08 WRX & Plastic Intake manifolds
 

An interesting article about the usage of composites (plastics) that have replaced aluminium in the new 08 Impreza:

[url]http://www.sportcompactcarweb.com/fe...wrx/index.html[/url]

To quote from the above article in sport compact car (on the 08 Impreza):

[QUOTE]---------------------------
The Legacy's EJ25 uses the same block and similar heads as found on STIs and '06 WRXs. The main difference is the intake manifold and associated plumbing. In order to reduce manufacturing costs, the Legacy motor uses an ABS plastic intake manifold and end-tanks on its top mount intercooler. This is good for weight reduction and lowering intake temperature, but comes at a cost for high-boost turbo cars. Unlike conventional aluminum intake manifolds and runners, plastic manifolds typically use a recessed rubber O-ring to seal against the cylinder head instead of a flat gasket. Under stock boost, this is fine, but for anyone who wants to upgrade turbos or just increase the stock boost levels past 17psi, the O-rings just might do a Challenger. The plastic intercooler end-tanks will have similar issues with high boost or prolonged beatings at stock boost.

While bolting up an aluminum STI manifold to the heads in order to hold more boost is a relatively easy task, getting the rest of the Legacy fuel, vacuum and boost plumbing to fit is not. Changing manifolds requires almost a complete engine bay fuel system swap since OE-quality fuel lines and connectors are near impossible to modify. That means the intercooler and throttle body will have to get changed out too.

Changing the stock VF40 turbo also presents another set of problems. Unlike the STI and WRX, the VF40 compressor outlet is a flange that bolts directly to one of the plastic end tanks. Not only is this not the greatest idea, WRX and STI turbo upgrades will not work with Legacy manifold/throttle body/intercooler set-ups. Again, you'll have to change the manifold or look for a Legacy bolt-on turbo from companies like Deadbolt Performance.

The lesson here is really to not be greedy with power. Keep the hardware stock and change out some intake and exhaust plumbing, and the new WRX should be more car than most can handle. If you have to have big power, then get ready for a huge financial commitment.

-------------------------------------------------------

Bottomline, it seems like the plastic setups will reduce weight but unfortunately, is not designed to cope with power that is well beyond the stock rating. The design intent in going with plastic, seemed to be cheaper manufacturing costs rather than any performance objective.[/QUOTE]



*****************************************************************************************************

But you also have to consider results from real world testing:
(From NASIOC)

[QUOTE]Quote:
Originally Posted by akoshy
Bottomline, the key point is the following, from the above referenced Sport Compact Car article on the 08 WRX and the GT before it:

In order to reduce manufacturing costs, the Legacy motor uses an ABS plastic intake manifold and end-tanks on its top mount intercooler.

[B]The design intent here is to reduce manufacturing costs not some kind of lofty performance target that Porsche etc were striving for.[/B]

That is a misleading statement there at the end. Just because they were able to reduce manufacturing costs does not mean they did not have performance in mind - that intake manifold is on the 276hp Legacy GT in Japan, for instance, which has significantly more power than the upcoming USDM WRX.

The statement also does not reference *what* costs were being reduced, and where. Reducing the cost of aluminum usage again does not mean that they reduced the performance.

In fact, almost all cars are designed to reduce manufacturing costs, with the exception of multi-million dollar specialty automobiles. No manufacturer says to their engineers "increase manufacturing costs, will ya?".

The SCC article unfortunately is a single voice that is relying on the opinion of aftermarket tuners whose experience levels we do not know. So far we have heard of one possible urban legend case that the o-rings on the plastic intake manifold have developed a leak, and we don't even know if that case was because of incorrect installation, manufacturing defect, or a host of other issues that come up with aftermarket tuning. And on the other side there is far more proven cases where the plastic intake manifold *has not* failed under higher boost levels. 400+ whp Legacies already exist. My personal car has been seeing 20+ psi for *3* years now.

Bottom line is that there is only speculation that the composite intake manifold is inferior to the older aluminum intake manifold design.

There has been no hard proven fact that it is. I would instead speculate that since the composite plastic intake manifold is a much newer and more refined design than the aluminum intake manifolds, it may actually have superior flow and performance.

Cheers,

Paul Hansen[/QUOTE]

oh happy times.. hopefully the sti is not the same

I've edited the post to present a more balanced view from US Legacy owners.

The Oz 08 WRX engine is going to be very much based on our 07 Liberty GT engine.

the plastic manifold has been around for years, the only downside to aftermarket tuners is the DBW throttle option only. everything else is win-win. infact i would consider the plastic manifold much more efficent then the aluminium ones to date.

The only concern I have is long term longevity in regard to heat and boost cycles.

I know for instance that the hot side plastic end tank on MY99 WRX radiator are not the best with increased pressures and temps, and fail over time. This may be more to do with design characteristics and lack of extra reinforcement/ribbing than the plastic itself.

yah i'd agree, although the material thickness on the new plastic manifolds is quite hefty, its not flexible or as susceptible to damage as the scare tactics make them out.
they still use tgv deletes, so the plastic manifolds dont actually directly connect all the way down to the heads.

Being plastic too, it should be a better heat insulator than the alloy manifolds...same sort of advantages as using phenolic gaskets :)

[QUOTE]but for anyone who wants to upgrade turbos or just increase the stock boost levels past 17psi, the O-rings just [B]might do a Challenger[/B][/QUOTE]

Hahah, harshness !!

I think people need to understand the properties of 'plastics' before thinking they are weak and no good.

A lot of plastics used in engineering are a lot stronger, lighter and better than steel or aluminium. People have a tendency to assume steel will be stronger.

Personally I fail to see what the issue is. Subaru release a car to be driven as it is, they don't release a car to be modded and run 25psi. If you purchase a car and start to mod it you have to expect that some parts will be under engineered for your task and will need to be replaced. In saying that, I would assume all parts have massive safety factors in built in the design, so they will be able to withstand the general publics mods.

It's a bit like saying Subaru are wankers because the stock 5 speed doesn't handle 500kW atw.

[QUOTE=WRXTASY]The only concern I have is long term longevity in regard to heat and boost cycles.

I know for instance that the hot side plastic end tank on MY99 WRX radiator are not the best with increased pressures and temps, and fail over time. This may be more to do with design characteristics and lack of extra reinforcement/ribbing than the plastic itself.[/QUOTE]

The plastic radiator tanks on every car fail (on the hot side) and i do agree that in most situations i dont think that they will pose a problem. The only car where i have seen a few fail is daewoo matiz's. This is where the coolant meets the plastic intake manifold (which is unusual). The plastic falls to bits and leaks coolant. Though the same thing could happen to aluiminium too if the owner failed to look after their coolant.