Intercooler pipe size...lemme know what you think!

[KATwo40]

Ok, guys, engine installation time is VERY near (couple weeks tops) and I have a final decision to make, regarding intercooler pipe sizes.

To summarize, this will be a 400whp application, using a T3/T04E .50 trim, running in the neighborhood of 20psi, with a fully modified intake manifold, fully built setup, blah blah blah.

So, let's see what you like for IC pipes!

[1WheelWonder]

I voted 2.5 hot and cold so we can be twins or bestest buddies like that, yeaaah.

[KATwo40]

lol

[Chezedik]

I have 2.5 and will be in the 350whp zone. How much difference can that extra few HP make?

[lrb_2000]

2" hotpipe is good for about 500hp.. so yeah.. I have 2" hotpipe, 2.75" coldpipe.

[NateDogg]

Think about it in physics terms like exhaust...

The hotter the charge the faster the charge travels with more energy yadda yadda yadda (requires larger pipe compared to colder).

So, if you think about that then you would use a larger hot pipe than cold pipe.

Now, 2.5" is good for 450hp. So, I would go 2.5" hot and cold. Or...you could even go 2.5" hot and 2.25" cold if you wanted a bit more response.

[KATwo40]

Think about it in physics terms like exhaust...

The hotter the charge the faster the charge travels with more energy yadda yadda yadda (requires larger pipe compared to colder).

So, if you think about that then you would use a larger hot pipe than cold pipe.

Now, 2.5" is good for 450hp. So, I would go 2.5" hot and cold. Or...you could even go 2.5" hot and 2.25" cold if you wanted a bit more response.

What about scavenging effect? When dealing with exhaust, the system never goes large to begin with, down to a smaller diameter pipe. If you made an exhaust manifold begin with 3" pipe and squeeze into a 2.5" pipe, the exhaust gas velocity would be very low, resulting in no scavenging from the head.

Air velocity from the turbo is not only a result of the physical "pushing" causec by the compressor itself. It's created by a combination of the "push" and the thermal expansion of the superheated gas as it exits the compressor outlet. If the hotpipe is too large, there will be more volume per sq.in. of pipe, but less velocity.

My main concern here is whether or not the 2" hotpipe will cause the gas to exceed the recommended velocity at my intended power goals.

Additionally, most OEM setups use a larger cold pipe than hotpipe, and not the other way around.

Thanks everyone for the input, and please keep it coming!
Modified by KATwo40 at 4:21 AM 11/15/2006

[bige240]

With that size turbo and that much PSI I would recommend 2.5" hot and 2.75" cold... the reason you have an intercooler and piping is to cool off the charge, and most of this is done by gradually letting the air expand which also causes a denser charge (cold air is denser that hot air). No matter what, you are gonna end up with 2.75" at the end, cause that is what the plenum size on a 240 is. the only way I would say go with something smaller is if you had a small turbo that could not pump enough air to fill the pipes which would make your turbo over work its self and blow.

[crzycav86]

I'd just use what everyone else uses, unless you want to experiment with effects on power. I posed this same question before I bought my piping, and I think that was the general consensus.

As far as I know, larger piping will be less restrictive, so it should make more power, but there's always the law of diminishing returns, and space availability. The downside is increased spool-up time.(This is stuff you probably already know)

I'd just go with 2.5 all around for simplicity. My kat has 2.25 hot and 2.5 cold.

[NateDogg]

What about scavenging effect? When dealing with exhaust, the system never goes large to begin with, down to a smaller diameter pipe. If you made an exhaust manifold begin with 3" pipe and squeeze into a 2.5" pipe, the exhaust gas velocity would be very low, resulting in no scavenging from the head.

Air velocity from the turbo is not only a result of the physical "pushing" causec by the compressor itself. It's created by a combination of the "push" and the thermal expansion of the superheated gas as it exits the compressor outlet. If the hotpipe is too large, there will be more volume per sq.in. of pipe, but less velocity.

My main concern here is whether or not the 2" hotpipe will cause the gas to exceed the recommended velocity at my intended power goals.

Additionally, most OEM setups use a larger cold pipe than hotpipe, and not the other way around.

Thanks everyone for the input, and please keep it coming!

Modified by KATwo40 at 4:21 AM 11/15/2006

Hotter gas travels faster because it has more energy, therefore you can afford to use a larger hotpipe to maintain a certain gas velocity than you would with a cooler charge.

As the gas travels through the IC, it gets cooled and looses energy and velocity.

IMO, best circumstance for the cold pipes is a 2.25" or 2.5" section from the IC go to a 2.75" section from the battery tray to the TB.

Most OEM's use a larger cold pipe because by slowing the velocity, you create a higher pressure against the throttle body, which increases transient response.

IIRC, the ideal increase in cross-sectional area to maximize transient response is 25%.