
A
little extra power 


Working
it out: BSA M20/21 basic engine alterations. 

Cylinder
capacity 
If
you are contemplating altering an M20/21 to increase its capacity or
compression ratio it is useful to be able to work out in advance the
effect of the changes you are thinking of making. The most common
alteration is to overbore the cylinder as this will produce quick results
in terms of a bit more power. Increasing the bore size substantially will
also bring the added benefit of a higher compression ratio. To calculate
cylinder capacity in cc use the following formula: 

3.142
x R x R x L where R is the radius of the bore and L is the stroke of the
flywheels. So for a standard M20: 3.142 X 41 X 41 X 94 = 496cc.
For an M21: 3.142 X 41 X 41 X
112 = 591cc. 

Compression
ratio 
To
calculate the compression ratio the volume of the cylinder head is
required in cc. This is usually done by
filling the cylinder head space with a thin fluid, such as
paraffin, from a graduated vessel containing a measured volume of fluid.
Once the head has been filled it is a simple calculation to deduce head
volume by deducting the amount of fluid left in the vessel from the
original total. In the case of the M20
and M21, which have identical head volumes this figure is 127cc. To
calculate compression ratio use the following formula: 

V1
+ V2 divided by V2 where V1
is the cylinder volume in cc and V2 is the cylinder head volume in cc.
So for a standard M20: 496 + 127 = 623 divided by 127 = 4.9~1. The
same formula applied to the M21 gives a result of
5.6~1 

These
figures show the effect on compression ratio of the 95cc capacity
difference between the M20 and M21 when the head volume remains unaltered
but the cylinder capacity is increased. 

M21
with a B33 piston 
An
easy alteration for the M21 is to fit a piston from the OHV B33 (of the
correct type) This piston at the std. bore size is 85mm dia and can be
fitted to a late unlinered M21 barrel simply by overboring. Using the
formula to calculate cylinder capacity the resulting volume is: 

3.142
X 42.5 X 42.5 X 112 = 635cc. If this new capacity is now used with the
formula to calculate compression ratio the result is: 635 + 127 = 762
divided by 127 = 6.0~1. so capacity has increased by 35cc and compression
ratio by .4 of an atmosphere over standard 

These
unlinered barrels will take a bore size of up to 90mm quite safely so it
is just a question of sourcing suitable pistons. From a practical point of
view compression ratio in this type of engine should not exceed 7.1. 

If
the cylinder head joint face is machined to increase compression ratio the
head volume must be re measured to ascertain its new volume before using
the above calculations. The easiest way to do this is while the head is
off the engine. Turn it upside down and set the gasket face level before
filling with fluid as described previously. 

