MERRY Christmas and Happy New Year, readers!
When I was in the US I saw one of the toughest naturally aspirated street engines imaginable. In fact, I’d be surprised if Sonny Leonard’s 727-cube 1475hp donk isn’t the most powerful naturally aspirated single-four-barrel pump gas motor in the world.
To Sonny, his 727ci street engine is like a ‘smallblock’ compared to his ‘Godfather’ 1005-cube race engine, yet it makes 1475hp and 1100ft-lb on 93-octane. His ‘little’ street engine – 278 cubes lighter than big brother – makes over 1000ft-lb from 5400rpm all the way through to 7700rpm. At 5000rpm, torque reaches 968ft-lb.
The engine defies just about all the conventional high-performance ‘rules’. For instance, the motor has a 4.751-inch bore and a 5.125-inch stroke. You read correctly, the stroke is longer than the bore diameter.
Usually high-performance engines are built with a big bore and a short stroke.
Piston speed is way beyond what textbooks regard as the safe limit. Piston speed in feet per minute is calculated as 2 x stroke (in inches) divided by 12 x rpm. At 6800 peak-horsepower revs, the piston speed of Sonny’s engine is around 6000 feet per minute, nearly double what is regarded as safe.
And the 5.125-inch-stroke Bryant steel crank and 7.267-inch-long Carrillo rods have a rod-to-stroke ratio of 1.418:1.
Yet the motor makes nearly two horsepower per cube and does dyno pull after dyno pull. Oh, and the forged pistons are BME flat-tops that give over 13:1 compression with the ‘Next Generation’ hemi heads.
So what’s Sonny’s secret to making this much power on pump gas? The answer lies in the superefficient NGH five-inch-bore heads and matching camshaft. The heads also defy what is theoretically possible. The 2.66-inch Manley titanium intakes flow 616cfm at 28 inches and the two-inch titanium exhausts flow 420cfm at 1.100 inches lift.
The SuperFlow formula for horsepower at 28 inches is cfm x 0.26 x number of cylinders. Theoretically, 616cfm should only make 1281hp, yet the engine makes 1475. And if you think the horsepower numbers are inflated, a 2200hp (same dyno) 1005ci naturally aspirated engine in a 2650lb Doorslammer runs 6.17@220mph. Also worth noting is that the intake-to-exhaust ratio is only 68 per cent. Sonny designed his own hemi heads, had them cast, and then they were CNC-ported in-house. And while not as spectacular as a tunnel ram, the billet intake
manifold with a single four-barrel EFI throttlebody flows so well there’s no flow restriction. The Accufab throttlebody flows 3500cfm.
Regarding combustion chamber efficiency and ignition timing, the more efficient a head is, the less timing is needed. Going back in history, ‘dirty’ closed-chamber big-blocks needed lots of timing, like 42 degrees of advance, and small-blocks 38 degrees. These NGH super-efficient combustion chambers only need 26 degrees advance to make 1475hp at 6800rpm. Even though the compression is over 13:1, there is no detonation with 93-octane pump gas or high-octane race fuel.
The super-efficient heads are a key ingredient; the other is the matching camshaft. The roller cam has around 280 degrees at 50thou intake duration and is ground with 0.630 inches of lift at the lobe, which is more lift than many small-block cams have at the valve.
The 1.75:1 T&D shaft-mounted roller rocker arms give 1.100 inches of lift at the valves, minus the lash. The keyed roller lifters are 1.062 inches in diameter. The one thing I can’t give away is the lobe separation angle and camshaft timing. To run 13-plus compression on 93-octane requires bleeding off cylinder pressure down low with overlap and duration.
You’ll have to buy a 727 to find out what the cam timing is. The engine can be bought in kit form if you want to save a few dollars, but the guys at Sonny’s have been putting big-inch motors together for more than 30 years and think it wise to get it assembled, dyno-tested, tuned and shipped ready to go as a crate motor.
Here are some calculated times and ET and mph predictions for Sonny’s little pump gas 727.
In a street-driven hot rod that weighs 2400lb with driver, the 727 could theoretically run 6.8@198mph. That would be one tough naturally aspirated street rod. Or a street-registered fibreglass-bodied Corvette on a tube chassis weighing 2800lb (Rod Saboury’s six-second street-driven ’53 Corvette weighs in at 2800lb and, co-incidentally, has a 1600hp Sonny engine in it) could go 7.12@189mph.
I can picture one of these motors fitting perfectly into a HQ-HZ Holden or a ’55-’57 Chev.
Weighing in at roughly 3500lb, you’d be looking, theoretically, at a 7.7@175mph ride, on pump gas naturally aspirated. s