GTR Plenum V Greddy Plenum

[KamikazeR33]

sheesh that even made sense to me

[sweetr33]

Hey Sk, is there going to be much difference if i use an 85mm throttle body? Or am i actualy better off trying bigger like 100mm?? Im not up with this airflow stuff so please inform me? I wont be racing around a circuit, it will be dily driven on street with occasional willowbank day for drags and drift. no circuit racing. does this make the single tb choice alittle better?

Steve, I have "tried it" both with a standard RB25 throttle body and a Foulcan throttle body.  I could fee lthe slowness instantly on the road, and on the track it was disasterous.  With the dog box we have ~50 milliseconds for a downshift and you MUST match the engine rpm with the gearbox rpm.  Otherwise you end up with twisted splines on the input shaft from the triple plate clutch and missing teeth on the gears, particularly 2nd.

It is quite simple to understand why this is the case.  With the multiple throttles there is very little air between the butterflies and the inlet valves.  So when you close the throttle the airflow to the engine stops instantly, therefore so does its power production.  Compare that to a single throttle body with a plenum in between the butterfly and the valves.  The usual rule of thumb is at least double the engine capacity in the plenum, so on a 3.1 litre that's 6.2 litres.  Plus that 6.2 litres is under boost just before you close the throttle, so at 1.5 bar it really has 9.3 litres of air in it.   In addition, after you close the throttle, the plenum goes under vacuum, around 0.25 bar is not unusual, that's another 1.6 litres.

So when you close and reopen the throttle 7.8 litres of air flow dulls the response.  On a 3.1 litre engine that's almost 5 revolutions of the crankshaft, at 7,500 rpm that's 666 milliseconds.  That's more than 10 times slower than the gearbox will handle.  To translate that into distance, at Bathurst it would mean we would be lapped twice during the race distance

Another way to look at it, Nissan designed the GTR to win races on the circuit, if they thought a single TB was better then they would have used it.  Let's face it, a single TB is simpler to make, therefore cheaper, easier to tune and maintain.  But Nissan went for multiple TB's, why?  Because for their stated aim it was superior.

Let me close off with;

[Sydneykid]

Hey Sk, is there going to be much difference if i use an 85mm throttle body? Or am i actualy better off trying bigger like 100mm?? Im not up with this airflow stuff so please inform me? I wont be racing around a circuit, it will be dily driven on street with occasional willowbank day for drags and drift. no circuit racing. does this make the single tb choice alittle better?

An 85 mm butterfly or a 100 mm butterfly, it makes no difference in the example I have given above. It is the volume of air between the butterfly and the inlet valve that matters. Obviously if the turbo can supply more air than will flow though the buttefly, then bigger is better. But I personally have used a standard RB25 TB and inlet up to 450 bhp and it is certainly not a restriction at that power level. I have seen a Zeni Tani built 550 bhp RB25 with the standard TB and inlet and it did not seem to suffer either.

Regardless of whether it is for road or circuit, as most people know, I hate cars with poor throttle response, turbo cars already have too much for me, even at their best. So I see no reason to make it worse and, if everything else is equal, a single TB WILL make it worse. Well up to a power level that I have yet to exceed anyway.

If you are intending to use a MAP sensor driven ECU (not an AFM driven ECU) then go with the single TB, it makes the tuning so much easier. Multiple TB's and MAP sensors are not happy together.

Hope that answered your question.

[ylwgtr2]

i concur with sk

[sweetr33]

i will be using a power fc with the single tb. Im alittle worried about the difficulty of adapting the gtr plenum to my rb25 head. Seems port design is different. i have no idea.

[lwells]

that's 666 milliseconds

See, its the devil's work!

Seriously, very good robust debate guys. Keep it up.. some of us are learning a lot

LW.

[RB26VL]

sorry to be off topic but every post i see from sydneykid is of excellent quality and as others said he is teaching everyone something new

keep it up

so also with my rb26 with 6 throttles bodies, and autronics be a problem to tune with more power? (currently got 350hp atw, 12.0 @ 118mph)

cheers

Brad

[Sydneykid]

i will be using a power fc with the single tb. Im alittle worried about the difficulty of adapting the gtr plenum to my rb25 head. Seems port design is different. i have no idea.

Power FC runs AFM's so the multiple throttle bodies would be OK.

You didn't really think that Nissan would make it easy to put RB26 (flagship) stuff on an RB25, did you? They simply didn't want the value of GTR's being deminished, so they made it as hard as possible. :headspin:

[Sydneykid]

so also with my rb26 with 6 throttles bodies, and autronics be a problem to tune with more power? (currently got 350hp atw, 12.0 @ 118mph)

cheers

Brad

Hi Brad, not a problem for the Autronic, if well tuned of course, they can handle multiple throttle bodies

[Kaido_RR]

SK? so how hard is it to mount the gtr plenum to a rb25det? (r33 model) what mods would need to be done?

And to add to that, do u know anyone that could mod to fit the rb25 and wat price am i looking at?

SK u have turn me off the greddy plenum and onto the gtr one apart from the cost factor, by the sounds it much better

[Sydneykid]

SK? so how hard is it to mount the gtr plenum to a rb25det? (r33 model) what mods would need to be done?

And to add to that, do u know anyone that could mod to fit the rb25 and wat price am i looking at?

SK u have turn me off the greddy plenum and onto the gtr one apart from the cost factor, by the sounds it much better

Honestly it is cheaper to buy an RB26 top end and sell the RB25 top end, than it is to fabricate the stuff to make it fit an RB25. Plus you get all of the RB26 advantages, solid followers, valve springs, valves, ports, combustion chamber, squish etc etc. And the other parts, cams pulleys, no VVT etc are much cheaper and easier to get second hand for an RB26 than an RB25.

I have done the numbers 20+ times for people, it always works out cheaper in the long run, you get a better result and it's much easier to sell if you ever want to.

[Kaido_RR]

thank SK, maybe i should start a new thread but i will ask u in here anywayz, will the head etc (top) be a direct bolt onto the bottom end of the rb25?

and wat price am i looking for the 26 top end?

[N I B]

450rwhp is easy on a totally stock RB25 intake so why bother.

[N I B]

Thort a hi tech racer would have flat shift anyway.

[DICKY]

kidney sid

good read mate, just for the record, croydon went 8.55 with 6 throttle body's