This is something I have been working on lately, may be of interest to some of the higher rev/power engines out there.

The OE Sti5 intake manifold design leaves a lot to be desired, (earlier versions such as the 93-96 are better but I still think they can be improved) the runners are all different shapes, bends are unequal and of various radius and the internal bore varies between cylinders.

All this just adds to the imbalance of air/fuel ratio per cylinder, the end result of which is a compromised mapping set up with regard to AFR and ignition advance.

The common plenum chamber is virtually non existant and every 180 degrees of rotation, one cylinder starts to intake its charge (from this small volume) whilst the previous one is still filling. This tends to make pulse tuning ineffective and can have a negative effect on the volumetric efficiency.

Consider a modern design of 'sporty' 16v engine with separate throttle bodys, it is not unusual to see 100bhp/ltr at 7k rpm. This equates to 200bhp/bar on a 2.0. I don't know of any 2.0 Subaru making 400bhp at 1 bar boost (2bar abs) but it 'should' be possible.

In an attempt to improve the normally aspirated performance (read VE)
I consider the inlet manifold is the biggest restriction on the Subaru.

Thinking back to my Ford tuning days, on the 2.0 SOHC you were capped at circa 160bhp on a common plenum manifold, regardless of how big the twin choke carb was or cams or valves etc. Bolt on a set of sidedraft carbs with larger individual runners per cylinder and an instant 40 bhp improvement with no trade off in torque Thats a 25% gain.

On turbo cars we (I) have become lazy, looking for power mainly from turbo upgrades, with little regard for VE, just pump it in...lol

So, enough waffle, whats the plan ?

Well ideally individual throttle bodys....but too expensive in my opinion, next option I considered was a split plenum and new cylinder runners, there are a few cars out there with twin plenums (one left, one right) but this creates a further imbalance due to the left left, right right firing order of the Subaru engine.
The plenums needed to be split front pair/rear pair to give 360 degree pulse separation. It has been done to a number of cars fairly recently, one of the problems with this configuration is working out a throttle body set up that doesn't pair up the plenums again ...... then I remembered I had an old Subaru SVX twin throttle body....perfect

So the plan formed to be a pair of large tubular plenums, sized to approx 4x cylinder volume to allow each cylinder to have instant access to a "gulp" of charge air, these would be linked to a pair of 60mm diameter TB's and feed out to 4 equal length, equal volume cylinder runners.

Easy when said quickly but how to actually build it ?

Well, using as many standard parts as possible, I think it can be done for around £100-150.....maybe ?

I have most of the parts to hand now and will post up some pics tomorrow.

Andy

 

Looking forward to see this one, you have a very good point there, turboing is an easy method, be interesting to see how you get on.

Good luck.

 

Suscribed ..... something like this for the older ver 3/4/5/6 scoobs would be great.

 

Carlos H.

 

Good grief! That's more than £150. Looks too good to be true.

There was a TSL engine at Autosport show this year with a central plen chamber and 4 equal length tubes to each cylinder, pic in the early pages of my project thread.
It was nowhere as neat as this though.

Really looking forward to the DIY AndyF version.
What an interesting project this will be.
Many have long inlet tracts to increase low rev torque. The Varioram Porsche system has a controlled 'telescopic' inlet (flat 6 remember), would such a system in principle benefit a turbo engine when 'under the turbo' level?

Graham

 

Similar idea Carlos but that one still has a single plenum which is fine on an inline engine but not so ideal when the intakes are opposite each other. There was a pic around recently of a twin tube set up. (most surprised if Carlos can't find it )

It would be nice to have a variable length intake Graham but i'm not going that far with this set up (yet )

Andy

 

Ive been thinking about doing something for a while, just wasnt sure on plenum design.

You going to use velocity stacks?

 

The TSL unit was in the form of a central plen chamber, with 4 tubes of equal length going to the cylinders, TB was front mounted for the FMIC.
Is there enough room to place 2 large tubes between the ancillaries and the TMIC if fitted? Loads of room if a FMIC used.
Graham.

 

I wondered what you wanted that for Andy

When I was doing my TB I also looked at the manifold but couldn't easily see any improvement with out some thing 'radical' as you are suggesting. Good stuff! Can I have it back now

F

 

Yes Floyd, I've been gathering the bits for some time now You can have half the manifold back if you want
It could be made specifically for a tmic but this time I'm going fmic as the turbo will be relocated there eventually.

Andy

 

Andy, am i guessing that at some pointperhaps you have cut up 2 99 inlet manifolds, and then used the better legs from each?

If you were to do that, which would be better, the flat legs or the curved legs?

Steven

 

Original manifold runner No2, just flattened off to fit the Air con pump ..tut tut !





Better as it rounds the bend !




A real gem of engineering, nice flat bit right in the line of flow !

 

Some nicer bits to build from







The air trumpets really are that big

 

What wall thickness have you gone with andy?

 

 

Nope, thats pairing left and right, not front and back !

3mm wall alloy bends Steven, using the OE shorter legs as they are rounder internally, the long legs are quite oval all the way up.

Andy

 

Cheers Andy, so thats the legs that would feed 2 and 4 normally.

You got me thinking again here, i had been thinking of an arangement similar to the collector on he set of gruppe s manifolds. This is where the throttle body would attach, then 4 equal pipes of suitable size and thickness to each inlet port, with 3 and 4 at the top and 1 and 2 at the bottom.

If that makes sense.

May just canabalise the 2 inlets i have.

 

great plan andy.

and some nice theory there whch you know I like.

be interested to follow this.

Can you explain why a single plenum with one throttle body would be a bad thing please.

Am interested in this, but at what cost.

would love to be able to increase VE as it is just a good thing all round.

 

Good thread. Am watching with interest.

Bob

 

Probably irrellavant now, but might be of interest!

Great thread Andy, we are all watching...

 

Andy, could u ask Quirt C. to post a bigger pic of the inlet manifold he sells .....



"maybe" we can modify and older style (early phase 1) manifold .....

 

It's not bad thing Adam, a big single plenum is better than a small single plenum however I prefer to have a split plenum so that by utilising individual runners with 360 degree (crank) separation, cylinders are not scavenging air from other cylnders via the plenum chamber and as such can pull as much air as that cylinder can handle at that time.

Andy

 

very nice, but I can't see you doing this for £150 after spending £60 on alloy bends, and £1600 on the SVX just for the throttle!

Paul

 

Will it make much of a difference though, designing new inlet manifolds?

I can't help think that Subaru have designed different shaped manifolds over the years and have came to their final design for each one for a reason.

The other thing is that the cars we have (the turbo version) *force* air in under pressure, so "scavenging" or trying to "pull in air" isn't really much of an issue?

Or is it a case of trying to get everything matched and even between all four cylinders (in respect to AFR and advance)?

 

alan, consider the fact that they have changed it each time, then think about the fact that they haven't changed the exhaust manifold. Implying they were happy with their design.

Do you think the standard headers can't be improved upon?

I think this is an excellent plan of andy's, and am definitely interested in the results.

It isn't just forced inductin that matters, off boost performance can be improved significantly, meaning it is less of a pain to use that larger turbo you might avoid because of lag concerns.

Not sure I would go the whole hog of twin plenums and throttle bodies, despite explanation above.

This is certainly the kind of tweak I would consider long after consider my project to be finished. Let's hope it is successful.

 

Alan, the 'easier' the air goes in the better, And iirc the length of the tube will effect power and torque

 

That is why Porsche did the Varioram inlet (admidedly along with other tweeks) to make their non turbo 3.8 engine produce about 340 bhp and lbft.
I don't think the 3.6 turbo has the same system possibly due to the internal pressure of the system at full boost.
Is there any advantage to a 'tb' to each cylinder, especially as they are paired and not independant on the SVX?

Will be facinating to see if there are real across the rev range benefits, or are the Magnus/GruppeS items more 'bling' onto the engine?
The Magnus system would be relatively easy to make in a decent workshop as long as you have a miller and a MIG welder.

I think the £150 AndyF budget is realistic.

Facinating.

Graham.

 

I thought long tubes were good for torque and short for top end. I had a K&N torque ram on my mini in the '80s. It consisted of an optimised length, long tube with a K&N cone filter one end and a plenum containing a stub stack at the other. I had to chose between torque or top end with the lengths. I see that Porsche have got the best of both worlds with the varioram thingy.

F

 

Graham,

Do you have an SVX twin throttle body loafing about?

it would cost £150 in welding alone if you cut all the bits yourself and took them to a fabricator to weld together.

Paul

 

Paul, and then the machine work..

and only if your making it out of steel, ally needs tig welding, forget trying to mig ally, you just cant get the heat into it - I know Ive tried using my mig at 250A on ally.. the wire speed you need at that current is stupid.. Hence I bought a tig.

Andy if you need access to a tig or something welding then I have one if you want to come use it or need something doing I appreciate the northern way of keeping costs down...

Im assuming the turbo is going at the front, centre feed bottom of an intercooler / chargecooler, twin exits to tb?? or twin turbo?

David