Foxbody Mustangs are a super popular platform to modify. And for good reason. Ford really did good with their 302 engine, and the aftermarket has taken it to new extremes. The best Fox 5.0L, in terms of power, came from 1987-1993, with the stock, Ford rated 5.0L engine producing 225HP and 300 ft-lb. Compared to todays standards however, this is pretty measly. Modern 4 bangers put out ~200HP, before boost. The turbo 4-cylinder variety surpass the 200HP mark with ease. Outlined below are some basic concepts and build paths to consider when building upon the smallblock Ford 302. Starting with some key strengths and weaknesses that should be realized and ending with a brief note on the ever popular 300rwhp, 400rwhp and 500rwhp goals. However, by no means is this a comprehensive, above all master guide. Rather, it is a basic guide outlining the principles of what options are actually involved in building a motor and reaching a specific goal.
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When starting a 302 build, there are a few handy things to know. First of all, before even touching the engine, it is recommended you follow the following modification list (Top 10 Essential Mustang Mods). Perhaps most importantly on that list is item #2, subframe connectors.
The Foxbody frame
The Foxbody frame is known to be very weak in part to it's unibody design and insufficient bracing. In fact, even just a factory 5.0L has the grunt behind it to twist and disort the frame. When increasing the power levels, the first check should be that the frame is capable of handling the added power. At a minimum, subframe connectors are needed. Options beyond this are to have the torque boxes reinforced (another weak spot) and if you want to go the full 9 yards, throw in a cage as well.Stock block limitations
Real world data has shown the stock 302 block limitation to be 500HP. Pushing power to near that level on a stock block is asking for catastrophic failure. If you're looking for a high horsepower build, it is highly recommended you start with an aftermarket block.
Think in terms of a colletive system
Another important aspect to consider when upgrading your Foxbody's engine is the effect the additional power will have on the rest of the car. More power means more stress on the clutch, transmission and axles, increased fuel system needs, traction control etc. In order to have a stout build, particularely at higher horsepower levels, there will be lagging effects on the rest of the car that must be considered. For reference, stock transmissions (both T5 manual and AOD automatic) will hold only marginally past the stock power levels. Stomping a 300rwhp car on a stock T5 will result a frequent case of transmission failure. (I ripped my T5 apart flooring it in 1st gear, on a stock car)
If you're very careful, you can even hit the 300hp mark with a budget GT40 build (for more info on GT40 and GT40P heads, refer here). Very careful assembly and matching components, usually some porting is necessary too, can have a budget GT40 head, GT40/Cobra/Explorer intake and an off the shelf cam lay 300 to the tire. Pretty impressive stuff. However, a 300hp GT40 can be done, but the majority of them fall between 250-290rwhp.
In short, 300rwhp is not all too hard of a goal to hit. It can easily be done using a stock 302 block and remain naturally aspirated. If you use the right parts, you ought to be able to keep RPM's under 6000 to hit peak HP. To hit the magic 300 mark, a solid overhaul of the topend will get you there. Upgrading the stock heads and intake to quality aftermarket pieces (Trickflow Twisted Wedge heads, AFR 165's, Edelbrock Performers, Ford GT40X - all are immensly popular) and a cam swap can net you the 300rwhp you're after (of course, if the bottom end is healthy too!). Generally speaking, 300rwhp is just about the limit a naturally aspirated 302 engine will produce (highest I have ever seen is 335rwhp) without spinning the engine to hell and back.
Of course, there is also always the option of just slapping on a turbo or supercharger but on an otherwise stock engine the gains versus cost can actually be a little disappointing. If you are very crafty at sourcing parts, you can put together a nice 300HP build for ~$2000 (doing labor yourself).
This is where things start to get serious. Let's start with stock block options. The stock block can handle ~400rwhp, but it is pushing it very close to the limit. If you're looking for 400rwhp out of a 5.0L engine, you'll need a top-end as mentioned in the 300rwhp section plus some sort of power adder. Slapping on a turbo or supercharger to a 302 equipped with aftermarket heads, intake and matching cam will net you 400rwhp fairly easily. Mind you, as already mentioned, at this level you're entering the danger zone of what a stock block can handle. Fuel pressure and timing has to be bang on (essentially the 'tune') if you want to keep the motor together for longer than a couple of passes down the 1320.
Pushing past the stock 5.0L/302 displacement, another (and better option, in terms of potential) is to switch to a larger displacement motor. You can stroke the stock 302 block to 331 or 347 cu and go from there (these are the most popular). A well built stroker (naturally aspirated) can net 400 to the wheels, but again, in comes the reliability of the stock block. Common strokers are 331 cu and 347 cu, respectively. Again, larger displacement and a good top-end package is key to power. The cost of a stroker is not too much more than that of building a simple 300rwhp engine. Of course, there will be charges for the new rotating assembly and machining (if you are using an aftermarket block, that too), but the top end, again, if selected carefully, is nearly comparitive in cost.
Another option when shooting for 400rwhp is to start with a 351. Ford production 351 blocks are good for ~750HP, and are almost a direct swap in place of the stock 302. 351W swap info can be found here.
In terms of cost, getting to these power levels will take a good amount of cash. Of course, everything can be built on a budget, but obviously the budget will be higher for a build like this. If you are starting from scratch, figure ~$3.5k for a naturally aspirated 347, utilizing the stock block (considers cost of HCI, rotating assembly). From there on, the price only goes up as you add perhaps an aftermarket block or power adder.
500rwhp is a tough goal. Nor is it cheap. '500rwhp' and 'budget' don't even belong in the same sentence together (I have already angered the racing gods just by saying it there...). First off, don't even bother considering use of the production 302 block. Not if you value your money. Push 500hp through a 302 and it'll push pistons through the firewall. At this point, the only options as far as blocks go, are aftermarket or 351W.
Looking to net 500whp naturally aspirated will be tough. In my opinion, it would take one hell of a 408 to hit that mark, N/A. Remember, there is no replacement for displacement! Going the 408 route will be a high end, lots of $$$ build. Mind you, once all is said and done, you should be able to reverse the rotation of the earth fairly easily. These things are monsters when properly done! One badass 408 with a kickass top end (that's a lot of ass!) can see you 500+rwhp/tq, but it'll cost a pretty penny.
Of course, you can always slap on a power adder to a milder build and crank up the boost. In a lot of scenerios, I would even go as far to say as this being the prefferred method. Pulling 500rwhp out of a naturally aspriated motor will be quite the challenge, in street trim at least. If you can spin the motor to 7500rpm, well, that's a different story. Aftermark block + 331 cu (331 being the minimum displacement needed) + heads + intake + cam + poweradder = 500+rwhp = a lot of money. Furthermore, at this level, you'll have to just about change or reinforce every other aspect of the Mustang to handle this much power.
The lesson here: you have to pay to play. Displacement + heads + intake + cam = power. Also keep in mind, for every major modification you make, there are multiple supporting mods to be done underneath it. If you're looking for more info on some mods, check out FiveOhInfo's Top 10 Foxbody Mod list.