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engine tuning


intake system

engine modification

exhaust system


The performance of an engine directly depends on the amount of mixture that goes through / is burned and is transferred into mechanical energy per unit time (power).

To increase performance, there are 3 possibilities:

A: Optimize combustion

B: Improve the mechanical efficiency

C: Increase the amount of mixture put through per unit time, and with it the volumetric efficiency.

Which possibilities can be achieved following these physical ground rules?

\b Possibility A

\b0 can only be done up to a point since the engine constructor has set most parameters.\line\b Possibility B\b0 can not be done without making big changes and many long dyno tests.\line\b Possiblitly C\b0 is the normal way to achieve this goal.

Possibility C gives another 3 options:

1: Displacement can not be replaced by anything, except yet more displacement !

(If a little is good, more is better, too much is perfect, Yankee saying)

2: With longer camshaft duration. This has it's limits, and these are easily found.

3: By increasing rpm. This "costs little" and "weighs nothing" but sooner or later it takes it's toll... What good is an engine that wakes up at 6000 rpm and dies off at 8000 rpm? Surely this is no fun during daily driving. And how are you going to get up the sidewalk at night without waking up the whole neighbourhood? A usefull combination of all given options give a "smart" solution.

This includes most possibilities. The remaining are chemical induction like Nitrousoxide and mechanical charging with a supercharger or turbocharger. Fuel Consumption:\line Nothing is for free, this is something we can't change. The mechanical science teaches us that with the same rpm and increased performance, a non proportional consumption increase corresponds with the increased performance. To illustrate this: If an engine at 5000 rpm, uses 10 liters and gives 50 bhp, the same engine at 5000 rpm giving 75 bhp, will not use 15 liters but something like 13/14 liters... } After every engine modification the CO% has to be checked throughout the rpm and load range to see if rejetting of the carb is needed. Any rich or lean mixture spots can seriously damage your engine. Setting up a carb properly is vital for the engine's life and is a specialist's job.


  Cylinders. Bigger capacity is best so go for 1300 barrels and pistons. Without modifications, these only fit on a 1300 crankcase.

Cylinder heads. The 1130 and 1220 heads have a much bigger inlet port then the 1300. To use 1130/1220 heads on the 1300 cylinders, the groove where the cylinder sits in the head has to be enlarged. Some metal has to be removed from the outer edge of the combustion chamber as well for the 1300 piston to clear the 1130/1220 head. These modifications are the smallest on the 1220 heads of course.

Valves. The 1300 has 38mm intake and 35.7mm exhaust valves. The 1015, 1130 and 1220 have 39mm intake and 34mm exhaust valves. So if you have the 1130 or 1220 head, optimal valve sizes are 39mm intake and 35.7mm exhaust.

Camshafts. The best camshafts are the long overlap camshafts from the G12/619 1220 engine. However, these camshafts are very hard to find. The next best thing is to use the camshafts that come with 1130 or 1220 heads. ECO camshafts have rockers with a trapezium head. Other camshafts have square head rockers. Don't mix camshaft and rockers. Crankshaft. Crankshafts for 1130, 1220 and 1300 are identical but the 1130 uses main bearings from different material, more suitable for high rpm.

Crankshaft work: Crankshaft balancing, lightening, aligning, reworked oil ways. Con-rod balancing and aligning. Improved bearing material.

Displacement increase. Overboring the barrels and fitting larger pistons will give more power but more importantly more torque.

Compression ratio increase. This can be done by a number of methods:

Reducing the height/length of the barrels. Letting the barrels fall deeper into the cylinder head. Fitting pistons with higher top. Always make sure there is sufficient valve-to-piston clearance.

Stage 3: Camshaft change. A camshaft with more duration / higher lift / more overlap / different ramp angle can improve power and torque. Camshaft profiles is an art in itself, and very few have mastered this art. Finding the right profile for the job can be very difficult. 2 cylinder engines: To change the camshaft, the engine has to be completely dismantled. When fitting a new or reground camshaft, be sure to use new / reworked followers.

4 cylinder engines: The camshafts can be changed without even removing the heads. When fitting a new or reground camshaft, be sure to use new / reworked rockers.

Stage 4:\b0 Cylinder head work: Enlarged ports, ports matched to manifolds / manifolds matched to ports.

Low resistance flow paths. Bigger valves with different angle or 3 angle valve seats. When modifying the heads and or valves, make sure the valves and valve seats are suited for running on unleaded fuel.

An engine build from the parts described above, with rejetted carb, large bore low back pressure exhaust system and low resistance air filter should make over 75bhp with a very wide torque spread. For the GS(A) (1300) engine the following displacement increases are possible: Capacity: 1436cc, Bore: 83.5 or 84mm This is the maximum displacement increase possible without altering the crankcase. \fs24 \fs20 Capacity: 1595cc, Bore: 88mm This displacement increase makes use of larger barrels. Therefore the crankcase has to be modified.

Note: GS(A) / Axel barrels are not hardened as many people think. They are the same hardness through and through. Therefore overboring the barrels can be done without risk when the walls remain thick enough and the right piston rings are used.

intake system

  To achive air flow with as little drag as possible first thing that opposes this is an air filter. Nornal air filters are not made for high performace applications, they are made to dampen intake noise. Custom air filter should be used instead, with liberal amounts of cold air flowing either directly from a bonnet opening or from a tube inlet.

Intake manifold is Helmholtz resonator, it has its own resonating frequency, where its action supplements piston's air pull. Longer intake pipes between carb and head give more low rev torque. Short intake pipes (often found on twin carb systems) moves the power higher up the rpm range and reduces the torque spread.

Intake manifold. From July 1981 onwards there are no more rubber hoses on the right hand side pipes. Manifolds without rubber hoses should be used.

Carburetors. There is no performance difference between solex and weber carbs. Any carb properly adjusted / jetted for the engine will give optimal performance.

ignition and other electronic devices

  Electronic ignition. This results in less maintenance and a stronger spark. Full engine management ECU controlled fuel injection and ignition. This is done with a programmable ECU (computer or black box) which is connected to all other hardware (sensors, regulators, etc). The ECU is programmed by laptop (usually) to give the right injector time and ignition timing according to engine load, rpm and environmental conditions. Getting an ECU from another car will not work since every engine has different fuel and ignition needs.

temperature regulation

  Any power increase will also increase the amount of wasted heat. This heat has to go somewhere or the engine will overheat. Therefor sufficient cooling needs to be available. On an air-cooled engine there is not much else to do then cool the oil. To reduce oil temperature there a 2 basic options: Increasing the amount of oil in the system. The easiest way of doing this is by fitting a larger oil filter.

On 602/652 engines this involves modifying the oil filter bracket or making a new one. Fitting a bigger or additional oil cooler. This can be done by taking of the original oil cooler and repositioning a bigger one in the cold air stream. Make sure there is enough protection from stones and such and use braided hoses. Another option is leaving the original oil cooler alone and fitting a sandwich plate under the oil filter. This is a plate with 2 hose connections which connect to an additional oil cooler. The plate can be fitted with a thermostat so the engine can get to working temperature easily and the new oil cooler only cools when needed. Before and after making modifications to the oil circuit, measure oil temperature and pressure. This way you know what is happening under the bonnet. On any tuned engine, oil pressure and temperature should be visible on a meter in the dashboard.

Higher then standard output engines put an additional strain on all other components of the car. Tuning is not just about more power. The rest of the car has to be up to it as well. Some questions which can help find the weak points on your car: Have their ever been repairs to the chassis? Is there any rust on the body? Are the brakes, tires, driveshafts new or nearly new? Does the gearbox make noise, or crunch sometimes? How about the legal aspects of modifications planned? If any of these points is not ok, you should get the car in good condition before you start to change anything.

exhaust system

  Exhaust system. Large diameter tubing and low resistance silencers, result in minimal resistance systems with low back pressure. Longer primary pipes before they join give more low down torque. Make sure both sides are equally long. Separate exhausts left and right should always have a balance pipe for decent cylinder filling at lower rpm. Lightened flywheel. This improves acceleration. Effect is the same as the amount of weight reduced on the flywheel x total gear ratio, weight reduced on the car. But lightening the flywheel will result in in lumpy idleing and less smooth pickup.

Advanced Ignition Timing


Moves peak cylinder pressure close to top dead center. Puts more energy to the piston and less to the exhaust, water, and oil.


Can build peak cylinder pressure beyond the threshold of detonation.

Retarded Ignition Timing


Can forestall detonation by moving peak cylinder pressure further past top dead center.


Can cause high exhaust and water temperatures. Power will be lost.

Richer Air/Fuel Ratio


Cooling effect stabilizes combustion on long wide-open throttle runs.


If too rich, fouled spark plugs and loss of power can occur.

Leaner Air/Fuel Ratio


Many modern engines will produce slightly higher power if the air fuel ratio is leaner by a small amount.


Not a recommended technique. Can generate detonation during long hard runs.

Higher Compression Ratio


Better efficiency and lower brake-specific fuel consumption (fuel-to-power ratio).


May not allow higher boost levels that could have produced more power than any gain from the increase in compression ratio.