NLA B28 vacuum advance

Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic

   I was just looking over these posts again. Thanks again for posting them, it is good to see actual data in one of these "learning sessions"! Couple observations, can't help myself. 
Looks like you are running 39 degrees total advance with another 16 worth of vac advance at cruise. If you put on that bushing, it will cut the mechanical down by 2-4 degrees, which will put you right about where you had it before you re-activated the vac advance, ie 36.5 total. It will also cut it down with vac advance to around 52 degrees. All of which sounds perfect to my ears, though the butt dyno is the final word. I would not be surprised if the old rubber bushing isn't somewhere in the bottom of the dissy. I have had them fall off before, that is why the bronze one is better than rubber. 

When it comes to mileage, bear in mind that you have probably already achieved the goal, at least at highway cruise. The advance is pretty much at max with either the copper or the silver springs by the time it hits 60 mph. So at this point really all it is is a drivability issue at part throttle. Butt dyno rules here. 

Again, I am really looking forward to hearing about the road trip mileage next weekend!

Thanks!

Chris  

Author	Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic
ccowx Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510	Post Options Post Reply Quote ccowx Report Post Thanks(0) Quote Reply Posted: Jun/07/2016 at 11:37am
	I was just looking over these posts again. Thanks again for posting them, it is good to see actual data in one of these "learning sessions"! Couple observations, can't help myself. Looks like you are running 39 degrees total advance with another 16 worth of vac advance at cruise. If you put on that bushing, it will cut the mechanical down by 2-4 degrees, which will put you right about where you had it before you re-activated the vac advance, ie 36.5 total. It will also cut it down with vac advance to around 52 degrees. All of which sounds perfect to my ears, though the butt dyno is the final word. I would not be surprised if the old rubber bushing isn't somewhere in the bottom of the dissy. I have had them fall off before, that is why the bronze one is better than rubber. When it comes to mileage, bear in mind that you have probably already achieved the goal, at least at highway cruise. The advance is pretty much at max with either the copper or the silver springs by the time it hits 60 mph. So at this point really all it is is a drivability issue at part throttle. Butt dyno rules here. Again, I am really looking forward to hearing about the road trip mileage next weekend! Thanks! Chris

bikerfox Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: Aug/02/2009 Location: USA Status: Online Points: 4473	Post Options Post Reply Quote bikerfox Report Post Thanks(0) Quote Reply Posted: Jun/07/2016 at 2:36pm
	Thanks, Chris, for thinking about the numbers. I told a buddy (who had built many chev/pontiac engines) about my total timing, 55 degrees. He was very worried about my engine and thought it was excessive. If I don't hear detonation when I drive, could I be damaging the engine while driving? Something I forgot to bring over to my buddy's house when we measured timing was the Crane Vacuum Timing Limiter Plate, which l have wanted to use for quite a while. What I really want to do is to increase initial timing from 7 deg. to 12-14 deg., as that's what my machinist said the engine will run really well. Problem is I have to find a way to reduce the mechanical advance, hence the bushing. We'll see how this weekend turns out.
	1969 Rebel SST (1970-1987) 1968 AMX (2005-2011) 1969 SC/Rambler (2011-2019) 1970 Javelin (2019 to ?)"Jane"

Red Devil Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/10/2007 Location: Canada Status: Offline Points: 1743	Post Options Post Reply Quote Red Devil Report Post Thanks(0) Quote Reply Posted: Jun/07/2016 at 3:30pm
	Have you tried running manifold vacuum instead of ported? The main reason to use the B28 can is so you can use manifold vacuum to get additional idle advance without having to run a lot of initial. If you reduce your initial & run manifold vacuum, you'd get back to the 34-36 or so total (mechanical + initial) and keep more idle timing. For the limiter plate, best to drill & tap the plate on the vacuum advance arm to mount the timing limiter. This will keep advance starting at a low vacuum and it won't affect your initial timing ... like this: The Crane mounting instructions affect the starting vacuum and changes your initial setting. Hope this helps,RD.

ccowx Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510	Post Options Post Reply Quote ccowx Report Post Thanks(0) Quote Reply Posted: Jun/07/2016 at 3:30pm
	First things first, yes, you can damage your engine with detonation you can not hear. An SC with a set of glasspacks is probably noisy enough at highway speed that you may not hear it. I would also agree that the 55 degrees is probably too much. That is why I was saying that putting that bushing on and cutting it down a bit would not hurt. For what it is worth, the Technical Service Manual calls for 45-49 degrees total, including vac advance. They tend to be a bit conservative, but it gives some idea. I think you are on the right track with your thoughts on raising the initial timing. Most guys seem to settle on 12-14 and then have around 24-ish degrees of centrifugal advance. That is what I have. If your engine was mine, the first thing I would do is put that bronze bushing onto the advance peg. That will get you down a few degrees right off. Then I would consider one of two options: Option A) Leave it as is and put the vacuum advance to manifold vacuum. That will give you more advance at idle, which will make it run better. You will also have the additional timing at cruise for mileage. You will probably want to limit the vacuum advance from 16 degrees to maybe 10-12 or so, using a limiter plate of some kind. This will give you maybe 18-20 degrees at idle and 46-48 at cruise. Option B) You can shorten the advance down from the 29 or so degrees you have now to about 24. Then you can put the initial timing at 12 degrees and have 36 degrees total advance. Same thing for vacuum advance, shorten it down to maybe 10-12 as above, but this time hook it to a ported vacuum source. This set up will give 12-14 at idle and cruise will again be 46-48. To shorten down the mechanical advance you can do a few things. The simplest one is to take a welder and zap the end of the advance slot, filling it a bit with welding rod. A bit of grinding to get the amount off that you want and make it smooth and you should be fine. You can also see if you can find a pole piece with around 24 degrees from the factory. Either of these require that you pull the distributor and dismantle it. You also need a welder. Something I have done is to modify one of the bronze bushings. I drilled a small hole in one side and threaded in a small machine screw. I used a dremel tool to cut off the threads short and put it in with a bit of Loctite. You can install it on the advance peg and turn the screw head to the end of the slot to shorten it's travel. If you need to you can file down the screw head to get the right amount of degrees. This way it is not permanent and you don't even have to pull the distributor out of the car, though it does make the job easier. I tend to like the second option, ie shorten the advance and use ported vacuum, but the other method works well too. There are situations that may cause you to choose one or the other, but most people seem to go with this. Remember, all of this is just semi-educated guesswork! The ever popular butt dyno will tell you what works for you and your car. Start conservative and work your way up. That way you are less likely to hurt anything. Thanks for keeping us posted! Chris Edited by ccowx - Jun/07/2016 at 3:50pm

bikerfox Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: Aug/02/2009 Location: USA Status: Online Points: 4473	Post Options Post Reply Quote bikerfox Report Post Thanks(0) Quote Reply Posted: Jun/07/2016 at 4:14pm
	Thanks, RD, for the pic. When I go that route, it will help. Just for fun, I found the bronze bushing and managed to press it on. You said, Chris, that installing the bushing will "get me a few degrees." How many degrees are we talking about? Also, looking at the mechanical advance (MA) setup w/bushing, it isn't clear to me how this bushing limits MA.
	1969 Rebel SST (1970-1987) 1968 AMX (2005-2011) 1969 SC/Rambler (2011-2019) 1970 Javelin (2019 to ?)"Jane"

ccowx Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510	Post Options Post Reply Quote ccowx Report Post Thanks(0) Quote Reply Posted: Jun/07/2016 at 5:36pm
	The bushing is worth about 2-4 degrees less MA. The way it does this is by taking up some of the travel in the advance slot. The overall MA is determined by the length of the advance slot. The advance slot is what that peg you just pressed the bushing onto rides in. The longer that slot is, the more travel the peg has and the more degrees of MA the distributor will have. If you remove your cap and pull the weights outward you will see the shaft rotate and the peg move along the slot. This movement is what moves your MA, with the rate controlled by the springs and weights. If you put the bushing on, it shortens the travel of the peg along the slot by the thickness of the bushing. If you modify the bushing as above and put the side with the screw head towards the end of the slot, it will shorten the travel even further. The one above changed my own distributor from having 32 degrees of mechanical advance to 24. Welding the slot so that it effectively shortens it does the same thing. The bushing can be removed, even with the dissy in the car, with a small screwdriver and a bit of cursing. I hope that helps, Chris Edited by ccowx - Jun/07/2016 at 5:40pm

bikerfox Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: Aug/02/2009 Location: USA Status: Online Points: 4473	Post Options Post Reply Quote bikerfox Report Post Thanks(0) Quote Reply Posted: Jun/07/2016 at 7:37pm
	Thanks, Chris. I didn't see your post before I left to do some errands, so I called Summit and asked about the bushing and number of degrees. The guy on the phone insisted that this bushing affects VA, not MA. He also insisted that all timing be in by 3k-3.5k. I told him that I'm driving an AMC, btw. I know that our v-8 engine timing is supposed to be all in by 2.5k-3k. Bottom line: he didn't know how many degrees would be subtracted from MA/VA. I think he meant well.
	1969 Rebel SST (1970-1987) 1968 AMX (2005-2011) 1969 SC/Rambler (2011-2019) 1970 Javelin (2019 to ?)"Jane"

ccowx Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510	Post Options Post Reply Quote ccowx Report Post Thanks(0) Quote Reply Posted: Jun/07/2016 at 7:54pm
	It is hard to be able to discuss things sometimes without a picture in front of you. Look at some of the terminology issues that have occurred even on this page! Good on him for trying. As for timing being all in by X rpms, that really depends on many things, not just brand. Stock the timing on an AMC seems to be all in by around 4000 or a bit less. I am sure that is very different from what Craig Breedlove's supercharged record setting car had! Cam, intake, compression, head work, etc will all affect the timing. That is what I mean by every car is different. Some cars might be fine at 3000-3500 and other may need it all by 2500 or less. The distributor will have a stamped number that gives the total that the slot is designed to give. See the sticky about distributor specs at the very top of this message board and it shows it. That number is meant to include the bushing is my understanding. For example, my Delco distributor had a pole piece with a slot that gave 32 degrees, including the bushing, probably 35-ish without. Thanks, Chris

ccowx Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510	Post Options Post Reply Quote ccowx Report Post Thanks(0) Quote Reply Posted: Jun/08/2016 at 3:27am
	Ok, I clearly have too much time on my hands! I have been piecing together a second ignition system using a spare Delco-Remy distributor I have. I have been collecting bits for probably a couple of years, but all of this recent interest has had me more interested in finishing it. One thing I had not put a lot of thought into was the vacuum advance limiter. This thread prompted me to take a closer look at the Crane version and I am not sure I want to do that. It limits the vac advance by pushing the pin and diaphram ahead from it's base setting. As Red above here pointed out, that changes your initial timing and also how the vacuum canister functions. One good solution is what he did above, by drilling and tapping a mounting hole ahead of it instead of using the mounting screw. Unfortunately, I don't have a small tap and die set, so I decided to see what I could do with what I had. Here is my answer, made from a piece of scrap metal I had that had a screw slot in it. I simply cut it out with a Dremel tool in about five minutes, scrounged up a couple of washers and done. The penny is for scale. I have it installed in the distributor and it seems to work fine. You can adjust it by loosening up the mounting screw without pulling the dissy and you don't have to change the initial timing when you adjust the vac advance. Not exactly a masterpiece of fabrication, but free, fairly user friendly and it looks like it will work well enough. Thanks! Chris Edited by ccowx - Jun/08/2016 at 3:41am

bikerfox Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: Aug/02/2009 Location: USA Status: Online Points: 4473	Post Options Post Reply Quote bikerfox Report Post Thanks(0) Quote Reply Posted: Jun/08/2016 at 7:56am
	That's a nice, homemade system, Chris! I like it!
	1969 Rebel SST (1970-1987) 1968 AMX (2005-2011) 1969 SC/Rambler (2011-2019) 1970 Javelin (2019 to ?)"Jane"