maf and throttle body diameter
 

got a question:

on the 986S, the maf is 3" diameter, but the throttle body is 2-2/3" (68mm).

on the 996, the maf is 3-1/2" diameter, but the throttle body is 3" (76mm).

one would think that they would be the same size for max airflow; why have a smaller throttle body?

hey king, thanks a good question. i would think that the reducing diameters (airpox to maf to throttle body to intake runners) would serve to accelerate the air, so there must be some benefit to having faster moving air vs the work required to accelerate it; perhaps to do with intake tuning and low rpm power development?

good point trk, you're a smart guy. to research further, i went to the ipd plenum page where a variety of dyno charts have been posted. if you go to the cayman page, they post dynos for an unmodified car, a car with an ipd plenum, and a car with an ipd plenum and larger throttle body. it's interesting to note the following:

- the larger throttle body does not make a difference over the plenum-only install until over 4900 rpm; presumably when the resonance tube and cams have switched over.
- both the plenum and the plenum with larger throttle body underperform stock up to 3500 rpm, by up to 10 hp less. i attribute this to the plenum. after that the ipd products do look to increase hp by 10 to 15 hp, and the overall area under the curve looks bigger with ipd.

conclusion? the larger throttle body doesn't have any ill effects, but the plenum causes a low rpm reduction in hp. in a cayman.

if you look at the ipd boxster products, they only offer a plenum with larger thottle body (74 mm thottle body, equal to the boxster maf diameter) and their dynos show hp improvement throughout the rpm range.

so, if there are no ill effects with a larger throttle body, why did porsche use throttle bodies smaller than their maf diamters?

Interesting I was thinking of somehow mating the 74 TB to the stock intake "tee". Not sure this is feasible as I am guessing the tee has thin walls.....or replace the tee with a 997 tee with silicone reducer boots feeding the left and right plenums

So you are telling me I need a plenum!?!

Sounds like the 68 TB is there to provide a de-tune factor.

i was thinking 'de-tune' factor also, but why do it to the 996 (thier flagship at the time)?

anyway, the purpose of this thread was the same as you - some thought on getting a larger throttle body on my car. findings are as follows:

- there is benefit to doing this
- porsche makes two 3"-ish throttle bodies that match the 3" boxster maf diameter; a 74 mm (996, 987, cayman) and a 76 mm (996 gt3, cayenne)
- the 76 mm won't work; different plenum connection, more expensive.
- the 74 mm would work, but i am uncertain if the plenums would match exactly. even if they did, i am uncertain if you are not just moving the restriction further downstream as a result.
- so, you can use 996 plenum and two reducer boots (but would have to re-route the aos) or cayman (edit to add that i think the cayman plenum has a resonance flap that the 986 does not, as well as two aos connections instead of one); there should be lots of take-offs from guys who have put ipd units on their cars.
- note that you would also have to change the piping from maf housing to throttle body as it is currently sized for the smaller throttle body; you would need a 3" pipe/flex duct of some sort.

the next step in my thinking was having a go at the plenum. there is something inherently wrong in paying $900 for the ipd product; i was thinking of the following:

- get a cayman throttle body and plenum.
- saw the 'T' off the plenum, leaving only the flange and aos connection.
- get a silicone guy to fabricate a 'Y' with appropriate diameters.
- hose clamp it to your two intake runners and modified plenum.

Hmm for the 996 the mass airflow sensor section diameter is probably common for cost savings ; parts interchangeability etc. I did not know that these sections were shared. The reduction in diameter here would bump up the velocity with minimal losses.....so very little hurt for major parts sharing cost savings......just a guess.

When you get to the throttle body 68mm with a butterfly valve stuck in there is a de tune vs. a 74mm. as long intake velocity does not fall off below optimal....just a guess

I looked into rapid proto typing an intake piece I am making....low volume costs
are huge...The IPD price is a bit easier to swallow after that but....

I have all this rolling around in my head too. Let me know how you make out!

I've been following this topic for a couple of years now; especially articles pertaining to the IPD plenum. Interestingly, most of the "tuners" that I spoke with initially (particularly the German ones) viewed this upgrade as a waste of time and money . Curiously, however, over time most them either began selling the IPD product, or developed a similar plenum of their own . Unfortunately, there doesn't seem to be enough supporting data regarding an M96 application to justify spending $1000 . I've heard a lot of "seat of the pants" and "butt dyno" endorsements. But no real evidence . It's up to you TRK to create a test bed ! If it's proof-positive, then a lot of us will buy one from you . :)

Last year I picked up a 996 TB and the intake T from a Porsche dismantler. I had the intake T modded by Pedro with the Techno-Torque. The original 986 rubber connector hoses in the intake are flexible enough that I was able to reuse them. It works. I haven't dyno'ed yet, but the car pulls harder through the midrange and top end. You know the surge a stock 986S gets around 5K? I'm getting that at about 4K now...
I really need to dyno this thing

very good to know. were you able to make the stock aos piping reach as well?

regarding the technotorque, i think it solves part of the problem but not all of it. when the air hits the 'T' it has to turn left or right around a sharp 90 degree corner. as a result, the air will cavitate in two places - at the split on the outside of the turn, and at the sharp corner on the inside of the turn. the technotorque puts a splitter on the outside of the turn, but doesn't do anything for the sharp corner on the inside of the turn.

if you think about it, what will limit airflow is the restriction caused by the inside of the turn; this is why the ipd product posts better results than the technotorque.

but it's all better than stock, and the TT is a better value than ipd.

For the AOS, I had to flip the T upside down, since the AOS is on the opposite side in the 996. I bonded a piece of PVC into my AOS as one fitting, then cut the factory fitting going into the T as the other fitting. Between them I have run a piece of rubber heater hose, secured on each end with screw clamps.
I'd like to ditch the factory AOS altogether and run a Moroso unit, plumbed in with stainless hose.

One more note, the bigger TB and change in the position caused by the different T causes the intake tubing to rub the engine cover. I had to use foam/aluminum HVAC insulation to create a barrier to prevent the intake tube from having a hole rubbed through it.

i have been taking a closer look at this mod. the IPD plenum looks like a high quality piece but i just can't justify the price plus the larger throttle body . Does anyone think that this part could be made out of epoxy or fiberglass construction. two molded halves bonded together?

Is it possible to interface a 74mm throttle body with the oem plenum or a TT2 using silicone reducers ? If, so, would it net a positive result even though the plenum is much smaller by comparison ?

Ahhhh somebody has already been there! Will Pedro modify a 3.4 intake tee with a special diverter piece? Or is this available for the 3.4 just as it is for the Boxsters?

You will definitely have to get some dyno work done!!

I think the walls may be too thin to modify.....see my #4 post. It looks like it is best to try a 996 Tee as jfro has done

If you don't mind me asking , what were the costs involved ?

It lowers losses to have larger tubes up to the throttle body. The pressure losses due to friction in a tube are directly related to the speed of the flow running through the tube.

Set the throttle body at something, 76mm, then run a 76mm tube 3 feet up to the throttle body and pull a vaccum. Measure the speed of the air at the inlet of the tube. Now run exactly the same experiment but start with a 100mm tube that gradually drops to 76mm at the throttle body. The speed of the air at the inlet is lower. If you look at the flow rate through each of these for a given vaccum level, the decreasing tube will have a higher flow rate. It's analogous to running a set amount of electrical current through a fat wire or a thin wire at a specific voltage. The thin wire heats up more due to resistance.

I think I showed the attached pictures before of flow entering a "T". When the center stream of the flow leaves the entering section and hits the back wall of the T, it stops or stagnates. The flow along the sides of the entering tube is pressed out by the stagnated flow from the center of the tube and makes it's way out towards the outlet of the T. In other words, the flow makes it's own wedge. Adding a wedge, without many, many experiments or fluid modeling, is more likely to hurt than to help with the airflow.

The more interesting thing about the picture is the big "dead" zones as the flow near the walls leaves the entering tube. This is known as separation. The tube in the T section may as well be half as large because the air isn't doing anything in the corners. If the entering tube where rounded as it merges with the T, the air could more easily follow the contours of the transition and use more of the volume of the tube. There is a potential for a HUGE benefit to airflow.

Maybe of interest??

TechnoPower Kit

TechnoPower2 Kit

With dynos (for the 3.2)..... looks good to me!

Are you using the CFX package in Ansys to model the flow?

Looks to me like the optimum flow regions are scribing a 'Y' pattern! The diverter being it the centre bottom stagnant zone. It looks to me that this shape would reduces losses but.... Don't forget that we are drawing air in and there are varying pulses. I would be concerned with any tendancy for the banks of cylinders drawing from side to side over the diverter. This may not happen to any large degree but in this case I would think a tee would be better.

I went and look at the ipd and low a behold they have relief hole put through the diverter. They look just big enough for pressure balancing side to side.

Is the Y pipe model next???

I'd be very interested in seeing a simulation like this for Pedro's techno insert in the T junction... Any chance you can whip one of those up?

blue - i get that it lowers losses to have larger pipes up to the constriction; my question is that, if the throttle body is the constriction, why did porsche put it in the system at all?

regardless, the ipd dynos show that a larger throttle body with a better-designed plenum is a winner on the 986 (the 987/cayman have a resonance flap on the oem plenum that is removed in the ipd product, hense i think the poor low rpm performance on the ipd kit for these cars).

the idea is to make the ipd product without the ipd $. as has been pointed out, the pedro product is flawed in that it doesn't address the turbulent flow that occurs on the inside radius of the 'T'.

the best solution i see is to fab a 'Y' pipe out of silicone that mates to the flange/aos end of a cayman plenum and throttle body.