Ported or manifold vacuum?

trevbeadle

trevbeadle

CCCUK Member
Just trying to confirm which components use which vacuum source as I am replacing a carb on a stock engine.The engine still has EGR, EFE, PCV and charcoal canister as well as hot and cold air filter housing and AIR ( I know no vacuum requirement for 'Smog'). As far as I can work out, the only thing that likes ported vacuum is EGR. My understanding is that ported vacuum is zero at idle. Whats the situation with vacuum advance on a standard fit distributor? I thought it only requires vacuum advance as the revs climb above 1500, i.e ported vacuum, yet all the diagrams I see it comes from manifold vacuum!!!
Here are the ports on the new Rochester carb I will be fitting with what I think are the correct ports labelled for each item. ( Canister also gets a small ported vacuum source teed of the EGR supply) The one I'm not sure about is the dissy one, it appears to be 'above' the throttle butterflies but maybe runs inside the carb to ported pressure or is manifold vacuum correct???
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IMG_1519.JPGand where does the S connector on the underside of the air cleaner connect to?


Many thanks
 
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Knodty

Knodty

Well-known user
Hi if I remember correctly diss vacuum port should be above the butterfliees. If not you will have vacuum operating the advance at idle which you do not want. The air filter tube should go to the rocker cover to the pcv valve. My engine is not stock but I think that is where they should go.
 
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Roscobbc

Roscobbc

Moderator
My understanding is that vacuum feed to distributor should either go manifold base (as does PCV port) - or - a port taking a vacuum feed from beneath the carb butterflies on a performance oriented engine. Maximum vacuum advance is at its most important in terms of smooth running and relative economy when engine is on cruise or de-cellerating. Getting 'on' the gas pedal creates an initial loss of vacuum which retards the ignition preventing potential 'knock' and labouring. Later 'emmission' engines may have a 'dual' vacuum advance/retard system where there is an additional vacuum feed pipe from above the butterflies - this will retard the ignition in certain cruise and warm-up cycles supposedly reducing certain emissions (NOX I believe). Ignore distributor certrifugal advance for purposes of above information.
 
langlj00

langlj00

Busy user
Distributor Advance is hooked to Ported Vacuum from the Factory (yes Ported is above the butterflies and only comes in when the the throttle is applied).
Most people tend to change the Distributor Advance to Manifold Vacuum for smoother running / cooler running. (See papers by Lars somewhere on the site).
My car is certainly on Manifold and runs better that way.

All the other connections look fine.
 
trevbeadle

trevbeadle

CCCUK Member
Thanks. Looking at the distributor I think its an HEI with an adjustable vacuum advance. When all this covid stuff is over I will have to get it to someone who can tune the carb properly for me, although the car starts now and runs but I have a radiator leak from somewhere! Doh! Maybe need to get that out next and get it repaired/re cored or I might be lucky with just a hose!!
 
Chuffer

Chuffer

CCCUK regional rep
Hi Guys , this thread has been very useful and informative to me too . It has set me looking into the way it is all fitted up on my car which has an Edelbrock Torker2 inlet manifold with 4 barrel Holley with vacuum secondaries on a replacement 305 Camaro IROC -Z engine , so all VERY non standard . I crankcase ventilation connected to passenger side valve cover from carb air filter but have PCV on drivers side connected to carbon canister with no sign of a connection to either the ported or manifold induction . Certainly there is no tee on brake vacuum line . Plus I also appear to have two outlets on top of canister not connected to anything :unsure: . That rather suggests to me I have no vacuum draw through PCV or carbon canister so have been running on a `pumped up` crankcase all this time ??
Need to get over to workshop again now Boris says we can come out play and have a better look . Any advise would be a great help.
 
Oneball

Oneball

CCCUK Member
You won’t have been getting excessive crankcase pressures as you have got a vent to atmosphere in the air filter so it’s unlikely to have blown any seals.

I’m not sure if the benefits of the carbon canister nowadays so might be worth seeing where the rest of that goes.

If I were you and it’s just getting used on the street I’d plumb the PCV system up like the earlier cars so it’s just connected to air filter and vacuum.
 
Knodty

Knodty

Well-known user
Hi Chuffer, with regard to the torker 2 manifold it can be restrictive on rev range I had mine originally fitted on my BB and it was a huge improvement on my stock intake but the power was limited between 1500 and 5500rpm. I know that is quite enough for a street car but it does depend on where you want to go With it. You could go with a Edelbrock rpm or air gap manifold but will most likely have to get another hood. Which was why I stayed with the torker 2 for so long. The performer manifold will rev past 6600rpm with good flowing heads btw.

Good luck with it
 
Chuffer

Chuffer

CCCUK regional rep
Knodty said:
Hi Chuffer, with regard to the torker 2 manifold it can be restrictive on rev range I had mine originally fitted on my BB and it was a huge improvement on my stock intake but the power was limited between 1500 and 5500rpm. I know that is quite enough for a street car but it does depend on where you want to go With it. You could go with a Edelbrock rpm or air gap manifold but will most likely have to get another hood. Which was why I stayed with the torker 2 for so long. The performer manifold will rev past 6600rpm with good flowing heads btw.

Good luck with it
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Hi Knodty , Thanks for your input . I am happy with the Torker 2 and research that I did some time back suggested that the Torker 2 gave a flatter peak torque curve across a wider rev band so that would fit in with the rev band you quote . That is if you mean max power is limited to between 1500 and 5500 rather than the the actually power output being limited at that point . The sweet spot on mine comes in at about 2200 rpm and then pulls like a train but I never get near to 5500 rpm to go easy on the old girl !! Also have standard cast iron exhaust manifolds .
 
Chuffer

Chuffer

CCCUK regional rep
Oneball said:
You won’t have been getting excessive crankcase pressures as you have got a vent to atmosphere in the air filter so it’s unlikely to have blown any seals.

I’m not sure if the benefits of the carbon canister nowadays so might be worth seeing where the rest of that goes.

If I were you and it’s just getting used on the street I’d plumb the PCV system up like the earlier cars so it’s just connected to air filter and vacuum.
Click to expand...
Hi Oneball ,
Thanks for input , I guess you mean air in via air filter on one side and gases out via vacuum through PCV to inlet manifold or carb on the other side . That is how I believe it should work but don`t appear to have at the moment . I may well ditch any conection to carbon filter and pipe PCV line straight into carb as I have found a blanked off intake on rear of carb above above butterfly level . Hopefully that should work .
 
Oneball

Oneball

CCCUK Member
Chuffer said:
Hi Oneball ,
Thanks for input , I guess you mean air in via air filter on one side and gases out via vacuum through PCV to inlet manifold or carb on the other side . That is how I believe it should work but don`t appear to have at the moment . I may well ditch any conection to carbon filter and pipe PCV line straight into carb as I have found a blanked off intake on rear of carb above above butterfly level . Hopefully that should work .
Click to expand...

Pretty much what I meant except the PCV should be connected to manifold vacuum so below the butterflies
 
Chuffer

Chuffer

CCCUK regional rep
Finally got my breather concerns sorted and now happy that all is as it should be . So now we can all drive as far as we like so long as not as far as Scotland or Wales and then do nothing when we get there , I will get the car out and enjoy driving it at long last :D
 
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