flubyux2

i was just wondering. the general rule of thumb seems to suggest keeping the intake plenum volume within 50%-75% of the engines displacment... so a 3.0 Liter should have a intake with a plenum volume of 1.5L-2.0L.

does anyone know of any downsides to a large plenum that is 125%-150% of the engine's displacment?

Matt scranton's celica drag car with a 3.0L had an intake with a plenum the size of a scuba tank... literally. it had to have been like 12L or something outrageous.

anyone have any info on this sort of thing?

0HP930

If by plenum you mean the common area of the manifold that feeds the intake runners then volume is not as important as the design allowing the runners to pull in all the air they need.

As long as the plenum is not restrictive and does not seriously disrupt the flow pulled into the runners, the length and size of the intake runners is more critical to the tuning and power output of the motor.

Using the inertia of the fast moving intake runner air to force air into the cylinder after the piston starts travelling upwards with the intake valve still open is where good induction systems can help an engine make extra power.

If your intake plumbing is too big and you didn't have any restriction problems to begin with then you are loosing power cause the slower flowing air will not provide the same induction tuning as a smaller and faster flowing system would.

So yes, there is such a thing as too big.

0HP930

A turbo can basically make up for a crap manifold by replacing port tuning with forced induction (as my own car is a testamony to).

Tuned induction on top of forced induction is the way to go though. Not only will it help fill the cylinders under boost but it also helps when off boost.

Its the main reason why I am switching manifolds on my 930.

https://www.tamparacing.com/forums/s...hreadid=199732

People manage to get over 500 HP from the stock 930 manifold but I am also concerned about improving my total power band and system efficiency.

figment

in a NA car size and design are much more important, its easy to go to big

and like OHP said a turbo will compesate for a too big of plenum

PoppaSmurf

theres a reason they call the veilside intake manifold for the MKIV "the surgetank"

Steve B

flubyux2

thanks kirk, yes that the one i was referring to.

well, i know that ideally, you want the port/runner volume to be at least equal to the total volume of the cylinder so that when the throttle opens, the necessary air is readily available and waiting to be fully injested.

and intake runner length is more suited to pulse wave tuning or resonant tuning whereas the intake runner diameter is more suited to flow volume and velocity... i know how those play into the whole power curve thing...

just wondering if a im designing plenums with too large of a volume... yes i do mean the chamber where the inlets of the intake runners begin.

the plenums im fixin to use have either a 4" ID or 4.5" ID, the intake runner diameters are kinda big, 1.75". the inlets of the runners are bellmouthed and radiused where they meet the floor of the plenum... then its welded together.

Sean... how fast was Joey's car when it had the man intake? was it sluggish off boost or in the low end?

PoppaSmurf

nope, the car felt torquey at all times....it made excellent low end power, but you gotta remember, a 60-1 is not a big turbo for that car, a 63mm or something of the like woulda been a better comparison