Any benefits of running taller velocity stacks/air horns on my 69S MFI?

[Regulatori]

I have a rebuilt 69S motor and currently going through the MFI system.

The 69S stack openings are huge (largest of the 69-73 MFI variants) at 46mm. It currently has the original plastic air horns which I won't use. It's old plastic that's fragile.

The car is going to be a weekend toy. Likely will have individual mesh filters and fiberglass rain hats.

Curious if there is any benefit to taller or shorter air horns since I need a set.

Thanks.

[Jeff Higgins]

Getting the tops of the air horns too close to the top surface of the rain hats will absolutely kill air flow. To a lesser extent, getting them too low, too close to the flat surface on the tops of the stacks will hurt it as well. It's amazing just how critical these relationships are to the air flow. Enough that we would notice on a street driven car? Not likely. But, unless there is some compelling reason to change any of this, I would assume that Porsche got it as "correct" as they could and leave it at that.

[Regulatori]

Originally Posted by Jeff Higgins

Getting the tops of the air horns too close to the top surface of the rain hats will absolutely kill air flow. To a lesser extent, getting them too low, too close to the flat surface on the tops of the stacks will hurt it as well. It's amazing just how critical these relationships are to the air flow. Enough that we would notice on a street driven car? Not likely. But, unless there is some compelling reason to change any of this, I would assume that Porsche got it as "correct" as they could and leave it at that.

That's the issue. You don't want to use the original plastic Porsche setup because they're brittle and chip easily.

Just seeing what people recommend for air horns since I have to replace them anyway.

Was thinking of PMO 46mm because they're readily available.

[nickelplated5s]

Listen to Jeff. I just had mine, 69, rebuilt by EuroMetrix. Fixed one with a chip too, replacements aren't available. Rain hats look cool but general consensus is stock is best. Go look on Early911S for muffler issues with MFI and other MFI threads. Wait a few weeks and I'll have a dyno sheet there although there are others. FWIW my pump rebuild was about 9 months. T Bodies only about 6.

[Regulatori]

It's been a long day and maybe I'm not explaining my question. Not RSR style 6" trumpets.

Forgot I also had a set of shorter metal velocity stacks like from carbs.


Just took a quick pic.
(promise the dog hair will be removed before install)

Short metal 1" vs Porsche original 2"

The factory 46mm in 69S only is ridiculous size for a 2.0 but it makes me smile. Like a little 906 DNA


Just kind of curious if there is any difference between a 1", 2", or 3"

(you can thank Henry at Supertec for the throttle body eye candy)

[917_Langheck]

Those air horns (stacks) are tuned instruments, and work in concert with the exhaust to move air through the cylinders. Unfortunately, their bandwidth is limited, with length working inversely proportional to rpm - short with high rpm, and long with low rpm. As such, there will be a difference between 1, 2, and 3 in. trumpets as they are added to the runners. So you'll need to calculate where your powerband is supposed to be, and correlate stack height with cam timing.

Per a Google ai summary:

A simplified, general formula used by engine tuners to find a starting length for a specific target RPM is: Length=Speed of Sound*2/Target RPM*Harmonic

Length: The effective length of the intake tract.

Speed of Sound: Approximately 1100 feet per second (or 335 meters per second), though this varies with temperature.

Target RPM: The engine speed (in Revolutions Per Second, not Minutes) where peak performance is desired.

Harmonic: This is a tuning factor (typically the 4th, 5th, or higher order harmonic) that accounts for the complex wave dynamics in a real-world engine manifold with multiple cylinders.

Old time hot rodders would just change stack heights until they found one they liked.