update: Advance/retard question for '78 911

Guys,

Those of your that were confused whether the single vacuum connection on a 1978 911SC distributor was for advance or retard. I have your answer.

I put a timing gun I my engine. Then I removed the hose and sucked on it while the engine was idling. The timing retarded by 4 degrees. With no vacuum I had 3 degrees. When I sucked it went to 7 degrees and the idle speed went up.

The funny thing is that this hose is connected to the top of the throttle body when there was no vacuum during idle.

I also check my timing at 6000rpm with and without a vacuum and it was 27 degrees both ways.

The book says it should be anywhere from 25 to 31 degrees at 6000rpm. I don't know if there is any advantage of 25 or 31 degrees? I'd think that you'd be better off with less retard as possible as you'd get the burning at higher compression which should result it better fuel economy??

Tony
'78 911SC

[QUOTE]Originally posted by tbitz:

I put a timing gun I my engine. Then I removed the hose and sucked on it while the engine was idling. The timing retarded by 4 degrees. With no vacuum I had 3 degrees. When I sucked it went to 7 degrees and the idle speed went up.

"the idle speed went up" this is indicative of timing advance. If you only have one vacuum hose to the distributor, it will advance the timing.

The concept is this:

Low vacuum at idle, that is why the vacuum source is above the throttle plate, little or no advance. As engine speed increases the timing must advance to stay ahead.

I have seen it illustrated this way: If you try to shoot a standing target you shoot right at it, if you are shooting at a moving target you must lead it, the faster the target moves the more you lead it.

While idleing your distributor shoots straight at the piston. As the piston moves faster the distributor must advance to lead the piston.

The vacuum is perfect for this purpose because the vacuum is a function of engine speed (the faster the engine the greater the vacuum), and advance the timeing by moving the plate the pick-up coil is mounted to.

The weights inside the distributor also perform the same function. The faster the engine runs the more centrifugle force is generated, and the force on the weights over comes the pressure of the springs, allowing the weights to move away from the center of the distributor and through mechanical design cause the rotor itself to advance.

Hope this helps.

OK, which is it, advance or retard? If you were at 3 degrees and it went to 7 with vacuum, then it advanced 4 degrees.

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Bill Krause
'79 911SC Euro

BIll,

You are correct. Advance the timing speeds up the engine, retarding the timing slows down the engine. That is all else being equal.

If the engine is idleing and up turn the distributor and RPM increase you just advanced the timing.

If you "manually" pull vacuum on the line and the engine RPMs increase you are advancing the timing.

Sorry guys, I got my naming wrong. The timing did go from 3 degrees with no vacuum to 7 degrees with vacuum. This is an advance of 4 degrees (for some reason I though this is called retard 4 degrees, Whoops!) It now makes sense, the timing "advances" 4 degrees before TDC.

But why have both a vacuum advance and a mechanical advance??

Also I checked to see how much vacuum I was getting from the hose on top of the throttle body and it was nil both at idle and at high rpm. Maybe I should check for a blocked hole?

Tony
'78 911SC

The vac advance/retard on the '78 SC is mostly a funcion for emissions. I have removed all that crap from my engine and have plugged the vac port. The mechanical advance is all you need.

The two different advance mechinesims offer two ranges of timing advance.

Vacuum is far more responsive in the lower RPM range, once the vacuum advance maxes out, but RPMs continue to climb, the mechanical advance offers additional timing advance. The engine will wind up real well, but the timing is a vital control system in regard to how well the engine revs.

The more advance you have the more responsive the engine is to the throttle, at a sacrifice to fuel economy, and a possible introduction of pre-ignition aka knock & ping, if advanced too far.

OOOps

[This message has been edited by GT911 (edited 04-08-2001).]

Double OOOps

It didn't post, it didn't post, now it posted three times.

I should learn what I'm doing....

But while I'm back let me comment (run my head) about emission control. The retard vaccum line is for emmisions, but the advance is very important for engine performance.

[This message has been edited by GT911 (edited 04-08-2001).]

Yes, as you advance timing, all other thing equal, your idle speed will increase. This fools some folks into thinking they are making the car run "better." This 'ear' method of timing will result in too much advance, pinging, and burnt pistons.

I share your curiosities about these distributors. They are different. I am used to the vacuum advance working throughout the range, by connecting to the airbox side of the throttle body. Porsches are apparently different. I am used to centrifugal advances as really a retard mechanism at startup. After startup they go to full advance. Backwards I guess, as Porsche centrifugal advances work throughout the range. Porsche advance curves must be pretty straight.

Well, anyway, the only stuff you really need to know, is timing at idle and timing at 6,000 RPM. If these are in spec, your system is working.

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'83 SC

It appears that my vacuum input on my distributor has no affect on idle. If it were a vacuum retard system, then I probably wouldn't mess with it, but now that I know it is a vacuum advance, it's worth getting straightened out. It's good to know there may be some low RPM performance that I may be able to restore.

Looks like I'll be pulling the dizzy out. I've read some good posts on how to clean up the advance mechanisms, but I wish I could see some pics before I open it up. I have this dreaded feeling that it's all going to go SPROINGGGG in my face.



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Bill Krause
'79 911SC Euro

The reason for both vacuum advance and centrifugal advance are this. The vacuum advance is load sensitive. The centrifugal is speed sensitive only. It doesn't know how much load the engine is under. By using engine vacuum for the rest of the advance, it allows the advancer to back off the advance slightly under high load situations when the engine is most likely to ping. There are 3 kinds of vacuum. Manifold, venturi and port. Manifold is highest at idle and decreases with throttle and is used for vacuum retard. Venturi increases with throttle and is used for vacuum advance. Port is highest at wide open throttle and is usually used for PCV systems and breather hoses. A vacuum retard in effect is an advancer since it goes from retard to advance as the throttle opens and Manifold vacuum decreases. It's there to make the engine burn cleaner at idle and really doesn't hurt performance since it goes back to normal timing the instant the throttle is opened.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

Tony,

Be slow to conclude that your vacuum advance is not working. Again, if you car's timing is good at idle, and also good at 6,000 rpm, close the deck lid and drive it.

Advance mechanisms often do have a 'retard' mission. When the starter is running, the engine is turning far too slow for any advance. Indeed, the best time for a spark to appear during starting is just AFTER TDC. Sparks happening before TDC will light the gasoline too soon, which can try to make the engine run backward. So, "static" timing is usually at TDC or a little after.

This tendency to run backwards is why I suggested in a nearby thread that 'dieseling' can bend rods. It can.

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'83 SC

I don't think this question was answered in all the replies so far.

As the air-fuel mixture burns at a constant rate (all things being equal) a mechanical advance is needed to ignite the mixture sooner as the engine speeds up so peak combustion pressure occurs at the most optimum piston position. The mechanical (often called centrifugal) advance mechanism advances along with engine speed up to a maximum amount before TDC.

Vacuum advance is not necessarily needed as the engine will run okay without it. However, at midrange cruising, air-fuel mixtures are typically leaner than at full throttle or idle, and this lean mixture burns slower. To take advantage of the slower burning mixture, the vacuum advance adds additional spark advance to that created by the mechanical advance mechanism, typically around + 10 degrees or so. This improves fuel economy at cruising speed. At idle, there is no vac. advance because the vacuum signal is from above the throttle valve which is almost closed and thus not affected by engine (manifold) vacuum. At WOT (wide open throttle), there is less manifold vacuum and this causes the vacuum advance mechanism to return to zero degrees advance. Vacuum advance is entirely independent of mechanical advance.

On late model vehicles, all timing functions are controlled by the engine control unit (ECU) which takes into account variables such as throttle position, engine temperature, engine speed, vehicle speed, transmission shift position, etc. to optimize spark advance.

Sherwood Lee
http://members.rennlist.org/911pcars

The first part of my message was chopped off:

In response to a previous statement:

"But why have both a vacuum advance and a mechanical advance??"

continues ......