How to get more flow from 993/964 plastic intake
 

How can I get more high rpm flow from above intake? Motor is 3.8 rsr p/c, 51.5 intake, 43?exh, in 964 heads with some porting on intake, 993 rsr cams. at this point torque starts to drop at 6k, max hp at 6.6k, and out to 7.7k hp has dropped ~25-30hp.

Would a larger throttle body help? I'm not sure what limiting factor is on these intakes at higher rpm.

I made a IPD like diffuser in the air intake on my (previous) 993. Instead of the T-shaped plenum just below the throttle body, it was Y-shaped, dividing the flow to each side more efficiently. I used an aluminum sheet, and formed it to fit inside. Look at the shape of a typical IPD plenum, and you will see the difference. A bigger throttle body would be good too. You can bore out your existing one.

I also flipped the whole intake 180 degrees, so the TB and MAF sensor was pointing rearward, and cut the plastic between the TB plenum ad the resonance plenum, so I could rotate the TB plenum. In this way I could fit a cone filter and velocity stack to the MAF sensor body, and tuck everything under the RS spoiler that I had on my 993. I also made a box around the cone filter, to scoop cold air directly from the spoler.

I never dynoed the engine, but there must have been some gains, even if the internals were stock 3.6L. And it sounded awesome!

Is the inlet runner diameter 51.5mm, as you say the ports are?

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Individual throttlebodies.

Anything else will be a choke point IMO

Fly911... Do you happen to have a pic of what you did that you could post?

There is another post on here somewhere where someone put an 80mm tb and got good results. No documentation though.

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If after asking nicely, the intake doesn't want to flow more air... I suggest using a few extra atmospheres worth of brute-force feeding :D

In all seriousness though, the 964 intakes flow well and based on your mod list, you might've outgrown them altogether. I think ITB's as Tippy mentioned is the only remaining choice. Everything else would be a compromise with your displacement and cam choice.

Yea, maybe... but I'm not looking for some step function in hp. I race in hp to wt, so just looking for something that may carry more torque higher in the rpm band to keep hp more flat rather than it going down.

Have you considered a 996 GT3 intake manifold? The runners are pretty big and you will probably need to have lowers custom machined to get the right taper from manifold to the heads intake ports. These manifolds were working on 3.6l engines running 8000rpm plus.

Agree with this approach, and something William Knight supports on anything 3.6 and above. On my 3.5, we went 993 intake but dyno tests found velocity stacks provided better peak figures with similar torque across the range.

The 964 intake runners are significantly longer than the GT3 intake runners. And as we know, longer runners = lower peak torque RPM.

i have a larger throttle body on my 964, works well. no AFM, is aftermarket ECU. just waiting on final dyno tune for numbers to compare to the ITB's it had prior.

Dannobee,
don't forget that with custom made lower intakes, you can make them any length you want.

Clutch Monkey -- what size did you use and could you post some pics?

Exactly! It's been too long since I've had to calculate these, but if you want peak torque to occur at 6500, the intake tract (from valve to end of the stack) needs to be around 10.5". Theoretically, of course. There's a reason why the GT3 runners are so short.

yea... did not realize the 996 cup ones were so short. I was think'n maybe shorten mine, but the resonance cross pipe is just about hitting top of fan shroud. interesting how on cup one res cross pipe on same plane as throttle hole.

What is ID of 996 cup throttle?

Anyone know what the OD is of the cross pipe openings and the intake runner ends?

http://forums.pelicanparts.com/uploa...1570049992.jpg

only pic i have
it's still running in (fresh build) so will have dyno to compare it to in a few hundred km's. had PMO ITB's on before this.
will find the spec for the throttle body when i get it back on dyno.
https://i.imgur.com/aQcUJBN.jpg?1

Sam,
Why the change from the ITB's back to the standard plenum?

the PMO's were worn out, and i wasn't keen on replacing them, but jenveys are expensive and cannot justify it at the moment.
in the heat here, on track, the plenum is going to be safer also, no throttles to go out of sync and cause a lean cylinder etc.
engine builder said the hp will be the same, maybe more than before.
my other 3.6 has jenveys on it though but the engine is not as built.

It'd be silly not to run ITB's on a 7700 RPM 3.8 if a racecar IMO!

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