[Fot] re TR6 Cannon Intake Manifold

[Enquiries Road & Track]

re valve seat pressures

valve seat pressures are normally considered as a pair, seat pressure and
nose pressure. more often than not, the cam grinder suggests what "over the
nose" pressure is probably needed and you then work backwards and find the
seat pressure.

you can make small alterations by shimming the bottom of the valve springs.
some brands of springs used to come with suggested shim thicknesses, but in
our engine shop we have a spring rate gauge to work this out

all of this needs some experimentation and definitely on a cam-by-cam
basis. generalist advice is often not helpful as it might seem

spring pressure is not the only consideration for valve float at high revs
so dont become fixated on this. valve train weight, spring interference
(inner to outer) and cam lobe ramp design, are all equally critical issues.

in days of old, very high pressures were used , often because the people
involved had no other method of controlling valve bounce at high revs.
today, analysis of working springs using a strobe light and some times high
speed video, ultra light weight valve gear, controlled inner/outer spring
interference, all combine to allow spring rates to fall dramatically on a
modern, highly accurately ground cam

on a TR6 with the stock "heavy" valves, pushrods & lifters, and an old
design cam something like 250lb over the nose is likely to be needed on  a
wild race cam.

all of my engines run the lowest possible pressures and the lightest
possible valve train. we have got nose pressure down to 150lb on several
TR6  race engines

Terry O'Beirne
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