2 replies to this topic

[Jordy869]

Hi all,

I was hoping someone can give me as much info as possible with regards to rebuilding an engine suitable for a supercharger. My 1275 needs boring out to 1293. Now as it’s being stripped and rebuilt what’s best for a supercharger.

As much info would be deeply
Appreciated.

Thanks

[johnR]

Deep pockets?

[Cooperman]

Installing a forced induction system on a normally aspirated engine is more complicated than it might at first appear.

 

The forced induction effectively raises the combustion pressure in the chambers and a lot of calculations and careful component selection is needed.

 

First of all it will be necessary to start with a lower then standard static compression ratio. However, pistons will need to be able to withstand the increased combustion pressures. The supercharer boost will then need to be selected and the drive pulley(s) chosen to provide the correct amount of boost.

 

A different dustributor advance curve is vital to ensure that the timing advance is matched to the new compressions. If the advance is not modified, it will cause premature detonation which will destroy the pistons in a very short while.

 

There are several publications about this on-line. When I engineered a Shorrock supercharger onto a MK.1 Austin-Healey Sprite with a 1275 engine I used an old Shorrock guide manual, but used modern toothed belt drive with the sprockets selected to give the correct pressure ratio. Of course, the Shorrock is an old design of supercharger, but it was used on Minis in the 1960's & '70's. The principles are still the same.

 

These days most people modifying for forced induction use a turbo-charger and for the A-Series engine the design is well-established. For this to work, an engine builder must know the maximum boost figure and the turbocharger should have a boost calibration certificate confirming the max boost which you have calculated is what the unit will give.

 

What you cannot do is simply have an engine re-bored, fit new standard pistons, bolt on a supercharger or turbocharger and drive away. I had an engine brought to me around 12 years ago where this had been done. It was turbocharged, but no calculations had been done and no engineering carried out to get it right. In fact, it was 'all wrong'. The engine had lasted for 5 miles before a piston broke and the resulting damage cost the owner a small fortune to correct.

 

It is not particularly difficult if you are a capable engineer, but accurate calculations and use of the correct matched parts is vital.

 

You might want to download this:   The ABC of Shorrock Supercharging - RJH-Heritage Switzerland

 

I hope this helps.

Edited by Cooperman, 02 March 2021 - 10:55 PM.