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.

what kind of realistic gains can be made by this mod ? if it is just a few h.p. like 5 or less i do not think it is worth the cost. if you can get about 10 to 15 h.p. then i think it would a good value. does anyone have any solid numbers for gains on a 3.2 engine ?

I'm using Fluent in these simulations.

A Y will be more efficient than a T for airflow in a steady state, that's correct.

Keep in mind that with three cylinders to a plenum, there is a pretty much constant flow of air into the plenum when the engine is kept at a constant speed. There is always a cylinder pulling air. The pulses that influence secondary pressure waves are important, but they are secondary in nature so the influence on flow is on a much lower order than the primary vacuum pulled by the cylinders themselves.

Here are three more simulations varying on the one I posted earlier. The first one just adds a sloped wedge to the original T. The second adds a large radius to the T junction and the last one combines the radius with the wedge.

http://986forum.com/forums/uploads01...1323780177.jpg
http://986forum.com/forums/uploads01...1323780193.jpg
http://986forum.com/forums/uploads01...1323780206.jpg

The volume flow rate results show that the T and T with wedge have almost identical flow rates. The radius-ed T with and without the wedge are both almost the same flow rates but are about 30% greater than the standard T.

Well done; well put. I was just considering any tendency to pull from bank to bank when the firing order is such that there is a strong intake pull from one side versus the other.

Looks like in theory the 'y' has some definite benefits...now comes the hard part part of testing varying shapes and geometries.

Am I correct in saying, rather than spending a $1000 + on the IPD set-up, it sounds like using a 74mm 996 throttle body, along with a 996 "T" that has been modified by Pedro, is the best and most cost effective way to improve the oem design ?

Seems right to me!

The IPD plenum rounds the inside corner of the T slightly so it's a little closer to the Y. Significantly more than the stock and 996 T. That's the important improvement, not the splitter.

That's what I'm showing. The rounded T flows significantly better than the standard T. The splitter doesn't do anything to the flow of either of them.

ok, update.

caveat - analysis officially works only for 3.2 S, but only because i don’t know anything about 2.5/2.7/2.9 intake systems.

whereby, airflow is limited by the smallest constriction in the system,

whereby, the only diameter that is inviolate is the maf housing which is fixed at 3” (to make it bigger would confuse the computer).

whereby, the throttle body is sized at 2.67”, introducing an unnecessary constriction in the system.

let it be stated that a 3” throttle body would increase intake efficiency.

now, the cayman, 996 and 997 all have 3” throttle bodies and share the same Porsche part number, so are interchangeable.

997 throttle bodies are hard to find, but there are lots of 996 units around (one currently on ebay for $100) and lots of cayman units around as folks take them off to install the ipd product (i just found one for $100 over at planet 9).

now then, with a larger throttle body we need a larger intake plenum. given that the throttle bodies are interchangeable so should the plenums. therefore we have 5 choices:

cayman ($300) - this dog doesn’t hunt. it has that funky resonance flapper, which results in a different aos arrangement and throttle body positioned further upstream.

996 ($60) - this would work, but the 997 piece is better.

997 ($60) - this is the guy you want. similar to the 986 but improved - larger diameter, matching aos connection, comes straight out instead of at an angle, improved flow characteristics. improved flow characteristics? as per earlier in this thread, it was shown that you want the inside radii as the air turns the corner into the intake runners to be as rounded as possible. examining images of the 997 plenum on pedros site (here: TechnoPower2 Kit ) it is evident that the 997 plenum has as much radius as possible given space constraints. in fact, the radius is better than that of the ipd product (images and dyno here: 986 Boxster / S | IPD Plenums ).

technotorque 3 ($300) - this is just a modified 997 plenum, with the addition of a splitter. as shown in this thread, the splitter does not contribute to airflow efficiency. this is further evinced by a guy on planet 9 who put a larger throttle body on his car and got a dyno; gains were equivalent to ipd but without the splitter or the cost (here: 75mm TB and Cayman Plenum upgrade for 987 - Articles ).

ipd ($850) - as above, i can see no benefit to paying $850 when i can get equal or more gain from a $60 part.

so, $100 Cayman/996/997 throttle body, $60 997 plenum, a 3” duct to the maf housing, a pair of silicone adaptors to mate to the intake runners, and some hardware (the larger throttle bodies are actually thinner and require shorter bolts). in for under $200 and a 10 hp gain (if you believe the above dyno).

RK you are really chasing this down. I had though the 997 "plenum" had much longer stem section that would move the throttle body further down towards the oil cooler....had assumed that there would likely be little room for it. What would be the effect of moving the throttle body further upstream?

You now need to make up a special short silicon pipe that goes from throttle body to MAF

....or maybe I have got it wrong

Okay looks like I am confused as to what the 997 piece looks like

it's the 987/cayman that has the long-neck plenum. the 997 is shown in the link to pedro's site below and is quite stubby. i think the 987/cayman have the long neck to distance the throttle body from the resonance flapper on the plenum, there to add more low rpm torque similar to the resonance tube on our cars. the problem with this is that that flapper is an obstacle to airflow at high rpm/airflow situations. it also must mess with the vacuum available, as the 987/cayman aos has two connections at the plenum.

Yes you are right...I started to think out loud without checking first

This looks like a viable option to try!

it's kinda exciting; if we can get 10 hp for under $200 it becomes the best bank-for-buck mod currently available. twice as much hp as the under-drive pulley for the same price, almost as much hp as a $1000 tune or $2000 exhaust setup.

King,

Seriously, what is the likelihood that you will actually make one ? I'm on the fence about spending $1000 plus on the IPD set-up, and I would very much like to see if your design works. If, so, I'll be your first customer . :)

I didn't know that there was a Carrera S throttle body at 76 mm and dedicated plenum. Thought there was just a 74mm and then the 82mm GT3 which would be too big I think.

Which one did the Planet 9 fellow use? 76 shouldn't hurt?? I think reducers are easy but a nice pipe from the tb to MAF may be trickier

It is very exciting!!! I was just going to put 996 stuff in there as a trial.

it's already been done by j.fro (see his posts earlier in the thread) i'm only fine-tuning his work by stating that the 997 plenum is a bit more advanced than the 996 (but interchangeable) and should produce better airflow, and that the pedro splitter is not required (hence lowering the price). and located some dynos to justify the work.

for me, the cayman throttle body is on it's way.

on my next parts order i'll get the 997 plenum (997 110 416 03; auto atlanta online parts diagrams are your friend when it comes to comparing parts and part numbers) and throttle body bolts. once they arrive i'll confirm diameters of the reducers and hose clamps required.

from what was posted by j.fro, and what is shown on the pedro board, the aos vent line appears to use different connectors so i'll have to confirm diameter and get some tubing for that.

i'll probably use some cheap 3" flexible duct to the maf housing until i get my new intake happening.

once everything is done i can post a list of parts required and my impressions of the end result (no dyno or anything). i'm slow, and shipping takes forever to my neck of the woods, so no results anytime soon ...

Yes I am with you ....plus it's cold up here!

The driving season and mod season are co-incident

Yes I saw his post after mine. I had assumed that my 997 plenum suggestion wouldn't fit as he had gone 996

It looks like it has nice radii as you pointed out. Let us know about the diameters when you get the 997. The hardest part will getting the right silicon parts to fit nicely and look like factory quality

No worries King, the Dangermobile has been put away for the season as well. But, as Spring draws near, I'd love to be able to implement your design . :)

I love this forum!!! Can't wait to do this myself!

ok, here's the quote from suncoast:

997-110-416-03 DISTRIBUTOR T 1 42.43
900-385-025-04 HEXAGON-HEAD 4 0.27 1.08
997-110-319-00 RUBBER SEALIN 1 5.49 5.49
N-011-524-27 WASHER 4 0.15 0.60
999-512-539-01 HOSE CLAMP 2 3.05 6.10

thats for the new plenum, four shorter throttle body bolts and washers, a gasket for between the throttle body and plenum, and two larger hose clamps for the larger plenum outlets. total $55.70 + $100 throttle body = $155.70 so far. all i need now is ductwork to the throttle body ($20 spectre flex duct from ebay until i figure out the geometry of my pending cold air intake) plenum outlet reducers (i'll wait for the parts to come so that i can confirm required diameters) and some tubing for the aos (only if i can't reuse oem; if not, i think i'll go with transparent so that i can monitor just what the aos is venting into my intake).

i still think i'll get in for under $200.

This is great stuff, I was this <> close to pulling the trigger on the IPD plenum. Now I have a whole new direction for a fraction of the price.

Thanks for sharing this guys.

TRK,

Build one - build one - build one ! :)