|09-05-2008 01:30 PM|
|hidollartoys||IMHO ported for the street. none for every thing else.|
|09-05-2008 11:21 AM|
I agree with Old Bogie, it really depends on all of the car/motor/drive train specifics to determine which of the two ports many possible different benefits are most needed...
here's a Pontiac 455 side by side (different build's) article link on manifold versus ported to add some insight....
|09-04-2008 08:59 PM|
|Regal Beagle||Thanks everyone for the replies. Its amazing how much I have learned over the last 2 days.|
|09-04-2008 08:49 PM|
Stiffer gearing has less need for vacuum advance as the engine is revving higher which makes it easier to have the centrifugal do the work. Higher gears and a heavier vehicle can use vacuum advance if street driven as the engine isn't spinning so fast. But you will need to customize the vacuum advance.
You can push vacuum advance further into a long duration cam, but you need to have an adjustable "can" so you can make it sensitive to the realistic manifold vacuum numbers. There's no sense in having a vacuum advance that functions with 18 inches of vacuum on idle when you have an engine that only makes 10.
A loose converter lets the engine rev up which once again allows the use of the centrifugal system at low speeds that also allow high engine RPMs.
However, regardless of cam, if you spend a lot of time cruising low and slow and don't want to constantly run in the lower gears to keep the revs up, a vacuum advance can be effective to squeeze the best performance possible under these conditions. But again, as the cam gets bigger, the vacuum will be less, which means the systems response needs to be customized to reality.
Whether or not to use manifold direct or ported vacuum is really dependent on set up. A lot of factors become involved these include the effectiveness of squish/quench, better needs less advance; location of the spark plug, centered in the chamber needs less; mixture turbulence, more needs less, mixture ratio, to a point more needs less, the weakest mixture in a cylinder needs more, this is usually the lower plane of a two plane manifold, single plane manifolds require specific solutions; headers will want more advance as they tend to over-scavenge the cylinder, however, with anti-reversion cones probably less is best. All these kind of things come into play, which makes exact specifics a learning situation.
Full up race engines tend to dispense with advance mechanisms starting with vacuum not because they have some trick unknown to Detroit' engineers, rather it's because as manifold vacuum goes down a vacuum system becomes less and less functional and it brings durability problems and failure modes that out weight it's usefulness. In far out motors, the mechanical advance can be eliminated again for durability and reliability issues that out weigh the usefulness of an engine that runs so fast so much of the time it just needs full advance. These type engines usually separate ignition and fuel flow from cranking, such that at start-up the starter is just spinning the motor without fuel or ignition. This eliminates the chance of backfire when just starting to crank it. Then the fuel is turned on to prime the motor then spark is added to fire it. With modern electronics the choice is there to have a starting retard or running full advance, or to even have electronic advance which has proven to be much less failure prone than mechanical mechanisms on the race track.
|09-04-2008 08:12 PM|
And do you hook your advance to ported or manifold vacuum?
|09-04-2008 01:23 PM|
Vacuum advance is not a part of "maximum performance" if by that you mean most horsepower at WOT. Vacuum advance is there to increase performance and efficiency at part throttle cruise where the mixture density in the cylinder is low. This causes the burn to proceed to slowly and the exhaust valves open before max temp and pressure is achieved. This throws unburnt fuel out the exhaust. The vacuum advance is intended to provide a longer burn time so combustion can be completed before the e-valve opens.
|09-04-2008 12:04 PM|
This is a subject that always draws out both sides.
Personally, I always ran ported on my truck. When I really got into researching it, I decided to do some real world testing. I "T'd" in a vacuum gauge and went for a long drive. Up some hills, down some hills, mild acceleration and hard acceleration. I took good notes on speed and my best guess on pedal position and vacuum readings on hills and cruising, following the same route one time on ported and one time on manifold vacuum.
My bottom line was that at cruise, I had almost identical vacuum readings, and identical timing added, whether on ported or manifold. During idle, of course I had max advance on manifold and no added advance on ported. At WOT, there was no vacuum signal so neither added anything.
When going from an easy cruise to a harder acceleration, on manifold vacuum, the vac. signal falls off very quickly, which is good since the mix goes rich quickly once the metering rods OR power valve are now open and giving a rich mixture.
From all I could tell, the biggest advantage is the added timing at idle, which, depending on cam selection, can be a great bonus in idle quality and a boost in idle vacuum. For me, the biggest drawback is when emissions testing comes around. Idling with 18 degrees of combined timing, my HC% levels are through the roof. If I just pull over before going in and yank the hose, my HC% is always within spec.
So for now, i'm running manifold vacuum for a little better idle quality since my cam is a tad tall for my converter stall.
and now back to work!
|09-04-2008 11:46 AM|
|DoubleVision||To give a slight explanation of it, I have delt with some GM vehicles that ran better with it on a ported source, but most ran best with manifold. Idle and off idle cruise mixtures are lean, and lean mixtures burn slower than rich mixtures, this is where the extra advance comes into play. When I was running my cutlass, I had it on a ported vacuum running a mild 350 chevy. The thing was a slug on low end, I kept wondering why it had no bottom end power, it had a RV cam and 9.5:1 compression, headers, a dual plane intake and a small 625 demon jr carb. it had really poor throttle response and drank gas. Then one day on here I complained about it and was told to set the base timing with the vac advance disconnected and plugged to 12 degrees, then reconnect the vac advance to a manifold source, I did so and it felt like it picked up 50 horses, it was really sensitive throttle response wise and a pleasure to drive. Fuel economy went up as well. Since then I`ve stuck to manifold vacuum unless it runs better on a ported.|
|09-04-2008 11:34 AM|
Thanks for the help fellas.
|09-04-2008 08:23 AM|
In general, and my rule of thumb - a Ford=ported vacuum. A GM=manifold vacuum. But of course there are exceptions just to make life interesting
|09-04-2008 08:02 AM|
|Greg T||Full manifold vac will aid in cooler city driving and better throttle response because your low speed and idle timing will be more advanced. Use manifold vacuum.|
|09-03-2008 11:19 PM|
At WOT it doesn't matter which way it's hooked up. The ported side just keeps the vacuum advance from moving the spark until you move the throttle. This will help part throttle drive-ability and economy and provide a more stable idle. When you are at WOT engine vacuum is at zero.
The vacuum adv. canister should only see vacuum at off idle cruise speeds.
This is my take...
|09-03-2008 11:18 PM|
Go with manifold vacuum
Here's a quote from a posting on this forum:
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively.
Read the full post "Timing and Vacuum Advance 101" here:
I was pondering this same question myself the other day and was searching the subject...
|09-03-2008 11:04 PM|
Ported Vacuum or Manifold Vacuum?
I was adjusting my idle mixture today and speaking to a mechanic friend for some assistance to be sure I got it right. My distributer was hooked up to the ported vacuum port but my friend says it needs to be hooked up to the manifold vacuum side of my Edelbrock Performer. I switched it over and adjusted the idle and got the car running and performing well much the same as when it was hooked up to the ported side.
My question is which side do I need to have my distributer hooked up to for maximum performance?