Greddy/Trust Plenum

[Steve]

Originally posted by Sydneykid

This is S of S here...

Lets put it this way; I reckon having the throttle body closer to all of the cylinders is way more intelligent than having a throttle body close to cylinder 1, but miles away from from 5 and 6, which is what is causing a few people problems with air distribution. Sure #3 and #4 are favoured, but then so is #1 and #2 in a forward facing manifold... but #5 and #6 don't get ANY air... Son of Sydneykid

Here is something else for you to think about, 5 and 6 GET MORE AIR than 1 on a forward facing plenum (thus lean out and cause troubles). The air gets backed up (for a better way of putting it) at the back of the plenum when using a front facing throttle body.

Next time you get to have a look at the HKS drag car, you will notice it has a front facing plenum. HKS realise that 5 and 6 get more air so they flow test the injectors and put richest to 6, next to 5 and so on. Why would they use a front facing plenum if it was better to use a side facing one? They are only interested in making the most out of performance. You can also argue that multiple throttle bodies give better response, they do, but less air in the pipe work gives better throttle response too (and less heat soak from under bonnet temps - as minor as that would be) It all adds up.

Sure you can say, but if I make the pipe work thicker and reduce its length - but that would hold true if you made the pipe work thicker with the stock plenum.

Bottom line, if the pipework is the same width but differnent lenght, the shorter will hold less air? how could it not.

If you create a forward facing plenum with the angle of join from the throttle body such that it directs air toward the front cylinders, that can help offset the richer state of the these cylinders. If you shunt it in from the side, 3 and 4 will always be leaner. I think this is what gradenko was getting at. That is how I interprated it anyway? If you have a look at the design of the cut and shut plenum gradenko posted, you will notice it follows this philosophy. This is of course if only using one thottle body instead of 6.

There are miriad possibilities when performing a mod, so the fewer the variables, the more indicatitve the results will be in a test type situation.

So far, I have never heard anyone say fitting a front facing plenum made my car worse to drive, I have only heard positive results.

I definately agree that 6 throttle bodies do it better than one, but I cannot argue with people that have changed to a forward facing plenum and found better results - too many of them.

It could be argued that its a waste of money, but I would suggest that every body has different driving style, and likes cars to drive differently. Two people can get in the same car, one love it the other hate it.

Please forgive me if I am wrong here, it is sometimes very hard to gleen intention from a post, but it seems (the vain of your post) you have closed mind to the whole idea, a closed mind cannot learn anything.

[sidewaymambo]

This thread is a very informative thread, and I could not agree more from different point of view, please keeping post..

[Roy]

Ok im going to run off at the mouth before crunchin numbers.

Below is only my thoughts based on some experience with solid modelling and fluid modelling.

Firstly consider the cfm (and in turn the velocity) of air as it passes the throttle body.

Secondly when looking at an RB20 inlet manifold, the distance between the throttle body and the bellmouth for cylinders 3 & 4 is approx 130mm (Actually favours cyl 4)

So considering that you have air coming into an engine at a given velocity, this velocity will drop when the large volume of the plenum is introduced just after the throttle body.

It would be wrong to think of the inlet air as your garden hose blasting thru the throttle body favouring cylinders 3 & 4. Closer to the truth is that the inlet air is slowed to a point where the inlet plenum is a reservoir of air waiting for the inlet valves to open.

Now up to a point, introduce some posotive pressure (boost) into the equation and this only helps to ensure equal cylinder filling. Now i say up to a point as if you increase boost, or more importantly cfm to the engine, then you are also increasing the velocity of the air as it passes the throtle body (same diam with more flow increases velocity)

Now as the velocity is increased, there is a point in time or hp, where the volume of the plenum wont ensure the velocity is dropped to a point where it does not adversely effect certain cylinders. Be it RB20/25 cylinder 3 & 4, or RB26 cylinder 5 & 6.

S of S is correct in that there is the Apexi GTR that runs 8s using the std inlet plenum. But it pays to mention that it no longer uses the multi throttle setup, instead uses a 100mm (?memory?) front facing throttle body. This changes the velocity in which the air comes into the plenum as it is larger orifice, and secondly increases the distance to cylinder no. 1

It pays to remember that the R31 GTSRs used the std RB20 inlet plenum and were able to produce approx 450hp whilst adhereing to Grp A regs.

So there will come a time if you have a turbo generating greater cfm/hp then std, that a new aftermarket plenum will help performance. But to say the std nissan gear needs replacing for "blah, blah..." isnt warranted, it is quite clearly good enough for almost all of us.

Thats not to say the Greddy plenums are nothing but cosmetic add-ons that hurt hp, far from it, but we all have reasons for doing what we do. I just dont think the hp benefits of such a plenum should be too high up on the list of its benefits.

The above is my thoughts only, and are exactly that, my thoughts. I am not attempting to pawn them off as fact, but to the best of my understanding they are close to the mark.

[Sydneykid]

V-MAx uses a 90mm Infinity throttle body... Its a drag car, so, 6 throttle bodies aren;t really of benefit... I now re-undertand that you get back pressure when the throttle is closed, then when throttle re-opens, it causes air flow to by pass cylinder #1. I guess this is exactly why the Top Secret Plenum is shaped the way it is (throttle body is on and angle and face directly at cylinder #1. I can now understand why on the modified standard plenum there is wall plate, so air flows to cylinder #1, instead of flying past.

[Gradenko]

Originally posted by Sydneykid

Before I start, I am not trying to shoot you down Granenko. Just one point confused me. IF you can explain exactly why a front facing throttle body is better, please go... I didn't really see any reason it would be from your post... but

I don't mind some friendly discussion, thats why I posted it . The reason why its better is easy, the front facing plenum saves me at least a metre of piping. I should have made that clearer. Theres a pic attached showing what the piping looked like before the plenum change. Why didn't I use a 120 degree bend at the thottle body in the first place? Cause it would have increased the cost of 'cooler installation at the time (since a new bov mount would have been needed), and I wasn't prepared to pay that much when there was a chance I might change the plenum in the future.

Originally posted by Sydneykid

Sure #3 and #4 are favoured, but then so is #1 and #2 in a forward facing manifold... but #5 and #6 don't get ANY air...

As others have explained, 1 and 2 aren't favoured. Rather, a pressure build up at the back of the plenum would tend to favour 5 & 6. I see your point about Nissan putting big money into the design of this plenum, but consider that their goals may not be the same as ours. Nissan's target was to lower the cost of ic piping and shorten its route to the factory intercooler.

Nissan would have a huge R&D budget, but I'm sure they used it to lower costs, rather than even out airflow and give the aftermarket a good base to work on. It's likely that crazy rich fuel maps and dropping timing at the first sign of any real air flow was their safe guard against bad plenum design.

The front facing plenum is no stranger to Nissan, look at the ca18det and sr20det. Or what about the 4g63, 1jz and 2jz? All big power engines with single throttle bodys on front facing plenums. I'm convinced packaging was Nissan's main reason for the cross over plenum.

Roy, I see what you're saying re: the standard plenum being sufficient for horsepower reasons. Thats one of the reasons I kept my plenum rather move to a new design. Another benefit of modifying the factory plenum is retaining the factory runner lengths. Nissan would have tuned the runners for max torque at lower revs (as most manufacturers do) and thats likely very difficult to improve on, so its something I'd like to keep.

Originally posted by Steve

If you create a forward facing plenum with the angle of join from the throttle body such that it directs air toward the front cylinders, that can help offset the richer state of the these cylinders.

Thats it in essence. If the thottle body was mounted flat to the front of the plenum, there would be a greater chance for air to bypass the front cylinders and pool at the back of plenum. By mounting the throttle at an angle, air is directed to the side wall and dispersed so no runner is favoured.

[Roy]

I'm convinced packaging was Nissan's main reason for the cross over plenum.

When looking at the std RB20/25 IC piping arrangement, the inlet plenum has been designed to minimise piping length/volume. With the std SR setups the piping is woeful running all over the engine bay. Maybe has somehting to do with SR/CA being used in FWD applications

So the tables are turned when putting a larger front mount on each of these cars, SR setups favours short piping runs like the GTRs where as the RB20/25 crowd end up with engine bays looking like std SR/CA engine bays.

So i agree Nissan packaged the std RBs so that effeciency was maximised and the for a std road car SR/CA/1JZ etc werent packages as well. They unfortunately werent thinking about us and our 600 x 300 x 75 ICs, which turn the tide the other way.

Oh and another thought, is that the longer the piping the softer the hit of boost. So with a big turbo and RWD, the longer pipes dampen the hit of boost which is only going to help traction. (Doesn't make it ideal, but does have a positive effect)

I have always liked the idea of a big water/air IC on RBs. Stable inlet temps and very short piping. Anyone handy with Alum and a TIG?

[Gradenko]

Originally posted by Roy

With the std SR setups the piping is woeful running all over the engine bay. Maybe has somehting to do with SR/CA being used in FWD applications

I've been thinking about it and I think Nissan went for a front facing plenum on the s13s because there isn't enough vertical room under the bonnet for a crossover pipe. You always hear about how tight it is to fit rb's are in there, so it seems likely.

Originally posted by Roy

Oh and another thought, is that the longer the piping the softer the hit of boost. So with a big turbo and RWD, the longer pipes dampen the hit of boost which is only going to help traction.  (Doesn't make it ideal, but does have a positive effect)

SR's don't really have a soft boost hit though. And the piping is ideal for jz engines with their front mounted intercoolers. (Maybe not so much on the Soarer with its side mount).

Originally posted by Roy

I have always liked the idea of a big water/air IC on RBs. Stable inlet temps and very short piping. Anyone handy with Alum and a TIG?

Read the recent AutoSpeed article about an air to paraffin wax ic? Has some benefits over air to water. Interesting reading.

[Roy]

Yeh, read it, but doesnt really appeal to me, some clever people out there though.

So is there anyone clever enough to make an inlet manifold for an RB20 similar to the old style SRs that still use long runners and face the throttle body to the front, bu tucking it up under the runners?

If thats my biggest grudge with Nissan then im doing alright, i cringe at what Renault have already in store for us.

[Sydneykid]

Thank you muchly for the reply and explaining your views exactly.

I Believe the reason they went for a front facing plenum on the SR20s is for 2 reasons.

1 - more under bonnet room between engine and front of car... whther it is the chicken or the egg, I dunno (front facing plenum or lots of room - which came first)

2 - The idea works well for a 4 cylinder engine, but not as effectively on a 6 cylinder engine? Possibly...

thats the way i see it anyways, altho i guess Nissan holds the true reason.. probably more do do with cost...

S of S

[Roy]

When i was looking into doing an RB20 plenum, i started by looking at race cars plenums including BTCC 2L, le Mans cars, and aftermarket plenums from Motec/Edelbrock etc etc.

Then remembering that the GenIIIs used a Nylon plenum, i started to look at using fibreglass/carbon fibre combined with machined alloy bellmouths.

So i was impressed when i saw this kiwis design, that followed my thinking

[T0nyGTSt]

Yeah a non metal plenum would be nice and cheap too.

T.

[GTS-t VSPEC]

Yeah, I've heard a few people are making plenum's from carbon fibre, can't wait to see the finished result:D

[Ronin 09]

Guys FYI - Trust are out of stock of the intake manifold for the moment in case anyone is interested.

[Steve]

They will have more on the 15th.

[JimX]

Wow, how off topic did this thread get? Well since the discussion is currently about comparing front-facing to top-mounted, I'll summarise my experiences.

I compared my dyno sheets from before and after the installation of a bigger intercooler and front facing plenum, and down low I have about 2-3 LESS rwkw on the post-mods version. So I actually lost a tiny bit of power down low. But once it gets into the mid range, the new setup absolutely hammers the old. I don't know how much of this is because of the new plenum, or the better intercooler, but whatever the case it's better than before.

In the upper rev ranges I have more power as well, but it dips and surges for reasons not yet fully known. Part of it was fixed with a stronger wastegate actuator, but there's more fixing to be done.

I pulled all my plugs out and checked them individually a few months after the install. They were all very similar in colour, but very slightly gradually lighter to darker from back to front. So I believe this plenum does favour the rear cylinders, but not to any great extent. Plug 6 was a perfect light brown colour, and they got slightly darker from there. So I don't think I have any leaning out problems.

Benefits I've found that I'm sure are the cause of the front-facing plenum are:

1, Better throttle response due to shorter intake tract.

2. Easier to get at spark plugs.

3. Easier to run cheaper top-mount injectors with a custom fuel rail. Also easier to take them out for cleaning/replacement etc.

4. Neatens up the engine bay (subjective I guess).

I suspect that it's also given me more mid-range power, but as the upgrade came with a cooler upgrade as well I'm not certain how much (if any) is the cause of the plenum.

I don't know if it's worth it (to me) to pay $1100 for a Trust item for the above benefits, but my home-made one cost me less than half that and I'm more than happy with it.

[benm]

So you lost 2-3 rwkw down low but how much did u gain up high ?

Edit: Reading over numerous threads over the past year to me it seems like power costs about ~$90 per 1 rwkw. So if you were wanting a front facing plenum 'for power' then an extra 13rwkw would justify buying a trust front-facing plenum.

[JimX]

Hard to say how much I gained up high because (a) the cooler was also upgraded, and (B) I ran more boost after the upgrade. Pre-mods was 166rwkw @.8bar, post-mods was 215rwkw @1 bar. 20% more pressure doesn't equal 20% more power though, so I think that there has been a definite noticable gain in there. Plus it dips and surges like a bastard, once I get that sorted out and whack the new injectors in I should be getting well over 220 with a more even curve.

[Roy]

Was the management changed/re-tuned when the new cooler and plenum went on?

[JimX]

S-AFC only. Because of the dips I'm considering getting a PowerFC. First step is to increase the dump pipe size to make sure it's not a strangled exhaust.

[Roy]

JimX, are you measuring boost at the plenum, befoire the intercooler?