Foward facing factory plenum mod

[R32-GTS]

I have a mate whos broken 2 motors after he completed his plenum mod. Whether or not this mod caused the leaning out on one cylinder is debatable and needs more evidence for it to be blamed.

[BOOSTMEISTER]

For any plenum mods it is a very good idea to have it flow bench tested...i.e. block off all but one runner and test air flow from throttle body out the 1 hole then do the same for the rest, if they are pretty much the same then go for it....

[EnricoPalazzo]

Is that how the measure on a flow bench, or do they measure each runner at the same time. Would make more sense to do it the latter id think?

[MetBlue_GTST]

Strich9ine, for the throttle cable, it would most likely need to be longer... For my Sub-Zero Plenum i used a silvia cable... Was the perfect length...

[warrick]

has anyone tried one of these?

cheers

warrick

[Gradenko]

rbs13, thats the single worst idea I've heard in a while.

BOOSTMEISTER, if you read my 2nd post again, I explained why flow of individual runners means nothing (or very little) in real world use.

Strich9ine, total cost would go like:

labour to remove plenum

$80 to mod

$85 & $15 for genunine Nissan plenum & throttle body gaskets. (Or make your own, I did).

labour to refit

The rest, ie, silicon connectors and intercooler piping, would have to be done anyway. The only fidldy bit is how you want to mount the bov. I asked for it between the ic and tb and I wanted it to recirculate air back to the afm-turbo pipe (plumb back). That increased costs a bit, but I'm happy I have a non-stalling car that is free of bov sounds (the rubber plumb back piping seems to have dampened the noise).

With the cut down stock factory L pipe, I was suprised they used it but the rational is that its 90 degree bend is a far gentler radius than what they could achieve with alloy donuts. And, the factory pipe already has a bov mount and mount for idle solenoid air feed, so there's less labour charge if they use it.

The throttle cable had to be custom made at $70, but it appears that if the throttle plate was mounted so that the tb linkages are on the drivers side, the original cable has enough length to reach it. Just remember to specify "linkages on the drivers side" when you have it made and it should be fine.

[rbs13]

why is it the worst idea.. say the front facing throttle body does stuff up the flow to the cylinders.. moving the throttle to the other side of the plenum next to the strut tower would make it even again wouldnt ? you just go and say its the worst idea but thats it?? why is it the worst? there is a vl turbo in melbourne with it done, done by P.E.T or something i belive?? i highly doubt that moving of the plenum is going to give you a hp increase only feel better, drive better etc so there is no point going into hp with a foward facing plenum.

[Gradenko]

SLY33, as far as testing goes, no, it hasn't had a wide band O2 sensor stuck in each exhaust runner (who can afford that?), but it has been tuned as best it can. I can't prove that the rear cylinders aren't running lean, but I can assure you that its running safe afr's (overall), has no sign of detonantion and is a better running engine than before.

[Gradenko]

rbs13, if I understand you correctly, moving to a side entry plenum from the drivers side will interrupt runners 3 &4, radically changing their effective length and flow potential. I'm sure a car could run with this setup but Its not something I'd be advocating.

[Skyrine-Dave]

yeh i think RBS13's idea is to have a mirror image of the plenum... not a bad idea if it could be made to work.

[Steve]

.

[Skyrine-Dave]

i want a front facing plenum for a few reasons:

- throttle response

- shorter pipe work

- high mount my turbo (wank factor) 8)

So if anyone can find decent front facing plenums that have been flow benched and work well... please send me some info

-Dave-

[SLY33]

guys the previous posts were not made by me and have been removed! i left my comp logged in and brothers friend had a go.

[Steve]

Well then, my response has been removed:)

[RNS11Z]

This is a pic of my engine bay. Seems to be working well for me.

[BOOSTMEISTER]

Yeh EnricoPalazzo that is right, I was thinking of something else when I was typing, when they do it you can set the throttle body up with some flexi pipe and adjust the angle of the throttle body so that you get the air flow right...etc

[Warpspeed]

Great work guys.........

There is a lot of mystery about plenums, different people tell you different things. But an awful lot depends on the state of tune of the engine.

A lot of the big horsepower guys tell horror stories of how the stock GTR plenum is crap, and how the rear cylinders run lean and how the resulting detonation blows up your engine. Well yes , maybe......

On the other hand I cannot believe the Nissan engineers screwed up so badly, and that they never actually bothered to measure air distribution or test full throttle mixtures on the dyno when they developed the GTR engine. Duh.

It all really depends on the power levels and the air velocity in the induction system. If you are running nearly stock power you can be pretty sure the stock plenum will work fine. And as Gradenko says, moving the throttle body is probably going to do it a world of good, especially with a really nice FMIC.

But suppose you decide to increase engine power from 250 BHP to 500 BHP ? The air velocity is going to double, and all the pressure drops go up four times. You just might start having a few air distribution problems then.

Now what about a 750 BHP car, three times the airflow, nine times the pressure drops in every bend, that stock plenum is just not going to be up to it at all.

On the other hand putting a huge drag race plenum on a stock engine is not going to be all that terrific either.

You just cannot test plenums on a flow bench, steady state flow through one intake runner at a time, will tell you nothing of how six pulsing cylinders are going to behave at different RPMs. You really must do it on a dyno.

[Guest INASNT]

Interesting read.

I was considering a new plenum as i think the stock intake pipe is going to foul on my new high mount turbo.

RNS11Z

You seem to be pulling a fair bit of power with your stock plenum mod in place. Have you had any problems as yet??

[Sydneykid]

Hi guys, this comes up almost once a week. Following are my experiences;

1. The standard plenum and throttle body show no restriction up to 450 bhp (the limit of the standard internals of an RB25) on the flow bench or on the car.

2. I have never seen a modded standard plenum (TB to the front) that doesn't have distribution problems. Pull the plugs out after a hard run and they ALWAYS show leaness at the rear.

3. Because of this poor distribution, you have to tune the engine overly rich on the other cylinders. This looses horsepower, if you put the standard plenum back on you ALWAYS get more power because you can tune the engine more evenly.

4. I have NEVER found any brand of RB20/25 aftermarket plenum that doesn't show poor distribution. Yes, that includes Japanese Tuning Shop ones.

5. I hear the stories of better response and no flat spots etc etc. But at the time of fitting the front facing TB, the car is always tuned better, has a freer flowing intercooler and/or pipes fitted, has other problems fixed etc etc. I have no evidence that shows an improvement in ANYTHING from fitting a front facing TB.

6. What we are trying to achieve with a front facing TB is to reduce the amount of air between the turbo and the inlet valve. So add it up, a bit of high school mathematics will tell you that by the time you add larger diameter pipework, bigger intercooler etc, moving the TB won't save you much in comparison.

7. The other important air volume for response is the amount between the throttle butterfly and the valve, ie the more air you put between the TB and the valve the worse the response. In fact this is more important for immediate response than the volume of air between the turbo and the TB. That's why GTR's have the TB's close to the valves. Think about the extra distance (volume of air) between #6 and the TB when you move it to the front.

8. You can achieve almost as much of a reduction in the amount of air between the turbo and #6 inlet valve by having the intercooler piping with a 120 degree bend at the TB. That's the way Fred had it on the R31 Group A car, and it went OK.

In summary, my suggestion is, until you upgrade the internals of an RB20/25, the standard plenum and TB is perfectly fit for the job. You would be better off spending your money elsewhere. Once you have got to the stage of upgrading the internals, then by all means change the plenum, but make sure you have an ECU that has individual cylinder fuel trim.

Sorry I raved on a bit there.......

[B-Man]

This plenum topic must be the single most subjective discussion point on RB25s around.

Everyone seems to have different ideas.

How can so many bright people have such differing opinions ?

The reason I think - is because no-one reallt knows - and you can not empirically measure flows, power, temp, etc - and statically measuring flow is not real world empirical measurement.

So guys - I think it's all guess work.

But if you are happy and comfortable in the justifications and reasoning for having or not having a front facing plemum - it doesn't matter - as long as you are happy.

To me - like I said, it's all guess work.

Until someone can prove empirically, in a real world situation which was is better for the differnt HP ranges - I just say - Who the heck knows ???

My 2c worth only.