ASK KEVIN: Whatever Happened to Intake Swirl?

engine block

QUESTION: Back in the late 1970s and early 1980s, it seemed like getting a so-called "swirl" of air/fuel into the combustion chamber was all the rage. Suzuki had its TSCC (Twin Swirl Combustion Chamber), and, not to be outdone, Yamaha had this unfortunate acronym: Y.I.C.S. (Yamaha Induction Control System). The purpose, as advertised, was to achieve more efficient burning resulting in (trumpets, please) "more power and greater fuel economy!"

As memory serves, the Yamaha Vision version of Y.I.C.S. was a cheap plastic dual chamber connected by hoses to the intake side of the cylinder heads. Kind of like little Johnny's ninth grade science project. To my knowledge, these systems went the way of the anti-dive fork. Where these just marketing ploys or was there solid science lurking somewhere?

Steve Klose

CycleWorld.com

ANSWER: Whatever systems those two companies developed in that period, they did not save Suzuki from their decision to shorten the GSX-R750's stroke to 44.7mm for 1988, with resulting reduced performance, as attested by their return to the previous longer 48.7mm stroke. Yamaha, too, had combustion trouble. The production FZR750 needed a tremendous 45 degrees BTDC ignition timing to burn its wide but vertically very thin 5-valve combustion chamber (I still have the rubber cast). The longer combustion takes, the more heat is lost from the hot combustion gas to the surrounding metal surfaces, and the more torque suffers.

This puzzled me, because Keith Duckworth had shown the world how to make short-stroke, large-bore engines burn rapidly and efficiently, through intake-generated charge tumble. Duckworth's revolutionary Cosworth DFV F1 engine of 1967 (picture above) fired at 27 degrees BTDC.

What I must assume is that only the appearance of Duckworth's new paradigm (a steep intake downdraft angle) was applied to Japanese sports engines of the 1980s, but without achieving the effect.

When I asked Ducati boss Claudio Domenicali about this, he replied: "Obviously, I cannot speak for the Japanese case, but here we have specific tests by which we develop the necessary degree of combustion speed."

He went on to say that Ducati had used the same basic tool pioneered by Harry Ricardo in his 1920s turbulence research—an in-cylinder anemometer (today this may be supplemented or replaced by CFD). By varying the intake downdraft angle and intake velocity, the charge entering the cylinder could be made to flow across the head, down the far cylinder wall, back across the piston crown, and up the intake cylinder wall. As the piston rose on compression, this “tumble” pattern of rotation would first preserve the energy of intake air motion, then break up into a multitude of flame-speeding eddies as the piston neared TDC.

Japanese manufacturers soon rectified their mistakes to become masters, and their early approaches to the combustion problem went into the archives.

Send your “Ask Kevin” questions to cwservice@cycleworld.com. We cannot guarantee a reply to every inquiry.