. The IMRC is a computer that keeps the secondary intake valves closed until 3250 RPMs. In 1996 and 1997, they were made with aluminum and got very dirty. In 1998, Ford changed to a plastic style IMRC valves. Ford has this actuator to maintain torque down low and have good horsepower in the upper end. The heads flow too big for normal street use, hence the reason why Ford chose the IMRCs for these heads. The IMRC controller has been known to fail as well, causing a huge loss of power. Some people will go as far as deleting the IMRCs with a kit, but this causes bucking, stuttering and a loss of torque; I personally do not recommend this, even on a non N/A car. Eric Duncan has done some tests on IMRCs and found you can actually gain quite a bit of rwtq by having the valves open closer to 4000 RPMs than 3250 RPMs.
From 1999 to 2004, Ford switched to the tumble port heads. The tumble port style is meant to prevent condensation on the intake ports from dropping into the cylinder bore, as well as having a homogenous mixture for a more controlled, predictable burn. These heads flow better than the “B” style heads until .450 inches of lift; then the “B” heads take over. The 1999/2001 heads flow 225 cfm on the intake side and 145 cfm on the exhaust side at .500’’ lift.
Ford Racing came out with FR500 heads. These heads are another version of the tumble port heads. The heads outflow the 1999/2001 Cobra heads by quite a bit with 228 cfm on the intake side and 162 cfm on the exhaust side at .500’’ lift. These heads have quite a bit of meat of them to port but I personally do not recommend these anymore. Ford came out with even better heads, IMO. Expect to pay around $700 per head for these “jems.”
In 2003 and up to 2004, Ford developed what is probably the best “bang for the buck” head for the 4V motors. These heads are found on 2003-2004 Cobras, Mach 1s, Aviators and Marauders. The heads are still tumble port heads but much improved compared to the other years. The exhaust port is squared off at the corners, which is probably the easiest way of telling these heads apart from the other heads. Ford decided to use dedicated left and right castings for these heads as well which help keep cylinders 6,7 and 8 a little bit cooler. These heads flow 233 cfm on the intake side and 168 cfm on the exhaust side at .500 of lift. The FR500 heads do outflow these from .250’’ lift and below but the price more than makes up for them. You can find these heads for as low as $450 for BOTH heads.
Head Specs:
1996-1998 “B” heads:
Combustion Chamber – 52cc
Intake port – 107cc primary/115cc secondary
1999/2001 Tumble Port heads:
Combustion Chamber – 54cc
Intake port – 177cc
2003 Cobra Tumble Port heads:
Combustion Chamber – 52cc
Intake port – 177cc
FR500 Tumble Port heads:
Combustion Chamber – 53cc
Intake port – 160cc
Non-PI vs. PI vs SVO
The 4.6L engine was introduced into the Mustang GT in 1996. The cylinder heads (and intake) in these initial Mustang 4.6L engines would come to be known as nPI (non power improved) heads. The Mustang GT’s were rated at 215 – 225 horsepower at the flywheel.
In 1999, Ford introduced the PI (power improved) heads and intake. For the purposes of this FAQ, we will only discuss the heads. The heads are superior to the nPI heads as the data below shows. These Mustang GT’s were rated at 260 (excluding the Bullitt which was under-rated at 265 FWHP).
The following data will show the differences between the two heads:
(numerical data courtesy of Sean Hyland)
nPI Heads:
Combustion chamber volume: 51cc *
Intake port volume: 146cc
Intake valve diameter: 44.5mm
Exhaust valve diameter: 34mm
Intake flow @ .500 lift: 156cfm
Exhaust flow @ .500 lift: 116cfm
nPI Cams: .480 lift (duration at .500 = 204 intake, 208 exhaust)
* Initial 4.6L engines utilized flat top pistons.
PI Heads:
Combustion chamber volume: 42cc **
Intake port volume: 159cc
Intake valve diameter: 44.5mm
Exhaust valve diameter: 36mm
Intake flow @ .500 lift: 156cfm
Exhaust flow @ .500 lift: 133cfm
PI Cams: .535 intake, .505 exhaust (duration: 192 intake, 184 exhaust)
** PI equipped
engines utilized dish pistons to keep the compression ratio at 9.7-1.
The primary drawbacks to the nPI heads as compared to the PI heads are the intake ridge, smaller intake port volume, and smaller exhaust valve. The intake ridge’s purpose is to induce in-cylinder mixture motion. However, it also acts a shroud to a valve that already is shrouded by its close proximity to the cylinder head wall. The intake port diameter is adequate for most naturally aspirated and mildly blown applications; however, the increased intake port diameter of the PI heads will support higher horsepower applications. Finally, the small exhaust valve acts as a hindrance to exhaust flow and it is a major reason why these heads wheeze out before the PI heads or the SVO heads.
The PI heads utilize a quench pad instead of a ridge to induce mixture motion. This quench pad does not provide as much of a restriction as the intake ridge in the nPI heads. The intake port volume is increased to 159cc and the exhaust valve got an upgrade from 34mm to 36mm. The PI heads also include PI cams that have a higher lift than the cams provided in the nPI heads. The cams also include a slightly smaller duration to enhance low end torque.
A popular upgrade for nPI equipped Mustangs is the PI swap. This includes swapping PI heads and the PI intake on an nPI 96 – 98 Mustang resulting in an approximate gain of 45 RWHP, but with an increase in the compression ratio to approximately 10.5-1 making these PI head swapped engines supercharger unfriendly.
Another common swap that involves less labor at significantly less cost is the PI cam and intake swap. The swap provides a gain of approximately 30 - 35 RWHP while maintaining the stock compression ratio. Because used PI cams and used PI intakes can usually be purchased for approximately $100 each, this mod is proving to be one of the best dollar for horsepower mods available for the nPI equipped Mustang.
SVO 2V Heads:
In 1998, Ford Racing came out with the 2V SVO head. These heads are a direct swap on an nPI (96-98) Mustang GT’s as the intake ports on the heads match the tear-drop shaped nPI intake ports. These heads alone add approximately 25 - 30 RWHP. When used in conjunction with the SVO intake, another 20 RWHP may be realized. The current price of these heads at about $850 each puts this mod out of the reach of many. The SVO head may be used on PI equipped (1999+) Mustangs provided the SVO intake is also installed as the ports on the SVO heads are not compatible with the intake ports on the PI intake.
SVO Head datA)
Combustion chamber: 51cc
Intake port volume: 173cc
Intake valve diameter: 46.8mm
Exhaust valve diameter: 35.8mm
Intake flow @ .500 lift: 193cfm
Exhaust flow @ .500 lift: 140cfm
Some other technological improvements with these heads are reduced intake curve radius and valve centerline. The intake curve radius was reduced to provide less of a restriction of the intake mixture flowing into the combustion chamber. The intake and exhaust valves were moved towards the center of the bore (combustion chamber) to unshroud them and increase flow.
Written by: Dennis “Bluebear” McCarthy
Port and Polish
Porting is a simple way of gaining HP by grinding out some of the intake/exhaust ports in a head and/or intake manifold. There are special tools that help do this and it’s not a “do it yourself” job unless you know how much to grind without worrying about the heads and/or intake breaking. Polishing, however, can be done yourself with a dremel power tool. Polishing is simply polishing up ports to a nice high shine. This promotes more airflow and allows the air to flow easier. The point of porting and polishing is to allow for the heads to breathe more and thus allowing for more HP; however, porting a head too much can result in a loss of torque as the engine cannot deliver enough velocity alone to “push” the air into the engine. Expect to pay quite a bit for a port and polish by a reputable engine builder when it comes to heads. This is a mod that should really only be done if you’re finished with bolt-ons and need that extra HP or when you have an extra set of heads lying around.
Valvetrain
The valvetrain in the 4.6Ls is unique in itself. The design is an overhead cam design, where the cams are actually in the heads. This gives the ability to rev to high RPMs without many modifications. Although the 2V and 4V seem different, they are actually quite similar when it comes to valvetrain components. Rocker arms, valve lash adjusters and timing chains are the same on the 2V and the 4V.
2V Camshafts
Cams are a mod that you should perform last. Typically cams take around 6-8 hours of labor to install and then you’ll need a tune afterwards to utilize all the power. Cam selection depends heavily on your application so you should consult with the company you’re ordering from. Cam gains can range from 15 rwhp to 30+ rwhp depending on the stage and other modifications done to the car. A rule of thumb though – any stage II or above cam will require springs. The best cams for the 2V are Competition Cams, Crower Cams and VT Engines.
2V Valves
The best valves for the 4.6L are Ferrea valves. You can also choose the SVO oversized valves that will provide more power. There aren’t many valve companies out there right now but hopefully other companies will start to manufacture valves. As of now, I recommend sticking with Ferrea.
2V Springs
As of today, the only manufacturer that I know who makes good valve springs for the 2V is Competition Cams. They make the beehive springs shown above. The problem with 2V springs is the various seat pressures they experience when compressed. The rule of thumb (as stated above) is any Stage II or higher cam will require valve springs. I also do not know the RPM limitations of the 2V springs. If someone would like to shoot me that information, it would be greatly appreciated.
4V Camshafts
4V Valves
4V Springs
The stock springs are actually quite amazing. Stock, they can rev to 7200 without any real issues. After that, they start to get loose and jump around. Most camshafts can utilize this spring without much of an issue either. It should be noted that the Mark VIII and Cobra valve springs are different and it would be worthwhile to upgrade if you have a Mark VIII. For those who want even more revving capability, Sean Hyland makes a dual spring that can rev to 9600 RPMs. To use this spring, you must have a cam that can open and close this spring along with using their special valve guides. Also, your heads will have to be milled out a little bit to make the spring fit as it’s bigger than the stock spring. If you want the best springs though, Al over at Boss330 racing has a 100% custom valvetrain that can be spun to around 10k RPMs with no issues.
Valve Guides
Valve guides go between the valve and guide it up and down precisely, hence the name “valve guides.” A common upgrade is to change them out for bronze guides. I always recommend switching to bronze valve guides whenever possible as you will have less friction, and we know that friction is the enemy of HP. If you are using Sean Hyland’s 4V double spring, you have to use their special valve guides that lock the springs in place precisely.
Valve Lash Adjusters
A valve lash adjuster goes between the camshaft and the valve. Their job is to take up slack between the cam and the valve. The stock valve lash adjusters are fine and really don’t need to be upgraded. They are the same in the 2V and the 4V. Stock valve lash adjusters can be taken up to 9600 RPMs without a problem. The 00 Cobra R has unique valve lash adjusters that are different from the 2V and 4V valve lash adjusters. They are smaller in design. I won’t get into too much detail about them but they provide a straight shot at the valve, much more so than the 2V and 4V lash adjusters. They also make solid valve lash adjusters for those of you who are drag racers. These will offer a very rough idle and are not recommended for a street car. However, they are the best option out there for those who need the best. They provide a safety aspect that the stock ones do not, since you can run an external oiling device without having to worry about valve float from too much oil pressure at high RPMs like you would the stock ones.
Adjustable Cam Gears
Adjustable camshaft gears are brand new on the market. They allow for the cams to be advanced or retarded in terms of degrees. Certain cams, such as Comp. Cams, have a 5 degree advance. With an improper setup, this will result in piston-to-valve contact. These gears will enable you to take the 5 degrees of advancement out of the cams. They are currently going for $400-$700 on the market so they’re not cheap. Consult your engine builder if you think you need them or not.
Rocker Arms
Rocker arms don’t need to be upgraded. Their function is to open and close the valves from the cams. They are the same on the 2V as they are on the 4V. It is recommended to polish and deburr them before installation.
Timing Chains
The timing chains on the 2V and the 4V are the same. The only difference is that the cobra chain is polished so it is slightly stronger (or so I’ve heard).
Intake Manifolds – 2V
When the 4.6L engine made its debut in the 1996 Mustang GT, Ford included a plastic intake along with this engine. In 1999, the PI (Power Improved) engine was introduced into the GT. This engine also incorporated a plastic intake manifold. However, this new plastic PI intake with its longer runners and larger diameter ports has been shown to contribute approximately 15 RWHP to the PI package.
Lately many people have experimented with swapping a PI intake on nPI equipped mustangs (96-98) and have achieved the above noted gains. Although the ports on the PI intake do not match the nPI heads, many people have successfully used hi-temp RTV sealant to seal the gaskets. Since PI Intakes can be purchased for as little as $160 brand new, this mod is proving to be an excellent dollar for horsepower mod for the 96 – 98 nPI Mustang.
In 1998, Ford produced an after market intake known as the SVO intake. This aluminum intake incorporated longer and larger diameter cross section runners. The intake also receives its air through a large plenum fed by a twin blade throttle. This intake is a direct swap on 96 – 98 nPI Mustangs or can be used with Mustangs that have the Ford SVO heads. Although considerably more expensive than the PI intake, this intake will add at least 25 RWHP to the nPI Mustang and being that it is made of aluminum, it is also more durable for nitrous or blown applications.
The Mustang Bullitt GT edition was made in 2001. This Mustang with its PI engine incorporates an intake very similar to the SVO intake discussed above. However, the “Bullitt” intake has ports that match the PI engine and is an excellent aluminum intake for the supercharged or nitrous equipped 99+ Mustangs or for those seeking higher NA (naturally aspirated) power as well.
There are also other after market intakes that are now available for the 99+ Mustang such as the Fox Lake P-51 and the soon to be released C & L lower intake. The P-51 intake has been show to give good gains for the NA and supercharged 4.6L engine.
After market plenums (plenum: the elbow between the throttle body [TB] and the lower intake), made their debut in 2003. People who are on a budget or who wish to retain their stock plastic intake can benefit from the use of an after market plenum and throttle body. The stock plenum and 60mm TB flow 410 CFM. After market plenums such as the Accufab plenum and 70mm Accufab TB flow 602 CFM and are good for 8 – 10 RWHP. Other after market plenum combos are the C & L and 75mm TB, Dragon, and Trickflow. The Accufab combo tends to dyno the highest (but at significantly higher price). The C & L combo is a very close second (usually within 1 RWHP) at lower cost.
Written by Dennis “Bluebear”
McCarthy
Intake Manifolds – 4V
Unlike the 2V cars, the 4V cars have many options out there for complete intake manifolds. Both the 96-98 intakes and the 99/01 intakes can be customized to suit just about any need you have. If you must have the best, sheet metal intakes can be made, but at a price of about $3500.
1996-1998 Cobras have an aluminum, side-entry throttle body intake manifold with long runners. The runners curve 180 degrees into the cylinder head ports and in between them are things called “IMRCs” or Intake Manifold Runner Controllers. The job of these is to maintain torque by only opening one intake valve and one exhaust valve until a given RPM (which Ford set at 3250 RPMs stock). The 1996 and most of 1997 IMRCs are made of metal which get gummed up quite a bit from the carbon that is found after engine combustion. Some late model 1997s and1998s have a ceramic style IMRC plate that doesn’t get nearly as dirty as the 1996-1997s do. IMRC deletes are out there but unless you have a poweradder that is constant like a turbocharger or supercharger, you will lose torque and have significant driveability issues under and until 3200 RPMs. Eric Duncan did some tests and found that you can actually gain quite a bit of torque by making the IMRCs open up at close to 4000 RPMs. This intake manifold will only fit on B style heads.
The 1999/2001 Cobra intake manifold is a long-runner design, just like that of the 1996-1998, but it no longer has IMRCs. These intake manifolds flow significant better over the 1996-1998 and that’s one of the reasons the 1999/2001 gets its HP difference from the older Cobras. This intake manifold will fit on any tumble port style head.
The 2003/2004 Mach 1 has a unique intake manifold that is similar to the 1999/2001 intake manifold. It is a long runner intake in that the air travels between the runners, then down the opening and back up again to the cylinder head ports. This intake seems to make great torque but isn’t too great at high RPMs. This is one reason why the Mach 1s perform better when shifting around 6000 RPMs instead of 7000.
The FR500 intake manifold is the most unique design of an intake I’ve ever seen. It is not a short runner, it is not a long runner – it’s BOTH. The FR500 intake manifold has short runners that are controlled by throttle blades that are closed until they are needed. The long runners are used until 3500 RPMs, which then the short runners open up at. Unlike the other intake manifolds, this intake is made of magnesium. This intake manifold will fit any tumble port style head but it’s going to cost you a whopping $2500. Gains have been known to reach 40 rwhp and beyond with the FR500 heads and cams to compliment them.