WHY IS EXHAUST VALVE ON FI HEAD SMALLER

[eric77super]

I understand that the 75 up Beetle has FI and its cylinder heads are VW part No 043 101 375H (driver side) and 375R on passenger side [has to do with Cyl. Head Temp Sensor] SO far so Good. But.....
Anybody know why the stock exhaust valve diam. was reduced to 30mm. The Wilson Book (page 50) sez "it "required for correct fuel injection operation". I can't figure that one Out? Anyone know why? What would happen if bigger valve was used, for example 32mm like in the 113 101 375A? I'm asking cause CIP1 has 2 types of replacement heads avail (you gotta tap your own CHT hole!) but the exhaus valve is bigger on option 2 and I figure what the Heck isn't bigger Better?????

eric

[VGM]

A bigger exhaust valve will" over scavange" the combustion chamber.Fuel injection was more efficient and did not require the larger size valves used for the earlier carberated systems.But I think the intakes are smaller as well on the FI heads so the balance is maintained between intake and exhaust flow.

[raygreenwood]

Thats all true, but the biggest reason had to do with keeping correct runner diameters...so they could keep correct runner volumes versus the length they had to reach to the plenum.
What does that have to do with valve diameter? Because its best to have the runner diameter (in cross section) and the open cross section the valve creates at full lift...as close as possible.
Too large of a valve opening will change the velocity in the runners. That in turn would necessitate changes to the plenum...and the TB to meter it all.
Bear in mind that velocity and vacuum signature ...even on L-jet and especially on D-jet....were critical to metering. The exhaust valve was also kept small ..to balance the overall flow...as suggested in the last post. Ray

[Piledriver]

...And to reduce power, to reduce heat, in order to reduce NOx emissions.

[caxambas]

I used the 32 mm exhaust valve heads + had the heads ported. Too much work keeps me from breaking engine in and testing. I will report findings/post complaints when I am that far. I will run fuel injection.
By the way, the type 3 had the 32 mm exhaust valves with the fuel injection.
Jos

[PJMS]

Just to make it even more interesting: Three months after VW started production of the L-jet Type 1 unit, with 33mm intake and 30 mm exhaust valves, they started fitting the small exhaust valve to the 1600 carb engines in Europe. Less than two years later, they then reduced the intake down to the 33mm, which is how all German and Mexican produced heads have come since then (also as replacement heads for all 1600 engines back to 1971). Brazil finally changed to the smaller valves in 1984, although the familiar Brazilian heads we see are of the pre-84 type with the larger 35/32mm valves.

Aparently, the period of 35/30mm valve heads saw an increase in engine failures due to over-heating.

[Tom Notch]

The smaller exhaust valve was for emissions.

[PJMS]

The smaller exhaust valve was for emissions.

That's also what I was told. In addition, VW sunctioned the replacing of just one head with the later version, so you could have an engine with one side 35/32, and the other 33/30.