Hey,

So made the "mistake" of port matching my primary intakes, or so I have read.

I have a 12a Y tall port center.

Getting conflicting statements on this reversion thing.

On stock ports/mildly modified stuff, it is best to leave the step in.

On fully bride/j/pports, match it up.

Lots of conflict imbetween those two "sum ups".

I am full bridgeport, freeflowing exaust (ports and system), and large ported intake manifold and 650 cfm carb.

I have enough meat in the centerplate to port the step back in. Many recommendations are to leave as little as 2mm of a step, which is easy to put back in, top and bottom.

Anyone want to chime in on they're experiences. I have been doing a lot of reading on it, so chances are, I have read that article/post.

All Paul Yaw's recommendations to keep the steps were on stock/lightly modified stuff.

Thanks,

 

Its absolutely amazing how much material I removed from the secondary ports.

It literally took me two, 3 hr shifts to get this done.

The primary's was pretty much a straight shot back on the tops and bottoms. Some smoothing of the radius, but not as much as the secondary's!

I will report back with my findings.

IMG_1562 by thomas telesco, on Flickr

IMG_1563 by thomas telesco, on Flickr

 

So as I was matching my carb to spacer I noticed that the primary butteryflys/intake openings provide a step when mated to the spacer.

Anyone think this will be fine? Does it matter where the step is, as long as its there?

IMG_1564 by thomas telesco, on Flickr

 

If I recall the 12a's late closing primary port timing causes the reversion. The step from the lim creates turbulence as the incoming charge enters the block. So any air reversion from the port closing gets slowed due to the turbulance. This helps low end torque because this collision kinda bounces the charge back towards the port entry. Since your full bridge, your gonna loose lowend anyways with all the overlap so it doesn't matter.

 

Thank you for your response.

Got it up and running this weekend. No tuning, as Im trying to figure out my hot start problem.

But when running it runs well.

Definitely needs jet re-sizing

I think just cause I race clearanced and opened up the bridges/exhuast, Ive lost some compression.

Starts up fine cold though? 1 pump of the gas, and it fires right up.

If I turn the car off when hot, and try to restart, (like changing jets!) she wont go.

If I hold the pedal down, and give it more air, it will start eventually. But most times the battery gives out before the engine starts. And when I say gives out, I mean I can hear it bog down and not spin fast enough. If I hook up a heavy duty charger, it gives it enough omph.

The battery is an optima red top, but it is 6 yrs old.

Started the car fine before the rebuild though?

Meh.

 

The reason you don't want to port match is because the cross section will make a large change right where there is a bend. Large cross sectional changes are Bad and trying to change cross section in a bend is Extra Bad. Doing so "lengthwise" in a port so that you increase the length difference between the short and long sides is Super Bad.

What you have made is a recipe for vortexes in the port where parts of it actually flow backwards. The energy to make the port flow backwards is stolen from the energy normally used to push air into the engine.

Measure the minimum cross sectional area before the bend, then fill in the bottom of the port with epoxy, then re-port it so that the cross sectional area does not change throughout the port. Ideally it will get smaller not larger but really it's already freaking small so just roll with it.

 

^ I understand.

I will get it tuned and drive it for the up coming summer.

Will take off the im and measure/re-fill next winter.

Will report back my findings.

Thanks.

 

That's what I would do too