Standard-to-High Output 5.0L V8 Conversion

For All 1986-88 5.0L V8 Cougars / Thunderbirds

Page Revised: 10 May 2019


Article by
Eric Dess

Your stock 1986-88 Cougar 5.0L V8 engine is only rated at 150hp (155hp in 1988 due to dual exhaust) from the factory. Sure, it runs great, it provides plenty of low-end torque to get you off the line, and it can get pretty decent fuel economy on the highway. But that's about the limit of its performance. You may be pining for more power but things can get pretty expensive quickly. What you need is a budget-friendly, economical solution. Well, how does a 75hp gain in one weekend sound to you?

It's relatively easy to convert your stock 150hp* 5.0L V8 into a near-perfect copy of the legendary 225hp HO 5.0 used in Mustangs. You don't even need to remove the engine from the car, making it an extremely efficient job.

Keep in mind a few things before you begin collecting parts:

  • This section assumes that you will be using STOCK Ford or Mustang parts.
  • It is also being assumed that you will keep your stock speed density setup. Afterwards it's possible to do the Mass Air Conversion.
  • To do the full HO (~225hp) conversion, you MUST have all of the parts below before you can start.
  • You must also convert to dual exhaust at the same time you do the HO conversion as the HO motor requires freer breathing.

Once you've assembled all the components for the HO conversion you can proceed using the guidelines below.

* This section is for factory SEFI cars only (1986-88). Earlier CFI cars (1983-85) would require a full SEFI conversion plus the information below.

Requirements for HO Conversion in EFI cars:


SEFI Fuel Injection

Since your car already has the same SEFI setup as the Mustang, you can simply reuse virtually the whole system, including the fuel lines, metal fuel rails and the stock fuel pressure regulator. See below about the fuel injectors.

E7TE-Casting Cylinder Heads
E7TE-Casting Mustang Cylinder Head

The stock 1986 heads are, to say the least, very restrictive. Narrow inlets and "high swirl" exhaust ports add to fuel economy, but at the expense of good power. These heads were used on the '86-'88 Cougar/Thunderbird, the '86-'91 Crown Victoria/Grand Marquis, and the '86 Mustang GT. In '87, the Mustang 5.0 engine received the F-150 truck heads (E7TE-AA castings, aka E7s), which added approximately 15hp; the Cougars still had the restrictive heads through 1988. You should consider using at least the E7 heads. You can find decent heads in a junkyard, at a swap meet, or online auctions or message boards. Aftermarket heads, such as the GT-40 and Trick Flow among many others, can also be used, according to your budget. By the way, you can put these heads on stock non-HO motors (without converting to HO) just as well. If you cannot find E7s, it's perfectly okay to use a slightly older Ford 5.0 head (E3, E4, etc.) but avoid reusing the E6s unless you cannot afford E7s or cannot find them.

Stock 19 lb/hr Fuel Injectors
19 Lb/Hr Fuel Injector

Your stock fuel injectors (1986-88 non-HO 5.0) are rated at 14 lb/hr and are light grey in color. The HO computer is set up for 19 lb/hr (orange or tan) injectors. You'll absolutely need to acquire 19 lb/hr injectors for this conversion; you can use them from a Mustang or a Mark VII. The good news is, they're fairly plentiful and usually very cheap. Just ask any Mustang guy that's switched over to 24 lb/hr; he may give you the old 19 lb/hr ones. Or you can check online. In general, fuel injectors are extremely reliable and usually do not malfunction (unless severely clogged), so if you get a good deal on a set, chances are you'll actually be getting a good deal. By the way, in case you're curious: if you mix the 19 lb/hr computer with your stock 14 lb/hr injectors, you will get a bad fuel starvation problem and drivability woes. You can't avoid getting the correct injectors for the HO conversion.

HO-Specific Camshaft
Mustang HO Camshaft

In order to work with the Mustang computer and injectors, you must also get a stock '87-'93 Mustang camshaft. It is the heart of the HO conversion. Ford re-ground cams almost every year for a little better performance and fuel economy, so stock HO cam part numbers changed, but they will all work with the HO computer and you will more than likely never notice any difference between them. It is therefore entirely possible to use a '92 HO cam with an '87 Mustang GT computer with no difficulties. Idle with the HO cam is very smooth, and top end is greatly increased over the stock Cougar cam. Generally you can use any 1987-93 HO cam with the speed density computer with great results. It is advised to avoid the 1986 HO cam as the later ones give a lot more usable power. There are lots of Mustang owners out there that would just love to get rid of the stock cam they have lying around (because they put an aftermarket one in)—perhaps you can wrestle one away from them. One note of caution: since we have flat-top pistons from the factory, anything more potent than the stock HO camshaft may result in serious piston-to-valve clearance problems and/or drivability problems. The only other confirmed combination that works with flat top pistons is the Trick Flow cam with TFS heads. By using an HO cam, your firing order will be different from your stock one. The firing order will now be 1-3-7-2-6-5-4-8, using the HO pattern. You'll need to route the spark plug wires accordingly.

EEC-IV 5.0 HO Computer
Mustang DA1 EEC-IV Computer

Here's the brains of it all, literally—the stock Mustang 5.0 computer. You should be able to find one at a salvage yard, swap meet or online for a reasonable price. Beginning in 1987 Ford began using a calibration code on each computer, with the code stamped on the top. For speed density, these are the computers that you can use (all listed for automatic transmissions):

  • DA1 (1987-88 Mustang)
  • DB1 (1987-88 Mustang)
  • DX3 (1987 Mark VII)
  • VL2 (1987 Mark VII)
  • DY3 (1988 Mark VII)
  • D9S (1988 Mark VII)

Just FYI, the Mark VII computers are top speed-limited to 105mph. The Mustang HO computers are not.

Higher Volume Fuel Pump
Mustang Fuel Pump

A stock Mustang/Mark VII uses an 88lph (liters per hour) fuel pump; ours are 65lph stock. To avoid a severe hesitation problem and/or a lean condition, you will need to change your pump to at least an 88lph unit. We recommend at least a 155lph, good quality aftermarket Mustang-style unit (BBK, Walbro, etc.). Avoid parts store pumps as they tend to have a high failure rate. You will only need the pump itself, not the hanger, and you can reuse your sock. The wiring may need to be modified slightly, and you may also need to zip-tie the new pump to your stock hanger, but you can get everything to work with your fuel system. Remember to change the fuel filter not long after changing out the pump.

In case you're wondering, you may be able to reuse your stock fuel pump if it's still good and has been recently changed. But as it is, even 65lph was cutting it close for a non-HO 5.0. You might experience a slight fuel starvation, in which case the pump will definitely need upgraded.

HO Upper Intake Manifold (optional)
Mustang HO Upper Intake Manifold

The HO upper intake should be used as it is better flowing and helps with delivering power to the 5.0 engine. But it is optional because, although there is a minor difference in casting between the non-HO and HO plenums, the difference is simply a few horsepower, estimated at 5-7 hp. The stock Mustang upper (as well as the Mark VII HO upper) has the letters "HO" stamped under the removable top plate. If you decide to go with this intake, you should install the matching EGR plate, EGR valve, and throttle body from the Mustang as well; this will give you maximum airflow performance. So you can still use your stock upper if you like, but you'll definitely get better airflow with the HO unit. By the way, the non-HO and HO lower intake manifolds are all the same; if you want to reuse your lower that's fine also.

HO Throttle Body (optional)
Mustang 60mm Throttle Body

The stock Cougar/Thunderbird throttle body is 50mm. The stock Mustang throttle body is better at 60mm. Your stock one will work if you can't get the bigger one, but your car may bog down slightly upon acceleration due to restricted airflow. Still, you can likely get away with not upgrading if you can't afford it or just don't want to at the time of the HO conversion.

A new aftermarket 65mm aftermarket unit will work with the stock speed density computer and give you much improved airflow. Anything over a 65mm-70mm throttle body on a stock motor will decrease low end torque and actually bog down the motor with too much air. Also, the HO upper has a 67mm bore; a 70mm throttle body may create slight turbulence because of this. The bigger units (75mm+) should be limited to severely modified motors and larger-bore upper intakes. Be sure to buy a throttle body made for 1987-up Mustangs—the 1986 throttle bodies were unique. And remember to get a throttle position sensor (TPS) to match the new throttle body, as your stock one may not work on it.

Mustang 60mm EGR Spacer

If you are going to upgrade your throttle body, whether the stock HO 60mm or larger, you will also need its matching EGR spacer plate. The EGR plate and throttle body must match for optimal airflow. The coolant rails are just like your stock ones, so you'll be able to reuse your coolant hoses on the new throttle body.

MAP Sensor (optional)
MAP Sensor

The MAP sensor hangs on the firewall, just above and to the right of the upper intake plenum (just to the left of the wiper motor). This reads the Manifold Absolute Pressure, and since you'll have a different amount of vacuum with the new camshaft, you may need replace yours. It's recommended that you try to use yours first. If you experience rough idle, bad starts, no starts, or general bad behavior from the engine, replace this sensor with one that matches the calibration code for your particular EEC-IV HO computer. Now there are tons of part numbers for this sensor so it may take multiple tries to get one that works. Incidentally, the 1989-up mass-air Mustangs have a BPS (Barometric Pressure Sensor) that is identical to the MAP sensor, save for the vacuum line. If you're ever going to switch over to mass air, you can use this existing sensor by simply capping the vacuum line and letting the MAP sensor open to the atmosphere.

9-Blade Fan (optional)
Mustang Fan

Technically you can reuse the stock fan and fan clutch, as they should be just fine to handle the demands of an HO engine—provided they're in good shape. The stock fan has 7 blades; the HO fan from a Mustang has 9 blades. While the 2 extra blades could help a bit, they're not mandatory. Still, check that your fan and fan clutch are in good working order. If they need replaced, it's perfectly fine to use the HO-spec replacements if you wish.


After all the parts are installed, you will need to reroute the spark plug wires to match the HO firing order (1-3-7-2-6-5-4-8). You will NOT need to change any fuel injector wiring in your stock harness for this conversion, no matter what anyone else tells you!

You can reuse your existing EVP, EGR valve, idle bypass valve, and radiator, as they'll be adequate enough for the HO conversion.

Stock timing is still 10 degrees BTDC. Once the car is running correctly, you can get some added power if you bump up the timing to 12 or 14 degrees. Try 1-degree increments at a time since every engine responds differently, but back off if you hear any detonation (pinging). Also remember that if you do advance the timing, premium unleaded gasoline (93 octane or higher) is REQUIRED.

As with any engine build, be sure to properly check the piston-to-valve clearance. Almost any other high-performance camshaft will REQUIRE performance dished pistons! Unfortunately, the removal and replacement of your stock pistons will have to be done with the engine outside of the car, on an engine stand. You could also consider having them fly-cut. So please consider this if you're debating on putting better pistons in your motor. In general, if you think the 225-hp version of your engine will be adequate, then the stock pistons will work great. But if you plan on adding a supercharger, nitrous oxide, or even a healthier camshaft, you will have no choice but to pull the motor out to change the pistons.

Don't forget that after the conversion (or even during), you must upgrade to dual exhaust to get the full 225hp benefit. Reusing the stock cast-iron manifolds and single exhaust will result in slight bogging and decreased power output. See here for more information about the dual exhaust conversion.