So has anyone been able to determine why Nissan chose to rob the 2 front cylinders of air flow, by sloping the plenum?

I find it hard to believe Nissan would spend extra money "re-designing" a plenum simply to accomodate the current hood and strut bar.

Moving/changing the strut bar with added brackets (ala Crawford) would have cost less than a Plenum re-design.

Even in a worst case scenario where the body design engineers and motor design engineers, were living on 2 different planets, and one day discovered the plenum wouldnt fit, still doesnt warrant a plenum re-design over adding strut bar brackets the way Crawford does...

I can think of more reasons for Nissan to do this "after the fact" due to design snafu's than as a deliberate design from the start.
Unless, the original plenum was so drastically out of bounds in its size/shape, that more than a strut bar change/move (hood change, etc) was warranted and the plenum re-design ended up being more cost effective.

Any thoughts?

 

I think they redesigned the lower plenum to allow more flow? I would still guess replacing/spacing the upper plenum would still be required. I know we are still waiting on Performance Nissan (Jason) to dyno with this new lower plenum.

 

the plenum needs to be sloped to maintain constant intake air velocity between all of the runners. as you take air away, the velocity decreases. the cross-sectional area must be reduced to maintain the original velocity. this is true of NA cars.

i assure you that nissan did this on purpose, and it wasn't a packaging or clearance problem. i am guessing that this is why the spacers are making more power than the plenums.

it is interesting, however, that the plenums are making power. i don't think that this was a design accomplishment, however, but more of a lucky find.

Chad

 

As a product designer these are my thoughts... You touched on some of them.

I believe the exterior designers brought an overall design concept to the table and someone at the top said make it happen. So they did...they probably looked at hundreds of side profile's and said this is it, this is what we want and what we're sticking with. Let the engineers figure out how to make it happen. So the next stage, making it "real", (ala engineering team) was probably slightly behind the exterior designers. Ever go to an auto show and say tha car is Purtty, but it has no engine, suspension or other "real" guts. This is most likely the case with the Z. Little eye candy to get the public to say YES, YES, DO IT! So to keep the story short, I believe the eng. Team over exaggerated the plenum slope to gain "safe" clearance for final production or for any unforeseen problems down the road. Once everything fit and was tested, strut bar, engine, hood, they said who cares about a little HP loss, the public won't care; it's got enought at 287. In the end, imo, Nissan thoguht it was too costly to re-cast and engineer the OEM plenum for ittle HP gain and that's why you see the 05 Z with a re-designed lower plenum. It was more cost effective to tweak existing parts then re-design and manufacture three new parts, ie strut bar, hood, top plenum.

On another note: To move the strut bar out and beyond the plenum is not the smartest thing to do, it's not going to hurt the car, but it's not the most efficient design either. The offset strut bars have a slight twisting motion on the bolts, not as efficient as the oem strut bar setup. I know the OEM mounts have a slight sheering action on the bolts, but much more force is directed into the strut towers vs the other setup, it's simply a better solution and why Nissan did it that way.

Theories...
Zquicksilver

 

Physics would say yes...I shared your thoughts in the begining and compared it to AIR DUCTS in an office building.

But someone from Crawford explained engine air-flow dynamics to me and it changed the whole ball game.

 

I'm with Chad on this one, Nissan needs to sell for total system efficiency, not just power. It also explains why the G35 has the same upper plenum and no strut bar, which I believe has little do do with cost because Nissan seems willing to spend a little extra on it's Infinity line.

Chris

 

wait a tic, who said that SPACERS were making MORE power than plenums?

I havent seen this, and unless someone can show me a couple dynos for each, I will remain skeptical.

 

I "think" you may have misunderstood... I think he meant OEM plenum. The spacers are making more power when combined with the OEM plenum than just stock

I would say the Infinity design team opted for no strut bar intentionally. It would ruin the "look" Infinity is after, which is not true sport, but luxury sport sedan... They used the same plenum as the Z because it is efficient as an overall cost in manufacturing. It's easy and cheap to come up with a different injection molded engine cover.

 

when i said that the spacers were making more power than the aftermarket plenums, i meant it. i don't have either one, (and don't push any of them) but i have looked at the dyno charts posted my motordyne engineering (hydrazine) in great detail, and it clearly shows that the spacers with the stock upper plenum make more power than the aftermarket plenums.

and in all honesty, the plenum manufacturers out there are not engineers. they make nice products, and have been able to stumble upon some horsepower. the bigger plenums make sense for forced induction applications, but for NA you are looking to keep the velocity constant, and limit pressure drop. why do you think that the VQ has been able to reach a volumetric efficiency of 1 or more. that is unheard of for a street motor.

Chad

 

So the spacer with stock plenum is the way to go?
I bet thats too hard for a novice to install.

 

As far as "the way to go", bang for the buck is excellent compared to most of the bolt ons I have seen and it retains the stock look and strutbar. I would like to hear if anyone has tried the 1/4" spacer with the 05 track lower plenum though, that little combo sounds interesting (but ups the cost considerably).

No, I think the average Joe could do it, the two main things to remember are 61 INCH pounds of torque (NOT FOOT POUNDS) in the correct order and don't drop anything down the port runners (and if you do, you have to get it out before you start the car). For more on the How To check out my site http://www.hypersprite.com/auto/tech-z-002/index.asp
also keep a heads up to the inventors classifieds, I hear there might be a GB coming up soon for the spacer.

I personally think this would be a great club Tune-In type of thing at someones house because most of it is easy wrenching with only a few little gotchas. Plus you can all chip in on renting an inch pound wrench for the day if no one has one lying about (not all that useful for most people, it will hardly ever come out of the tool box).

Chris

 

just to clarify...i think only the 1/2" spacer is the one that makes more power than the aftermarket plenums. i completely forgot that i am on a Z forum, because i drive a G. for us, there are no height limitations, so i would naturally get the 1/2" spacer.

 

Thanks Hyper

 

Thats complete ********. They aren't engineers? Whoa I think you need to go do some research about some of the people designing the plenums. The independent dynos are not out. Nothing has been proven spacer wise. Don't make claims that have no backing. Plus alot of people claim that the spacers do essentially the same thing that the aftermarket plenums do so how would they make more power? Hydrazines dynos had flaws and sections where the power "unexplainably" dropped. Plus if I remember correctly the spacer with the Crawford plenum made more power then the spacer with a stock upper plenum.

 

the key to dyno testing is a large sample survey, and an average. if you dyno back to back with different mods, you may see different increases/decreases from run to run. the spacer dynos clearly showed an average increase in power across the board. these dynos were also performed on another board members car, so it was an independant dyno in a way.

i noticed that you are from tennessee...is it a coincidence that you brought up only the crawford plenum?

 

nice...

 

I agree. Look at the design of some of the plenums, they are basically all right angles, very simple design, looks like cake mold. Sorry but that's not optimal anything for flow or pressure. Maybe they can bend pipes with the best of them but intakes are a different and much more complex beast. The larger spacers have consistently shown more gains than the aftermarket plenums. Just take a look at the dynos, there are several...

 

+1, and i have direct experience.

 

Which Plenum was used for the test? Oh wait Crawford; so why do you think I mentioned Crawford? Because it was used in the test. If you can't put 2 and 2 together like that don't say anything. Plus I don't even have anything Crawford on my car.

All the dynos come from manufacturers which typically use their lowest and highest dynos to make claims. Until there is further independent testing they aren't proven. And to about the air velocity it is more important to get volume. The engine is pulling the air through the runners so air velocity in the plenum doesn't effect anything.

 

So, it sounds like the best think to do is get a spacer and keeep the stock Plenum.
Correct?