As I am in rant mode, one of the things that really irritates me is that guy that always jumps up and says removing the thermostat makes the water flow too fast thru the radiator and thus causes overheating. He obviously did not study thermodynamics and closed loop systems....
Any way, I found this in some data I had saved. It came from Grapeaperacing
.com and I think that site is now gone which is sad because it had lots of good, valid tech info. Oops, I am wrong, he is back with a new site.
This is long, but, it should interest some of you do-nut shop racers
www.grapeaperacing.comThermal Efficiency
Before we go into what a cooling system
does and how to modify it, you must first understand
what the engine does. Plain and simple, an engine
makes heat energy and turns it into mechanical
energy. Any heat generated that does not get used
to make power is wasted energy. How well an
engine converts the heat it generates into
mechanical energy is known as it's thermal
efficiency.
The cooling system takes heat from the
engine, heat that ideally could have made power, so
the cooling system actually takes power from the
engine. It is a necessary evil, without a cooling
system, the engine will overheat and the internal
parts will have a very short life.
A cooling system will also reduce the
chances of detonation. With new cooling systems
and coolants, it is possible to run today's engines
hotter, which increases thermal efficiency. If you
take less heat away from the engine, there will be
more energy available to make power.
Any heat that is radiated off the engine, and
out the exhaust system is also wasted heat energy
that did not get used to make power, which reduces
thermal efficiency. The average engine has only a
25-30% thermal efficiency, so 70-75% of the heat
generated never gets used to make power. An
average 250hp gasoline engine is actually burning
enough fuel to make about 1000 hp, making it a very
inefficient machine.
Cooling System Goals
Most people seem to think that all a cooling
system needs to do is keep the engine from
overheating. But what is not realized is that if the
engine runs too cool, thermal efficiency is lost and
power is reduced. Many will argue that an engine
has more power when it is cold, but that is only due
to the fact that the intake air is colder and denser,
actual BSFC is higher. Remember that an engines
whole job is to make heat and turn it into mechanical
energy. Running the engine as hot as possible
(limited by the detonation limit) will increase power
and provide a lower BSFC.
If the coolant begins to boil, steam pockets
will form and detonation will limit power (by forcing
you to retard timing to less than optimum or run the
engine cooler). Most of today's high output street
motors using a water/ethylene glycol mixture will be
limited to about 200° F before detonation becomes a
problem (unless other steps are taken).
Another goal of modifying the cooling
system is to even out the temperatures of the whole
engine, which is not easy to do. All it takes is one
hotter cylinder to run into detonation to limit the
engines power. It only takes 1 cylinder to limit all of
them. Most high performance engines are close to
detonation
to begin with, so a good cooling system
is a must.
Nucleate Cooling Phase
As coolant flows through the system it
absorbs heat from the engine parts that it comes in
contact with. As it does this some of the coolant will
boil and form tiny steam bubbles (absorbing a lot of
heat in the process) on the internal engine surfaces.
When these bubbles get larger they become a flow
restriction and the flowing fluid pushes them away
from the surface and that process starts over again.
The process is called the Nucleate Cooling
Phase. When the coolant boiling point is too low or
the flow rate is too slow, these bubbles can become
too large and form steam pockets that insulate that
surface from being cooled. This usually happens
around the combustion chambers, the hottest parts
of the engine. Once the steam pocket forms the
surface will rise in temperature (even though the
coolant is not overheating) and cause that part to
overheat, which can cause detonation and / or other
problems.
Types of Coolant
I'm sure that you've read or heard
somewhere before that water is the best coolant.
This is true as far as being able to absorb heat for a
given flow rate, water does do that the best. Water
also boils at a lower temperature than other coolants
and can develop steam pockets easier, so it's not
the best coolant in that respect.
A water / ethylene
glycol mixture will boil at a higher temp and resist
steam pockets better than plain water, the down fall
is that it has to have a higher flow rate, but that is
easy to accomplish.
The 3rd common form of coolant is
propylene glycol, which has the highest boiling point
and can run higher than 250° F (average
temperature as seen on a gauge) without forming
steam pockets, but it must flow at more than twice
the speed of a water / ethylene glycol mixture (which
means major changes to most cooling systems).
System Pressure
The pressure in the block is higher than the
radiator pressure; this is because the pump is
building pressure due to the thermostat being a
restriction. This pressure raises the boiling point of
the coolant and reduces the chance of steam
pockets, so never run with out a thermostat (or some
form of restriction).
The radiator cap will usually hold 15-18 psi,
if the radiator holds the system at 15 psi, the boiling
point of plain water will be raised to 250° F. The
water pump can then make an additional 40-45 psi
in the engine and bring that boiling point close to
300° F. So as you can see, pressure is important.
Stock Cooling Systems
Most stock cooling systems pull coolant from
the radiator and push it through the each bank of the
block; it then goes up through holes in the head
gasket(s) to the heads and out the front of the heads
to a common exit point. This ok for a stock engine
that has no problems with detonation, but the
cooling is very uneven. The front cylinders will run
coolest and the front combustion chambers will run
the hottest.
Most stock pumps will also favor one bank.
The stock pump used on a small-block Chevy for
instance will always favor the passenger side bank.
This means that cylinder 2, 4, 6 & 8 get more flow,
so the 1, 3, 5, & 7 bank runs hotter.
With the center exhaust ports right next to
each other, you can see that combustion chambers
3 and 5 will run the hottest, it is in these two
cylinders that detonation will usually first start.
It seems a little backward to start the coolant
at the block instead of the heads; it would make
more sense to bring the coolest coolant to the
hottest parts first. This type of reverse flow system
has been tried with much success, but it is harder to
get it working properly and not worth it for car
companies to research when the stock system
worked good enough on a stock engine.
Mechanical Water Pumps
As I said before, stock pumps rarely flow
evenly between banks. On the small-block Chevy
you can restrict 1/2 of the block inlet to the even
cylinder bank to get more even flow, but the better
solution is to use an aftermarket high volume pump
that has worked out such problems.
Stock pumps have a stamped steel impeller
and tend to cavitate easily when turned more than
6000 rpm, so overdriving the stock pump offers little
to no advantages and can actually aggravate any
cooling problems. Most aftermarket pumps will use
a cast iron or an aluminum impeller that better
resists cavitation. Weiand, Howard Stewart and
Milodon make very good water pumps for most
popular applications, which improve flow, resist
cavitation better, and require less power to drive
than stock pumps.
Electric Water Pumps
Many aftermarket companies offer electric
water pumps. These pumps do not flow well or build
sufficient pressure in the block. They are only good
for limited drag racing use, and when used they
need a high pressure cap to help prevent steam
pockets. At best these pumps can flow 30 GPH and
only build about 5 psi additional pressure in the
block. An electric pump should never be considered
on a street or any type of endurance engine.
Even a stock mechanical pump has less
than 10hp parasitic power loss, so the advantages
outweigh the disadvantages of an electric pump.
Better aftermarket designs only take 5-7hp at ~6000
rpm, so there is not much to be gained by switching
to an electric pump.
Coolant Flow
Different coolants require different minimum
flow rates, but contrary to popular belief, you cannot
make the coolant flow too fast. This rumor was
started because people removed the thermostat to
gain flow, because they had an over heating
problem, and it only aggravated the problem. The
real reason they ran into problems is that removing
the thermostat also removes the restriction that
builds pressure in the engine, so they gained flow,
but reduced the boiling point of the coolant in the
block.
Running a higher flow thermostat and a
higher volume pump to maintain pressure, will give
no such problems. If you think about it, making the
coolant flow twice as fast will also make it flow
though the engine twice as often, so there will be
more even temperature across the engine.
There has been, and still is, the rumor that
of the coolant flows too fast, it will not have time to
pick up heat. That is nonsense, as long as there is
coolant contact a surface, the rate of heat transfer
will be the same. Coolant that flows twice as fast
also flows through the block twice as often.
Basic Flow Modifications
Most stock systems on a V type engine will
have a common outlet for both banks. The outlets of
each bank flows directly at each other than must
take a 90° turn to return to the radiator. If one side
gets hotter (which is sure to happen) the pressure of
that side will increase. The increased pressure will
increase flow in the hotter bank and decrease flow in
the cooler one. The faster moving coolant will cool
the hot bank better and the slower moving coolant
picks up more heat in the colder side. As you can
see, the hot side is getting cooled and the cooler
side is heating up. This happens until the banks
reverse, the side that was cooler is now hotter and
has more pressure. The cyclic flow will continue
until the engine is shut off. Smokey Yunick was the
first to do studies on the cyclic flow and traced the
problem to the outlet. By tapping the front of the
heads, and bringing the coolant together in a Y
eliminated the cycling.
Radical Modifications
To truly equalize temperatures throughout
the engine is not possible with today’s technology,
but we can improve the situation some. To get the
best results you must start fresh and build totally
custom cooling system.
The first step is to tap off the pump and put
coolant to the back of the block so the coolant enters
at both ends. This helps equalize the cylinder
temperatures, but the heads will still be hotter
toward the front.
To equalize the head temperatures you
must tap outlets at the back of the heads so that all
the coolant does not have to pass the front
combustion chambers. To further equalize, you can
tap inlets and outlets in the center of the block and
heads also. At that point the coolant will be flowing
basically from bottom to top and is about the best
you will get without reversing the flow.
Reverse Flow Systems
As I said earlier, it makes sense to put the
coolest coolant to the hottest parts first to bring the
temperatures down as much as possible, the
already heated coolant can help bring the
temperatures of the coolest parts higher and make
everything more even. To do this the coolant must
flow in reverse (compared to most systems). The
problem with reverse flow systems is that the pump
tends to cavitate easier (even with a good
aftermarket pump). To limit cavitation, a higher
boiling point of the coolant helps and so does a
higher system pressure.
Cooling Fans
Stock Clutch Fans
The stock clutch fan is often tossed in favor
of an electric fan. In reality this may not be such a
good idea. A factory clutch fan and shroud flows
more air than just about any electric fan set up. I
have seen many people go out and buy electric fans
because they had over heating problems. Then find
out the problem is worse. If your car is running hot,
find out the cause, the fan is probably not it. A new
fan clutch is a whole lot cheaper than an electric fan.
If the problem is a clogged, or too small
radiator, buying a new fan is no solution. Bottom
line here is that the factory did a lot of research and
development on cooling systems and they use a belt
driven clutch fan and shroud because it flows the
most air. In turn, this allowed them to run slightly
smaller radiators. I have no problem with using
electric fans, but they do not move as much air as a
stock clutch fan and shroud set up. Set up your
whole cooling system with this in mind and you'll be
fine, but just put one on your car and plan on loosing
some cooling. If your cooling system was borderline
adequate before, an electric fan might just push it
over the limit. If you are having an over heating
problem, don't even consider an electric fan until you
find the problem.
Flex Fans
My opinion of flex fans is, they are next to
worthless. They can be better than a solid (no
clutch) stock fan, and that is the only good thing I
have to say about them. They are noisy, and offer
little to no benefit over the factory clutch fan. They
claim to move a lot of air at low-speeds and flatten
out at high speeds to cause little drag. The air
hitting the blades is what flattens them out and that
takes power to do, so they must have some drag.
Maybe not a lot, but certainly more than a clutch fan
that is near freewheeling.
I personally just do not like them. Flex fans
are popular in certain race classes that require an
engine driven fan due to the fact that they are light
and can take very high rpm. They were popular for
a while on the street, probably because they are so
cheap and people always insist on buying the "race"
parts for a street car.
It is very important to do research on these
kinds of parts. Race cars do always use parts
because they are the best, they are usually the best
for what is allowed by the rules in that class. Some
of the newer designs of flex fans have the blades
curved toward the rotation, this looks like a better
design because centrifugal force will assist in
flattening out the blades and they should reduce
drag over other types of flex fans. I have not had
any experience with them, so I can't say for sure, but
they appear to be a better design. Actual testing will
tell for sure, but I have not personally tried one.
Electric Fans
Electric fans can offer some advantages.
They are compact, which can really helps when
there are space limitations. They are reliable and
simple, so it can make for a clean neat installation.
They may not move as much air as the stock set up,
but if the cooling system is planned out well, they
can flow enough to get the job done just fine.
Hotter running engines have better thermal
efficiency, which means that heat losses are
reduced and intern more heat is used to make
power.
If you can safely raise the operating
temperature of the engine, you will have less heat to
get rid of in the cooling system. If you have less
heat to get rid of it means that you can use a cooling
system with a smaller capacity. This is where
electric fans work great, when the factory fan moves
more air than you need, an electric fan with less flow
can work just fine.
Another benefit of electric fans in the ability
to control them however you want. My ECU for the
injection system controls my fans and I can over ride
that with a switch in the car to keep the fans on or
shut them off if I want. Many aftermarket companies
also make thermal switches to control fans.
When you add and electric fan, there is
always the option of pushing or pulling air through
the radiator. So which is best? For the most part,
the pulling air through the radiator works better. It is
not a question of the fan being more efficient as a
puller, if the fan was totally sealed to the radiator so
there was no leakage, the pusher would be the
ticket, but even with a shroud, there is some
leakage. A fan does not just flow air through itself
straight. A fan spins and causes the air to spin as
well. Centrifugal force throws air outward all along
the fan as well, but the intake side of the fan is pretty
much limited to the area of the fan. When the fan is
in front of the radiator, a lot of air goes thrown out
and never makes it through the radiator at all. So
when you compare total air moved, with a pusher,
less makes it through the radiator than the same fan
as a puller. A shroud really helps with a pusher, so I
recommend a shroud on all pusher fans.
Curving the blades toward the direction of
rotation like the new flex fan designs might help
electric fans as pushers. The curved blades could
cup the air and limit the amount thrown outward by
centrifugal force. This is just a theory though, some
experimenting would tell for sure.
