Someone tell me why not - Intake Plenum

[sightless]

I removed the plastic cover over the front bank this afternoon to have a glance over, and come to think of an something.


Why must the intake pipes go over the rear bank? could they not go up and over a little and go over from there?

Following is an example of my pro paint skillz



I'm thinking that the size of the runners (right term?) is measured to a required length., although I've only heard people mention that it was overly important in relation to NA cars.

1) My first thought is that it would open op the intake a bit by not having such a nasty kink right before the TB
2) Make the the rear spark plugs more accessible
3) Be able to show off cool blink like Glys cylinder heads

[bradc]

No real reason why not

[Turbo_Steve]

No real reason why not

Going to have to disagree there:
Inlet plenum and runnels are a very specific and complicated design: we're deep into fluid dynamics here, but essentially each runnel will be a 'tuned' length and width. The runnels themselves, and the inlet plenum, are designed to total a specific volume and to flow in a particular way. This is for two reasons:

The first is more NA: it controls the flow into the cylinder. However, even at pressure, if the runnel design does not flow correctly, you will be restricting the engines ability to breathe, and ergo spool and eventual power will be compromised. The ideal runnel will maximise the airflow speed into the bore: it is this that has the maximum effect on inspired volume.

The second reason is more complicated, and potentially, more devastating.
In certain circumstances, the valves closing on each chamber set up a "reverse pulse". This is much the same as you snapping the throttle closed, which requires a dump valve to relieve the pressure to protect the turbo. However at the inlet plenum, this isn't required as it's essentially a sealed box. However the reverse pressure wave setup by the valve closing can reduce pressure at the other valves (think of a bow wave behind a boat). The runnels will be designed to surpress this, and the chamber to disperse the pressure wave.

How important these two factors are depends entirely on the usual million variables around an engine, but it's worth bearing these thoughts in mind before you go and fab up any old box and wonder what happened to the bottom end torque

[bradc]

Quite a few EVO's and Skylines run similar plenums to what sightless is proposing, short little runners going into a bit cylindrical plenum, I can't imagine that all of those would be perfectly designed.

[scott.mohekey]

I'm not 100% sure on this, but the cars with modified plenums/runnels will be aimed at producing high power, so low end torque is probably not as important. From what I've learned, shortening the runnels tunes them for higher rpms.

[Oblivion]

I've often wondered about this too. I dont suppose its actually been tried before?

[Davezj]

i found a great site to explain all of this but the link is at home at the moment.

it was aimed at the supra owners but it was still a turboed car. It seemed to suggest from what i remember that the style of plunum they would recommend (note the choice of words used, "they recommend") is effectively half an isosolies triangle with the throttle body coming in at the wide end of the triangle and the inlet manifold conections on the long 90 degree side.

don't pick me up on the cylinder number it is just an example and a crappy drawing and igrone the dots they are just for spacing.


........../|() Cyl 1 inlet
......../..|() Cyl 2 inlet
....../....|() Cyl 3 inlet
..../......|() Cyl 4 inlet
../........|() Cyl 5 inlet
/______|() Cyl 6 inlet
( )
Throttle
Body output

[Turbo_Steve]

smohekey has hit the nail on the head: those are pretty much all big power Evos (Where the plenum starts to be a restriction). Shorter runnels will reduce the pulse surpression and ruin the cars low / off boost manners. fine on a race or drag car with mental cams. Pretty useless on a road car.

[stuey]

I've seen on quite a few cars with shorter induction tubes or direct throttle bodies per cylinder to increase power, would all of these increase top end hp at the expense of low end power ? How about on many V8's where the plenum sits atop the V with very short runners ?

Just curious as they do seem quite long from the throttle body to the inlet compared to other engines, or is this design why low end torque is good on the 6A13.....

[Davezj]

Here is the site i was talking about.

i think someone else posted the link up on here about a different topic. makes interesting reading

http://www.max-boost.co.uk/max-boost...kemanifold.htm

[Goku]

Tyler has that design from the first post on his VR-4, I'd say down at the lower end of the rev range it has no power what so ever.

[bradc]

But then he has a massive turbo on his. I'd be happy to trade a bit of power below 3000rpm with a bit more power from 5000rpm upwards.

[Goku]

yea, but I would say even a non turbo galant would take him before he hits 4k

[sightless]

This is all theoretical of course, I cant make anything along these lines

I'm confused turbo_steve, your saying in your second example that when a valve closes on the exhaust side it creates a small bit of back pressure that can go over to another cylinder?
Whereas with the longer runnels its not so easy for it to go up those and back down another? where does that pressure go normally? although I imagine its not very much pressure.

BOV on the plenum?

On another note, I understand quad thottles have trumpets inside the plenum, and the length of the trumpets has an effect, would that be a way to change the low down power loss by tuning trumpets into the plenum? and also create some of this wizbang fluid dynamics? (lets not discuss how to tune test that though just yet)

[scott.mohekey]

It's when the inlet valves close, not the outlet (exhaust). When the inlet valves closes, the air flow is stopped abruptly, which causes an echo pulse to run back up the runnel.. kinda like sonar, but not.

[sightless]

ah inlet sorry, that's what i was thinking but the fingers walked the wrong direction.

If you were to "aim" trumpets in a direction?

[Turbo_Steve]

smohekey again steps into the breach (thank you sir) with his correctnesses

Essentially the length of the runnels allows a pulse from the inlet valve shutting (which is a pretty powerful standing wave in a volume of air) to be delayed inside the runnel. Dependant on the design of the manifold, this is either to reduce the energy of the pulse, or to delay the pulse sufficiently to ensure it doesn't interfere with aspiration.

The shape of the plenum itself will usually be involved in this as well, though it's usually more of an issue on turbo'd cars than NA, presumably due to a lack of positive pressure behind the closed valve on an NA car....but I'm guessing. Notably, NA cars tend to have "squarer" inlet manifolds, whilst turbos tend to be more cylindrical, though this is only my personal observation and totally available for debunking (probably with great ease).

The heads, cams and inlet will all have been designed together to work in a particular way. If you're going for huge forced induction power, then a whopping plenum with wide, short runnels is optimal, especially if you have wild cams and a huge turbo: if you're going for big power, chances are you've already raised the rev limiter, and you aren't going to be mucking about below 4500rpms if you can help it.

Making big bhp is easy: it either blows itself up or it doesn't. Making big bhp civilized is difficult: Most deagsters cost less to build than the price of something like an RS6, but make way way more power. Many racing cars are cheaper and quicker than production Porsches.
Big power costs money. Big reliable power costs serious money. Big, reliable, refined power takes an international corporation millions of dollars to get right, and so they reflect that in the price tag of their vehicle.

[bow dn]

I removed the plastic cover over the front bank this afternoon to have a glance over, and come to think of an something.


Why must the intake pipes go over the rear bank? could they not go up and over a little and go over from there?

Following is an example of my pro paint skillz




I'm thinking that the size of the runners (right term?) is measured to a required length., although I've only heard people mention that it was overly important in relation to NA cars.

1) My first thought is that it would open op the intake a bit by not having such a nasty kink right before the TB
2) Make the the rear spark plugs more accessible
3) Be able to show off cool blink like Glys cylinder heads

Yeah dude i had the same idea make it easier to get to the plugs less pipe work too, i was trying to find a inlet manifold i could play with but not alot of people want to part an engine

[scott.mohekey]

Some manufacturers make plenums with adjustable runnel length. I'm not too sure how they work, but the idea is to address the fact that optimal runnel length is depends on target rpm.

There's an excellent article covering this stuff on the autospeed.com website. I forget what it's called but if you search for plenum I think you'll find it.