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350 HO crate motor / vac advance/ timing issue
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Joined: 28/07/2007(UTC)
Posts: 76
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Guys,
I have a 350/330Hp crate motor in the ol HQ and have been trying to sort out a small timing issue. I have searched the net and found that the specifications for this GM crate engine say not to use the vac advance on the HEI distributor and only rely on mechanical advance. Could this be correct???
Quote from 350 HO Engine (12486041 Base) Long Block Specifications:-
"Set initial spark timing at 10 before top dead center (BTDC) at 650 rpm with the vacuum advance line to the
distributor disconnected and plugged. This setting will produce 32 of total advance at wide-open throttle (WOT).
The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only
the internal centrifugal advance to achieve the correct timing curve."
I did set it up initialy with base timing at 10deg BTDC but used full manifold vacuum and yes it pings...... I will try again today with firstly ported vacuum and then with vacuum disconnected to see if there is any major difference.
To me , it just seems unusual to rely on mechanical advance only but if one of you tech gurus could shed some light then it would be much appreciated. I might be able to tune this thing properly.
First long trip 330km used 17 litres per 100km sitting on 110kph Ouch.
Cheers Trev...
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Rank: Member
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Joined: 13/01/2006(UTC)
Posts: 303
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I would be seeing how much advance you are getting...is it getting 32 degrees? "NORMALLY" You need to change from manifold vac to ported vac. You can buy advance springs really cheap but I would get the dissy regraphed or you will never get good economy u
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Rank: Member
Groups: Registered
Joined: 28/07/2007(UTC)
Posts: 76
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Just ran some tests and came up with the following:-
With no vac advance connected,
1000rpm - 10deg btdc
1500rpm - 19deg btdc
2000rpm - 28deg btdc
2500rpm - 32deg btdc (max advance)
With the vac supply connected to a ported vacuum there was virt
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Rank: Member
Groups: Registered
Joined: 28/07/2007(UTC)
Posts: 76
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Just had another run with the ported vac fitted. No ping at all. Yet I get a ping with full manifold vac connected. I was of the idea that ported vac only drops to 0 at idle, and is the same as manifold vac at higher revs. (corvette forum suggested to use
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Rank: Member
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Joined: 28/03/2008(UTC)
Posts: 69
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Full advance at 2500rpm sounds about right to me.
Ross
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Joined: 17/04/2005(UTC)
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Ported and manifold vacuum are almost opposite each other. Manifold vacuum is high at idle, and low at wide open throttle. Ported vacuum is low at idle and high at wide open throttle.
The vacuum is used to modify the centifugal advance based on engin |
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350 HO crate motor / vac advance/ timing issue
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