Ported or Constant Vacuum Source? ... Again

[vintage chromoly]

I understand that engines need tuned.
I'm asking if the increase in load and cylinder pressure is slower than the vacuum canister release time, and as such, is not a concern.
All engines have more cylinder pressure when under load, regardless of how radical they are.

 

[oldbee]

Vintage that’s a pretty good question actually. IMO though , it’s pretty quick letoff. In the real real world, I’m thinking it’s a trade off (porter/manifold). Could be wrong, but I worked with both many times.

 

[threewood]

Question....You want to use manifold vacuum to add advance to the distributor to make up for less mechanical advance at idle. So, why is manifold vacuum needed if you have already added advance on the mechanical side? I.e. 20 degrees initial vs. 12 degrees initial plus 8 degrees vacuum advance.

 

[383man]

I'm not understanding how you can blame this on todays gas. I'm assuming you are talking about ethanol. To my understanding, ethanol increases the octane, so todays gas would be less prone to detonation than "yesterdays" ethanol free gas. Can you elaborate (or correct me), Ron.

When I say todays gas I am basically referring to the low octane rating as compared to the 60's and 70's octane rating 's that were at the pump when these muscle cars were new. I know even in the 70's we had 104 octane Sunoco 260 and much better in the 60's. What we have today just don't work to well unless you build the eng using todays technology to run good on the lower octane rating. And because of the ethanol in the gas today it will boil around 190 degrees and causes all kinds of headaches in the hot weather.

My point being my buddies 340 may have ran fine on the higher octane gas that was at the pumps in the 60's and 70's. I could have given it the timing it wanted without all the spark knock and pinging. When I built my eng in my 63 I used what I could that we have today so it runs ok on 92 pump. Course I run aluminum heads to start and I built good quench in the eng which helps a lot. The comp rating is 10.6 and I run a custom grind cam to keep the cylinder pressure from being to high also. And I run a lot of quick timing advance to help keep the eng cooler. Ron

 

[hunt2elk]

Question....You want to use manifold vacuum to add advance to the distributor to make up for less mechanical advance at idle. So, why is manifold vacuum needed if you have already added advance on the mechanical side? I.e. 20 degrees initial vs. 12 degrees initial plus 8 degrees vacuum advance.

Say your car wont start when its hot with any more initial than 16*, but runs better with better throttle response at say 24* initial. If you have enough idle vac and you hook to the manifold port, you can achieve this scenario. I have one of my cars set like this, but the other has a bigger cam so I don't have enough idle vac to pull in the can so I had to hook to ported.

 

[threewood]

Say your car wont start when its hot with any more initial than 16*, but runs better with better throttle response at say 24* initial. If you have enough idle vac and you hook to the manifold port, you can achieve this scenario. I have one of my cars set like this, but the other has a bigger cam so I don't have enough idle vac to pull in the can so I had to hook to ported.

That makes sense. And at least for me they are easier to tune when hooked to ported. When I was hooked to manifold and removed the vac line to time it the car would about die so I had to dicker with the curb idle before and after timing it.

 

[Dragon Slayer]

I am no expert on this, but doesn't the manifold and carburetor design also play into this. Early into this the original author states ported vacuum was moved above throttle plate and therefore at idle NO vacuum is seen. That implies no vacuum advance. Yet we know that you have to remove the vacuum hose from the distributor at idle when setting initial, because the ported does have vacuum at idle. It is less than manifold, but it still is there.

Having looked at plenty of the carbs the location of the port does vary therefor the vacuum seen at idle on the port can vary.

Chrysler training video on this subject specifically talking about the Vacuum Retard canister (used on the Hemi) and other cars; explains in broad terms what goes on with vacuum at the various stages at the ported outlet. I am not sure it agrees with the GM concept.

GM and Chrysler design requirements can be different. I don't argue fully with the theory, but I do think the design of the carb and the location of the ported vacuum play.

In the end don't real race cars run without vacuum advance, dial in lots of initial and go all in early in the rpm cycle? So why even get hung up on this. If it is a cruise car, vacuum helps out with efficiency, and sure it can effect acceleration performance if other factors are off.

 

[91r/t]

I am no expert on this, but doesn't the manifold and carburetor design also play into this. Early into this the original author states ported vacuum was moved above throttle plate and therefore at idle NO vacuum is seen. That implies no vacuum advance. Yet we know that you have to remove the vacuum hose from the distributor at idle when setting initial, because the ported does have vacuum at idle. It is less than manifold, but it still is there.

Having looked at plenty of the carbs the location of the port does vary therefor the vacuum seen at idle on the port can vary.

Chrysler training video on this subject specifically talking about the Vacuum Retard canister (used on the Hemi) and other cars; explains in broad terms what goes on with vacuum at the various stages at the ported outlet. I am not sure it agrees with the GM concept.

GM and Chrysler design requirements can be different. I don't argue fully with the theory, but I do think the design of the carb and the location of the ported vacuum play.

In the end don't real race cars run without vacuum advance, dial in lots of initial and go all in early in the rpm cycle? So why even get hung up on this. If it is a cruise car, vacuum helps out with efficiency, and sure it can effect acceleration performance if other factors are off.

Biggest reason I use mine is to ad timing at idle where my stroker wants to idle and run. It likes 28ish degrees at idle and runs cleaner, but there is no way it starts hot there. Timing is set to 18 degrees initial in the distributor and then the vacuum advance is adjusted to add 10 degrees to the engine connected to manifold vacuum. Race cars like our Dart with the same 500 stroker runs at 34 degrees fully locked, but you don't usually start the race car when its hot as the starter would bind and kick back like the street car. We run it and then shut it down and it sits and cools between runs. Also we run that car at WOT most of the time, so a timing curve isn't really needed to ramp up. We mash the throttle at the line, run WOT to the end, and shut it down back in the pit to cool and wit for next run. That's one of the biggest reasons I have mine connected to my street 68 Charger, along with the added timing at cruise to help clean up the burn and add some better economy to my car.

 

[Nacho-RT74]

Ok... another question on this.

I have a Mercedes W116 with M110 engine ( 280 DOHC )... distributor gets a double action dist vacuum pod. I still have to wonder when works one or the other, but if you have a clue, let's talk about that, making a diff point of view from ChryCo and GMC

 

[theman440]

For years I ran my mostly stock 383 4bbl with the vac advance plugged. Timing was set at 35 degrees total, ran great with no spark knock. After reading an article on the subject on Hot Rod Online I decided to try hooking up the vac advance to manifold vacuum and see what happened - I had to re-adjust the carb (mixture & idle speed) but no other changes - car runs great and still no spark knock. best of all the fuel mileage went way up . . . I can't belive i'd been so foolish for so long.

 

[Dragon Slayer]

For years I ran my mostly stock 383 4bbl with the vac advance plugged. Timing was set at 35 degrees total, ran great with no spark knock. After reading an article on the subject on Hot Rod Online I decided to try hooking up the vac advance to manifold vacuum and see what happened - I had to re-adjust the carb (mixture & idle speed) but no other changes - car runs great and still no spark knock. best of all the fuel mileage went way up . . . I can't belive i'd been so foolish for so long.

Wouldn't you have the same effect if you chose ported?

 

[theman440]

At cruise yes, but the extra timing at idle lets the engine work easier (better idle vacuum) so you can close up the throttle blades and you get better adjustiblity in the mixture screws.

 

[powermyster]

so in setting the timing should the initial timing be done with the advance connected. Provided you have sufficient manifold vacuum.

 

[vintage chromoly]

Initial is set with the vacuum canister disconnecred

 

[powermyster]

Initial is set with the vacuum canister disconnecred

That’s how I would normally do it. But if I’m pull-up in another 15 deg at idle after connecting vaccum it may be too much for my motor me thinks. It’s while since I checked but I think I’m running 18deg initial

 

[theman440]

If your engine is not completely stock "initial" timing goes out the window.

 

[threewood]

Agree. When I dial a motor in I use a tach and vacuum gauge to get best vacuum / idle quality. I won't try and set to a desired advance number. After that is set then I look to total and modify the mechanical plate. I can see how the vacuum advance can play with that.

 

[91r/t]

Yup that is what I did and when I finished I was around 27 in/mg. No way that bugger starts there hot. It was locked in at 23 degrees initial on the distributor plate when i got the car with the vacuum advanced plugged off, and it wouldn't start there hot, so forget about 27-28 degrees initial and starting hot for me. That's when I jumped over to the vacuum canister for some assistance.

 

[threewood]

Yup that is what I did and when I finished I was around 27 in/mg. No way that bugger starts there hot. It was locked in at 23 degrees initial on the distributor plate when i got the car with the vacuum advanced plugged off, and it wouldn't start there hot, so forget about 27-28 degrees initial and starting hot for me. That's when I jumped over to the vacuum canister for some assistance.

Perfect reason to use it too. My motor is an oddity as it will easily crank over set to 25°.

 

[91r/t]

Perfect reason to use it too. My motor is an oddity as it will easily crank over set to 25°.

I think my compression may be the reason too as it is 10.8:1. Runs great on pump gas with the closed chamber aluminum heads, but I think its a bit much to crank over hot with that much timing.