[Fot] Distributor centrifugal advance curve for race engine

[van.mulders.marcel at telenet.be]

Without the possibility of computer ignition mapping, the most important thing is the maximum total advance. This can start from 3000rpm or higher, depending on the distributor springs. 
2 degrees is already making a significant difference for horsepower. On a dyno one can find at what advance the engine is making the best power, but it is all important to find out if the power is not dropping after 20 or 30 seconds (detonation). 
And the the circumstances on a race week-end can be different : if one is arriving at 30° during a dyno session, it is possible that only 28° is safe on a particular day. Total maximum advance can differ considerably (compression ratio,...). 
I believe that a fresh engine tends to pre-ignition more easily because of glowing spots : protruding headgasket(wich should be avoided of course), sharp machining edges...In an older engine the surfaces of the combustion chambers will be smoother 
Marcel 


Van: "fot" <fot at autox.team.net> 
Aan: "fot" <fot at autox.team.net> 
Verzonden: Vrijdag 19 maart 2021 23:53:55 
Onderwerp: Re: [Fot] Distributor centrifugal advance curve for race engine 



When I had crank fire ignition I did test it on the dyno and found that a slight retard as RPM's went up helped (at least that's how I remember it - up to 34 / 35 at 3000 rpm but down to 32 at 6000). But with my more vintage points system, the advance is "all in" at 3000 rpm. I just set to 30 degrees or 32 degrees or if I'm feeling particularly lucky / brave / stupid 34 degrees at max. Whatever it ends up at low rpm is where it ends up. When racing I'm never under 3000 RPM anyway. 

Regards, Tony 
On 3/19/2021 5:27 PM, timmmurphh--- via Fot wrote: 





What advance curve are folks using in their race engines? I have 87mm pistons, a Kastner G3 cam and about a 12:1 compression ratio. I am using about 19 degrees advance at idle (no centrifugal advance) and 30 degrees advance at 3000 RPM. I don’t know what I have at higher RPM’s because I never looked. I’m currently doing an engine rebuild so I will have to look what I’ve got at the higher RPM’s when I get the engine running. I’m curious as to what others are using and if anyone has done any testing on a dyno with different advance curves. 



Thanks in advance for your input. 



Tim Murphy 

#317 BRG TR4 

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