|Topic Review (Newest First)|
|04-16-2019 02:58 PM|
|geeteeohguy||I agree with Lemans Guy 100%. I also set my cars up like as he posted: 6-10 base with limited vacuum and mechanical advance. No more than 45 degrees total at light cruise or 34-36 WOT. I agree with Big D about 99.9% of the time, but not this time. Because if you physically twist the distributor in the block to an advanced position, it WILL effect the total timing. You can actually twist it far enough it'll go so advanced the car won't run if you want to. The centrifugal advance allows for the distributor shaft to make it's arc through the curve, but if you are telling me that twisting that distributor in the block 90 degrees won't effect total timing, I don't buy it.|
|04-16-2019 12:47 PM|
100 % correct,...you can go 4000 RPM or 10,000 RPM but if set up as you described the Total advance is 36....that is all...
Only if one disregards the centrifugal advance total, ignores it and sets base to high will you have the problem of too much Total timing at high RPM
Vacumn advance is never considered when discussing total timing.....total timing is ..........Base + Centrifigal = Total timing
|04-16-2019 10:37 AM|
"...Sure it does. The base timing. Where you actually bolt down the distributor. With no vacuum advance connected. If you set your base timing at 20 degrees BTDC as opposed to 10 degrees BTDC, and then hook up the vacuum advance, and go drive the car, the car will run stronger in the top end and start easier. Timing that is over advanced retards performance.....pistons don't do well when the spark plug fires as it's still moving up in it's bore under compression stroke. Much better when the spark plug fires when the piston is near TDC!..."
We are obviously not on the same page.
At higher rpm, say above 4000 rpm, at WOT there is not enuff vac for the vac advance to be a player. Therefore, ONLY the total max mechanical advance is involved. With a positive advance stop, adjusted correctly, the initial timing, at idle, has no effect on the TOTAL mechanical advance at WOT, once the dist reaches it's max mechanical advance.
For example: If your engine runs best at 36° total, at WOT above 4000 rpm, then you need to set your initial so that the total will be 36° at WOT above 4000 rpm. Where you have to set your initial for this will be determined by how much mechanical advance your dist has. If it has 24°, then your initial will have to be 12°. 12° + 24° = 36°
All this is with the vac advance unhooked & the vac source plugged. Once the mechanical advance is set correctly, THEN you can set up the vac advance like you want it.
And if you want more initial mechanical, then you'll have to limit the amount of mechanical advance with a positive mechanical advance stop of some kind. Otherwise, your TOTAL mechanical will be too high & could cause detonation at full advance.
Once again, I'm not talking about the idle & cruise advance, when using a vac advance. I'm talkin about the total mechanical advance at higher rpm WOT, when the vac advance is not adding any advance.
|04-16-2019 07:42 AM|
Very helpful thread. Thank you
I have the 10* vacuum canister installed and will do some testing later this week and will report back.
|04-15-2019 06:47 PM|
Pa Pontiac Jim made an excellent point about 34 total even 32.....each car is different.....so the range is really 32 to 40I always set them right in the middle at 36 and then you can adjust the base easy from their, up or down as needed. Some big Pontiacs take 38 even 40, just be sure you know what you are doing if you go that high...you don’t want pinging.
I have seen 65 tripower 389 that wants 34, and a 460 stroked that wants 38, mine likes 36,....34 is even safer.
|04-15-2019 06:41 PM|
A good discussion and well made points all around. You are correct that if one set base timing at 20 and then was adding in their 16 for 36 with centrifugal weights they would get more timing early and not as much later. Of course their is some overlap with when springs come in etc.
And too high of a base timing will cause hard starts and kickbacks. Also vacumn cans pulling 25 degrees is the norm I see and like GTO guy said all athat advance makes your car run poorly.
I get nice running on the distributors set up with about 10 or 12 base and 10 vac added. That gives a nice smooth and cool idle. You have 24 or 26 centrifugal and you have a nice set up for total. Use springs that bring it in 2800 to 3800 and it will run good. 3200 is nice and usually get table.
If you bring them in too fast you can sometimes get pinging during acceleration, again back to the point too much too early.
If you run a race billet no vac advance for the street you are giving up 10 degrees of timing during light throttle cruise and at idle. Why do that?
The light throttle is a leaner mixture than acceleration and you need the spark earlier,...and the idle cooling with the 10 added vac really helps.
You can run 6 BTDC, I would then run a 10 degree vac can hooked to full manifold vac, for better idle cooling.
The problems come in when someone does what geeteeoh guy said, runs up their base timing to 20 cause they heard it is better and they have no idea what their centrifugal timing is. Nor how to leverage vacumn for better stronger and cooler running.
BigD knows has to set his total advance and it is limited, so then you set the base.
On the distributor machine I always set centrifugal first....limited and known.......then I know base, get vacumn set up,....spring are always last.
|04-15-2019 03:10 PM|
|geeteeohguy||Can't edit my post. I reversed 20 and 10......transposed them. The car will run stronger in high rpm and start easier at 10 degrees. And it will start hard and run out of breath at a lower RPM at 20.|
|04-15-2019 03:08 PM|
Originally Posted by bigD View Post
|04-14-2019 02:44 PM|
"...extremely high initial timing...all it does is lead to hard starting and a weak top end..."
Just curious. The timing at idle, has no effect on the engines power in the upper rpm range, unless your dist has too much mechanical advance. That can be limited by some type of positive advance stop, as previously mentioned in this thread.
|04-14-2019 02:02 PM|
|geeteeohguy||I'll add two things here: There is no need to run extremely high initial timing, as all it does is lead to hard starting and a weak top end. I run my '65 and '67 at spec, about 6 BTDC. Also, I have installed the Crane and Accel adjustable cans, and they are JUNK. The allen screw strips after the first use and the can is rendered useless. This happened to me on every one I tried. You are better off using a stock can and installing a limit bar or one of Lars Grimsruud's advance limit brackets that he sells for cheap. Also, reading ANY of Lar's timing articles will put you at the head of the class regarding ignition timing. All cars need and want their own specific curve. I cannot believe that amount of Pontiac guys I run into that think they have to run 16-20 degrees of initial timing, have no manifold vacuum advance, and wonder why they break piston lands and pound their engine bearings into the copper.|
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