Plenums.... why bother?

[willwal98]

opps, sorry. Updated

it is more of a lower rev thing. At high revs the pulses are that close together it's pretty much a constant flow of air anyhow.

[oldcorollas]

at higher revs, the pulses get much much stronger
so while the flow thru the TB at the front of the plenum might be nearly constant, the extra energy in the runners themselves makes it more complicated

[river]

Hi,

river: at maximum power output - yes, a short, straight distance between throttle and valve is ideal. But at off-throttle/idle this setup is less than ideal.

As some chick used to say..... please explain?

seeyuzz
river

[indeecent]

As most have said, plenums do have an effect on the whole effect of the intake...

mainly by the pulses (oscilations) caused by the valves and filling the spaces

there is a rough guide to this tuning...

from memory (good idea to double check it all) it was 34cm for peak power of 11000rpm, and you take 2 to 2.4cm from that length for every 1000rpm below that

so for 6000rpm peak power the runners should be close to about 22-24cm long

the size of the plenum for ther largest gain comes down to what boost its running etc etc, as the idea is to fill the cylinders while its at a low pressure point (correct if im wrong)

how we got ontot runners while looking at plenums is beyond me, i think of them as two very seperate devices placed next to each other

[Screamn_Sleeka]

So does this all me ITB's arnt the shizzle or what?
Facts and "science" aside, isnt the atmosphere the best storage of air.

[oldcorollas]

indecent, as rpm goes up, the time between intake events decreases, requiring the runners to get shorter, not longer.

screamin, when you have boost, atmosphere is not the best storage

the discussion here is if a plenum or extractor style tube is best for boost... no?

hmm... so what do the higher echelons of turbo racing (indy/chap.. whichever, Lemans, old F1) run for their turbo systems?

[thechuckster]

As some chick used to say..... please explain?

i don't own a fish-n-chip shop, so here's my take on this...

with very short runners between the throttle-plate and intake valve - there's no-where near enough air to fill a cylinder - so at cruise/idle when you're at very high vacuum levels, each 'induction event' will cause the air-pressure in that small runner space to dramatically jump around - in terms of vacuum level (or manifold absolute pressure - MAP if you want to be geeky).

If this is on an EFI engine using a MAP sensor, you have significant pulses/variation in MAP levels during cruise/idle making it harder to tune the car.

This highly variable MAP makes it harder to tune quads or ITBs for low-speed cruise or smooth idles..

You usually overcome this pulsing on EFI cars by taking a MAP source from each throttle and joining them together before feeding to the MAP sensor. I don't know how dual carbi's deal with this, but i suspect you run some kind of balance pipe between the two carbis

...



would you like salt with that?

cheers,
Charles

[Screamn_Sleeka]

sorry, i only speak N/A... so the plenum idea only for boost?

[thechuckster]

no - both ... unless you're mostly driving at >75% throttle

[indeecent]

oldcorolla's :

indeed, i seem to have written it wrong , intake isnt my speciatly, more of an exhaust side man...

taken from http://sdsefi.com/techinta.htm

Runner lengths can be adjusted within the space constraints to help boost torque within the desired range. Short runners are good for high rpm torque as in a racing situation. Long runners are more applicable for street use at lower rpms.

sorry for the misinformation

[Screamn_Sleeka]

sorry if this is getting off topic, but is this similar to the "ram tubes" "velocity stacks" etc, on carbies, like DCOE's which is the closest thing to ITB's. The Long ones are sold as racing and the short ones are sold as street, is this jsut a con then?

[oldcorollas]

sorry if this is getting off topic, but is this similar to the "ram tubes" "velocity stacks" etc, on carbies, like DCOE's which is the closest thing to ITB's. The Long ones are sold as racing and the short ones are sold as street, is this jsut a con then?

yes.

the length of trumpet needed depends ont he manifold and carb length also, as well as the desired operating range of the engine....


anyway, someone had to link this eventually...


and what they said about it...

Intake and Packaging
Since this engine has four turbochargers, there is a lot of air to force down into the intake tract. Moran is trying some reverse technology to fill the cylinders, "Obviously, we wanted an unrestricted, even flow of air into the engine, but also wanted to keep the hood as low as possible. To do this, we just looked at airflow systems that work in tight spaces--and what I saw were exhaust headers. Our intake looks similar to an exhaust because I took the basic design of a tubular exhaust header, reversed the airflow, and starting making changes to optimize the flow. We don't know if it'll work as well in real life as it looked in testing, but at least we're trying something different.

[willwal98]

as oldcorollas said yes it is a similar theory. You adjust the length of the ram tubes for your rev range, cams etc.

And I gotta say, damn you have some rep....

[Joshstix]

Just as a side note that Pacer with the slant six up the top is running a proper plenum these days I believe. It has been features in a mag more recently than that issue of zoom.

[thechuckster]

i wonder what a slant 6 with EFI is actually like to drive - i learnt to drive in a manual valiant with a slant six (which was eventually replaced by a 265 hemi) ... very agricultural motor...