Discussion in 'Buick FAQ' started by LARRY70GS, Jan 2, 2005.
thanks larry! got my 430 running 14*initial,20*adv, 34*total.
HEY Larry do you have a chart that shows you octane mixture i use 110 with 93 wonder how much to use thanks
Simple average. 20 gallon tank, say you use 5 gallons of 110 with 15 gallons of 93. 5 X 110 = 550, 15 X 93= 1395. 1395+550 =1945. 1945 divided by 20 = 97.25 octane
a related article
Attached is an article I wrote for The Build Sheet on making your own advance bushing. One issue I have since discovered is that it is possible to make a bushing that allows 20 degrees of movement, but it could turn out to be 16 degrees of advance and 4 degrees of retard!!!! Found this out after I installed the springs and the bushing was not at the end of its travel. So, if you use this method, throw the springs on as you are measuring your progress while making the bushing!
This is really the way to go on distributors designed for very low initial advance (like the 0 degree initial spec on a stock 68 400) since they have a huge slot for a ton of mechanical advance.
Hey Larry thanks , who's better than you????
Since we`re on this topic was wondering what do you guys think of MSD`s Cap-a-dapt design for a billet MSD distributor?
A great addition to the thread :TU:
I bought the moroso recurve kit for my 350 HEI but I ended up modding the thing myself because the kit was inadequate. It did not come with bushings so I was on my own with the total advance. First I took the dist apart and welded up the plate as an alternative to bushings. The trial and error process was taking too long. I am impatient about things like this so I thought of an alternative to bushings. I made two adjustable stops out of machine screws. I put the nuts on the screws and held them with vise grips and used a bench grinder to make the head into a cam shape. I used nylock nuts on the screws so they didn't losen up. I'm pretty sure I used holes that were factory. I did have to grind off a tiny part of the under part of the rotor to clear the screw heads. Now total advance is easily adjustable with the rotor off.
The springs did not get my advance in when I wanted it either. If you look closely in the pic the weights are modified as well (the pointy part). The shape and size of the weights as well as the cam the ride on have a lot to do with how fast the advance comes in. The moroso weights and springs did not do the trick. Now the advance is all in by 2000.
I also made am vacuum advance stop similar to the one larry posted. It works fine.
Vrey creative, Andy. Good work
thanks Nicolas, glad you like. :TU: now all I need is another 200 or so horses :3gears:
Limiting Mechanical and Vacuum Advance ala Factory
FYI, The way the factory limited the mechanical advance was to use different cams on the distributors. This is the part with the slot for the bushing, and the 8 sided cam for operating the points.
On the underside of the cam arms, there is a 3 digit number stamped, usually 500s or 700s, (i.e. 728). There are many different numbers. I have found that the smaller the number of the last 2 digits, the less mechanical advance. Is also seems to follow that number closely, that is, a 728 cam seems to have 28 degrees advance, a 518 has 18 degrees, etc. At least this is what I am seeing on my Sun distributor machine.
The difference in these cams is the length of the slot for the advance limiter pin, which is what the bushing would go on. The longer the slot, the more mechanical advance.
You should be able to use any 8 cyl GM point cam from a clockwise rotating distributor. I don't know about counterclockwise units.
That means you can use Buick, Chevy, and Cadillac cams. These range from the teens to about 34 degrees mechanical.
It seems that the Cadillac distributors have the shortest curves, with the most vacuum advance.
The distributor specs from any manual should tell you which ones to look for if you want a specific advance. Just look at total mechanical advance for that unit, then find one in the junkyard.
This would allow you to use the stock bushing.
The vacuum advances were done in a similar fashion.
The flat part of a Delco vacuum advance is stamped with 2 numbers. The 3 digit number is the end of the GM part #, and the 2 digit number is the amount of vacuum advance built into the unit.
Therefore, an advance with 236 and 16 on it would be GM part number 1115236, a 16 degree vacuum advance.
This also applies to the GM Hei units, but I am not familiar with the numbers.
I mention the slot, and pin in my original post, but that information makes a great addition, and I appreciate it. Most vacuum advance units have entirely too much advance to be used with lighter springs. 8-10* should be the max. That's easy to limit with a stop plate. Thanks again.
This may be a engine 101 question and Ill go back to reading if it is, but why do most Buick V8's like 30-36*? My initial thought was it would be more Cam related than just engine specific.
I'll be applying this method shortly... Just worried about the outcome. Minimally modified 72 CA emissions SBB350 (save the components it arrived without installed or connected to vacuum lines when I received it)
Thanks for the great info.
Buicks have a fast burn combustion chamber, and as a rule require less advance than other engines. 30-36 is a good figure, but some engines might need as little as 28* And yes it is cam related, as well as combination related. That's why the range. You need to find out what your engine likes. There's no one size fits all. Best way to find out is on a chassis dyno, or the dragstrip if you can test and tune. With a mostly stock engine, I think you'll find 30-32 is best. Getting the advance in fast(at or before 2500 RPM) will always give good results, and you'll be able to feel it in the butt dyno :laugh: Just remember to modify the vacuum advance also.
Of these 4, which on edo I need. I have a 455 in my 69 with no smog stuff on it.
720-925A Delco Distributors pre-1969, Includes 2-Weights, 1-Bushing and 3-Sets of Springs (heavy, medium and light). $9.99
720-926G Delco distributors 1969 and later, Includes 2-Weights, 1-Bushing and 1-Set of Springs. | View Product $8.99
720-928G Delco Point Type Distributors, Includes 1-bushing, 3-sets of springs (heavy, medium and light). | View Product $7.99
720-929G Fits HEI Distributor, 1974-79 w/o Computer, Includes 2-Weights, Special Center Plate, 3-Sets Springs,and 2-Pairs of Nylon Bushings. | View Product $7.99
Ok, now I'm confused. All the Buick books I look at say the timing should be set at anywhere from 0 to 2 to 4 degrees BTDC. Where is everyone getting these 30 degree things from?
Most of what I've read here makes sense, but in the books it says for instance for a 69, 0 degrees BTDC, for the 455, like 2 degrees BTDC. What does this equate to in the overall scheme of things? Sorry I'm so dence, but I don't have a dial back light and can't afford to spend another dime. I have a dwell meter and a regular timing light though. o No:
Edit: Ok, by the way, this is great info you've posted Larry. And others as well. I didn't mean to put my confusion before that. It sounded greedy. Sorry. :TU:
Of all the kits listed, the 720-928G looks to be the best one. The 929G is for an HEI. You have a points distributor, correct? You do not need the weights, or center piece in the other kits. You can use your current weights. The advance limit bushing is nice to have. The kit states, it allows full advance between 2000, and 2800 RPM.
The reason I like the Crane adjustable vacuum advance kit is that it comes with better, more versatile springs (IMHO). It is more expensive because of the adjustable vacuum cannister. It does not come with an advance limit bushing though.
Initial timing will vary according to which distributor you are using. Distributors are different for different engines, and different years. The initial timing specified depends on the total amount of mechanical advance in the distributor.
An engine requires more advance(earlier spark) the faster it spins. The distributor has a mechanical advance system inside that advances the spark with rising engine RPM. If you remove the distributor cap, and rotor, you will see 2 weights and springs. The weights are forced outward by centrifugal force, as the distributor turns faster. The springs restrict the outward movement, and determine at what RPM the weights reach their full travel. When the weights reach full travel, we say all the mechanical advance is in. In a stock distributor, with stock springs, this occurs at about 4600 RPM.
The actual amount of mechanical advance in a distributor varies, and is built into that particular distributor. It is not easily changed. To do so requires disassembly of the distributor, however, you can restrict the mechanical advance with the bushing in the Mr. Gasket kits. You must determine how much mechanical advance is in the distributor you have. You can do this with your regular timing light, and a set of light springs(like the ones in the Crane kit) Just follow the directions in my original post.
No problem Ken I'm not sure you really understand how the advance systems work in a distributor. We might be speaking 2 different languages. A chassis manual explains the basics very well. I think if you read up on distributor basics, what I have written will make more sense.
Ok, I've read it, and read it again, and then printed it out, and highlighted the important bits, and it's slowly sinking in... I'm a little bit confused abut the bronze bushing that is supposed to limit some of the built in advance by going on a pin in a slot under the weight plate. Is this something that is only for points distributors, or only HEI's or both? I got the crane kit, with the vacuum canister, and then I also ordered kit 929G from mr gasket to get the bushing I thought, but it doesn't seem to be any bronze bushing included, just four nylon ones. This is the correct kit for a 74-79 GM HEI according to mr gasket's site, am I missing something here?
I sent them an e-mail to mr gasket's tech support:
"I am in the process of re-curving the HEI distributor I have installed. I have heard that every distributor has a built in amount of mechanical advance in them. You can limit this advance by adding a bushing on a pin that moves in a slot under the weight plate. I am looking for this bushing, I've been told it comes in certain re-curving kits. I purchased kit #929G and I am not sure if the nylon bushings supplied are the bushings I am looking for, or if they are meant to go on the weights? The bushing that goes on the pin underneath the weight plate has been mentioned as being made of bronze, do you have any idea about this? According to your site, this 929G kit was the correct kit for my 1975 GM HEI distributor. I have not taken the HEI apart to see this slot and pin myself, this is just what has been described to me, and it is possible that the HEI does not have this feature, but it is described like it would, so if you could please help me out with this question, that would be great."
What's the situation here, anyone knows?
Yes, your HEI uses the same pin and slot arrangement as described here; you just ended up with a kit without an advance bushing. The bushings you did get are for the weights. (And, by the way, not all HEIs use those, so you may not need them)
Personally, for an HEI I would go with Andy's method at the top of this page.
Added in edit--
I think maybe HEIs might have two pins and two slots, and two of the nylon bushings could be for weights and two for the advance pins...
Need to go out to the garage and find an HEI to look at...