3 1/2 Derby Camshafts

Colin Gibson · Posted: Sat Mar 09, 2019 2:42 pm Post subject: 3 1/2 Derby Camshafts

Does anyone have experience with camshaft design that might improve the 3 1/2 litre engine performance in terms of power output? I have to replace my original camshaft due to extensive corrosion pitting on the cam lobes and base circles and was wondering if there are any modifications in terms of lift, dwell and overlap that could be designed into the new cam that could provide a fast road performance cam? Is the re-entry ramp profile desirable as few cars seem to employ this design today, so I assume there was some reason that this profile went out of favour? I have been told that the 3 1/2 engine responds well to increased compression ratio with the use of a raised oval piston crown design which fits into the bathtub in the head. I am contemplating using this type of piston also, which would limit the amount of increase in lift on the new cam. Any information or experience would be much appreciated.

Christopher Carnley · Posted: Sat Mar 09, 2019 5:04 pm Post subject:

Hi Colin,
You will notice that the follower tappets are actually in the form of ball races, hence the pear shape of the cam lobes. For its purpose the cam design is ideal.
Believe it or believe it not, the average power output of the 3 1/2 litre engine fitted with air cleaner and full exhaust is 85BHP.at 3600 RPM
Removal of the air cleaner/silencer and exhaust raises the output to about 105 BHP.
The published figures of 110-115BHP are spurious.

The 4 1/4 litre cars struggled to achieve 90BHP.

By increasing the diameter of the exhaust system to 2 1/2" removing the front box, the engine is not so constipated, but the 3 port exhaust restricts power and development. Increasing the diameter of the exhaust valve also helps breathing.
As the combustion chambers will be turbulated these require machining before high compression pistons may be used, as in the "one I did earlier" which raises the c.r to 8.5:1

Camshafts based on the one in the R-R "R" 12 cylinder aero engine have been tried experimentally, but problems with valve spring breakage were met.

Colin Gibson · Posted: Wed Mar 13, 2019 7:20 pm Post subject:

Chris,
Thank you for your comprehensive and detailed reply to my question.
It seems that the new alloy head we have bought has the turbulating lip on the lower edge of the combustion chambers and that the high compression ratio pistons are not currently available from stock and are not on order from the manufacturer. As a result we have elected to rebuild the engine using the standard flat top pistons with a standard 3 1/2 litre camshaft. The car is only intended for touring work, so I think we can live with the original design that was used by RRB when the car was built in 1936.
Best Regards
Colin