Performance Gains of Downdraft Head vs Sidedraft Head

[nicholastanguma]

Set aside the packaging problems normally associated with having to mount the carb and air filter so high up in a downdraft head configuration, and instead just focus on the performance aspect.

In the 80s and 90s, when the Sound of Singles racing series was in its glorious air cooled, carbureted heyday, as far as I know the hottest engines were the Rotax and Honda 4 valvers using custom downdraft heads (sometimes dual carbed, no less).

Not necessarily downdraft carbs, mind you, but downdraft heads.

Just how much better for performance is a downdraught head vs a standard horizontal head?


350cc Rotax with a single carb downdraught head




675cc Rotax with a dual carb downdraught head




VS


Standard horizontal sidedraft head

[BCjohnny]

Cosworth were doing it in the sixties with the old pre-crossflow (and it was also done on the later X-Flow head by others) Ford 4cyl cast iron head, by brazing in 'high port' sleeves

So it's nothing new and it's obviously worth it

[PackardV8]

A true downdraft head has the intake ports between the two cams.





The Gen I Lamborghini and the Ford Indy DOHC come to mind, but there have been many others.