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Sometimes you just have to do things even if at first they don’t make a lot of sense. That’s exactly what happened when Gibbons Motorsports was approached to build a high-horsepower, modern LT1 engine...with a carburetor.
The new LT1 engine is the fifth generation of the famous Chevrolet small block. It first appeared in the C7 Corvette beginning in 2014, and it’s an incredibly modern design packed with all kinds of technology. The architecture is based on maximizing the efficiency of the performance V8. That means we get technological advancements like gasoline direct injection, a variable-vane oil pump, displacement on demand, all sorts of computer controls and even an 11.5:1 compression ratio.
All of that made getting a carburetor to work on the LT1 a bit more trouble than you might think. The owner wanted the unique combination because he already had a new stroker rotating kit for an LT1 from Manley. And as far as the carburetor goes--well, he just wanted something a bit different, and probably felt more comfortable tuning a carburetor at the drag strip. So brothers Pete and Jake Gibbons set about making it all work.
The biggest challenge didn’t come from physically bolting on a carburetor, but figuring out how to make the engine run safely after deleting the ECU. There isn’t nearly as much aftermarket support for the LT1 as there is the LS family of engines (yet). So Gibbons had to get creative. Variable valve timing doesn’t work without the ECU, so Gibbons deleted it by modifying an LS3 timing set to work. That, however, also required a different front cover to move the cam position timing sensor closer to the cam gear so it could pick up the signal properly. The LT1’s mechanical fuel pump had to be deleted and replaced with a lower pressure electric fuel pump. The spark timing is now handled by an MSD LS-series ignition controller. It works great, but it doesn’t like the LT1 sensors, so Gibbons had to make some adaptations to get the LS sensors to work. And there was the fact that nobody actually makes an intake manifold for the LT1 with a 4150-style carburetor flange.
You know, little things like that.
But, they figured out how to make it all work and on the dyno the engine even made 615.9 horsepower at 6,400 rpm. That, by the way, is 160.9 better than it came from the factory. Check out more details in the photos below.
Making one of the world’s first carbureted Gen V LT1 engines required quite a bit more than simply bolting a Holley double pumper on top.
To get the compression down, Gibbons used a piston from Manley with a 20 cc dish. It is shelf-stock pistons to save money, so you will notice it still has the “divot” in the center of the piston for the gas direct injection. There’s no benefit in a carbureted application, but it doesn’t hurt, either.
The crankshaft and connecting rods are also from Manley. The stroke is now 4.000in, up from the stock 3.622in, and the bore is 4.075in inches (+0.010in). That brings total displacement to 417in, or 6.84 liters.
The heads are actually a set of used stock units. The combustion chambers are sized at just over 59cc, and you can see the direct injection ports in each chamber directly opposite the spark plug holes.
The rockers are an upgrade over stock because Gibbons is running a camshaft with both higher lift and duration, but they are still stock ratio at 1.8:1. A big difference between the LS1 cylinder head and the new LT1 is there is no rocker stand. Instead, the rockers bolt directly to bosses in the head oriented to the correct valve angles. This also allows all the rockers to be the same. No offset rocker arms here.
We aren’t using the original fuel injectors that spray fuel directly into the combustion chambers at over 2,100 PSI, so Pete Gibbons drives in these plugs to properly seal the combustion chambers.
There might be options by the time you read this, but while Gibbons was building this engine there were simply no carburetor-friendly intake manifolds available for the LT1. So Pete was forced to use these adaptors from ICT Billet which allows an LS3-style intake manifold to be bolted up. It isn’t a great solution, but this was the only solution, so they went with it.
The LS3 carbureted intake bolts to the adapter brackets and allows use of the carb.
The engine will be going into a first-generation Camaro, and to make sure it fit Gibbons swapped out the original cast oil pan for a unit from Holley designed to work with the F-body chassis. To make extra clearance the Holley pan integrates much of the pickup tube right into the casting.
Spark is handled by a set of MSD LS coil packs. The mounting holes are different, so Pete Gibbons had to weld up new mounting bosses on the LT1 valve covers. They are fired by an MSD LS-series spark controller which allowed us to dial in exactly the spark advance curve the engine wanted.
And here’s the purpose of it all. The carburetor we ended up using is a 950 cfm Holley double pumper.
The end results speak for themselves. The stroker LT made 615.9 horsepower and about 560 lb-ft. of torque when put to the test. The horsepower figure alone is over 160 horsepower more than the LT1's stock output!
