Opening Up Exhaust Manifold

I have a pair of primo 4V exhaust manifolds. Exhaust holes are currently 2".

Has anyone opened them up to 2 1/4"? Sure looks like there in enough meat on them.

Purpose for doing so...?

That's what Ford did for the Boss 351 manifolds. It's the same casting number for 2" and 2 1/4" so I don't think it would be a problem.

Steve

Use patience, determination, die grinder, and sanding rolls. Chuck

May as well just match port it to the heads. No sense going any further than nescessary.

Purpose for doing so...?

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Matching Boss 351. It is more of a sound thing than a power thing.

Ah. Makes sense. Any good machine shop could do it.

i see no benefit, you cannot get into the manifold, so only opening up the first 2-3 inches of the port will not give any great benefit.. just a lot of work and no HP gain...

putting on a decent set of headers would give you more..

my 2 cents..

i see no benefit, you cannot get into the manifold, so only opening up the first 2-3 inches of the port will not give any great benefit.. just a lot of work and no HP gain...

putting on a decent set of headers would give you more..

my 2 cents..

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IIRC, there is a choke point about an inch thick at the outlet of the manifold and it opens up after that, so increasing the diameter would help some, but agreed headers would give you more. If you want to appear stock with just a little extra flow though, the extra 1/4" would be the way to go.

Steve

i see no benefit, you cannot get into the manifold, so only opening up the first 2-3 inches of the port will not give any great benefit.. just a lot of work and no HP gain...putting on a decent set of headers would give you more..my 2 cents..

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I agree the way I drive there will not be much if any benefit. I was trying to go headers but for all the typical reasons(cost, clearance, heat, fitment...not much HP benefit in my build I decided I would stick with the manifolds.

And, just trying to be true to the 351 BOSS Specs I am building to, thought I would widen the exhaust output. It does neck down pretty significantly. If I decide to open them up, I will take before and after photos.

For the engineers,

2" DIA = 3.14 sq in opening.

2.25" Dia = 3.97 sq in opening.

27% Net Increase

27%! That is nothing to sneeze at. Maybe I will knock .005 seconds off my 0-60 time.

Wolv,

You may be better off using shorty headers. Sanderson says their collector output is 3" for the FC4 "block huggers".

Put a 2-1/2" exhaust on it with straight-thru mufflers and you might see a good power increase.

Get the ceramic coating on the headers for heat reduction.

This is the setup I'm concidering.

If you just want more noise though, look at straight-thru mufflers like Magnaflow.

Wolv,

You may be better off using shorty headers. Sanderson says their collector output is 3" for the FC4 "block huggers".

Put a 2-1/2" exhaust on it with straight-thru mufflers and you might see a good power increase.

Get the ceramic coating on the headers for heat reduction.

This is the setup I'm concidering.

If you just want more noise though, look at straight-thru mufflers like Magnaflow.

Click to expand...

I was considering this.

8 months ago, I asked Sanderson to provide my email address to 1 or 2 people in California who installed these on a 351C, preferably Mustang, and ask them to email me. I wanted to see how the install went and take a look see myself. They said they could not do such a thing. I then posted an inquiry on the 335 forum asking if anyone had used the shorty headers. I posted twice over 6 months without a response.

So, this is one reason I decided against the shorty's. They can't give me one guy?

The other was cost, and the other was they would not guarantee their ceramic coating if I used their coated headers to break in my motor.