964 Resonant Intake

[gliding_serpent]

Very interesting.

The 3.2 intake manifold also uses resonance tuning with the connecting pipe between the two intake plenum chambers (never thought of that before), in addition to resonance tuning of each intake plenum runner. What it lacks is the variable intake lengths.

Such resonance waves of pressure occur within a tube at any point between two areas of pressure change. So in the case of the 3.2 runners, it is between the valve and the plenum vacuum chamber, between the two vacuum chambers, between the throttle body and the AFM, and between the air box an the end of the intake snorkel.

I wonder if Porsche tuned the length of each component to synchronize at the same RPM? Does anyone know this?

[winders]

Originally Posted by winders

Electronically actuated but the flapper is opened or closed (depending on year) by vacuum.

Originally Posted by Steve@Rennsport

To be correct, electronically triggered, but vacuum actuated.

Steve,

Really?

Look up the word "actuate" in the dictionary. Here is one definition:

to put into action; start a process; turn on:
to actuate a machine.

It's easy to argue that the ECU of my race car "actuates" the flapper.

Also, "trigger" is a synonym of "actuate" and vice versa.

Would you argue that the switch on the dash of my race car that turns on the fuel pump does not actuate the fuel pump because there is a relay in between the switch and the fuel pump?? By your logic, the switch "triggers" the relay and the relay "actuates" the fuel pump.

[gliding_serpent]

Semantics police! I interpret what Steve says as meaning that electronics triggers/actuates/initiates/commences the event, but vacuum opens the valve (does the mechanical work).

[winders]

Originally Posted by gliding_serpent

Semantics police! I interpret what Steve says as meaning that electronics triggers/actuates/initiates/commences the event, but vacuum opens the valve (does the mechanical work).

That is EXACTLY what I said in my post original post..........

[panzerfaust]

Originally Posted by Costa P

pf, there have been various ingenious designs with infinitely variable length inlets from Audi, Ford (Tickford), BMW, etc

AutoZine Technical School



It seems though that their complexity & cost ultimately brought about their demise.

yes i remember those big cumbersome ones

here is the sexy beast guillotine trombones... mazda and other jpn manufacturehad a whole host of trick intakes during the 80's-90's there production cars. I had an mr2 equipped with one in the 8o's

[Costa P]

Originally Posted by gliding_serpent

Very interesting.

The 3.2 intake manifold also uses resonance tuning with the connecting pipe between the two intake plenum chambers (never thought of that before), in addition to resonance tuning of each intake plenum runner. What it lacks is the variable intake lengths.

Such resonance waves of pressure occur within a tube at any point between two areas of pressure change. So in the case of the 3.2 runners, it is between the valve and the plenum vacuum chamber, between the two vacuum chambers, between the throttle body and the AFM, and between the air box an the end of the intake snorkel.

I wonder if Porsche tuned the length of each component to synchronize at the same RPM? Does anyone know this?

Yes, quite right. The 3.2 manifold was the 1st generation resonant manifold on the 911s.

Yes, you will get pressure oscillations in a tube which connects two areas of pressure change. But, the pressure oscillations will only "resonate" when the two areas of pressure are exciting the air in the tube at the natural frequency of that particular installation

Example, those beautiful variable length trumpets will have pressure oscillations running up and down (between the air box and the inlet valve). The inlet valve excites the air and the air box reflects it back...etc etc

You need both ends of the pipe to be excited (at the right frequency) for the pulsations to reach maximum amplitutude (resonance).

The interconnecting tube in the resonance manifold is subjected to an alternating suck-suck, which at the appropriate engine speed, will reach maximum amplitude (resonance).

The air box can not contribute to "suck" from that end of the runner, it can only reflect the pressure pulse (some of the energy is lost and only a portion is reflected).

The spring-mass analogy (in my earlier post) elaborates on this.

[panzerfaust]

you can see cross over tubes on exhaust pipes too. newer bmw boxer motors have them as well as old gt40 crossing one pipe to the other bank to take advantage of the exhaust gas pulses lacking in a 90 degree crank verse race derived 180 cranks.

interesting that the 3.0SCRS 954 which was developed near the same time as the 3.2 which had more power with less displacement. it didn't have any elaborate resonance features and just reverted back to a system used previously. perhaps it was expected that the race team would redeveloped the motors and low end torque wasnt a high priority due to the low homologation numbers needed.

never driven a 954 (how many really have?) but some say its really hollow at low rpms.

[Bill Verburg]

Originally Posted by panzerfaust

you can see cross over tubes on exhaust pipes too. newer bmw boxer motors have them as well as old gt40 crossing one pipe to the other bank to take advantage of the exhaust gas pulses lacking in a 90 degree crank verse race derived 180 cranks.

....

they put 180º exhausts on the GT40 because the engine is a 90º V w/ 90º crank, therefore odd fire. In an odd fire motor there are always 2 consecutively firing cylinders on the same bank, this is where the American V8 sound comes from. The 2 consecutively firing cylinders on one side are bad from a performance perspective because they temporarily overload the collector on that side. Solutions vary fro the 180º exhaust seen on the GT40s and in current Nascar V8s to a simple H pipe crossover used on most modern street V8s

Ferrari and the new GT350 have 90º blocks w/ 180º flat plane cranks and are thus even fire engines, these do not benefit from the crossover pipe, Porsche flat 6 are also even fire w/ 180º block and 120º cranks.

[gliding_serpent]

So the 911 engine foes not benefit from a crossover design? Or less gain?

[Bill Verburg]

Originally Posted by gliding_serpent

So the 911 engine foes not benefit from a crossover design? Or less gain?

The benefit of a crossover on a 911 engine is for noise when dual mufflers are used. There is no tuning benefit.

When headers are used to enhance performance the performance benefit comes from 2 sources
1) flow improvement, i.e. reduction in back pressure w/ associated intake and engine management improvements. The goal here is to move the largest mass of spent fuel/air mixture in the shortest amount of time.
2) pressure wave management whose goal is to put a low pressure wave at the exhaust valve for the longest period of time while the exhaust valve is off it's seat. This only happens over a relatively narrow range of rpm and is affected by header configuration and intake and cam configuration. The use of mufflers almost always cancel the positive effects of this type of tuning. This type of tuning relies on regularly spaced exhaust pulses that are timed and are usefully far apart. At high rpm the pulse spacing/timing can get to be as big an issue as odd fire pulse congestion at lower rpm.

[Steve@Rennsport]

My experience on the dyno is that a crossover, whether its a common inlet muffler or a separate pipe, makes a significant improvement in mid-range torque without a HP penalty.

Positioning as well as diameter is critical.

I've seen this effect on everything from 2.0 to 4.0 engines since 1976.

[gliding_serpent]

Originally Posted by Steve@Rennsport

My experience on the dyno is that a crossover, whether its a common inlet muffler or a separate pipe, makes a significant improvement in mid-range torque without a HP penalty.

Positioning as well as diameter is critical.

I've seen this effect on everything from 2.0 to 4.0 engines since 1976.

My understanding is that the crossover (or h pipe) adds torque below peak torque, or that is the non 911 specific dogma. No real benefit in an engine that lives above this.

My understanding is that you also want it as close to the start of the collector as possible... and I am thinking that with our short secondaries, that is less of an issue.

Where do you like your crossover Steve? I was thinking of integrating it into my muffler, which would use 2 inch ID tubes.

[gliding_serpent]

Originally Posted by Bill Verburg

The benefit of a crossover on a 911 engine is for noise when dual mufflers are used. There is no tuning benefit.

When headers are used to enhance performance the performance benefit comes from 2 sources
1) flow improvement, i.e. reduction in back pressure w/ associated intake and engine management improvements. The goal here is to move the largest mass of spent fuel/air mixture in the shortest amount of time.
2) pressure wave management whose goal is to put a low pressure wave at the exhaust valve for the longest period of time while the exhaust valve is off it's seat. This only happens over a relatively narrow range of rpm and is affected by header configuration and intake and cam configuration. The use of mufflers almost always cancel the positive effects of this type of tuning. This type of tuning relies on regularly spaced exhaust pulses that are timed and are usefully far apart. At high rpm the pulse spacing/timing can get to be as big an issue as odd fire pulse congestion at lower rpm.

Crossovers extenuate high rpm sound correct?

The theory of the exhaust is the same as the intake, you are just looking for the reverse pressure waves. I am having Turbo Thomas in the UK make me custom headers to my specs, and I am specifying 31 inch primaries (which is common for aftermarket headers for our cars). I did the math using stroke, displacement, desired rpm for peak torque (near stock), cam profile, etc, and it came out to 31. The reality is that with a muffler, it really does not matter much as you say. This is probably part of why the original headers for 3.2 cars do just fine. But still, my intake is stock, and my exhaust will be tuned for the same/near same targets for peak torque... so I suspect it will be worth something.

What kind of blows my mind is the Bisimoto Pulse chamber muffler thread. Using blind end helmholtz resonators to increase flow expulsion is smart.

[Bill Verburg]

Originally Posted by gliding_serpent

My understanding is that the crossover (or h pipe) adds torque below peak torque, or that is the non 911 specific dogma. No real benefit in an engine that lives above this.

My understanding is that you also want it as close to the start of the collector as possible... and I am thinking that with our short secondaries, that is less of an issue.

Where do you like your crossover Steve? I was thinking of integrating it into my muffler, which would use 2 inch ID tubes.

There is no free lunch, a crossover like a tri y can broaden the torque curve but it is always at the expense of peak hp.

[Duc Hunter]

Originally Posted by Steve@Rennsport

My experience on the dyno is that a crossover, whether its a common inlet muffler or a separate pipe, makes a significant improvement in mid-range torque without a HP penalty.

Positioning as well as diameter is critical.

I've seen this effect on everything from 2.0 to 4.0 engines since 1976.

Would you see a similar effect from 2 headers feeding 1 muffler? My 996TT has 2 mufflers and a X-over pipe....my 3.2 and 3.6 have headers into one muffler (3.2 Fabspeed, 3.6 B&B).