Ducati head - bore shrouding?

[superbike]

Is there a rule of thumb for valve shrouding in a 4v head? Say for instance if it was 10 percent of a 40mm valve we would make sure we had 4mm clear all around the valve ?

Below is a picture of a Ducati head very full of valves, 42mm diameter inlets. When flow testing I decided to offset the head on the flow bench mounting bore by 1mm. The flow went up 4 cfm through the range.

Would this suggest that 42mm is too big and that a 41 valve may perform better ?

[Guy Croft]

That's neither here-nor-there actually Chris.

To make a valid comparison you need to overlay the 41 & 42mm valve-in flow test data. Sorry!

GC

[superbike]

That's neither here-nor-there actually Chris.

To make a valid comparison you need to overlay the 41 & 42mm valve-in flow test data. Sorry!

GC

GC i actually thought that was the case.

[ADsim]

Hello ,
I read a SAE paper that was related to this fact I think :

2004-01-1922 : Influence of Valve-Wall Distance on the Intake Flow in High Performance I.C.E. University of Calabria (Italy)
The author tested two different high perf head (Alfa?) with differences as follow :
No change to exh valves position.
Head B have inlet valves moved a little away from exh valve and axis of exh valve inclination is 0.2‚° lower.
These two heads were tested with two bores having ratio of Delta bore A / bore B = 0.021 (if largest bore was 100m , smaller Bore would have been 98). Head A flows better than head B.
Conclusion is that flow coefficient is lower as low lift when wall distance from valve is smaller. At high lift , air flow distribution is less influenced. Decrease have been observed when wall interference was less than 2.5 mm (gradually! not sharply).

GP Blair conducted same job with a FZ600 head tested with liners of 66 and 59 mm bore (62 mm stock) and observed a mean 10% difference is flow coefficient (higher difference at high lift).

The question is always : does curtain inlet area increasing with valve diameter give better hp than flow coefficient decrease during operation cycle ?

Basic rules can be found in Hyundai SAE paper 2004-01-0998 giving trends based on :
- valve diameter to valve bore
- inner seat diameter to throat diameter (under seat)
- height of straight portion of throat between base of seat insert to base of guide boss in head (this is a image of port incline relative to valve angle in head)
- outlet head diameter to total valve inner seat area.

Mean flow coefficient is observed through a generic valve lift law with major factors :
- port head area to inner seat(s) area : mean Cd (flow coefficient) increase when port to seat area increase.
- Throat height to inner seat diameter : mean Cd increase with throat height to inner seat diameter.
- Throat diameter to inner seat diameter : mean Cd increase with this ratio.
- Valve diameter : mean Cd increase with valve diameter (a bigger engine have ability to better breath).
- Valve diameter to bore : mean Cd decrease with increase of valve diameter relative to bore (wall interference).

Equation coefficients are given in paper for insertion in a spreadsheet and you will be paid of your $11. A lot of papers exist on head subject but (for me) but not very generic as this one.

Regards
Alain