Foward facing factory plenum mod

[Gradenko]

rev210, I believe the 120 degree bend is similar to BOOSTMEISTER's piping route.

(taken from this thread.)

[GTS-t VSPEC]

Relocating the carbon canister, that is a good idea:D

[rbs13]

lol.. maybe i should take my carbon can off.. its not conected :S.. thats how i bought the car.. there are some lines running off it but not all..

[gts4diehard]

I have an r 32 with rb20 det. The factory plenum changes for number 5 and 6 cylinders beacuse of the brake master cylinder, so I not so sure that it would create the same mixtures for cylinders 5 & 6 as it does the others. So i dont think the factory manifold is that perfect at creating equal air flows for all 6 cylinders either - unless it has some sort of deflector internally.

[justified2.4]

Many risks listed here such as A/F ratios effected on the front cylinders as well as adverse effects on the rear cylinders...

However many Nissan engines, (i.e. GT-R, Silvia, etc) all use straight runner style plenums as well as the Greddy and other aftermarket intake plenums.

My argument is that these cars show little to no issues using these intake styles, although I do agree, the GTS-t as well as the Toyota Supra in various models utilize this center oriented intake manifold so there must be some reason why they opted to use this design. But I feel theoretically that the straight runner should not show a significant difference, especially when more airflow provided via front mount intercooler and typically larger diameter piping when changing over to the straight forward design.

Off topic, can those that have them post pics of completely custom intake plenums on the GTS-t, including the sub-zero and shop fabricated. I have one that was made by a guy here in the US that I will post this week once I get it.

[Sydneykid]

Hi justified2.4m, comparing the inlet on a inline 4 cylinder (CA or SR) with an inline 6 (RB) is not valid due to the 50 % extra length of the engine and therefore plenum. Secondly a GTR has 6 throttle bodies exactly the same distance from each valve, that is not the same as having one throttle body at the front of the plenum.

Hope that clarifies

[B-Man]

Hi,

What if one was to extend the standard inake runner up into the forward facing plenum - an extenstion if you wish of the runners, and then bellmouth the ends (that are inside the plenum) ?? This might go a way to allieviating the leaning out of cyl 5 & 6 -

However, it also may disrupt the air flow in the plenum dramatically.

[Gradenko]

B-Man, that kind of design would require a completly new upper plenum, which blows out the budget somewhat. While bellmouths will help to increase flow potential of idividual runners, they would do nothing for flow distribution. But with a fresh start, an upper plenum could be designed and tested for better distribution.

Spending that kind of money would only be worthwhile above 450hp.

[B-Man]

Hi Gadenko,

I already have a forward facing plenum

But I am concerned with the lack of air flow to cylinders 1,2, 3 & 4 (ie leaning out in cylinders 5 & 6) as has already been mentioned numerous times.

I am aiming for around the 450 HP mark - and am researching options.

It might be possible to 'un-weld' my plenum from the top and insert some 'ram-tube, bellmouthed ends' - which is why I asked the question. I'm not sure if this is possible though as I am not a fabricator/fitter/turner/engineer.

This might be more cost effective for me that say: reverting back to standard plenum OR buying a Trust/Greddy forward facing plenum.

Maybe I am being paranoid - but what I know I want is reliability !

Cheers,

[Sydneykid]

Hi B-man, there are a number of things we do to get around the rear cylinder leaness issue. Remember the leaness is, at worst, around 5% on #6 and 3% on #5. So there are a couple of minor tricks you can employ to ensure you don't have localised detonation.

Firstly we put the highest flowing injector in #6, the next in #5 etc. There is always a difference in flow rates of individual injectors, as little as 2 to 3% can help a lot.

Depending on what ECU you have, you can also add a little fuel to the rear cylinders via the injector trim. Again 2% to 3% on the trim, plus the extra flow from the selected injectors, ca njust about overcome it.

Lastly don't get too adventurous with the A/F ratios. Sure it will make a little more power at 12.5 to 1 than 12 to 1, but the reliability goes down exponentially.

Hope that helps

[Steve]

My injectors all flowed within 1%

[B-Man]

Hi Sydneykid,

When you say the leaness is at most 5% on cyl 6 - what do you mean exactly ? 5% of what ? (Sorry for my igonrance)

I will be getting the same injectors as Steve fitted (550cc Sards) - and if they flow 1% difference - and Steve is talking the flow rate of the injectors - But you are talking 5% of somthing else - This makes me confused ?

If you mean 5% fuel flow between injectors and I only get 1% difference in the injectors then this is ~ 400% difference in what you recomend. (OMG..)

Maybe I am missing something ? (Highly likely BTW)

I understand what you mean about AFRs - ie back it off a little - that's for certain !

I will be running PFC - so individual fuel trim will not be possible for me.

Thanks for the feedback

[Guest RedLineGTR]

So it all basically comes down to tuning. If the tuner is good u can try avoid major problems with the rear cyclinders.

[Sydneykid]

Hi B-man, I have a set of 550 cc rated injectors that we tested recently. The lowest flowing one is 560 cc's and the highest is 575 cc's. So the lowest is 101.8% of its rated capacity and the highest is 104.5% of its rated capacity. The difference being 2.7%.

So if I put the highest flowing injector in #6 and the lowest in #1, then #6 will get 2.7% more fuel tha n#1 for the same length of time of injector opening.

Hope that makes sense.

[B-Man]

Yes, I think I understand the difference in flow rates between injecors - but how does this relate to the flow rate of air through the plenum and TB ?

And, I dont relly get this :

there are a number of things we do to get around the rear cylinder leaness issue. Remember the leaness is, at worst, around 5% on #6 and 3% on #5

I'm not trying to be a smart arse - It takes me a number of goes before I get it ..... -

I do understand that there may be a difference in air flow rates between the Cyl 1, 2, 3 & 4 as to 5 & 6 due to the length of the forward facing plenum - ie the momentum of the air flow induces higher O2 density in cyl 5 & 6.

But what is the percentage difference in air flow (hence O2) versus the % difference in fuel flow needed to correct the varying air (O2) levels ?

Cheers,

[Roy]

If only a company like Injection Perfection idenitified the potential market for inlet manifolds on the RB series engine.

Not only are RB engines plentiful in one of the most popular imports, the RB20/25 are popular for engine conversions.

If a company sold a high quality, cast aluminium plenum, using 6 throttle bodies then i would jump at the prospect of getting one for my Rb20.

Added fashion of having front facing plenum, added performance of 6 throttle bodies etc etc.

[Gradenko]

Roy, I had thought about doing a custom 6-throttle setup... until I ran the numbers.

6 x 48mm DCOE TB's: $1,770.00

Fabricating a plenum: $800-1200

Airflow distribution tests: $$$ (modified exh manifold for wideband O2 sensors, results used to redevelop plenum)

Even if you choose to even out airflow distribution problems with an aftermarket ecu, you're still looking at a substantial outaly. Almost cheaper to buy a new rb26 head ($1200) and intake manifold from Nissan.

[Sydneykid]

Hey Roy, it's simple. If you want that sort of stuff, buy an RB26 top end. It's way cheaper, and Nissan spent millions of $$$ getting it right, far more than anyone else, and is ever likely to.

Hey B-Man, You have got it right, the difference in airflow changes with RPM, changes with boost, changes with individual cylinder temperature, even minor differences in cylinder efficiency, including piston ring, tension, valve seat width, valve seat recession into the head, etc etc.

Predicting it is a waste of time, only way to do it is to measure it.. The cheap way is by looking at the spark plug colouration, the next is by taking infra-red temperature pyrometre readings from the primary pipes on the exhaust manifold. Lastly, the best method is wtih an individual lambda sensor in each individual primary exhaust pipe.

A number of people have done these tests, and the results seem to indicate a good starting position is to assume 3 to 5% over average air flow in number 6. 1 to 3% in number 5 with minus 1 to 2% in number 1.

I would treat these as rule of thumb, that you should carry o8ut your own tests on your specification of engine.

Hope that helps.

[sewid]

Originally posted by 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.

I was going to say the same thing. Seems to make some sense.

[justified2.4]

here is a pic of the custom intake I bought from a guy here in the US, we were working on making a plenum that just bolted onto the lower half of the manifold but I got impatient.

more pics here