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Post by jugulator on Aug 5, 2014 9:23:46 GMT -5
Ok, here is the part I don't understand.. the line coming from my carb to the vacuum advance only has vacuum when you Rev the motor and get the RPMs up and no vacuum at an idle. So how would hooking this line up at an idle change a thing? the vacuum you get from the carb when you rev the motor is venturi vacuum .. we don't want to use that,, you get mainifold vacuum at idle,,, ..................lenny
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Post by scottgropp on Aug 5, 2014 10:22:27 GMT -5
Leadfoot,
You might want to research it yourself and get advice based on your research. I'm looking at some of the things on here I'm not sure you are going to gain much with some of the wrong information.
Buy or borrow a cheap vacuum gauge. They aren't much money. You need to focus on your own vacuum measurements at different places (ports), with the engine running, checking at various RPM's and throttle positions, and then at cranking speed also. Depending on your engine, your vacuum may surprise you based on what you have been told on here. Once you know your own setup, you can determine where to connect the vacuum, if you want vacuum at all. If you really want to learn more, change some things on your engine and test again - pull a plug wire, change valve lash, and see what happens. Use a longer hose for all of your tests, and test when the car in gear, then take it out and perform the same tests in gear/under power. That is how it will work on the track, so that will be the most accurate place to learn.
Also, there are articles on vacuum vs. mechanical advance, and what they do. You tube and wecrash might not be the best learning location. Understand the constant ignition/burn rate of air/fuel mixture and how it reacts with crank degrees and rpm to set timing.
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Post by jugulator on Aug 6, 2014 8:48:25 GMT -5
scott,, the guy has a stock 351M,,, he just wants a simple way for it to run good,,
this is the easy way,, and its not going to cost him anything,,
.......lenny
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leadfoot455
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Post by leadfoot455 on Aug 6, 2014 17:19:31 GMT -5
Thanks everyone for all your help!
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Post by scottgropp on Aug 6, 2014 20:15:22 GMT -5
scott,, the guy has a stock 351M,,, he just wants a simple way for it to run good,, this is the easy way,, and its not going to cost him anything,, .......lenny Oops, my bad, I forgot that giving wrong advice and having him blow up his only engine would be a good thing, right? I always think people ask questions to get the correct information. Close is close enough I suppose, until BOOOOOM.
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ynotracing
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Post by ynotracing on Aug 6, 2014 21:48:23 GMT -5
I think my gropp dizzy has the vacuum advance cut off.....
I'm all for doing it the easy way but save a few bucks and get the combo and u will be amazed ....
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Post by justforfun1973 on Aug 6, 2014 22:54:13 GMT -5
I think my gropp dizzy has the vacuum advance cut off..... I'm all for doing it the easy way but save a few bucks and get the combo and u will be amazed .... next year for me!
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Post by jugulator on Aug 7, 2014 10:27:21 GMT -5
scott,, the guy has a stock 351M,,, he just wants a simple way for it to run good,, this is the easy way,, and its not going to cost him anything,, .......lenny Oops, my bad, I forgot that giving wrong advice and having him blow up his only engine would be a good thing, right? I always think people ask questions to get the correct information. Close is close enough I suppose, until BOOOOOM. why do you have to criticize ...? vacuum advances have been use on stock motors from the 60s to the mid 80s,,, and I bet,,, that not one motor has blown up because of them,, #2,, just to let the readers know,,I'm 57,, started mechanics out of school,, I am a retired ford A tech of 30 years,,,ASE certified in everything,, building derby cars for 24 years,,,, My son has so many trophies,, he doesn't know what to do with them,, oh,,,, and our SBC,,, has a vacuum advance,,,, .........................lenny
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Post by scottgropp on Aug 7, 2014 10:50:25 GMT -5
Why criticize wrong info? I can't even believe we have reached that point on here.
Engines have blown up from improperly setting up the ignition. That includes using vacuum advance improperly when designing the entire timing package.
I'm 39, never went to Nebraska Wesleyan University, then UNL College of Law, and have never been to a tech school class or took an ASE certified test in my life. But I have many trophies from my derby driving days, and have built engines that have won almost every major show in the country, some multiple times, and who knows how many other shows I can't even keep track of anymore.
The funny thing is, Leadfoot and I were sending each other PM's after my post on here. He bought a vacuum gauge and tested some things, and couldn't figure out how it matched the advice he was given on here. Good for him doing the work to learn.
So carry on with bad info. I've said its killing the sport for years, but that never seems to stop people.
Maybe I should join in. Just set the timing at 25 degrees BTDC. Sounds good to me.
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99x
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Post by 99x on Aug 7, 2014 23:18:29 GMT -5
Noon at 2000rpm is what I'm at with idle around 1000rpm. Thats my preference and if I burn it up then i go grab another from the scrap yard and do it again. Usually lasts long enough to make one hit or a second place, No advance on it either, but either way I like junking fancy sh!t with junk sh!t that looks fancy :-)
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dm440c
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Post by dm440c on Aug 8, 2014 13:58:31 GMT -5
hey one thing- Leadfoot you have part of it backwards but the question you asked shows you were using the ol' noggin.... the vacuum advance does NOT advance at high RPM, it's the other way around. You later asked the right question though because you wondered how the heck that could be true if manifold vacuum drops off as RPM and engine load go up. (edit- at first I missed the part where Scott PM'd you, I imagine he explained this already)
Vacuum advance as the factory designed it works like this- there is a "timed" port on the carb that effectively has no vacuum at idle but picks up vacuum signal as the throttle plates open up. Plotted on a curve it has a quick ramp up and a slow ramp down. The point of it is to boost fuel economy at part throttle cruise conditions by advancing ignition timing into high areas that would otherwise cause detonation if there was any real load on the engine. The vacuum signal works perfect for this function because if you are cruising along at a steady speed it is boosting timing to get max fuel economy but if you jump on the throttle it will drop out as load & RPM go up to avoid detonation.
Keep in mind the real boss of the timing is the mechanical advance weights which are always working and have a fixed relationship to RPM. Based on the weights, springs, and slot lengths any specific point on the RPM curve has a corresponding point for timing advance above initial (idle). So, if you disconnect the vacuum advance you still have a functioning ignition system. You may not get the same fuel economy going down the road and depending on the particulars of your carb settings and other ignition settings you may notice a change (better, worse, or none) from the seat of the pants in how the engine "feels". Generally, someone who knows what they are doing with carb and ignition tuning will not use vacuum advance on an off-road application because they can get it right without it(edit- this is mainly a reference to off road applications like mud trucks or race cars, etc., demo derby is a special application due to high heat considerations that most of these other off road applications don't try to account for).
So, should you use it? I've often argued that the average derby driver doesn't need it used as designed form the factory. If you try it and "feel" like your engine runs better, and you don't know how to fix that by tuning your carb and the other distributor settings, go ahead and use it if it makes you feel better. Also, as Lenny talked about, there is some trickery you can do to make it work for you on hot starts but I would advise anyone attempting that to get an education on the whole subject, not that the risk to damage anything is very high but by setting the initial low and having stock settings for total mechanical advance more likely you will not have high enough total advance and it will be pretty doggy as you start getting up there in RPM. Certainly with this concept you would use straight manifold vacuum rather than the timed port on the carb and almost certainly have to use a custom curve for the mechanical advance to get it nice and crisp from idle to top speed. Fun stuff to play with if you like backyard mechanics.
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Post by jugulator on Aug 9, 2014 6:58:24 GMT -5
wow DM,, you typed a lot there.. ok,, everyone can do what ever they feel is right,, since I was told I'm wrong,, ............lenny
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dm440c
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Post by dm440c on Aug 9, 2014 7:36:25 GMT -5
I'm a pretty fast typist I like the information exchange process on here, so I like to explain some of the theory behind the conclusions. ...and to underscore a point, Lenny's idea will work but it's not how the system was designed to work so to really do it right I am advocating modifying the advance curve on the distributor to bring all of the timing factors back to their happy place. Which I think is in between the lines of what Scott was saying. I would suggest that Scott has a point as well about possible danger although I would also suggest that while a kaboom event from improper timing settings is possible, it would be unusual in the context we are mulling over today. Regardless of the probability, he is right in that messing with the unknown has risks and the reader should ponder this a bit before they go charging out into the garage to test what they just read on Wecrash. ALWAYS.
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Post by jugulator on Aug 10, 2014 17:10:24 GMT -5
well how about that ?... aug 9th 2014,,, another 1st place win for my son,, 76 caddy,,, 350 chevy,, with a VACUUM ADVANCE how did he WIN I must be doing something wrong,,, AND,, we don't use a cooling system,, AND,, we didn't spend 5000 bucks on a motor,, so,,, how does it go ??.. oh yeah,, put that in your pipe and smoke..
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ron17t
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Post by ron17t on Aug 10, 2014 20:00:39 GMT -5
You really don't like radiators do ya Jugulator? Congrats on the win, that couldn't have been a JM show right?
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