msd pro billet dist. 8361, anyone use the vacuum advance with a stout cam? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 07-29-2010, 08:38 PM
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msd pro billet dist. 8361, anyone use the vacuum advance with a stout cam?

The vacuum can needs 15 inches to pull timing advance my BBC only makes 7 to 10 inches. Has anyone found an adjustable vacuum can that fits this dist. I chatted with sales at summit racing they had no luck finding anything that fit.

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Old 07-29-2010, 09:45 PM
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You can get vacuum cans with many different start- end points.
NAPA: even CTC if you know the origional car application the can you want , came from.
You can get a fully adjustable can from Crane or Mr.Gasket.
You need to adjust the rate as well and the stop limit.

Engines with big cam duration/overlap that have low manifold vacuum at idle require
a different setup than stock or near stock motors with moderate cams.

Modify the mechanical advance curve for less travel, more inital.

You want around 24+deg base timing at idle and 38deg (BBC) at max mechanical advance. shorten the advance travel stop from 20ish(stock),
to around 13degrees limit.
You want the mechanical advance to top out around 3000rpm.

24+13 =37

Then: vacuum advance setup:

Vacuum advance is primarily for part throttle cruising efficientcy.
Vacuum advance amount and rate has to be found by trial and error drive testing and tuning but this is a general guide line starting point.
You want to limit the vacuum advance max travel limit to a max of 15deg.
That means reguardless of how much vacuum is applyed at any time the vac advance does not go beyond 15deg. (limit to 15deg)

Then at hiway cruise steady part throttle driving you want to adjust the
rate to get +/-10deg of vacuum advance at cruise.
Adjust as nessessary to avoid throttle roll in pinging.

With this setup, what ever vacuum advance you happen to get at idle is a bonus. The vacuum advance will drop when idleing in gear as the manifold vacuum drops off.

initial base + mechanical + max vac advance should not exceed 51deg combined total at any time. Usually a combination of the three adding up to 46-48deg is just right.

To get the right advance curve for radical cammed motors requires
more than just swapping advance springs and bushings.
You need more initial base timing at idle than this will allow.

What cam is in this motor?
Car? gear ratio? converter?
If this is the car with the big Isky solid cam and you want a clean, consistant idle with max throttle response you will want as much initial base timing as you can throw at it. Right Up to locked out timing.
Short of that you will need to tear down the distributor and modify to allow
A ton of initial at idle 24deg+++++ and a short limited advance curve. 38deg max.
If the car is hard to start with this increased idle timing get a start retard and or ignition power interrupt switch. If using a GM starter make sure you have the required rear support bracket/brace installed.

If you depend heavily on vacuum advance at idle for the increased idle timing be aware that it is going to drop when idleing in gear and drop right when you floor it from idle exactly when you need it to not drop.
Ideally you want to choose a carb power valve that will remain closed at idle in gear when manifold vacuum is lowest. usually a 3.5 or 4.5" power valve is best. You will not achieve max street performance and spark plug life and consistant sharp throttle response will be compromized.

You need that 24deg++ base timing. Ya that involves modifying the distributor.

The best available octane pump gas will be required
Sunoco 94

Last edited by F-BIRD'88; 07-29-2010 at 10:14 PM.
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Old 07-29-2010, 10:24 PM
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Most all these distributors components like the vacuum advance can is based on a previous OEM distributor.
Your vacuum can style and mounting point pattern is probably the same as GM HEI, GM pre1975 points or Ford or chrysler distributors.

I'm guessing the old pre 1975 GM points style is the same vac adv.
Post a clear picture or two of it, for ID.
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Old 07-29-2010, 11:15 PM
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Quote:
Originally Posted by F-BIRD'88
Most all these distributors components like the vacuum advance can is based on a previous OEM distributor.
Your vacuum can style and mounting point pattern is probably the same as GM HEI, GM pre1975 points or Ford or chrysler distributors.

I'm guessing the old pre 1975 GM points style is the same vac adv.
Post a clear picture or two of it, for ID.
have you mapped your vaccum advance unit?
Amount of vacuum required to start advancing..
" " " " reach full vac travel.
Amount of advance at full rod travel
Amount of vacuum advance @ 10" of applyed vacuum.

Go for a ride with a vacuum gauge and measure the amount of intake manifold vacuum created when driving at 100K (60mph) 50K and 80K
note the manifold vacuum at idle in neutral and while idling in gear.
Note the vacuum gauge at full deceleration at from hi speed. (let off the gas) Now you can map out a vacuum advance timing curve and fine tune it.
If the motor has a high compression ratio and you run crappy gas in the tank you will not be able to optimize it. There is a notable difference in the pump gas real anti knock quality. I found Sunoco 94 the best. Pioneer 93 second best in our area. Ya get what you pay for.
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Old 07-30-2010, 07:42 AM
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Thanks f-bird that's the best explanation yet! This should be a sticky, the msd part number is 8463 I can't post a link to summit because I'm posting from my phone. The vacuum can from bolt holes to rod is shorter than most I've seen, I emailed summit , they said they didn't have an adjustable vacuum can that fit? They may not know that they do. The combo is a 454 BBC, ls6 short block, 254/263@50 duration 620/629 lift, 3500 convertor, 3.73 gears , 325/50 bfg radials, 10.5 to 1 compression.

Last edited by fast times; 07-30-2010 at 07:57 AM. Reason: spelling
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Old 07-30-2010, 10:51 AM
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OK I am as we speak right in the middle of setting my mechanical advance limit. I agree with you Fbird my setup likes allot of initial so I must limit my mech. I have a off shore HEI that is most likely the same as thousands out there, not sure how it compares to the OP's but it may help others too with the same. I am shooting for 34 total on my vortec head 383 with 221 duration @ 50. I would like 20 or 22 initial/base so if I can add say 12-14 with the mech then lets try that. This HEI will not accept a bushing kit so I am using a small machine screw/nut through the rotor plate, the head of the screw will butt against the advance plate stopping its movement. This HEI had over 20 deg mech to start with. I would like to see if I can ball park this stopper with the dist. out of the engine by using a degree method. I have installed a pointer on the rotor plate and measured the movement of the pointer from no mech to its limit. The diameter of the dist body is 132mm, the travel of the pointer is 17mm, If say 6 deg on the pointer makes 12 at the crank right? or is it the other way around? how close am I? I would like to use some math to see if I have the right deg movement...anybody?
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Old 07-30-2010, 11:15 AM
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Well I am answering my own question C/D= 3.14 were C is circumference and D is diameter so 132 X 3.14 = 414.5 divided by 360 = 1.15 mm per deg approx so I get 19.55 deg movement is that 9.8 crank Deg? looking for some confirmation. Am I on the right track here or will this little exercise shoot itself in the foot somehow?
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Old 07-30-2010, 11:23 AM
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Ya the distributor runs at half crankshaft speed.
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Old 07-30-2010, 11:29 AM
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I should note that I respect the initial OP's question on vacuum advance and although my post does not address this directly yet, this may be useful if he is setting up the mech limit first off to get to what Fbird is making posts on, I will try and keep it relevant and add some vacuum adjustment details as we go along if need be
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Old 07-30-2010, 11:57 AM
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Quote:
Originally Posted by F-BIRD'88
Ya the distributor runs at half crankshaft speed.
OK I have tweaked the stopper to get about 21mm movement that should equate to approx 12 deg at the crank thanks to mechanical. I will post when I get it in and set the light on it. Thanks
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Old 07-30-2010, 05:34 PM
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Ended up with 17 deg added by mech on the running engine with the 21mm advance travel. In this case I will set my base at 17 as well to get my 34 total without vac advance. My cam is not that big so this is OK for now but I may want to get my base up further sometime, that means getting back in there and changing the stop point...again. SO 21mm/17deg = .81 wonder if we can use this ratio? would setting the mech movement limit to 15mm travel have given the 12.15 deg I was looking for? (15 x .81 = 12.15) just a ballpark procedure maybe

One thing I found was that my adjustable vac can had a slow leak, it would bleed down really slow, the leak was were the sleeve for the hose meets the vac can, the OEM one I replaced it with is one molded piece or at least braised at this connection and tested good, by the way it adds another 22 without a limiter so back in I go once I get a new adjustable can.
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Old 07-30-2010, 06:18 PM
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Hmmm, still looking for an adjustable can for 836
1 msd dist. as stated in the thread title.
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Old 07-30-2010, 07:00 PM
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Yeah got ya, did not mean to fill your thread with my own ramblings should have started new one on this, was relavent to a point but not helping you much...my bad.
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Old 07-30-2010, 07:11 PM
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Quote:
Originally Posted by fast times
The vacuum can needs 15 inches to pull timing advance my BBC only makes 7 to 10 inches. Has anyone found an adjustable vacuum can that fits this dist. I chatted with sales at summit racing they had no luck finding anything that fit.
Can you show a photo of it w/a ruler in the shot- there may be a GM (or other) can that'll fit it.

Summit sells a can- http://www.summitracing.com/search/B.../?autoview=SKU

But doens't say it is adjustable, and I take it you've checked yours for an adjustment?

EDIT- Does it look like a GM points unit?

Last edited by cobalt327; 07-30-2010 at 07:24 PM.
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Old 07-30-2010, 07:30 PM
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A little hard to tell from these images. The HEI can has a longer bracket than either of these- and the two are not interchangeable. I bet the points-type GM can will fit the MSD distributor, though.

ACCEL ADJUSTABLE GM POINTS-TYPE


MSD REPLACEMENT

Last edited by cobalt327; 07-30-2010 at 07:48 PM.
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