924Board.org

AFBCamaro · Joined: 03 Nov 2002 Posts: 199 Location: Orlando, FL

I understand the slight advantages of ported vacuum, i see no loss in anything from using full vacuum instead. From the looks of it the distributor on the porsche has only around 14 degress of total advance(thats a very small amount). and 6.5 of vacuum advance. Now that i see the specs there is no dought in my mind that you wont notice a thing hooking it up to full vacuum. I have mine set up this way(camaro) and it runs far better than with ported. Then again im emissionless.

dpw928 said "If you hook to manifold vacuum, you will get centrifugal but not vacuum advance at WOT" ...isnt that what you want?

the vacuum advance is mainly for light load or cruising to increase economy and running characteristics.

From you guys experience does the 924 engine really need a vacuum retard to aid it that bad? Like I said I dont have alot of experience on smaller engines so dont get mad at me!

_________________
1982 Porsche 924 in mild restoration process.
1991 Chevy Camaro RS "Freigh Train"

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

AFB,

During acceleration you want all the timing advance you can get without causing preignition. If you hook up to manifold vacuum you will not get the initial advance. Look at the curves in the Haynes manual. The vacuum advance kicks in first. The centrifugal advance needs higher rpms to advance the timing. Obviously both would be better.

It is possible that the manifolds were made without the ports as the vacuum will probably pulse which makes this discussion moot.

BTW does 16 years of owning and wrenching my 924 count as experienced?

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

Paul · Posted: Sun Jan 19, 2003 3:07 pm Post subject:

IMO, the purpose of a vacuum advance on a carb engine, is to vary the advance based on the load on the motor. Mechanical advance cannot do this, it is strictly based on rpm.

At heavy loads, advance must be retarded to avoid pinging.

So at steady state, say 60 mph, vacuum and mechanical advance max out. When you punch the throttle, vacuum drops and so does the advance. As rpms increase and load decreases, vacuum and advance increase.

Users of Weber carbs almost always converted to mechanical advances and ran the carbs rich to help avoid pinging under load.

jpab924 · Posted: Mon Jan 20, 2003 11:01 am Post subject:

Paul · Posted: Mon Jan 20, 2003 11:53 am Post subject:

Here's a couple of sites that will help, the second one is quite funny.

http://www.aircooled.net/gnrlsite/resource/articles/distributor.htm#stock

http://www.imperialclub.com/Repair/Lit/Master/001/cover.htm

Lizard · Posted: Tue Jan 21, 2003 1:52 am Post subject:

"At heavy loads, advance must be retarded to avoid pinging."

don't you have it backwards here or have I just not had enough coffee
_________________
3 928s,

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

OK, just to add to the confusion I will put my unsolliceted 2 cents in here. First off, I beleive you guys are not reading the fine print in your Haynes manuals. All the info given in the distributor curve graphs is in distributor degrees and speed not crank speed and degrees. You need to double the numbers to get crank numbers including RPM's.

Next, you need to remember the stock advance curve is tailored more towards emmissions and drivability than performance. As AFBcamaro said, Chevys run fine with little vacume advance. Most stock chevy distributors have about 20-22° of mechanical advance. When you add this to 10-14° initial advance you end up with 30-36° total advance. This is considered pretty close to optimum for most applications. It depends on combustion chamber design among other things as to what is optimum for a paricular setup. The less efficiant the chamber the more total advance is needed, ala Porshe 924.

If you look at the graphs in the Haynes manual you will see the 110HP model has a total of 27° mechanical advance (crank°) but added to the recomended -3°ATDC initial advance you only get 24° total advance. So you can see this system is very reliant on the vacume advance to make up the rest for performance. If a person where to advance the initial timing to 8-10° BTDC you would end up with 35-37° total advance. This would be much better for performance with no vacume advance needed.

Alas, there is still a problem with this seemingly simple fix. If you look at the graph again, you will see that the distributor does not reach full advance until 4600 RPM's! This will not due for a performance oriented engine. For most engines, timing should be all in by 3000 RPM if not sooner. To fix this, different flyweight and\or springs would have to affixed. This should really be done on a distributor machine of some sort as the 924 distributor is a bit of a pain to dissasemble using a trial and error method.

Keep in mind this info is tailored toward the 924 not the 931. Also remember vacume advance at cruise is a good thing as this is when you can go beyond the 30-36° total advance (up to 50°) and improve fuel milage considerably over a mechanical only setup.

Todd

jpab924 · Posted: Tue Jan 21, 2003 2:30 pm Post subject:

I made a mess of this post....see below

jpab924 · Posted: Tue Jan 21, 2003 2:30 pm Post subject:

[quote]It depends on combustion chamber design among other things as to what is optimum for a paricular setup.[quote]

So what would best suit a dual carb setup with no source of ported vacuum? More mechanical advance right? Sounds like a job for the speed shop. How much mechanical advance would I need....or is there a certain degree range I should try to meet?

I remember reading somewhere about using a 77 distributor ( I don`t remember why, I can`t find my graph its missing

) And validity to this endwrench?

Thanks for the info endwrench. Very informative.

AFB, are you up and running yet?

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

As far as combustion chamber design goes, the 924 is a real looser by todays standards. With this said, and this is just theory mind you, you will probabaly be looking for around 36-38° total advance for best performance with stock compression. The '77 distributor is a good idea as it has more mechanical advance and a faster curve. It is all in by 3800 RPM. Not great but better than 4600RPM. The '77 distributor gives a total of 35° of mechanical advance so you would want to start with a timming setting of 0° or 2° advance. This would put you in the ballpark. If you hooked the vacume advance to manifold vacume at only the front of the distributor can this would give you an additional 4° for cruise and improved fuel milage. You may be able to improve on this by swithching to a later model vacume advance wich would give you about 10° extra advance for cruise. I don't know if this is physically possible though.

The only thing that may make it better would be a lighter set of advance springs to bring all the advance in by 3000RPM or so. Think I'll look for a '76-77 distributor

Todd

Paul · Posted: Wed Jan 22, 2003 12:13 pm Post subject:

The problem with hooking the advance to the manifold is that you will get full vacuum advance at idle. The port at the throttle used for vacuum advance is blocked shut when the throttle is fully closed (at idle).

jpab924 · Posted: Thu Jan 23, 2003 7:01 am Post subject:

well that buggers things up a bit. I am running 9.3:1 euro spec pistons. I asked about the 77 distrib. because I have one at home somewhere. I am going to have to look at the part #`s to figure out which one it is.

Thanks for the info guys. I`ll email a few of the weber vendors and see what they have to input.