ported vacuum vs manifold for advance

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

bsmith123

bsmith123

10 Year 15 Year
Joined
Oct 9, 2007
Threads
164
Messages
3,068
Location
Norfolk United Kingdom
I read an interesting thread on a hot rod site that basically stated that putting the distrib advance to manifold was better than putting it to ported vacuum

According to the poster ported vacuum was an emissions trick that came along about the time of AIR to help the engine cope with leaner mixtures but it resulted in higher temps for the engine and less power or at least throttle response

any thoughts on this from the MUDDERS?
 
If this is a vacuum advance distributor, I believe you will be running full vacuum advance constantly if connected to manifold vacuum as opposed to the ported vacuum from the carb - this would probably defeat the purpose of the vacuum advance and may induce pinging as well. Mechanical advance would still be rpm dependent regardless.
 
vacuum

yes it seems the function is to retard idle timing so advancing it may not be a bad thing? I think how fast it came in after that would still be a function of the vacuum present with part throttle but there is also the mechanical advance to think about

When emissions became a priority to vehicle manufacturers, a method had to be found to reduce emissions
at idle. The amount of Hydrocarbons emitted out of the tailpipe can be drastically altered by changing the
timing: Retarding the timing reduces Hydrocarbon emissions. But retarded timing adversely affects gas
mileage at cruise. So a method was needed to retard timing at idle, yet maintain it at normal levels for
cruise. The solution was seen to —turn off“ the vacuum advance at idle, yet have it operate normally under
all other operating conditions. To do this, a small hole was drilled in the carburetor throttle body just above
the position of the throttle plate at idle (NOT in the venturi area), and this hole was connected to a vacuum
nipple on the carb. When the throttle plates are closed at idle, they act as an —off“ switch to block the
drilled hole from manifold vacuum. As the throttle plates are opened up, the hole becomes fully exposed to
manifold vacuum, and normal manifold vacuum is realized at the nipple. Thus, you have a manifold
vacuum —on-off“ switch, turning manifold vacuum —off“ at idle, and restoring it to normal operation once
the throttle plate is cracked open. Vacuum advance can be eliminated at idle for good emissions, and
instantly restored to normal operation at cruise. At both cruise and Wide Open Throttle (WOT), manifold
vacuum and ported vacuum are exactly the same: There is high vacuum at cruise, and virtually no vacuum
at WOT. The difference in vacuum occurs only at idle.
 
Last edited:
Advance

Well, the way I am looking at it the only reason advance has been retarded at idle was to reduce emissions. Basically the Vacuum advance is shut off.

The mechanical advance will advance per rpm but not react to engine load for instance putting the gas down at low rpm


I am looking at it this way---as the throttle is opened advance is added, both mechanical and vacuum. The more it is opened the lower the vacuum so the vacuum advance will drop off and as the rpms build the mechanical will come in and you will not see that big of a change, maybe kinda canceling each other out?

As you decelerate the throttle starts closing, mechanical drops off but vacuum advance is added in, until idle then shut off again

Does that make sense to you guys?
 
Edit: I meant to post to this more comprehensive thread. Oh well.

https://forum.ih8mud.com/40-55-series-tech/35514-ported-vacuum-vac-advance-2.html

OP:

I believe the carb that came on Marvin is a 1/75 to 10/75 Federal carb:

Mine:

ext94.webp

SOR's page:

http://www.sor.com/cat042d.sor

SOR's pic:

042-01H_rt-big.jpg


The smaller port is keyhole shaped and about half-and-half above and below the edge of the throttle plate.

Mine again:

int95.webp

Note the Pismo sand...

int97.webp

So, this is ported vacuum, suitable for a large-cap vacuum advance dizzy, correct? It seems different than what you guys are talking about drilling out a port, which sounds round and completely above the throttle plate.

I blew some air through this. Opening the throttle a very small amount opens this port completely.

Damn thing is pissing me off. I'm about ready to send it out to someone who knows what they're doing.

Any chance a leaky side-cover gasket could be the source of a vacuum leak? It really only weeps a drop off the inspection cover once in a while. It's not much.

I rebuilt this other carb that I thought was the same, but after I got it bolted on I realize it doesn't have the ported vacuum. This is SORs pic. It's a 1/'75-12/'75 CA model:

042-01H-CA_rt-big.jpg


I'm going to rebuild the other carb, get a proper timing light with an advance feature and take another shot at making it run.

PS: Is there anything I can/should do with the aux accel pump? Tee it into the vac advance maybe? It's not used in my rig otherwise. I ran a vaccum hose to the air cleaner to make it look like it was connected. They don't have the diagrams to compare. I think the smog ref would know it was bogus if it ever got that far.

JimC, looking for a Rubithon ride? :steer: I might have some frequent flyer miles somewhere too.
ext94.webp
int95.webp
int97.webp
 
Last edited:
The slot right at the closed primary plate is the off idle or transition slot that provides idle fuel when you start to open the throttle. It keeps it from bogging down until fuel is pulled from the main nozzle

The ported vacuum hole is small, round and a few mm above the closed throttle plate
Carb2 026 copy.webp
 
bsmith123 said:
I read an interesting thread on a hot rod site that basically stated that putting the distrib advance to manifold was better than putting it to ported vacuum

According to the poster ported vacuum was an emissions trick that came along about the time of AIR to help the engine cope with leaner mixtures but it resulted in higher temps for the engine and less power or at least throttle response

any thoughts on this from the MUDDERS?
Click to expand...

Vacuum control of ignition timing has been around a long time; a lot longer than people were worrying about emissions. Its original purpose was to allow fine tuning of advance to accomodate engine load at a particular RPM. At high load WOT situations, it retards the spark as RPM drops to prevent preignition or pinging. At low load, high RPM situations it advances the ignition for optimum power. Some systems use ported vacuum with a vacuum advance. Others use manifold vacuum with vacuum retard to accomplish the same thing. The trick is getting the amount of advance at a particular manifold vacuum right. I don't know that hooking a toyota retard distributor to manifold vacuum will accomplish this, but you can try it. You might have to adjust the base timing at idle from the stock 7 degrees to maximize performance.

The purpose of vacuum retard on mid 70s Toyota engines was to implement its "lean burn" cycle to meet CAFE standards
 
I don't have that little hole. Here's a picture with some more light.

int98.webp

So that's manifold vacuum at that port? Not sure that explains the problems I'm seeing, but makes me more sure I need to understand exactly what my timing is doing.
int98.webp
 
Manifold vacuum is below the throttle plate. You have the ported vacuum port. It is above and to the left of the transition slot at the shadow line. Ported vacuum is the opposite of manifold vacuum at idle: It is zero at idle and increases with increasing air velocity through the venturi. Manifold vacuum is high at idle and drops with increasing throttle opening, but increasing RPM tends to also increase it.
 
chuckost said:
Sorry to jump in here, but great info thread. I have a 9/78 cal fj40. What/how does the vacuum retard accomplish?
Click to expand...

78 year model has a two stage vacuum advance. Yours could be a '79 year model and I think it has a single stage advance, but maybe two. It is controlled electronically to make the engine run better under more different circumstances than is possible with a strictly vacuum mechanical mechanism.
 
78 is dual advance. 76 and 77 retard. Both vacuum advance and retard are electronically controlled to provide better corporate average fuel economy and reduce emissions. The two goals go hand in hand after the mid 70s.
Two different ways of accomplishing the same thing.
 
Pin_Head said:
Manifold vacuum is below the throttle plate. You have the ported vacuum port. It is above and to the left of the transition slot at the shadow line.
Click to expand...

I'll take another picture, but I don't think so. I don't think there is a 3rd hole in that barrel.
 
You must log in or register to reply here.

Share this page

Back
Top Bottom