LT1-7

Say you have 3" pipes from the turbo but the intercooler inlet and outlets are only 2.5". Is performace going to suffer allot or will I barely see a difference? I remember when I installed my FMIC it only had 2.5" piping and it seemed to spool up the turbos way quicker than my stock IC

maxcooper

The stock pipes are 2.75". It is generally undesireable to change the diameter a bunch, so keep the number of section changes to a minimum. Most turbo compressor outlets are 2.5", so using a 2.5" pipe from the turbo to the IC makes sense to me in your circumstances.

There is an air speed threshold to stay under (just a rule of thumb). Corky Bell talks about it in the Maximum Boost book. I calculated it out once, and 2.5" pipes seemed to be fine for a high guess at the max airflow I calculated for the FD.

-Max

HWO

most guys run 2 1/2 from turbo to intercooler becuase its the compressor outlet size, and 3 inch from intercooler to TB.

3inch into the TB seems to be the way to go, it allows the car to breath/inhale more somehow

SPOautos

Here is my take on this and I'm sure if someone doesnt agree they will say so lol

You're running boost which means there is air built up in the pipes because the engine cant breath as much in as your throwing at it. You have 6 "chambers" that are about 13.17 cubic inches each. This means your engine is going to be taking in air 13.17 cubic inches at a time. You have a lot more than 13.17 cubic inches of air from the lim, uim, tb, elbow so to me the IC pipe wont affect how much air your engine will take in. Now if you were flowing 1200 cfm and using pipes that would only flow 800 you'd have a problem. But I dont think your going to run into that problem with 2.5" pipes. If your engine was sucking in more air than your pipes could flow your boost would start droping off in the higher rpms.

Is my thinking wrong????

STEPHEN

HWO

when i asked RICE RACING about using 2 1/2 inch from the IC to the TB he said "if you want to make power you need 3 inch"

so there has to be some explanation behind it

BicuspiD

I would think 2.5 piping should be sufficient for a ton of horsepower. a larger pipe will increase volume, which might affect initial lag , and it will decrease velocity, which I'm not sure of the effect in a rotary. Not good for piston engines, you want consistent speed of air through, not pulsing. Plus remember if the intercooler if working properly, the air is colder, so its denser. You would think the opposite would make sense.. 3" inlet to the IC to supply the volume of air needed to fill 2.5" outlets , since cooling the charge " makes the same amount of air smaller"

Figure out the amount of boost you are running, estimate HIGH cause boost is addictive, and what turbo, and you should have come up with a CFM number the turbo will flow. No need to exceed that flow capacity with the piping IMHO.

nocab72

No doubt! I always laugh (to myself of course) at the people saying they are going single and want to run a max of 14psi.

How long does that 14psi max last? Till the first time they goto adjust their boost controller to 14psi, overshoot, hit 16 and see how much more to life there is once you approach the efficency of that single you just spent $4k bolting on?

K

peejay

That is true for a positive displacement supercharger, but not for a turbo.

Turbos don't create boost via backpressure. They physically compress the air by basically scooping it and slinging it at a curved wall. They need to be flowing the right amount air in order to be able to do this effectively, which is where the science/art of turbo sizing comes into play..

HWO

logically yes, but have you EVER seen a car with that set up????

RICE RACING

What happens is that you pick up on VE, using the larger pipe, also you modify the air speed, this is critical in every type of engine (forced or NA) at a certain range of air speed you get most efficient charge mixing and port efficiency, well the same happens in the delivery piping, in that if a certain velocity is exceeded then you end up with a drop in VE for that section of the system, which accumulates on the the total VE of the Engine system as a whole. It is related to temperature of the gas or fluid as well, the hotter it is the less pipe diameter you need, because it actually does "flow" better despite the increased pressure.

Without getting into the technical stuff (you got to remember there are MANY variables) If you are talking post IC, then for engines making 400 to 700 bhp @ gauge boost pressures of 1 to 2 bar then a 3inch Inlet pipe covers most of them, in actual fact closer to 3.5 for 700bhp & 2.5 for 400bhp (would be ideal), but you get the picture. This is very different to what type of "delivery pipe you would use for a NA 300hp rotary, anything under 6inch or twin 3inch causes a losses in VE ! It is esentialy a function of (power, or air volume flow rate), pressure, & temperature, the actual density "Is not seen" by the pipe system.

You can go to large for sure, then down side is increased lag, and lower mixing efficiency (which affects engine power by increasing the BSFC) if you have any addition injectors in that section of the sytem.

A three inch post IC pipe is a realy good compromise for best efficiency in the top end with minimal losses at lower engine speeds for a 13BT street (and alot of race) applications.

LT1-7

thanks guys.....lots of info here

peejay

I think I might understand why... the airflow even at the throttle body isn't "smooth" because of the ports opening and closing. after the IC you don't really want the air moving TOO fast, since it uses more energy to accelerate/decelerate the air column the faster it's moving, so in a sense the larger piping is acting similar to a surge tank. but before the IC you are far enough away from this that the airflow is more linear.

does this sound correct, or am i way off base here?

i agree it "seems" to make more sense that you have the larger pipes before the intercooler, since the air is less dense... but i looked at the "Big Picture" and realized this.

maxcooper

Perhaps the temp difference of the before/after IC pathways evens out. RICE RACING mentioned that hot air flows better than cold air, so perhaps the flow is related most closely to the number of molecules flowing per unit time, which will be the same before and after the intercooler (n molecules in means n molecules out, after all). The CFM will be lower after the IC since the volume occupied by those molecules will be smaller with the temp drop, but perhaps the temp difference balances out the difference. It seems like it might be "molecules per minute" that really matters, with CFM just being a useful simplification.

-Max

SPOautos

Thats how turbos compress the air but what does that have to do with running boost. Your turbos are spinning and compressing air even when your engine is under vacume. If your engine is inhaling more air than the turbos are putting out then how would the system pressurize???? If its not pressurized you dont have boost. The "boost" is the built up air that your engine isnt taking in.

What am I missing??? Maybe we're talking about two different things

STEPHEN