jmj951

I've been reading up on variable intakes since I realized that my 996 and some older 911's have this technology built in, and I thought that the way we use our 951's as street-driven DE machines, we could really benefit from this technology. This is one of the ways Porsche flattened the torque curve throughout the RPM band, so it should be possible to apply it to any 944/951 as well.

The stock intake manifold on this car is a compromise between low-end and high-end performance, whereas a variable intake could optimize torque for both low-end and high-end bands by controlling the airflow available through an additional, secondary intake pipe for each cylinder. I've seen the SFR and Lindsey intake manifolds and wondered about the advantages and tradeoffs. Best I can tell from my limited research, these manifolds will help the top end, but will definitely hurt low-end torque because the intake air velocity will be lower than with a stock intake, which will negatively impact the dispersion of fuel and (to a small degree) the volume of air that enters the combustion chamber (due to a forced-induction effect that can be created before the turbocharger even kicks in). One of my personal interests for this car is to figure out what it takes to optimize the low-end torque of this car, and it seems that one way would be to build a custom intake manifold with a basic controller using engine RPM to open/close the secondary air intake pipes.

At first I thought that there wouldn't be much benefit in a turbocharged application, but then I read that the 997 turbo is using this technology as well, so I would like to give this a shot. Realistically, what materials can be used to fab an intake manifold (at least a working prototype that doesn't have to last forever) that would allow me to mount and control valves (either one valve per cylinder, or possibly just a single valve)? Realistically, it's going to be a long time before I take up this project, but I'd like to start thinking about my plans in detail. I'm thinking carbon fiber, but I've never worked with it before.

BTW - similar concept applies to exhaust manifolds - smaller diameter exhaust pipes result in faster air movement, which not only pushes the turbo faster but also pulls more air through the engine due to the higher inertia of the air (I'm assuming there is some overlap in the closing of the exhaust valve and opening of the intake valve). Perhaps it would be even easier to build a complete secondary exhaust and crossover that could do this...but now we're talking dual wastegates and...probably more complicated than the intake project. BTW, this finally helped me understand why backpressure was so important for low-end torque - it's all about the velocity/inertia of the air.

V2Rocket

you could use a split plenum with different size throttles kind of like a BMW 318i, even though that motor only had 1 length runner.

one smaller throttle would have a longer air path and the larger throttle would have the short, high-rpm air path. i imagine you could do a progressive throttle opening where like depending where the throttle position is, the small bore would be open fully and the large bore like halfway and then at WOT above a certain rpm both are fully open.

Chris White

Sounds like an interesting intellectual exercise – but not enough of a real world gain to make it worth the effort.
A couple of things to put you on the right track –
• The dispersion of the fuel is irrelevant to this design issue – with the injectors right by the intake valve the fuel dispersion is controlled by the injector. In older carburetor designs the fuel distribution was an important element in manifold design.
• The ‘tuning’ of the intake runner length has to do with a resonant effect. It’s not a ‘velocity’ thing; it’s similar to any tuned resonant pipe (pipe organ, Trumpet, ported subwoofer enclosure).
• Any resonant system has a specific bandwidth – once outside the tuning point the efficacy drops off. In acoustic terms you typically get half an octave in each direction of additive effect and then the benefits are either nonexistent or possible detrimental. (for those that are really into this stuff you can tune the Q based on the resonant design. Higher gains can be made with a narrower Q.)
• The common variable intake designs use two lengths on runners tuned for different resonant frequencies. Since the design of each manifold is for a limited RPM the designers can tweak the design for a higher Q without worrying about what happens outside of the tuning range.

The problem you will run into with your quest for better low end torque is that you are dealing with several tuned systems of limited bandwidth. If you optimize your intake for better low end you will find that the turbo is outside of its efficiency range (you mentioned the newer 911 turbo intake manifold – remember that the same car has a variable vane turbo that helps increase its efficiency bandwidth).

The 944 turbo stock design is a really well tuned system (intake, turbo and exhaust) for its intended use – midrange torque. You can improve the efficiency at particular points like higher RPM or higher boost pressures but it would be difficult to better the design for overall performance at the same boost/rpm range.

As a general design parameter you can expect a 3000rpm range of good flat torque with the right turbo, cam, intake and exhaust. 3500 if you really get it right – outside of that the torque will fall off. A variable intake may help this a little – but you would need the turbo and exhaust to support it.


And finally - good luck finding the room to fit it in…

pettybird

how about a complete 968 engine, and relocate the turbo to the passenger side down low?

V2Rocket

Chris White

That would have a lot worse low end torque than a 951 engine.

333pg333

Why is that?

67King

That is a dual runner, not a variable. Ford was doing that on all of their 4V engines in the 90's. Each set of runners tunes at a different RPM, giving a flatter curve

I'd love to add to what Chris said, but he prettymuch nailed it. VCT would b cool, though.

pettybird

+1...

I'm talking the whole 3 liter engine, not just the manifold. Variocam, varioram, etc. Swap in low comp pistons, fab the exhaust and go.

If you meant no torque swapping on the 968 intake I'm still not completely sold, but I'd understand better.

Chris White

There is nothing about the 968 engine that was designed for low end torque!
I owned a 944 S2 cab and a 968 cab back to back – even the S2 engine had better low end than the 968.

The head, intake, cams on the 968 are optimized for higher RPM performance. Sure, with a lot of changes you can get the 968 engine to make some low end turbo torque….but by then its not really a 968 engine anymore!

DanaT

Here is a factory example (not for our cars) of what you want. It uses a very "simple" vacuum operated valve to open a second set of runners.



-Dana

ALFAPU

My old MR2 turbo had the same thing (called TVIS), however when making 300hp or more, this was the first thing to get tossed in the trash. The TVIS butterfly valves are located on a removeable plate that bolts to the end of the runners just before they run into the head. The MR2 turbo uses a 3s-gte 4 cylinder motor with 2 runners per cylinder. One runner is constantly upen, the flow of the other is regulated by the TVIS valves. Here is a picture of the plate with the valves open:
http://www.gtfours.co.uk/what/tvis/tvisplusmanifold.jpg

Other Toyota motors such as the 1mz-fe and 3vz-fe use a simplified version of TVIS called ACIS. The ACIS system utilizes a single butterfly valve mounted directly inside the intake plenum.
http://img.photobucket.com/albums/v4...e/IAC-ACIS.jpg

They all use manifold vacuum to operate the valves, but the line is controlled by the ECU and a vacuum actuator. It is not uncommon to bypass the ECU and install an RPM activated switch to control the vacuum actuator. This requires 2 dyno runs, one with the valves closed and one with the valves open. The point at which the two RPM bands cross is the RPM range for the switch that is used.
http://www.turbomr2.com/MR2/Reference/TVIS/TVIS.htm

I should mention that the higher horsepower JDM versions of the same motors do not use the TVIS or ACIS systems. At higher power levels, the shaft and butterfly valves become more of a restriction than a benefit. Later Toyota came up with variable valve timing called VVTi and gave up on TVIS and ACIS completely. Now just about every car manufacturer has a similar valve control system, the most commonly known is Honda's VTEC.

thingo

I'd have to disagree about the torque of the 968, I found the low end one of the great pleasures of the 968 on the track, and one of the reasons it blows away a stock 951.

CarbonRevo

I'm surprised no one has mentioned the dual runner intake found on US Spec H22 Honda Prelude motors.

Below are some pictures showing the intake.

Picture 1 shows what the intake would look like bolted together. With throttle body in top left of the picture and the intake flange in the foreground.

Picture 2 shows what the intake looks like with the top part (throttle body and plenum removed, as well as the middle butterfly assembly shown in picture 4).

Picture 3 is another angle of picture 2.

Picture 4 is the piece of the intake that is between the two larger parts (plenum and flange).

At all rpms/loads the square intake runners are always open. Then, under certain rpms/loads (and when in V-TEC, IIRC), the butterfly valves open up and give a sort of dual runner intake. It's a variable intake (such as what you guys are discussing). However, aftermarket intakes ditch this design for a single runner aspect and they tend to pick up 6-8hp (on a motor only making 190-200hp to begin with).

Just figured I would throw this out there...

Chris White

The immediacy of the response was good – but the quantity was not that great!