Over the last several years we’ve had the pleasure of working with Steve Morris Engines to cover a number of their engine builds; the company is well-versed in many combinations and has made huge power over the years with everything from this pump-gas ProCharged big-block Chevrolet to the massive quad-turbo V16 of their own design. One thing we don’t see much of from their engine shop–and something we’re definitely enjoying–is this supercharged LSX engine designed around one of Kenne Bell‘s liquid-cooled twin-screw superchargers.
Based around an LSX block and LS3 cylinder head configuration, the engine displaces 427 cubic inches and is designed to run on 93-octane pump fuel purchased from the station right down the street from SME.
One area that must be considered during the design process for a street-going engine like this is the camshaft. In an engine with a positive-displacement blower like the KB, it’s super-critical to get the valve events correct to ensure maximum horsepower while retaining quality drivability characteristics. SME paid close attention to the design, and has developed a hydraulic-roller camshaft that they now stock due to its solid performance.
“Positive-displacement blowers require a different profile, for sure. You spin it, it moves X amount of air, period. With a centrifugal, it moves more or less air based on how the engine flows, and you have to adjust the profile for that,” says Morris.
“We’ve designed a spring, retainer, and keeper package that uses our roller trunnions in the stock-style rocker arm and it works real well. It’s proven stuff that we’ve tested and developed over the years.”
As the majority of the work SME does is with centrifugal superchargers and turbos, Morris doesn’t get the chance to work with these as much, but he’s definitely pleased with how this build has gone. Boost pressure is not equal across power-adder sizes; a 3.6-liter twin-screw supercharger moves far more air–and makes far more power–than a 2.8-liter twin-screw supercharger showing the same ten pounds of boost pressure on the gauge.
“There’s a quality and volume of air that’s different with bigger superchargers. I’m pretty impressed with it–it’s a 3.6-liter blower that’s moving more quality–and volume– of air than a smaller blower would if we spun the smaller blower faster to try and make the same power. It’s an efficiency thing. Just like a bigger turbo will move more volume of air than a smaller turbo, at the same boost number so does a bigger screw blower. As long as the power to drive the blower does not increase more than the air/power it generates, a bigger blower will make more power,” says Morris.
“Three things matter in this combo: the blower is really good, we know and build the right combination–a little hint here for all of you, boost is a restriction number, not a flow number– and the third thing: we don’t tune like the average tuner. I think of every engine like it is my wife. I think I know what she wants to be happy, but what I think does not matter. I just give her what she wants even if it does not make sense! If I force my wife to do what I think she should do, the results are not too good. It’s the same with an engine and tuning. You get some guy that says you can only do this, or have this much, but without actually testing and trying things their results are just sub-par,” says Morris.
With respect to the tuneup in this engine, Morris says it needs to be set up differently than a centrifugal or turbocharged combination.
A basic explanation from him says that the timing curve is much flatter with the twin-screw supercharger, whereas a centrifugal-powered combination would have a steeper ramp as the boost builds. Timing is also altered accordingly to ensure engine stability.
“Anything like this twin-screw that comes in with a bunch of boost right away, the fuel and timing maps always looks different from a centrifugal blower or turbo. The boost is built up so quickly,” he says.
“It all boils down to timing per pound of boost (cylinder pressure), and the detonation threshold. As long as you know what the detonation threshold is and what is tolerable in the engine for timing, you can run it there,” he says
Ultimately, this LSX-based engine pumped out 1,136 horsepower on pump gas–on only 11.4 pounds of boost pressure, and Morris says there is far more left in it. Currently they are battling a belt slip issue that’s idler-related, and are working hand-in-hand with the folks at Kenne Bell to develop a tensioner system to withstand the requirements of the supercharger on this engine.
“We’re going to lean on it and put race fuel into it, lean on it a bit harder yet,” says Morris.
We’ll be watching!