The "real-world" compression ratio can be as low as 6.8 to one !! .... the heads are closed chambers BUT they are huge ... around 90 cc's.

It was a DAMN good motor for what it was designed for but it is as far from a HP motor as it can be.

Those UGLY heads ? ... are way WAY ahead of the competition "back-then". And those heads still are better than most truck/industrial/motorhome heads even made these days.