valve head diameter vs. runner diameter

[regis101]

Greetings. Working on a 2.0 FI bus engine. The intake runner measures 34 mm OD. The port at the intake side of the head measures 33 mm. The intake valve size is 37 and the exhaust valve is 33. At what size do the valves become too large to make a difference. Maybe there is a rule-of-thumb to go by? The extent of intended porting is mainly to clean up and somewhat straighten the ports. The engine is for a 78 westy retaining the stock FI.

[Michael Ghia]

I was asking a buddy of mine at work about port size in comparison to Valve head size just yesterday... He said that the ideal size is for the port size to be 89% of the valve head size.

Does this help?

MG

[regis101]

Thanks, there, MG. I also have a friend who ports. Actually helped him build the flow bench. My past knowledge comes from Detroit iron. The same parameters would hold true for the aircooleds, I guess. My goal is to build a nice 2.0 for our camper. I'm probably limited on potential due to the bus FI runner diameter. I am going to research the Porsche 2.0's for runner diameter. They used 42 mm intakes and I think 36 mm exhausts. I am hoping the runners are indeed larger. Then it's off to the Porsche swap meets I go. Would be nice to find out what the Europeans did with factory Bosch FI. Just don't tell me they scrapped it. Ahh!

[kdanie]

Yes, 2.0L 914 runners are larger ID than 2.0L bus runners. I don't know if they are a bolt on though.
ken

[Michael Ghia]

The inlet manifold pattern on 914 2.0 heads has 3 studs instead of 4 like the VW T4. Basically the middle 2 are replaced with one.

MG

[regis101]

A reputable source has stated that the bus plenum and runners are good for 120 horses. The bus injectors are supposed to be good for 37.5 hp each. So I'm thinking that I should be able to squeak out 100 hp for our westy. Flat tops, some cam, 8.5-9.5 CR, mild porting, good ignition and good exhaust. Another 20-30 hp, and I'd be happy for awhile. Should also be able to make it clean for smog checks. Increased CR will make it dirty, but I feel confident that it can be leaned out enough to pass smog. Can always get a bigget Cat.