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exsystem
Samba Member
Joined: March 17, 2018
Posts: 142
Location: Delaware County, PA
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 Posted: Sun Aug 19, 2018 5:43 pm Post subject: New svda dizzy and coil |
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Installed a pertronix svda dizzy and coil today. Disconnected the vacuum tude and set the advance at 7.5 @ idle and 32 @ 3000 rpm. Plug back the vacuum hose and all set right? Well just to check I ran the timing light and slowly reved up to 3000 rpm and it was advancing as it should but I was getting 42 degree advance. Is this right? I thought I shouldn't have more than 40 degree advance at any time?
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vamram 
Samba Member
Joined: March 08, 2012
Posts: 7300
Location: NOVA
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| Posted: Sun Aug 19, 2018 5:43 pm Post subject: Re: New svda dizzy and coil |
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42* is fine.
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gt1953
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Joined: May 08, 2002
Posts: 13848
Location: White Mountains Arizona
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| Posted: Sun Aug 19, 2018 9:06 pm Post subject: Re: New svda dizzy and coil |
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That sounds about normal actually. That is what the SVDA does, running on flat ground the advance at 3k would be about that. Your throttle is light and the vacuum goes up. Reminds me of the olden days we had a vacuum gauge, green was economy and red was not so efficient or under acceleration.
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ashman40
Samba Member
Joined: February 16, 2007
Posts: 15982
Location: North Florida, USA
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| Posted: Sun Aug 19, 2018 11:34 pm Post subject: Re: New svda dizzy and coil |
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| exsystem wrote: |
| I thought I shouldn't have more than 40 degree advance at any time? |
You want to make sure your vacuum advance is working, but there are no suggested methods for setting timing which include vacuum advance as part of the timing adjustment. Vacuum advance is too unpredictable.
The suggestion for rpm-based ignition timing (mechanical advance) is to make sure you do not exceed 32BTDC. If you are running a race engine and you monitor detonation very closely you could theoretically push it to 35BTDC for a high octane gasoline engine. But since most of us don’t, 32BTDC is a safe limit for rpm-based timing.
To check total timing (initial + mechanical) you need to disconnect the vacuum advance system since vacuum advance is NOT rpm-based. Vacuum advance is “load-sensing”. In your driveway revving the engine there is no load and the vacuum advance will add an unspecified amount of timing advance. While you are driving down the road and downshift to pass someone you load the engine and the vacuum advance will dial-back the timing. You cannot predict the vacuum advance which is WHY it is NOT used as part of setting rpm-based timing. I don’t think you will find any legitimate source that suggests setting timing while vacuum advance could be active. Even DVDA distributors adjust timing with both vacuum hoses connected, but at idle vacuum advance should be inactive so not contributing to timing reading.
Different distributors produce different amounts of vacuum advance. If you know what the specs are for your distributor’s vacuum advance you can test for this at idle with a hand vacuum pump with a gauge. Apply the specific amount of vacuum that produces the max vacuum advance and confirm the timing increases by that amount. If the increase in timing increases your rpms above 1200rpm some of that timing advance could be coming from the mechanical advance. Another example of why vacuum advance is not usually part of setting timing... it can be hard to separate from rpm-based timing.
If you are looking at the timing at high rpms with the vacuum advance connected, all you can expect is that the timing MAY BE higher than the timing with the vacuum advance disconnected. Unless you know the amount of vacuum being produced you cannot assume anything... like you “shouldn't have more than 40 degree advance at any time”.
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exsystem
Samba Member
Joined: March 17, 2018
Posts: 142
Location: Delaware County, PA
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| Posted: Mon Aug 20, 2018 5:52 am Post subject: Re: New svda dizzy and coil |
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| Thanks for everyone's help! |
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