Roots 14/1671 supercharger bypass/blow off valve
Application:Offshore race/pleasure boat,BBC,800-1300HP @6-7000 RPM,engines are well built w/details available.
Problem:As
these engines are used for pleasure, 92-93 octane is used. Due to lack
of preferred inner cooler design,no inner cooler is used and engines are
over engineered in other areas to control detination.However,throttle
settings are constantly changed which is beleived to add heat,flex
rotors and encourage detination.
Solution:Find method of installing
pressure relief as throttle plates are closing/closed, w/o destroying
distribution. Method considered is either going through the burst panel
or finding best location in plenum tent, plumbing from tent to bored
hole next to distributor to bypass valving to throttle body base plate,
using 1.25" tubing.
Suggestions for the Newbee ?
Those are pretty low pressure drops, indicating a comparatively large
valve in both cases. Unless your oil is VERY viscous, you very likely
don't have to worry about laminar flow.
Which one to use? I
don't know anything about your application other than what you've told
me, so can't advise you. What are you trying to do? The rule of thumb
is that a control valve (manual or automatic) should represent at least
30% of the frictional pressure drop in the piping system such that it's
the controlling resistance even when open. Like all rules of thumb, it
can be broken successfully by those skilled in the art when necessary.
A well matched turbo can get very quick response with greater durability
at the same sustained power output because you don't have to generate
the extra cylinder pressure to produce all the power lost in driving a
supercharger.
In my experience boats rarely require full throttle
from a standing idle. they are normally at least at a fast idle when
you nail it.
Most of my experience comes from circuit racing and ski racing.
With
fully submerged propellers, I normally put the water scoop just behind
the propeller and that way protect it to some degree from debris If you
run legs that is probably not an option and a presume your pick up is
built into the leg.
I run all water through several less critical
to thermal shck devices before it goes into the block so as to preheat
it somewhat.
I size the system to run at a maximum of 80 deg C under the most severe stable conditions.
I
size the discharge side slightly smaller than the inlet so as to build
some pressure and thereby avoid possible localised boiling ans build up
of salt deposits.
I check water pressure in the head and never
allow it to go over about 100 psi as it is then to inclined to leak past
the gaskets and enter cylinders or the sump.
I would not pull to
much vacuum on the crankcase on an endurance engine as it lowers the
boiling point of the engine oil and can damage gudgeon pins. Enough to
stop leaks and to eat up blowby, but not enough to suck in seals or
gaskets and let the oil on the bottom side of the piston crown and the
pin boil or lose it's lubrication capacity. It does help clean fuel and
water contamination from the oil. Maybe 5 psi vacuum.
A dry sump
might be value at the cost/benefit range you seem to explore. Even a
single stage scavenge and the OEM pressure pump system can be a good
gain. A 5 stage system costs a truckload more and gains only a little
extra over a 3 stage.
I think best value is one pick up from the
back corner (that is back of the boat) of the sump on the side the oil
throws toward and one from the valley with all other returns from the
valley blocked off is the best value.
With a positive
displacement blower like a roots, you do not need high compression for
response as the instantaneous boost builds effective compression in an
instant.
With a centrifugal pump like a turbo, you do need reasonable compression to get response before boost builds.
With
a twin charger, you can run low compression, moderate boost from a slow
turning blower then a truckload of boost from a real big turbo that
would otherwise be to laggy, but you do increase cost and weight and
complexity.
In a boat, you have the option of a really efficient inter cooler due to the infinite supply of cold water.
Rod
to stroke does not have any significant impact so long as it is not out
to lunch. The difference between 1.7 or 1.8:1 will not matter, but 1.4
to 1.5 certainly will matter.
A good oil system that cools the
oil well and controls surge and windage is critical. Oil squirters to
cool pistons certainly helps high compression, high boost motors so long
as you can keep the oil cool.
Except for a stable slow idle and
somewhere to hang the ring gear, a high speed power boat does not need a
flywheel. I use aluminium or a flex plate. The flex plates tend to bend
from bendix drive engagement if the ring gear gets rusty, so that
depends on amount of use, conditions of storage and maintenance.
For
this sort of application, I would use a blower drive hub with inbuilt
torsional vibration damper (most commonly called a harmonic balancer).
It would be steel, a tight interference fit on the crank and have two
1/4" keys or one 5/16" key, carefully hand fitted to a gentle tap fit in
the crank and the hub. It would also be as long as possible to maximise
bearing area in the crank and hub.
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