Technical Advices > Induction and Exhaust Systems
Induction and Exhaust Systems
7.1 Induction and Exhaust Systems
Making more horsepower is all about getting more air and fuel into the cylinder. Now there are many ways to accomplish this, some more technically challenging than others. Put differently, horsepower is all about optimizing "volumetric efficiency" (VE).
Since almost all historic racing clubs do not allow the use of turbocharging, supercharging or fuel additives, I will not spend a great deal of time on these issues except just to review how they affect volumetric efficiency.
Turbocharging and Supercharging - Here the idea is to simply cram more fuel mixture (air and gasoline) into the cylinder under mechanically induced pressure greater than one atmosphere. Using these methodologies the only limiting factor is how much additional fuel and air can be reasonably burned, and this is usually dictated by the ability of the engine components and the cooling system to handle the additional heat load.
Fuel Additives - There is a distinction that has to be made here between those chemicals that can be added to fuel to increase resistance to knock, and those that can be added to increase VE. The best known of these is MTBE which has an oxygen content of 18.2% and is in use in California in reformulated lead free gasolines sold to the public. Others are Methanol (49.9% oxygen) and Ethanol (34.7% oxygen). These are all readily miscible with gasoline. There are also other chemical agents such as Propylene Oxide and Aniline, both known to be very dangerous and require special handling, that can be used as octane enhancing additives. Some of these additives have extremely low boiling points, such that on a hot day over 95 degrees Fahrenheit they would simply boil on there own. This adds to the danger of their use, as in a vapor state they become highly volatile and explosive. If you choose to use these take special precautions. Quite obviously, additives like Nitrous Oxide and Nitromethane will also provide a performance boost, but these are easily detected.
7.2 Intake Manifold Variations
Now lets get back to what can be done to increase volumetric efficiency through non-chemical means. The Otto cycle combustion process is all about "timing". This includes cam timing, valve opening timing, and as you will see intake charge timing. Just as a properly designed exhaust system utilizes the "scavenging effect" to extract spent gases from the cylinder, the same effect can be put to good use in maximizing the amount of fuel/air mix goes in to the cylinder. This will deal with a number of issues in the fuel induction system.
The whole idea is construct an intake arrangement whereby the intake pulses arrive at the intake valve just before the valve opens, at the desired RPM range. This has a supercharging effect which increases the amount of air/fuel entering the cylinder by as much as 20%.
To examine this we will look at three areas:
- Intake port diameter and length
- Plenum volume
- Ram Pipe length and Diameter (Helmholtz Theory)
- First intake port diameter and length - In general the diameter should be approximately 85% of the valve seat diameter. While this is am approximation, it is pretty close as this is based on assumption that the valve itself will provide some restriction even at full valve lift. If therefore the valve seat diameter is 31mm, then the port diameter as it approaches the valve seat should be around 26mm.
There are two types of heads in popular use on Fiat Abarth rear engined cars, the standard/modified head and a 8-port aftermarket head like the PBS 8P.
Now on a PBS head, with individual intake port runners, you have the ultimate flexibility in setting up your intake system. There might be a temptation to say that "big is better", but be careful. Large diameter ports may seem like they would flow a large volume of air, but they may lose a great deal of their velocity. I believe that in order to maintain low/mid range performance that the intake runner should maintain a certain amount of taper so that just before the intake valve seat the port is around 27mm, then blending out to the 31mm seat diameter. This means that the air will travel a path from the velocity stack bell (40mm) through the secondary venturi (30mm), through the throttle plate (40mm), down the intake runner to an area just before the valve (27mm) and through the valve seat. I have calculated that the theoretical length of the runners to achieve best performance around 7000 RPM should be 8-9 inches. The 8P design comes very close to this, and it would be difficult to change it. The design of the PBS 8P head promotes good midrange performance, while not restricting top RPM operation. I know of many engines that reliably run up to 9000 RPM.
The standard Fiat head is a different engineering exercise entirely. In reality we have an arrangement of two tri-Y intakes. If we divide the head into the right and left side, then inlet port of the head forms basically the bottom leg of the "Y" and the two short runners to the intake valves form the upward extensions of the "Y". This is identical for the left and right hand sides of the head. Just like in an exhaust manifold, this tri-Y arrangement has the effect of broadening the optimum RPM range over which the intake system is most effective. The intake manifold should be considered as a "plenum volume" common to all four cylinders. Unfortunately, as this head was designed originally for a 27 horsepower engine with limited RPM capability, the standard dimensions of the head do not lend themselves to a high performance application. It is possible to apply the port diameter theory in terms of making sure that the port diameter immediately prior to the intake valve seat is around 27mm. This will induce a secondary venturi effect just before the intake charge enters the cylinder. As far as port length goes, this is a combination of the two parts of the "Y" on each half of the head, being the very short individual port sections and the longer bottom of the "Y" section. Even so, this is not as long as ideal. As each cylinder that forms part of the Y fires alternatively to the other, the entire runner (both parts of the "Y") may be considered as the runner length for either cylinder. One could argue that if you had a two barrel carburetor with both venturis opening at the same time, then you could separate the two tri-Y areas. As we will see later on this may not be the case. The opening in the head should be of such dimension as to insure that adequate air flow is present and should match that of the bottom of the intake manifold/plenum.
Plenum Volume - Plenum chambers are designed to diminish the pulsing effect of the intake system, and this is particularly effective when more than one cylinder is fed from the same plenum. Plenums are not generally effective for more than 4 cylinders, although multiple plenums could be used for 6 and 8 cylinder engines.
For an 8-port head like the PBS unit there really is no plemum at all. Each port/intake runner combination acts as a standalone system. It is my view that this could be enhanced by adding a connection between the four intake runners just prior to entering the head itself. This would not be unlike the multiple tuned port arrangement in the BMW M44 engines, where an opening between the ports is opened at a certain RPM. This would then modify the effective RPM band to provide most efficient operation over a larger RPM band. Therefore the tunable range of the intake system of the 8P head will be fairly tight.
For Fiat Abarth heads, based on the standard Fiat head, the intake manifold provides for plenum chamber of sorts. To be ultimately effective around 7500 RPM the volume of the plenum chamber should be around 400cc on a 1000cc motor. This is the ideal number but is not critical. The plenum has the dual effect of dampening down intake pulse effects and also adding effective length to the intake runner. It is this very combination of intake port and intake manifold that then make up the effective port length that allow engines with standard heads to produce good power right up to 8000 RPM. Something that the original designer I am sure never intended.
Ram Pipe Length and Diameter (Helmholtz Effect) - The Helmholtz Theory was originally derived based on the harmonic effects of audio. He had postulated that a tone, or noise, was a combination of a primary frequency and a number of other secondary additive audio frequencies. Others applied this to fluid and plenum theory. There are three portions to the Helmholtz theory, namely plenum volume, ram pipe diameter and ram pipe length.
In the case of the PBS 8 port head the Helmholtz principle can be applied to each runner of the intake manifold. As such, about the only part that can be conveniently changed is the length and possibly the diameter of the velocity stack. Most velocity stacks concern themselves with providing a smooth air entry into the carburetor by ensuring a proper minimum radius on the bell of the stack. It may be that there is some advantage in actually making a velocity stack in which the bell opening, while having the suggested entry radius, might actually be smaller than the diameter of the secondary venturi. This would in effect increase the velocity of the air entering the carburetor and the adjustment in length would insure proper timing arrival of the pulse. Even with the PBS intake manifold there is a small difference in overall runner length for the outside cylinders, so one might consider using slightly shorter velocity stacks on these cylinders.
For the standard Fiat Abarth head, using a 2 barrel downdraft carburetor, the Helmholtz principle may have some further implications. In essence the volume of the area below the throttle plates, up to the back of the valve head is a plenum chamber. Therefore the velocity stack on the caburetor, along with the area of the secondary venturi in each carburetor throat makes up the entry to the "plenum". We can vary the tuned entry length both in diameter and length to get greater effectiveness at certain RPMs.
There may be an opportunity for further gains by employing yet another plenum chamber which would enclose the top of the carburetor and the radiused carburetor throat entry. Again, this plenum chamber should at a minimum be 500cc in volume on a 1000cc engine.
There are some basic generalizations that can be applied.
A longer the intake tract will work better at lower RPMs
A smaller diameter intake tract will promote better torque at lower RPMs
A shorter intake tract will work better at higher RPMs
A larger diameter intake tract will promote better HP at higher RPMs
Therefore, we need a good combinations that will promote good torque at around 5000 RPM, yet still provide good HP in the 8000-9000 RPM range.
7.3 Exhaust System Design
Exhaust systems, sometimes referred to as "headers", have been a subject of fascination for me for some time. Like other things, the design of header systems is sometimes is regarded as a "black art". However, when examined closely there are some basic rules that can be applied. This is not to say that the formulas and ideas that I am to present are the absolute truth on the matter, as every engine will have its own particular characteristics. The concepts put forward will however get you close to an optimum exhaust system.
Before I go too much further I have to give credit for much of the technical material in this article to the authors of a 1966 Hot Rod publication called "Supertuning". Bill Allen, the 1969 SCCA D-Sedan National Champion, used the same information to design an exhaust for his winning NSU. As the displacement of that car is similar to that of the Abarths that we still race today, I thought it a good place to start.
On all engines, including Abarths, a tuned exhaust system provides significant advantage, but this advantage is limited to a particular RPM band, depending on the design. Making a good tuned exhaust is not particularly difficult. The object is to get the reflected wave of one cylinder to help scavenge The next cylinder in the firing order. To better understand this it must be understood how waves are generated. The primary exhaust has a very high positive value. When this wave reaches to end of the exhaust pipe it is reflected back up the exhaust pipe as a negative pressure wave, which in turn is reflected again when the exhaust valve closes. Given the right timing, this secondary wave will help in extracting the residual exhaust gases. This wave movement provides further benefit in engines with camshafts with long duration and large overlap. In this case the intake and exhaust systems can drop below atmospheric pressure. When this occurs, the fresh mixture will begin to flow into the cylinder even before the piston initiates its intake stroke. This contributes to Volumetric Efficiencies in excess of 100%.
The real problem is in determining when the primary exhaust wave reaches full force, There is no sure way of determining this, but there is a fairly good empirical formula. I apologize in advance for not providing metric equivalents, but it became far too complex I should also mention that this model only deals with 4 into 1 type systems, although the computations are fairly close for 4:2:1 systems as well. It is:
L is the length of the exhaust pipe primary from the head of the exhaust valve to the end of the merge collector (not including exhaust pipe)
N is the desired peak RPM
V is the speed of the wave in the exhaust gases (1700 ft/sec)
A "tuned exhaust” is generally only resonant over a relatively small RPM range, about 1500 RPM. If we consider that 8500 RPM is our absolute maximum, then subtract 1500 RPM, this leaves us with a midrange RPM of 7750. This would make the effective range from 7000-8500 RPM. At all other times the exhaust manifold would be less than 100% efficient. You may prefer to tune the exhaust somewhat lower in the RPM band, as with most Abarth engines peak torque is achieved at around 5500 RPM.
Let's model an Abarth 1050 motor as an example. Using the above formula with a peak 8000 RPM, we find that the ideal primary pipe length is 25.5 inches. Remember that this overall length includes the exhaust track in the head and the length of the merge collector, but not the secondary exhaust pipe. This is an approximation, but it will get you close. If you are using a cam with a lot of duration (say 300 degrees or more) then the pipe should be 1-2 inches longer.
Of great importance is the diameter of the primary pipes of the tuned exhaust. The diameter of the pipe will directly influence the velocity of the gas flow (note: not the wave) . The diameter should be such that, at the engine's power peak, the mean velocity of the exhaust gases is about 300 ft/sec. At this speed there is a balance, between the internal friction on the pipe on the one hand and the benefits of increased gas speed on scavenging on the other. When gas velocity is high enough, the sudden rush of exhaust products from the cylinder will tend to pull much of the residual gases along, and may even leave a slight vacuum in the cylinder, which will further aid cylinder filling during the intake stroke. If the primary pipes are made too small the the gas speed becomes so great that that the scrubbing along the inside of the pipe impedes gas flow and creates back pressure. The 300 ft/second point appears to be a good compromise.
The next step is to calculate the gas speed. Here is the formula:
V= (piston speed/60)X(D2/d2)
V is gas velocity in in feet per second
D2 is the piston diameter squared
d2 is the inside pipe diameter squared
Piston Speed is in feet per minute
Now then, for our Abarth 1050 engine lets start out with the following characteristics: 8000 RPM, 2.913 inch (74mm) stroke and 1.25 inch (31mm) pipe diameter. To determine piston speed in feet per minute we take the stroke times two (remember there are two strokes, one up and one down, for each turn of the crankshaft), namely 5.826 inches. This is them divided by 12, being the number of inches in a foot. This gives a result of .485 foot. This must then be multiplied by the desired crankshaft speed of 7500 RPM to give the piston speed. This computes to 3884 feet per minute or 64.73 feet per second. We then finish off the last steps by squaring the piston diameter of 2.657 (7.059) and the pipe 1.25 inch (31mm) pipe diameter squared is 1.562 inch (39.7mm. We proceed then by dividing 7.059 by 1.562 for an answer of 4.519, then multiplied by the piston speed of 64.75 ft/sec. The exhaust gas velocity computes to 292 ft/second.
Well, this is within 8 ft/sec. of our target. Almost perfect!!
Therefore we can now indicate that a 4:1 exhaust system for an Abarth 1050 motor having a 67.5mm bore and a 74mm stroke, with a mean peak RPM of 8000 RPM, will require exhaust primaries of between 27 and 29 inches, 1.25 inches in diameter. This would give a peak power RPM range between 7000 and 8500 RPM.
As the primaries come together in the collector, care must be taken to make sure that the tubes are fitted so that the next firing cylinder is adjacent in the collector. This will greatly increase the scavenging effect. Once in the collector, the exhaust pipe can be a minimum 15-20 inches in length. You will only find the "right" length by trial and error. There are different schools of thought as to whether the exhaust “pipe” should be a megaphone, or straight. Generally a tapered exhaust pipe will promote a slightly wider power band. You will remember that Abarth used such an exhaust megaphone on many of his race cars. On some engine designs, for sake of ease of installation, you may find having to use a “resonant multiple” for the exhaust pipe. So if a pipe of 22 inches (560mm) works well, but does not exit the car, then the next choice would be 44 inches (1120mm). The TCR exhaust is a good example of this.
A word of caution!! There is a temptation to make the exhaust primary tubing too large in diameter. Bigger is not always better, unless some thing else has been compensated for.
There we have it. Easy, right !! Well actually the math is easy. Now comes the hard part of finding the right tubing sizes, and making sure that the final design has equal length tubes, and will actually fit inside the engine compartment.
A Note of Interest - I went back and computed a complimentary intake length for this exhaust system and it indicates that the total length of the intake path should be 13.2 inches (33.5cm) from the bell of the velocity stack to the back of the head of the intake valve. If you look at a 1000TC intake manifold with a 36DCD7 carburetor and velocity stack, I believe you will find that this is very close to what Abarth used.
7.4. Carburetion and Fuel injection
The standard head, be it a Fiat 600/600D 850 or A112, have all been equipped with various 2 barrel, downdraught carburetors. The two manufacturers most often used as Weber and Solex.
The most often seen Weber models are 36DCD7 and DCN/DCNF carburetors.
The 36DCD7 can be found in two versions. One has both venturies opening simultaneously, whereas the other version has a progressive secondary. I have had experience with both types. At full throttle there is virtually no difference in performance, however the progressive carburetor may have a slight advantage in terms of fuel economy at partial throttle.
The DCN series of carburetors come in various varieties and have and have been used for many years on marques such as Ferrari and Maserati. For these engines the carburetors were cast in lightweightt aluminum, whereas most versions are cast from the conventional alum/zinc mix. All DCN type carburetors are non-progressive
Both of these types of carburetors should be mounted with phenolic spacer, to isolate it from heat generated by the cylinder head.
A good starting point for jetting would be as follows
For a number of years Solex supplied a carburetor specifically suited for competition purposes. This unit, the 36-40CCI carburetor was designed for competition. Again this is a non-progressive design, made from lightweight aluminum alloy.
These carburetors are always popular and when they do become available they are quickly purchased. They have been out of production for over 25 years, but they are still one of the best designs.
Abarth equipped the TCR head with dual Weber 40DCOE carburetor. This is probably the most widely implemented carburetor design for competitive applications.
With an individual throttle for each cylinder, this provides the ultimate in fuel control. The PBS 8P head, which also incorporates individual runners, is perfectly suited to use these carburetors.
A good starting point for jetting would be as follows
F11 Emultion Tube
There are other carburetor types that have been used successfully on Fiat/Abarth competition vehicles. These include Del’Orto units from Italy and various flat slide carburetors.
7.4.2 Tuning Weber Carburetors
All Weber carburetors have four circuits.
The Idle and Cross-over circuits are inter-connected, in that the idle jet is responsible for the fuel supply for both circuits. It provides fuel to the idle mixture screw (responsible for idle supply up to about 1500 RPMO), and the crossover ports, which are hidden by the butterfly and come into play as soon as the throttle open, because the incoming air literally "sucks" the fuel into the airstream. The cross-over ports provide additional fuel in this critical phase and If the idle jet is too small, then there is insufficient fuel to satisfy the needs of both the cross-over ports, and of course the idle mixture screw as well, as both circuits are active simultaneously. This mechanism take care of the "sudden" rush of air when you open the throttle butterfly. If the fuel is not sufficient at this point, the engine will run lean and stumble momentarily. Of course once the butterfly is fully open, the vacuum on the intake tract drops, and the motor is running on the main jet and air corrector jets. The crossover ports and the mixture screw have done their job and are no longer active, at least until you close the throttle and it all starts again.
If the idle mixture jet is correctly sized, then the idle mixture screw should provide a stable idle when it is 2-4 turns from being fully closed (seated on the seat) at an idel speed of 1000-1200 RPM. Be careful when you screw it fully in, on the seat, so as not to damage anything. I suspect with the large venturi that you need to change the idle jet to the next larger size. It probably has a 45 in it now. I would change it to 50, or perhaps even a 55. Then you can back off the idle speed screw and you will then have to reset the idle mixture screw and it should come back to 2-4 turns from fully closed. This should get rid of the stumble at 1500-2000 RPM. I would also increase the main jet on both the primary and the secondary and raise it 5 points from what it is now. So if they are 125, I would change them to 135. The LAST thing that you want is to have the engine run LEAN at high RPM. It will run great for a short period of time, before the pistons melt. The carburetor also has an accelerator pump that squirts a pre-determined amount of fuel with the opening of the secondary venturi. Much like the cross-over ports help the primary venturi with a small additional amount of fuel, the accelerator pump does the same for the secondary venturi. If this were not so, then there would be an instant when the secondary venturi would run lean (on first opening) and this would cause a hesitation in the power delivery. It is important to get this part of the carburetor correct, as this is what gives the engine that "crisp" response so important when accelerating out of a corner.
Just in case you friend does not have the right jets, get on the Web and go to www.mcmaster.com or some equivalent Canadian company and order an assortment of miniature drill bits and a "Pin Vise". I would order drill bits from 1 to 2.5 millimeters in .05mm steps, or some 30 drill bits, and a .45, .50, and a .55mm drill bit as well. You are now equipped to deal with any jet in any carburetor (except those that use needles, but that is another story). They should be between $1 and $2 dollars each. Make a little holder from a block of wood and label each of the drill bit by its size. A 125 jet has a hole 1.25mm, and a 130 jet has a hole 1.30mm. As you can see, as a last resort you can make your own jets. If you need to go smaller, a soldering iron and some regular soft solder will close the hole, and then you can re-drill it for whatever size you need. The drill bits are also your "gauge" as you can with them measure any jet to make sure what size it is.
As far as high speed running is concerned a good rule of thumb is to start with 125 Mains and 175 Air Correction jets (a numerical spread of 50) if you have no idea where to start. The fuel and air as joined together in the "emulsion tube". In the cavity in the emulsion tube the air/fuel is mixed (emulsified) and aerated before it is fed to the opening in the secondary venturi. The air stream passing through the secondary venturi literally carries the fuel with it into the combustion chamber. I like F11 emulsion tubes for DCOE carburetors, but regardless of the type of carburetor, the emulsion tube is the LAST thing that you fine tune, if necessary. The engine should at least run on this combination. Next I recommend that you use a Air/Fuel meter to monitor high speed fuel situation. The key number is 13:1. Yes, a stochiometric mixture would be 14.7:, and this is fine if you are tuning to pass a smog test, or for ultimate fuel economy, but it is much too lean for a competition motor. If course if it reads 10:1, then you are much too rich. Be careful, you need to test the entire RPM range, as it could be that slow running is OK but wide open throttle is too lean, or vise-versa. If you do not have an A/F meter (also known as a Lambda meter), then you have to use a bit of intuition.
If the engine misses at high RPM, then it is entirely likely that the main jet is too small, particularly if the exhaust pipe is very light in color (white to very light grey). The engine is leaning out at high RPM and if this situation persists, then you will do major damage. You can have the same miss if the engine is grossly too rich, except that the tailpipe will now be black and it will be spewing large amounts of black smoke (unburnt fuel) and it may foul the plugs. If you are standing behind such a car, it will not be long before your eyes begin to water.
This brings us to how to adjust main jets. Adjustment steps in the main fuel jet are generally done in .05 sizes (from a 125 to a 130). Of course the larger the number the more fuel it will pass. Main jet changes affect the carburetor fuel delivery throughout the entire RPM range, once the carburetor is past the cross-over circuit (from about 2200 to ????? depending on how brave you are).
Air Correction jet use a different formula. In short, 4 steps of air correction change is roughly equivalent to a single incremental step in fuel jet. Change the air correction from 175 to 195, and that would be the equivalent to changing the fuel from 130 to 125. Confused? In essence you are letting more air into the emulsion tube in relation to fuel, so the mixture is "leaner". Here comes the interesting part. Whereas a fuel jet change affects the entire RPM range equally, changes in air correction jet have greater effect at RPMs above 5000 RPM. So if the engine is running fine everywhere, but a little lean at top RPM, then you could fine tune it by going 5-10 points smaller on the air correction jet.
Tuning Webers is as much an art as it is a science, as every now and then you will have an engine that needs something totally different, but this is VERY rare.
As we are talking about competition purposes, I will only discuss those variations of fuel injections that are directly applicable to competition vehicles.
There are basically two type, mechanical and electronic. From a design perspective they both employ a “throttle body” per cylinder. On the mechanical side the only one that was actually implemented on an Abarth vehicle was the Kugelfischer type. This had a “slide” type throttle, instead of butterfly type, and a complex belt driven mechanical, high pressure pump.
The latest ECU driven, electronic fuel injection systems therefore have a real advantage over the earlier mechanical system. If you look at the early Kugelfisher systems, these were really only totally efficient at near, or full throttle. It is for this reason that the injectors were placed well upstream. This aided high RPM performance, but did little for lower RPMs and made for a very narrow usable power band.
It would be possible to use an ECU that could be programmed to fire two banks of injectors, either singly or together. This would allow me to place one set of injectors just past the Injector Throttle Bodies (ITB) or as close as practical to the valve, and the other set above the inlet trumpets. (See diagram). I found a company called Extrabody that was working on something similar and we are now collaborating on a solution that could be produced as a kit for the PBS 8P cylinder head.
The Extrudabody ITB units can be mounted on a manifold designed for a Weber DCOE carburetor. This makes the system simple to mount to the 9P head. Because the system is modular, additional extrusions can be added, either before or after the ITB portion to custom tune the intake runner length to meet almost any design criteria. As the second drawing illustrates, the system can also cater for the two injector idea that I described earlier.
Using one of several aftermarket ECUs, a map can be derived that automatically sequences between the two injector banks, either using both injectors at the same time, or switching from one bank to another.
Now, I understand that this type of system is not allowed in many racing clubs, but where it is, it hold some reasonable promise, particularly when you add to this the ability of the ECU to control spark advance as well.