Less Vacuum with ported head?

[wheelman]

Cody,

My reasoning was that he ported the head but not the intake so the head will flow better but the intake won't. In this case it seems to me that the intake restriction will be the same as it was prior to the work so it will not really flow much more but the head will thus generating more vacuum in the intake than the stock head would.

 

Besides my understanding is that vacuum at partial throttle is generated by the restriction from the throttle plate more than anything else so again the vacuum would only increase from porting the head because of the increased ability to flow. I'm thinking that there is very little vacuum in a manifold at WOT and the vacuum seen at a vacuum port in this situation is generated by the venturi affect and not intake track restrictions. Please correct me if I'm wrong.

 

Wheelman

[blueovalz]

It appears the reasoning for vacuum is narrowing in on the truth. With the primary restriction to air flow being the throttle plate (be it FI or carbed), runner configuration will have little to do with the vacuum signal at idle. Once the plates are opened, the primary restriction is reduced, as is the vacuum signal from the intake manifold. At WOT, this primary restriction becomes more equal to port configuration in restricting flow into the motor (in general). The vacuum signal is directly affected by the intake system's ability (or lack of it) to fill the intake manifold with air that was drawn out of it by the pistons, and the port for measuring this signal is usually at a point that reflects plenum pressure, not runner pressure. The only time I can see where the intake manifold runners (or ports) will affect the vacuum signal seen by the plenum is when these runners are so restrictive that they now become the primary restriction to air flow over the throttle body. In this case, the vacuum signal would actually drop because the manifold is being choked down by the runners, which are down-stream of the vacuum port. So any vacuum signal difference seen by porting runners (as insignificant as it is) should improve the vacuum signal rather that reduce it by making the path out of the manifold even more efficient than the path into the manifold.

[Guest ON3GO]

intake manifold was ported also.

 

mike

[Clifton]

intake manifold was ported also.

 

mike

 

Still doesn't matter. The restriction at idle is still a nearly closed throttle plate.

[Cody 82 ZXT]

So are we saying that practically all vacuum is created by the downward pull of the piston and the closed throttle plate. The intake and head(s) make very little to no differnce at idle since air flow is to low to be effected by the difference between ported and non ported items. All of this is assuming the cam, cam timing, and engine timing are all the same.

 

What do ya think?

[Vintage-TechZ]

I agree that a fresh motor will make a big difference.

 

But Mike, is the throttle plate ACTUALLY sealing /fitting well ? HAs it been EVER so slightly been dicked with in any way ?

 

Or perhaps its been dissasembled and cleaned...out of adjustment. NOT closed or SEATING perfectly ?

 

Did anyone try to port inside the T/body and buzZ a few spots near the plate strike zone ? This will indeed make an effect.

 

I ALWAYS go to the most simple screw-ups/changes of late with subtle changes like these to figure them out .

 

I could be way off and TOO simple minded here (easy for me lol.) So don't be too mad at me for guesstimating !

 

......................Vinny