Ask Away! with Jeff Smith: Swapping a 4.8L LS into a 1955 Chevrolet

I have a ’55 Chevy that was my dad’s and now it has come to me. The car has a very tired 283 small-block in it with a TH-350 trans. I want to update it with a better engine and I just ran across a used 4.8L LS truck engine out of a 2002 pickup. The only reason I’m even thinking about this engine is because I can get it really cheap from a friend who bought a wrecked truck just for the interior and he’s parting out the rest of it. I can get the engine for $250 because nobody else wants it. I don’t know much about what it would take to convert to the LS engine, but it can’t be too tough because LS engines are everywhere. I’ve heard that converting to a carburetor would be less hassle and less expensive than using the factory EFI. Is this engine worth putting into my ’55 or am I wasting my time?

J.P.

Jeff Smith: We actually have a bit of experience with the LS 4.8L engines. Because it is in a 2002 truck, that makes it a Gen III engine. This is the smallest of the family of LS engines with 4.8L equating to 293 cubic inches. The rest of the Gen III family of truck engines ranged in displacement from 4.8L (293 cubic inches) – 5.3L (325 cubic inches) – 6.0L (364 cubic inches). Even though your engine is the smallest of the bunch, don’t let that dissuade you.

The most common 283 from the early to mid 1960s was the 195-horsepower 2-barrel carbureted variety, while the more rare 4-barrel models made 220 horsepower. Likely, the 2-barrel version is the one in your 1955. Conversely, the factory stock hp rating for the 2002 4.8L engine in a Silverado pickup is 285 horsepower, nearly 100 horsepower more than the 2-barrel version. What’s even more interesting is that in this same year, the larger 5.3L engine only makes 15 horsepower more at 285, and the 6.0L was rated at 300 horsepower.

The big difference in these engines would be the torque rating. Larger engines always make more torque, which is what you feel when you accelerate at low engine speeds. The little 4.8L motor will not deliver nearly as much torque as the much larger 6.0L, so that’s what you will give up. But for the dirt-cheap price, assuming the 4.8L motor is in decent shape, that $250 price makes the engine very attractive. It will cost a lot more than that to do the conversion.

The next big question is whether to retain the factory fuel injection and computer or to modify it with a simpler carburetor. Let’s take the carburetor option first. Some will say that you are taking a 21^st Century engine with excellent fuel and spark control and retrograding it back to the 1960s. While that is certainly true, there are advantages. First, this conversion will not be overly expensive and relatively easy. I’d recommend a dual plane intake from Holley  or Edelbrock. If the old 283 already has a 4-barrel carburetor, that’s one less thing you have to buy. If not, a simple 600 cfm Holley 4-barrel carburetor would work fine.

You will also need an MSD ignition controller for this engine. For the Gen III engines, this will require an MSD controller. Connecting this ignition could not be easier unless someone else did it for you. You can run a pre-configured ignition curve by choosing one of several plug-in modules supplied with the controller or you can do it yourself using MSD’s free Pro Data+ software using a laptop.

You will still need some kind of electric fuel pump to make this conversion since none of the LS engines offer a provision for a mechanical fuel pump. You would not need an expensive in-tank pump but depending upon the style of electric pump, you may need a fuel pressure regulator.

If you are considering retaining the high-tech advantages of full electronic engine control, that’s also not as daunting as it might appear. The most important point is to make sure and get the entire engine control harness with the engine. You might also consider including the 4L60E trans with the engine. This is an electronic 4-speed automatic with a 0.70 overdrive which will be a nice cruising addition. Plus, the 4L60E uses a deeper first gear than the TH350 to give the little 4.8L motor some help in acceleration.

The 4L60E is roughly 3 inches longer than the TH350. We’ll assume that someone has added a transmission crossmember since the original 1955 Powerglide automatic used a mount attached to the bellhousing. If there is a crossmember already in place, all you may have to do is relocate it slightly rearward to accommodate the 4L60E. Trans control will be accomplished by the engine ECM. If you choose to retain the TH350, you will need a spacer and an torque converter adapter.

Bolting the engine into your 1955 might also be easy if that later 283 has been converted to side engine mounts. The original 1955 Chevy used mounts bolted to the front of the engine along with the aforementioned bellhousing mounts. If the original factory mounts still exist, that’s not a deal breaker as Trans Dapt or Danchuk make conversion mounts. Even if you decide to use a small block after all, this conversion is still a great idea. Then, for an LS engine, the only other piece you’ll need is the plate that adapts the LS engine to the small block Chevy motor mounts from Trans-Dapt.

With the LS bolted in place, now comes the detail work. For example, you will need an in-tank fuel pump. The easiest way to accomplish this would be a dedicated fuel tank and pump assembly from Aeromotive using the Phantom system. The Phantom places a fuel pump on a steel strap so that the pump’s pickup is very close to the bottom of the tank. The pump is enclosed in a 6-inch diameter layer of fuel cell foam located in a plastic retainer that retains fuel around the pickup like a small reservoir. This system works extremely well. We have years of personal experience with this kit and it works great. It’s also easy to install since you bolt the whole tank in and just plumb the fuel lines. But you also pay a price for this ease of installation since the entire tank assembly runs over 700 dollars. Of course, there’s also the Phantom kit that you install yourself.

The next minor hurdle is that 2002 truck engines all came with electronic throttle control. So make sure you get the electronic throttle pedal assembly with the engine. This will require mounting the electronic pedal to the firewall, but that should not be difficult. Earlier engines used a cable throttle but an electronic throttle will actually make the conversion much easier. If for some reason that 4.8L truck engine came with a cable throttle – look into a conversion cable assembly from Lokar. They offer a ton of options for cable throttle that will get you where you need to be.

Exhaust modifications will also be necessary. It’s likely the stock truck exhaust manifolds will not clear the steering column. Before you look into headers, consider Holley’s cast iron exhaust manifolds. I’m not sure they will work on a 1955 Chevy, but it is possible since they do clear an early Chevelle and these are generally a tighter fit than a 1955 Chevy. Another option is a set of headers. Almost everybody offers headers for a 1955 Chevy LS swap, so it’s just a matter of searching for the best deal. Patriot offers a non-coated shorty LS swap header that might work. Patriot’s catalog lists about four footnotes that are worth reading before making a decision. One of the footnotes calls for an LS1 car style oil pan.

Since we’re on that topic, it is entirely possible that the bottom of the 4.8L oil pan will drop below the crossmember on your 1955 Chevy. That’s bad should the car ever bottom out since that cast oil pan will easily crack or break. If so, there are several options available including the aforementioned LS1 Camaro oil pan that you might find on the used market. Chevrolet Performance sells a muscle car pan for conversions like yours but the sump depth is in between the truck and the Camaro pans. A little research there will help make that determination.

So now that we’ve really made this question really complex, don’t be intimidated by all these details. Sure, the LS engine swap is much more complicated than just switching it with another small block Chevy. Yes, you could just buy/build a 350 cubic-inch small block and bolt it right in place of the tired 283. There is certainly nothing wrong with that idea. After all the stuff we’ve outlined here, that might sound much easier and if your automotive hands-on experience is limited, a nice 355 cubic-inch Mouse motor might be the better way to go. But if you like the idea of ramping up that 1955 with something a little more modern, then there might be an LS swap in your future.

Be adventuresome!