The small-block Chevrolet is a great engine – period. We just wanted to get that out there, right off the bat. Built correctly, they can be a punch-packing powerhouse that will burn tires with ease. At the same time, however, it is nearly impossible to beat the power and torque of even a mildly-built big-block Chevy. They run smooth, they run strong, and they drink gas like it’s going out of style. There is a reason that many years ago, someone coined the term, there is no replacement for displacement. To get a rundown about what parts are needed to undertake a swap like this, we contacted Casey Mohr of Original Parts Group, to get some professional input.
Once you feel the torque of a big-block, and hear the sound of a 3-inch exhaust, why would you want anything less? – Tom Brown
We’re sure you realize that a 1955 Chevrolet did not come standard with a big-block – it wasn’t even an option at the time. So this car – like many others that came with a small-block, is ripe for a swap to a dimentionally-larger “motorvator.”
There are many things to consider when installing a big-block in a car that previously-housed a small-block or six cylinder, and we are here to deliver some insight to make sure that you have a good idea of what you are getting into before making a commitment to do the swap.
“If you’re working with a car that had a big-block as an option from the factory, then the swap is sometimes as easy as pulling one engine out and sticking the other in,” Casey explained. “It is really easy to complete this swap in first-gen Camaros, ’64 through ’72 Chevelles, and Impalas.”
For the most part, this upgrade is a plug-and-play install, but there are a few key components that need to be changed in order to accommodate the new engine. Casey put together a nice list that will help make for a smooth swap for any of the cars mentioned above.
- Frame mounts (usually different for small-block to big-block)
- Motor mounts (sometimes different between small-block and big-block)
- Oil pan (may need to be changed for frame clearance depending on where the engine came from – i.e: car or truck)
- First-gen Camaros have common steering shaft clearance issues, especially when running headers (it gets pretty tight in the engine compartment)
- Some wiring may have to be extended
- The accessory drive on the front of the engine is specific to big-block
- A big-block typically uses a larger radiator (some small-blocks that had factory air-conditioning might already have a larger-capacity radiator)
- It is also recommended to change the front springs for higher-rate springs designed for a big-block car
If you’re working with a car that had a big-block option from the factory then the swap is almost as easy as pulling one engine out and sticking the other in – Casey Mohr
When it comes to engine fitment, the frame stands are what determines engine positioning, regardless of displacement. All traditional Chevrolet V8 engines (pre-LS), from 1958 and later, have the same motor mount-boss configuration, regardless of displacement. But, the frame brackets are another issue.
For 1964 Through 1967 Chevelle
For these years, dimensionally, the frame bracket/stand for small- and big-block cars are the same, except that the big-block bracket has an indentation for the motor mount safety lock feature. This means the big-block frame bracket will work with both big- and small-block motor mounts, but the small-block frame bracket will not work with big-block engine mounts because of the safety lock feature. When installing a big-block engine in a 1964 through 1967 Chevelle with small-block frame brackets, you can use the small-block motor mounts.
For 1968 through 1972 Chevelle (Includes Monte Carlo)
The frame brackets and engine mounts are interchangeable for all V8 engines, except the 307 cubic-inch version. But, when installing a 350ci or big-block engine in a 1968 through 1972 Chevelle that originally came with a 307ci engine and still has the 307ci frame stands, you can use 307ci engine mounts.
Nova And Camaro
These years of Chevrolet’s Nova take special subframes and a lot of modifications to install a big-block. For instance, not only will the stock frame need replaced, but the inner fender and shock towers will need to cut from the car. Get ready to do some cutting and welding.
Since the Nova and Camaro feature similar subframes, it stands to reason that the frame bracket for the engine mounting also be simliar. All ’67 and ’68 small-block and ’69 307ci and 327ci engines use the same frame bracket. The 1969 302ci and 350ci engines used a shorter and narrower bracket. While ’67 and ’68 engine mounts will physically attach to the ’69 302 and 350 cubic-inch frame bracket, this places the engine at the incorrect location, and can result in driveline vibration. This is because the engine will be sitting too low, possibly throwing the driveline angle out of spec, and the engine could also move on the bracket.
To obtain clearance under the hood, big-block engines were moved slightly forward from the factory, and offset 1-inch to the passenger’s side of the car by using asymmetrical brackets. Big-block frame brackets from ’67 and ’68 cars can use the same engine mounts as ’67 and ’68 302ci and 350ci engines. In ’69, big-block frame brackets were redesigned to use a new-for-the-time thicker and narrower engine mount, which was also used on 302ci and 350ci engines. Big-block bracket pairs are easily distinguishable, because the left bracket is noticeably taller than the right, to achieve proper engine offset.
If you’re going to keep the factory heater, you’ll also need a new heater core and big-block-specific heater core cover. Factory big-block cars without air conditioning benefitted from heater hose outlets that were very close to the passenger’s side fender, as opposed to being next to the engine block like small-block cars.
This generation saw the last big-block option from the factory in 1971. The engine mounting changed beginning in 1972, and used what is called a clamshell-style mount. This uses a steel bracket that is bolted to the engine, and the insulating mount is contained in a steel housing that is bolted to the frame of the car.
There are different “motor mounts” used, and they have different mounting heights depending on the bracket’s original vehicle use. The height of the mounting bracket as measured from the block mounting surface to the bolt hole center line can be either 2-1/8, 2-5/8, or 2-3/4 inches. Depending on the application accepting the swap, choosing the correct mount is hit or miss as hood clearance can be an issue. Another option would be to use frame brackets and motor mounts from a ’68 to ’72 Nova.
It’s also likely that you would want to change the transmission as well. While you could run a Turbo 350 or a Saginaw four speed, the more durable Turbo 400 or Muncie four speed would be better suited to handle the extra torque and power. If you will be changing the transmission, the crossmember is another area to consider.
All ’67 through ’69 six-cylinder and small-block-equipped Camaros use the same transmission crossmember. The big-block crossmember for manual transmissions looks similar to the small-block version, but the transmission mounting slots in the big-block manual transmission crossmember are in a different location than those in the small-block version.
The transmission crossmembers in the Chevelle are interchangable, unless you wish to keep it looking factory original. Some manual transmission crossmembers had a bracket welded to them to support the Muncie shifter if so equipped.
Get The Right Parts
If you’re planning a swap to a big-block into your car, or need other parts for your Chevelle, Cadillac, GTO, Bonneville or other GM car, the OPGI website can help you find everything you need.
Mounting a big-block where a small-block once resided also means upgrading accessory brackets and pulleys. Pre-1969 vehicles used a short water pump, and most ’69 and later engines used a long water pump. We could probably develop a complete article about bracketry for this swap, but just keep in mind that brackets for the power steering pump, alternator, and air conditioning will vary. Although some will tell you the pump reservoir is interchangeable, the hoses actually use different connections.
Those of you contemplating a big-block upgrade in a Tri-Five also have room constraint issues to contend with, but you will still need many of the accessory items like pulleys, power steering parts, and other bracketry. We reached out to Joe Lutz over at Hot Rod Dynamics, and he gave us details. “On the ’55 through ’57 Chevy cars, a big-block will contact the firewall when mounted in the stock position,” Joe explained.
Joe let us know that there are two ways to make the big-block fit: you can either move the engine forward to clear the firewall, or take the time to modify the firewall to clear the engine. Both routes have their own challenges, and one isn’t necessarily better than the other, it just depends on the way you want to go about doing it. It’s about looks as much as it is about performance.
Of course there are different challenges associated with either route, so it is up to you to decide how you want to tackle such a project. “When you move the engine forward, the oil pan can interfere with the steering linkage,” Explained Joe, “it also crowds the front of the engine to the radiator and core support, making it very difficult to fit a serpentine belt system and cooling fans.”
One way to make things work with the engine pushed forward is to relocate the radiator forward of the core support. Joe offers an easy solution that condenses other performance features into one unit, while still making more room forward of the engine. “We offer an engineered cross-flow radiator system with fans, A/C condenser, transmission cooler, and an optional twin snorkel cold-air induction system that integrates into the radiator support,” Joe explains. “This system locates the radiator in front of the core support, and the fans inside of the core support for maximum engine clearance.”
Once the project is complete, the final outcome will feel like it was worth the work and effort, as it’s unlikely that you’ll regret adding the larger engine once you feel the added torque and power. The big-block also adds another element of customization to your car, especially when it wasn’t an option from the factory.
We hope this gives you a window into the world of big-block swaps, and informs you enough to give you an idea of what you are getting into, or if need be, talk you out of a project that may be more than you can chew.
If you do decide to take on a swap like this, give OPGI a call so you can be sure to get all of the correct parts you need for a cubically-swollen engine upgrade.