Technical Vacuum or no Vacuum ?

Could or how can I run a Bubbas Chevy Dist. with no vacuum advance on a basically stock 255 Merc. engine ?
Should I trade it for one with a vacuum advance ? Maybe Charley can chine in here ? Thanks. Jim T.

 

As long as you have centrifugal advance it will work. Some prefer them that way.

 

I run one of Bubba's mechanical advance dizzys on a '53 Merc that's in my 40 Coupe. Mine does run a pair of Strombergs, but it does the job beautifully.
In fact Bubba suggested staying away from the vacuum advance dizzy on my engine. (it's pretty much stock except for the carbs and mild cam.)

 

If you are actually going to drive this rod, you need to seriously consider a vacuum advance distributor.

 

I am running an original loadamatic on an 8ba.
Years ago I decided to try one of Bubba’s centrifuge distributors. It
Wasn’t as responsive as my vacuum original loadamatic.
But I’m sure hundreds of happy customers would disagree.

 

For something mainly street driven I'd usually prefer to have a vacuum advance in addition to the mechanical advance. But either way a lot depends on the mechanical advance curve built into the distributor.

As a rule of thumb, with mechanical advance only, there is less advance built into the distributor that is offset by increasing the initial (static) timing to provide the correct total advance at higher engine speeds.

With a vacuum advance distributor there tends to more mechanical advance built in. The initial advance is less (which can make it easier to start a hot, high compression engine) but the additional mechanical advance should give you the desired total advance.

The vacuum advance also responds to engine load and will temporarily retard the timing as needed to control spark knock, or detonation.

 

Thank you all I will start the engine with the vacuum advance dis. then change over to the Chevy mechanical distributor. Thank you all . This is my first V 8 flathead.

 

You planning on having a second one?

 

I had a factory engine that came with a centrifugal advance only! It ran great, smooth and powerful! But it had a ferocious appetite for fuel. Used twice as much gas as similar engines with vacuum advance distributors.
I believe the vacuum advances sole purpose is for fuel economy!
Depends on what is important to you!




Bones

 

Also for emission's. Some of the later dual vacuum advance units actually retarded your initial timing.

 

True, but the OEMs added the vacuum advance years before they worried about emissions. Granted they did utilized them at the start of emission control. About 1968 , I believe. Those did give performance problems! During that time, I made a lot of money , converting the daul diaphragm back to single. But it just helped a little, as there were also other things the OEM did that hurt performance and mileage.






Bones

 

I'm not challenging your example , but most mild performance V8's will experience a 10-15% increase in fuel economy from installing a vacuum advance . IME .

 

I might have got over zealous on the twice….lol but, my car got 11 mpg…no matter what! My sister’s car , with the same size engine ,with the vacuum advance and her driving it got close to 20 mph. There could be other factors here, but I thing the elephant in the room was the vacuum advance!






Bones

 

Yes, but for engines related to the HAMB guidelines it is solely for fuel economy.

 

For myself, I prefer to have even a 500-600 hp engine run as clean and efficient as possible.

 

Bones, I realize it’s out of the realm of our discussion, but I started out in the industry working on school buses. Mostly gas GMC’s and Chevies, with a few IHC and Fords thrown in. The GMC and Chevies all had 366 BBCs in them and the older ones didn’t have vacuum advance on the HEI distributors. They actually had an OEM plastic bracket in place of the vacuum can to hold the pole piece in the bottom of the distributor in place.

 

I can't see any reason to run a street engine without both vacuum and centrifugal advance.
A pure race engine? Those may only have to be on their best behaviour at full throttle, as long as they kind of idle and respond when you step on the pedal they may be good enough with completely locked advance. Street engines have to behave well all the way through the rpm range, especially at idle, small throttle openings and low rpm as that's where they spend most of their life, and even with the cheap fuel you guys have over there it isn't fun to visit the gas station so fuel economy becomes a big factor.

 

Flatheads won't tolerate near as much ignition advance as an OHV engine so I can sort of see the reasoning.

But generally speaking running most any engine on the street without vacuum advance is just ... retarded.

 

I've got a Bubba mechanical advance only on the 8ba in my '39 p/u. Its stock except for EAB heads and possible a Merc cam but no guarantees on the cam and a single 94 that CharlieNY redid. Regardless, its a daily driver unless its raining and it runs like a top.

 

I won't try arguing with the "experts". because I'm sure they have tried it both ways and have data...
But I have a Bubba's Chevy distributor(no vacuum) in my flatty powered(mild) F-1 PU. Got about 30k miles on it so far, I think it works great. I have gotten as good as 20mpg with this non aero truck. Got no complaints. Would buy another in a heartbeat!

 

Here's a good reason (to help you out) , someone gifted a like brand new non vacuum advance distributer that , (other than not having vacuum??) it works beautifully in your hobby car engine . Even at today's fuel prices ,if would take me 3+ years of fuel savings to break even for the cost of a new distributor.

 

I have both. I am planing to use a dual carb setup after I break in this engine and possibly Edmonds heads.

 

No, if anything it's the other way around, in most cases. That's why it gets disconnected in the first place. A lot of people don't really understand ignition timing in general, and also try to perform a tune-up without fully understanding what they are trying to accomplish. Everybody wants to start dinking around with the carburetor first thing, when it's always done last, basically.

I've been following a bunch of different automotive forums for a long time, and it's always the same timing questions, always the same assumptions, always the same misunderstandings ... and always the same results.

Generally people often don't want to get too involved, they aren't interested in learning or they just want somebody to 'splain to them simple, easy quick answers to their complex troubleshooting woes to something they just pulled out of the weeds. "Turn this screw 2.5 turns out, and close the hood"

 

Andy, I too, worked on those engines, when I first started out , a fleet of sanitation trucks. The truck engine is really more closely related to a race car engine, than a passenger car engine, mainly because it usually it is working hard most of the time! That is why you see four bolt mains heavy duty rods, etc. in truck engines.
The timing is also reflected in regards to power not necessarily economy. That is why Chevy elected to omit the vacuum advance on your trucks. I have seen vacuum advances on trucks and not on trucks.
But as with any engine in any situation you want the timing advance as much as possible before it spark knocks . That may be a little over simplified , but gives a person a general idea of timing.
That is where the vacuum advance comes in, at high vacuum, around 60 mph at 1/3 throttle.
You put your foot on the floor, there is no vacuum , so the vacuum advance has no advance at that time. That is why Chevy omitted the vacuum advance on those 366 truck engines, you had to run them on the floor!






Bones

 

Marine or boat applications, no such thing as low load cruising so there's no vacuum advance there either.

 

I couldn't agree more. that is centrifugal and vacuum advance.

 

On all my old pontiac motors I always ran a dual point conversion plate and mechanical advance. Honestly I noticed more on the low and mid range with this set up on a nicely built street motor.

 

A more in-depth discussion... there's multiple threads here on this.
Vacuum advance | The H.A.M.B. (jalopyjournal.com)

 

An easy way to understand both is, centrifugal advance knows only speed, not load! Vacuum knows only load, not speed. To get accurate timing, for the street, both are needed.
For racing where all out performance at top rpm is your only concern, locked out timing with no advance can be used.





Bones

 

It should do fine.
I assume you have the stock Loadomatic or LOM distributor. This must be matched with the stock carburetor otherwise it will not function correctly.

If you do not have the correct stock carburetor that is designed for the LOM vacuum only advance, you are much better off running Bubba's or anybody's mechanical distributor.