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.

[Shaun @ Tru6]

On the plastic air horns, has anyone 3D printed them? Doesn't seem like a difficult part to make.

[scarceller]

Correct! The longer the trumpet the lower the RPM tune point, because the sound wave travels the length of the trumpet before reflecting back down. In theory, short trumpet length is for higher RPM.
This book explains the principles really well, the author is very well respected. The book also has great section on camshaft selection and how every other choice in engine design is done around the cam choice.
https://www.amazon.com/Tuning-Engine-Definitive-Performance-Economy/dp/1859606202
Again, amazing book!

Originally Posted by 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.

[kltarga72]

I'm not a mechanical engineer like others that are responding to this thread which is very interesting. However in the early 70's I had the opportunity to drive the early 911's whcih two of my good friends had. One owned a 69 -S and the other had a 69- T. Those two models were night and day difference in performance. The T was a sports car and the S was a race car. The T was much more user friendly in everyday traffic, and the S only wanted to cooperate when driving aggressively and over 4000 rpms. My two 1972 T and S I owned were similar in driving response but that 69-S was a race car.
Keep that in mind when deciding on modifying the 69 performance, The Bosch MFI systems were superb from the factory.

[nickelplated5s]

Post 18 on the thread. Not for a 2.0 but gives you an idea about cams. presumably stock horns. I'd buy them or Shaun probably would if you want to sell.
https://www.early911sregistry.org/forums/showthread.php?178870-Differences-in-Solex-Cams-vs-E-amp-S-CAMS

[Jeff Higgins]

Originally Posted by 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)

You actually did a great job of explaining yourself, so give yourself some credit. Your photo helps illustrate what I was trying to explain. I think I'm the one who kinda fell short.

The taller air horn on the right is meant for use in the stock air cleaner housing. It sits inside that big old tub on top of the stacks, then has the big old air filter housing over the top of it. All of that provides plenty of clearance above the air horn to allow for undisturbed airflow into the horn.

Your rain hats will sit much lower over the tops of those tall air horns, thereby disrupting air flow. Using the shorter air horn on the left will come close to mimicking the air space above them that Porsche achieved with the stock horns and air filter housing. They are also high enough above the flat surface on the tops of the stacks to not be affected by them. In other words, you have the air horns that you need right there. Just use those and you will be fine.

[motorstereo]

“…This book explains the principles really well, the author is very well respected. The book also has great section on camshaft selection and how every other choice in engine design is done around the cam choice.”

The amazon link does not work for me, is this the book? “Tuning the A-Series Engine: The Definitive Manual on Tuning for Performance or Economy” by David Vizard

[nickelplated5s]

The ones in my pic are stock 69S. The Regultarori stack pic on the right is not stock. Pick your poison but I'd wait for my dyno sheet post.

[scarceller]

Yes, that's the book. Hardcover yellow book, filled with tons of good engine build guidelines.

Originally Posted by motorstereo

“…This book explains the principles really well, the author is very well respected. The book also has great section on camshaft selection and how every other choice in engine design is done around the cam choice.”

The amazon link does not work for me, is this the book? “Tuning the A-Series Engine: The Definitive Manual on Tuning for Performance or Economy” by David Vizard

[targa72e]

I did some dyno testing with the factory air box and was to see how much power I lost with it on. I was using the stock trumpets for Zenith carbs. I have since switched to custom rain hats and use a paper filter that is taller than the usual K&N. I have not dyno tested this configuration yet.

https://forums.pelicanparts.com/porsche-911-technical-forum/1151593-dyno-summary-testing-early-2-4l-2-5l.html

john

[nickelplated5s]

Carbs v MFI = apples and oranges not to mention the cams or displacement. Regulatori has a 69S, presumably 2.0. Why he didn't go 2.2 as I did, no clue. Mines stock S cams and a reshaped space cam. In other words my dyno runs will have the best data. It will take time (money) as we'll be playing around with exhaust too. It is a balanced system.

[Jeff Higgins]

Originally Posted by targa72e

I did some dyno testing with the factory air box and was to see how much power I lost with it on. I was using the stock trumpets for Zenith carbs. I have since switched to custom rain hats and use a paper filter that is taller than the usual K&N. I have not dyno tested this configuration yet.

https://forums.pelicanparts.com/porsche-911-technical-forum/1151593-dyno-summary-testing-early-2-4l-2-5l.html

john

I have found the air filter setup on my 3.0 MFI inducted motor that makes the best power to be the stock plastic ('72 and on) air filter housing with both the snorkel cut off and the front of the air cleaner section hole sawed out to match the I.D. of the filter itself. That way it breathes from both ends.

On a chassis dyno where I was able to tweak the main rack adjustment to at least somewhat compensate for the variation in intake air flow, it made more power than other filter options. Rain hats will not fit on the later plastic stacks, so I had to kind of jury-rig a temporary setup (and the deck lid wouldn't close). I borrowed a set of the sausage shaped ITG filters (they will clear the deck lid, but with no rain hats, the foam filter elements are exposed to our Pacific Northwest rain). And I ran open stacks, with no filters at all. The modified stock housing made marginally more power than any of the others. Not enough to write home about, but at least it no longer killed power like that long, single side snorkel.

Granted, this does increase induction noise in the cabin by a good deal. The factory actually refers to the filter housing as an "intake silencer". Their reason for this designation becomes very apparent when you open up the front - it sounds like it's going to suck your hat up against the back window when you open the throttles. So, for long road trips, I made up a cover from a margin tub lid and a mouse pad that I just stuff in the front, held in by the air filter. Takes a couple of seconds to pop it in or out.

[Cloggie]

Originally Posted by Shaun @ Tru6

On the plastic air horns, has anyone 3D printed them? Doesn't seem like a difficult part to make.

This was a little more complex as the air cleaner was too tall for my PMO's, so I made the whole assembly. Filament was PAHT-CF, oil and gas resistant and I think it is good to something like 180c temperature-wise.

Air horn length was set more by ensuring clearance from the air cleaner which according to my very clever Porsche engine builder, needs to be at least 1 diameter away and nothing from the airbox too close to the horn lip. Hence the differential between the top of the horn and the sealing surface, the steel air cleaner being roomier there.

Standard airbox just clips on with some plastic clips I printed up.

D.

[nickelplated5s]

What would you need for a template? 3D isn't my thing but you might have a nice side business. As Regulatori stated 69S is only a one year version. Having gone through mine there are shops I know who would, perhaps, buy them.

[Henry Schmidt]

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