911R intake manifold

[IMONBOOST]

Correct me if I am wrong, but one of the main reasons for the taller manifolds on high rpm cars is to move the injection of fuel farther from the intake valves to allow more time for fuel atomization at very high rpm.

[jluetjen]

IMONBOOST;
That's an interesting point that I hadn't considered. That would also be consistant with the "trick" of mounting injectors so that they face upstream rather then downstream or perpendicular to the airflow.

It just goes to show that intake design is a complex subject. There are lots of details to consider. I wonder what other ideas we may pick up.

[GT3STIG]

Jluetjen
What can you tell me about adding tall velocity stacks to a 2.7, Solex cams, S heads, PMO tall intake, RS p/c, single plug and sport muffler.
Thanks.

[wayner]

I'm no expert on the black magic of engine tuning, but when I raced motorcycles I had a mechanic who was.

When I semi-retired and switched to vintage racing he helped me build that motor too.

He used a text book formula and based on cam timing, calculated the size and distance that the exhaust collector had to sit away from the heads, and then he built a pipe and custom collector for it.

Then he calculated the intake base once again on the cam we were using.

On my first run down the back straight at mosport, the organizers had a timing gun out.
When I pulled into the pits my mechanic pushed the carbs back over 2" from their stock position and I picked up a full 20 mph at the end of the back straight!

His work was based on the theory that at the optimum RPM, by engine was sucking in fuel and air, slamming the intake valve closed and igniting it, meanwhile the intake flow that hadn't made it into the cylinder apparently according to him was bouncing off the closed intake valve and passing back through the carb, thereby picking up another rule load making it overly rich. ...But that's not all. When the valve opened once again, he stated that it picked up a third charge as it passed back though the carb once again.

His theory was that pushing the carb back much farther kept the resonant air between the carb and the cylinder.

he hd a specific formula to figure out how far to push the carb back for optimal tuning, but space constraints meant we had to compromise.

Another technique that some of the other tuners experimented with was placing a custom intake box between the carb and head in order to combat the same effect. The box contained a one-way read valve such as what was used on two stroke motors.

Thats all I know is that my bike became 20mph faster with whatever voodoo he used.



Here is my 911 motor. A different witch doctor built it.
I know very little about other than the fact that it is very street able until all hell breaks loose at 5000RPM.
The intakes seem a bit longer than normal to me but I'm a novice so I'll let others comment in this motor.

[GT3STIG]

Thanks Wayner!
Great write up and I will re read several times until it all sinks in.

[Trackrash]

There is a formula that relates the "tuned" length of the intake tract to the RPM at which a "ram" effect will occur.
I measured the intake tract length on my 911 and IIRC it related to a ram effect at about 7000 RPM.

[TRE Cup]

i have an original 911R intake manifold i can measure tomorrow. will post the length and intake/exit diameters

btw- these being magnesium and so tall, they were prone to breaking a lot (as evidenced by the patched one i have)

[TCracingCA]

Basically I feel Porsche was trying to hit air flow velocity closer to the piston bottom with a long runner, so as to get the effect of cramming in a little more airflow past essentially filled (like a ram effect) resulting from reaching terminal velocity and the residual that would essentially race into or pack the cylinder by this effect and less with this particular engine to develop low end torque! Thus don't get alot of the comments here in this thread, because that engine was not meant to run on the street, but on a race track and the overall engine would be made to run mostly at the optimal RPM, and every engine has an optimal Rpm and Range of operation! 4000+ views? on this one? Wow! But there are some smart guys here sincerely!

[Trackrash]

Additionally, if the ram effect is optimal, meaning not impaired by the air cleaner or bends etc in the intake tract, it may add 5 to 10% or more torque.

Problem is at some other RPM, like around 3500, the effect is the opposite, so you loose power.

Maybe someone should compare the intake length of the SCs and Carreras and compare them?

[tharbert]

Holy thread revival, batman! 11 years!

[TRE Cup]

The intake manifold with casting # 901 108 319 00 is 98 mm tall with 46 x 38 ports top and bottom

[wayner]

Originally Posted by Trackrash

Additionally, if the ram effect is optimal, meaning not impaired by the air cleaner or bends etc in the intake tract, it may add 5 to 10% or more torque.

Problem is at some other RPM, like around 3500, the effect is the opposite, so you loose power.

I think that is what my mechanic was trying to do when he said we needed to move the carbs back even further (but could not do to space constraints) was to maximize ram effect as you call it.

By moving it back as far as we were able to he got it out of the range of opposite effect at least.

[wayner]

I remembered sitting in the lemans winning Mazda prototype when I was a youngin.
I also remember taking turns with my buddy pressing on the gas pedal while the other person watched the intake manifolds go up and down.

I just found a technical article on it.

"telescopic intake mani- fold system (TIMS), the first of its kind ever used on a racing engine."

TIMS is explained a few pages into this.
http://www.freelancemotorsports.com/wp-content/uploads/2013/10/Mazda-4-Rotor-Le-Mans-Engine.pdf

[TCracingCA]

This manifold is a Race manifold and as I said above, they are looking for a particular optimal operating range and enhancing that with design of a tube essentially that is getting close to maximum velocity on the flow into the cylinder at a particular rpm operating range! Whereas for a Street designed manifold, you can design the tube, venturi's etc. to not cause the air flow to hit maximum velocities or if the volume is such to not reach ultimate maximum velocity based on cross section diameter and such, then you can with a longer runner of proper design gain low end torque or a broader optimal rpm range. One guy here hit on closed valve bounce is what I will call it and that is not good in a street design, thus even this unit was designed with a volume slightly more than what a cylinder could stack for the ram effect and he covered that topic better than I could have ever explained it. Thus this 911R manifold is not a unit to throw on a street car!

[Flieger]

Originally Posted by TCracingCA

This manifold is a Race manifold and as I said above, they are looking for a particular optimal operating range and enhancing that with design of a tube essentially that is getting close to maximum velocity on the flow into the cylinder at a particular rpm operating range! Whereas for a Street designed manifold, you can design the tube, venturi's etc. to not cause the air flow to hit maximum velocities or if the volume is such to not reach ultimate maximum velocity based on cross section diameter and such, then you can with a longer runner of proper design gain low end torque or a broader optimal rpm range. One guy here hit on closed valve bounce is what I will call it and that is not good in a street design, thus even this unit was designed with a volume slightly more than what a cylinder could stack for the ram effect and he covered that topic better than I could have ever explained it. Thus this 911R manifold is not a unit to throw on a street car!

I'm not exactly sure what you just said, but the length of the runner doesn't affect the average speed of the mass flow. What it affects is the resonant frequency of the tube. This means that at certain rpm's the pulsations of the airflow (which travel at the speed of sound) will cause a high pressure area at the intake valve. At others it will be a low pressure. By chaging the length you are able to keep the high pressure area at the inlet valve across the rpm range.

This also applies for the exhaust, but you want low pressure. I haven't seen a variable length exhaust yet.

[TCracingCA]

When I mentioned longer runners in my last statement, I was just talking in general! All of this type of stuff has been studied from the long crossram manifold Chrysler cars to the staggered stack McLaren's of the Can Am days! Yes a given tube length would affect pulses, harmonics, frequencies everything you said. I don't think even after re-reading what I wrote above about saying anything about the general tube length changing speed, but I did say cross section or diameter would. Also if you neck something down or up you can tune things to put the pressures and velocity where they would be beneficial to the characteristics that you are needing or wanting.

Ps here on this forum, I have to be careful how I write something, because there is a higher probability with Porsche guys of having someone truly smart reading something that I write!

[pdtweeks]

Originally Posted by Flieger

I'm not exactly sure what you just said, but the length of the runner doesn't affect the average speed of the mass flow. What it affects is the resonant frequency of the tube. This means that at certain rpm's the pulsations of the airflow (which travel at the speed of sound) will cause a high pressure area at the intake valve. At others it will be a low pressure. By chaging the length you are able to keep the high pressure area at the inlet valve across the rpm range.

This also applies for the exhaust, but you want low pressure. I haven't seen a variable length exhaust yet.

Isn't variable-length exhaust what the 2-stroke Karting guys use to tune "on-the-fly" ??
I have seen karts at Road Atlanta and assumed that's what they were running.

[Flieger]

Originally Posted by pdtweeks

Isn't variable-length exhaust what the 2-stroke Karting guys use to tune "on-the-fly" ??
I have seen karts at Road Atlanta and assumed that's what they were running.

I've never seen it, but that isn't saying much. 2 strokes benefit much more from intake/exhaust resonance than do 4 strokes (which is why they have the complex expansion chambers and other things), so it would not surprize me. To take full benefit of it though I think you'd need an automatic system- either with a cable and cam mechanism or electronic.

[Steve@Rennsport]

The reason Porsche used tall manifolds on the 906/911R engines was an attempt at reversion containment.

With long duration, wide LC cams, big ports (for the displacement), and large diameter headers, these engines suffered low intake velocities and major reversion issues which affected drivability & torque below 5500 RPM. Moving the throttle away from the intake valve and lengthening the whole intake path helps contain this undesirable phenomenon.

We see this all the time on the dyno as it manifests a cloud of fuel vapor above the stacks from 4K to 5500. Extending the intake stacks higher (when there is room) really helps as does adding additional spacers between the manifolds and the heads. Further, using a better header with longer tubes (and sometimes smaller) helps improve exhaust velocities toward reducing the problem. I've seen 20-25 lbs-ft gains from these measures.