Single Vs Dual Plane Manifold

[inline6]

I have an inline 6 engine currently with a single plane intake manifold where the 6 runners are fed from a common plenum.

I have heard that dual plane manifolds make more lowend power and single plane manifolds make more topend. Why is this?

I have heard that a 6 cylinder with two plenums is perfect for a helmholtz resonator and wonder if this is the reason?

 

[GregLocock]

Last bit first - not really. You can tune a helmholtz to any frequency.

Your typical log type single plane runner doesn't see much of a VE boost from the runner lengths, but if you make the runners long enough you will see a boost. The falcon I6 had switchable runners to give good torque and good power, swithcing at 3200 or 3500 rpm.

Cheers

Greg Locock


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[NormPeterson]

I have heard that dual plane manifolds make more lowend power and single plane manifolds make more topend. Why is this?

Most single plane vs dual plane discussions that I've ever heard were concerned with V-type engines where a dual plane manifold limits each cylinder to having access to only half of the rated carburetor flow. A separate effect is that lower plane of a dual is typically forced by hood clearance to have more tortuous paths from carb to head port, which wouldn't apply to an IL-6 with the intake split into two "halves".

I have heard that a 6 cylinder with two plenums . . .

Suppose you were to draw the intake flows for all six cylinders as positive half sine waves, each stretched out over a bit more than 180° along the X-axis and all plotted in the proper phasing on the same chart. Then consider the instantaneous combined flow that would result from various manifold arrangements and the engine's firing order.


Norm

 

[inline6]

Does a dual plenum not seperate some of the cylinders from each other to reduce the negative impacts that some cylinders pressure waves can have on others?

The BMW S38 had a valve to change the plenum/runner characteristics

http://www.bmw-m.net/techdata/m5engine.htm

the wording seems to suggest to changed the effective length of the runners but from the graphics i dont see how the length changes. r does it work by isolating 2 pairs of 3 cylinders to create a stronger helmotz effect?

 

[BrianPetersen]

For an inline six, there is not normally a "dual plane" manifold.

A "dual plane" is V8 terminology. It's like that because one section of the manifold joins the front and rear cylinders of one bank with the middle pair of the other bank (which are the even-firing group of four) and vice versa. This can only be done practically, by having one set of runners on top of the other, hence the term "dual plane".

With an inline six, if you want to split the manifold, you join the front three together, and you join the back three together. It's not a "dual plane" because they're not on top of each other. This avoids cross-talk between adjoining intake strokes. Whether that is a good thing to be doing or not ... is hard to say, and probably varies depending on RPM.

The other choices are to use a plain log manifold, or a long-runner manifold with six separate runners from a single plenum like the old Chrysler Slant 6 used.

 

[inline6]

i am wanting to use a switchable system if there is merit in tuning a dual/twin plenum setup for low to mid rpm and then it will act more as a conventional single plenum for topend.

 

[patprimmer]

The only advantage into splitting into 2 plenum's is that you can have 240 degrees between each intake stroke so long as every second cylinder only is connected to the same plenum. The viability of that of course depends on the firing order.

The advantage is a stronger pulse signal at a carburettor discharge nozzle to get fuel flowing out the main metering system faster. Of course individual runner manifolds with one carby choke per runner take this to the extreme and you get very heavy pulsations, reverberation and fuel stand off effects.

This has the disadvantage that only half the carby capacity is being used as only one cylinder sucks at a time.

If they all draw of a large plenum, all the carby capacity feeds each cylinder as it sucks. This of course also depends on runner CSA and plenum volume and runner volume.

As EFI meters fuel electronically, this is not such an issue, but it depends on how the EFI senses airflow. Really strong pulsations confuse MAP sensors and even MAF sensors I think unless the MAF is at the feed into the plenum or the MAP sensors have their own restriction and separate small plenum to act as a damper.

I think this technique is more applicable to carby systems. Any good EFI system uses tuned length individual runners off a plenum which when compared to a carby, has a relatively large throttle body controlling airflow into the plenum. EFI can use a relatively large throttle plate as it does not depend on airspeed through a venturi to draw fuel into the engine.

Regards
Pat
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[Lou Scannon]

Assuming air/fuel ratio is properly sorted out among cylinders, the differences between common vs separate plenums (& assuming separate plenums are serving cylinders that are equally spaced in the firing order) are really only significant at WOT. My understanding is that pulse tuning is more effective when the pulses are distinct (240deg separation) rather than overlapping (120deg separation). Pulse tuning can increase cylinder filling, in a narrow rpm band (at other rpms, the effect may be minimal or negative). Of course, if the effective port length can be changed on the fly, the improvement can be realized in multiple rpm bands.
At less than WOT, pulse tuning doesn't really matter, since the operator is not asking for maximum power anyway.
In addition to pulse tuning, the inlet ducting, throttle body, plenum design as well as valve events & exhaust system design all play important roles in determining the engine's volumetric efficiency; so if you are desiring to change the engine's breathing characteristics, you really need to approach it as a system.

 

[inline6]

would twin plenum help alleviate some of the nasties from a 300+ camshaft?

 

[patprimmer]

I don't see how. A hell of a lot depends on other details of manifold design. I had a VW boxer that ran a 310 deg cam in my wifes shopping car. It was fine with zero nasties.

Another was a six cylinder Holden and unless you really knew how to drive it You took of in a wild wheel spinning screeching cloud of smoke or it stalled or lost a cylinder or two. Both where IR manifolds with Webbers with one choke per cylinder

Regards
Pat
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[NormPeterson]

If you can figure out just where to put the valve (or valves), it is possible to fatten up the midrange. Mazda's KL-series V6 had two valves and three switch-over points. If a solenoid controlling either valve went inop you didn't have to be all the way at wide-open throttle to tell.


Norm

 

[BrianGar]

If your running a variable geometry manifold on an engine that spends a lot of its time at high rpm(Is this a racer?) then all the internal flaps and redirect runners do nothing for you but get in the way of airflow, and add weight.

If you are tuning an inline 6 Bmw engine for performance, the V. geometry intake is the first part you throw in the bin.

Before you go pulling your hair out studying intake design, intake pulses, runner lengths, plenum volumes, cam timing, runner cross-section, reversion, and possibly spending a lot of money, you need to know one thing.
Its all a compromise.

You need to figure out at what revs your engine spends most of its time at, and tailor to that specific area, and match your transmission to that also.
'Generally speaking' different Intakes dont 'make' torque. They are used to move it to a particular point on the rpm range.
Be it high up, or low down.
V.geo intakes try to do both - ideal for all rounder engines, but not so ideal depending on what your idea on an all rounder engine is.

One last thing Id suggest you do is find someone with an Inline 6 Bmw with a V. geometry intake - nearly all the newer ones have them.
Drive it around a bit.
Now, tell a friend to go and either disconnect, or leave the V. geometry mechanism connected - so you dont know which he has done.

Drive it again, do this a few times and see if you can pick out the trip on which it was disconnected...you may not notice the torque shift vs rpm range at all.
At a guess, its only going to be a shifted difference of about 200rpm.

Im in no way telling you not to study it all as its very valuable information. But, its also important to realize the real world differences too.

Another thing Id advise you to read about is torque and rpm, and how they fit into the whole horsepower figure.
Its amazing how many dont know the relationship between the three, or where the hp figure comes from.
Im sorry if you have already knowledge of this - its hard to tell from a few posts,


Brian,