Foward facing factory plenum mod

[EnricoPalazzo]

Well SydneyKid does deal with these things every day, however the fact that hes saying things that disagree with what others believes makes it a bit controversial to some.

Im staying out of this, however i believe by changing the tb to the front of the plenum will cause some bad implications with airflow on the cyclinders.

[Simonster]

I have a RB25 which has a custom plenum. The only reason I changed it is I wanted to have a cooler with short pipes and I don't like the"look" of the corss over plenum.

This is what I had made. The guy that made it for me has been building racing engines and cars for 24 years.

My goal for it was not to increase power, but to enable me to run the cooler without the crossover pipe as I like the "GTR" look.

I'm still waiting for my turbo to arrive so I can't give any benfits or drawbacks, just my reason for wanting one.

[JimX]

Originally posted by Simonster

I have a RB25 which has a custom plenum. The only reason  I changed it is I wanted to have a cooler with short pipes and I don't like the"look" of the corss over plenum.

That was one of my considerations too, Other benefits it gives are making it much easier to get at the plugs and injectors. I'm soon going to drop a set of RX7 injectors in, and I will be very glad that I don't have to remove the plenum to get at them.

Small considerations maybe, but considerations nonetheless.

[Sydneykid]

Sorry B man but I just gotta disagree with your post "To me - like I said, it's all guess work."

What I said in previous posts is most definitely not "guess work". I have tested the flow rates on our Superflow bench of standard RB20/25 inlet manifolds, plenums and TB's and there is no restriction of any substance up to 450 bhp. That's not guess work, it's a FACT.

I have changed at least 5 X RB20/25's from front mounted TB back to standard and every one of them has made more power. Not just maximum, but a higher average in the useable rpm range. That's not guess work, it's a FACT. Magazines have done similar tests, and guess what, they have found exactly the same thing. They don’t like doing it though, their advertisers hate it.

Every RB20/25 with front mounted TB's I have checked the mixtures on, plugs, exhaust gas temperatures, lambda etc they all have distribution problems. Some are really terrible. That's not guess work, it's a FACT.

I have measured the amount of air between the inlet valve on #6 and the turbo outlet on 3 X RB20/25's with front mount TB's and none of them have substantially less air than if they were standard with 120 degree pipework at the TB. Certainly not enough to make any difference to response. That's not guess work, it's a FACT.

The only guess work here is from people who say they get better response from fitting a front mount TB, when they do so many other things at the same time that they couldn't possibly know for sure what made the difference. If in fact there was a difference at all, the placebo effect is alive and well in car mods.

I have had exactly this discussion with numerous people and I am certainly not the only one who has this opinion. Have a look at the HPI R34GTT, that car has had over $50K spent on it. Do you really think Ben wouldn’t have changed the inlet manifold if he, Martin or Jim thought it would make a worthwhile difference? It makes 295 rwkw and does 11.9 ‘s at 120 mph, all with a standard inlet manifold plenum and throttle body.

Sure there are places for front mounted TB’s and aftermarket plenums, I have used them myself, often. But not on engines with standard internals.

Hope that clarifies………….

[Steve]

Originally posted by Sydneykid

The only guess work here is from people who say they get better response from fitting a front mount TB, when they do so many other things at the same time that they couldn't possibly know for sure what made the difference.  If in fact there was a difference at all, the placebo effect is alive and well in car mods.

Think Gradenko only modded the TB nothing else.

I would love to see back to back on the dyno stock vs aftermarket comparsions. Which magazines/issues are we talking about? Do you have any before/after dynos to demonstrate?

I think this comparison was probably the best I have seen to date as it was a throttle body change - nothing else. Just a shame there wasnt a before and after dyno result.

Also sydneykid - you state that a forward facing plenum would be worthwhile if taking the car beyond 450bhp, then use Ben Ellis as an example, but he is putting down more than 450bhp, perhaps he does need to look at doing one:p might just put down more than 294rwkw.

Ben if you are reading this - lets see a back to back plenum tech file...PLEASE, would be interesting on a car putting down healthy power:)

Perhaps then sydneykid could have a rest:D

[Sydneykid]

Thanks Steve, I need a rest before the heavy weight end of the race season kicks in.

I know you are aware of my thoughts on engine kw (bhp) versus rwkw (rwhp) but just for those who have missed out to date....

A standard R34 GTT puts out 206 kw (275 bhp)

On our dyno that end up as 150 rwkw (200 rwhp)

ie; it looses 56 rwkw (75 rwhp)

Some people would say that's 26%

So if it now makes 295 rwkw (390 rwhp), just add 26%

Then that's 370 kw (490 bhp)

Now I don't subscribe to that logic. I believe it is a constant loss, ie if it looses 56 rwkw (75 rwhp) when it is standard, why should that suddenly increase. Energy can neither be gained nor lost (according to Newton), so where does the extra 20 kw (30 bhp) go? Not in heat, otherwise you could cook your hamburgers on the gearbox and diff. Now I know Einstein disproved that with E=MC2, but there ain’t no atomic reactions going on in my Skyline.

So I reckon it has 350 kw (465 bhp), which is about the limit for a standard internals RB25.

Ooooops I opened the “which is the right way to calculate bhp from rwhp" topic again, sorry….

[B-Man]

Hey Sydneykid - sorry my post was not aimed at your post or your findings in particular.

It is just a general observation I have found.

Maybe I should have said "To me - like I said, it's all seems like guess work". (

Measuring flow when the engine is runnig - the valves openening and closing and all the other factors, pressures, temperatures, forces, - how can one (accurately) determine what is really going on ??

I hope that clarifies where I am coming from.

[RNS11Z]

Originally posted by 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??

Nope, no problems. As sydneykid said, all depends on the computer you have and how its tuned. During the dyno tuning session we checked the plugs many times. We are how ever making the 6th injector work that fractional more rich then the others. Reason: Better be safe then sorry.

[B-Man]

Here's another opinion I sought from a custom fabricator here in Sydney - this guy has an RB25 and RB26 head and has studied the stanard inlet runners, plenums, inlet port sizes, flows, velocities, etc -

According to his maths, Nissan got it all wrong - the intake ports on the head for the RB motors are too BIG - he fabricates a intake/plenum that actually reduces the size of the ports on the head - fabricates 'tuned length' ram tubes with bellmouth ends, enclosed the ends in a cylindrical plenum and reckons he gets 40HP more at the wheels with just this conversion.

He sells this custom plenum conversion for $3000

And the pipework from the cooler comes from underneath and in the centre of this cylindrical custom plenum.

There's another opinion.

Not I'm not saying that this guy is right or wrong - Just another way of forcing air into the cylinders.

It's all good - lots of opinions is what makes life interesting.

[BU5TER]

I think the greddy plenum is great its well mathed up with my TD06 truck turbo

[Steve]

Originally posted by Buster

I think the greddy plenum is great its well mathed up with my TD06 truck turbo

:bahaha:

[JimX]

Originally posted by Sydneykid

The only guess work here is from people who say they get better response from fitting a front mount TB, when they do so many other things at the same time that they couldn't possibly know for sure what made the difference.  If in fact there was a difference at all, the placebo effect is alive and well in car mods.

While I can agree with most of your other sentiment, I don't understand the total vilification of the front-facing plenum in the manner I have quoted above. Using the same logic, a bigger intercooler would not introduce more lag.

It's common sense really - lose 1 metre+ of intercooler piping and how can it NOT improve throttle response? If you honestly think that it can't, then there must be something else with the airflow of the plenum that completely negates the loss of 1 metre of plumbing. I would be interested to know what that is, if so.

Edit: I remember mentioning in the past that I had lost around 3kw in the very low rev ranges due to the front facing plenum. What I had totally forgotten about was the 2 degrees knocked off the timing, which I think would account for most or all of that power loss. I'll see what happens when I play with the timing again after the PowerFC arrives.

[rb20-calais]

awesome discussion, good posts as usual Sydneykid. I am always amazed by the sheer knowledge on these forums.

Evan

[Sydneykid]

Hi JimX, you asked "lose 1 metre+ of intercooler piping and how can it NOT improve throttle response?"

The volume of air is the important measure, not the length of the pipe. Most people use larger diameter pipework (than standard) and loose more than the benefit of the shorter pipe. I know you know that the formula for volume in a cylinder is Pi X R2 X H. So any increase in diameter is going to have a power of 2 larger effect than a decrease in length. Increase the diameter from 62 mm to 75 mm (a very common upgrade) and you loose the benefit of the 1 metre in your example.

Also have you thought about what it does to response when you have one cylinder (#1) so much closer to the throttle body than another (#6)? One cylinder will respond quickly and the other, well it's obvious. That is another reason why Nissan equipped the GTR with 6 throttle bodies, all the same distance from the inlet valve.

Not only have I experienced this a number of times is the real world, I have also done the mathematics on a number of cars. Don't get me wrong, I keep the pipework as short as possible and I also keep its diameter as small as possible (keeping restriction in mind). But moving the TB to front of the plenum isn't ever on my list.

Hope that clarifies.

[Guest RedLineGTR]

as always sydneykid always comes up with the good...very happy with this info...might consider this option soon.

[rbs13]

wouldnt the 6 throttle bodys on the gtr make the imperfections of the manifold design non existant? the plenum would just be holding the air ready for the throttle bodys to open and the air go through. but then wouldnt a BIG plenum hold lots of air ready for the throttle bodys to do their stuff.. and wouldnt the piping/cooler hold air unless its being pushed out by the bov or pushed back onto the turbo and wasted? what i mean is that having six throtlle bodys in the same position like the gtr set up would stop 5 and 6 leaning out.. or would the shape of the plenum just make this happen naturally?

[T0nyGTSt]

From what I'm getting here... the distance between the throttle body and the inlet valves is what is most important.

It is irrevelant how long the inlet tract from filter to throttle body actually is?

T.

[JimX]

Tony, that would be the case if the intake before the TB was constantly under pressure - which it isn't unless you've got the hammer down. So while you're on boost, you're mostly right. But to get on boost quicker, a shorter intake is better.

[Gradenko]

Sydneykid, a lot of your points are valid, but simply aren't a factor with a carefully designed factory plenum mod.

Originally posted by Sydneykid

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.

Which is a major reason why I chose to stay with the factory plenum (albiet modified) rather than go for a full custom job. I also kept the standard throttle body.

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.

I haven't come across any problems yet. Note, even with the factory plenum, certain cylinders show signs of leaness. You've suggested in the past that water cooling issues has some part to play in that. The poorly designed exhaust manifold may also contribute to the rear cylinders running hotter. I don't think its fair to blame known problems on this plenum.

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.

If that was the case, shouldn't I have lost power? Keep in mind that for all intents and purposes, the plenum I have is the same as what Nissan deisgned with regards to runner lengths, runner diameters and plenum volume. All I've improved on is the throttle body location. Comparing modified factory plenums to enormous volume, short runner, custom plenum's just isn't right.

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.

In my case, I changed nothing but the plenum and turbo-afm pipe. Intercooler was already in place and if anything, the current tune is a little more conservative than the last (ie. not on the bleeding edge).

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.

I was mindfull of this even when I had the original intercooler piping done. All the alloy piping is no larger than 63mm and the factory L pipe is 65mm to the match the throttle body inlet. For anyone installing an intercooler and using sanely sized piping (please let me know if mine isn't), a modified plenum saves a lot of volume in comparison to the stock one.

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.

Of course, individual throttle bodies is the pinacle in achieving perfect throttle response (although BMW's infinitely variable lift valves goes one step further!), but how many people are going to pay for the privallege? The basis of your point (distance from valve to throttle body) apply's equally to all front facing plenums, not just our cut'n'shut specials. Mistubishi, Toyota and Nissan have no problems using front entry plenums on their engines. Sorry, but its not a valid "against" point.

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.

Would Mr Gibson not be working within class restrictions? Its possible that changing the plenum volume or structure was not allowed by regulation. He may have found (like I have) that a front facing plenum is a better choice given the reduction in heat soak and length of piping to the throttle body.

[Roy]

Do you have any dyno charts??? This will go along way in appeasing those that are sceptical (not that you have to, only those considering it still sound as though they require convincing)

By shpowing dyno graphs im not expecting it to show any difference in power, but hopefull you will have some graphs with boost pressure shown on them, a means of telling if the thing is more tractable/ throttle responsive. Shorter pipes reducing boost threshold etc etc then it shold hit full boost sooner... shouldnt it? (or torque for that matter)