Agustin Jimenez
Technical Editor
May 7, 2015
Photos By: Courtesy of Ford

Did we really just say that an LS engine swap could be a recipe for disaster? Yup, we sure did! Don’t get us wrong. It’s a very popular swap for muscle cars and hot rods, as well as trucks and 4x4s, and it is easy to see why. It can be had fairly cheap and is pretty darn reliable in most situations. So why isn’t it the best option when looking to repower your Mustang?

What is LS Engine Swap

The LS engine has plenty of aftermarket support and is a viable alternative to building power reliably in an application that has a very undesirable engine. Heck, the LS engine is fairly compact and fits nicely in the engine bay of a Mustang. Well, that’s where it starts to get a little tricky. Not only will you run into several challenges but it is a surefire way to anger the Blue Oval purists who would never dream of desecrating their beloved Mustang with a little corporate Bow Tie under the hood.

We’re not going to bash the LS engine family and claim it’s not one of the greatest engines ever built, but we will tell you why it isn’t the best option to make your Mustang haul the mail down the road.

Here's the top 10 reasons why an LS engine swap could be a recipe for disaster in a Mustang!

1. Fuel injection system

The stock fuel injection system is not compatible with a Mustang’s EFI system so you’ll inevitably have to spend extra money to run a custom OEM wiring harness and computer or a standalone aftermarket fuel injection control system. The fuel plumbing as well as the fuel pump will also require custom work to make an LS live in a Mustang, so you can expect that to drive up the cost significantly, which makes an affordable takeout LS-series engine a fair bit more expensive in the long run.

2. Drivetrain Challenges

The C4, AOD, AODE, and 4R70W transmission will not bolt up to an LS engine without a custom transmission adapter, so you’ll have to reach in your pockets for a GM transmission that will fit in the transmission tunnel. You’ll also need to fabricate a set of custom LS motor mounts or swap in a K-member that will accept an LS engine. By comparison, a 5.0L Coyote V-8 will bolt up to a 4R70W transmission and will work with the stock Modular motor K-member.

3. Resale value

While you might not consider selling your late-model Mustang right now, it could be worth more money in the future for someone who wants to pop in Side B of their favorite mix tape and relive their glory days. This probably mostly applies to all-original numbers-matching Fox-body Mustangs, but we wouldn’t be surprised if SN95s and New Edge Mustangs also become sought-after pieces soon - so it might be worth beefing up the stock engine versus swapping in a modern powerplant, especially one that uses the wrong firing order and is all about the Bow Tie life.

4. Would Require Custom Headers

The stock 1979-present Mustang Exhaust system will not work with the LS engine, so you can expect to purchase some pricey swap headers or build an even pricier set of headers around your setup to work with your application. If you’re thinking you’ll end up adding a turbocharger or two, you’ll have to factor that into your decision as well since you’ll likely have to scrap your entire exhaust system to accommodate a turbo setup.

5. Metric to SAE adapters

Unless you plan on running the stock sensors and an aftermarket dash, such as a RacePak dash, you’ll have to scrounge up some metric-to-SAE adapter fittings to get the stock gauges to interface with the LS engine. Even then, the gauges will require a little modification to work with an LS engine.

6. There isn’t a lot of Aftermarket Support

While the LS engine does enjoy extensive aftermarket support, there isn’t as much support for it when it’s dropped in the engine bay of a late-model Mustang. The 5.0L Coyote on the other hand is getting plenty of aftermarket support from Ford Racing Performance Parts as well as many aftermarket companies that cater to making late-model Mustangs get moving in a hurry. The aftermarket has already offered all the parts necessary to complete your 5.0L Coyote swap without your having to be a competent fabricator/welder.

7. May Not Be Emissions Legal

While you can make an LS engine smog legal, you’ll have to jump through a lot of hoops to get it certified as a smog legal swap. If you live in California this can be next to impossible for the average garage builder. While a Chevrolet Performance E-Rod LS crate engine would allow you to be emissions compliant, the stock engine is easier to keep smog legal while still being able to build some decent power. The 5.0L Coyote V-8, on the other hand, is a viable option to swap into any late-model Mustang because you can actually make it smog legal without too much hassle, provided you use all the required smog equipment from the engine you are swapping in. A wrecked donor car would be a solid option for an engine swap. If the 5.0L Coyote is a fair bit out of your price range, you can always find an SN95 or a New Edge Cobra as a donor vehicle and swap that in since 1979-2004 use the same platform.

8. Why Would You Swap In a Used High Mileage Engine?

While it is true that you can find LS engines for around $1,000, it’s not the same as installing a freshly rebuilt engine. Picture spending at least over $5,000 to swap an LS engine into a Mustang and then realize you will have a stock high-mileage junkyard engine making about as much power as a modern V-6 Mustang. Then consider that the engine likely came out of a beaten-down work truck or a wrecked Camaro/Firebird or even a Corvette that was driven by someone with the same tendencies as Cole Trickle every day of its life. It really doesn’t make sense to swap an LS engine into a Mustang when you can buy a rebuilt small-block Ford 302ci from companies like Late Model Restoration for $1,000 and spend the rest of your budget buying some sweet go-fast parts.

9. You won’t have the distinct Ford exhaust note

You can always tell when a Mustang is sporting a Bow Tie under the hood because of the unique camshaft profiles as well as the firing order, which is usually a dead giveaway. It’s not that GM LS series engines don’t sound good; they just have a different firing order. The No. 1 cylinder is on the driver side rather than the passenger side as is common on all 5.0L, 5.8L, and modular Ford engines. In case you weren’t aware, the 5.0L HO and the modular 4.6L engines share their firing order with the 5.8L V-8 engine, which is why all late-model Mustangs have that distinct deep and throaty exhaust note that makes a mean sound when you crack open the throttle.

10. Modular Ford engines respond very well to boost

There’s nothing wrong with the 4.6L Modular V-8 engines, and chances are it’s already in the car unless you have a Fox-body or newer 5.0L Mustang. Even Fox-body guys have swapped to modular engines when it comes to making serious power at the track. You don’t have to worry about splitting the block at 500 hp when it comes to adding boost to the 4-valve 4.6L mod motor, and it’s not out of this world to see people running 600 hp on stock bottom ends. They can take some serious boost and abuse, so building a junkyard Two-Valve or Four-Valve 4.6L V-8 can be more economical and fun than swapping in a junkyard LS engine.

And a Bonus Reason! Drive-By-Wire Pedal

Unless you’re swapping in an early LS series engine, you’ll most likely have to retrofit a GM throttle pedal to your firewall for the drive-by-wire function to control the engine. If you’re buying the GMPP controller kit, it’ll come included, but you can expect to pay more for the kit than the typical price of a used LS-series engine. You will still have to figure out a way to mount it to the firewall, and if you’re sourcing all the parts from a junkyard or the classifieds, you’ll need to make sure the ECM has the ability to control the throttle.