[DeTomaso] 351C (ignition) timing

[Göran Malmberg]

The trick with vacuum advance is to rise the advance during light trottle
operation, like cruising at 60 mph for long distance, which is a case
where you will save fuel. this is due to that the high vacuum charge will
burn at a slower rate, therfore needing more advance. As soon as you
open the trottle for acceleration the diaphragm-spring willovercome the
manifold vacuum and return the distributor point mouting plate to
mecanical only advance possition, exactly as a non vacuum distributor.
So there will bo no loss of top engine performance. If the distributor
design is poor, you may have erratic point operation from a flexing
mounting plate, since it must be able to turn to give vacuum advance.
Since the point plate move, the rotor will not poin in exactly in front of
the cap pole at highest vacuum, but 10 dgr is not a problem.
The vacuum curve could add some10 dgr of advance between the finger
and thumb, but for best performance (which is not really needed) you
should use a vacuum guage to sort out the highest light duty vacuum
while altering the ignition timing. Depending on camchaft the engine will
produce different vacuum, and in such case the diaphragm-spring will
have to be exchanged. The number of vacuum degree can also be
altered by different thick schims. However, the factory setting will in
moost cases be okay.
I use digtal advance setting for my engine, and have a total of 50 dgr
at 2000-3000 rpm that is only in at the first step of trottle opening since
I use a TPI sensor.
Regards
Goran

----- Original Message ----- 
From: "Art Stephens" <artstephens at verizon.net>
To: <detomaso at realbig.com>
Sent: Monday, April 13, 2009 6:53 AM
Subject: Re: [DeTomaso] 351C (ignition) timing


>     It sounds like we sacrifice performance by running vacuum advance?  I
> would imagine we have no vacuum advance at WOT?  If that is the case, 
> let's
> say we subtract 8 degrees of vacuum advance from the 36 total,  leaving us
> with 28 at WOT.  So if we make max HP or torque at 36 degrees,  it seems 
> we
> would be best off if we did not use vacuum advance,  and had a total of 36
> from initial and centrifugal only?  Or,  is there a problem with running 
> 36
> degrees of initial and centrifugal,  and then running vacuum advance in
> addition to the 36?  Can we safely get away with running 44 or 46 degrees 
> at
> part throttle cruise?
>
> Art
>
>
>
> ----- Original Message ----- 
> From: <JDeRyke at aol.com>
> To: <davel at emspace.com>; <detomaso at realbig.com>
> Sent: Sunday, April 12, 2009 12:25 PM
> Subject: Re: [DeTomaso] 351C (ignition) timing
>
>
>> IMHO you're on the right track: for best acceleration, try to get the
>> centrifugal advance all-in by 3000, with not more than 36 degrees total:
>> initial
>> plus centrifugal plus vacuum (if used). The exact amount will vary with
>> the
>> engine setup. Total initial advance is easily varied by moving the
>> distributor body but be very sure the primary mark on your harmonic
>> balancer hasn't
>> shifted so its still true TDC. The springs only vary the rate of advance,
>> not
>> the amount, but total centrifugal advance is usually variable by means of
>> a
>> pin that sticks thu the fixed plate in the distributor and has a sleeve
>> that can be varied in thickness. Total vacuum advance is often variable 
>> by
>> sticking a small allen wrench thru the hose connection pipe and adjusting
>> a
>> bleed hole. Vacuum advance sometimes can
>> increase mileage at cruise but seldom WOT power. Finally, be aware that
>> electronic timing lights vary (especially the dial-back types), so 
>> amounts
>> of
>> all three advances that work fine on your engine-with your light, may not
>> work exactly the same for someone else. And I added to your post-title,
>> 'cause
>> timing the camshaft is a whole 'nother subject which can affect best
>> ignition timing, too. Good luck- J Deryke
>>
>>
>> **************
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