Air Cooled Hot Rod: 3 Valve Head Actually Better Than 2 or 4 Valve?

Engine tech, for those engines, products, and technologies of yesteryear.

Moderator: Team

Post Reply
nicholastanguma
HotPass
HotPass
Posts: 144
Joined: Fri Apr 16, 2021 9:27 am
Location: LA and SF

Air Cooled Hot Rod: 3 Valve Head Actually Better Than 2 or 4 Valve?

Post by nicholastanguma »

For more than 30 years now many Japanese air cooled engines, from little singles to big singles to huge inline 4s, have successfully been using 4 valve heads, and most of these engines are known for the typical Japanese level of bombproof reliability. Honda XR650L, Suzuki DR650, Yamaha XT600, Suzuki LS650, these are just a few of the decades old examples of four valve big singles, for instance. Look at the old air cooled Zook GSXRs and Bandits for examples of the big inline variety.

The new Harley-Davidson Milwaukee Eight pushrod V-twin is now using four valves per cylinder, too.

But sometimes in air cooled applications multivalve heads are known for overheating, especially at the exhaust ports; from what I understand, Porsche battled this issue for years. The Japanese clearly have some things figured out, however, so I can't definitively say 4 valve heads in air cooled engines will always be prone to overheating.

However, I've heard rumor that the new Milwaukee Eight four valve heads do indeed tend to overheat once hot rod fun like big bores, high compression pistons, and stroker cranks are introduced to the equation.

Fueling, the company that was making 4 valve heads for H-D Evo V-twins in the 80s and 90s, is also the company that licenses 3 valve head technology to Ford, so clearly they must know at least a little something about multivalve heads. According to this video the Milwaukee Eight heads have so much tendency to overheat when their engines are hot rodded that Fueling has been testing bolt-on 3 valve replacement heads.

https://www.youtube.com/watch?v=AY0yeZdcwok

With a three valve head that one big exhaust valve or that one big intake valve will, as far as I can tell, pretty much keep rpms limited to what a 2 valve head could manage. Nonetheless, engines are just complex air pumps, so cramming in as much air intake or exhaust outflow is theoretically always better than less. And correct me if I'm wrong, but it's more important to get IN as much intake air than get out exhaust gas, so I'm assuming 2 intake valves and one exhaust valve is probably the way to go.

For an air cooled hot rod, then, is a 3 valve head actually a better proposition than either a 4 valve or a 2 valve head?
HeinzE
Member
Member
Posts: 110
Joined: Wed Apr 27, 2016 1:48 pm
Location:

Re: Air Cooled Hot Rod: 3 Valve Head Actually Better Than 2 or 4 Valve?

Post by HeinzE »

3 valve heads have actually been used in a few motorcycle engines already. The Honda 400 Hawk made in the 70's and early 80's used
a two intake valve-one exhaust valve configuration. I believe that the Ducati ST3 also used a three valve scheme, hence the ST(3)
moniker. I've been told that Ford also used 3 valves, or maybe still does, in some trucks, but I don't really know that much about
Ford trucks, so someone more familiar with them might want to chime in.
As far as flow and rev capability goes, I imagine the 4 valve is a better choice, but I did see a lot of burned exhaust valves in the
early Honda and Kawasaki 4 valve multis, so attention to cooling between the exhaust valves is definitely a priority.
Geoff2
Guru
Guru
Posts: 1980
Joined: Mon Nov 09, 2015 4:36 pm
Location: Australia

Re: Air Cooled Hot Rod: 3 Valve Head Actually Better Than 2 or 4 Valve?

Post by Geoff2 »

The Rudge used 3 valves, one int, two exh goes back to the 1920/30s.
Truckedup
Guru
Guru
Posts: 2728
Joined: Mon Jan 21, 2013 2:41 pm
Location: Finger Lakes

Re: Air Cooled Hot Rod: 3 Valve Head Actually Better Than 2 or 4 Valve?

Post by Truckedup »

Geoff2 wrote: Tue May 18, 2021 3:49 am The Rudge used 3 valves, one int, two exh goes back to the 1920/30s.
was that the era Rude that had one set of valves vertical and the other set at an angle of maybe 45 degrees?
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire
englertracing
Guru
Guru
Posts: 1547
Joined: Thu Sep 29, 2011 8:55 am
Location:

Re: Air Cooled Hot Rod: 3 Valve Head Actually Better Than 2 or 4 Valve?

Post by englertracing »

nicholastanguma wrote: Mon May 17, 2021 7:21 am For more than 30 years now many Japanese air cooled engines, from little singles to big singles to huge inline 4s, have successfully been using 4 valve heads,.
https://youtu.be/eaRop_ZMwo0

Or 55 years.....
Post Reply