2.5" or 3" i/c piping

[evil_si]

I would think it would be better to have the cooler (and therefore lower volume) air flowing through the smaller pipe

I don't think this statement is true - if the air is cooler, it becomes more dense, and since the entire system is pressurised with probably less than 1psi difference between pre-and post-cooler, then the volume of air after the cooler is actually going to be greater than before it.

hot air expands cold air contracts,
everybody has different theories on this and most setups will work fine,

guys that deal with gas turbines will tell you, you need 3" from the turbo and 2" to the throttle body.

personally id run 3" all the way if there is room for it and dependant on the length of the piping,

I run 3" on hot and cold sides on my rx7 and so does the 2j altezza both with excellent results,

[deaf_rattle]

here are 2 pics of the set up ill probably be running with (most mk2 guys run their pipe work like this)

[deaf_rattle]

as you can see, going front facing manifold will be alot of work.

[QikStarlie]

looks like 2" piping in those pictures.
no gains to be had going to 3" piping with that throttle and manifold. 60mm throttle will flow less than a bit of 2.5" tube

it'l blow a head gasket either way

[Malcolm]

no gains to be had going to 3" piping with that throttle and manifold. 60mm throttle will flow less than a bit of 2.5" tube

That's just plain not true. There will be greater pumping losses with smaller piping: just because one part of the inlet tract is a small diameter, it doesn't mean there's no gains to be had by having larger diameter elsewhere. That doesn't necessarily mean there will be a big difference, but it'll be there.

[deaf_rattle]

it'l blow a head gasket either way

ha, not for awhile i hope
hks mhg and arp head studs should look after that.

[deaf_rattle]

looks like 2" piping in those pictures.

definately 2.5"

[xsspeed]

That's just plain not true. There will be greater pumping losses with smaller piping: just because one part of the inlet tract is a small diameter, it doesn't mean there's no gains to be had by having larger diameter elsewhere. That doesn't necessarily mean there will be a big difference, but it'll be there.

he speaketh the truth.

Over a length of pipe there will be an associated pressure loss per unit length. The higher the velocity the higher the pressure drop.
There are also pressure drops that occur at fittings such as bends or transitions in size. The losses at transitions are going to affect you whether you have larger or smaller pipes, however the effect can be mitigated by 'smoothing' the transition. Sweeping your bends with a larger radius also helps.

Of course you come to a compromise between velocity and pressure drop.

I would say, finger in the air sorta thing, that 2.5" would be substantial for what you are looking to get out of the motor. Plus it will look tidy and will allow sweeping of bends moreso than 3", not to mention easier to work with for the fabricator

[QikStarlie]

yeh i know how it all works. but in this situation, dout there's anything to gain. i was just stating it in the short version

[xsspeed]

didnt mean to sound like a dick spelling it out was more thinking of when people look at this thread and need further explanation when I was writing that.

I can see your point too, as at the end of the day the turbo is just a pump, so approx the same m3/s is gunna make it to the engine, just comes down to mixing velocities then

[Malcolm]

yeah that's not really the case because it's a centifugal pump, not a fixed displacement, and even with a fixed displacement pump (ie twin screw supercharger), you'd be suprised by how much power can be lost due to small pressure drops through the system (especially on the inlet side). Anyway, I think we're bordering on a thread-jack - sorry matt

[xsspeed]

approx the same m3/s

I was very careful in writing this just for you, must be the mechanical engineering lol
Also sorry Matt

[mjrstar]

i'd be less worried about going to 3'' pipe and more interested in making a foreward facing plenum to save about 2 metres of intercooler pipework...
not a 7m but the 1g in my starlet is along the same lines.

[deaf_rattle]

yeah, i have thought about it, but while running the stock ecu, i dont think the gains would be worth it.
you need to relocate the alternator for one.

maybe down the track ill look at that.

[docTRD]

i rekn go 2.5" piping, if u can get a kit then even better will save a lot of hours labour making up all those custom bends. im going to be running 2.5" on my 1jzgte cressida as i don't want it laggy and going 3" will be more expensive and I wont be able to use a piping kit so yea it can get $$ going too big.
my 2c

[deaf_rattle]

the kits are $$$ and i already have the i/c, so yeah ill go with some custom plumbing. although id imagine it will turn out roughly like the kit anyway.

i guess with the 1jz, you already have the ffim. lucky!
haha