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Do I really need vacuum port?


cruznude

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So a while back I bought a new dizzy from Datsun parts etc, but it's actually from what I understand a z car and modified for the 620. So I also installed same time a Weber 32/36. Remind you no smog rules here for this year of a truck. Oh it's a 79 kc. Now I have my vacuuming advance for the distributor coming off the bottom of the carburetor. Is this correct. And do I really need that vacuum port or just plug it up. That's what I've done already. Thanks

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There are three timing devices on your car.

Static............. this is the setting that your distributor is set at at idle. Depends on gas used, compression etc.

 

Mechanical.... This advances with RPMs. At higher speeds there is less time to burn the gas and air so you start the burn sooner (advance)

 

Vacuum advance.... purely load dependent advance.

 

 

Engines need more advance at light, or part throttle, and less at full throttle. Less gas and air in the cylinder takes longer to burn so you want to light it sooner (advance) so it has more time. Full throttle the cylinder is tightly packed with gas and air and it burns faster in less time, so you want to light it later.(no advance)

 

Vacuum advance is primarily important at any speed where you are at part throttle and is load dependent. A race car would not use it. Any engine at part throttle would benefit from better mileage and power.  

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I always prefer a purely mechanical advance on a Datsun L motor.

 

With a properly re-curved distributor, you can run more initial timing, which makes the engine idle smoother and gives better snap throttle response.

 

There are downsides to running without a vacuum advance, but they aren't huge.

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I feel like any car/truck without the load-sensitive advance (vacuum) is more "dull" at part throttle acceleration.  Lifeless.  Boring.  Add the right amount of vacuum advance and it gets peppy, fun, exciting again.  

The exceptions to this are oversized carbs like a 38/38 or DCOEs.  The 38/38 can run great with a vac advance unit, but not the original.  DCOEs (and similar sidedraft carbs) destroy your vacuum signal so much that vacuum advance  just becomes a tuning obstacle.  

 

More initial advance is one of those things forced by our fuel.  As the chemistry continues to be modified for high pressure injectors, the anti-aeration additives result in a fuel that doesn't want to ignite under low pressures.  When rpms rise, dynamic cylinder pressures allow the fuel to ignite much easier.  

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I feel like any car/truck without the load-sensitive advance (vacuum) is more "dull" at part throttle acceleration. Lifeless. Boring. Add the right amount of vacuum advance and it gets peppy, fun, exciting again.

The exceptions to this are oversized carbs like a 38/38 or DCOEs. The 38/38 can run great with a vac advance unit, but not the original. DCOEs (and similar sidedraft carbs) destroy your vacuum signal so much that vacuum advance just becomes a tuning obstacle.

 

More initial advance is one of those things forced by our fuel. As the chemistry continues to be modified for high pressure injectors, the anti-aeration additives result in a fuel that doesn't want to ignite under low pressures. When rpms rise, dynamic cylinder pressures allow the fuel to ignite much easier.

Can you please direct me to further reading/products for the 38/38 vacuum advance. I've always felt my advance wasn't right, but didn't know better.

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Idles just fine. I jb welded the mechanical. Starts can be a bitch but the easy work around is a separate starter button then engage the ignition. Completely locking the distributor is more common in the v8 world but it worked fine on the l distributor.

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You have no idea what you're missing and what side effects you caused by locking your distributor.  The guys who lock their V8 distributors don't win races.  Did you know that your timing retards when you lock a distributor? Typically 6-8 degrees from where you set it by the time you hit redline.   It doesn't stay in a stable, locked position.  You lost an fair amount of torque, likely caused detonation, and lost top end hp - at least from the potential you have.  It may be better locked than worn out and damaged with a functional advance, but its not likely.  That's just a bad idea on all fronts.  

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Once revved up above 3k the maximum timing can be 32-36 (depends on the engine) but what about idle and in between???? This might be ok if racing and never below 6K but for driving on the street I just don't get it.

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admitted the vac advance is wasted and i'm sure i'm loosing out on mileage and possibly some power but before i locked the mechanical it would ping at any setting. 

it is 11.5:1 compression.  Once i get my meth injection installed and i feel a little safer about pinging i'll run a new unlocked distributor to see if there is any difference.

 

It idles fine, starts can be a bit rough if i don't use the starter button then engage the ignition but other then that it runs awesome, no more pre detonation and i came in 4th at my first AX running this setup. 

 

I'm not going to argue about the technical aspects as your right by all book standards but in my long and varied experience i've found the book to occasionally be wrong or skewed/exaggerated.

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"The book" definitely needs to be amended.  But the laws of physics still apply and any engine develops different cylinder pressures at different rpms, so timing needs will be radically different between idle, 3k, and 7k.  A locked distributor doesn't make anything better for any engine.  Easier perhaps, but never better.  Tons of people still use this approach.  I've rebuilt in excess of 15000 distirbutors.  I can't think of one occasion where a locked distributor is a good idea.  Even when using a computer controlled distributor, a 4 degree advance curve (measured at the crank) eliminates a timing retard scenario that plays out from gear slack and timing chain slack between idle and redline.  Its rarely addressed, but its there.  

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You have no idea what you're missing and what side effects you caused by locking your distributor.  The guys who lock their V8 distributors don't win races.  Did you know that your timing retards when you lock a distributor? Typically 6-8 degrees from where you set it by the time you hit redline.   It doesn't stay in a stable, locked position.  You lost an fair amount of torque, likely caused detonation, and lost top end hp - at least from the potential you have.  It may be better locked than worn out and damaged with a functional advance, but its not likely.  That's just a bad idea on all fronts.  

Well put.

 

Chevy small block language does not translate well to Datsun L motor tuning. Actually, it was one of my biggest hurdles as an engine builder. Often times, a person would have to be convinced that the tricks they learned racing their SBC were hurting them when they applied them to their L motor. Two different languages entirely.

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Plug the vacuum port on the carb and do not hook up the vacuum adv unit, leave it alone, if you remove it the adv plate can move and screw up you timing, 12* BTDC at idle and 34-36* BTDC at 2,800.   Make sure that you mech adv system is healthy and not sticky.

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If you have a carb from the late 60s and newer you probably have a ported vacuum signal for the vacuum advance. Earlier engines ran vacuum directly from the intake. Ported vacuum is zero at idle because the port is above the throttle plate when closed at idle. This means that there is NO vacuum advance at idle and this reduces emissions. As you step on the gas the throttle plate rises above the port and vacuum is applied to the distributor.

 

This is why some old engines you have to disconnect the vacuum advance and plug the line to set your timing. Otherwise the high idle vacuum would throw it off. Newer engines don't have vacuum on the distributor ai idle so it isn't mentioned when setting the timing. For all the Datsun engines I have seen (mostly '68 and up) you can leave the vacuum advance connected when setting the timing.

 

Take note that your engine MUST be at the proper idle position to prevent any chance of the throttle being open enough to send a vacuum signal. Also starting as little as 900-1,000 RPMs the mechanical advance may be coming in.

 

 

The EGR valve is also a ported vacuum signal. There is no EGR at idle either.

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Chevy small block language does not translate well to Datsun L motor tuning. Actually, it was one of my biggest hurdles as an engine builder. Often times, a person would have to be convinced that the tricks they learned racing their SBC were hurting them when they applied them to their L motor. Two different languages entirely.

When I first got my car running my uncle fucked with everything thinking an L with side drafts and electronic ignition was the same as his small block Vega with a Holley and points ignition. Man, was I aggravated. Sometimes explaining anything to him is like talking to a brick wall.

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Worn or poorly rebuilt carbs can allow ported vacuum levels to rise from nearly nothing at idle to a signal big enough to allow vac advance.  Because of the few bad eggs I've seen, plus the possibility your idle is set high enough to allow vacuum, I always recommend disconnecting the vac unit when setting timing.  Period.  

 

We kindly refer to SBCs as bellybutton motors.  Everyone has one.  Owning one is like training wheels for a car guy, and some guys never take those training wheels off.  If the SBC guys could learn that tuning revolves around reciprocating mass, combustion chamber shape, and intake runner volume, they could learn leaps and bounds.  

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