C15S Valve Timing

[C15S]

1962 C15S.  The timing marks on the cam and drive gear are correctly positioned as shown on Pg 74 of Ruperts.  The timing mark on the drive gear is partially covered by the tab washer, but I think it is correct.  This is further verified by the fact that both pushrods are at equal height when the mark on the cam gear is 180 out.  I placed a degree wheel on the rotor and verified the inlet valve just starts to open at 51 deg BTDC - and this, to me, makes no sense.  Ruperts, Pg 224, also states the inlet valve should start to open at 51 deg BTDC.  How is it the inlet valve is coming open while the piston is still coming up??  I have even noticed there is very small mist of fuel being blow OUT the carb when I try to kick start. 

[DAVE BRADY]

Hi,

I find it best not o think about the nuances fluid dynamics and combustion flame front velocities and trust that BSA new what they were doing and focus on ign. timing and carb.
It also does not seem to make sense that the inlet is probably  just starting to open whilst the exhaust valve is not quite closed.

Dave.

[JulianS]

The valve timing on the C15S is the same as used successfully on many BSA singles and twins right up to the end of production. On the twins they called it the Spitfire.

The inlet and exhaust timings work together and the overlap helps scavenge the exhaust and induce the fresh charge into the cylinder.

[C15S]

Got it!  Thanks to both of you for the info.  Makes sense now.

[Steve.S]

I found your original post very interesting, and agreed with your comments.
I had never heard of IVO at 51 degrees for a non race engine, but I have read that in the 60's full race car engines could have as much as 60 degrees. But they probably wouldn't run much below 3000 rpm and you wouldn't want to drive these around town. I know that motorcycle engines were always sportier than car engines, but even some Ferrari engines use only 40 degrees.
However, BSA certainly knew far more than I ever will, and Service Sheet 428 shows the following:

C15 26 degrees
C15T 41.5 degrees
C15S 41.5 degrees
C15SS 51 degrees

But after engine No. C15T 1251 and C15S 2112 they both changed to 51 degrees, so I doubt if it's a typo.

Like you, I find it difficult to believe, but what do I know?
Finally, you say that you can't see the timing mark on the half time pinion, but I've always found that on a single cam engine the mark is at 12 o'clock.

 

[Derek996]

FYI. BSA Gold Star Clubmans cams have an inlet opening of 65º BTDC, the Tourer uses 43º BTDC but the trials and B31/33 are only 25º BTDC. So 51º is not so radical.

[scifi]

It might be opening at 51 BTDC, but it is not VERY open until way past TDC.  The exhaust will only fully close at about 50 ATDC. 

The inlet period for lawnmower and stationary engines is only 180 degrees.  But our engines have 180+50+50 = 280, to get more power per cc.
This lengthening of the Inlet open period is to reduce the cam loads on the valve-train at high rpms, and ensure the inlet valve is quite a bit open for all the downstroke inlet period.