Intake Runner Air Velocity Question

[average guy]

looking at the 2 intake ports in head and
i'm looking at 2 bbl intake runners.
i would say if i blew foam into runner and
took it out the cross section would be
smaller than pair of ports at the cylinder head.
questions: does this speed up the air?
if i keep the runner 'full size' up to the
carb base plate will it be a 'dog'?
do i want a smaller runner?

have a nice day!

 

[mender]

looking at the 2 intake ports in head and
i'm looking at 2 bbl intake runners.
i would say if i blew foam into runner and
took it out the cross section would be
smaller than pair of ports at the cylinder head.
questions: does this speed up the air?
if i keep the runner 'full size' up to the
carb base plate will it be a 'dog'?
do i want a smaller runner?

have a nice day!

Tough question without seeing the ports; do you have casting numbers of the intake and heads or at least the application that they came from? What's your planned usage?

Typically 2 bbl intakes are about economy of production and little else, so I wouldn't use what you see as any indication. That being said, a smaller runner will give a higher velocity for a given volume of air passing through.

Whether that engine will benefit more from higher velocity than an increase in flow from a larger cross section is the question. As usual it's a package deal.

 

[average guy]

mender
that's a good take on the situation
'Whether that engine will benefit more from higher velocity than an increase in flow from a larger cross section is the question.'
their small valve heads and rest of engine stock.
'a torque engine'. so full size runners would probably work against
the way it was designed.
i think save full size runners for another project.
thanks for smacking me in the face.

 

[mender]

Any time.

If you want to practice your porting skills you can do a quick bowl clean-up and blend any obvious casting issues; the more air going through the motor at any given rpm, the more torque it will make so your time won't be wasted.

 

[average guy]

mender
does that statement point back to 'full size' intake runners.

'the more air going through the motor at any given rpm, the more torque it will make'

intuitively i say yes and yes to 'stacks' exhaust ( no restriction at all )
on the intake stroke each cylinder is a 'pump' and
i think there is plenty of 'draw' there.

Have A Nice Day!

 

[mender]

Generally speaking an engine will respond better to a small efficient port than a larger "normal" port with the same flow, so smoothing and blending without hogging is the best path to power. Usually the runner sizes are not too far off what would be appropriate but the execution can leave a lot to be desired, especially where the runners join the plenum.

http://www.castheads.com/view.php

On the exhaust, a header or manifold that has long tubes of equal length that flow smoothly into a collector before having to push against atmospheric pressure works better than stacks.

There are "restrictor" racing classes that require 2 bbl intakes and carbs, and the engines can develop a surprising amount of power; what are your plans for this engine?

 

[average guy]

mender
cfm is in minutes
rpm is in minutes
it takes 2 rpms to fill each cylinder
on intake stroke
5000/2 rpm = 2500 rpm
292/4 = 73 cu in
want to use 350 cfm carbs
probably one per pair of cylinders/intake ports

now I'm stuck

got unstuck a little.how many cubic inches in a cubic foot? 144? no that would be square inches.
1 cubic foot = 1728 cubic inches-wikipedia

 

[mender]

It'd be a lot easier if you just tell me what you have for an engine and what you're wanting to do with it; chances are pretty good that I've built something close to it already and can give you specific recommendations.

 

[average guy]

mender
'the easy road gets harder and the hard road gets easier'
the formula you see around is:
CFM = (RPM x CID) / 3456.
they just leave out the 2 revolutions to complete
four cycles.
answer is: 5000 x 73 /3456 = 105.61342 CFM
so a 350 CFM carb on each pair of intake ports
is a little too much carb.
more like one 350 CFM carb per pair of pairs.
you know the displacement here's another hint
intake ports are 'stacked' one over the other.

Have A Nice Day!

 

[mender]

Like I said, chances are good that I've done one, even an old brute like a Y-block Ford 292 (no I didn't peek). I might still have the dyno results from the 312 I built for a guy's '57 T-Bird about a decade ago.

 

[average guy]

mender
my contribution to the little following of this engine
is to come up with intake setup.
the clean way to build this is four independent manifolds.
MAYBE build each side onto a bolt down plate later.
left head & right head intake ports are not straight across from each other.
anyhow I'm going to build one manifold for pair of ports and
that's it for me.
i'll let the enthusiast's cast up set's of four somewhere.
i sure hope there's 1 bbl 150-200 CFM out there that's reasonable.

Have A Nice Day!

 

[mender]

Individual runner manifolds (IR intakes) have a different carb requirement than a plenum style intake.

With a plenum style manifold (virtually all domestic intakes) the carb has a more or less continual draw into the plenum, which acts as a buffer or reservoir of air/fuel mixture. With IR, the carb has to be able to supply a cylinder worth of air during the intake stroke without unduly restricting the flow. That means that the usual rule of thumb for carb sizing is way too small.

In short, four 350 cfm carbs will actually be pretty close to what the engine will need for your planned intake arrangement. I'll do some checking and comparisons to the IR equipped engines I've built. In the meantime, have a look at the cfm ratings of side-draft webers on IR systems for 2 to 2.5 litre engines; that'll be pretty close to what you need.

 

[average guy]

mender
good.did find Autolite 1100 in smaller CFM.
have 2100 2 BBL already so that's a real good thing
for mocking up this intake.
the situation is pair of intakes are basically 3" x 3" and
the carb pad is 4" x 6" .
i'm up for just putting rectangular box on top
of 3 x 3 runner.
i do want to keep runner 3 x 3 all the way.
NO bologna slicing it because the cross section area will
get all messed up.
so it will need to have a CURVED runner.

Have A Nice Day!

 

[turbo]

Do you want max intake velocity? Then do not polish the intakes. You need a rough enough surface to provide a small layer of microturbulence so that the mass of the flow zips through as easily as possible. Taught to me by a very successful drag-racer over 25 years ago. You can hog out the intakes and enlarge them, but resist the temptation to make the interiors really smooth. That will result in laminar flow at the air/fuel-intake boundary and cause drag. Just a thought.

 

[average guy]

turbo
thank you for thinking about airflow in cylinder heads.
having a problem with bend in manifold.
the heads are at generally a 45 degree angle.
the carb obviously sit level.
if i keep the cross section equal the length of the manifold.
then put a 45 degree sweep in it.
i think the air/fuel mixture on inside of the curve is
getting there first.it has less distance to travel.
any solution to this?

Have A Nice Day!

 

[mender]

No solution needed; for what you're doing there won't be a problem.

the situation is pair of intakes are basically 3" x 3" and
the carb pad is 4" x 6" .
i'm up for just putting rectangular box on top
of 3 x 3 runner.
i do want to keep runner 3 x 3 all the way.
NO bologna slicing it because the cross section area will
get all messed up.
so it will need to have a CURVED runner.

If you make a small plenum under the carb that blends into the intake runners you won't need to have a curved runner if I'm remembering how the top of the intakes look. For carbs I'd check the Rochester 2G or even the Weber 32/36 (primary and secondary throats for better street use) unless you need to use a particular carb. If you want to do a Stromberg look-a-like but with EFI you could use single throat throttle bodies from the Chrysler Intrepid and hide the injectors. Just a thought.

Turbo, hogging/enlarging the ports is what drops the intake velocity; surface finish will have a very minor effect on flow and the rough surface is mostly to get the raw fuel off the walls and back into suspension.

 

[average guy]

mender
1 i mean like an oval track field.you know they put the
start and finish at different places for each lane.
keeping the runner 3 x 3 and curving it.the inside of
the curve is less distance than the outside.
but i'll pass that up for now.
2 i just had an evil thought.
i've been paying attention to Autolite 2100 bolt pattern
and carb pad which i thought would be @4 x 6.
i'm going to go measure the two VENTURI'S .
maybe they'll fit right on 3 x 3 runner.
3 you said runners, you know this is individual manifold for
each pair of intakes, right?

Have A Nice Day!

 

[mender]

So it sounds like you're having a mini-plenum serving each pair of intake ports, right? That reduces the size of carb needed and more or less eliminates the need for a curved runner; just make sure there are no abrupt turns or edges in the air path.

 

[average guy]

mender
here's the venturi size/CFM chart for Autolite 2100
.98=190
1.01=240
1.02=245
1.08=287
1.14=300
1.21=351
1.23=356
1.33=424
which CFM size for each manifold?
formula for 2 cylinders 73 cubic inches says 105 CFM.
then Holley says go up 50%.
but holleys talking about 4 bbl on top of regular manifold.

Have A Nice Day!

 

[mender]

So it sounds like you're having a mini-plenum serving each pair of intake ports, right?

If so, pick the cheapest carb that flows between 245 and 351.

Also, there is quite a difference in how a 2 bbl and a 4 bbl are rated; 2 bbl carbs at measured at a higher pressure differential so their numbers are quite a bit higher for the same size venturi. Translation: you can't size a 2 bbl carb using the 4 bbl formula.

 

[average guy]

mender
right.
have stock 351 CFM it came with.
just need 3 more.
how is the curve from the level carb to the 45 degree head
face going to work?
it is HVAC ducting or raingutter shaped.
the air/fuel mixture on inside of curve will have shorter path.
your saying don't worry about it?

Have A Nice Day!

 

[mender]

Right, don't worry about it; that small a variation will make no measurable difference.

By adding a central divider in your mini-manifold you can make it a true individual runner set-up. That would boost the torque output slightly by lengthening the intake tract several inches and will work a bit better with the 350 cfm carbs. I'm assuming you'll be fabbing the prototype and the production pieces will be sand cast?

 

[average guy]

mender
i'm just making the 'plate and heliarc' mule or prototype.
i'll let the young guns cast them up.
can i weasle out of the plenum divider?
there is a deal with the intake bolts.
i'm sure you've seen lots of heads and intake gaskets.
the OUTBOARD boltholes are where you'd expect them.
the inboard holes are about 1 1/2" from intake ports.
note: i think i can use this 350 CFM for pattern.
point is without plenum divider what is carb CFM
recommendation now?

Have A Nice Day!

 

[mender]

The 250 would likely work better but the 350 should be good enough for testing.

Speaking of testing, just to see what the effect is you could make a base plate that fits under the carb that has a "tongue" divider if you don't want to put it in the manifold.

 

[average guy]

mender
here's Microsoft Paint .png file of FRONT VIEW of manifold.
( ignore little gap in 45 degree line )
3" face of intake ports matches 3" width of runner.
somebody could contour slanted wall of runner with epoxy
if they were motivated.
it just needs the 1" box/carb pad on top and
the plate at bottom to bolt plate to head.
i'm hoping with 4 of these manifolds and
4 Autolite carbs ( facing forward not sideways )
that the horsepower will go from 180 to 185 on the dyno.

Have A Nice Day!

 

[mender]

I'm going by this picture:
http://www.trustmymechanic.com/automotive/images/tmmfor292lb.jpg

What are you doing about the exhaust crossover and the thermostat housing/water passages? How much room do you need for the carbs and linkage wrt the valve covers?

 

[average guy]

i'm building something like attachment.
the cooling crossover can be plate and tube
for hoses to connect to.
in the middle it needs a tee. water from heads coming
in sides and then out the front for large hose.
needs to be box with room for thermostat.
lip for thermo can be machined in tee or pipe piece that
goes over top of thermo.
heat riser crossovers can be left open, there just stoves.
there's plenty of room for carbs. manifolds and carbs go straight up and
heads & valve covers are heading away from them at 45 degrees.
there's also the @ 1" box between manifold in drawing and carb. it serves as carb pad
and transition from 3 x 3 manifold to @ 4 x 6 carb pad. that can be blended
inside with epoxy too.

Have A Nice Day!

 

[mender]

If you make the side walls from 1/8 sheet you should be able to form them so that the air path is curved and fairly smooth and include the centre divider without too much trouble - still don't see the issue with that re: the bolt holes. Do you have a better picture of the bolt pattern? I'd set up all four carbs on a plate across the valve covers to make sure of the spacing and aesthetics then jig the carb base plates where you want them over the intake port plates and just fill in the space. Don't know what your plan is.

I can see using epoxy to get a good finish for the plug, and you might build in some taper and allow for shrinkage.

 

[average guy]

mender
here's pic of factory heads.
that picture you have i don't know what they are.
see the inboard bolts with a stud or two in them?
there a good inch or more from faced casting around intake ports.
no big deal, just a bigger intake flange.
you brought up smoothing contours in manifolds.
that's going to be a 4 x 6 x 1 box on top of 3 x 3 runner.
that first one inch ought to be blended into 3 x 3 WITH EPOXY.

Have A Nice Day!

 

[Fahlin Racing]

Those heads are pretty wild IMO, never seen horizontal ports before.