Low profile intake manifold tuned for the 3rd resonance

[ptuomov]

Any clever ideas for a low-profile intake manifold? Tuned to 3rd resonance at about 7000 rpms. Needs to be able to handle boost. The hood line constraint very tight, so it has to be low profile. Yet, I don't want any short-radius turns in the runners. Also I don't want to do a simple cross-ram manifold, because the uneven firing order would lead to resonance problems. Could do either ITBs close to the intake port or single/dual throttle bodies at plenum entry/entries, the current plan is ITBs. Do need a manifold and not just an air box with filter because this is fed with turbos.

Here's a photo of where the manifold would have to live, the bore spacing is by memory 122m which is close to that of the BBC.

EngineNoManifold.jpg

[nickmckinney]

I wonder how far off a 4.6 2V Edelbrock Victor would fit.

[ptuomov]

Far off. The Mod Ford has 100mm bore spacing while the 928 has 122mm bore spacing. The 928 has American big block dimensions.

[BrazilianZ28Camaro]

You're concerned with wave tunning on forced induction engine?

The boost pressure offset an non-tuned runner lenght by far.

You shoul'd be concerned with fuel distribution in first place.

You could build a intake with short runners and a big plenum feed by the front/center ,just like the Alien intake NRE build.


JMHO

[nickmckinney]

We have a project where we sawed this Victor in half to make it fit a wider engine, the motor needs a larger plenum anyway and we are fabbing a custom sheet metal upper elbow for the throttle body. The square bore holley pattern plenum will then be rectangular. With the ports at what look like a flat plane its the closest intake I know of, you are going to have to fab no matter what for that motor IMHO.

[ptuomov]

You're concerned with wave tunning on forced induction engine?

I am concerned about wave tuning on all kinds of engines, boosted or not. Why wouldn't the waves matter in turbo engine? Now, with turbos there's one more degree of freedom (manifold pressure can be chosen) so it's more complicated.

The boost pressure offset an incorrect runner lenght by far.

I agree that one can boost over many flaws. But it would be nice to maximize the potential.

My impression is that a lot of N/A race motors are tuned for the 3rd harmonic whereas at least here in the US high powered turbo cars seem to have short runners that look like N/A runners tuned for the 4th harmonic. Or maybe the 3rd harmonic but at a high rpm. If I am correct, I don't know why that is. Let me speculate, however:

The resonance frequencies change with density, pressure, and temperature. When air is compressed, the density and pressure effects cancel, and only temperature needs to be considered. There's some effect on the optimal runner length from the higher temperature of boosted charge, but I believe that's minor. Let's ignore this.

Friction and losses from small radius turns appear more significant with denser charge in the sense that it's relatively more important to make the runner paths as straight as possible with a boosted engine than with a N/A engine, even if that means compromising a bit on the runner length. Things seem to fall apart real fast in boosted applications if the air speed gets too fast for what the intake port and runner can handle. That's pure speculation, can anyone confirm?

For a N/A engine, runners are tuned for the mid point of the used rpm range. For a turbo engine, one might do better maximizing the dual torque peaks right before the boost threshold rpm and at the very high end when the engine is running out of turbos. Basically find out the point where the selected turbo work the most effectively and then tune the intake to work poorly at that point. Conversely, design the intake to work well at the points where turbo doesn't work that well. The turbochargers can fill in any torque hole in the mid range. That's one strategy. I can't think of an intake design without moving parts, however, that works well under 4000 rpm and then great again above 6000 rpm while not doing much in the 4000-6000 rpm range. Any ideas?

Finally, if the turbo engine is intercooled and intercoolers have excess capacity, it sometimes makes sense to "detune" the intake. That is, intentionally design the manifold such that the charge density drops in the runner. Why? Just turn up the boost to compensate. What's the benefit? One can get hotter air to the intercooler, which means more heat is removed. Then the temperature drops further in the runner as the pressure drops. It's like the air-cycle air conditioning in air planes. For knock-limited engines, this can result in same mass in cylinder with lower temperature, which shifts the knock constraint. This is what the new 911 Turbo successors do, I believe.

And all these things have to be taken into account while dealing with space constraints. So far, the best idea I've come up with is to tune for the third harmonic at high rpm where I am other wise running out of turbos. This means shorter than typical runners.

Still, I am very, very intrigued by those second order harmonic Honda four-banger manifolds that Joe McCarthy has posted about. If I could get straight enough runners, wouldn't say 7k rpm 2nd order manifold work better than anything else?

You shoul'd be concerned with fuel distribution in first place.

The injectors are in the intake port. With any sort of non-zero runner length, fuel will not be blown out of the runner. So fuel distribution will be automatically correct as long as air distribution is equal between cylinders.

You could build a intake with short runners and a big plenum feed by the front/center ,just like the Alien intake NRE build.

The space constraints under the hood are such that I can't do a front feeder. I need to use a rear feeder or dual side feeders.

[MadBill]

I'm with Brazilian. Boost is overwhelmingly more effective than pressure tuning. Also, over a wide powerband the tuning benefits of runner length cancel out and as well the resulting torque peaks and valleys will make for driveability issues on a high-power application. Better to just go with very short runners and a 'whatever fits' plenum.

[BrazilianZ28Camaro]

Lets add into the equation, the exaust reversion at overlap;

How will it interact with the wave tunning on several RPM bands

I believe the most important aspect in regards of forced induction intake is, make it with generous ports, and a plenum big enough to equalize the internal pressure to improve air distribution to all cylinders.

You could design the same short runner T-ram style intake, but with a rear feed, if theres room to it.

[ptuomov]

I'm with Brazilian. Boost is overwhelmingly more effective than pressure tuning. Also, over a wide powerband the tuning benefits of runner length cancel out and as well the resulting torque peaks and valleys will make for driveability issues on a high-power application. Better to just go with very short runners and a 'whatever fits' plenum.

Well we know short runners work with a turbo, but does it maximize the engine?

I can smooth out any natural torque peaks and valleys in the mid range by compensating with the manifold pressure.

Right now, I am running the stock manfold which has the following runner lengths, including what is inside the head:

Inside cylinders: 12 inches + 4 inches in the head = 16 inches total runner length
Outside cylinders: 8 inches + 4 inches in the head = 12 inches total runner length

For some reason, the factory made the inside cylinder runners longer. The story doesn't tell why.

[ptuomov]

Lets add into the equation, the exaust reversion at overlap; How will it interact with the wave tunning on several RPM bands

I am not going to pretend that I understand all this, but here's my impression of what's going on.

I think that the most important aspect of intake wave tuning is the pulses in the intake runner between IVC and IVO. When the piston is near BDC with the intake valve open, it starts to generate a pressure wave in the intake port and slowing down the flow thru the port. The optimal IVC timing is when the flow at the intake valve has just stopped and is about to reverse, which is also the time when the intake port pressure is pretty high. After the intake valve is closed, the pressure wave travels back and forth in the runner, flipping signs every time at the open end of the runner. Each time the wave goes back and forth it become weaker. The goal I believe is to have a high pressure wave arrive at the valve right at IVO event. If you tune for the third harmonic, I believe that this means that you tune the runner such that the wave travels the length of the intake runner 12 times between IVC and IVO. If you tune for the second harmonic, then it's eight times.

Now, what happens on the exhaust side matters to the optimal IVO timing. Basically, you want to open the intake valve immediately when the pressure in the combustion chamber is lower than the pressure in the intake port. Whatever happens on the exhaust side will therefore have an indirect effect on the tuned intake runner length. If the exhaust back pressure is high and making the combustion chamber pressure high, then you want to delay the IVO. Delayed IVO in turn means that the intake wave should be timed to arrive later, and that in turn means that the runner should be longer.

However, the question remains how much longer. I think the answer is "not much." Suppose that the length between IVC and IVO is say 440 crank degrees. Suppose further that something that we do on the exhaust side makes it optimal for us time the IVO 5 crank degrees later. That's 1.1% increase in the tuned runner length, which is basically nothing compared to all the other errors and omitted factors in my computations. That's why my conclusion is that what's happening on the turbo exhaust side doesn't really matter for the optimal runner length.

[ptuomov]

Just as a counter example to conventional wisdom (and my own thinking) that turbo cars should have short intake runners...

Here is photo of the Peugeot 205 Group B turbo car that made 700+ hp from 1.8L engine in 1989 Pikes Peak I believe:



Those runners must be tuned to the first harmonic at peak rpm?!



Here's another configuration. This looks like it has also very long runners, but still much shorter than the engine configuration in the first photo.

How do we explain these?

[SWR]

I've seen the intake length / tuning be the deciding factor if a drag engine would manage to spool a turbo or not... so saying that tuned lenghts do nothing because you have boost... ...you have a denser medium, which means you have a much larger difference in potential power (trapped charge) if you're on a pulse giving you an additional 8 psi or in the valley of one (subtract 8 psi)... that's why turbos are usually run with shorter intakes. You want it to tune at 3rd or 4th pulse, so that the "valley" between 2nd and 3rd will not show up smack in your peak hp register..

But first.. negate the backpressure to 1:1 or so at your peak hp rpm. Then you can tune it.. a 3:1 backpressure will kill all tuning at the first hint of IVO, you'd just dump exhaust all the way up to the throttlebody..

[SWR]

Here is photo of the Peugeot 205 Group B turbo car that made 700+ hp from 1.8L engine in 1989 Pikes Peak I believe:



Those runners must be tuned to the first harmonic at peak rpm?!

I guess second pulse, at about 7000-7500rpm...

[Warpspeed]

It depends on the intended purpose.
A street engine needs good low down turbo response, and long intake runners can certainly help with that.
If it doesn't make good torque off boost, there isn't going to be any exhaust flow down there to start the fun.

If it's a race only engine, something like a standard single plane manifold with an air hat and two throttle bodies, one going in from each side, will be about the best you can do with very limited space.

[BrazilianZ28Camaro]

I'm not a turbo expert by any means, but I think the maximum achievable gain in power with intake ram filling would be in the 6% range on a N/A engine.

I also believe a straight shot, correct shape designed intake runner under a generous plenum on a turbo engine will offset that gain by far.

The longer runners in the 1.8l engine just proves me forced induction makes power, no matter what intake is used.