[Fot] FW: TR6 Cannon Intake Manifold

[Marcel Van Mulders]

Thanks Keith and Michael. About valve springs and seat pressure : it's a
question about
keeping the valve train parts together but also to avoid that the valves
bounce off the seats after (the first) landing. I suppose this valve bounce
is not so clearly reflected in the power  dropping at high revs, . On
youtube are very interesting high speed cam video's on valve bounce, spring
surge...

  _____  

Van: Michael Porter [mailto:mdporter at dfn.com] 
Verzonden: woensdag 16 september 2015 10:57
Aan: Van Mulders Marcel
Onderwerp: Re: [Fot] FW: TR6 Cannon Intake Manifold


On 9/16/2015 2:18 AM, Van Mulders Marcel wrote:


Thanks to the Fot members for their reactions : most are warnings that
triple 45 Webers may be too big on a TR6 engine. But it is a race engine
(2720cc) and the owner has already bought 3 good secondhand 45 Webers with
identical progression holes : 36mm main venturi's seem  to be a reasonable
starting point in this case? If necessary, he can go down to 34mm in 45
Webers.
 I've seen TR6 racecars with 45 Webers and I wonder if the Cannon manifold,
with 40mm diameter bores on the carburetor side, can be reamed to 45mm? 
Another question : the valve seat pressure on my TR4A racecar is 35 kgs/77
lbs. I've also a cylinderhead with only 25 kgs/55lbs seat pressure. The
springs on both cylinderheads are 5.25kg/mm (290lbs/").
Do you think 25kgs seat pressure is enough?



On fitting 45s to the 40DCOE manifold, yes, I think there's enough meat to
match the manifold to the carburetor exit.  It would mean increasing the
radius by 2.5mm (~ 0.100"), but I wouldn't do it until 45mm soft mounts can
be obtained--if only 40mm are available, then it's a waste of time and
money.  I would recommend that the shop doing the machining find a suitable
diameter end mill with a 10-15 deg. taper, just to minimize the finish work
required and to make a smooth transition to the smaller manifold bore.

I don't actually understand the complaints about 45DCOEs being too big for a
moderately-sized six-cylinder.  For all-around use on the street, 40s would
be more than enough, but on the track, 45s are fine.  In fact, I recall 45s
with 38mm main venturis being used commonly, but mostly on engines tuned to
7200 rpm or so.  The advantage of Webers is that they have lots of spares to
tune to carburetor to the application, although the expense of having such
spares on hand for tuning is rarely part of the equation.

As for valve seat pressures, they're not as important as having just enough
spring pressure to keep the follower tracking the cam profile.  For
normally-aspirated engines, anyway (seat pressures become more of an issue
with supercharged engines, when too much boost can push a valve off its seat
if the static spring pressure is too low).  The lower the combined weight of
the valve train, the lower the spring pressure required.  The steeper the
ramps on the cam, the greater the spring pressure required to overcome the
acceleration of the valve train parts.  So, it's kind of a balancing act,
and one that can usually only be handled on the dyno, while looking for
power drops at high rpm due to valve float.


Cheers.

-- 





Michael Porter

Roswell, NM





Never let anyone drive you crazy when you know it's within walking
distance....
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