OHV Intake Manifold Height Difference

[DPDISXR4Ti]

Is there any height difference between the Aluminum and plastic intake manifolds? I need the shortest manifold for my swap, hence the question. On a related note, has anyone ever shortened/re-welded the Aluminum manifold? I'm assuming there's not any after-market manifold that sits lower, right?

 

[RonD]

Just a heads up
Intake passage(runner) lengths are like exhaust manifold passage lengths, they are calculated lengths to add free extra power
When an intake valve closes air flow stops and creates a pressure wave that travels back up the passage and then is reflected back, if this returning pressure wave hits the intake valve port when valve is open again then its like a mini-turbo boost that pushes in more air/fuel, ain't much but ain't nothing either
The calculated length sets where in the RPM band this "boost" occurs

Short wide runners move best torque up in the RPM range
Long narrow runners move best torque down in the RPM range
IMRC(intake manifold runner control) is a way to change the "length" of runners on the fly
Google: calculate intake runner lengths


Like scavenging exhaust manifolds, its free extra power
So heads up on changing runner lengths, as it WILL change engine performance
The MYTH of "back pressure" in exhaust systems comes from people removing factory exhaust manifolds and installing home built headers, they lost power because they didn't realize the "tuned" factory exhaust created low pressure to suck exhaust out
If you build a custom intake it WILL change when you get the "boost", or if you lose it completely

 

[DPDISXR4Ti]

Short wide runners move best torque up in the RPM range
Long narrow runners move best torque down in the RPM range

I wouldn't mind moving the torque peak a bit to the right in the curve, so I'm not terribly concerned if that happens. At the end of the day, it's mostly a packaging concern for me (i.e. stuffing a 4L engine in a car designed for a 2.9).

 

[bobbywalter]

The 4.0 don't rev like a 2.9.. .watch your gearing.


You can rev it higher .. but they can tear oem rods/bolts up over 6g and float. Easily addressed...but keep it in mind. A short runner box may help with the breathing.

The early style is lower if I remember right.

 

[DPDISXR4Ti]

The 4.0 don't rev like a 2.9.. .watch your gearing.
The early style is lower if I remember right.

Yes, I fully realize that the 4.0 is a "truck motor" that runs out of steam quickly.

Thanks for answering my question, even if only an educated guess. Ideally I don't need to re-fabricate the manifold and can achieve the fitment goal by just dropping the sub-frame a bit.

 

[RonD]

Body lift can also give you more room under the hood, even stock new spacers can give you back the room that has been lost over the years

 

[bobbywalter]

Looks like he is running a merkur.


There is an intermediate composite intake....tom morana has stuff worth looking at as well.

 

[bobbywalter]

Look up petroleum junkie. His setup on the 2.9 is awesome.

 

[RonD]

oops, merkur was unibody so no body lift