Why vacuum retard?

[gungadin]

I was wondering why some bosch distributors have both vacuum advance and retard. Especially for the SC engines.
How many degrees of retard does this represent?
I guess I don't know where the retard side is connected either.
Has anyone explored an alternative for the OEM Bosch vacuum units?

[unclebilly]

I think they are either vacuum advance or retard - I know my 77 2.7 california edition is vacuum retard and calls for something like 14° After TDC.

[emcon5]

Emissions.

Tom

[cgarr]

I know on the 2.7 the retard is only to get the idle down, as soon as you crack the throttle it advances.

[gungadin]

Huh?
I have an SC distributor and it has advance and retard on the vacuum.
I have a '76 distributor and it only has vacuum advance.

So if the vac retard is only for emmissions then perhaps an advance only vacuum pod will work?? "to a degree"?

[porsche930dude]

iv read you can just disconnect the retard and plug it and you have to turn down the idle. But its better .. Some how

[Laneco]

Someone once told me that the retard on the SC helps it start easier and as we all know, the advance is for the go-pedal side! I cannot 100% vouch for that but the fella who told me was quite a knowledgeable person.

angela

[gungadin]

yeah both those things make sense to me.
I guess I was wrong about the'76 advance. Forgot it is CW rotation.

I may actually try a single acting vac pod on an SC distributor.

[Won]

Originally Posted by porsche930dude

iv read you can just disconnect the retard and plug it and you have to turn down the idle. But its better .. Some how

I guess depending on how you lower the idle. The vacuum retard line on mine hardened and broke off one day and without knowing about it, I lowered now ~1300 RPM idle back to 900. That meant I had to lean it out A LOT as well as adjust the idle bypass screw. Absolutely no power at top end (found out about the vacuum line half way through an autocross, probably not a good move ).

Also, the popping through the exhaust at off throttle was completely gone with vacuum retard disconnected.

[RoninLB]

1977 smog test was at idle.

The '77 retard is to meet smog test. It also leads to improper combustion and sometimes hotter than necessary engine operation in mid range rpms.

A vac advance comes on when car is coasting without much load. The added advance beyond the mechanical curve allows better combustion and gas mileage. Advance curve is tailored to engine etc.

In my original set up I disconnected vacuum line forever and retimed total advance to 35deg max. Idle wound up around 5deg btdc.

[psalt]

I was wondering why some bosch distributors have both vacuum advance and retard. Especially for the SC engines. How many degrees of retard does this represent?
I guess I don't know where the retard side is connected either.

Vacuum retard is a device to help meet the curb idle HC emissions test. When you retard the ignition timing, exhaust temperature goes up, burning more hydrocarbons in the exhaust. It also transfers more heat to the engine, raising the engine and oil temperatures, especially at hot idle. Modern ECU's severly retard the ignition after firing up the engine to light up the converter quickly and reduce warm up time and emissions. The SC vacuum retard is around 5 crank degrees at idle. The vacuum retard hose is generally blue or braided grey and connected at the back of the distributor module, the advance line is usually orange. If you take off the cap and watch what happens when you apply vacuum to the nipples, it should be obvious ( remember the direction of rotation ). Another way to tell is that the advance uses ported vacuum ( no vacuum at idle) and the retard uses manifold vacuum (active at idle). Vacuum advance is a good thing, vacuum retard without active idle control only causes stalling, air box explosions, overheating when stationary and can be deleted, but you will need to reset the idle speed with the bypass screw.

[Paulporsche]

I plugged the retard line from my dizzy last fall. I use the old, points style diz. The engine now idles much smoother, especially during warmup.

[T77911S]

someone told me disconnect the retard and run it with out. i tried and it ran worse, and it was harder to start.
my timing is suppose to be set at 0 deg. i removed the the vacuum and set it 0 deg. then at 5000 rpm i am not getting enough advance (the dizzy has been rebuilt so the mech. advance is working fine). with the vacuum connected and set to 0 deg, i am getting 35 deg at about 5000rpm. as said above, once the throttle is cracked, the 15 deg of retard is gone.

what i have read is retard causes a cooler combustion chamber and advance makes it hotter, that is why you have to richen up the mixture when you advance the timing. also, advancing the timing heats up the combustion chamber to the point of causing spark knock or detonation.

[Paulporsche]

What if you set the advance for 35 @ 5000 (or isn't 6000 supposed to be used for this?) and forget about what the advance is @ idle, once the retard is disconnected?

I don't have a clue about combustion chamber temps.

[T77911S]

i tried setting at 6000 rpm. i get about 15 deg advance at idle at it would not start.
all the advance is in around 4500 -5000, so why stress the motor any more than i have to.
i only get about 20 deg mech. advance, so i guess the other 15 deg advance is coming from the retard.
will a car start easier with advanced timing or retarded- in relation to TDC.

i hope i am not hijacking- sorry gungadin if i am.

[gungadin]

Originally Posted by T77911S

i hope i am not hijacking- sorry gungadin if i am.

No way!
Please add to the discussion

[psalt]

what i have read is retard causes a cooler combustion chamber and advance makes it hotter, that is why you have to richen up the mixture when you advance the timing. also, advancing the timing heats up the combustion chamber to the point of causing spark knock or detonation.


The ideal ignition timing produces peak cylinder pressure (and temperature) at around 15 ATDC to get maximum work from the engine. The burn time is somewhat of a constant so you need to advance the timing as rpm increases because there is less time available. If you retard the ignition, the temperature and pressure in the cylinder goes down, the temperature of the exhaust goes up, less work is done and more of the heat of the fuel goes into heating the engine. Many engines with vacuum retard emission controls also include a temperature switch to advance the timing when the engine starts to overheat . Generally, retarding the timing on a engine causes head, block and coolant temperature to increase. Detonation is a different issue, the temperature rise can melt a piston, but you will never see it on a temperature gauge. The 911 engine is knock limited by the design of the hemi head with domed pistons and air cooling, so most of them are detuned for the fuel spec.

If you set your advance at high speed and it has too much advance at idle, the springs in your distributor could be weak or the wrong spec. Try setting it statically or reducing your idle speed to see if the advance is coming in at idle speed. Most engines start best with advance from TDC to 10 BTDC. The problem with the SC is the lean calibration and the non active idle control. It takes more throttle to idle when the retard kicks in on a cold engine, the CIS cannot respond resutling in a stall, stumble, or a backfire. The probability of a intake backfire increase as you retard the timing. Vacuum advance or retard should have no effect on performance at WOT.

[RoninLB]

Originally Posted by Paulporsche

What if you set the advance for 35 @ 5000 (or isn't 6000 supposed to be used for this?) and forget about what the advance is @ idle, once the retard is disconnected?

see my above post

[john walker's workshop]

later 2.7s actually had a inline red plastic block-off plug between the retard unit and the hose when new. most have been lost over the years and people just attached the vac hose to the retard unit, thinking that's the way it should be. california cars may have had the retard hooked up, i don't know. they idle smoother without the retard. set the total advance somewhere in the 32° - 35° range where it doesn't ping and whatever ° it idles at is where it idles at. adjust speed and CO and there you go.

[Gunter]

gungadin:
If your engine is a US '79, why would you have a distributor with advance and retard?
The US '78-'79 has advance only, no retard.
Post the Bosch number on the distributor so we can see what you have.
O 237 XXX XXX ??????