The spring within the suction chamber loads the piston in a downward (closed) position. The tension of the spring is selected such that the piston reaches its fully open position when the engine reaches its maximum air demand, that is maximum brake horse power output, not maximum RPM, at any given engine speed up to maximum BHP:
If the spring is too weak, the piston will be elevated to a level higher than its optimum position causing the engine to run too lean.
If the spring is too strong, the piston will not rise to its optimum position causing the engine to run too rich.
Why is this so? Surely, if the piston is higher than it should be, the engine will run richer, and if the piston is lower than it should be, the engine will run leaner?
Wrong! Remember the volume of air the engine draws is proportional to its speed.
So… If the piston rises too high the throat area will be larger than optimum, the vacuum between the piston and the bridge will be low. That is the air velocity across the jet will be low, drawing less fuel, thus causing the mixture to run lean.
The opposite occurs if the piston does not reach optimum height the throat area will be smaller than optimum… The air velocity will be high between the piston and bridge causing a larger volume of fuel to be drawn through the jet assembly causing the mixture to run rich
There are a number of springs available for the SU type carburettor each with a different compression loading. They are:
SU Carburettor Springs
I am unsure if Hitachi, via Nissan, released their variable choke carburettor with a range of different compression rate springs. I believe however, that the HS6 SU springs will fit the Hitachi carbie if a substitution is required.
I have not measured the spring compression rates of the Hitachi carburettor therefore I am unable to compare the ratings between the recommended spring used in the SU carbie and the Hitachi spring. Red springs are recommended where 13/4” SU carburettors are bolted onto the inlet manifold of an unmodified L24 or L26 engine.
DASH POT/DAMPER ASSEMBLY
The dashpot and damper assembly are also located within the suction chamber. The damper assembly, which is actually a one way valve, is contained within the dashpot filled with oil. The valve and oil work such that they impede the lifting of the piston, but allow it to fall rapidly once the speed of the engine decreases.
The dashpot serves two purposes. Firstly, it acts as a damper to prevent the piston following air fluctuations at low engine speed thus keeping the piston steady. Secondly, when the throttle opens it prevents the piston rising in unison with the opening of the throttle. If the oil in the dash pot assembly is too thin the piston will rise too quickly causing the air/fuel mixture to lean out.
Air and petrol, in a hydraulic sense, are both fluids and air is less dense than petrol. Therefore, air has less inertia than petrol. So when the throttle is opened more, air will be sucked into the carbie but the petrol will take a little longer before its flow rate catches up with the new air flow rate.
By damping (retarding) the piston movement with oil an accelerator pump action occurs, ie. as the throttle is opened, the movement of the piston is retarded a sufficient amount to cause a momentary enrichment of the mixture, enabling a sharp pick-up in engine speed.
What type of oil should be used?
Too often people use light duty (sewing machine or general purpose) oil in the dash pot assembly. This type of oil does little if anything to impede the upward movement of the piston as the throttle opens.
Engine oil can be too viscous (depending on climate). After 2 hours of driving it ends up in the bottom of the piston, the majority of it sucked into the engine. This happens because it is too thick to pass through the damper as the piston falls causing the oil to flow out of the top of the dashpot.
I use a mix of 20W-30 to 20W-50 and sewing machine oil. The ratio is three parts engine oil to one part sewing machine oil.
When you use the aforementioned oil mix if you attempt to raise the piston when the engine is cold you will find that a lot of force is required to move the piston to its uppermost position. When you release the piston, it will drop to the bridge quickly (less than half a second).
THE FUEL METERING NEEDLE & JETS
There are literally hundreds of needles available for the SU carburettor, the majority with profiles manufactured to suit particular vehicle engine systems. There are two types of needles available for the SU carburettor, biased and unbiased.
The biased needle has a collar and spring attached to the top of the needle. To eliminate droplets of fuel forming on the needle it is located within a bushing located in the underside of the piston. The needle is a loose fit within the bush and is loaded by the spring in a downward direction causing the needle to lightly contact the side of the jet. All anti-pollution SU carburettors are supplied with biased needles.
Fig 3 Two needle types
Figure 3 shows the two needle types, the biased needle being the one on the right.
The needle profiles are measured at 3mm (1/8 inch) intervals along the centre axis as indicated in Figure 3.
The vehicle speeds given below are a generalisation and assume that the throat area of the carburettor will not restrict the airflow therefore affecting the volumetric requirements of the engine (piston fully open at max brake horsepower)
The first two dimensions (1 and 2) govern the idling mixture. The next five dimensions: 3 to 7 govern the pick up in fourth gear, from 30 to 70 kph (approx 20 to 40mph). A cruising speed of 60kph (35 mph) will lie somewhere around the fourth dimension, a cruising speed of 80kph (50mph) will occur around the sixth dimension. The dimensions from 8 to 13 affect top end rev range of the engine. The last 3 dimensions, with 13/4” diameter carburettors, do not actually take part in the fuel metering process.
According to the S.U. Fuel Systems Catalogue, the following needles are the most suited to the 240 and 260z:
OTHER FACTORS AFFECTING THE TUNING OF THE ENGINE/CARBURETTOR
SIZE OF CARBURETTOR
An alteration in the size of the carburettor should only be necessary if the breathing capacity of the engine has been altered substantially. This situation may be necessary if larger inlet valves are utilised simultaneously with alterations to the head, ports and cam, and/or an increase in engine capacity.
I installed a camshaft that gave additional lift to the valves and increased the inlet and exhaust valve duration.
I found that the red springs were inadequate, that is the engine speed would never reach redline, and the bottom end power was inadequate. At around 5500 rpm, it would miss-fire excessively.
I then uprated to yellow piston springs and found that the engine would rev to around 6500 rpm and then start miss firing. The bottom end power increased somewhat.
By finally changing to green springs the engine revs out well past red-line, about 95kmh (or about 60mph) in first gear (I have a 3.36 diff and a 2.9 first gear), and has plenty of bottom end torque.I did not need to change the fuel needles.
I don’t have access to a chassis dynamometer so, the spring changes were done by trial & error. Had I made other internal changes to the heads, ie. larger valves or lumpier cam, I may have needed to replace my existing carburettors with twin 2” or triple 13/4” carbs.
AIR FLOW INTO THE CARBURETTOR
The medium, through which the air passes and enters the carburettor throat, can greatly affect the air fuel mixture entering the engine.
Air filters tend to reduce the airflow and lean out the air/fuel mixture entering the engine. The density of the filter element reduces airflow.
There are two types of filter element available on the market at present. These are the paper element type and the oil impregnated foam element type. Each of these have advantages and disadvantages.
Paper element filters tend to give less air flow restriction for a given element surface area but can allow more micro-fine dust particles into the carburettor which can build up inside over extended periods of time.
Oil impregnated filters tend to give greater air flow restriction for a given element surface area but are better at filtering out the smaller dust particles, provided of course they are maintained properly. Therefore, by fitting an oil-impregnated filter to the inlet side of the carburettor it may be necessary to enrich the mixture to accommodate the change of filter type.
Ram pipes, also known as ram tubes or velocity stacks, are horn shaped devices that can be fitted to the inlet side of the carburettor to improve the performance of the engine.
Fig 4 Ram Pipes
Figure 4: The length and shape of the ram pipe determines the rev range over which the engine’s power curve is affected. More air flows into the engine due to the following factors:
- the difference in the cross-sectional areas at the inlet and engine side of the ram pipe;
- the cross-sectional shape (taken along the centre axis of the ram pipe);
- inertia of the air entering the ram pipe (hot day = air less dense = less air into engine, cold day = air more dense = more air into engine).
The following explanation refers to Figure 4.
Air is passing through the ram pipe into the carburettor and into the engine. A vacuum is present at the mouth of the ram pipe when compared with the surrounding atmospheric pressure. The air velocity at the mouth of the ram pipe is low when compared with the air velocity at the mouth of the carburettor. The volume of air available at the mouth of the ram pipe is large when compared with the nominal throat diameter of the carburettor.
As air is drawn into the engine, it passes from the mouth of the ram pipe (A) into the carburettor its velocity increases due to the reduction in pipe diameter. The air also has more inertia due to this increase in velocity. The inertia of the air passing through the carburettor enables an increase in engine performance.
Air is compressible, so let’s consider what happens within a theoretical cylinder inside an engine as the piston reaches the bottom of the inlet stroke and the inlet valve closes. The engine is fitted with a carburettor only.
Say the volume of the cylinder is 250cc. Ignoring friction losses, a normally aspirated cylinder without a ram pipe will suck in 250cc of air/fuel mixture, depending on valve timing. The air/fuel mixture within the cylinder at the bottom of its stroke will have a density approximately equal to the atmosphere surrounding the engine. The inlet valve then closes and the air/fuel within the cylinder compresses and becomes denser as the piston rises in the cylinder.
Consider the same cylinder/engine under the effects of a carburettor fitted with a ram pipe. The higher velocity air/fuel mixture also has greater inertia, as the piston reaches the bottom of its intake stroke and before the inlet valve closes a greater amount of air/fuel will enter the cylinder. This may only be a couple of cubic centimetres (cc’s).
Effectively the higher velocity air, due to its inertia, is pushing more air/fuel into the cylinder, creating a Ram Effect. As the inlet valve closes the additional volume of air/fuel is trapped within the cylinder. With more air/fuel mixture in the cylinder, the engine develops more power. I have fitted a pair of two-inch ram pipes to my Zed and have found that the vehicle’s performance improved over the rev range 3000 to 4500rpm.
You will probably need to readjust the engine idle speed to accommodate the change in air fuel mixture. Once completed the improved performance characteristics of your vehicle should be noticeable under acceleration.
§ Why do the variable choke carburettors connected to the Zed engines have a pipe linking the inlet manifolds?…
This pipe connects the inlet manifolds to minimise air pulsations through the carburettors caused by engine cylinder firing order.
The 240Z and 260Z firing order is 1, 5, 3, 6, 2, 4
The timing diagrams in Figure 5 depict (approximately) the air pulses through the front & rear three cylinders of a theoretical engine six cylinder engine with no balance pipe installed.
Without the balance pipe the piston follows the fluctuations in manifold vacuum in turn affecting the air/fuel mixture feeding the respective cylinders. This situation is more noticeable at low engine speeds
The balance pipe enables the presence of a continuous vacuum in the inlet manifold reducing the pulsation effect caused by the opening and closing of the inlet valves.
Note: With a balance pipe installed between the two inlet manifolds piston fluctuation will be minimal for both carburettors.
Zed owners should not be concerned about the pulsation effect because if the engine is idling at 750 RPM each inlet valve opens/closes 350 times/minute, or 5.83 times/second. There will be 35 pulses per second in a six-cylinder engine. At 3000 RPM the pulse rate will be 140pps. Higher engine speed therefore reduces the pulse effect.
©2001 Mal Land – The reproduction of this document (on any form of media), without written permission of the author, will incur legal action.
Tuning S.U. Carburettors 4th edition
by G.R. Wade
Published by Speed Sport