Tech Bulletin

[Dick Roberts (Web admin)]

Posted by Neil Tech Sec Thompson on July 16, 2004 at 13:28:55:

All competitors are requested to read carefully the following bulletin on behalf of the 2CV Racing Club Technical Committee

If you wish to offer feedback please do so to me (Christian Callander) and Neil Thompson (Chairman of the Tech Committee). Do not reply to all.

Investigation of Engine Failures at Snetterton.
-Available data suggests the most likely explanation for valve failures and contact between piston & valve is 'Valve Bounce.'

-The revised cam profile results in increased valve acceleration. If the spring is not able to apply sufficient force to close the valve and keep the follower in contact with the cam, the closure of the valve will be uncontrolled. The resultant impact increases the stress applied to valve seat and valve, and initiates other mechanisms of failure the conclusion of which is contact between valve and piston.

-Measurement of valve springs has identified softening and variability in used springs.

-Calculation performed by Kent Cams suggests valve bounce is possible when spring rates are low.

-Consultation with Kent Cams identifies failure mechanisms affected by cylinder head temperature. The most frequent failures were of the right side - which is the hotter - which is consistent with the theory.

-Competitors have encountered symptoms consistent with Valve Bounce on engine test and fixed the problem by replacing springs.

-A similar failure occurred at Oulton Park prior to Snetterton. Initially thought to be an isolated event, this was not brought to the attention of the tech committee until after Snetterton.

-The technical committee recommend that all competitors using the Kent '04 cam check or replace inlet valve springs.

-If competitors do not have the facility to test springs they should replace used parts with new standard springs. New standard valve springs should be suffcient but should be monitored and replaced periodically as necessary.

-Fitting double valve springs is a permitted modification. New double springs is the most robust solution and competitors are recommended to follow this course if they do not intend monitoring or replacing springs.