304 Stock intake limitations

[shane_3800]

Hey guy's, I'm just after some extra thoughts on this.

So I have a spare banana manifold, so I cut the plenum floor off to take some measurements.
What I've got is the plenum entry is 41mm dia which gives a 132mm square cross sectional area. Then the rectangle exit to the head is 27mm x 48mm giving 129.6mm square.
The wall thickness on the runners is 5mm.

So I've read a few old threads around on the net and I saw someone claiming 268rwkw using the OE intake. So if that is true which seems high so I've been conservative and guessed it about 375hp at the fly wheel.

With my calculations a 70mm TB should be able to support 550hp plus at the flywheel.
I'm thinking if I bored the runners out to 47mm which is 173mm square which is a 76% increase, that should see the intake able to flow 550hp.

I'm thinking that my math is flawed and the intake manifold will never be able to hit these numbers.
What do you guy's think?

Also my plan to bore it is to use a flex cable from a bike brake or something like that and drill a hole in a rod to attach the flex cable with a grub screw. Then use emery paper in a slot in the bolt.

 

[Immortality]

The problem with the banana manifold is the long small runners (no way to port the whole runner) and the oven like plenum at the bottom of the manifold which puts way to much heat into the intake charge.

 

[shane_3800]

The problem with the banana manifold is the long small runners (no way to port the whole runner) and the oven like plenum at the bottom of the manifold which puts way to much heat into the intake charge.

Yea but lets just say my idea works and I can port it. Now I've cut the plenum floor out and I can weld in some plate alloy and ceramic coat the underside. Do you think my numbers are right?

I'm thinking it still won't like to rev past 5500rpm maybe a little increase due to the cross sectional area increase.

 

[shane_3800]

The only other way I can think of to have a TB at the front on a VN-VT to be able to run an airbox on the PS is to LS it.

 

[Immortality]

To make that power you are either going to need to rev it which the manifold doesn't like or you are going to need lots of cubes. I don't think I've ever seen a banana's manifold make that sort of power without some sort of forced induction.

 

[shane_3800]

To make that power you are either going to need to rev it which the manifold doesn't like or you are going to need lots of cubes. I don't think I've ever seen a banana's manifold make that sort of power without some sort of forced induction.

Well a 396 is 77% more cubes than a 304.

 

[Immortality]

To put it in a different way, a friend who is a tuner has done a few 304 with banana manifolds. On one particular example, 304, pocket port and decked heads, good exhaust with a 224°@.050 cam the manifold was showing increasing vaccum at WOT and high revs. This is a clear indication the manifold is becoming restrictive. So even with extra cubes the manifold is going to quickly become a restriction.

 

[shane_3800]

To put it in a different way, a friend who is a tuner has done a few 304 with banana manifolds. On one particular example, 304, pocket port and decked heads, good exhaust with a 224°@.050 cam the manifold was showing increasing vaccum at WOT and high revs. This is a clear indication the manifold is becoming restrictive. So even with extra cubes the manifold is going to quickly become a restriction.

Yes I get that. But everyone says the runners are the restriction so if I bored them from 41 to 47mm that would be roughly 75% increase in corss sectional area.

Do you know any rough power figures from those builds?

 

[shane_3800]

CFM Air Flow - Throttle Body - Accufab Racing

What is CFM? The air flowing through a product is measured in Cubic Feet per Minute, CFM. It's important to flow test a performance throttle body and find the CFM rating. A 75mm Brand A t

accufabracing.com

Based off this chart a 70mm TB should make about 550-600hp flywheel.

 

[Not_An_Abba_Fan]

How are you going to bore a curve?

Only way to overcome the restriction is force it.