Head Variations

[motoddrob]

Just noticed the exhaust port differences between my original S1 1200TS head dated 480, and the S2 1200TS replacement head dated 681. The later head has a much smaller exhaust port, 32mm where as the 80 head measures 36mm. Also the valves sizes, 81 head has original valves and measure 38mm, 35mm, the 80 head has a set of Wolfgang's stainless valves and measure 39.5mm, 36mm. I believe that a smaller exhaust valve is beneficial, but how does a smaller port work compared with a 36mm port when mated to a 40mm header??

 

[redax5]

you are installing a set of stainless valves into what type material valve guides ?

 

[motoddrob]

Hi Red,
Not installing stainless valves, the 81 head has the original valves fitted.
The old head has the stainless valves fitted. When they were installed back in 2000 the guides were also supplied by Wolfgang. Pretty sure they're steel of some sort [magnetic]
Anyway that's history now as that head will end up as a door stop.
The question is about the differences in exhaust port sizes, is fitting this head going backwards from a performance point of view given the reduction in port size. Also the valves are 1mm smaller than the stainless ones fitted in the old head.
I'm interested in knowing why they reduced the size if the ex port in the later head. Do the 81 jotas have the same head fitted?, Pretty sure this head is off a S2 1200

 

[redax5]

Piet will know the answer to the head variations a lot better then me but what I have found is that the factory changed the valve size and exhaust port size somewhere in the run of 81 Jota heads
I have found early 81 Jota heads with 38mm inlet valves and the smaller exhaust ports but they also seem to be dressed with the earlier push on carb manifolds same as a 78/79 set up
when the factory changed to the rubber coupling manifolds the exhaust port seemed to "grow" as did the valve sizes

most of the Series 2 x 1200 engines I have seen have the smaller valve sizes, factory probably trying to use up existing stock or they may have figured it was a waste of time using the larger valves and ports with A11/A12 camshafts

if you want full on performance then you go to 41.5mm inlet valves, open the inlet ports to 36mm with offset porting and use 34.5mm exhaust valves with the larger exhaust port
that is what we have in the track bike, sort of, couple of other modifications in there as well
is also how Mike's Motodd engine is set up, his engine is nearly identical to Paul's track bike including the machined crank, is going to be a weapon when it is finished
you will have to swap bikes with him one day, be interesting to feel the difference

 

[sfcpiet]

Rob,

Wouldn't worry too much about exhaust port size, haven't found it to make much of a difference.  Stock SFC 1000 made good horsepower with the smaller 34mm exhaust valve but with a larger inlet, 40.5mm.  If you go any larger, it requires rather careful set-up in regard to valve-valve clearance at overlap, difficult with stock length valves with decent thickness shims under C4 or similar cams. 

Factory used several diffent combos over the years, must be a good reason for reverting to a smaller exhaust valve size with the introduction of the 120s.  My personal favourite is the SFC 34/40.5mm pairing, works well enough with pretty much every normally available cam for a road engine.

If you want to sooth your conscience, you can open up the exhaust ports to match the headers.  Will keep you from causing any other mischief for a couple of afternoons...

When I wished to purchase "Jota" valves many years ago, Cico advised me to use only a larger inlet valve and keep the stock exhaust, he claimed the works endurance bikes were running 34/41 at the time.

piet

 

[motoddrob]

Red, Piet,
In reality this head will be fitted as is with the original valves it came with. I had the inlets refaced and hand lapped all 6 valves. I don't have any more money to spend on the bike till after my house is finished so it is what it is. I was just surprised to see the later head with the smaller ex port, must have been a reason to reduce the size.
Hopefully in a couple of years I'll send the head to you Red for ceramics, valves and porting etc. In the mean time I need to get the Motodd back on the road with no more costs involved.