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Foxbody Mustang Engine Builds And Swaps Essential Information

Written By: Connor MC

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Although the Foxbody might be "old" in the scope of Mustangs, that doesn't mean it can't be revitalized. Swapping in some modern muscle will not only add power, but also make finding parts easier.

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The following page has been developed to provide insight and information in regards to building and modifying your Foxbody's 302 V8, common stroker sizing and how to swap a 351W into your Foxbody. Use the quick links below to jump to a particular section.

Foxbody 302/5.0L Engine Build FAQ

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.

Getting Started: When starting a 302 build, there are a few handy things to know. First of all, before even touching the engine, it is recommended install 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 500 HP. 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 collective 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, particularly 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 300 rwhp car on a stock T5 will result in a frequent case of transmission failure. (I ripped my T5 apart flooring it in 1st gear, on a stock car)

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Foxbody Engine Builds - What Does It Take? (300 RWHP)

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.

GT40 stuff aside, 300 rwhp 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 top end 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. Real world data has shown slapping a centrifugal style blower on a stock Foxbody to produce an extra 70 horsepower. Really, the fundamental aspect of building your Foxbody's engine is to get as much air into it as possible, and this is done via the heads and intake. If you were to take a GT40 setup, THEN throw a blower on it - that would be a heck of a ride and not cost that much either, relatively speaking.

300-450 RWHP

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 cubic inches 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 351W. Ford production 351 blocks are good for ~750HP, and are almost a direct swap in place of the stock 302. 

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 ~$4-5k 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.

500 RWHP

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 now...). 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. Yes, there are guys that are running around with 500 horsepower stock block engines, but the one time a combustion event doesn' go 100% perfectly - that motor is ruined. It is not impossible to use a stock block for 500 horspower, it just isn't practical or sensible.

At this point, the only options as far as blocks go is aftermarket (Ford Performance, Dart) or production 351W block. Looking to net 500 rwhp naturally aspirated is not an easy feat. In my opinion, it would take one hell of a 408 to hit that mark. 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 aspirated motor will be quite the challenge, in street trim at least. If you can spin the motor to 7500 RPM, well, that's a different story. Aftermark block 331 cu (331 being the minimum displacement needed) heads intake cam power adder = 500 rwhp = a lot of money. Furthermore, at this level, you'll have to just about change or reinforce every other aspect of the Foxbody 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. Case and point - the transmission. It is well known that the factory T5 transmission is barely able to handle the power of a stock 5.0L.

Foxbody Engine Stroker Sizing

Below is a quick outline of what displacements are produced using different bores and sizes. Keep in mind, the stock 5.0L Foxbody V8 engine is a 4.0" bore. When you 'stroke' an engine, you do not change the bore... well not really. A common misunderstanding is that a 347 uses a different (larger) bore size than a 331. In actuality, both 347 and 331 cubic inch engines use a 4.030" bore (which is 0.030" larger than the stock 302 cu bore). Changing the cylinder bore size is done to clean up the bore after years of use. (A factory 302/5.0L block has a 4.0") That is why you see engines that are speced as '302 bored 0.030 over'. What that means is the cylinder diameter was increased an additional 0.030" to remove imperfections during a rebuild. The stroke length of the engine is what differentiates between a 347, a 331 and the stock Foxbody 302. When you 'stroke' a motor, you increase the stroke length, and thus the displacement.

To calculate displacement, you use a very simple formula. Simply calculate the volume of each cylinder, and multiply by the number of cylinders. In a few neat lines, engine displacement formula is:

Engine Displacement Formula
(volume of cylinder) * (number of cylinders) = displacement
[pi*(bore/2)^2*stroke)] * (number of cylinders) = displacement

4.000" bore
3.00" stroke: 301.6 in3 (4.94L)
3.25" stroke: 326.7 in3 (5.35L)
3.40" stroke: 341.8 in3 (5.60L)

4.030" bore
3.00" stroke: 306.1 in3 (5.02L)
3.25" stroke: 331.6 in3 (5.43L)
3.40" stroke: 347.0 in3 (5.69L)

4.040" bore
3.00" stroke: 307.7 in3 (5.04L)
3.25" stroke: 333.3 in3 (5.46L)
3.40" stroke: 348.7 in3 (5.71L)

4.060" bore
3.00" stroke: 310.7 in3 (5.09L)
3.25" stroke: 336.6 in3 (5.52L)
3.40" stroke: 352.1 in3 (5.77L)

Foxbody 302 to 351 Swap: Why Swap?

Swapping a 351W in place of your 5.0 is a somewhat budget-friendly way to make a lot of power, and to subsequently go very fast. A 302 (5.0L) based engine is not incapable of making good horsepower, but it certainly has many more limits than a 351W motor. Why? Well, first off, the displacement issue. Whoever coined the phrase 'there is no replacement for displacement' is, frankly, a genius. A 351W engine has an extra 49 cubic inches over a stock 302 cu motor. That is an advantage right there. If you recall, an engine is basically a large pump. A larger pump can move more air, and thereby is more powerful.

Secondly, a 351W has a ton of room to grow. A 302-based motor can be stroked all the way to 347 cu, which is only 4 cu difference from a 351. So why not just stroke the stock 302? Well, you could certainly do that, and you'll get excellent results. However, Ford production 302-based blocks are weaker. Production 302 blocks are not as thick, and can only be stroked to a maximum of 347 cu for this reason. And because they are weaker, 5.0L blocks have a lower horsepower limit before cracking (for the record, it is shown to be around ~500HP). On the other hand, 351W blocks have thicker webbings, and also beefier main and rod journals, making it overall a much stronger block and crankshaft. The aftermarket world has shown that Ford-production 351W blocks are good up to 750HP. So we start with a block that is 351 cubic inches, and can hold up to 750HP. Want to go bigger? Not a problem. You can stroke a 351W all the way up to 427 cubic inches - creating a behomoth with such grand power it could easily reverse the rotation of the earth!

Finally, there is the aspect of cost. In the first paragraph, the term 'budget-friendly' was thrown out there. A 351W swap is a budget-friendly avenue because of the availability of parts, and the surprising minimal amount of work needed to slip one into a Fox or SN95. No need to buy an expensive aftermarket block, just head to your local scrapyard. Furthermore, 351Ws utilize the same bellhousing pattern as a 302, so any transmission that will bolt up to a 302 engine will also mount to the bigger Windsor, no modification necessary.

Reasons for 351W swap (summarized):

  • No replacement for displacement
  • Stronger block (~750HP)
  • Affordability and availability of parts (i.e: scrapyard)
  • No fabrication work necessary
  • Same bellhousing pattern as a 302

What Kind of Power Can I Expect from a 351W Swap?

In truth, there is no guaranteed answer regarding this question. All I can provide is some guidelines and approximate estimates, based on other enthusiasts real world results. To make decent power with a 351W, it is assumed you will spend a little money to dress it up, not just drop in a 351W straight out of a truck. If you do the latter (drop in a stock 351W from an F150 or the likes), prepare yourself for disappointment. In stock truck trim, a 351W will be slower than your stock 302 powered Mustang. Why? Well, it is engineered for a truck, not a sports car. How do we change this? Easy, heads, cam and intake swap!

A mild 351W build will easily surpass 350HP. Throw on some good aftermarket heads with intake, and a cam to tie them together, and 500HP can be had out of these engines. It really depends on how deep your pockets are (as is the case with most things). To hazard a guess, I would say the average 351W falls around ~450HP mark, with plenty of room to grow as more cash becomes availabile later down the road. If you're throwing a 351W into your Fox or SN95, be prepared to leave tire marks, and potentially, skid marks. These engines, when done up, are wicked fun!

What Parts do I Need to Complete the 351W Swap?

The 351W swap is what we like to call an 'almost direct-fit swap'. Sounds like a typical marketing gimmick to me... Truth be told, it isn't too difficult to swap. Most of the 302 parts swap over, but there are some that do need to be changed. All the major stuff will drop right in, no modification to the engine bay or k-member needed. What parts do need to be swapped? Let's dive in.

Oil pan: The stock 351W oil pan is a front-sump, whereas the 302 uses a rear-sump. Thus, the 351W oil pan needs to be changed to a rear-sump (like the 302) in order to fit in your Mustang (otherwise it will not clear k-member). Ford Racing has a good 5-quart kit available, or there are larger offerings from Canton or Moroso, in the order of 7-quarts and with the option of a wind-age tray.

Oil pump, shaft, pickup: A new oil pump and shaft is cheap insurance, whereas a new pickup will be necessary (remember, we went from front-sump to rear-sump), the original 351W pickup will not work.

Flywheel and harmonic damper: Ford's 351W uses a 28oz imbalance versus a 302's 50oz imbalance, thus meaning you will need both a new flywheel and harmonic damper. In this case, you're looking for a 157-tooth 28 oz flywheel to use with a 10.5" clutch . Also, the flywheel bolt pattern differs. An alternate option is to have a machine shop trim the 50 oz imbalance down to 28 oz and re-drill the mounting holes in the correct locations.

Distributor: The distributor too needs to be changed, as the 351W has a larger oil pump drive shaft as well being physically taller. Grab a dizzy from any EFI 351W (5.8L) truck out of a scrapyard, or any aftermarket vendor will be more than willing to sell you a brand new one as well. It should also be mentioned, if you are using a roller cam, swap the stock cast-iron distributor gear for one of steel.

Exhaust manifold/headers: Fox style headers, be it short or long tube, will not work due to clearance issues. Instead, you must seek a set of headers labeled as '351W swap headers'. It is easier (and cheaper) to get shorty headers, but I believe there are some long-tube conversions out there as well. Check out companies like Ford Racing, Kooks, Hooker, BBK etc. There are even sets available on eBay, however I cannot attest to their quality (but check out their feedback, it could be worth it).

Intake manifold: A 351W has a greater deck height and separation, thus rendering any 302 lower intake useless. It simply will not bolt up because they are not physically wide enough to bridge the gap. Rather, you will need to find a specific 351W lower intake. However, this isn't really a problem as all major aftermarket companies offer them (Ford Racing, Edelbrock, Trickflow, Holley etc). On a side note, 302 heads will work without any issues, just the bolt-holes will need to be enlarged to fit 1/2" fasteners.

Accessory brackets & crank spacer: The last items that are not a direct swap are accessory brackets, namely the power steering and AC bracket. Depending on what accessories you are running, some of the 302 brackets will not fit. Ford Racing sells a replacement kit with the right geometry for around $60-$70, as does March. Furthermore, a crank spacer may also be necessary to help align your accessories and serpentine belt. Some guys are able to make it worth with just the regular 5.0L parts, whereas others can't. As mentioned, it depends on what accessories you wish to run, and where you want to place them.

The above parts are what is needed to get a 351W to both fit and run inside of your Fox or SN95 Mustang. However, there are still a ton of other factors that need to be addressed, but are not directly involved with the swap. For instance, if you plan on making more power (which is the whole idea of using a 351), you'll need a fuel system to match. Larger injectors or carb, more powerful fuel pump, throttlebody etc.

Will a 351W Fit in My Engine Bay?

It depends. As mentioned, a 351W is a direct, drop-in engine in terms of mounting location. It will bolt up to the stock motor mounts and will fit in the engine bay width wise without issue. However, seeing as a 351W has a bigger deck height and is thus a taller engine, depending on what intake manifold is being used, there could be some clearance issues under the hood. There are two ways to fix this. You could either purchase a hood with a larger cowl, or you could change the motor mounts to drop the engine a bit farther down (or do both). Recommended motor mounts are the OE units from any 87-93 convertible Mustang. They are stronger than regular mounts, and will better deal with the added weight of the engine (5.8L is heaver than the 5.0L). There are also aftermarket solid motor mounts out there that will drop the engine by 3/4". (HP Motorsports comes to mind for this, but I'm sure there are other companies as well.)

How Much Will a 351W Cost Me?

Cost is another highly subjective question, as not only is it region dependent (i.e: scrapyard prices), but owner dependent as well. If you have a whole top end laying in the garage and only need a 351W shortblock, then it is a pretty cheap build. If you are starting with nothing, then expect to spend a pretty penny as you will need a better top end than what is stock if you wish to make any power. For this reason, when I list my 351W swap estimate in a few moments, it is a quote for only 351W necessary parts. It is assumed you have the supporting mods already covered (top end, fuel needs, labor costs, bits & bobs). Here goes:

Part Cost Notes
351W engine $400 scrap yard
Oil pan $135 5 qt, new, w/pickup & dipstick tube
Oil pump and shaft $75 New
Flywheel & balancer $200 New
Distributor $300-$500 MSD
351W headers $500-$800 Hooker, BBK
Accessory brackets $100 March
Total $1710-$2210  

Again, please keep in mind these are only estimated costs, and do not include other things that could be necessary such as machine work, install labor, gaskets etc. It by no means is a comprehensive monetary value, but rather a rough estimate of what it would cost to accumulate the parts needed for a basic 351W swap. If you want to make 500HP, you'll need some nice aluminum cylinder heads, which of course are not factored. Furthermore, not all 351W blocks are roller blocks - early blocks were flat-tappet. There could also be the cost of getting correct lifters and what not to convert a flat tappet to roller if that is the route you wish to go.