It's a topic as old as the hills, but some people might find this post useful by gathering information in one place, especially those new to the RB series. There are so many options. I'm just outlining what I've found while researching, not arguing for or against.

I'm not the most technically minded and although I've looked at my RB engine a lot over the past few years, I've always come away a bit confused, not quite clear, and thinking it just had too many darn pipes to get my head around. I drew some (very) rough diagrams to help me understand what might be the most cost effective IC choice and piping layout for me. I hope those diagrams are correct enough to get the point across. They are definitely not to scale (otherwise we'd all have ~400rwkw hahaha), but I've just discussed what I've read and learnt about each, and also drawn up one which I haven't seen before (and I wonder why). If I've said anything incorrect let me know, I wouldn't want to lead anyone down the garden path.

The diagrams were made in Paint. Go jump if you think they are poor I'm sure there's better out there, but maybe not a lineup like this. I've also included links to photos of the setups where I can. I can't vouch for the links working as they aren't mine and are from various sources.

Diagram 01 is just the layout of the stock RB25DET engine. Nothing complicated, just to show the parts. The front of the car is towards the bottom left.

Diagram 02 is the same again, but shows the air flow. The air enters the compressor turbine either via a POD filter (boxed or unboxed) OR the stock airboxed (stock or modified). The air then passes through piping underneath the airbox down the front bumper to the side mounted intercooler. It is cooled here. Then it travels back up over the top of the engine block and into the intake manifold (/plenum) for combustion. The exhaust exits the chambers through the exhaust manifold (red part in Diagram 1). Exhaust exits the engine through the dump pipe, and as it goes it spins the fan in the compressor (intake) side of the turbo faster, creating more boost. I won't go into an explanation of vacuum's and operating during lag and during boost, there are WAY more qualified people for that! Anyway, on with the show.

Looking at the picture below, you can see why if your new to the engine it's hard to see what's happening - it's mostly hidden under the airbox and inside the bumper.



Diagram 03 is the basic layout when the common front mount intercooler is installed (FMIC), such as a Trust M Spec kit. The air follows the same process, but the intercooler extends across the front of the engine bay and the outlet piping comes up from the driver's side of the bumper. It get back to the inlet manifold, the average piping kit snakes its way over the top of the radiator fan, passes over the engine block and mates up to the intake. First point to notice, this is a lot more piping than the standard SMIC setup. This is why many people report an increase in lag after fitting a FMIC with on other modifications to suit. There is all that extra piping to fill up with pressure before the engine will respond. These kits can sometimes require cutting of the car frame/metal to fit the pipes. Usually not much. This kind of kit nearly always requires the stock airbox to be ditched, and a POD filter used instead. Some people have modified their airbox to keep it. Some fit a separator of cold air box around the POD. Both of the latter are used in states where having a POD _AND_ a FMIC at the same time is illegal. Some argue that PODs should never be left exposed because they will suck hot ambient air from the engine, others say it makes little to no difference. Some love the induction noise of an exposed pod, others want to avoid it and box up the pod to help achieve this.


And a slight variation with less 'square' angles to avoid the heat of (/hitting) a high mount turbo:


Some people also don't like the fact that the piping passes over the engine so close, saying it heats up the piping, increasing air intake temps and as a result robbing power. Heat resistant tape can be used to avoid this, as can heat proof coating, but this is more expensive. Another consideration is that if you have a large or high mount turbo, this piping route can pass right over the top of the turbo, which some say will have the same effect as passing over the engine bay, but much more so (since the turbo gets much hotter).

My diagram is particularly dodgy here, as more often than not the inlet and outlets are oriented at the bottom of the intercooler. This is the most common intercooler setup, is proven to achieve the aims desired and can be bought very cheaply these days, and often fitted by the mechanically inclined.

Diagram 04 is an intercooler kit which reuses most if not all of the stock piping, and (hopefully!) requires no cutting to the metal. I've only seen these kits on eBay so I couldn't say an example of a brand. However, they are not available from the likes of Trust/Blitz/HKS etc (but please note they are similar to Diagram 05 which ARE). I dunno much else about it, so just enjoy the (un)pretty picture This kit has the benefit of looking stock upon cursory inspection.

Inlet and Outlet same side


Diagram 05 is very similar to the setup in 04, but the outlet of the intercooler is on the opposite side to the inlet. On one hand, the air travels through the FMIC all in the one direction (as opposed to D04), but on the other hand, at one point the air has to do a VERY acute angle, which some people don't like. Manufacturers which make this kind of kit are Blitz (LM model) and ARC. This kit also has the benefit of looking stock upon cursory inspection. Some people think the return pipework on this kind of kit hangs too low and is at higher risk of hitting stuff. Also, the plastic shielding will have to be fudged with to fit one of these.


And another variation where one of the pipes (the inlet I think) comes behind the core. This would be better in terms of how long the pipework hangs, but again, sharp bend in the piping at one end and I haven't seen any big brands make this sort.
One pipe behind the core


Diagram 06 is one I've learnt about only recently. It is the same as the Trust style setup to the point when the air exits the intercooler. Instead of passing up over the radiator fan and across the top of the block, it passes up over the driver's side of the engine block and does a reasonably sharp (~120 degrees) angle into the manifold. This _should_ avoid most of the heat issues possible with the basic Trust type kit, and also makes a significant reduction in the length of pipe in the system, which purportedly reduces lag. Intercooler kits including pipes like this are available from Hypertune, Plazmaman or can be made up by a local shop if you give them good enough instructions or they've done it before. The piping kits can also be bought separately, allowing you to use an intercooler core of your choice, just make sure the parts will all fit together! An example of this kit can be seen in the Gibson Motorsport R31.

Gibson Motorsport/SydneyKid Style

Diagram 07 is a forward facing plenum setup. These theoretically avoid the heat issues of 03 and 06, with the added benefit that it's now easier to access the coilpacks/plugs, and there is less piping than any other setup described so far. The main issue I'm aware of, and it's been measured and recognised as fact, is that since the air isn't entering the chamber from the centre, an inbalance will form between the front and rear cylinders. Unless the manifold is VERY well designed, this could cause problems. Some ECUs can vary the spark/fuel to each cylinder individually, and this can be used to minimise these effects. Some people modify the stock manifold so that it's intake is at the front. I don't think I'll get flamed too badly if I say most consider this a bad idea, as it mucks up the flow, and will exhibit the issues stated previously, only much worse.

Forward Facing Plenum

Diagram 08 is one that I found halfway through writing this (hah!). I know very little about it have only seen this one picture like it. It is basically a Trust kit, but the airflow is opposite to usual. I'm not sure what to make of it, but it doesn't make the sparks/coils any easier to get to, and the pipe lengths look similar (or longer) to a Trust M style kit. It just shows there is lots of choice in modding the RB

"Reverse Flow" Style

Finally, Diagram 09 is something that occurred to me. I'm sure someone's come up with it before but I've never seen anyone talk about it, and never seen it in real life (admittedly I haven't looked to hard). It's basically the same as 06 and 07, but instead, the air would entering the chamber from the outer side of the manifold, instead of looping over the engine to meet the stock inlet. At first this would appear to be a very direct route for the air to enter the chamber. It would avoid passing over the engine bay (so no hassles getting to the sparks/coils). It would have a little less piping than setup 06, and just a little more than a forward plenum.

The only possible issues I can see are:
- Clearance with the bonnet and other engine components
- My suspicion that the intake was designed to curl over the top of the block to create some kind of "ramming" effect on the air just before it enters the manifold.

I think it would be really interesting to see if the last type works. Anyway, I think that's a nice wrap-up of the intercooler and piping options that the average street-goer might choose. I'm sure there are more wild and whacky setups for racing etc.

Edit: Numbered the diagrams, added more pics

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wow. nice detailed write up. good stuff!

I'm keen to know if anyone's tried the setup in diagram 9. I'm SURE I'm not the first to think of it, and hope it's been tried already. Maybe SydneyKid or someone with experience in track.

Kick - Was hoping this would generate some discussion!

I tried number 7. Stock turbo full boost by 2500rpm now. Don't know why for sure. Didn't bother to check the a/frs. Car feels good, drives good.

I vote this goes in the sticky topics of tech section !

Sticky!

stock car... and wants front mount number 8 is the way to go in my opinon ive done it to a r32 gtst before and works great

Diagram 05 is awesome, thats what I am running on my R32.

Looks dead stock in the engine bay, also put stock air box in to give it a very stock look.

Didn't have to cut/move or break anything to fit it in and it doesn't even sit that low. I like it because the return pipe to the intake manifold always gets air and it stays cool, so its cool upon entrance to the intake plenum.

Also retained the stock guard lining as well.

Mine sits fairly heigh to be honest, you can see the bottom pipe through the front bar. Its a BLITZ kit, easiest and BEST mod I have ever done.

Nice write up mate, very informative!

Thanks I suspect if the mods think a thread is worthy, they sticky it themselves? I'll be putting my money where my mouth is shortly and getting number 6 (with a new turbo too though, so won't be able to make any worthwhile comparison ).

Edit: gonna get a Blitz one now haha

I have #4 on my stagea (trust kit) and it is fantastic... no noticable difference to stock at all

this is by far the most effiecient way of doing it on a skyline (if fitted correctley DOES NOT affect ground clearance), we did back to back tests BLITZ LM SPEC vs TRUST TYPE R on a r33 skyline and the intake temps on the BLITZ were far better even after heat wrapping the cooler pipe that crosses over near the radiator fan.


if you think about it too the air leaving the majority of the core only needs to change direction via the lower 90 degree as the air is already travelling down the end tank... another thing to note with BLITZ cores is that they vary the fins inside the delivery tubes. They are tightly pack along the bottom and become "less dense" as it goes up the core this improves cooling as the charge is distributed over the core better.