High performance plenum system for aftermarket EFI ?

[Mark Henry]

EFI Systems

I've seen lots of ITB system builds as above and I'm no newbie, for T4 and I've built my own motorcycle ITB's and have used Jenvey's and CB on other projects.
What I don't like about ITB's and EFI is running in alphaN or a TPS only system. It was great when I was younger but now that I'm older I want it to run more like a factory performance street car...with power of course.
Right now I have a good running hot street 3.0 twin plug with Weber 40IDA and MSD with a Patrick/Jarvis dizzy, runs great no complaints other than it smells like a carb car.

I just acquired a complete EFI system with twin plug ignition.

I also have a 2nd 914/6 project with a 2.7 engine that I'm planning to build, it will need the other intake and ignition system, so it's either carbs or this system but one of them is getting the FI.

First I know I could just buy a 3.2 intake and be done with it, but that's going to be like $1500 Canadian. Or I could mod a Boxster intake, but I'm a cheap and I like to fabricate things, so I was also wondering if one can make a better tuned plenum system?
BTW my cars are both 914's, so for the most part you can't see much other than the tops of the rain hats.

Here below you see a system for a Subaru being built (aircraft conversion), but I think I would build it more like the later Porsche intakes that have the 2 separate plenum halves. The 996/997 runners are ballpark 14" I have both a 996 3.4 and a 997.1 3.8 intake here, I'll pull them down and properly measure them.





These are mild steel that are then powder coated, I imagine I could do them in aluminium.

Question is would you think the power loss will be so bad that it's a bad idea? Change my mind.
How long would you make the runners?
How straight would you keep the runners? Or would you curve them more like 996?
Mild steel powder coated or aluminum? Why?
Has any one already done this? Linky please?

If it such a bad idea I can always do a triumph TB build, sort of like what I did here: 914World.com - The largest online 914 community!


I'm not in a rush, I might not even start this till next winter, I just want to see what ideas come from this group.
Yea or Neh on the plenum?

[Walt Fricke]

Don't turbo plenums of this sort taper, so the farthest cylinders get as much air as the nearest? Or isn't that a consideration for NA?

[Mark Henry]

That's a turbo thing, but it's not much of design issue more because you can with a turbo, my buddy did it only to avoid interference with other parts under the hood.

[dannobee]

Optimal intake cross section area and length on normally aspirated engines are mathematically computed and have been known for several decades. Nothing new there. Here's just one of many online calculators.

Intake Runner and Peak Torque Calculator

Generally speaking, you want the plenum volume to be as low as practically possible (for a flat six anyway). If you find that increasing the plenum increases horsepower, then the throttle body is too small. If it's too big throttle response will suffer. Since we're connecting to halves of a flat six, consider each bank of three to have its own plenum, with a big balance tube between.

As the RPM capability of an engine increases, the optimal runner length decreases.
As the engine displacement increases, the intake runner cross section needs to increase.

As far as injector placement, the higher up in the intake runner the injector is, the more power the engine will make. There is no disputing this fact. The only reason that production cars have injectors mounted close to the intake valve is for cold start emissions reduction and idle characteristics, not for performance. Use that info to meet your objectives.

Porsche does a very good job with its intake manifolds and I've found that when checking cylinder head flow rates on a flow bench, the flow will often INCREASE at mid-lift with the intake runner attached. I'd use the Porsche intakes as a guide if I were to build an intake; they've already "done the math."

The latest in intake material tech is plastic. It's very likely that you could 3D print one if you have the capability. Aluminum would be a second choice because of light weight and thermal conductivity.

With turbos, all bets are off. You could pretty much connect everything with a big box and short runners and given enough boost, it'll still make good power. And if it isn't enough, you simply add more boost. Yes, intakes can be optimized with turbo cars, but packaging concerns tend to be the driving factor.

[Mark Henry]

Originally Posted by dannobee

Optimal intake cross section area and length on normally aspirated engines are mathematically computed and have been known for several decades. Nothing new there. Here's just one of many online calculators.

Intake Runner and Peak Torque Calculator

Thanks I'll play with the calculator later.

The MY/96 head has a wider intake manifold pattern so other than a reference and maybe the odd part I don't think a boxster plenum will work for this purpose.

This is a NA plenum discussion only please.

Thanks Mark

[stownsen914]

I've noticed that some builders of very high output NA 911 motors are putting GT3 plenums on top of their ITBs. The factory has put plenums even on non-restrictor motors. I don't know enough to comment intelligently on specifics, but there must be something to it.

[Jim2]

Originally Posted by Mark Henry

I just acquired a complete EFI system with twin plug ignition.

Ahhhh, so that's where that system went!

[Mark Henry]

Yep... Chris made me a deal I couldn't refuse

[winders]

Originally Posted by stownsen914

I've noticed that some builders of very high output NA 911 motors are putting GT3 plenums on top of their ITBs. The factory has put plenums even on non-restrictor motors. I don't know enough to comment intelligently on specifics, but there must be something to it.

Porsche put a resonance intake system on top of ITBs on the 993 RSR normally aspirated race engines.

Gunther Werks uses the 996 GT3 intake with ITBs on their specials 993 cars and Singer uses that same intake on most if not all of their cars.

The idea being to gain midrange torque using the resonance to fill the cylinders faster/more efficiently in certain RPM ranges which improves drivability. I am not sure what happens at the top end.

[Mark Henry]

I'll look at the GT3 but;
-one I bet it's too expensive.
-two I think it would be too tall for my 914 engine bay.

I don't want to lose a lot but I'm Ok with a slight loss at WOT.
My bet is I won't lose much because the 40IDA's I have now are a bit on the small side for my 3.0 big port, twin plug, 120/104 cammed engine, it should have 46mm webers.

The 40IDA's I have are better suited to the 2.7 I'm going to build.

[Mark Henry]

So looking at the GT3 manifold I'm going to build something like it.
I'll build it in mild steel similar construction to the pics I posted above, maybe try a couple designs. If I get one I like I may copy it in CF or aluminum, but for now I'm not worried about using powder coated steel as a material.

[targa72e]

Hi Mark,
intake style is somewhat dictated by the camshaft choice. Individual throttles help with reversion for cams that have a lot of overlap. Common plenum with single throttle limits cam choice to those that typically have less overlap. Plenum over ITB is a way to run more aggressive cams and use resonance tuning to help broaden and enhance the power curve. GT3 intake is often referenced for larger engines and 964 plastic runners for smaller. When used on top of ITB the front crossover where the throttle would normally be becomes a air filter inlet. The rear cross over tube has the resonance flap that is opened or closed depending on RPM. I do not know enough about intake design to know if there is a reason Porsche did not use a single center chamber or if it was a packaging decision.

john

[Mark Henry]

Originally Posted by targa72e

Hi Mark,
intake style is somewhat dictated by the camshaft choice. Individual throttles help with reversion for cams that have a lot of overlap. Common plenum with single throttle limits cam choice to those that typically have less overlap. Plenum over ITB is a way to run more aggressive cams and use resonance tuning to help broaden and enhance the power curve. GT3 intake is often referenced for larger engines and 964 plastic runners for smaller. When used on top of ITB the front crossover where the throttle would normally be becomes a air filter inlet. The rear cross over tube has the resonance flap that is opened or closed depending on RPM. I do not know enough about intake design to know if there is a reason Porsche did not use a single center chamber or if it was a packaging decision.

john

The rear cross over tube has the resonance flap that is opened or closed depending on RPM.

I'm well versed in the 996 and 997.1 plenums, I have one of each here on the wall.

This EFI system was spec as a turbo system (930) IIRC this system has extra relays that can be programmed to turn on/off certain items like fans, etc. and likely this flap. I know this resonance flap runs off of vacuum and has a "can" under the plenum but it also has a electric valve controlling something to do with this as well.

I think I'll dry fit the 2.7 for a jig and work from there.

[winders]

With the smaller displacement engines like the 2.7L, the 964 and 1995 993 plastic intake setup is going to be better than the 996 or 997 intakes. They are not plenums, by the way.

[Mark Henry]

It's going on a 3.0 big port twin plug, RSR style Mahle pistons, web 120/104 cams that's presently running perfectly on 40 IDA webers and MSD with a PMS/Jarvis TP dizzy and wires.

The EFI system is a SDS EM5-F with twin plug coil packs, that I'm sending in to be upgraded to 2021 software spec and add the port for full data-logging.

The 2.7 engine I have will only be used as a jig, but the plan is to one day build it for a 2nd car I own with the Webers and MSD.

[winders]

Jeff Gamroth has used the 3.6 plastic intake with great success on the 3.0L engines. I would think the 996 GT3 intake has too much volume to work as well.

[914efi]

I built this for my 914-6 almost 20 years ago, hard to believe. Not sexy, but it worked well and is designed to fit a 914. I found a place near me in RI [Tubodyne, I think] that bent the aluminum tubes. Hardest part was making the injector blocks, early version had the injectors higher as seen in the photo.

Today, the machining would be much easier. I think the high injector position was fine after all. Happy to share CAD files. If nothin else will tell you what will fit. Still have the jig for welding the tubes.

shown

[Mark Henry]

Cool thanks,
One of my 914world good buddy's is going to send me a set of 964 aluminum intake halves.
They were in a topside fire so just the two halves, I'll have to source or make the center bits.
I also realize if I go this route I also have to make the head/injector part, rails, etc.

[winders]

Are you going to use a flapper setup so you can optimize mid RPM torque? Does you ECU have the ability to control the vacuum valve?

[Mark Henry]

Not sure, I know it's vacuum activated but is it just a solenoid opening an on/off valve?

I do have a few relays for NOS and meth injection I can re-purpose.