What would the net effect be of enlarging the plenum size of a 2.5 N/A intake? What about changing the runner length?

James

 

If you got to big with tht runners, then your gonna loose air speed which will hurt performance. going longer will help torque. There is a plus to both. Companies spent tons of time and money trying to figure out what works best. If I where you depending on what else your doing, Get a MAF kit and upgrade your exhaust and do a port and polish, This will help everything flow

 

Got the MAF and added headers too. Was looking to change the cam to maybe the S3 cam by Milledge. As for the plenum, I was thinking of changing the plenum to one with a slightly larger volume and making the intake track only have 1 - 90 deg bend instead of the winding route it currently takes with the stock setup. From my old days with V8s they always preached anything to straighten out the airflow path wouldn't hurt. Thinking was since air has mass, any change in direction adds to turbulance and hinders flow.
I wonder what the effect no plenum would have? Just runners with a throttle body attached. Essentially an upside down tunnel ram.

 

Yes with your setup I would think honing and taking the bends out of the intake would be good. It will make less restrition. Try finding a used intake on E-bay and play with it.

 

I've got a spare one laying around along with an extra throttle body to play with. Maybe over the winter.... Milledge also recommended shaving the head about .01 to .02 to up the compression to true 10.5:1. Don't know if I want to do that though. My long term goal is to add a supercharger so I want to only do mods that will benefit me now and not be a hinderance later if I'm able to go for the gold and add forced induction. The usual party line is to sell and buy a turbo but I have an N/A sunroof delete in otherwise good shape. For what I'd spend extra (after selling) for a turbo I could add the charger.
Also half the fun is in the tinkering......

 

I agree I love messsing with my 924S. I doing a pefromance engine rebuild and adding a 75hp shot direct port nitrous setup. It will be fun

 

Here check out my baby

http://www.cardomain.com/memberpage/723719/1

 

Enlarging the plenum can delay throttle response if you get too carried away with it. ITB’s can solve that one and should be good with a blower too. If you really want a trick intake, you could look into making composite plenum with tapered runners. (Hell, you could throw in staged injection while you're at it!)

GO MOTOR!!!

 

Hrsteel, I'm going brain dead working here today. What's 'ITB'? So if an enlarged plenum delays throttle response would reducing it increase throttle response? Making it essensially an upside down header. The 4 runners with the throttle body on the 'collector'. What would the torque curve look like? The S3 cam would move the increased torque curve up a couple hundred rpm. Would the volume decrease then be something to still benefit the cam increase but maintain low end power?

 

ITB stands for Individual Throttle Body. Throttle response is typically related to plenum size because of the mass of air between the throttle blade(s) and the intake valves. As this volume increases, it takes longer for a pressure change at the throttle to reach the motor. Larger plenum -> less throttle response, and smaller plenum -> quicker throttle response. A larger plenum can damp out reverberations from the motor and increase mass flow through a restriction upstream. This is of particular importance with racecars required to run an intake restrictor. ITB’s allow for the use of a really big plenum for power, without sacrificing throttle response.
Making an “upside down header” is probably not the best way to go. The relatively small intake coupled to a nearly non-existent plenum would choke itself to death at right about idle. Off the top of my head, I would guess that a reasonable plenum size would be around two liters or more.
One of the most important things in intake design is runner dimensions. As runners get longer, low end power tends to improve. The reverse is true as they lengthen. Shape is important too: Straighter is better; make it easy for the air to find the motor.
Regarding your particular set-up, I can’t give any hard answers because I don’t know the profile of the cam in question. It sounds like a pretty mild cam that shouldn’t be too finicky about intake mods. Hopefully I’ll have some better answers when I get a motor up on a dyno.

 

HRsteel, I checked out your GATECH web page and thats is awsome wish I could help you guys.

Anyway so The longer the runners the more low end torque, I wonder what would happen If I made runner spacers that go between the runners and the head, maybe about 1" long with a swirl pattern to help mix the air/fuel right before it gets into the head, Since the injectors are attached to the intake manifold on my 87 924S, I wonder if it would mess things up moving the injectors a inch back?? Would it screw up injector timming??

 

When I spoke of the 'upside down header' design, I meant to say the runners would be the same length as stock, just reworked to come together like a header with runner volume staying the same. The difference would be plenum volume maybe reduced to half stock. I've heard it said the runner length is tuned to a frequency of 1. Is this 1 cylinder volume=1 runner volume? The thought was to bend them into a gentle S ending up facing forward. This would reduce the abrupt 90 deg or sharper turn going to the cylinders, especially number 1.

 

fast924s, adding a spacer to lengthen the intake runners probably help Volumetric Efficiency (VE) lower in the motors rev range. Although you would have more air getting to the motor, you would also have to increase fuel flow to extract the benefits form the additional air. The injector placements shouldn’t have too large an effect so long as they remain mostly aimed at the back of the intake valves. A big part of the reason that the 2.5L intake is shaped as it is has nothing to do with the motor: Porsche wanted the engine bay to be as short as possible to improve aerodynamics. Thus, they basically had to smoosh the intake flat to keep it from popping out of the hood in an “unsightly” bulge. Adding an inch to the bottom of it may cause it to interfere with the hood. (Not sure, but I would measure first.) Notwithstanding any of the preceding challenges (which you will find with any custom mod), I would caution you against lengthing the intake. These motors already have a torque curve that peaks rather low in its range. Longer runners will improve low end VE, but it will hurt high end VE. The result would most likely be less peak and total HP.

I am glad that you enjoyed our motorsports site. You can indeed help us with our cause; We are loosing about a third of our funding in the comming year, and we all know how much racecars cost. (Yes I am shamlessly begging for money.) All contributions are tax-deductable.

 

Is it hurt due to not being able to overcome the inerta of the 'column' of air in the runner as easily at high rpm? Like you stated about the intake overall design. Sometimes these things are designed with other considerations in mind and the effect on performance being an acceptable sacrifice in the big picture.

On the flip side from low end performance, what runner/plenum configuration is optimum for top end HP?

 

A little while back I saw a picture of a Jon Milledge carbon fiber intake for a 2.5 N/A. Looked like he really shortened up the runner length. The cost was ridiculous (~$4-5K) but it might give you some ideas to start with. I'll see if I can find a picture of it.

As for runner length, generally shorter runners will boost your top end power. Of course there is a point of diminishing returns and only some good modeling(expensive) and testing will tell you where this is.