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Disty Vacuum advance - manifold or ported - lets hash this out


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#1 GeneralDisorder

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Posted 23 April 2004 - 12:18 PM

Ok - so I've done a bit of research on this one, and I've come to the conclusion (like I thought in the first place) that ported vacuum is what we want for a stock distributor on an EA81. Some people have claimed that full-manifold vacuum is better, however this just doesn't make sense.

The problem is that the disty's mechanical advance mechanism will only advance a total of 15 degrees. Full manifold vacuum drops off at high engine speeds, and the mechanical advance is supposed to take over. But with a distributor designed to accept ported vacuum (where the vacuum rises with higher engine speeds), the mechanical advance is not great enough to reach full advance. You will only reach about 23 degrees of advance (8 plus 15), versus the 33 that the engine was designed for. Thus you will not be running nearly as effecient as if you had the extra 10 degrees from the vacuum advance - which you will only get if you run ported.

Maybe the disty could be modified to run correctly with full manifold vacuum, but a stock one surely is not designed for it. The drawbacks of never reaching full advance, and of having higher idle emmissions from haveing TOO MUCH advance at idle (which will make the engine appear to run smoother), are not assets to the EA81.

I suspect that this goes for EA82 carbed engines as well, but I'll leave that one up for discussion.

Any comments? I would like to know if my logic is somehow flawed...

GD

#2 Snowman

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Posted 23 April 2004 - 01:30 PM

It seems pretty odvious to me. The disty needs more advance and hence more vacuum as RPM increases. Manifold vacuum decreases as RPM goes up. Ported vacuum increases as RPM goes up. Which one do you think I've got it connected to, and which one has it always been connected to on all the soobs I've ever seen? Ported.

#3 baccaruda

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Posted 23 April 2004 - 04:08 PM

how do you hook up to a ported source? i've always just looked for a source on the manifold...

#4 Snowman

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Posted 23 April 2004 - 04:16 PM

On the Weber 32/36 DGV series, the single ported vacuum source is on the front of the carb. I think it's about halfway up the carb, but I'm not sure. It's pretty odvious where it is. On the Hitachi carb, there are two ported vacuum sources, both on the front of the carb, about 2/3 of the way up if I remember correctly. One is for disty advance, the other is for EGR. I don't know if there is a difference between the two, as I switched them back and fourth with no noticable change in engine operation.

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Posted 23 April 2004 - 11:00 PM

Look you guys, you seem to be fixated on a simple modification. I think more effort goes into posts like this where the only thing you have to do is switch the vacuum hose from one port to the other, than other posts where serious modifications are taking place.

By the way if we only altered our Soob based on orignal factory design then we would NOT have: bigger tires, lifts, CIS-Fuel injection, desmogging, larger exhaust, Rear drivers, Nissan transfer cases, 5 speeds where 4 speeds used to be, turbo's on non-turbo engines, EA82 pistons in EA 81 blocks, Webers, and on and on and on and on. I think you get the picture.

Bottom Line, my 82 brat with full manifold vaccum runs better than on ported. Simple switch of the hose proves it. I can also run more initial timing advance, and have more low end torque, and have no pinging issues. Its an old, tried and true hot rodding trick. It costs no money, can be completed in a second and can be switched back if you don't like it.

Flame on

Bill

#6 GeneralDisorder

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Posted 23 April 2004 - 11:25 PM

I did try it - I got less top end from it - for the reasons I stated above. I tried it both ways, and although I am not saying that it is totally wrong to do what you have done, I AM saying that what you have done is incomplete, and ignorant (of the principles of operation) at best. For it to run correctly in this manner, a recurve of the mechanical advance, and a redesign of the vacuum mechanism would be required. Your "modification" is making your idle smooth in spite of another problem I would guess. This is what I experienced - the engine would run with a high amount of advance at idle in spite of a quite large vacuum leak that I had not discovered. It ran MUCH smoother with the manifold vacuum because it MASKED the other problems that I had not fixed. But after doing some research on the subject, and reading a lot of FSM's, I determined that this simply is not correct, and switched it back. I looked for my rough idle elsewhere, and did in fact find, and correct the problem. Now it runs just as smooth on ported vacuum as it did when I had it switched to manifold for a brief test run.

Why is it so difficult to understand that the engineers DESIGNED the friggin distributor to run on ported vacuum? Do you think that the exact same design and configuration would work for both a vacuum source that rises in pressure as RPM's increase AND a source where it drops? No - of course not. The same design could not possibly handle both, as it has no way to know which it will be using. It may run, but it is not running correctly.

You sir, are wrong. And a simple "switch of the hose" proves exactly nothing - other than you can switch a hose. Where is your technical data to back up your claim? I have provided mine for all to see and critique.... please prove me wrong if you can - I want my Brat to run better too. That's what this forum is for. I don't, however, like disinformation - intended or otherwise.

GD

#7 Russ Hill

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Posted 24 April 2004 - 04:53 AM

You need ported vacuum for the disty. it provides a short advance before the centrifugal advance spins up. If you want more advance, increase the initial timing until it gets hard to start or pings. But I wouldn't on a Suby. I doubt you'd get enough extra ponys to make it worth the hassle of running higher octane gas. Try the 2wd disty it has a quicker advance curve.

#8 TomRhere

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Posted 24 April 2004 - 05:15 AM

I agree with GD.

Way back when, before all of the emission stuff started, circa '71-72, the disty was given manifold vacuum for the vacuum advance. After emissions started showing up on engines, the vacuum source was moved to a ported one, to help clean up the exhaust.

Older vacuum advance units had different actuating arms, (read over-all length), and the point of connection on the advance plate of the disty was different. IE; the lenght and shape of the slot in the arm was different between like models of engines vs different type of vehicle the engine was in. (read truck vs car, and camero vs impala). Haven't messed around with any of the newer ones to know if there is still a difference there.

Also, the mechanical advance has changed as well. Which you can still modify yourself, if you can find the spring/weight kit for your type of engine.

#9 GeneralDisorder

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Posted 24 April 2004 - 10:30 AM

Actually - the FSM's I have don't note any difference in the Distributor advance curve for 2WD vs. 4WD. Only a single graph is given. By chance, I have a 2WD disty in my wagon (it was a 2WD car!), and I can't say that I notice any difference. The Hitachi units seem less prone to malfunctions arising from shaft wear as well, so given the choice, I think I would stick with the 4WD disty. I have yet to see any hard evidence to show that a difference exists, and I can't come up with any myself..... maybe someone has an FSM that shows this elusive disty? At the very least it seems that not ALL 2WD distys have this "better" curve..... craig - your pretty quick with a scanner - you want to show the graph in your 83 manual?

GD

#10 c150L

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Posted 24 April 2004 - 11:43 AM

Sorry if this is a repeat as I have not read all posts entirely. Manifold vacuum would make sense if one wants to loose any and all disty advance when ever they stepped on the gas pedal. This loss of advance will eventually come back as the engine RPMs get nearer to the the rpms that that particular throttle setting will allow. As I understand as stated in some posts, this manifold vacuum drops at higher RPMs. Guess i have not seen this, but it makes sense that it would. So, we hook up to manifold vacuum on our disty. We're sitting at the stop and go lights, full advance, jump on that gas to beat the bozo next to us that is going straight in the turning lane, loose all advance in the ignition. Would we have more power with less advance? (I don't think so, but correct me if I'm wrong.) So, here we are, cruising 55 on the highway. Hooked up to manifold vac, full advance. Need to pass. Step on the gas, vac dropps, no vacuum advance on the disty. Again, do we gain power by loosing our advance? I still don't think so. (CBR) When somewhat into stock car racing, many many moons ago, I threw in so much advance into the disty for racing that we could not even start the car. The manifold vac theory would seem to be the opposite of what I would expect. Less advance equals more power. I still don't think so. I vote for ported vacuum for the disty.

Not OT, but didn't Chevy (GM) do something with the vacuum advance that it used manifold vacuum to retard the advance? (Maybe it wasn't GM.) Like a dual advance/retard setup on the disty or something? Been a long time since, but seems I have run into distys somewhere that used a manifold vac line to a disty.

#11 GeneralDisorder

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Posted 24 April 2004 - 11:55 AM

Actually - even some of the early subaru's used seperate advance, and retard lines. I don't think that any of them were connected straight to manifold vacuum, but who knows for other Brands.

I agree with what your saying in principle tho - the engine will run very nice at idle - since you have a ton of advance. But as you increase throttle, you will lose advance - exactly the opposite of what you want under load.

GD

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Posted 24 April 2004 - 03:14 PM

yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy

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Posted 24 April 2004 - 03:19 PM

Why is it so difficult to understand that the engineers DESIGNED the friggin distributor to run on ported vacuum? .........
You sir, are wrong. And a simple "switch of the hose" proves exactly nothing - other than you can switch a hose. Where is your technical data to back up your claim? I have provided mine for all to see and critique.... please prove me wrong if you can - I want my Brat to run better too. That's what this forum is for. I don't, however, like disinformation - intended or otherwise.

GD


Whoa, tone down. I get this funny picture in my head with your hair disheveled, great beads of sweat on your forhead all because of a little experiment with a vacuum hose that you don't agree with. Try a little more kindness and less emotion and anger and I might listen.

Your buddy Bill

#14 Qman

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Posted 24 April 2004 - 06:26 PM

I agree. Let's stick with the idea not the individual.

#15 GeneralDisorder

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Posted 25 April 2004 - 12:46 AM

That wasn't mearly directed at you taprack - sorry if it sounded that way. It was directed at all who claim what you claim - your "side" if you will. This is simply a discussion, and if It sounded like an attack on you personally, I'm sorry - I was mearly trying to create a "heated debate". It appears I have inadvertantly gotten it a bit too hot in here.... please accept my appologies for that. You sound like an intelligent fellow, and I was hoping that you might see my point if I made it a bit finer. I made it too fine apparently.

Just for the record - I'm not angry - I simply used a debate technique that I probably shouldn't have. Known as a "strawman" argument. It's more or less where you introduce a non-sequitor into the argument designed to provoke an emotional response. Probably not the wisest decision on my part. Once again - sorry.

Still tho - I would like to see any technical data you could come up with. Feel free to prove me wrong. I still think you can't..... hehehe.

GD

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Posted 25 April 2004 - 12:58 AM

General disorder, I'll see what I can come up with for information on "my side". I think I'll start out with recording my Brat's own vacuum readings at several different RPM's. I don't know that many people on this board have checked their own vacuum and it can be very helpful in tuning your engine. I'll then refer to some articles old and new and get back with ya.

Bill

#17 archemitis

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Posted 25 April 2004 - 09:19 AM

i dont think this is reely debatable, as the peeple who know whats up run them from the factory position. and the peepl who run them on the manifold, just dont get it. retarding advance at higher rpms, is NEVER a hotrod trick. more advance=more hp, up to that certain point. these motors dont have enough compression to have any problems with pinging at 33degrees. get a vac guage on your car once, and watch it nearly hit 0 when you floor it. then suck on that vac hose, and watch the disty move. thats the only way to tie it together in my head.

i vote for ported, not manifold

#18 4wheeln2

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Posted 25 April 2004 - 10:29 AM

the vacuum advance is for part-throttle driveability. it allows more advance during light throttle/ light load cruising operation. the advice to disconnect and plug the vacuum advance diring setting the initial timing is so the vac advance will not affect the overall timing. ie- the vac advance is adding or subtracting timing advance while you are really only interested in setting the (mechanical advance) timing. the disty's mechanical advance has a fixed amount of advance like maybe 24 degrees (just a guess) and if you set the initial timing at 12 deg btdc (before top dead center) the result is that you have 36 deg total timing. aand you would get full advance (36 deg) at the rpm that the spring pressure is overcome by the centrifugal action of the advance weights. likely close to 4k, but every disty is different, and by checking advance at 500 or 1000 rpm intervals, it should be easy to plot an advance curve. ideally, for a street driven, or off roaded vehicle, part throttle driveability (ie throttle response) should be the main focus. you want the advance in as soon as possible, ther curve miught start adding advance at 1200 rpm and have all 24 deg in by 2500-3000 . the stock curve is probably struggling to get a full 24 or whatever the disty is designed to provide by 3500-4000rpm. the vacuum advance should be left as is (factory stock) and the disty should be used to alter the timing curve. any use of the vac advance to provide more advance would only improve a vehicle's performance in a narrow band of operation, if at all, and then only because the vehicle was not properly tuned to begin with. true there are a lot of smog related items that can provide a performance boost if removed- but most of the benifit is from lost weight. biggest cork to performance is clearly the exhaust system. then the intake, then the cam and heads. you're going to have to work really hard to get another 25 horses out of an ea81 ea82. they are only 80hp or so to begin with. the better plan is to marginally improve performance by 15 or 20 horses across the rpm range, rather than trying to get 50 horses at the top end. the other guy will be outrunning you for 4000 rpm of a 5000 rpm powerband. final word- fix the vaccum advance, it promotes mileage, throttle response and driveability. you guys could also consult some of the old-school hot rod toype magazines, as they all still regularly run articles that explain in much better detail the function of complicated engine systems, and most of the knowledge is very general in nature and does apply to our subes. Pat

#19 edrach

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Posted 25 April 2004 - 04:32 PM

Another thought that hadn't come up on this discussion and I don't know if it's applicable. On the older EA82s and EA81 engines, almost all the vac. advance diaphragms are shot anyway and you don't get any advance from that; you're only working on the centrifugal anyway. Take that from an old junk yard dog who's found the 19 out of 20 distributors don't have a working vacuum advance unit.

#20 GeneralDisorder

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Posted 25 April 2004 - 04:40 PM

Absolutely correct Ed! That's why I have these guys rebuild mine with brand new rubber:

www.philbingroup.com

Cost is about $35 for any vacuum advance module....

And the figure for the mechanical advance is 15 degrees maximum. So total advance is limited to 23 degrees with only mechanical and initial advance settings.

GD

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Posted 25 April 2004 - 05:31 PM

you guys could also consult some of the old-school hot rod toype magazines, as they all still regularly run articles that explain in much better detail the function of complicated engine systems, and most of the knowledge is very general in nature and does apply to our subes. Pat


How does it NOT apply to Subaru engines? What exactly is different from hotrodding a VW air cooled which follow the same or similar techinques of hotrodding then that of the EA81? What in automotive modification history will not work on Subaru? There is nothing special or so different on the horizontally opposed watercooled engine that it cannot benifit from OLD SCHOOL hottrodding.

The same principles that cause many of us to modify out suspensions and think outside the box are the same prinicples I'm using on my engine. Now granted, it may not work, and you may spank me off the line in a drag race, I might even change my vacuum line to ported (did I say that). But don't think our engines are so unique that nothing in the world of Hotrodding will work on them and the only confiquiration that will work is completely BONE stock.

With that kind of logic I would think all you guys that spent $300-$400 on Webers just dumped your money down the drain because the engine wont respond to that modification. Small valves, low compression, smog cam,tiny intake and choked exhaust will negate any improvement the weber would/could afford.

Let me experiment and find out for myself. If we only did what somebody else said works then there would be no Weber, no 4 inch lift because all the 4x4 shops said it can't be done. I've read tons of magazine articles, web sites and the like and from what I've read, Full vaccum works in principle and practice. Is it the best..maybe not. I like it and thatswhat matters to me. I don't beat you down for going ported. Give me a break for going full. And read a little hotrodding, you might learn some new/old tricks.

Happy rodding

Bill

#22 GeneralDisorder

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Posted 25 April 2004 - 08:20 PM

How does it NOT apply to Subaru engines? What exactly is different from hotrodding a VW air cooled which follow the same or similar techinques of hotrodding then that of the EA81? What in automotive modification history will not work on Subaru? There is nothing special or so different on the horizontally opposed watercooled engine that it cannot benifit from OLD SCHOOL hottrodding.

Well - it does apply - sort of. It applies to the engine itself, yes.... but not the distributor per-se. The distributor that was designed for these engines was (to your mind set) badly designed. Now that being said, IF you wanted to run full manifold vacuum, there is nothing wrong with that in principle, but the distributor would have to be redesigned to accomidate that "mod". Other makes / models may not need a redesign to take advantage of your "mod", but the Subaru design does. Simple fact of it is that running full manifold vacuum will flop the vacuum advance curve over. It will make the advance curve almost a mirror image of what it's supposed to be.

The short answer is that while this may be a worthwhile mod, you need to do a bit more work to get there than a VW, or whatever else you might own. No one is argueing that this isn't a good mod, I'm arguing that this isn't a good mod *using the stock distributor*.

Also - I have some more technical data - taken from a friends 83 FSM. The mechanical advance peaks at only 1700 RPM's or so, not the 3k or 4k that a lot of people seem to think it does. It actually comes in pretty quickly. One interesting thing I noted was that the 83 vacuum module was good for about 12 degrees of advance as compared to the 10 degrees from the 80/81 FSM....interesting.

And you are (in my opinion) 100% correct on the Weber thing. I don't believe that a stock motor benifits much at all from a Weber. However, that beign said, there are still benifits to having one. It's simpler to work on, and the progressive linkage gives it a more "sporty" feel. It also increases the low-end torque by allowing you to instantly open both barels by fully depressing the pedal. On the Hitachi, the secondary is vacuum operated, and doesn't kick out the low-end torque because of that.

As for HP, and top end, I think a couple horses are gained from a Weber, and a better flowing exhaust, but it's mostly the exhaust that's doing it. I'll be building an engine here shortly with a bigger cam, decked and ported heads, HC pistons, custom exhaust, and (you guessed it!) a recurved disty. that will be more capable of utilzing the Weber.

GD

#23 4wheeln2

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Posted 28 April 2004 - 07:14 AM

the last couple of threads were a little confusing to me. I'm not sure if I was misunderstood when I suggested reading old-school hot rod magazines. I do read those mags, have been doing so for 20-25 years. knowledge is power. My intention was to promote this general knowledge as it applies to all cars, including subys. I didn't mean to hurt anyones feelings or insult anyone in any way. I was simply trying to add my .02. that said, I will suggest again to anyone interested in modifying the timing curve on any vehicle to do so after a little research on what exactly spark timing affects, and especially, what role the vac advance unit plays in the overall timing picture. As I tried to explain before, the vac advance unit is there to add extra spark advance (cause the spark to arrive inthe combustion chamber sooner relative to the piston arriving at top dead center) when the engine is in a light load/or cruising mode. ie low demand, part throttle, downhill, etc. the additional spark advance at this point allows more complete combustion, at a low demand time when there is little danger of detonation(also known as pinging or preignition) detonation is the uncontrolled burning of the intake charge (not initiated by the spark plug firing) due to increased cylinder temperature. usually, some sharp edge of the spark plug or combustion chamber glows red hot and ignites the intake charge prematurely- while the piston is still on its way up the cylinder bore. the result is like hitting the top of the piston with a hammer as it arrives at the top of the bore. the first to go is the crank and con rod bearings, piston ring lands, etc. detonation is a condition that will ruin an engine in short order. look for grey specks on you spark plug insulator- that's part of your piston that got melted. like I said before, if your car runs better with the vac advance hooked up differently, it's probably because there is some other problem that you need to address. about the advance all being in at 1700 rpm, I would be more inclined to trust a timing light, and tachometer to plot the timing curve of the stock disty. perhaps the FSM is incorrect. I don't think you will find any stock disty that has all the advance in by 1700 rpm. again, just my .02 the reason you have spark advance at all, incidentally, is because as engine speed increases, there is less time for the intake charge to arrive in the chamber, and for the spark to ignite it, and the flame front to propagate (burn the mixture) and create the energy to push the piston down the bore. lastly, if you have read tons of magazine articles,websites etc, taprack then perhaps you misinterpreted them as you misinterpreted my reference to hot rod magazines. I would like to see ANY reference to running full vacuum as a performance enhancing modification. I apologize for sounding so harsh, but it is important to have some understanding of ignition timing in order to know what effect your experiment is actually having. I agree that it is important to think outside the box, and challenge conventional thinking, but I think if you understood what role the vac advance plays in the overall timing scheme you would hook it up as the factory had it, and enjoy all of the benifits that it provides for part throttle driveability. We might also simply take a survey of some folks on the board. I think the results would support the stock vac advance case. Pat

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Posted 28 April 2004 - 09:32 AM

http://www.carcraft....958/index2.html

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Posted 28 April 2004 - 09:34 AM

http://www.chevytalk...ng_Advance.html




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