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SBC engine


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roscobbc

Committee Member

Fri 29 Dec 17 12:17

Re: SBC engine


Vacuum line to distributor really needs to come from a direct source I.e inlet manifold rather than port on carb.


--------------------
CCCUK Chairman & Editor, Vette News
'68 coupe - 4 speed - 3.08 - 489 cu in - 570 bhp @ 5850 rpm - 606 ft/lbs torque (821 Newton Metres) @ 3850 rpm
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roppa440

Club Member

Fri 29 Dec 17 21:46

Re: SBC engine


You need the 23 degrees initial timing so you have to restrict the total timing so it does not go over 36 mechanical only.

Where on the carb are you connecting the vacuum advance?

If you use a port on the base of the carb that sources vacuum from below the throttle blades instead of above them that should give you manifold vacuum and that will vacuum advance the timing at idle to give you enough initial without effecting total mechanical advance.

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Dellb

Club Member

Sat 30 Dec 17 09:53

Re: SBC engine


Hi All
The amount of expertise out there is quite awesome. You guys have really helped.
Back to my problem.
My current vacuum setup is

Main 12mm take off below throttle plates under secondary bowl is to the brake servo.
6mm take off half way up the primary metering block is to the distributor vac.

6mm take off under primary throttle plates was blanked, but is now connected to a PCV system that I have installed on the engine. I am keen to keep this, because not only does it keep the engine clean, but I believe it takes the pressure off the rear main bearing seal.
From the inlet manifold there is a connection that services the following.
Gearbox vacuum actuator
Headlights
heating and AC controls (currently being used for the vac gauge)
My plan is to blank the current distributor connection and tee the distributor into the gearbox vacuum actuator.
I will then set the initial to 15 degrees and see what the vacuum does to it.
For info, my engine is not a stock 350, but is a 400 SBC.
Rgds
Delb


--------------------
Derek L G

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roscobbc

Committee Member

Sat 30 Dec 17 10:30

Re: SBC engine


Dellb wrote (View Post):Hi All
The amount of expertise out there is quite awesome. You guys have really helped.
Back to my problem.
My current vacuum setup is

Main 12mm take off below throttle plates under secondary bowl is to the brake servo.
6mm take off half way up the primary metering block is to the distributor vac.

6mm take off under primary throttle plates was blanked, but is now connected to a PCV system that I have installed on the engine. I am keen to keep this, because not only does it keep the engine clean, but I believe it takes the pressure off the rear main bearing seal.
From the inlet manifold there is a connection that services the following.
Gearbox vacuum actuator
Headlights
heating and AC controls (currently being used for the vac gauge)
My plan is to blank the current distributor connection and tee the distributor into the gearbox vacuum actuator.
I will then set the initial to 15 degrees and see what the vacuum does to it.
For info, my engine is not a stock 350, but is a 400 SBC.
Rgds
Delb

It sounds as though you have matters in hand - your plans for the vacuum take-off are fine and will give a 'proper' vacuum signal rather than 'ported' signal from side of carb. 15% initial seems a good starting point to start with. If the distributor is stock you may want to consider replacing with a performance unit that has an adjustable advance curve and total figure - or - as an interim thing initially source a pack of different strength springs and bob weights to alter the advance 'curve' - perhaps bringing the advance-in much earlier (Moroso used to supply one of these). You'll need to be careful using an auto transmission though not to 'load' the engine and create detonation with too much advance - even more so if you are using the stock torque converter and a 'low' numerical axle ratio.



Last edited by roscobbc on Sat 30 Dec 17 10:32; edited 1 time in total. [2 %]
--------------------
CCCUK Chairman & Editor, Vette News
'68 coupe - 4 speed - 3.08 - 489 cu in - 570 bhp @ 5850 rpm - 606 ft/lbs torque (821 Newton Metres) @ 3850 rpm
Image

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roppa440

Club Member

Sat 30 Dec 17 23:36

Re: SBC engine


Using vacuum advance is handy because when the engine is put under load the vacuum drops off and so it does not cause detonation. It's the same a WOT.

You really need an adjustable timing light that allows you to set the total mechanical timing to 36 degrees max all in at 3000 rpm with half that timing in at 2000 rpm.



Last edited by roppa440 on Sat 30 Dec 17 23:37; edited 1 time in total. [6 %]

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Dellb

Club Member

Fri 12 Jan 18 20:45

Re: SBC engine


Hi All
I just thought I would take time to complete this thread. I have listened to the advice given, and it was spot on.

I purchased an advance timing light, and it is brilliant. It takes all the guesswork out of setting and measuring the timing.

I have set the carb as discussed, and the engine now idles well (it will never be smooth with this cam) is responsive with crystal clear exhaust.

Many thanks to you all for your advice.

Derek


--------------------
Derek L G

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roppa440

Club Member

Sat 13 Jan 18 16:37

Re: SBC engine


Well done! Clap Thumbs up

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