transbytip
Senior Member
A friend has a multi carb 8ba engine with 3 97 carbs wanted to know what to do for vac. running stock 8ba dist. any ideas for vac. thanks Cliff.
Vacume source
- Thread starter transbytip
- Start date
Licensed to kill
Well-known member
ON my '35 truck there is a spacer plate under the carb with vacuum ports. He could put one of those under the centre carb.
Automotive Stud
Well-known member
The stock 8ba distributor won't run correctly with multiple carbs from what I understand.
Licensed to kill
Well-known member
Not sure why that would be. Manifold vacuum should be the same regardless of the number of carbs, all other things being equal.
51 MERC-CT
Well-known member
The stock distributor does not run off manifold vacuum.
It runs off carb. venturi vacuum. As previously stated.
Black Fifty
Senior Member
And if you switch to a later distributor with mechanical and vacuum advance here is some good reading. I have found multiple sources that support this article's conclusion.
Happy Reading!
http://www.superchevy.com/how-to/ad...now-about-vacuum-advance-and-ignition-timing/
Mike
Sorry about the source magazine.
Happy Reading!
http://www.superchevy.com/how-to/ad...now-about-vacuum-advance-and-ignition-timing/
Mike
Sorry about the source magazine.
Phil Gillespie
Well-known member
Can recommend a Chev distributor Delco Remy. Charlie NY does a great job with these.
You can get adjustable Mech Advance and also ajustable Vaccum Advance.
You will not be disappointed.
Phil NZ
You can get adjustable Mech Advance and also ajustable Vaccum Advance.
You will not be disappointed.
Phil NZ
Black Fifty
Senior Member
Bored&Stroked
Well-known member
Not exactly . . . think about the amount of vacuum drop when you have just two small throttle plates . . . or 6 throttle plates. The vacuum signal changes.
This is why when using Holley type carbs (with power valves), you have to ensure that you have the correct power valve value/setting - given your carbs, cam, engine, etc.. A vacuum gauge that you can read while going down the road is your friend! LOL
Licensed to kill
Well-known member
By "venturi vacuum" I assume you mean Ported vacuum and if so then the problem is with the carb model not having posted vacuum, not the number of them.
Licensed to kill
Well-known member
That's why tri-powers are set up with progressive linkage (in most hot rod applications) and vacuum secondaries on most street/factory setups. The outboard carbs do not operate in unison with the centre carb but work as secondaries that come on when full power is demanded. Your still running around on just the centre 2 barrel carb when driving normally.
Bored&Stroked
Well-known member
Not trying to be a pain in the rear, but that has nothing to do with the original question about having the correct vacuum signal to run the stock distributor. In a vacuum only distributor designed for ported vacuum (actually any given vacuum signal), a specific engine combo and a single carb (49-53 flathead) - the system is tuned to provide the right advance at the right vacuum/load condition. If you start changing any of the system components (manifold, number of carbs, cam, etc) - then the original distributor and advance/retard curve doesn't work as designed. That is why dang near everybody on this site who has hot-rodded a 49-53 engine (dual carbs, cam change, etc) - recommends that you change the distributor to one that has at least mechanical advance (RPM driven) and additionally a tunable vacuum advance (great for mileage, etc).
The main reason why many factory tri-power setups have vacuum operated outer carbs is not related to keeping the vacuum signal high for something like a distributor. The primary reason is for fuel economy - in that the vacuum aspect of bringing in the outer carbs (or in the case of a 4-barrel - the secondaries) is so that they have a load-sensing tip-in, versus a straight mechanical linkage tip-in.
Regardless of the number of carbs, how they operate (mechanical or vacuum), it is critical that any vacuum advance/retard system be tuned to the specific system it is used on. You can't expect the same vacuum signal at the same RPMs and associated load conditions to apply across all the different manifold/carb configurations - they vary greatly. Also, you cannot "tune the curve" in most vacuum systems - you can't change how much advance and/or how it comes in (unless it is computer driven). Even with a modern Accel type vacuum cannister on a GM distributor, you have limited abilities to tune it (no concept of advance bushings, advance weights, advance springs, etc).
Okay - enough of my babble . . . I'm starting to bore even myself!
The main reason why many factory tri-power setups have vacuum operated outer carbs is not related to keeping the vacuum signal high for something like a distributor. The primary reason is for fuel economy - in that the vacuum aspect of bringing in the outer carbs (or in the case of a 4-barrel - the secondaries) is so that they have a load-sensing tip-in, versus a straight mechanical linkage tip-in.
Regardless of the number of carbs, how they operate (mechanical or vacuum), it is critical that any vacuum advance/retard system be tuned to the specific system it is used on. You can't expect the same vacuum signal at the same RPMs and associated load conditions to apply across all the different manifold/carb configurations - they vary greatly. Also, you cannot "tune the curve" in most vacuum systems - you can't change how much advance and/or how it comes in (unless it is computer driven). Even with a modern Accel type vacuum cannister on a GM distributor, you have limited abilities to tune it (no concept of advance bushings, advance weights, advance springs, etc).
Okay - enough of my babble . . . I'm starting to bore even myself!
Last edited:
40cpe
Well-known member
No, venturi vacuum in a Holley 94 develops a maximum of something like 4 inches vacuum as opposed to manifold or ported vacuum of 15-20 in. Venturi vacuum is created in the venturi of the carburetor, not in the manifold.
No, venturi vacuum is NOT the same as ported vacuum. And manifold vacuum is not the same as either ported or venturi vacuum. If you look in a Motor Manual from the day, you will find a cutaway drawing of the carb used with the LoadAMatic vacuum only distributor. You will see special passages that give a blended vacuum signal to the distributor. Many of these carbs had a "spark valve" that looks similar to a power valve but helps with the proper vacuum signal.
Not trying to be a pain in the rear, but that has nothing to do with the original question about having the correct vacuum signal to run the stock distributor. In a vacuum only distributor designed for ported vacuum (actually any given vacuum signal), a specific engine combo and a single carb (49-53 flathead) - the system is tuned to provide the right advance at the right vacuum/load condition. If you start changing any of the system components (manifold, number of carbs, cam, etc) - then the original distributor and advance/retard curve doesn't work as designed. That is why dang near everybody on this site who has hot-rodded a 49-53 engine (dual carbs, cam change, etc) - recommends that you change the distributor to one that has at least mechanical advance (RPM driven) and additionally a tunable vacuum advance (great for mileage, etc).
The main reason why many factory tri-power setups have vacuum operated outer carbs is not related to keeping the vacuum signal high for something like a distributor. The primary reason is for fuel economy - in that the vacuum aspect of bringing in the outer carbs (or in the case of a 4-barrel - the secondaries) is so that they have a load-sensing tip-in, versus a straight mechanical linkage tip-in.
Regardless of the number of carbs, how they operate (mechanical or vacuum), it is critical that any vacuum advance/retard system be tuned to the specific system it is used on. You can't expect the same vacuum signal at the same RPMs and associated load conditions to apply across all the different manifold/carb configurations - they vary greatly. Also, you cannot "tune the curve" in most vacuum systems - you can't change how much advance and/or how it comes in (unless it is computer driven). Even with a modern Accel type vacuum cannister on a GM distributor, you have limited abilities to tune it (no concept of advance bushings, advance weights, advance springs, etc).
Okay - enough of my babble . . . I'm starting to bore even myself!
Actually, you can tune those vacuum only distributors by changing the springs on the points plate.
flatheadmurre
Well-known member
Doesn´t matter much if you try and tune a venturi vacuum distributor too take the higher vacuum of a manifold.
Since manifold is taken on the other side of the throttle plate it will work backwards any way....
Since manifold is taken on the other side of the throttle plate it will work backwards any way....