My assumptions are a racing engine that is pretty cammed up.

Of course, the "best" induction systems out there are slides and ITBs. There is a relatively new ITB product out there with a shaftless butterfly design.

These Porsche parts were developed by Reno Rennsport and AT Power in the UK, and they make more power than conventional ITBs. They are probably as close to slides as you can get at maybe about half the price.

I may be the person referenced above that implemented an 80 mm TB.

My application was a cammed up 3.8 (rsr-like profiles). I already had excellent S Car Go 1 3/4 inch OD headers.

Instead of using a stock 993 or 964 TB on my 993 plastic intake, I used a much larger 80 mm BBK product designed for 5 liter Ford Mustangs. I had to do some modest fab work on the TB and on the intake section to which it mated-- that wasn't very difficult. So throttle linkage, attachment and porting. Check.

Because I run Haltech EFI, the engine could be tuned for the greater flow. And of course Haltech could deal with the BBK-style (really ubiquitous) TPS that fit on the BBK. Check.

I used a ~$20 rubber boot with a 90 degree bend to mate to a 4" alu tube, where I then mounted an aftermarket cone filter. Check.

I made custom carbon fiber box for the cone filter. Check.

I made some simple modifications to the geometry of the throttle linkage to have better throttle response. Check.

The set up already had an RPM switch- based set up to leverage the 993 intake flapper valve. Check.

All of this was really easy.

I did not dyno the engine using a before and after approach. My friend/engine builder usually does road/track tuning. What I can say is that we made more power than we would have with the smaller stock-style TB, and the power was fine north of 7300 rpm ( I tried to shift in the 7k-7.3k window to be conservative on wear).

I felt that my investment in time and modest dollars (probably ~$350 all in) was a worthwhile cost versus performance step between stock parts and ITBs or slides. The cost was offset by selling the stock 993 TB.

I guess the other thing I will add is that I have seen some clever induction approaches implemented by Jeff and the guys at Rothsport. I doubt any of them are as inexpensive as what I did, but if you are looking to manipulate the curve shape but have to operate within some type of box (rules), then it might pay to ping those folks up in Oregon.

If it has 993 rsr cams (294 deg adv intake duration), 3.8l, peak torque 7200, it should have an intake runner area of 3.2", 10.5" long (50mm tube, 267mm long). For peak torque at 6500, 2.8" intake runner area, 12.00" long (48mm tube, 305mm long). If you want peak torque at 7700, intake runner area of 3.4", length of 9.75" (53mm tube, 248mm long). Remember, the runner length is measured from the valve to the end of the stack. There is no magic bullet here though. You're only going to gain so much, but as you've found, you can lose quite a bit by not having the intake runner volume at least in the ballpark. Max effort NA race engines make about 1.60 lb ft torque/cu. in. If it's not quite "max effort," adjust accordingly. 1.40 lb ft/cu in is usually good for an engine that serves dual purpose. If we use 1.6 for your example, 370 lb ft is all she's got (give or take a few). If peak HP is at 6600, that's 465 hp. If you can change peak HP to 7500, we get 527 hp.

ITB's or not, the calculations are the same. And there are these things called "rules." If it's not legal in your class, you sure as hell won't get away with ITB's. They're just too visible.

There are a few different GT3 intakes, mostly based on engine displacement. I don't have first hand info on tube length or diameter, but knowing that intake spacing is 118mm, I'd be inclined to find a pic or two and carefully map them out, knowing our dimension of 118mm.

dannobee -- that is great info! thx...

minimal gains for a lot of $$/more points for failure
i have ITB's on my roadcar for fun but not sure i'll ever put them back on my track car at this time

As a previous Fox Mustang guy - I dig the TB :) ...and your plexi IAC block-off-plate! :)

It's posts like these that keep air-cooled Porsche's behind modern day Honda's......

Why would you build such a bad@$$ motor and choke it with a common plenum?

It's like putting points on an F1 engine. :D

There is always a lot of talk about the benefits of ITBs, but nobody ever talks about the benefits of a properly sized plenum chamber. Where the ITBs help part throttle driveability, plenums help wide open throttle mid to top end power. At wide open throttle or near there, there is no benefit to ITBs whatsoever. All the air sees is an unobstructed path to the combustion chamber. The only place where ITBs shine, is on part throttle at reasonably low engine speeds with a very aggressive camshaft. Under those conditions an ITB setup would make the car much more driveable and easier to tune. In a straight line ITBs are a useless expense. On a road race circuit with wider gear ratios, where you might have to drop the rpm down a ways, ITBs will help. But unless you need to improve your lap times by one or two tenths to get into the winners circle, it is money better spent elsewhere.

because i want my engine to live
plenum far safer
plenum also far smoother when jumping on the throttle- no sudden stall in air speed because the plenum masks that with the volume of air ready to go.
ITB's are fun but finicky. better on the street
yep all this

Ollies930 -- I did not know that... I always though itb's were best across all rpm band.

I took a look at a 92sec lap at MidO...
-- Besides in brake zones, I was partial throttle 16 sec, and an additional 7 sec if you count times I was blipping throttle to help car rotate.
-- Total time less than 5k rpm, 16sec.

Is the spacing (left right) between the head intakes the same on the 996/997 motor as the 993?

I suggest taking some care when considering all of the variables and feedback brought up in this thread. In my experience, depending on your application, it may make sense to seek out input from folks with experience and expertise that don't spend time in Forums like these.

There are lots of variables when it comes to induction.

If your application is racing, then I might suggest getting some input from folks with race engine building experience. For example, if I were thinking about various induction approaches (and curve shapes) for a large displacement air cooled, I might consider folks like Jerry Woods, Jeff Gamroth and Mat Lowrance. All have extensive experience with a variety of engine applications and induction approaches. All have racing experience covering the air cooleds since nearly the beginning. Likely between them there is between 100 and 150 years of experience. They all know and respect one another, and I don't think any of them guys hang out in these Forums.

Another fellow I'd speak to would be William Knight.

There are a lot of arguments in favor of ITB's for certain applications, and a lot of cons for large single-plate butterflies.

And of course I am sure most contributors here know that in their water cooled racing engines of the last 20 years ago, Porsche has done some things a bit differently from what it did in the past. For example, for some (perhaps a lot... not sure whether all) of the Cup engines, Porsche has used a plenum and a single plate TB. For the R/RS/RSR cars, a plenum with ITB's.

I have a friend with several water-cooled R/RS/RSR factory cars and a pretty extensive array of induction and exhaust set ups they used. Many of these parts are absolutely unobtanium, I think many folks reading and or posting in this thread would be shocked at some of the things Porsche has done with their racing induction systems as of late. Very, very few people I know have ever even seen any of these types of parts.

Everything else being equal, my favorite racing induction system right now is the AT Power ITB set up. That assumes some cost constraint amongst other variables. I only have experience driving big displacement with either a stock or large single plate... even with the linkage geometry mods I have done (which help immensely), the part throttle response is a compromise.

I had William Knight advise on my build - great guy to work with.

Plot below shows a 3.45 on AT Power ITB with William’s cam spec. Thick lines were a 993 plenum, thin lines are stacks without ITG filters.

http://forums.pelicanparts.com/uploa...1570459971.jpg