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Distributors & Ignition Timing Facts


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#1 KernowCooper

KernowCooper

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  • Name: Dave
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Posted 01 August 2013 - 11:18 PM

As us Mini owners are always interested in changing bits on our engines I thought it worth while to just list a few facts about the effects on our engines effiency if we modyify the engine to run with a standard distributor.

 

Lets take a Mini 1000 for example which produces in standard form 38-40bhp depending on year, now if we modify the engine with the following.

 

998 Cooper Camshaft

12G295 Head with correct compression pistons for Cooper spec

Twin HS2 SUs

Cooper Maniflod and Correct sized exhaust for the small bore engines

 

 

Now we assume were goiing to make about the same as a 998 Cooper at 55bhp?

 

You maybe disapppointed when you put it on the rollers!

 

A 72-74 Mini Distributor No 41254 would have a totally different advance curve

of 17.6degs @ 2000 rpm and 30degs @ 5000 rpm

 

Where as the 998 Cooper distributor No 41032 had more advance

of 23.9degs at 2000 rpm and 34degs @ 5000 rpm

 

So you can see your standard distributor does not provide enough advance to cope with the increased volumetric eficiency of your modified engine and will be short of crankshaft advanve where the max volumetric efficiency is which is also where maximum torque will be produced.

 

Ignition timing is devided into 3 sections

1. Static Timing, the amount of timing when the engine is stationary

2. Idle Speed Timing, which is where the engine idles at the smoothest possible

3. Total Advance, where the amount of the static timing and the mechanical timing combined give the total advance

 

Ok now lets look at whats diferent betwen the 2 distributors

 

They have different weight primary and secondary advance weight springs and the centrifugal advance is different with different advance settings stamped on the cam plate. Which vary in the range 10-17degs.

 

What the deg marking mean stamped on the cam plate

 

There is a difference when talking about distributor advance and crankshaft advance, for example

 

The Distributor turns at half of the Crankshaft speed so when we see 14degs stamped on the cam plate of the advance mechanism we have to do the conversion of double the 14degs of the distributor advance as the crank runs at twice the speed so we have 28degs of mechanical advance + our setting of 6degs of static advance which gives our engine 34degs total advance, which is where abouts the A series produces its maximum torque and power. (With todays fuel this may have to come back a few degrees)

 

So if we use our standard 72-74 Mini distributor in the modified engine we can assume it has been set at 4degs static advance and the cam plate would be stamped 13degs x 2 which is its max advance of 30degs so we wont have enough advance engine to produce max torque/power on our improved engine. And increasing the static timing to 8degs might well give the required max advanve but at max volumetric efficiency the timing will not be correct.

 

I hope that given a insight why when you modify your engine its important to look at the distributor and ignition timing.

 

 

I also get asked and see road cars brought in with the Vacuum advance disconnected, in short cars used on the road NEED VACUUM ADVANCE,  Under crusing condition where the mixture is obvious weaker to give good MPG more advance is required to burn a weak mixture efficiently. Running without Vacuum advance can cost your between 15-20% in lost MPG. There is no advantage on a road car not to run Vacuum Advance.


Edited by KernowCooper, 28 June 2020 - 04:49 PM.





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