Theoretical idle vacuum question

[My427stang]

I wanted to circle back on this one to let everyone know where we ended up

461 inch FE Cobra Jet, all iron. Ported iron CJ heads, ported iron CJ intake. 10.4:1, quench was .046, 292/236 .620 hyd roller, 114 LSA on 106

The engine made 515 HP at 5800 with the small carb, with a dyno carb it made 526 @ 6000 but was basically flat from 5800-6000, valvetrain stayed stable to shift point. (Comp Beehives, Brent Lykins' non adjustable rockers, oiling through pushrods) Torque values were 530/536 at 4500 with each carb. I think we could have got a smidge more out of it, but it was 97 and humid, and after lunch we were happy enough with the results.

Idle vacuum was 15-16 inches and very stable, should be a nice little street toy. Going in a resto 68.5 CJ Mustang coupe, 4.11 with a TKO-600 5 speed, .64 OD. All parts hidden the best they can be.

Basically looked like a bone stock CJ from the outside, even painted with the proper paint, flattening agent and a run where the original motor had one got to love the resto crowd!

I am doing another one now, basically the same combo, but using Trick Flow heads, painted and name ground off, and a worked PI intake. I pulled a couple degrees off the intake lobe, but nothing significant, same lobe family. Should be interesting what happens on this one. The CJ intake manifold and CJ heads have a huge cross section compared to the Trick Flow and PI intake, but the Trick Flows flow way better, both quickly and a lot more, peaking at approx 335 cfm at .600 It's going to be tough to get the intake to feed it, but we'll get close enough, I am interested to see what a smaller port with more flow does, both with power and vacuum

[cjperformance]

Cam wise, being the same everywhere(you did say a couple deg off of the Intake) you will see more vac and more hp. The intake manifold change being smaller CSA may or may not hurt peak hp. Do you know average csa of both intakes? Hopefully the Pi intake is near the mark csa wise.
The smaller csa port with more flow is far more efficient. So you get better combustion quality at idle/low rpm. At higher rpm the more efficient port just helps the entire intake runner/track work better/more efficiently.
I have been thru this with Cleveland stuff switching heads from iron 4V(big csa)to CHI and AFD alloy heads(less csa, more flow) but using the same cam, intake, carb etc and yes, more idle vac, more hp, nicer to drive everywhere in the rpm range.
Yours sounds like a cool build/project.

[My427stang]

We may change the intake, haven't decided yet. I'll get some CSA numbers ASAP.

The issue is the difference in flow is so significant, and the money to port, it may be better for the owner to get a Blue Thunder med riser intake, which matches the ports cleanly and breathes a lot deeper. Although I haven't checked it myself, the PI as is, flows about 260-280 cfm per port versus a 335 cfm head, and although I agree it will be a fast port, FE runners are kinky, and not in the good way LOL

We will see, I'd really like it to be efficient, because although the all iron twin brother did good, as you mentioned, I'd like to see the difference with a faster and better port that travels higher before the short turn without any self-imposed restriction