Plenums.... why bother?

[ed]

oh, and remotely on the topic (for completion) - an article on intake resonance and tuning...

http://www.v-eight.com/tech_forum/viewtopic.php?t=22

[ed]

copy and paste:

I am currently doing my PhD in the optimisation of automotive airboxes, and can confirm that this is one of the most frequently misundersttod aspects of airbox performance. Many believe that the airbox should speed the air up, to 'ram' it into the cylinders more quickly, but as your friend pointed out, this is not true.

<boring PhD mode on>

The airbox tries to slow the air down to as low a velocity as possible. This results in the air having a higher pressure, increasing the pressure difference between inlet and cylinder and so pushes more air into the cylinder. Apologies to those who know all this, but I am just chuffed that my studies are actually relevant to a discussion!

One of the other main difficulties with airbox design is to try to ensure that all the cylinders get the same airflow into them. If you look at the Lola T370 and The lotus 72 with the horribly ugly airbox, you can see that for the air to flow in the inlet and fill the 'front' cylinder pair it would have to change direction very quickly... this probably didn't happen and so the front cylinders probably ran rich because they didn't get enough air.

The ligier airbox posted by philippe charuest looks like a much better design - the air changes direction relatviely gradually and then feeds a plenum at the bottom of the duct. Although this would probably still cause the front and back cylinder pairs to run rich, the problem wouldn't be anywhere near as bad as the other two I mentioned.

<boring PhD mode off>

[ed]

http://www.v-eight.com/tech_forum/viewtopic.php?t=136

[kingmick]

mick - youve been neg repped mate! come on... quit being an armchair expert and actually contribute to the forums with information, or i will continue to neg rep you and start deleting your posts.

bullshit tease and carry-on dont hold water in this section any longer mate. pick up your game

LOL my comments in here were purely saying good thread, didnt think anyone had to add! lmao
Delete threads away and i can also help im way to busy to worry at the moment.

[river]

Hi,

Good stuff ed.... but alas I must spread the love.

However, I still have questions. Excuse my questions if they sound stupid, but this is an area where I have little knowledge.

If the air-box is designed to slow down the air, then wouldn't a super/turbo charger do the opposite? Or, is the purpose of super/turbo charging designed to increase air pressure, as opposed to velocity?

Why do people remove their air-boxes and put trumpets on their carbs (yes, I talk carbs here ), or some people just whack filters over the carbs without trumpets? I remember the older generation F1 cars having a stack of trumpets and no plenum.... but was this before they understood the dynamics of plenums, or (as mentioned) at high rpm (which the F1 engines would be doing for most of the race) such direct intake of air works better than having a plenum?

And now a really dumb question..... if, as you mentioned (via your V8.com link), that a redesign of an air box gave an air induction increase of 6.2%, then would some form of fan at the front of the plenum be able to increase the induction. We mock the "electric supercharger", but would some form of assistance actually provide a small increase of inducted air-mass and therefore provide a similar percentage of increased power?

seeyuzz
river

[Mr Revhead]

havnt read the whole topic...

but one thing iv picked up is ppl getting a little confused....
plenums are after the t/b
surge tank (toyotas name) are before, such as on the 20v engines
cant recall any stock boosted engines with either.
but then i havnt thought very hard about it

[willwal98]

And now a really dumb question..... if, as you mentioned (via your V8.com link), that a redesign of an air box gave an air induction increase of 6.2%, then would some form of fan at the front of the plenum be able to increase the induction. We mock the "electric supercharger", but would some form of assistance actually provide a small increase of inducted air-mass and therefore provide a similar percentage of increased power?

River: I think an electric supercharger would provide an increase, but i doubt it would be worth the cost the produce it and upgrade the cars electricals to handle it, compaired to a standard blower / turbo.

Mr Revhead: Not quite sure what you are saying there. If you are saying that there aren't any stockly boosted engines with plenums then there most certainly are. Not sure what the surge tank bit is, can't say i've seen one.

[Mr Revhead]

im saying i cant think of any, not they dont exsist
by surge tank i mean like a plenum but before the t/b

[BigWorm]

River, I think the air slowing down in the plenum would still be applicable to forced induction, as it's in relation to the air speed in the intake piping. As I'm sure you're aware the airflow will be faster in a smaller space, & as it enters the plenum, because it's a larger area, it will still slow down and have a higher pressure than say as it goes through the throttle body.
The air pressure would come down to the volume of air being delivered by the turbo/supercharger vs. the restriction of the air getting into the cylinder, whereas the velocity would be governed by the area through which the air travels.

As for stock boosted engines with plenums or surge tanks, I can't think of any motors that don't have them! All the F/I Toyota motors I can think of have single throttle bodies with plenums (1J, 2J, 7M, 3T etc) and the only multi throttle F/I motors I can think of are the Nissan RB26DETT and the GTi-R SR20.

[Mr Revhead]

hmm at what point does a large diameter intake become a plenum?
but nevermind theres obviosly some with, and some with out.,
4agze doesnt, nor 3sgte iirc. but, that doesnt really matter with the topic

[BigWorm]

Hmmm, 4AGZE doesn't really count as the throttle body is before the SC & intercooler & all the piping, the 3S certainly has what I'd call a plenum. I'm just struggling to think of what doesn't have one.
What do you mean by large diameter intake?

[RAd28]

well i think i found the answer in ed's thread.. regarding slowing the charge down to increase pressure, HOWEVER.. and there always seems to be a however... velocity MUST come into it at some stage, because you wanna get as much air into the cyclinder while the valve is open as you can... although perhaps this velocity comes with pressure difference... hmmmm.... i like that answer

thanks ed... btw.. 4 posts in a row? couldn't keep up with your self huh??

[Bitch]
and KIDS.. OC, KM, for grown men (i assume your both grown men) you bitch more then me and my little brother! and your hijacking my thread... start your own "flame" thread, and just go for it... bitch away... obviously more then welcome to post useful posts, even non useful posts... though try and keep the "i know something you don't know'" posts out, for the record, i saw nothing wrong with KM's post, OC seems to me your were just looking for a bite... KM if you have somethign useful to add please take the time to share your wisdom, or atleast hint whether or not we're in the right direction, doesn't take much long then any post you've already made, kids or no kids, if you were so busy you wouldn't bother with toymods at all.
[/Bitch]

[/END BITCH ALL]

[oldcorollas]

... although perhaps this velocity comes with pressure difference... hmmmm.... i like that answer

thanks ed... btw.. 4 posts in a row? couldn't keep up with your self huh??

ya think? velocity comes from pressure difference? how does that answer your original question of using extractor style manifolds?
heh heh, way to go.. make fun of the mod that just helped you

though try and keep the "i know something you don't know'" posts out, for the record, i saw nothing informative in KM's post, OC seems to me your were just looking to finally get some information out of KM in public... KM if you have somethign useful to add please take the time to share your wisdom, or atleast hint whether or not we're in the right direction, doesn't take much long then any post you've already made, kids or no kids, if you were so busy you wouldn't bother with toymods at all.

fuxed for you.
LOL@ hint

mate, if you weren't looking for decent answers, why start the thread in the first place?

[BigWorm]

well i think i found the answer in ed's thread.. regarding slowing the charge down to increase pressure, HOWEVER.. and there always seems to be a however... velocity MUST come into it at some stage, because you wanna get as much air into the cyclinder while the valve is open as you can... although perhaps this velocity comes with pressure difference... hmmmm.... i like that answer

Velocity & pressure are directly proportional, if the air (or fluid) slows down, pressure goes up, if velocity increases, pressure drops. When it comes to getting it into the cylinder, this is the job of the runner, & it's the runners area/size that will determine the velocity. I think this gives a good explenation of the plenum vs. the runners, the plenum is there to slow the air down & equalise the flow/pressure/volume to all the cylinders, & it's the runners that then cause an increase in velocity to get the air into the cylinder.

[ed]

note: 2 diferent types of systems must be considerd and defined...

1: the situation where the plenum sits behind the throttle
2. where the throttles are in the runners feeding from the plenum

in 1, there must a large enough volume for the aims previously mentioned to be achieved, but smal enough volume not to completely ass fuck throttle response

in 2, the ideal plenum would be atmosphere. infinitely large, damped, with maximal pressure differential to the port/chamer.

river, the reason people ditch air boxes on carbs and ITBs is to et direct access to amosphere, but often at the expense of cold air induction.

the trumpets are simply to reduced air duct termination losses, check here:

http://www.v-eight.com/tech_forum/viewtopic.php?t=23