Dyno tuning with different SUs and velocity stacks
We were recently doing some engine dyno testing of a 3/4 race "fast
engine destined for a Morgan. We had what I thought were some
The engine is an 87 mm bore Hepolite piston engine with standard
valves, 10:1 CR, 280 degree seat to seat cam (similar to the
D" cam), a "pocket ported" head with a 3 angle valve job. The
done with both HS-6 carburetors on a long manifold (these carbs and manifold
were dead stock and in perfect shape with the float bowls mounted on rubber
using MG-B mounting insulators) and with a fully race prepared set of H-6
carbs on a race prepared short inlet manifold. The engine had "Tri-Y" style
headers and was run both with an open exhaust and with a straight through
English style muffler.
The first and most important thing that we found was that the
first day of testing we could not get the engine to repeat power
runs accurately enough to
learn anything at all until we modified a 45 DCOE Weber "soft mount" and
it in between the HS-6 carbs and the inlet manifold to insulate the carbs from
engine vibration that apparently was upsetting the fuel delivery to the
engine. As soon as this was done, we were able to start to get some testing
The next surprise was when we added velocity stacks to the HS-6
carb set up. We tried the old aluminum parabolic stacks that
I had seen on the
Competition Department TR-4s, and also a set of stacks that I had made many
years ago that look like Weber Velocity stacks. Either stacks just killed
the power above 5500 rpm. I mean it just took a nose dive, whereas
stacks the power would peak at about 5400 rpm and then stay almost level
until 6,000 rpm. Unfortunately we did not have a set of "stub stacks to try.
Apparently the 13.2" length of the intake and carbs is
very much a "tuned
length, at least on a 3/4 race engine. At least that is what I concluded.
What a shame, because the Velocity stacks look so cool!
We then found that with no stacks, the stock HS-6 carb setup made a higher
torque peak by almost 8 ft. lbs. than the race prepared H-6 setup .Above
5,000 rpm the H-6s on the short manifold was slightly superior.
The H-6 carbs did
not need the "soft mounts" and the "weber like" velocity
stacks worked well on
It made very little difference whether the muffler was on or
the engine had a straight open exhaust! But remember the muffler
we were using is
a very free flowing unit, we have used it on engines making up to 150
hp and this
engine was in 120s.
Our best runs made 123 hp with a very broad peak that gave over 120 hp
from 5,000 to 5700 rpm and 138 ft lbs of torque with over 130 lbs. from
3300 to 4700 rpm and with the peak around 3900 rpm.
All of the testing was done on 91 octane pump gas. The best power was
with 34-36 degrees total advance measured on the outside of the
I had a similar experience a few years ago with HS6 carbs. I
not get any power over 6000 rpm on the track. I just could not
them to flow enough fuel. I put air cleaners on to add some
restriction and gained 300 rpm on the straights. I did not, however,
go so far as to add the weber style spacers.
My work with velocity stacks was always with
FULL RACE engines. The purpose
of the stack use was to help contain the fuel standoff which
appears at the
bottom of the rev scale (around 4000 revs) where the long overlap
camshaft tends to reject the air/fuel charge. But we were fortunate
very good gains in the entire rev range.
My back to back tests
with the GT-6
engine showed a improvement of 6-8 horsepower at 8000 rpm.
In these tests, I
held the velocity stack up to the carburetors with my hands
as the engine
was a full throttle and top revs, then removed the velocity
watched the torque arm fall, pushed the stacks back into position
watched the torque go back up and stabilize. We were all pretty
standing right next to the engine at full power, but trusted
builder (me). Again this is with full race engines and big
think Gregs tests were great and give added information to
all that there
are variables in this world of engine development and also
points out the
wonderful information available from dyno testing.
So Joe, adding the air filter reduces the amount of air and therefore
indicates to me that there is either insufficient fuel supply (too small
a needle and seat) or a lean mixture at that higher rev point
Subject: velocity stacks and stand-off
While we were dynoing this 3/4 race engine (280 degree duration
installing the velocity stacks in every case made the fuel "stand-off" disappear.
Without the velocity stacks there was a cloud of fuel in front
the carb inlets that you could feel the wetness on your hand
up to over 1 ft
away. The fuel seemed to be in constant motion into and out of
throat. the fuel did not seem to "blow away" into the
dyno room. When the
velocity stacks were installed, the cloud was no longer there,
and the engine
made slightly more power from the point where it came on the
cam, ie. about
3200 rpm up to around 4500 rpm. Above that rpm the power was
always less with
the stacks than without them.
As I mentioned before, we did not have any "stub stacks" as
David Vizard as being a benifit on Minis and MG engines. So they are a
questionmark as far as this engine is concerned. It is interesting and
puzzliing when emperical results do not agree with what should the theoretical
results. Everthing I have ever read says that the stacks should make things
better everywhere, not worse. The square corner of the carb inlet should act
to in effect reduce the size of the inlet and reduce the air flow. Maybe the
power level of this engine does not require any more air and so that is why
did not help.
On Weber carburated engine, every time we have gone up in venturi size,
the power has gone up. But again this was on a full race engine making well
over 170 hp on this same dyno.
By the way even on this engine, with either set of carbs, the air valves
were up all the way they could go by 4500 rpm. So from this point on the
actual taper of the mixture needle is no longer having any effect on the fuel
metering. as only the exact point of the needle where it enters the jet is
controlling the mixture and that is no longer changing as the revs change.
course the size of the needle at this point is important and is controling
amount of fuel flow into the engine.
Regards, Greg Solow <email@example.com>
That's pretty interesting. I might make note
that the higher revs the
higher the air velocity the higher the velocity the lower the
therefore the flow of fuel will be HIGHER even though the needle
risen any higher in its taper. Funny stuff huh!!!.
IOW, when the piston hits the top of it's travel, you suddenly
have a fixed
venturi with a fixed jet ... just like a real carb <g>
But with no method of preventing over enrichment like the air
on a Weber!
Just wondering; what was the shape of the outside lip on the stacks you
used. They should be rounded back toward the outside of the stack (fully
radiused). If these happened to be of the variety with a sharp outside
edge, this could be one source of trouble.
If you check the airflow into various stacks, I believe you will find
quite different characteristics between the sharp edged ones and the ones
with a fully radiused outer end. I dont have the explanation, but they
result in very different air-flow patterns.
The "parabolic" velocity stacks have an extremely
large radius bell that
flares out a full 90 degrees but does not roll back on iteself.
diameter where the edge of the bell ends is 3 5/8" whereas
the throat of the
carb is only 1 5/8" in diameter, so I don't think that the
fact that they
don't roll back on themselves is hurting the power or airflow.
Since Kas set up TR-4 engines that made over 160 hp with 1 3/4 SU carbs
on them, I suspect that at the 125 hp level, the air flow requirements of
the rest of the engine are such that they are not close the limit of the
carb size. If that is the case, it would make sense that increasing the air
flow potential of the carbs would have no effect on the power output of the
engine. Other factors like "tuned length " of the inlet tract might
greater effect on power than air flow potential of the carbs.
We also did some back to back testing of high lift rockers vs. standard
lift rockers and found that even with an almost stock head, the engine
really liked more lift. Going from total lift at the valve of .375" to
.415" was worth about 5 hp from 4500rpm on up to 6,000 rpm, which was the
maximum at which we ran this engine.
Consequently, we are now working on a new cam grind which will still
have seat to seat duration of 280 degrees, (which idles well at 800 rpm with
SU carbs) but will have faster lift rates and more total lift while still
being relatively gentle on the valve train so it will not require hemongus
spring pressures like a real "race" cam does.
The early velocity stacks that I did had a sharp edge and no
roll back of
the outer edge. They did not make the big difference that we
with a rolled over edge. In tests we found the inlet air was
coming from the
back of the carburetors. I'll explain our tests another time
if interest is
demonstrated. It's no big deal.
The tests I did for the air movement was first on the flow bench.
the stack on the air inlet then used smoke to get the indication
air was coming from. For a simple test I put my hand over the
stack and it
didn't make a bit if difference until it was about 2" away.
indicated that the air was coming from the back of the stack
manifold side. In the dyno with the engine running under full
about 5000 I used a flat board and slowly moved it up to the
happened until I was ABOUT AN INCH away from the stack again
the air was coming from the manifold side. After this I then
made the stack
with big roll on the edge, that is when I picked up the big change
improvement. Like I said, simple minded tests. I would think
that stacks on
Webers that had rolled edges would improve the flow. You could
that by mounting in a lathe, heating the edge with a torch, turning
lathe on the back gears so that it was low revs then use a wooden
roll the edge over. OR just buy some that do the job. The latter
What all this did was relieve me from worrying about the air
inlet being too
close to the inner fender well. But it also showed the worth
of having a
heat shield between the headers and the carbs and manifold.
Ah yes, free horsepower from ram air. This is a favorite subject
of mine. Lots
of experiments and study of the effect.
The prime thing I found was that cooler air, near the roadway,
was a better
giver of free bhp than anything else. It takes a lot of speed
get an ram effect. Now if you use a corrugated flexible tube
you just tossed
all chance of ram out the window for as the speed increases the
wall drag of
the tube and reduces the effective diameter to half of or less,
disappointing. Smooth tube works though.
If the ram tube is led directly to the carburetor inlets you
can have the
problem that the car will not run very well at anything but light
Why? The slight ram and I mean slight will give the carburetor
than the float bowl fuel sees, thus the float bowl will not flow
Fix this by making a box for Webers that includes the float
chamber inlet and
on S.U.'s just connect the float lid overflow pipe to the air
tube with a
piece of hose. Still problems with miss fire etc, poor high speed
Probably it is the turbulence in the box and the carburetors
are not getting
equal air, and in fact may have choked off some of the inlets
Cut a hole in the back of the box the size of ONE carb butterfly
(if that is
better but still a little problem, increase the hole size 50%)
certain the box extends well past the inlet of the rear carburetor
When you are all finished, you'll have spent some money, some
labor and testing and for this probably will not gain a single
because these boxes (car shape) do not operate at the speeds
needed for true
Cool air? Yes, now you're talkin'.
Never Be beaten by Equipment
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