High Output 5.0L V8 Transplant

For All 1983-88 Cougars / Thunderbirds

Page Revised: 10 May 2019


High Output 5.0L V8

Article by
Eric Dess

There's sometimes a point when, as much as you love your Cougar, you might be pining for more power. Maybe the stock 5.0 is shot and you need a new motor anyway. Or perhaps you have the underpowered 3.8 V6 under the hood right now. Considering the availability of the HO 5.0 from wrecked Mustangs, it sure seems logical to put that engine under the hood of your Cat. This section will tell you how to drop in the venerable High Output Ford 302 into your Cougar, whether you currently have a V8 under the hood or not.

There were relatively few Ford Motor Company vehicles from our era that received the factory HO 5.0 motor. Now there's plenty of information out there about these cars, but in general here's what you need to know to make your life simpler for this swap.


The 1986 Mustang/Capri 5.0 is different from all succeeding years; it actually has a lot in common with our factory 1986-88 engines. In fact the only differences were the camshaft, fuel injectors, factory headers and the EEC-IV computer. This factory-rated 200 hp engine would make a decent starting point, although you'd probably want to consider an upgraded HO conversion while the motor is out. The pistons are aluminum flat-tops. (The Capri was dropped by Mercury after 1986).


Starting in 1987, the Mustang went to the legendary 225 hp version that we all know and love, with improved heads and camshaft. The Mark VII got this same treatment starting in 1988. These initial HO engines were of the speed density variety. Internals were beefed up and the E7TE casting heads were a big step up from the 1986 HO engine. Pistons were now of the TRW forged variety.


Beginning in 1989 (technically 1988 in California) the Mustang went to the mass air setup. (The Mark VII never received factory mass air and was discontinued after 1992). The motors themselves didn't really change but the engine harness, ECU and some related sensors were updated. The 1993 HO engine received hypereutectic pistons, which are good for daily use but will not hold up to forced air induction or nitrous oxide. Also, a revised horsepower rating method employed internally at Ford reduced the HO 5.0 down to 205 hp; in reality none of the components changed, and most Fox enthusiasts still refer to it as a 225 hp engine.

Concurrently, the Cougar and Thunderbird received an HO 5.0 from 1991-93, rated at 200 hp. It was a different version than that used in the Fox Mustang. Physically the blocks and most of the internals resembled the 1993 engine, but there was a different camshaft grind. All of the accessories and brackets were also revised to hug the block for better NVH. The most important change was the dramatically lowered upper intake manifold, designed to clear the low MN12 hood, and the curved intake attachment for the throttle body.


In 1994 the all-new SN95 Mustang debuted and with it was a revised HO 5.0 engine, rated at 210 hp. Virtually all the parts needed to stuff the motor in the engine bay were already pre-engineered for use in the 1991-93 MN12 Cougar/Thunderbird, so Ford wisely used that engine setup for the SN95 Pony. This lasted until the end of 1995, and with the debut of the 4.6L SOHC engine in 1996, the HO 5.0 was history in Ford cars.


However, the 5.0 HO engine did live on in the Ford Explorer and the Mercury Mountaineer. Again, new brackets and accessories were necessitated due to the Explorer's tight engine bay space. Radical new curved manifolds (and later, headers) were designed to clear the massive engine crossmember, and a new oil pan did the same. Most people know this engine for its new exclusive intake manifold setup, again lower and now less restrictive, outflowing even the aftermarket Cobra intake package. Rated at 215 hp, this engine is the newest factory 5.0 HO available and, due to its rugged nature, is one of the more desirable ones.

Even though there were quite a few vehicles to sport the HO 5.0 from 1986-2001, not all of them are optimized for your particular engine bay or engine management system. Generally the easiest to swap would be 1986-93 Mustang engines, and 1988-92 Mark VII engines. It is a little difficult to get a 1994-95 Mustang engine setup to work in our cars; ditto for the MN12 engines. The Explorer/Mountaineer engines need downgraded to Fox accessories and brackets. The wiring and computer to run the 5.0 is different between cars and body styles...fortunately part of that problem can be resolved using a harness from our own cars.

Now you must realize that no two Cougar engine bays are alike. So a 1985 5.0 and a 1987 3.8 V6 have much different engine bays, as far as the configuration goes. The key to having this whole transplant work smoothly is realizing how the 5.0 engine bay looks. Any 5.0 engine bay from any Fox automobile from 1986-on (including Mark VII, rear-drive Continental, Capri, and of course the 'Stang) is arranged in the same basic way: battery tray and starter solenoid on the driver's side, throttle body and air cleaner box on the passenger side. Your engine bay MUST be configured to this arrangement for the HO 5.0 to fit. Now just because your engine bay doesn't look like this now, doesn't mean you can't make it that way. As long as you get the necessary components from the donor car or another Cougar, you will be assured that the 5.0 will fit and work properly.

The following will help get you generally oriented with what you'll need for the swap:

Turbo Cars (1984-86 Cougar XR7, 1983-86 Thunderbird Turbo Coupe)
If you have a 1984-86 XR7 (or Turbo Coupe) your engine bay is already configured for the new HO 5.0 motor. The 1984-85 cars must either cut a hole in the passenger side inner fender for the air cleaner assembly so fresh air can get in, or use a K&N Filtercharger to get around that. Your vacuum tee can be reused, if you wish, and the battery tray is just fine.

1986-88 Factory 5.0 Cougars/Thunderbirds
You're all set.

1988 3.8L V6 Cougars/Thunderbirds
Your engine bay is already set up for the new motor. You will need to replace sensors on the firewall; the stock vacuum tee can be reused.

1983-85 5.0L V8 Cougars/Thunderbirds, and 1983-87 3.8L V6 Cougars/Thunderbirds
Your engine bay is completely opposite of what's needed, which means you will have to do some additional work. It's not bad, really; it just requires a little more time and a few more parts. Your current engine crossmember (aka "K" member) will not have to be changed. You'll just need the appropriate V8 motor mounts (see below).

The 5.0L HO V8 Engine

Why the HO 5.0? What's the difference between that and a Cougar 5.0?

There are actually quite a few differences between our stock SO (standard output) motors and the HO (high output) motor. Below are the major differences:

1987-95 HO 5.0 (Mustang / Mark VII) Non-HO 5.0 (Cougar / Thunderbird / Crown Vic / Grand Marquis)
Higher-flowing E7TE heads Standard "swirl port" heads
Higher-lift HO cam (performance/fuel econony) Lower-lift cam (fuel economy/torque/towing)
TRW forged pistons with reliefs* Aluminum flat-top pistons
60mm throttle body stock 50mm throttle body stock
19 lb/hr fuel injectors 14 lb/hr fuel injectors
Better breathing upper intake Standard flow rate upper intake
9-blade fan 7-blade fan
Factory unequal-length headers Factory cast iron exhaust manifolds
HO/351 W firing order Unique non-HO firing order
225 hp** 150 hp (155hp in 1988 only, w/dual exhaust)

* 1993-95 HO 5.0's have hypereutectic pistons from the factory.

** 1993-95 HO 5.0's were rated at 205-215 hp using Ford's revised HP rating system. The output is still approximately the same as the pre-1993 HO engines.

In short, the HO has all the good stuff. Not that the stock non-HO motor is bad; it's just not as good as the HO engine.

Where do I get one?

If you live where salvage yards are plentiful, a stock HO 5.0 shouldn't be that hard to find. Finding one that hasn't been beat to death, on the other hand, is a whole different story. If you luck out and find the motor still in the car then you can pretty much judge by the car's condition how the engine will run. And remember this: some salvage yards will sell you anything just to make a buck. Buyer beware. Be sure you're dealing with reputable people, first of all, and then see what kind of warranty there is on the motor. If there is no warranty offered then shop somewhere else. Also, be sure to ask if there is a core charge. That means you get charged extra until you bring your old motor back, when they refund you the fee. It's more money up front, so plan on that. If you cannot find any where you are, there are lots of places online to shop. If you're building a 302 from scratch, Summit Racing still carries bare 5.0 blocks that are already set up for roller lifters.

What should I expect to pay for a used HO 5.0?

This is practically anyone's guess but whatever you do, don't get overanxious about finding one for a good price. If you're patient enough you will find one, even if it means spending an extra $100-$200 US for one you know is better. Mileage is a concern; obviously, $500 for a motor with over 100,000 miles is not a good deal. We suggest trying to find one with the lowest possible mileage for the lowest possible price. Sometimes that's not easy and you'll need to compromise.

What do I use for a transmission? Can I reuse my old one?

Very generally, a 3.8L V6 and 5.0L V8 automatic transmission from these cars can interchange, as most internal specs are the same and they both have the same small-block Ford bolt pattern. This includes the C5 (3-speed) and AOD (4-speed overdrive). Technically they can be used with an HO 5.0 but the converters will have a lower stall speed, which shouldn't really hamper performance too much but may lead to a slight drop in power output. A rebuild may want to be considered, as well as a shift kit and higher-stall converter. You could also consider using a Mustang HO AOD or even a performance AOD. The existing throttle valve (kickdown) cable can be reused if you're keeping the same transmission with the HO motor. For linkage, see below. If you're swapping over to a 5-speed you can use all Fox Mustang parts.

How about the suspension? Any changes needed?

Yes, if you're going from a V6 or I-4 to the V8. You will need at least Cougar/T-Bird 5.0 front springs. Rear springs can be used from V8 cars as well, or you can use cargo coils (heavy duty variable rate). You can safely use any front and rear springs from any 1983-88 Cougar/Thunderbird V8; they will all fit and return a compliant ride with the new engine. For more information on springs, see here. You should consider installing at least the V8 rear sway bar from a Mustang/Turbo Coupe/XR7. The front sway bar would be optional but it does help a bit. For more information on sway bars, see here.

And what about the fuel pump?

This is a touchy situation as there were multiple kinds of fuel pumps used in these cars over six model years. Some earlier cars had two pumps (one in the tank, one on the framerail), some had one (in the tank). The HO motor in the Mustang and Mark VII were rated at 88lph (liters per hour) from the factory. No Fox Cougar or T-Bird had a factory fuel pump with a rating that high; ours were just 65lph at best. It is highly suggested that, someway, somehow, you upgrade the fuel pump to one with a rating of at least 155lph. For those people with the single factory pump you can simply use a good quality aftermarket fuel pump (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. Now for those of you that have two pumps it might be worth your while to change your fuel tank over to one from a one-pump car, and run some new fuel lines. That is certainly the easiest solution to bypass the second pump although it does require more initial expense.

What else will I need?

Besides the engine itself, you will need quite a few more things. Assuming that you do not already have a 1986-88 factory 5.0 under your hood and that you're staying with a computer-controlled engine with an HO 5.0 computer, first and foremost you will need 5.0 wiring harnesses. For Fox cars there are three harnesses: the main EEC harness coming from the computer inside the passenger compartment and out through the firewall to the motor; a separate fuel injector harness that plugs into the main EEC harness; and a separate oxygen (O2) sensor harness that also plugs into the main EEC harness. You will need all three harnesses. It is highly recommended that you use a 1986-88 Cougar/Thunderbird 5.0 harness, regardless of computer style (speed density or mass air) and even though it was never HO-style in nature. This is because the main harness will mate up to your existing dash harnesses better than, say, a Mustang or Mark VII harness. Although those could be used they would need repinned in a few places. The 1994-95 Mustang 5.0 and 1991-93 Cougar/T-Bird 5.0 harnesses are generally incompatible with Fox cars and should be avoided. The 1986-88 Cougar/T-Bird 5.0 harness generally will not need repinning (unless mass air is used; see this section for help with that). Of course, this all depends upon the model year of your car and your dash style.

There may also be a need to change the harness on the driver's side of the engine bay. This harness couples behind the dash and extends outward for some engine controls, such as the alternator harness, fusible links for the starter solenoid, and so on. They also contain the wires needed for the headlights, side markers, turn signals, windshield wiper motor, and other accessories. This is probably the trickiest part of the swap, as what you have now may not mate up with the SEFI 5.0 engine. It's possible that it will...but also possible that it's not.

You must also get a compatible EEC-IV computer module to use with your motor. You will not be able to reuse your stock one. Each Ford EEC processor has a calibration code printed on top, usually 3 characters long. For speed density, a computer marked with DA1 or DB1 (1987-88 Mustang) will work. You could also use a 1988-92 Mark VII computer (DX3, DY3, VL2, D9S) although you will be speed limited to 105mph with most of them. For mass air, the ever-popular A9L (manual) or A9P (automatic) are the computers of choice. Most of these computers are easily available on eBay if you can't find one at the salvage yard, but you can also get remanufactured computers at parts stores (though we hear the quality is hit-or-miss so buyer beware). Don't forget to also strip the firewall of all related sensors, wiring, and vacuum lines from the donor car.

Next, you will need all the brackets and accessories for the motor as well. This is due to the serpentine belt configuration. If you're replacing a stock 5.0, you can reuse your old accessories and brackets if you wish. But technically, the brackets and pumps should come with the motor. The stock Mustang metal fuel lines on the motor will also be necessary; if unavailable, use them from a 1986-88 Cougar/Thunderbird 5.0. The radiators used in 3.8 and 5.0 cars will be adequate enough for the new 5.0; owners of 2.3 4-cylinder turbo cars are strongly urged to buy a new radiator made for a 5.0. You can reuse your stock 7-blade fan and fan clutch, but can upgrade to the HO 9-blade fan if you wish.

As far as motor mounts for the new engine, you have to be careful. You can use Fox Mustang-style mounts in 1983-85 Cougars ONLY. This is because the Cougar's engine crossmember (a.k.a. K-member) was very similar, if not identical, to the Mustang. Beginning in 1986, the Cougar got its own special K-member (which may have also been shared with the Mark VII). So that means you'll have to get motor mounts for a 1986-88 Cougar 5.0, which is not easy. Fortunately there is a Fox Cougar/T-Bird enthusiast that is manufacturing high quality motor mounts for our cars; see this website for more information and ordering.

If your Cougar is not a 1986-88 5.0 car to begin with, you will also need the following from a 1986-88 Cougar/Thunderbird 5.0 ONLY:

  • Battery tray
  • Starter solenoid and ignition coil.
  • MAP sensor (firewall).
  • A/C lines (stock condenser will work fine. Stock accumulator may work fine, depending upon the model year).
  • TAB/TAD sensors (located in the rear passenger side of the engine compartment, behind the strut tower)
  • Air cleaner box and tube(s)


After removal of the old engine and before installation of the new engine can begin, you'll have to do a little more work with the motor out first. You will need to install the main EEC harness. Be aware that the hole in your car may be oval, and some harnesses are more square (and vice versa), so there may be some trimming or covering up at the EEC harness hole in the engine bay. Also, you can install the new computer inside the passenger compartment footwell and hook the wiring to it. The injector and O2 harnesses will have to be installed after the motor is in. You can simply flop the new harness onto the windshield to keep it safe and out of the way.

Next up are the TAD/TAB sensors, behind the passenger side strut tower. These control the EGR/EVP. Parts from a Mustang are not quite exactly like those on the Cougar/T-Bird 5.0's; however, they can be made to work. Mark VII sensors require more work to use correctly as the harnesses may be different. After those are all in, you can install the battery tray on the driver's side. Everything else (starter solenoid, ignition coil, air cleaner assembly) will have to wait until the new motor is in.

Once you've got the motor in, bolted down, the transmission installed and the transmission crossmember back in, you can start to bolt everything to the motor. We would highly suggest getting the wiring situation resolved first because it's the most tedious. The most important thing to remember is that each sensor or connection has its own unique configuration, and can only reach so far, so that means you really can't hook the wrong wires to the wrong sensor. Once all that is over with you can install the ignition coil first. It goes on the driver's side strut tower toward the front. You should have some holes in the area so just bolt the bracket down and you're set. You can also install the starter solenoid, almost directly above the coil, but on the side fender. On the other side, you can install the air cleaner assembly now, including the mass air meter if applicable. If your car did not have a factory hole in the front passenger apron (1983-85) then you'll either need to cut one using the 1985-88 air cleaner box as a guide, or run an open-element air filter such as a K&N or similar filter. A mass air meter will require additional wiring that should be installed at this point (again, see here for more details).

For the exhaust system, it's totally up to you, but we would suggest a full dual exhaust conversion since the HO motor was made for duals. Using the stock Cougar exhaust with cast iron manifolds is pretty restrictive and will result in a power loss and slight bogging on acceleration. See the Dual Exhaust Conversion section for more info.

If you need to adapt your fuel lines Ford makes a nice kit (p/n N803431-S) that makes life easy for you. It's two unions and 4 clamps, used to convert your existing lines over. Cost for the kit is around $10. If you find your car cannot use this kit, you may have no other choice but to run all new fuel lines from the tank to the engine.

Now for the throttle and transmission linkage. If your old transmission linkage was rod-activated, you're going to have to buy a TV (throttle valve/kickdown) cable, because the stock HO 5.0 throttle plate won't accept a rod system. And your cable may or may not work, depending upon the transmission. Same may be true for your throttle linkage as well; there may be a new cable in your future. You may be able to use a ball-type connector from a Holley carb, sold at parts stores, to adapt your cable. Fortunately both cables are pretty inexpensive and easy enough to install.

Of course, these are the major sticking points for the conversion. There will be many small things that need addressed but you'll figure things out. When all this is taken care of, hook the battery up, and turn the key. You should hear the sweet, sweet sound of a rumbling small block Ford. Yes, you have finally done it. Now go enjoy the fruits of your labor.


Don't forget to now use the HO firing order (1-3-7-2-6-5-4-8).

Stock timing on the HO 5.0 is 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.