911R intake manifold

[rs911t]

I've been browsing old posts about the 911R and came across this photo.



I only noticed because of my recent carb rebuild, but the intake manifolds are much higher than stock.

Why? What does the taller manifold do for performance?

[pwd72s]

Longer tubes are for peak horsepower at high RPM levels...at the expense of torque range. These look high enough to produce some harmonic wave technology as well, giving a bit of positive pressure just as the intake valve opens. Sort of like free turbocharging...keep in mind, at the expense of torque range...the 911R was really too peaky to be a street car.

[rs911t]

Thanks Paul. Sounds like a cool effect, but only really functional at continuous high rpm, eh?

[gregk1]

So if Irun the shorter manafolds on my rs spec 2.7 with S cams I will do better in the lower RPM torque range???

[jluetjen]

I think that the idea is right but you might have it backwards...

1) Longer intakes are tuned for lower rev's (relatively speaking). Why? Think of a trombone: longer pipe = lower frequency (pitch). This is why most EFI'd cars on the road have intake manifolds which curve over and appear to be at least 5 or 6 inches long -- and most car's on the road are not tuned for high rev's. Note that this doesn't take into account harmonics (2x the freq, 3x the freq, etc) of which there are more in longer pipes and fewer in shorter pipes. So it's actually possible to get longer pipes to be tuned to more places in the rev range then shorter pipes.

2) Why would a 911R have longer manifolds then a stock 911T?
a) Packaging? Note that the 911R doesn't have a full airbox over the carbs like a T does. Without the airbox they may have used longer pipes because they could.
b) I've been told that longer intakes are less prone to "reversion" which can be a big problem on cars with cams with lots of overlap (something that the 906 cams in the 911R had a lot of ). As I understand it at low rev's the pulses up the intake will hit a resonance and create a cloud of fuel droplets floating above the mouth of the carb - a bad thing! By making the intake track longer the frequency of the intake no longer matches that resonance and so the cloud disappears. At higher rev's this would not be an issue since the air would have a lot more velocity and inertia into the cylinders.

But keep in mind that tuning intakes is not a linear function. I recently read that Honda RA122E/B 3.5 Liter V12 F1 engine used variable length intake trumpets. But it was not as simple as long = low speed and short = high speed. Rather the funnels were raised and lowered over a 25 mm range (~1 inch) 3 times as they engine rev'd from 8000 RPM to 15000 RPM in order to fill in some troughs on the power curve. (Classic Racing Engines - Page 209. GREAT BOOK!!! It's also got at least 5 Porsche engines in it!)

By the way - going back to trombones. Trombone players can hit at least 2 or 3 different notes at each slide "position" based on how tight they purse their lips. So in engine terms, 1 intake length may be correct for 2 or 3 different engine speeds. (Forgive me if there is anyone on this BBS who plays or played the trombone. I spent my musical career as a drummer - so feel free to step in and correct me!)

[jluetjen]

I've been thinking about this overnight and want to try making a model of it. Does anyone know the length of a stock 911T intake manifold? How about a "High Rise" manifold? Here's what I have so far based on parts in my garage...

* Carb System:
- Head (Valve to manifold face): 4 inches
- Manifold: ???
- Carb (40mm IDS): 4 7/8 inches
--- manifold face to choke: 2 1/2 inches
- Stacks: 2 3/8 inches

MFI:
- Head: (same as above) 4 inches
- Intakes: 9 1/2 inches

Hmmmm

[silverc4s]

PMO's custom intake manifolds are also taller than stock. More like halfway between the two extremes of stock & R as shown here..

[steveW]

Don't forget that by moving the carbs away from the heat source is also an effective way to stop fuel percolating.

Cheers

[jluetjen]

Nobody has a set of intake manifolds off of which they could measure the height???

Excuse me -- this is the technical BBS isn't it???

[avendlerdp]

I do. I'll get back to you asap. I have a '70 T.

Alex

[RPMClassic]

I believe the 906/911R intake manifolds are 4 1/4" tall, PMO's regular intake manifolds are 3", my stock ones were less then 2". PMO has a taller manifolds are pretty close to the height of the 906, but they don't offer them with a small port size for the 2.0 and 2.2. Good luck.

[jluetjen]

Thanks Guys! I'll get back with you regarding what I can learn (without access to Wave software!).

- John

[pwd72s]

John, maybe my confusion is because of difference between tuned injection stacks and carb manifolds? I've read that the taller the injection stacks the higher the RPM's need to be before the free positive charge at the intake valve happens. (I'm nutshelling this, tuned stacks are a magical science) Thus, I assumed the same happened with manifolds...high rise tunnel ram type mainfolds are used mainly on the higher revving 1/4 mile cars in hot rod circles...again, not really street engines. Maybe Wayne, with his MIT degree in engineering would care to weigh in. I'll admit, my knowledge is HOT ROD magazine level...not the writers, the readers...

[TimT]

John, Ill try to get some measurements tomorrow, my carbs are at my friends shop, he uses them as a benchmark to check against when cars come in that have carb problems. Anyway Ive stored a spare set engine with stock T intakes, and my carbs have the tall PMO intakes. I also have the 3/8 (1/2" with gaskets) insulators..

[Joe911]

The intake manifold on my car are 3 1/4" tall flange to flange + PMO spacers. I have no idea what they came on orginally.

[jluetjen]

When I ran this through "Ask Jeeves" I found this
interesting piece which got me started thinking.
Here's another one which also looks pretty informative (Check out the subjects on harmonics, "Flutes vs Clarinets" amd "Pipes and Harmonics"). I'm going to have to think about this stuff pretty hard, but maybe some others on this BBS can understand it better then I can.

(PS: I've fixed the links )

[pwd72s]

John, I clicked, but all I got was a notice to "ask jeeves" the questions again...

[jluetjen]

Ok; I've re-read this stuff a few times and I've been thinking about it a lot and I think there are a couple of conclusions that can be drawn without the use of Wave software or other expensive tools.

As they say -- a picture is worth a thousand words. And I think that this is a great diagram.



We can ignore the "Open Cylinder" diagram on the left since it doesn't apply and focus our attention on the "Closed Cylinder" and "Conical shape" drawing in the center and on the right. Why?

* Open Cylinder: None of our induction systems have a hole in the side like a flute. So we'll skip this case.
* Closed Cylinder: A 911's intake is about a closed at the end as a clarenet is. So this would seem to apply, but are the sides really parallel? Generally no. See the comments below regarding the conical shape.
* Conical Shape: Give that the standard induction system (T/E) is at least 40mm at the top and about 32mm at the bottom, I'd argue that this is roughly a cone -- at least as much of a cone as a Sousaphone is.

OK, so I worked through the fairly simple math and figured out the harmonic frequencies for some of the different intake configurations:



Some Conclusions:
1) Harmonics repeat regularly (duh!). So every 543 Hz a stock T intake system will hit a harmonic if the system is a closed cylinder. When it hits a harmonic is when a pulse of pressure will hit the intake valve while it is open (a good thing for charging the cylinder!). The same applies in the other cases.
2) Since closed cylinders only have odd harmonics, the gaps between them are greater. (Note that I accidently included 18th harmonic on the cylindrical table which should not be there)
3) Conically shaped intakes will have all the harmonics and so the gaps will be half the size as closed cylinders. Hmmmm - It sounds like if you want the intake system to be in tune more often, it should be conically shaped.
4) Longer intakes (like a 906's or the 911R pictured above) have the harmonics closer together, so they tend to be "in-tune" more often. For example, the harmonics of a stock 911T conical intake are 271 Hz apart while the harmonics of a 906/911R intake are 234 Hz apart.
5) (Note that while the diameter is not listed, larger diameters have lower pitches and also affect the gas speed - a whole nother subject!)
6) Although it may not be obvious from the first diagram at the top, but conical ly shaped intakes also seem to have another benefit since the intensity of the sound ("Pressure" on the diagrams" will fall off according to 1/r^2 where r is the radius of the portion of a sphere enclosed within the cone (roughly the length of the intake). So while the intensity of the vacuum pulse going up the intake will diminish, the intensity of the pressure pulse reflected down the intake will INCREASE as it approaches the valve! Cool!

So, an engine with the ports approaching the size of the carb (let's say with 39mm ports and 40mm IDA's) will start to act like a closed cylinder and lose the even harmonics and the intensity affect of a conical intake. A double whammy. It may still work, but have some "holes" in the rev range. But switching to 46mm webers could make a big difference in a situation like that.

Now lay this against the frequencies of the different engines across the rev range which I've included below.





The first thing that I noticed as I worked these out is that 911 motors seem to need to have their valves open a certain amount of time to be happy. Specifically, once the intake valve is open less then .00011 seconds, the HP drops off pretty quickly. Interesting...

Anyhow, try mapping one of the intake systems against one of the rev ranges shown and you can start to see how often the harmonics come on. Where you chose to put the harmonics (by playing with manifold lengths and intake trumpets) depends on what you are trying to do as far as boosting torque or filling in a hole.

A couple of caveats though -- this is obviously a gross simplification. Real intakes (especially those with carbs) have butterflys and venturi in them. I have no clue as to the affects of these items.

Does any of this make sense? Am I all wet? Did I miss something?

[Ernie W]

This may help in your calculations,
The 906 engine used an intake manifold that is 98mm high, measured from the face of each flange, top to bottom. although the 906 and 911R manifolds differ by the way the cross linkage attaches to the manifold, the 906 engine is installed reverse from the 911R so the linkage goes from the #3 intake stack to the #6 intake stack on the 906, flywheel end, and the 911R has the cast attachment points from the center #2 stack to the center #5 stack. The 906 had two versions of intake manifolds, a later one with reinforcements cast around the lower flange mounting holes, and one corner of the maifold is cut on an angle, (#1 stack) to accomadate the frame cross tube above the engine. Both early and late style 906 manifolds are the same, 98 mm. high. and use the same part number 901.108.318.00 One part fits both left and right sides.

Ernie Wilberg, R Gruppe #8

[Ernie W]

Early style intake manifold, without cast reinforcements around the head stud holes on the bottom flange. The linkage is very similar to the early 911 Solex set up.