Race Engine Forum UK

Hi there, I run a 1608cc TC in a 125T. The engine is not radical and is prepped to meet FIA Schedule K. Thus I run CB points. I am having difficulty in getting the required dwell (mid to high 50 degs?) I am down to a gap of .012inch and still the reading is 48 deg.
Apart from the obvious (faulty dwell meter) is there any thing I should be looking for?

Can't find fault with your method,

16 thou" should give 55 +/-3 deg dwell angle (the period the points are closed and charging the primary circuit of the coil) so unless it's just a sloppy distr shaft - I cannot explain, sorry!

G

I have had a similar problem on a Ford SOHC distributor. The problem was in the points as the 'head' on the moving side side was loose on it's rivet. So it remained closed initially even when pushed open but would still set at what ever points gap you wanted as it was pushed back into position by the feeler gauge.

Make sure that the fixed point is completely parallel to the moving point. You can check this by examining the points opening and closing from the side and from top view using a magnifying glass. You have to make sure that both points when closed ar fully in touch for the entire round surface. If not try bending the fixed point.

Since you run a competition vehicle you should run the points a little closer, try 0.32-0.35mm with a filler (a tight 35), so that you have a stong spark at high revs. The life of the points will shorten seriously that way, but you shouldn't mind (change every 1500km), also use the Lucas Sport coil DLB 105 for a strong spark.

I dug another distributor out of the depths of my spares and prepped it with new points and condenser cleaned and lubed etc. It also appeared using the shake rattle and poke it test to have a little less shaft movement. First attempt at .014 gap has offered me 54deg dwell, so much better.
Now two questions. (a) Does this sort of dwell variation have a very noticeable effect on outputs? Torque in particular because that is the issue being chased at the moment. (b) What are the easy swap in distributor alternatives from the Fiat/Lancia stable that will be newer or NOS that will keep me in Schedule K. must be block mounted :)

Nope, no idea of the effect but there will definitely be a measurable loss if out of spec, though you'd need a very good dyno to pick it up I guess, I have never done a dwell/dyno examination but since dwell (well, more properly, points opening phase) affects ignition timing and of course spark intensity I would simply suggest that you optimise it and leave it at that.


GC

As long as you keep dwell between 55-60 deg you will have no issues in the high rev range 6000+rpm. I used to race with 58-60 dwell and the Sport coil my 1608 up to 7800rmp with no problem, with the nylon rotor arm S144 marelli distributor (competizione) block mounted, fount on some lancias and the 124 Abarth Rally. It also has a good advance curve giving max at around 3000rpm. Try and find one of these, the key search item is the nylon rotor arm or a "T" BIG bacelite one. The cam acceleration lobe is also better without any flying points phenomena.

Enjoy,

To correct the above, the distributor I am talking about is ex cam mounted not block mounted.

FIAT125T wrote: Does this sort of dwell variation have a very noticeable effect on outputs? Torque in particular because that is the issue being chased at the moment.

I'll be bold and say yes, I'm sure most will agree. Can you quantify it though? Well, It would be very difficult with a points system due to the RPM variable. As RPM increases dwell (time) effectively reduces. Hence, all you really want to know is that you have "enough" dwell to suit your new (higher) RPM torque band. Any more than that is expecting too much.

Do you have a fault or are you just chasing every last Newton meter?

For the forums information, I have spent some time this weekend eliminating another variable in the "lack of torque quest". It now appears that the inlet cam had jumped a tooth. I have only had time to drive it quietly on the road since so unsure of the end result. Next weekend is the first of our big summer race meetings so I will find out for sure then no doubt.
thank you