Can you use a SB vacuum advance on BB motor?

 

Although it wouldn't surprise me if the factory had different part numbers on the canisters, I can tell you that in an aftermarket setting, they aren't marketed that way. MSD pro billet doesn't know that you pulled it out of a BBC and stabbed it into an SBC.

 

Car was not running well after engine rebuild would not shut down, stuttering, idle rough etc.... took it back and they said VA was bad, they only had a SB VA and put that in. Said It should work unless I "get on it". Plan to have the correct BB put in, just curious of the performance differences.

 

Check out this paper by our very own Lars

http://outintheshop.com/faq/Vac%20Adv%20Spec.pdf

As you can see it isn't BBC vs SBC, but application specific

 

All distributors will fit both small, and Big block.

 

Yes, physically either will fit, but the vacuum and mechanical advance curves are different for different part numbers. This article I attached is from T. I Specialty and pertains to t.i distributors and we can assume there are differences also with points distributors.

 

Hi Nicole1,
I know from following some of your threads you have had some questionable concerns involving certain services performed on your 68.
Rightfully so I'd say by some of the "stuff" you were told by mechanics.

If the car is dieseling after shut down, it needs higher octane or the timing is incorrect. Did the engine builder go over octane minimal requirements?
The vac advance has nothing to do with shut down.
The later year C-3's...1971 on had anti-dieseling solenoids to thwart this.

The vac advance must receive full vac advance at idle.... 750-800 rpms at idle depending on build and base vac the motor is producing.
If you pull vac with a miti vac and all in by 15 and your motor only produces 11 then your not using full vac.
Just the same if you have too much vac added on top of advance curve the car will feel like its hitching or jerking at highway speeds.

If you are using a points style original distributor any distributor as has been stated can be used.
1. is it on TDC at #1 cylinder? Yes move to #2.
2. is the dwell set 28-30 degrees? Yes move to #3.
3. Set your timing with 36 degrees in at 2500-2800 rpms.
Remember to disconnect the vac advance can and plug the line.
Reattach the vac line when this is achieved.
4. Check carb screws and make sure the mix is right.
A properly warmed up engine should respond to the mixture screws.
Readjust curb idle as required to maintain 750-800 at idle

If that's completed take out for a test ride.
If there's no other fly's in the ointment the vehicle should perform as prescribed. You should have also been told by the engine builder a break in procedure???

The vac can if the internal diaphragm was ruptured would cause a possible vac leak and cause some idling issues.
A Napa 1786 vac advance can is the correct replacement for a 427/390
engine built within reasonable stock range.
If the cam is really radial then that's another discussion.
Hope to have been some help.
Marshal

 

That's complete nonsense.

First, run-on after shutdown is a function of idle speed - not vacuum advance or timing. If it runs on after shutdown it means that the throttle plates are open too far at shutdown, allowing the engine to "diesel." Adding more timing, like running manifold vacuum to the vacuum advance with adequate initial timing, will allow the throttle plates to be closed more, and can eliminate run-on.

There is no "small block" or "big block" vacuum advance. All GM points distributors use the same style of vacuum advance. You select the vacuum advance with the "pull-in" specs that match the vacuum characteristics of your engine. Whether it's a SB or a BB is irrelevant.

The vacuum advance is completely non-functional when you "get on it." You can be running any vacuum advance at all - or none - when you "get on it" and you will get identical results. Vacuum advance is irrelevant under power.

E-mail me for my timing and vacuum advance papers for a full explanation. In the meantime, install NAPA part number VC1765 vacuum advance and use one of my limiters to limit the vacuum advance to no more than 12 degrees. Make sure you are running around 16-18 initial timing with 36 total (both checked with vacuum advance disconnected). Run the vacuum advance off manifold vacuum and then set your idle speed for lowest speed that will produce reliable idle.

Lars

 

I have extremely minimal knowledge on this subject, quoting text from garage that did engine install after rebuild. As others noted, I have had the car back 4 times. Every visit they blame a different issue for the poor idling and performance. It's better than it has been but still not running as good as it was PRE-rebuild. Not driving her recently, as we are stripping interior.

I assumed with the $$$ we put into the rebuild we would have produced a better, more powerful car, that was the goal. I know what they say about "assuming". I sure do not know enough to talk "shop" with them. This shop was recommended by several local forum members. Seems like a nice, knowledgeable guy and mostly repairs vettes. He recommended the tranny, exhaust and engine rebuild person. But every person (other then the tranny guy) we have used, has produced sup par work. IE; If you have seen my other threads on the exhaust etc. I would think I am being difficult, but forum members seem to concur.

Cannot afford to take it to another garage to evaluate and make it right. I am sure I am to blame for not educating myself more, but just like my patients, I don't expect them to have extensive understanding of their complex medical conditions. They pay me and I provide a service. Thought that was why I was paying others to do the work!!??

 

You're in a tough spot, and that's unfortunate. Your expectations are certainly reasonable.

You can do what others have done and just ship the car out to me here in Denver and I'll set it up right for you. I don't need the interior...

Lars

 

Help me understand. If all distributors are the same, why did GM list different part numbers for mainshafts, cams, weights, springs, and vacuum advance cans? Why do different engines have different part numbers shown in the parts catalog if they are all the same? Why do distributors have different numbers on the bands?

 

the main dist basic dimensions are the same.
they specify the advance curve for specific weights springs, vacuum cams for the motor and car they are going into.
and then apply a part number.

 

I would LOVE to have you look at her and get her running right. You helped me a few months back, when I was not sure what carb she had on her. You informed me it was off of a pickup. PO has done lots of things to this car we are discovering. Will take time and people with far more experience than us to get it right. Denver is not that far away from us in Central OK.

 

I think you're just working with people who do not understand the finesse of proper setup and tuning. Throw the car on a trailer and bring it out here for a long weekend and I'll set it up for you - it's less than a full day drive from Edmond.

Lars

 

Lars,

That would be great! She is without paint at the moment, hopefully we will get her painted soon and we could do that before the snowy season comes. We have been to CO many times. Most destinations we head to take us right through Denver. Lake City CO was my dad and mine's favorite place. We would trout fish and take 4 wheelers up to the Alpine loop between Silverton and Ouray .... amazing beauty.

Could you email me a ballpark range of cost, just so we can budget?

c1nicolei@yahoo.com

Thanks!