Foward facing factory plenum mod

[Gradenko]

Otherwise known as the cut'n'shut special

The front facing plenum has been the subject of much discussion on the forum, and the modified factory plenum is surrounded with its own stigma. I set off to install my own modified plenum to find out if it was a good idea or not. Firstly, have read of this thread if you haven't already. Specifically my posts starting from here.

Pre-requisites for this mod:

• You have a front mounted intercooler.
  
• You have a stock turbo or aren't currently making more than 500hp (reported flow limits of factory runners).
  
• Some sort of engine management.
  

The aim of the front facing plenum is to gain throttle response by cutting down the number of right angle bends in intercooler piping and lowering the volume of piping involved.

Reasons for modifying factory plenum instead of going full custom:

• Idle solenoid and a/c compensation all intact.
  
• Nissan would have tuned the intake runners for max torque at low/med revs. Exactly what we want and would be hard to improve on.
  
• Plenum volume isn't so large as to cause response loss with our stock turbo.
  

"What about airflow distribution?", everyone asks. I haven't noticed any problems with airflow. Some inaudible detonation was picked up on at the top end, which needed base timing retarded slightly, but theres no evidence its caused by the plenum. A different person tuned the car this time around, so it could come down to something as simple as tuner sensitivity. Also, ye ole 5 knob s-afc isn't the most articulate ecu inteceptor.

Of course, its early days yet, so I'll report any problems as they happen.

Improvements to throttle control were picked up on straight away. Previously, holding the accelerator pedal at a small throttle opening would see the car accelerate linearly unitl it got onto boost when it would rapidly accelerate. This meant constant modulation of the throttle if you only wanted linear off boost acceleration. With the new plenum, I can actaully hold the throttle constant and keep the engine just off boost indefinitely or press a little harder and hold it at a different boost level. No more creeping acceleration syndrome.

I used to have an interesting problem where if I was hard on the accelerator and lifted off slightly just as it came on boost, there seemed to be a massive reversion in air and the throttle would snap shut. This made the car jerk about and was't comforting in the least. Anyone else felt this with a fmic? Well, its gone now. Absolutley perfect control at all loads and throttle points.

Another improvement is with throttle response, an amazing difference here. Its not going to rival a current model LS1 for crispness, but it is a hell of a lot quicker to come on boost. You can be accelerating gently off boost at about 3000rpm and simply twitch your right foot for it to come on full boost in an instant. Full throttle in 4th gear on the dyno saw full boost (13psi) come on by 2700rpm. Best way to descibe it is that it feels like a (slightly) bigger cube turbo engine.

I won't give any dyno figures (because its completly irrelevant), but after long periods of driving, there is a huge difference in the seat of the pants feeling. Previously, the alloy piping would heat soak and rob quite a bit power. With the new shorter route, intake piping is only luke warm to touch (as opposed to bloody hot previously) and it makes a difference on the road. I've since wrapped the pre throttle body piping in insulation to further reduce heat soak from the radiator pipe.

If anyone has any questions about the install, ask away . If anyone in Perth has the extra long fmic piping and wants to see what all the fuss is about, pm for a ride.

Heres a pic of the previous setup. http://www.skylinesaustralia.com/forums/at...=&postid=516510

And attached is a pic of the current setup.

[Gradenko]

And another pic.

[JimX]

Nice! How much did it cost to mod the plenum?

[Steve]

Nice before and after, thanks.

[Gradenko]

JimX, I bought the plenum used, but my mechanic tells me he would charge ~$80 to mod one.

[Roy]

Thats not the price for a shop to do is it?

Labour to remove plenum, cut/fit/weld/finish new throttle body location and weld up old location + cost of any required aluminium. Its going to add up to more then $80 for most of us, surely.

Good to hear you have had success with the mod:D

[JimX]

Well, the other costs of removing/refitting the plenum are going to be the same regardless of what you put on there. I think $80 sounds fairly realistic, as long as the mechanic knows what he's doing (or else he'd take a few hours of working it all out).

[Roy]

Well, the other costs of removing/refitting the plenum are going to be the same regardless of what you put on there.

What i was getting at was as a stand alone mod for someone wanting to change the throttle body location, its going to cost more then this unless you are willing to do a lot of the work yourself... inc throttle cables etc

[AeroGrace]

Originally posted by Gradenko

I used to have an interesting problem where if I was hard on the accelerator and lifted off slightly just as it came on boost, there seemed to be a massive reversion in air and the throttle would snap shut. This made the car jerk about and was't comforting in the least. Anyone else felt this with a fmic?

i've experienced this too, especially when on boost on the lower gears.... (FMIC connecting to the stock plumbing in the engine bay)

Gradenko where abouts in perth did you get that done?

[Skyrine-Dave]

i'd like to consider a mod like this.... but the thoughts of leaning out thre rear cylinders and losing a piston is jsut too great...

[EnricoPalazzo]

Yeh there is alot of debate about people

a. loosing power from this mod

b. Causing huge cylinder starvation of the rearest cylinders.

[T0nyGTSt]

Originally posted by Skyrine-Dave

i'd like to consider a mod like this.... but the thoughts of leaning out thre rear cylinders and losing a piston is jsut too great...

That's a bit harsh and that might be levelled at all front facing plenums.

T.

[Skyrine-Dave]

its mostly coming from my wallets point of view. Uni students cant afford to fix things that go bang.

and its not starvation of the rear cylinders its leaning out due to too much air flow... cyl 1 might be starved of air.

But hey i am not an expert in plenums and i doubt anyone here is, unless you go an flow bench them and provide results.

/me thinks Warpspeed has his own flowbench.....

like i said.. i would like a front facing plenum too

Dave

[Skyrine-Dave]

just another thought... ***For discussion only***

wouldnt the factory plenum be designed to distribute the air flow from its orginal position in the best manner possible ?

by changing the inlet, that would have some affect on the air flow characteristics

Dont jump at me, i am hear to learn too and provide discussion.

and TonyGtst i think aftermarket front facing plenums such as the Trust item would be flow tested and made to suit the forward facing application... hence lots of R&D

[T0nyGTSt]

I would tend to agree with you about the vertical runners and the horizontal travel path of the air.

Also one thing that some people do not see is that most of these plenums reuse the original runner tubes.

The Greddy model does not (ie. it bolts to the cylinder head).

Does that make a difference?

I also submit this one:

http://www.workshopx.com/development.htm

Down the bottom. The carbon plenum. Interesting?

T.

[Skyrine-Dave]

yep interesting and looks nice.

i think the design for most front facing plenums is to be tapered at the back... to reduce the amount of airflow to the rear cylinders to try and even up the air flow. Would be good to hear from someone knowledgeable in this area as i have always been intrigued with the design of plenums.

-Dave-

[Gradenko]

AeroGrace, I installed the plenum myself and Global Motorworks did the piping and finish work with assistance from Perfomanze and SST (all in the same complex).

Despite the controversy surrounding the modified factory plenum, everything I've looked at says it shouldn't be a problem. I've yet to run temp probes to individual exhaust runners to check for lean running, but my tuner is confident with the result. As for power loss, although I cleaned the injectors and replaced the soft afm-turbo pipe at the same time, I'm fairly sure the plenum didn't lose power. Or so my passengers tell me.

As T0nyGTSt mentions, there can't be anything fundamentaly wrong with the front facing design if so many OEM's are using it. When I first got the plenum, I spoke with a head porter about having the plenum flowed. He explained that since we can only flow one intake runner at a time, the final result means nothing as in practice all 6 are pulling air from the plenum at the same time. Air reversion (caused by valves shutting) travel back to the plenum as a wave and interact with other wave fronts causing high and low pressure areas. Very difficult to simulate and measure. A flow bench will tell you the max flow potential of each runner, but not the flow distribution to each runner.

Skyrine-Dave, you are right in that the flow characteristics are different. I believe the reason for the factory design, as brought up in that other thread, is for packaging purposes. A cross over pipe is the shortest route to the stock intercooler in the guard. Any consideration to air distribution can be summed up by the picture attached of a stock plenum: yes, the throttle body does open up straight into runners 3 & 4.

I like some of the aftermarket designs, but I wanted to keep the same plenum volume and runner length as I had (stock turbo, remember), and spend as little as possible. I don't really want to turn this into a "how to design the best plenum" thread, just wanted to offer an alternative.

[rbs13]

then why dont ppl relocate their throttle body to the other side of the plenum.. next to the strut tower.. iv seen this done before on a vl turbo.. it would work the same as the stock one you would think. just the same place only at the back of the plenum.

[JimX]

I think people are getting a little too hung up over flow characteristics of front-facing plenums. Mine is just a tube with some holes cut in it, not flowbench tested or anything, much like the cut-n-shut factory plenums.

I have very slight leaning out on the rear cylinders, but the only way I can tell is by comparing plugs 2 or 3 apart. Any 2 plugs next to each other look pretty much identical. Well actually the way it was tuned it is just richer up front rather than leaning out at the back. Plug 6 is a "perfect" light brown colour, and plug 1 is a slightly darker shade of light brown It's not like it goes white to black.

With this apparently "crappy" setup, I was still able to get +30rwkw with an S-AFC tune. If significant air starvation of cylinder 1 was really happening, there is no way I could get an extra 30rwkw out of the car.

I sold my S-AFC the other day and until my PowerFC arrives I'm back on the stock ECU mapping. It feels like my turbo has been removed

[ix9]

Gradenko,

So you mean it'd only cost around $80 to relocate the throttle body to the front of the plenum?

After the installation of a Front Mount, this would sound like a pretty cheap figure to improve throttle response.

I saw you used the old crossover pipe on the return from the FMIC, was there much fiddling involved ? Extra Piping to be made up ?

How about throttle cables etc, how did you go with that? Surely you needed a new longer one ?