High compression engines and reduction starter motors

I have just accquired and fitted a 4.2 that was rebuilt for performance with 9:1 pistons,
but measured compression of 185psi on all cyls.

It came minus starter motor, and the used S3 XJ unit I fitted failed, it now has the gigantic S1 XJ unit fitted, but struggles somewhat to spin the motor to start (at 8* btdc static timing, seems better at 14* btdc, which is how it came to me)

My choice is to have my old S3 unit rebuilt for $180, not including clutch, which is NLA,
or fit a Reduction unit supplied by Jagdaim, the largest independant supplier of Jag parts in Oz, for $240.

my respected aotolec suggests a rebuild, as he finds aftermarket RSMs often poorly made.

my main question is whether the RSM will spin the motor “easier”
that may influence my decision

One thing I have noticed is the main (original 420G) wires are only about 6mm diameter.
I have noticed most other older vehicles have wires about 8mm.

is it a known problem for these original battery-starter-earth cables to be too light a gauge?

our new forum computer function has just alerted me that someone has asked similar before

My S3 4.2 engine is now 9.55:1 and the standard S3 starter motor has no problem turning it over. The engine is still quite stiff with only 250 miles on it since a complete rebuild. It has an after-market Link computer but only the standard, used coil.

Greg

Dear Tony,

I run a fuel injected v12 which has a higher compression ratio than your car. The car has a datalogger fitted, so I can see what is going on. The gear reduction starter “GRS” will turn over at up to about 160rpm, whilst the standard heavy Lucas item for this car turns over at 120rpm. In that respect, the GRS represents an easier electrical load, but that is not the whole story.

I can also see that the car starts quite esaily under a relatively wide range of ignition advance.

What is stopping your car from starting more quickly is the fuelling. Petrol needs to be atomised and then delivered to the cylinders in a relatively narrow band of air:fuel ratio and a carburettored car has relatively little airflow over the venturi to achieve this at low rpm. What the GRS “buys you” is a little more time for that to happen without straining the battery further than is necessary.

If the price were the same, then I’d chose the GRS every time when selecting a replacement on a carburettored car, but if the fuelling is well sorted on your motor, then the GRS brings nothing to the table.

The earth return to the battery wants to be of the same gauge as the starter cable if possible.

kind regards
Marek