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I do not have a degree in fluid dynamics, but this statement does not match my understanding. My understanding is the percussive wave is created by the valve closing against the onrush of the air, the wave reverses direction there and travels back to the bell-mouth, which reverses it again and sends it back to ward the valve.
This is irrespective of whether there is a plenum or it is open to atmosphere.
At least that's what I understood.
Picture from those glorious Can-Am days... and how they ran inlet runners of different lengths to generate a wider power band (more useful in road racing).
Do not forget the motion energy of the incoming charge that is one reason why we over here in the old world prefer to tune the length of the intake duct
to the 3rd harmonic. From long time experience for a road going high performance vehicle I prefer to tune for a rpm in the middle between peak torque and peak power rpm.
The stock S4 intake manifold have runners of unequal length for the same reason, for a wider power band.
Åke
A very good explanation - yes!
And yes again, it is common practice to develop the intake runners and the intake manifold to perform best at an rpm that is between peak torque and peak HP.
The stock S4 intake manifold have runners of unequal length for the same reason, for a wider power band.
I completely agree. I did the same when I developed a 16v intake manifold for myself.
Well Carl knows about my side project; I am the only company in the world making a reproduction 964 3.6 engine case and the tooling is almost complete after a year. Basically, the only company that builds Porsche engine cases on the planet is Porsche....and me!
So I could easily do his runners. The money is in the tooling, clearly, but after that the raw casting would probably cost less than $100 my cost.
Here's an idea: cast some aluminum oil filler necks with proper breather ports. The plastic ones are hard to modify and don't always seal great. The early magnesium alloy ones are in short supply.
Originally Posted by Catorce
Well Carl knows about my side project; I am the only company in the world making a reproduction 964 3.6 engine case and the tooling is almost complete after a year. Basically, the only company that builds Porsche engine cases on the planet is Porsche....and me!
So I could easily do his runners. The money is in the tooling, clearly, but after that the raw casting would probably cost less than $100 my cost.
I have my hands full with the cases right now. The runners I could probably do, but there would need to be a demand for the oil filler necks en masse for me to want to tool for them.
I have my hands full with the cases right now. The runners I could probably do, but there would need to be a demand for the oil filler necks en masse for me to want to tool for them.
It won't ever make money for you, that's for sure. I did hear that various used part suppliers have waiting lines for those filler necks, though. Just an idea for a hobby project. Guaranteed it won't be a big profit opportunity.
Update on the intake manifold... all the mock-ups are nearly done in the computer and we are now starting to run the formulas to determine what would be optimum for your 6.5L motor. Then we will know whether we can achieve optimum results and still stay under the hood or not...
I was just told that we should be having that conversation in the next coupla days.
Fitting long runners to ITB's like Ake's isn't really much more difficult than fitting them to any flange. It's the same problem and the solutions are the same also.
A short-runner intake manifold or ITB system produces a less peaky torque curve than a long-runner intake manifold or ITB system. I don't think there's any question about that. That's why modern turbo cars often use short runner intakes, they intentionally want to remove the torque peaks since the engine is knock limited anyway. If anyone is confused about this, take a look at Simard's 7L ITB motor's torque curve -- it's almost perfectly flat because of the short runners. Louis Ott's ITB motor also has short runners and has the same flat torque curve. The reason to run long intake runners is to get the peaky torque curve peak at the rpm you want.
A word about terminology: Helmholtz tuning in intake manifold design terms of art is limited to manifolds where the plenum acts as as the resonating chamber. Given how pulses work, this means three or four equally spaced pulses per plenum. Flappy closed, the stock S4 intake manifold is considered a Helmholtz resonator intake manifold, flappy open it's not considered one. If you just run all eight pipes to a single plenum, it doesn't act like a Helmholtz resonator and you don't get the characteristic tuning peaks. For ITB systems or single plane intake manifolds that connect all runners to one plenum, the plenum volume doesn't really matter from pulse tuning perspective as long as it's not too small and as long as it distributes equally. That's because it doesn't act as a Helmholtz resonating chamber anymore when you connect eight runners to it.
The reason for running ITBs is that they allow you to run more camshaft overlap. For example, Ake's planned cams would run terribly at cruise with a plenum manifold fed from a single throttle. If you don't run big overlap cams, then you can do about as well with a single throttle manifold as with ITBs. And you shouldn't even think about running big overlap cams with exhaust manifolds, in a cross-plane V8 you need headers to run high overlap cams. So the system where ITBs shine has high overlap cams and long-tube headers.
I thought I should pipe in on this since I have done a lot of research on the intake configuration subject. First, Tuomo is spot on. What I wanted to accomplish is to get great torque above 4500 RPM. You also need a cam set with more lift and duration than the GT cam offers. Any intake system with a common plenum for all cylinders and a throttle at the inlet of the plenum will have rough idle and part throttle with the necessary cam. The volume of the plenum and length of the runners is important to prevent torque holes in the operating range. With ITBs, you do need to insure that the plenum is large in relation to the engine size, but that is all. Carl is correct that figuring out the air box and filter system is a bear. Probably the worst of all problems to deal with. The short runner aspect isn't as much of a problem as it first seems. The 6.5L engine can produce transmission destructive torque at around 4000+ RPM with any intake system. My criteria was to reduce the mid range torque and increase the above 4500 RPM torque so I could have a reliable and fun to drive 928. My runner length from the valve to the bellmouth intake is about 11". I can't remember what the 3rd harmonic resonance RPM would be but it is pretty high. The key is that you don't have to run RPMs to the resonance point to get good benefits. I made the inlets tapered with the maximum bellmouth 2x the TB size (50mm). That broadens the resonance effect while reducing the overall effect. I do get a beneficial effect from around 5500 RPM to my self imposed redline of 6600. I use the "skirt" of the resonance effect. The edges of my bellmouth fold over at the outer edge which really helps with inlet flow when the top of the airbox is close. My inlets are angle inward which allows for a straight shot to the inlet valve and permits a little longer runner and entire bellmouth isn't close to the top. You can have the airbox top as close as 3/4" without loss of flow although like Carl mentioned you can suck the top right down to the inlet. The curve of the rounded bellmouth with the lid close generates really high velocity as the air enters from the side and curves down into the inlet. This makes low pressure around the periphery of the bellmouth. I have removed the plexi cover altogether and use the underside of the hood as the top cover. Weatherstrip seals the edges of the airbox.
For my use the ITB is the only way to go. The airbox design and fabrication is the hardest to do. Tuning is lengthy and difficult at low throttle openings. I have run my ITB engine about 40k miles without a problem after initial setup.
Really good to hear from you Louie, glad to hear you still have your car and it's going well. Cheers Greg
Originally Posted by Louie928
I thought I should pipe in on this since I have done a lot of research on the intake configuration subject. First, Tuomo is spot on. What I wanted to accomplish is to get great torque above 4500 RPM. You also need a cam set with more lift and duration than the GT cam offers. Any intake system with a common plenum for all cylinders and a throttle at the inlet of the plenum will have rough idle and part throttle with the necessary cam. The volume of the plenum and length of the runners is important to prevent torque holes in the operating range. With ITBs, you do need to insure that the plenum is large in relation to the engine size, but that is all. Carl is correct that figuring out the air box and filter system is a bear. Probably the worst of all problems to deal with. The short runner aspect isn't as much of a problem as it first seems. The 6.5L engine can produce transmission destructive torque at around 4000+ RPM with any intake system. My criteria was to reduce the mid range torque and increase the above 4500 RPM torque so I could have a reliable and fun to drive 928. My runner length from the valve to the bellmouth intake is about 11". I can't remember what the 3rd harmonic resonance RPM would be but it is pretty high. The key is that you don't have to run RPMs to the resonance point to get good benefits. I made the inlets tapered with the maximum bellmouth 2x the TB size (50mm). That broadens the resonance effect while reducing the overall effect. I do get a beneficial effect from around 5500 RPM to my self imposed redline of 6600. I use the "skirt" of the resonance effect. The edges of my bellmouth fold over at the outer edge which really helps with inlet flow when the top of the airbox is close. My inlets are angle inward which allows for a straight shot to the inlet valve and permits a little longer runner and entire bellmouth isn't close to the top. You can have the airbox top as close as 3/4" without loss of flow although like Carl mentioned you can suck the top right down to the inlet. The curve of the rounded bellmouth with the lid close generates really high velocity as the air enters from the side and curves down into the inlet. This makes low pressure around the periphery of the bellmouth. I have removed the plexi cover altogether and use the underside of the hood as the top cover. Weatherstrip seals the edges of the airbox.
For my use the ITB is the only way to go. The airbox design and fabrication is the hardest to do. Tuning is lengthy and difficult at low throttle openings. I have run my ITB engine about 40k miles without a problem after initial setup.
I thought I should pipe in on this since I have done a lot of research on the intake configuration subject. First, Tuomo is spot on. What I wanted to accomplish is to get great torque above 4500 RPM. You also need a cam set with more lift and duration than the GT cam offers. Any intake system with a common plenum for all cylinders and a throttle at the inlet of the plenum will have rough idle and part throttle with the necessary cam. The volume of the plenum and length of the runners is important to prevent torque holes in the operating range. With ITBs, you do need to insure that the plenum is large in relation to the engine size, but that is all. Carl is correct that figuring out the air box and filter system is a bear. Probably the worst of all problems to deal with. The short runner aspect isn't as much of a problem as it first seems. The 6.5L engine can produce transmission destructive torque at around 4000+ RPM with any intake system. My criteria was to reduce the mid range torque and increase the above 4500 RPM torque so I could have a reliable and fun to drive 928. My runner length from the valve to the bellmouth intake is about 11". I can't remember what the 3rd harmonic resonance RPM would be but it is pretty high. The key is that you don't have to run RPMs to the resonance point to get good benefits. I made the inlets tapered with the maximum bellmouth 2x the TB size (50mm). That broadens the resonance effect while reducing the overall effect. I do get a beneficial effect from around 5500 RPM to my self imposed redline of 6600. I use the "skirt" of the resonance effect. The edges of my bellmouth fold over at the outer edge which really helps with inlet flow when the top of the airbox is close. My inlets are angle inward which allows for a straight shot to the inlet valve and permits a little longer runner and entire bellmouth isn't close to the top. You can have the airbox top as close as 3/4" without loss of flow although like Carl mentioned you can suck the top right down to the inlet. The curve of the rounded bellmouth with the lid close generates really high velocity as the air enters from the side and curves down into the inlet. This makes low pressure around the periphery of the bellmouth. I have removed the plexi cover altogether and use the underside of the hood as the top cover. Weatherstrip seals the edges of the airbox.
For my use the ITB is the only way to go. The airbox design and fabrication is the hardest to do. Tuning is lengthy and difficult at low throttle openings. I have run my ITB engine about 40k miles without a problem after initial setup.
Louie, 11 inches if intake tract sounds very short to me. That is about what we use for high spinning motorcycle engines. Looking at your power curve the optimum tuned (3rd harmonic) intake length should be about 15-16 inches. I know that is quite difficult to achieve under the hood but why are people so shy of modifying the hood?
Your air box looks very nice and you are all right that the air is entering the bellmouth from the side.
It seems there are two different schools regarding the design of intake system in the old vs. the new world. Over here I believe we are more familiar with individual intakes thinking of all the Weber carbs being installed on almost any high performance engine since the 50´s. Over there on V8-engines you are still using a single four-barrel carb with what we think a weird kind of intake manifold.
Åke
Louie, 11 inches if intake tract sounds very short to me. That is about what we use for high spinning motorcycle engines. Looking at your power curve the optimum tuned (3rd harmonic) intake length should be about 15-16 inches. I know that is quite difficult to achieve under the hood but why are people so shy of modifying the hood?
Yes, I agree, from when I've done the calcs for my intake manifold builds, 11" is on the short side.
Can you get away with it on the torquey 928 engine? Depending on the driver and his driving style, yes.
As to hood mods... we are designing two intake manifolds now for Adams' 928, one is the optimum intake with a thru-the-hood height allowed, and a second with everything kept below the hood. I should know soon what HP losses the computer models tell us we will suffer (if any) by staying below the hood.
To the hood itself: an owner could apply an add-on hood scoop or bump to the hood; or replace the whole hood with one already designed with the riser in it, which is what I am working on. I have asked Tom at 928 dreaming to design something that looks like it was designed with the rest of the car at the same time - something that looks like it belongs there... I am anxious to see what he comes up with. If I like it, I'm going to get quotes into having it made for us.
Here is one of Tom's designs with a hood riser in it that I don't think most owners would find offensive. I have asked Tom to limit the riser to no more than 2". Sitting in a 928 and checking some hood heights, I found I could see over a 2" increase very easily. Anyway, he will show me some new ideas soon, he said.
05-11-2017, 02:46 PM
