Vacuum hose on or off for timing distributor?

[David_S]

I have a Nissan P40 engine with Pertronix ignitor replacing points. My question is whether I should set the timing with the vacuum line on or with the vacuum line off and plugged.

The vacuum is operated off full manifold vacuum. i.e. it is NOT located above the throttle plates so at idle there is substantial vacuum. The Nissan Workshop Manual does not specify which.

If I set the advance to the specified 10 degrees at idle with the vacuum line off and plugged, the advance increases to nearly 20 deg when I reconnect the vacuum line.

In the past I have generally left the line on and set the timing to 10 deg and have had no problems but I was working on the erroneous assumption that it made no difference. Now I am curious as to the function of the vacuum advance which if I understand it right advances the timing under light loads such as idle or cruise when there is high manifold vacuum. I would like to know before I activate the electronic ignition timing on my Megasquirt ecu and have to set up an ignition timing table.

[Gwagensteve]

I could be wrong but if you have an igntition timing table and therefore some amount of timing control through the computer all vacuum/mechanical advance must be locked out.

If you are timing the car conventionally, base timing must be set with vacuum line attached. Vacuum advance line must always be in the manifold downstream (engine side) of the throttle butterflies.

Hope this makes sense.

Steve.

[cloughy]

Disconnect and block it

[David_S]

Steve, yes you are right about an ignition table. When I get round to controlling ignition with the Megasquirt ecu I will need to populate one. As a starting point I was intending to base the advance on the vacuum and centrifugal advance figures given in the Nissan manual. And, of course lock out the existing centrifugal and vacuum advance mechanisms. But that is in the future.

My immediate problem is to set the timing for the truck running off a conventional distributor. And it is not true that all vacuum advances use the manifold vacuum produced downstream of the throttle plates. Wikipedia states

One source for vacuum advance is a small opening located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. This is called Ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle. Other vehicles use vacuum directly from the intake manifold. This provides full engine vacuum (and hence, full vacuum advance) at idle.

So it would appear that with ported vacuum it does not matter whether the hose is attached or not for setting the timing at idle but with manifold vacuum it does. If I set the timing to 10deg with the hose disconnected and plugged when I reconnect the hose the timing goes to 20 deg. If I set the timing at idle to 10 deg with the hose attached when I reduce the manifold vacuum by depressing the throttle say the advance will drop until the centrifugal advance kicks in.

At least that is how I think it works. Anyone got any other thoughts?

In the absence of any instructions to the contrary in the Nissan Manual I think I will set the idle timing with the hose attached.

[David_S]

Disconnect and block it

But why? And is it the same for ported and manifold vacuum options?

[Gwagensteve]

Steve, yes you are right about an ignition table. When I get round to controlling ignition with the Megasquirt ecu I will need to populate one. As a starting point I was intending to base the advance on the vacuum and centrifugal advance figures given in the Nissan manual. And, of course lock out the existing centrifugal and vacuum advance mechanisms. But that is in the future.

My immediate problem is to set the timing for the truck running off a conventional distributor. And it is not true that all vacuum advances use the manifold vacuum produced downstream of the throttle plates. Wikipedia states

One source for vacuum advance is a small opening located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. This is called Ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle. Other vehicles use vacuum directly from the intake manifold. This provides full engine vacuum (and hence, full vacuum advance) at idle.

So it would appear that with ported vacuum it does not matter whether the hose is attached or not for setting the timing at idle but with manifold vacuum it does. If I set the timing to 10deg with the hose disconnected and plugged when I reconnect the hose the timing goes to 20 deg. If I set the timing at idle to 10 deg with the hose attached when I reduce the manifold vacuum by depressing the throttle say the advance will drop until the centrifugal advance kicks in.

At least that is how I think it works. Anyone got any other thoughts?

In the absence of any instructions to the contrary in the Nissan Manual I think I will set the idle timing with the hose attached.

Good tech David_S -

I think that might have been a problem when I converted my 1.0 sierra to vacuum advance and electronic ignition.

Steve.

[cloughy]

Disconnect and block it

But why? And is it the same for ported and manifold vacuum options?

Because your setting BASE timing, with no alterations, you should only effect it when the vac or mechanical advance do their thing

[WRXZook]

As above... disconnect and block . Basic ignition timing is set with the vacuum advance line disconnected and plugged. If you do not do so, the basic timing will be too far retarded and the centrifugal advance curve will continue to be retarded across its range.

[David_S]

Because your setting BASE timing, with no alterations, you should only effect it when the vac or mechanical advance do their thing

Yep, that makes sense and ties in with the timing table I have yet to set up for the electronic ignition. This table has MAP (manifold air pressure) on the vertical axis corresponding roughly with vacuum advance and RPM on the horizontal axis corresponding with centrifugal advance. i.e the first is load sensitive as measured by the throttle opening (low throttle high vacuum more advance, wide open throttle no vacuum no advance) and the latter is dependent on engine speed (higher revs more advance).

What I had not realised until now is that the existing vacuum advance works over the whole rpm range and not just at the low end. Thus it will advance the timing when cruising down the highway at mid range rpm and light throttle as well as at idle.

I can see fun times ahead when I switch to electronic timing and try to tune the beast without a dyno. Any suggestions there apart from use a dyno which I would if there was one available locally?

Thanks, guys, for your input.

David

[F'n_Rover]

Because your setting BASE timing, with no alterations, you should only effect it when the vac or mechanical advance do their thing

Yep, that makes sense and ties in with the timing table I have yet to set up for the electronic ignition. This table has MAP (manifold air pressure) on the vertical axis corresponding roughly with vacuum advance and RPM on the horizontal axis corresponding with centrifugal advance. i.e the first is load sensitive as measured by the throttle opening (low throttle high vacuum more advance, wide open throttle no vacuum no advance) and the latter is dependent on engine speed (higher revs more advance).

What I had not realised until now is that the existing vacuum advance works over the whole rpm range and not just at the low end. Thus it will advance the timing when cruising down the highway at mid range rpm and light throttle as well as at idle.

I can see fun times ahead when I switch to electronic timing and try to tune the beast without a dyno. Any suggestions there apart from use a dyno which I would if there was one available locally?

Thanks, guys, for your input.

David

do the dyno ! i've been trying to get a good ignition map by ear and feel........ not a huge amount of success, though still does go better than stock timing. amazing how much free power is available from playing around with timing.

also - why keep the dizzy? not a big deal to remove and go coilpacks now

[David_S]

also - why keep the dizzy? not a big deal to remove and go coilpacks now

One step at a time. When I go to electronic timing I will just use the dizzy/Pertronix at first as a timing device and later thought of going EDIS. Have to figure out first how to install a toothed wheel at the front of the crankshaft. Any problem with the EDIS VR sensor being dunked in mud and water?

David

[PGS 4WD]

Sorry if I have repeated whats been said as I didn't read the entire thread. Vacuum advance varies from engine to engine. The most common vac advance has one hose and is ported vacuum and should not have vacuum at idle, just part throttle, so won't effect timing if it is on or off at idle. If the vacuum is direct to the manifold the timing will actually retard as you open the throttle rapidly due to vacuum dropping with load. Pull the line on and off with the car running and see if the timing alters to determine how yours is operating if not sure. There are exception such as some Mazda, Nissan and Mercedes cars I can think of off hand that have two opposing vacuum lines to the distributor, these are usually set with one off, but it is vehicle specific.

We have a VN engined turboed Suzuki that uses factory sensors and we haven't had an issue under water.

Joel

[David_S]

Well I have done a 1000 km over the weekend both on and off road after setting the timing with the vacuum line off and plugged. Noticed a distinct improvement particularly in the pickup when starting from idle or planting the boot when entering an obstacle. Previously there was a slight stumble as, presumably, the timing retarded 10 deg as the vacuum dropped with the throttle wide open.

Thanks guys

David