Subject: Intake manifold
From: Rotary Engine
Date: 10/8/2007, 4:19 PM
To: AARotary Engine



     >> Paul
     >>
     >> I noticed many intake designs have constant diameter intake
    pipes while
     >> Guru Racing and a few others taper theirs down to the intake port.
     >>
     >> I will make my intake manifold out of carbon or some other heat
     >> resistant
     >> glass epoxy GRP so I am free at the moment as to the course and
     >> profile they
     >> will be.
     >>
     >> I thought to go from 60mm id at the intake plenum with the
    adjustable
     >> length
     >> portion being able to extend another 120mm within the intake intake
     >> plenum.
     >> From the plenum to the slide throttle, a distance of about 320mm I
     >> would
     >> like to taper the tubes down to 50mm gradually over their length.
     >>
     >> Injectors are right after the slide throttle so fuel mist
    separation
     >> is not
     >> an issue. Other than being a pain in the butt to construct the
     >> tapered pipe
     >> are their any disadvantages or advantages to such a design?
     >>
     >>
     >> Thanks
     >>
     >> Doug
     >>
     >> It is mentioned in:
     >> Scientific Design of Exhaust and Intake Systems. Philip H. Smith
     >> and John C. Morrison Published by Robert Bentley. ISBN 0-8376-0309-9
     >>
     >> http://www.rb.com
     >>
     >>
     >> --
     >> Paul Lamar ...No rotor no motor.
     >
     > Doug,
     > Concerning information given to me on inlet manifolds.
     > My understanding is that any reduction (such as a Venturi) in an
    inlet
     > is a
     > restriction. However if a small restriction increases velocity
    without
     > choking the flow, it has an advantage for VE.
     > Alternatively any increase in size (especially toward the engine
    end) the
     > flow will slow and will be a disadvantage.
     >
     > I'm advised that THE OPTIMUM SHAPE is a megaphone shape starting
     > larger at
     > the outmost end and ending at the port, in a even taper. The air
    flow
     > starts
     > out larger than the port, builds pressure and speed, but like you
     > said, the
     > hard part is building the darn thing!.
     >
     > Probably the same reason why those tapered exhausts work so well in 2
     > strokes .
     > George ( down under)
     >
     >
     > Do you have any mathematical theory to support this contention or is
     > it an
     > opinion? If it is an opinion do you have (or the person that
    expressed
     > their opinion
     > to you) have any empirical data to support their contention.
     >
     >
     > Paul Lamar ...No rotor no motor.

    Hi Guys;
    Attached is a pic a local fellow sent some time ago detailing a
    comparison of various intakes.
    I would love to know the source - unfortunately he could not find it...
    I'm guessing the front top surface of any laminar flow airfoil revolved
    into a body of revolution like a bellmouth
    would be beneficial in terms of drag reduction by delaying the
    transition point from laminar to turbulent (then separation)
    with favourable pressure gradients. 2D airfoil software (such as xfoil)
    shows this. 3d multi-element or CFD results would be nice :)

  I have heard of performance increase claims in the 15-25% range -and
    many rules of thumb for tapered gradients.
    Check out something like the following for claims of big results:
    http://www.bpinitiatives.com/products.html
    There is no question you can reduce drag but the big question is how
    much turbulence results in optimal power
    and at what range of intake velocities? as is being done with some of
    the latest turbulent flow intakes?

    Fuel routing is greatly simplified if all the injectors are in a closely
    spaced row nearer the housings.
    I vote for the adjustable length intake for maximum flexibility with
    varying ambient conditions and just in case our exact
    calculations and reverse engineering don't match reality :)
    Cheers
    Cary

    That is really interesting Cary. Thanks for the data. We knew the
    bell mouth was
    worth about 5%.
    --
    Paul Lamar ...No rotor no motor.


The real effect that a "bellmouth" has is to act like an effectively
longer pipe. It doesnt really "make for faster flow" because the vacumn
signal at the source, i.e. port, is a function of VE. The "bellmouth"
allows for a more rapid transition to laminar flow in a shorter distance
which is the same as having a longer pipe allowing for the Re number to
drop until is it turns into fully developed flow. You can get the same
results but having a long pipe but this is limited due to space and
acoustics.

The other more important thing is the acoustics more so that
anything. You need to tune your lengths based on wave theory (helmholtz,
impedence modelling) since acoustics will actually spit fuel air back
out if you are not careful. Go look at the video on youtube.com of the
Renault F1 v10 running on the dyno and you will see the vapor getting
spit right back out of the "bellmouths" due to acoustics forming an
almost inpenetrable layer. Just simply adding a bellmouth to something
may end up hurting you if you are starting to fall off of ou tuning
peak. It would be real interesting to see someone run the numbers on a
rotary running at the 6000rpm you guys are always talking about. I have
a SAE paper that shows how to calc impedence methods and a simple way of
doing a realworld test with microphones and a manifold.


Rob Woods.



You need to read the web site Rob: http://www.rotaryeng.net/intake.html

The spit back is common to all tuned manifolds I have seen.

That is one reason why I designed this air box for this car.
If you have a picture of an earlier race car with this feature please
send it to me. I think it was a first.

Paul Lamar

Rob,
I don't know if I can agree with all your statements, however I agree with
harmonics are at play, as you mention.
To my knowledge a sharp edge will cause turbulence entering the manifold
tube, to eliminate this the rounded edge is used. This provides smoother
flow into the tube - the further the rounded edge is as it returns back to
the outer tube wall the smoother the air will access the inlet tube without
turbulence. The better entrances will have at least 180 degree rounded
edge - the more the better in my opinion.

As mentioned previously a tapered inlet (megaphone type) is the better
option and I notice is Cary's document this is proven to be the case.

My personal option is that the primary advantage is better entrance flow
minizing turbulence and restriction.
George (down under)



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