Effect of induction system on ideal ignition timing?

[djaychela]

Unfortunately due to time restrictions (in order to compete on Wales Rally GB this year), I've had to shelve my plans to rebuild my ex-works Group A Skoda 136 engine, and instead have bought a running engine from someone. This is on DTA management and Weber Alpha throttle bodies, which are not allowed in Group A, so I will have (unfortunately) to re-fit the original MPi inlet manifold and single throttle body. While I realise this will probably have an effect on the power output of the engine, while re-mapping I was wondering if you had any ideas if the ignition advance map would need to change to suit the new setup? i.e. would it be better to reduce the advance to be on the safe side and get it totally re-mapped?

Do you have any experience of this sort of thing (although I realise it would probably be the reverse, taking a single-throttle-body manifold and then replacing it with individual throttle bodies)?

[Guy Croft]

OK

It won't burn anywhere near as effectively on a single throttle body - so would tend to need more advance at some load sites - cannot tell you which ones, so it will probably run absolutely fine but be down on power for that reason and also because the inlet tract will a lot less effective as it was with split runners.

Because of the supreme importance of having the map presets entered properly I know what I'd do - get on to Phil Lanes at GG Motorsport or Northampton Motorsport (they are both site members) and get the revised setup checked out.

The fuelling needs checking too - it will no doubt need leaning out.

My take on mapped units: I would never advise anyone to take a chance with a 'mapped unit' - get those settings wrong = wrecked engine from detonation/overheating/over-underfuelling. Very, very sensitive issue with me that one.

G

[djaychela]

Thanks very much Guy - useful info, as ever, and I understand your wariness over "mapped" units - when I start the unit up to check it I'll have my LM-1 monitoring the mixture, and certainly won't be doing anything with any load put on it.