4 Cylinder to 8 Cylinder
By: Ron Lee
Changing a 4 cylinder Mustang to an 8 cylinder Mustang is a very involved project. You can just drop a carbeurated 302 and transmission into a 4 cylinder Mustang and call it done, but the car will be somewhat dangerous to drive, and the rear axle probably will not live a long time. The 4 cylinder front brakes are much smaller than the V8 brakes (and therefore are not meant to stop the extra weight of the V8 car), the 4 cylinder suspension is not made to handle the extra weight of the V8 components, and the rear axle assembly is not made to handle the power of the V8 engine. Such a car will make a fine drag race car (with a stronger rear axle assembly), but needs the rest of the factory (or better) V8 related parts to make an everyday reasonably safe (and very fun) car to drive. By far the best way to convert a 4 cylinder car to a V8 is to have a wrecked V8 donor car sitting there to take all the parts from. Ideally the V8 car will be an 89-93 vehicle with 87-88 a close second, 86 a further third, and the earlier years much less desirable.
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I have changed a 4 cylinder Mustang to an 8 cylinder car, and it is amazing what is the same between the two cars and what is different between them. The radiator, fuel pump, rear lower control arms, and rear brakes are all the same between the two cars. The rear upper control arms that come with the 4 cylinder cars are actually superior to the V8 ones (they have stiffer bushings in them). The brake lines are basically all different between the two cars and I never could fathom any reason for the differences.
On the front brakes, the front rotors on 87-93 V8 cars are 11 inch diameter while the 4 cylinder rotors are only 10 inches in diameter. That makes a huge difference in braking power, especially from the higher speeds that the V8 engine is capable of. Now for the rest of the news. Not only are the rotors different, but also the spindles, calipers, caliper locating pins, splash shields, flexible brake lines, and flexible brake line brackets. You either change all of it together, or none of it. Preferably you change the front brakes at the same time you do the front suspension, it will save you time. One last comment on the brakes is that the brake booster is different between the two cars, along with the two short little brake lines that go from the master cylinder down to the proportioning valve. The V8 brake booster is thicker, giving it more internal volume to respond better with a lower engine vacuum level.
On rear axle assemblies, all 79-85 Mustangs, and all 4 cylinder Mustangs came with the 7.5 inch rear axle assembly. All 86-93 V8 Mustangs came with the much tougher 8.8 inch rear axle assembly which should be the minimum for all V8 Mustangs. The 8.8 inch rear axle assembly will bolt directly into your 4 cylinder car with absolutely no changes whatsoever, except for the brake lines. The brake line from the front to the rear of the car stops on the right side of the rear axle for 4 cylinder cars and the left side for V8 cars. This means that you need that brake line from a V8 car to complete the installation. Incidentally, you will have to remove the right front fender to remove this brake line from the car. Sounds weird when you tell people that you had to remove the right front fender to replace the rear axle. I got quite a few funny looks from that one. The rear lower control arms are identical between the two cars, and the 4 cylinder rear upper control arms are better than the V8 parts. You should definitely add the rear sway bar from a V8 car, it will improve the handling of the car dramatically. You should also add the V8 rear springs at the same time. Ride height will remain the same, the V8 springs are just stiffer which will also improve your car's handling. The quad shocks will help control wheel hop if you are going to use the factory rear control arms. If you use aftermarket rear control arms, which I highly recommend, I'd leave the quad shocks off.
On the front suspension, the springs, struts, sway bar, sway bar end links, and lower ball joints are all different between the two cars. Your car will sag down in the front considerably if you do not change the springs, and more important handling will be adversely affected. The V8 sway bar is much larger in diameter and that is highly desirable to improve handling. The struts are stiffer to work with the stiffer front springs. The sway bar end links have polyurethane bushings to improve handling versus the rubber bushings used in 4 cylinder cars. Remember that springs are very dangerous to work around, and have someone else remove them if you're not sure of what you're doing. Now would be an excellent time to install polyurethane control arm bushings and strut bushings if that is in your plans. They will tighten up your car's handling by leaps and bounds.
On the steering, the rack and pinion, power steering lines and steering pump are all different. The V8 cars also include a small oil cooler for the power steering that bolts in front of the radiator. The rack and pinion on the V8 cars are 15:1 ratio, which is much quicker than the 20:1 4 cylinder rack and pinion ratio. The pumps have different outputs, different mounting brackets and are not compatible with the other rack and pinion units. The 4 cylinder power steering line also includes a power steering shut off valve near the power steering pump that the V8 cars do not have.
On odds and ends. All drive shafts are the same except for the front yoke on 4 cylinder automatic car driveshafts. Tail pipe brackets and rubber insulators are the same, except assuming you are going to run dual exhaust, you will need two of them. The muffler bracket and rubber insulator on the four cylinder car is the same as the left side bracket on a V8 car. However, if you're going to run a dual exhaust, you will have to get the right side muffler bracket and rubber insulator from a V8 car (the bracket is different). The V8 cars come with pinion snubbers that the 4 cylinder cars do not have. The fuel lines are totally different, but the tanks, sensors, fuel pump, clips, and filter are all the same. The 4 cylinder fuel lines come up in left front side while the V8 fuel lines come up in the right front side. The speed controls are different because the V8 speed control cable is longer than the 4 cylinder speed control cable. The gas pedal cables are also different for the same reason. The V8 cars have a small vacuum canister under the right fender for the engine electronics that the 4 cylinder cars do not have. Many of the vacuum lines are different between the two cars. The K member, the main frame support under the engine, are the same for all Mustangs except those with inline 6 cylinder engines. The transmission support is the same for all Mustangs except for the 4 cylinder automatic cars which are unique. One caveat to the transmission support is that it was changed in 86 to allow for dual exhaust. Pre 86 ones do not have the double hump and therefore only allow a single exhaust. So installing dual exhaust will require a transmission support from an 86-93 V8 car.
On electronics, they are totally different, almost everywhere and every single item. The wiring harness under the dash is the same, but that is about it. The computers are different along with the wiring harness that goes from the computer through the firewall and then various places. The wiring harness going from the coil area up and around the radiator to the alternator and the engine wiring harness are also different. The engine wiring harness is also different. The barometric sensor is different. The V8 also has a EGR vacuum regulator, TAB solenoid, and a TAD solenoid mounted on the back side of the right shock tower that the 4 cylinder engine does not have. The coils and starter motor relays are the same. One last comment is that the computer, wiring harnesses, and sensors should all from the same year, or you may have to do some adapting to make it work. Don't be daunted by all the various wiring connectors, they are all unique and will only plug into the correct mating wiring connector (except the 8 fuel injector connectors).
On instrumentation, the 4 cylinder tachometer will not read right with a V8 under the hood and vice versa. The 89-93 V8 Mustangs also have the 140MPH speedometer so hopefully you can get the whole instrument cluster from one of those and just bolt it right in. The V8 tach will read one-half of the RPM with a 4 cylinder and a 4 cylinder tach will read twice the RPM with a V8. Get the right tachometer, but honestly it's easier to just change the whole instrument cluster.
Did I mention that the engines are different? The transmissions as well. The two 5 speeds look similar on the outside, but the 4 cylinder 5 speed transmission will not live very long at all behind a V8. All the internal parts are much smaller. One last item to mention should be the wheels and tires. There is no requirement to have wider wheels and tires, but they sure make a big difference in handling and traction.
Good luck! If you have any questions, feel free to ask!
©Copyright 1998 Ron Lee