Cylinder heads are a very important part of a two-stroke engine.
Their design can be very critical to how an engine works and how reliable that engine will be.
There are plenty of theories to complicate matters, some work, some donít, some are relative to Lambretta engines and some are not.
Things could get a bit complicated so we wonít bore you with the theories but only what works for MB Developments engines.
Usually the higher the compression ratio the higher the horsepower would be.
This would be fine a few years ago using 4 star fuel or higher-octane fuels.
Unfortunately only unleaded low octane fuel is freely available now.
Today compression ratios are governed by this fact; low octane fuels need lower compressions to make the engine reliable! By doing this some loss of speed may occur. It's swings and roundabouts, reliability over speed.
CYLINDER HEADS THE MB DEVELOPMENTS WAY
As mentioned head design is very critical to how your engine will work.
Different types of pistons have different dome heights depending on model and manufacturer.
With this in mind MB Developments cylinder heads are reprofiled to suit the shape of the dome used.
This then gives a perfect squish clearance, which is a very important part of the engines design.
The squish clearance is the gap between the piston and head at top dead centre.
Most road going Lambrettas should run a minimum of 1mm and a maximum of 1.5mm squish clearance to achieve maximum benefits from the engine.
A squish clearance below 1mm could mean at high rpm that the crank, con rod, piston and bearings could stretch, expand and then the piston could hit the head, causing a failed engine.
A perfect squish clearance would compress the air fuel mixture into the combustion chamber to give a maximum spark and power once ignited, without the piston hitting the head at full rpm.
A squish clearance of more that 1.5mm would mean that the trapped mixture left in the squish band wouldnít be compressed into the combustion chamber, causing the mixture not to fire correctly and a lack of power would occur!
On most Lambretta engines machining a head to the dome of the piston will give a correct squish band for power.
When a compression test was done you would find that, even with 4 star fuel the compression ratio would be far too high, for race use let alone road use! This was the cause of many an engines failure over the years!
To lower the compression ratio the combustion chamber would need grinding out and polishing to lower the amount of ccís of compressed volume.
Over the years MB Developments have machined, tried and tested thousands of cylinder heads.
We find that our method works and has cured many a faulty tuned motor.
Different scooter tuners apply different methods of cylinder head design.
Lambretta machined their heads to a curved dome but it wasnít true to the dome of the piston, it had a smaller squish clearance to the out side of the cylinder bore and a larger gap towards the centre of the bore.
This effectively lowers the compression but would also looses power. Some dealers prefer this method.
A method being used at the moment by other dealers is to fit a fat head gasket to lower compression.
The need for this to happen has come around with unleaded fuel and incorrectly machined cylinder heads.
Yes, this lowers compression but increases the squish clearance as mentioned this is incorrect!
Other dealers take an average of the dome in a straight line and cut the head straight.
This would mean a trapped mixture could be gained on the outside of the bore, with a tight squish clearance in the middle then a larger clearance towards the centre of the bore again effecting compression ratios and the way the spark could ignite.
Most dealers leave the combustion area alone then retard the ignition more or over jet to compensate for the over high compression.
Whatever method used they all work in their way but MB Developments stick to our well-proven method.
There are two ways to check compression ratios.
This is the old way used by most engine manufacturers.
It measures the volume of mixture from bottom dead centre to top dead centre using a formula.
This method and theory is used to find an engines cubic capacity.
This is the method the Japanese use.
This measures by a formula the trapped mixture from top of the exhaust port, as this is when a true trapped mixture occurs.
A compression ratio measured the uncorrected way would show a higher reading than that measured the corrected way, this can get a little confusing when reading other people and companies technical specs so be aware!
MB Developments have used a mixture of the two to find suitable compression ratios for road going scooters.
What amount needs to come out varies from engine to engine, this has mainly come about by trial and error, experience and a lot of compression ratio checks.
CYLINDER HEAD DESIGN
Motor cycle designers over the years have used a number of head designs based on different theories.
Lambretta have also used different designs based on their theories.
But lets face it a Lambretta has never been a full race two stroke engine itís a glorified shopping / commuter transport machine, aimed at reliability with high mileage and high miles per gallon.
The Lambretta manual and the tuning manual show various head types with different compression ratios; donít worry too much about this as it means nothing to us in this modern age.
A lot of difference in ratios occurs though different head gasket thickness.
With this in mind we will look at what is available for a tuned Lambretta engine.
125 ENGINES, 53mm DIAMETER CYLINDER HEADS
There have been two types.
1. Side plug, side squish (offset) combustion chamber, as used on most Li 125 engines. Part number 19111015
2. Side plug, centre combustion chamber, as used on the GP125 and 125 special. Part number 19411015
For most purposes both heads are unusable, as the compression ratio would be far too high.
It has been known to use them with 175 conversions as a last resort but more alloy needs grinding out compared to other heads.
150 ENGINES, 58mm DIAMETER
There have been two types.
1. Side plug, side squish (offset) combustion chamber, as used on Li150, 150S and GP150 engines. Part number 19911015
2. Side plug, side squish (offset) combustion chamber, Part number unknown
There seems to be some confusion with both types of cylinder heads, as part numbers were not always cast into the heads.
They are very similar except one has a larger combustion chamber volume than the other, 150 heads are ideal to use on 150 Ė 175 conversions.
The lower compression type is best as very little needs taking from the combustion area, but both can be used.
Both scooters Indian, API and the Spanish factories have produced 150 heads and are all basically the same, all types are usable for all conversions.
175 ENGINES, 63mm DIAMETER
There is one type.
1. Side plug, side squish (offset) combustion chamber. Part number 19711015
ē The Italian pattern firm MDP has produced a 175 head over the years but was inferior in quality.
ē A 175 head is available from India and is considered the same as an Italian head, these heads are very good.
175 cylinder heads are ideal to use on 150 - 175, 150/175 - 190 or 175 - 200 cylinder kits as they have lower combustion chamber volumes.
These heads usually just require polishing after machining.
200 ENGINES, 67mm DIAMETER
There are two types.
1. Side plug, centre combustion chamber, as used on the GT/ TV 200 engine. Part number 42011002
2. Side plug, side squish (offset) combustion chamber. As used on SX200 and GP200ís. Part number 42911002
The GT head is to be considered useless these days.
An old conversion was to weld up the plug hole and replace it in the centre of the combustion chamber where it should be for maximum efficiency. MB Developments offer special cylinder heads for this conversion with a lot more scope.
SIDE SQUISH 200 HEADS
MB DEVELOPMENTS CAST CYLINDER HEADS
MB Developments offer special cast cylinder heads if required.
1. Centre plug for use with cowlings.
2. Centre plug for use with out cowlings.
3. Centre plug liquid cooled.
These cylinder heads are available as blank heads and can machined to suit any engine.
They are made from heat treated alloy making them stronger than any other head available and are less liable to distort.
They are designed to be centre plug heads only and can be used as a bolt on improvement over the standard style cylinder heads.
They were originally made for our racing engines but are ideal for any road going Lambretta.
They can be machined to suit any engine size, piston style, and combustion design or compression ratio.
The plug needs to be in the centre position as this design gives maximum performance over the side plug design as used by Lambrettas.
The only problem is it is difficult on most engines to get to the spark plug.
The best method is to remove the rear shocker then access is easy using an extended socket and extension providing the exhaust isnít in the way, this can be done without removing the runner boards.
The heads are much more suited on cut downs or racing type Lambrettas.
But when the engine is reliable and set up correctly it is usually not a problem on full bodywork Lambrettas.
Please E-mail us with any questions or comments.