Heat Risers - the under considered weakness of aftermarket
center mount setups
I've been working on ways to make
center mount carbs work well, from idle
to WOT. Anyone that has run one knows this just doesn't happen.. ever...
unless it runs so rich you kill mosquitos for miles around. While
ridding the world of mosquitos is not a bad idea, making it from gas
station to gas station and changing your plugs at each fill-up isn't the
way most of us want to drive our VWs. There are plenty of people that
would run center mounts if they could make them work right, especially
the casual off-roader that is not at full throttle all the time, and
streeters that want something easier and different.
So in efforts to deal with this, I am doing R&D on heat riser tech for
headers. As a starting point, I have geared up the Beetle with a genuine
Weber 32/36 DFAV progressive carb, and the best intake money can buy,
which is the one that aircooled.net sells. I bought the intake years ago
and have misplaced the stabilizer so I made one and added a spring perch
to add a little return force for the throttle cable.
So, what's the deal with heat risers? Why does this happen?
If you look at what happens under the carburetor, air and fuel are
dispensed from the carb and are delivered under the throttle plate. At
low throttle angles, i.e. closed position, there is a high vacuum under
the carburetor in the intake manifold.
For a steady state process, the
ideal gas law states:
PV=nRT
Where P=pressure, V=volume, n and R are constants that are not important
for this discussion, and T=temperature.
I'm leaving standards and units out because all I want to express is that
as P drops, T drops. So when your mixture makes it under the carb, under
low pressure, it gets COLD. When it gets cold, it does what the weather
does when it gets cold. Warmer air with more vapor meets a cold
region, and the vapor condenses.
Engines move a certain mass of air under any steady condition. If
the size of the passage it moves through gets larger, the speed of the
air slows down. The aftermarket setups ALL have a larger cross
sectional area than the stock setup, and are at a further disadvantage.
Slower moving air with fuel vapor in it has more opportunity for its
fuel to condense.
Now given, the process is not exactly steady state, but the equation
above is significant, because under any of the conditions of low
throttle plate angle it does have a significant effect, and can be
modeled this way in approximation. The point is that the constant
low pressure condition under the carburetor makes the intake cold, just
like the cold side of an air conditioner or heat pump. It acts
like an expansion valve.
So with the geek stuff aside for a moment, the air/fuel mixture gets
cold and the gas rains down on the intake runner. The only good fuel in
the engine is VAPORIZED fuel. That's what we want, not fuel pooling in
the intake runner.
This problem is not limited to our setups. Most water cooled
engines have water jackets in the intake, and their purpose is to warm
the intake, not keep it cool. Some others, mostly 4 and 6 cylinder
inline engines, have a heat riser setup from the factory as well.
Many of them suffer from heat riser deficiencies with the use of
aftermarket carburetor systems with different headers and/or intake
manifolds. Nothing is ever engineered as well as the factory
setups, so to reach nirvana, we have to do some extra work most of the
time.
How I am going about this R&D:
First I needed the setup. What's the worst running center mount setup
that is a mainstream item? Barring actual problems with the parts, it's
safe to say that the progressive and the center mount IDF or DRLA setups
are the hardest to make work right, mostly from the fact that they are
big carbs on big intake volumes. Let's look at the two.
Weber
IDF and Dellorto DRLA center mounts - they come it two flavors. One is an isolated runner
setup and the other is a plenum setup. There are a few of each kind on
the market. I wouldn't wish the plenum setup on my worst enemy, due to
the effects of a plenum on a synchronous two barrel carburetor on these
engines. There is not much way to definitively set mixture screws since
both of them feed the same intake space. Besides that, in order to get
the right idle speed, you have to crank the idle speed screw so far in,
you start feeding the idle stage with the progression circuit rather
than the mixture screws.
Progressive center mounts - these are necessarily plenum, just by nature,
which will be fed by either one barrel or both, depending on how far
your foot goes. One mixture screw, one space to deal with, and pretty
simple, but it does have it's own challenges.
So I chose to start with the progressive, because it is pretty
common and a lot of people use it. These setups typically run super
rich just to run fair, and tends to be even worse in cold weather. Most of
these that you see installed do not have any heat risers connected to
the exhaust, because:
1. The owner bought it second hand with the intake and the original
owner looked at the riser tubes and said, "Duh, gee Tennessee, I don't
need these..", and threw them in the scrapper.
2. The owner bought it new and looked at the riser tubes and said, "Duh,
gee Tennessee, I don't need these..", and threw them in the scrapper.
3. The exhaust has no heat riser connections, or has them just welded in
place, and the risers have not been drilled out into the exhaust. Most
standard headers with the heat risers on them are not opened up yet.
This eliminates the need for block-off plates when the heat risers are
not used, and saves the cheap manufacturers an extra step to drill them
out. They just weld them on, put the cheap paint on the header and throw
them in the box. The off-road guys tend to get exhausts that have
no heat riser connections.
With these and other possible reasons, heat risers are not hooked up
most of the time.
Volkswagen's original setup for the center mount carburetor included a
heat riser setup along with a temperature and/or flow controlled system
to source hot air to mix with the cold air on the air cleaner. For the
heated air intake to the air cleaner, some had linkage that tied to the
thermostat linkage via an arm that was attached to the thermostatic
cooling flaps, some had a weighted flap that would open more for cool
air under heavier air flow, and some had a thermostatically regulated
vacuum actuated flap in the air filter that responded to the air
temperature inside the air cleaner space. Each one would source hot air
from the hot cylinder head
and mix it with cold air to go to the carburetor. While this is helpful
for atomization of the air/fuel mixture in the carb, as it leaves the
carb under low throttle positions, it still gets colder from the dynamic
process under the carburetor and there is more
needed. I tend to think of it more as an assist in really cold temps,
thought it was in action in most climates.
For the heat riser setup, it is basically a heat exchanger. The intake
base is either steel or cast aluminum, with steel tubes attached under
the intake runner center section. On the dual port engines, aluminum was
probably used because it conducts heat so well. The hot exhaust gas is piped through the intake, below the
runners, and isolated from the charge in the runners. The hot gas
through it exchanges temperature to the center section intake runner, and that
section has a hot spot right at the base where the downpipe from the
carburetor flange meets the runners that go out to the heads. The area
of primary influence is right at the base in the center area.
Below is the center section of a dual port intake system.
On dual port engines, the end castings are heated too. They're heated
from the cylinder heads because VW used a metal gasket for the end
castings. Due to the fact that the end castings are aluminum, they
conduct heat fast as well and they stay warm. As your intake runner
volume goes up, the speed at the same volume flow rate goes down, and
this heat becomes more critical. With single port engines, the intake is
one single piece, and the steel ends don't conduct quite as well as the
aluminum, but they still do pretty well, and the fact that the
runner size is smaller helps keep things in vapor form anyway.
So that's a lot of heat going into the intake system, and while that
seems strange, recall that many water-cooled engines use water jackets
through the intake manifold. It's not to keep them cool, it's to HEAT
them. A hot flow boundary discourages
condensation of any vapor in the intake. In fact, you will find that
many of these have an insulating spacer between them and the carburetor
itself, to reduce the heating of the carburetor. Hot carburetors are not
a good idea, so keeping it separated when you have an intake
with a lot of mass or just a lot of heat in it helps keep things groovy.
So.. how does the heat riser system work?
In order for a heat exchanger system to work, a transfer of heated gas
or liquid must occur. A heat exchanger's job is to transfer heat energy
between two isolated systems of flow. In this case, one system is the
exhaust gas, and the other system is the intake charge leaving the
carburetor and going to the heads.
In order for the cold air constantly
flowing through the intake to get heated, the exchanger must
continuously add heat to the intake manifold, which requires heat to be
added to the intake continuously from the exhaust system. In order for
this to happen, heat energy must constantly be applied, and so the hot
exhaust gas must move through the heat riser circuit. In
order for that to happen, there must be a pressure difference between
the two ends of the heat riser connections.
So.. how did VW do that?
Look at a stock muffler. The heat riser connections are shown in this
picture. Note that one connection comes directly from the exhaust port
near the head, and that the other goes into the muffler chamber. The way
it is designed, the hot exhaust gas is pushed by the exhaust connection
near the head, and the pipe in the muffler chamber is oriented in a way
as to be subject to a draft effect, not unlike the effect of following a
large vehicle closely at high speeds. The low pressure condition created
behind the vehicle doesn't just give you a low wind resistance zone.. it
also has a pulling effect that takes mass down the road with it, just
like leaves on the ground following a car that just ran over them at
speed. There are many examples of this happening, but this one will do
to illustrate the effect. It's related to the low pressure condition
brought on by the venturi effect.
The picture below shows how the low pressure region was set up.
The fast moving exhaust gas around the tube causes a siphoning effect.
So with a pipe with different pressures at each end, you get flow, just
like a straw, or a garden house, or lots of other stuff. This works to
pull hot exhaust gas through the isolated chamber at the bottom of the
intake, and some of this heat energy is transferred to the intake and
works to warm the intake and cause a warm boundary around the air/fuel
mixture, and voila, fuel is encouraged to stay in suspension.
It matters a lot more with a long intake runner, because of two things:
1. There is a part of the long intake that is distinctly isolated from
any connected heat source, i.e., the heads.
2. There is a long run of this space. Long runs translate to time and
opportunity for this condensation or fuel dropout to happen.
When you have dual carbs, the intakes are SHORT, and they are constantly
warm from the heads within a short period after startup. The heads get
HOT, and so the intakes stay warm.
What about just using the heat risers that you find on most of the
aftermarket low tech headers out there?
Look at the way they are made. They are just a set of connections at the
heads. What's gonna happen there? Well, let's look at the situation.
The gas pulses are evenly applied, due to the location of the fittings
and the firing order. It's a decent pulse at idle, but other
times, as the engine speed increases, it becomes much less effective.
If it is not moving exhaust gas in a way that nets flow, it becomes
useless.
The exhaust gas is gas.. not solid.. so let's look at the concept of
specific heat for a minute.
Specific heat, by definition, is the amount of heat per unit mass
required to raise the temperature by one degree Celsius. PER UNIT MASS.
How heavy is a gas? Not really heavy at all. And what we have is hot gas
moving through a tube. It's job, hopefully, is to create a hot boundary
for the gas to flow through. It's going to need to heat the charge a bit
but that's a lot of cold gas charge going through the intake... that's
ALL the air and fuel that the engine is getting, split to the left and
right banks.. the exhaust is MOSTLY flowing through the exhaust pipes,
with a small percentage flowing through the heat riser.
But the exhaust gas is hot! It is over 1000 degrees at exit. So you do have a significant difference in temperature. This is
called a "delta" or difference between one state or part of a system and another.
So what we have is a relatively small percentage of the exhaust going
through this heat riser. And it's job is to do a good job of keeping the
intake charge from raining down on the intake runner, with pretty
significant tendency toward doing just that. For a point of reference,
an engine taking in a 50 degree air temperature can and will form frost
on the region below the carburetor. This suggests a pretty good drop in
temps below the throttle plate.
With that in mind, we need something that is going to
heat up the charge
significantly in its short life in the intake... something to discourage
if from turning into a liquid again in the intake runner. What we need
is a pretty good heat exchange.
The hot exhaust gas is low density, with relation to the intake runner.
So in order to maximize the amount of heat that the intake runner can
soak up.. we need a good flow of hot gas through it.. not some pitiful
little bit.
If you look at the aftermarket exhaust headers that do have heat risers
on them, they are designed as mentioned above at the picture, with both
connections right at the exit from the heads' ports, namely at #2 and
#4. The firing order is 1-4-3-2. So that means that the exhaust pulses
from the heads's ports are timed evenly and are theoretically equal in
pressure. Look at the events over time and you can see that they net
ZERO flow. To me this is a fail. Idle works ok since the
pulses are slow, allowing more time for the gas to move back and forth.
When the engine speeds up, as mentioned above, it allows less and less
exchange of gases, and less and less energy exchange.
So obviously SOME hot gas moves back and forth through the heat risers
with this setup.. back and forth.. but how much? Well at idle, it might
be a little, but it is just not that much. And at speed, it isn't much
at all. The amount that can propagate through the tube is a function of
the difference in pressure and the time window it has to do it in. As the
engine speed picks up, the pressure difference and the time diminish.
I'm being repetitive, but I want it to sink in.
Since it doesn't matter as much at WOT because the air and fuel is
moving pretty fast, it might be easy to think that idle is the key area
that this needs to happen in. But it's not. There is a significant need
for it at early throttle independent of RPM because the air flow IS LOW,
and the air speed is low. We need it during much of the first half of
the throttle action. Further, we don't want to let the intake cool
off too much even with heavy throttle, because heating the intake up
takes time and if your engine returns to an idle or low throttle
position with the intake now cold, it'll have the cold intake condition
until the heat risers warm it up again.
What we need is a net flow through the tube, all the time. By net flow,
I mean that exhaust from one riser constantly flows toward the other and
there is not a zero average of flow from one side to the other and vice
versa. We need a lot of heat
energy to flow through that heat riser, to keep the intake runner hot. A
lot of that heat is getting absorbed and cooling the intake runner, in
order to keep the air and fuel heated up enough to stay in vapor form.
The heat is transferred to the air/fuel mixture constantly.
So.. back to the original design.. note that the German engineers knew
this, and implemented just that - flow. Flow means that the heat energy
that is stored in a system that has relatively low specific heat can and
well be transferred in part to a material that has a relatively high
specific heat in an amount that can make a difference.
In determining where the heat riser return should go and how it should
be configured, it's safe to say that it will function best in a place
where it gets the most flow over the tube it. For this solution there are
two variables that will matter the most.
- Location in the exhaust
- Configuration of the tube
When looking at the flow of gas or liquid in a pipe, fluid dynamics
teaches us that the flow is a gradient that has highest flow in the
center of the pipe, with flow volume tapering off as you get close to
the boundary of the pipe wall. One illustration I found shows this with
some good graphic representation.
You can see that as the gas or liquid travels through the pipe, it meets
the wall with friction, and is slowed down as you get close to the
boundary.
The point in the exhaust that has the most flow is the collector at the
minor diameter, so in fitting the tube, it should be considered closest
to this point.
So with this in mind, there were three options to look at in putting the
pipe in the stream.
The first one would work from the edge, and wouldn't have that much
effect until you really got gas flowing through it.
In the second one the tube would be in the fastest part of the stream,
and has the opening parallel to the flow of exhaust gas. I see
this type of configuration on some aftermarket crankcase scavenging
systems, and I would like to believe that there is a method to the
madness.
In the third one the tube would be in the fastest
part of the stream, and has the opening perpendicular to the exhaust gas
flow, and on the trailing end of the tube. As gas flows over
the tube, it will develop a low pressure condition under the discharge,
and suck the exhaust gas out of it.
This is at a cost, namely some restriction. The truth is that most
aftermarket headers other than all out merged racing headers have a
larger collector than they need, and this may actually do the system a
favor by reducing the cross sectional area of the collector.
After prototyping these three conditions, I was able to confirm that
option three works best. I played with orientation and entry
depth, using a flow meter and a vacuum gauge, and was able to determine
this with some rough measurements. The testing I did was with
configuration three.
How I tested the system and measured the results:
I have a temperature sensor unit from a PC that is basically a front
mounted panel with flat temperature leads embedded in flat plastic tabs.
These were perfect to attach to the intake manifold and air inlet of the
carburetor. The air fuel mixture was measured with an Innovate
LM-1 air-fuel meter. The results were recorded in real time, under
roughly the same conditions. The ambient temperature outside was
measured with an outdoor thermometer. Two headers were used, same
make, one with typical heat risers, and one with the modified setup.
To test the three conditions, the standard header was installed first
with the heat risers blocked, and second tested with the heat risers
opened. The third test was performed with the modified header.
So here are some results. During the testing, I did not change ANYTHING
but the two headers and exhaust heat risers. No mixture, jet, timing, nothing. The only
uncontrolled variable was the temperature and humidity outside, but
those were all fairly close anyway.
Note the AFR numbers and their relationship to the intake skin
temperatures. As the intake got warmer the numbers go down and
stabilize. The significance of the AFR numbers is that the wideband
oxygen sensor establishes its numbers by measuring the oxygen
concentration by percentage in air. Standard percentage is about 20.9%,
and as it gets used up in burning the fuel. The meter derives numbers
for how many pounds mass of air the engine uses per pound mass of fuel.
The AFR number represents how many pounds mass of air are used per pound
mass of fuel, based on the oxygen concentration left. Lower AFR means
richer, provided that fuel is being burned reasonably efficiently. Since
the jetting and other factors did not change, the same ratio of fuel was
delivered by the carburetor in all tests, and the amount that was burned
for power here changed by about 20%, which means that the engine at this
point COULD be 20% more efficient with the heat risers the way they are
in the last test, versus the first test.
With NO heat risers
Temp 48
Humidity 46%
At reference throttle position, AFR is 14-15
At steady temperature state, air inlet temp is 81 degrees
At steady temperature state, intake jacket is 54 degrees
Idle AFR is 14.8 average
Engine notes: Rough idle, erratic driving behavior.
With STANDARD heat risers
Temp 52
Humidity 42%
At reference throttle position, AFR is 13.2-13.8
At steady temperature state, air inlet temp is 82 degrees
At steady temperature state, intake jacket is 71.5 degrees
Idle AFR is 13.4 average
Engine notes: Better idle, better driving behavior.
With MODIFIED heat risers
Temp 60
Humidity 40%
At reference throttle position, AFR is 12.4-12.8
At steady temperature state, air inlet temp is 89 degrees
At steady temperature state, intake jacket is 91.5 degrees
Idle AFR is 12.9 average
Engine notes: Smoother idle, good driving behavior.
That's all I have done for now. I want to do one more modification to
the exhaust to determine if it makes any more heat, and then after that
it's time to tune the carburetor. I am pretty sure I can squeeze another
10% efficiency out of the fuel system, and gain mileage and performance
in the process. Another benefit to this is that with a hot intake, you can
run extremely lean mixtures at low load conditions, and boost efficiency
while reducing combustion temperatures (lower EGT).
I have a couple more tricks up my sleeve - I would like to see the
intake get to about 150 degrees, which may be asking too much, but I
know I can get it higher than it is now. That temperature is not the
charge temperature, it is just the temperature of the intake. I know if
I can get it there, I can do a lot more for efficiency than I am able to
with what I have now. Getting it there is a no brainer, but doing it
without restrictions or heroics is another story. This needs to be a
bolt on in my opinion, and that makes it a harder challenge.
To a few people this is no revelation, they've done it, in different
ways, usually with good results, but this is the first time I have seen
data. My controls were loose, but I don't think they were loose
enough to mean you can't see the trend.
Results will vary, with engine combo, geography, fuels, intake and
exhaust components, etc., but I think that the trend will be the same,
namely improved performance and tuneability.
