8ba with chev dist. timing info

334charlie

334charlie

Member
I am making a 62 chev dist. for my 255 8ba shop truck i nead help with timing info. Does 4* inital and 24* mecanical at 2500 rpm soundright and my big question what adout the vacuum advance or retard? I am using an excel adjustable vac. advance. I have a sun dist machine to work with.The goal is clean burning efficient daily driver.This truck has 2 97's, mild cam and 5spd.
 
4* initial and 24* mechanical would give 28 total. Before you break something. try 4* initial and 16 mach all in by 2000. Give the vac about 88 for cruise. Better place to start. You can always add more.
 
334charlie said:
I am making a 62 chev dist. for my 255 8ba shop truck i nead help with timing info. Does 4* inital and 24* mecanical at 2500 rpm soundright and my big question what adout the vacuum advance or retard? I am using an excel adjustable vac. advance. I have a sun dist machine to work with.The goal is clean burning efficient daily driver.This truck has 2 97's, mild cam and 5spd.
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Thats exactly where we set most of our conversions. The chevrolet distributor will usually be limited at about 24 degrees mechanical advance from the factory. Look under the cam and the numbers will tell the limits on the unit.. ex: A 728 cam is 28 degrees advance and a 738 would be 38 degrees. We use a brass bushing to limit the flatheads.
Let me know if ya need any help , glad to help.......:D
 
334charlie said:
Bubba what numbers do you think if i add vacuum advance?
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The vacuum issue is another story.
Typically vacuum is 12 degrees and mechanical is 12 degrees (distributor degrees) Problem is the mechanical is at lets say 3000 rpm=12 and 18 inches of vacuum is also 12 . Problem is you will never have 3000 rpm and 18 inchs of vacuum at the same time , especially under a load of any kind.
So they overlap each other .
You may have 6 degrees mechanical and 6 degrees vacuum advance making the total 12 (24 engine) etc.....
OR you could have 10 degrees vacuum and 2 degrees mechanical (low rpm) and still have the 12 .
The vacuum is typically good for 2-3 mpg on a standard vehicle .
Kinda hard to tune on a flathead .:eek:
 
My friend Richard (FLA) have been working on this same problem. Unfortunately a stocl low compression engine will require a much different curve than a 276 Hi compression long cam engine. His engine with an L-100 cam likes a max of 20 degrees at WOT. That's what we set the max mechanical advance at 2000 rpm. At present we're running 8 degrees vacuum. All this on regular gas. Right now Richard is making some recording equipment that will record the mechanical and vacuum advance under ALL driving conditions, thus we won't be guessing at the results. Unfortunately It won't apply to a mild or stock engine. The biggest problem is the apathy of some owners drivers, they don't experiment with changing the curves to find out what is best for their engine.
 
Question ? is vac advance really nessesary on a flathead (chev distributor ) or just too much trouble.

Steve.
 
Back from test drive vacuum at idle 17 cruising 12-15 step on it 0. I also tested the excel vacuum chember starts to move at 7 when set for max advance .starts to move at 11 when set at minimum this makes me think i should limit the vacuum chamber with a stop so it would be more responsive.
 
Good work and pretty good vacuum.
Dont forget to factor your rpm band in the deal as well.
And consider your gear ratio as a factor of cruise vacuum. Keep us posted and let me know if ya need any parts , springs, weights or distributor cams etc.....whats the number under the cam on the delco you now have .......??
 
Is the vacuum advance necessary. Absolutely not. The engine will run pretty good. Might burn more gas. But that's not the question. ill it run right. NO!
 
There is a nice article in "Street Rodder" about ignition timing and the new gas. Just remenber one size doesn't fit all
 
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