[Fot] Six cylinder cams--some real facts

[Enquiries Road & Track]

there seems to be very little factual info in the recent thread and i'm
hoping to fix this by starting this new thread and challenge other racers
to share some some real info

i previously offered the comment that cams cant be considered in isolation
and a race engine is quite different to a street engine, with few practical
compromises

it would probably help everyone if interested people listed what 6 cylinder
cams they have used/are using along with a few other comments and what
their thoughts are. of course, every cam grinder/vendor espouses the
virtues of their own product, but only on forums like this can we get any
real world experiences.

I'll start with some experiences gleaned over 40 years with these engines...

the cam lobes are rarely perfectly aligned with the lifter bores and vice
versa. triumph 6 cylinder cam lobes are meant to be perfectly flat and so
are the lifters. the rotation of the lifters comes from offsetting the
lobes relative to the lifter centre. if you test this during disassembly/
assembly using engineers blue on your lifters,  you will see what is going
on and probably be horrified as to how bad some are. experiences with
flogged out lifters and/or lobes often point to problems here. i have had
occasions to move the cam to correct this (by changing the thrust plate
fractionally), but its a trade-off and brings about other issues. some of
the the turkish made cam blanks i've seen have poor control over lobe
separation and the cam grinder cant correct this.

the factory cam grind  can be identified by the number of rings machined
into the front bearing. when we could get factory blanks out the back door,
they had no rings on them, but neither did the early 2000 cams. from
sometime in mid70's, the cam 'stock" got thicker which is a good thing.
early 'thin" cams (Mk1 GT6 etc) cannot be modified very much as the base
circle encroaches on the stock . these days, radically welded up cams to
fix this shortcoming are rare, so the problem has basically gone away by
using new blanks.

the oiling flats machined into the front and rear journals are perilously
close to the end plates and some cam makers stuff this up. anyone
assembling the engine should check this lest they end up with unexpectedly
low oil pressure. a real problem if you have switched to a slightly thinner
alloy front plate.

most modified 6 cylinder heads stop flowing at about 0.490" valve lift (on
inlet) so there is not much point in a cam/rocker system that opens the
valve more then is needed. To choose a real 'race" cam, you really need
this head flow info for your engine. without it you are just guessing.

most race spec TR cams seem to rely on long durations, typically 300
degrees plus (this is valve motion, not 050" duration which is more like
245 degrees).  if you have a factory/Lucas  PI engine, then cams like this
are still quite street drivable with the injection properly set up. The
factory TR5/6 PI cam is a struggle on carbies, bordering on horrible,
because it produce so little vacuum at idle.

The factory TR5/TR6 PI cam has valve duration of 280 degrees and it is
"mild". This cam grind, which is commonly copied, is great on any PI street
engine, especially if the compression is raised as far as you can get it
with the fuel available to you. On 98RON fuel, i have run these at 12;1
with conventional (but modern) cast pistons. distributor re-curving is a
must

The factory 2500PI  cam has valve duration of 260 degrees and works just
fine with carbies. we used to fit these to US imported TR6's and raise the
compression at same time get another 10-15HP.

it is not possible to grind a modern aggressive long duration, high lift
cam, on any original TR cam without an awful lot of welding, which
typically bends the cam to a point of useless. it is hard to even find
blanks that will do this , but they are out there.

these modern design, aggressive  race cams can't be run with a conventional
triumph lifter. the lifter diameter needs to increase about 040" as a
minimum to stop what is known as edge riding. The cam acceleration rate
(how fast it can lift or close) is mostly governed by lifter diameter.
oversized lifters require block modification which is not hard.

For a race cam, the lobe will look like a house brick with some rounded
edges. anything that looks like your original cam lobe, probably wont cut
it in a race engine.

I have run hundreds of modified 6 cyl engines without cam bearings and with
zero issues. having said that, the latest cam grinds will benefit from the
added support a block with bearings will give. also, if done properly, this
should improve oil pressure by reducing losses from cam journals.

A recent 2500 engine for a mostly track car............
98RON fuel, 12:1 comp, long exhaust primaries, highly modified head (intake
flows 150cfm at 25"@0.550" lift)
cam 308 intake valve duration, 250@ 050". max  lift on intake 0.495"
comment. sounds fantastic & pulls like an ox all the way to 6750 (imposed)
limit. almost too lazy. for a race engine. currently being changed to more
aggressive cam


Terry
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