Port Limitations

The limiting factor for real horsepower "overload" for XV/Viragos lies in their ports. There is too little material around them for enlargement, and they bend severely, choking off airflow.

On Gen I's, the frame -- and to a lesser degree -- the fuel tank limits where the port can end, the manifolds/flanges that can be used, and the size and configuration of carburetor(s). I see 920R/TR1 enthusiasts cobbling up adapters for FCR's -- but they leave the basic ports intact.

My assessment? For a little more flow, enlarge them slightly to match carb flanges and pipes; for a lot more flow: straighten and move them, maybe enlarge them. The valves are pretty big, and the ports are plenty sizable, just crooked as a cork-screw.

Link to More After Pictures . . .

Pressure Pulse

Topics

First, Some Pictures:

One can see that the bolt centers are 75mm apart. This is pretty standard for big bikes. 40mm carbs sit in the rubber flanges bolted on. One can see that the port is only 38mm -- there's room to enlarge.

But only to the edges of the blackening I put on them. Not much. And see how tightly the port turns.

Without modifying the cam "gallery," the best center location for the intake port would be where the caliper end is. This is with a fair amount of weld build-up and re-angling the spigot face. It also (on Gen I's at least) butts right up against the frame -- no room for carbs (there may be a work-around). A better port face center is canted out into the fins. Caveats? Carbs must be down-draft, or it's time to go EFI. Leg room may be a problem.

Two shots of the angle taken by the exhaust port. I see a 37mm ID for the port outlet, and 41mm for the gasket ID -- lets let that be the limit for the time being.

Some room to go bigger. Typically, the ID of exhaust headers is larger than the ID of the exhaust gasket, so you may want to make some impressions and trimming first.

The caliper end shows where I feel the best center for the exhaust port would be. Fins would need to be welded up. The spark plug would be approached -- better than swinging toward the cam chain tunnel. And avoiding making modifications to where the cam and valve train reside.