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Adding A Vac Unit To An Alden Yellow


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#1 absx2

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Posted 06 December 2017 - 08:11 PM

I have a Alden 100AY to fit to a +.060" 998 with a Piper 270 cam and I have retro fitted a vacuum unit from a standard 998 dizzy but now i`m not sure if Alden have different curves for vac and non vac.

 

Does anyone have the answer please ?



#2 KernowCooper

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Posted 06 December 2017 - 11:15 PM

I spoke to Aldon direct last year and they sent me the curves for the yellow and the red and there was only one curve for each so no different curve for Vac/Non Vac.

 

The curves are here on TMF in the FAQs

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Edited by KernowCooper, 06 December 2017 - 11:16 PM.


#3 Cooperman

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Posted 06 December 2017 - 11:18 PM

I don't think that will work. Any distributor without a vac advance will be curved for a specific spec. engine, so adding a vac unit will simply change the curve to something completely different.

Best bet is to speak with Aldon, give them your engine spec and be advised as to what will work best. They are very good at doing this.



#4 absx2

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Posted 07 December 2017 - 07:31 PM

Thanks chaps



#5 carbon

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Posted 07 December 2017 - 07:38 PM

Absx2 - there are also lot of differences between apparently identical vac advance cans. The three variables are:

- start vacuum, when it starts to advance, in inches mercury

- end vacuum, when it stops advancing, in inches mercury

- total vacuum advance, in distributor degrees

 

These are normally stamped on the can, for example 3-10-8 would be:

- start vacuum is 3 inches mercury

- end vacuum is 10 inches mercury

- total vacuum advance is 8 distributor degrees

 

So at full advance this vacuum can would give you an extra 16 degrees of crank advance. For a fast road build I would recommend avoiding cans with much more than 16 degrees of crank advance, just to be on the safe side.



#6 absx2

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Posted 08 December 2017 - 05:01 PM

Thanks guys. 

I called Aldon today and unfortunately for me he confirmed that there is a difference between the curves on the vac and non vac so if I wish to keep the vac unit I will need to return it for recalibration along with my engine spec.

 

Is the vac unit worth the bother I wonder ?



#7 KernowCooper

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Posted 17 December 2017 - 10:44 PM

Well seems the advice I got was wrong last year ? sorry about that, the advantage of running a vac unit on the road is, increased mpg, and more engine torque on a part throttle, if your running on the road then yes use a vacuum unit



#8 Moke Spider

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Posted 18 December 2017 - 02:44 AM

Thanks guys. 

I called Aldon today and unfortunately for me he confirmed that there is a difference between the curves on the vac and non vac so if I wish to keep the vac unit I will need to return it for recalibration along with my engine spec.

 

Is the vac unit worth the bother I wonder ?

 

I find this 'odd' I gotta say.

 

Usually, the mechanical advance is sorted and then vacuum, if wanted, is done 'on top of' the mechanical advance.

 

Mechanical Advance is worked out on Wide Open Throttle, under which conditions there is no or very little vacuum and this curve is the same whether vacuum is fitted or not.






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