Screamin’ Eagle 135ci Crate Engine Dyno Test

Harley-Davidson continues to push the performance of air/oil-cooled V-twin engines. Earlier this year we dynoed the Milwaukee-Eight 117 V-twin engine in the 2023 Harley-Davidson Breakout. At the time, it seemed very impressive with a recorded 94 hp and 118 lb.-ft. of torque on our in-house dyno.

Just recently, we had a chance to ride with the new Milwaukee-Eight 121 VVT in the 2023 CVO touring models. Utilizing the same piston diameter as the 117ci, but increasing the stroke from 4.5 inches to 4.625 inches, the larger 121ci engine is claimed to produce 12 hp and 13 lb.-ft. more than the 117ci M-8. As 121 VVT test models become available, we will run them on the dyno for a direct power comparison.

But before riding the 2023 Harley-Davidson CVO Street Glide and CVO Road Glide 121, we had the opportunity to dyno the highly anticipated Screamin’ Eagle 135ci Stage IV crate engine fitted in a Fast Johnnie Street Glide ST. The biggest and most powerful street-compliant engine offered by Harley-Davidsons is available as an upgrade for 2021 and later Touring models equipped with an air/oil-cooled Milwaukee-Eight engine.

Featuring Screamin’ Eagle Extreme CNC-ported cylinder heads, an SE8-517 high-lift cam, new high-compression forged pistons, a high-performance cam bearing, high-performance tappets, patent-protected 4.31-inch steel sleeve cylinders, a new 4.625-inch flywheel, new 68mm throttle body, CNC-machined 68mm intake manifold, new 6.2 grams/second fuel injectors, and Screamin’ Eagle Pro Billet Cam Plate and Oil Pump, the 135ci crate engine will be the most powerful street-legal H-D we have tested to date.

On the Cycle World Dynojet 250i dynamometer, the 2023 Fast Johnnie Street Glide ST with 135 crate engine produced 126.1 hp at 5,490 rpm and 141.2 lb.-ft. of torque at 3,700 rpm. That’s 34.1 percent more hp and 19.7 percent more torque at the rear tire than the 117ci mill in the Breakout. For those wanting the most powerful street-legal Harley-Davidson engine available, it will cost you $7,999.95. But there is one stipulation: It’s 49-state US EPA compliant, meaning it’s not for sale in California for use on public roads.