turbine diameters
 

i have a quick question....i am still trying to figure out the perfect size hot side for a turbo. is it true, that the smaller the turbine that slower the spool up? i mean, if you think about it if you have a small sail on a sailboat, you will only be able to capture enough wind to make you go a limited speed. but if you have a bigger sail on the sailboat, more air is caputred - making you go faster. also, isnt the bigger the diameter of a wheel = higher top speed ? maybe i sound like an idiot but it makes sense to me. i have an air research t3 42/48 and it full spools around 4500 on a b18b. lets say i had a t3 60/62, what would be mine spool time? but with the 60/62 you pack more of a punch at lower boost vs a 42/48. maybe i am on to something, a little help?

Re: turbine diameters
 

you have it backwards. something's wrong if the 42/48 is fully spooled at 4500 rpms

Re: turbine diameters
 

hold on, the t3 42/48 and the 60/63 use the same turbine whee believe it or not, the difference is in the exhaust housing, the .63 housing flows more due to its bigger channels.

Re: turbine diameters
 

but the compressor side is also 60, so it would push that much more air in cm2...right?

Re: turbine diameters
 

no help?

Re: turbine diameters
 

accordepicenter has it right.

For most T3's, the exhaust wheel is the same - to vary the spoolup / top end characteristics, the manufacturers use a different A/R exhaust housing. There *are* differing T3 turbine wheels available, but for the most part, they're the same.

The A/R is the ratio between the cross-sectional area of the passage that begins the "spiral" in the turbine housing and the distance between the middle of this passage and the center of the turbo. When talking about the same sized turbos with the same sized turbine wheel, the "R" (radius) is usually fixed, since the turbine wheel is going to be the same diameter / radius.

So what the A/R really tells us is the cross-sectional area of the "spiral" in which the air flows to spin up the turbo. Higher A/R, bigger cross-sectional area.

To see how this might affect spoolup and top end characteristics, consider this analogy, which i've found useful when explaining this to people who aren't familiar with the concept.

Take a pinwheel (the child's toy that just has a llittle spinning turbine like thing on a stick), and two things:

1) A 12" long McDonald's drinking straw
2) A 2" diameter, 12" long PVC pipe

Now, using each of these, point one end at the pinwheel (perhaps an inch away from it), and blow through it.

Which one spins the pinwheel up faster?

The answer is 1) - the McDonald's drinking straw. Why? Since you can create a relatively low volume of airflow from your lungs, the smaller cross-sectional area creates higher air velocity in the straw than in the 2" PVC pipe. In the pipe, you probably can't create enough velocity to even make the pinwheel move.

Of course, we also need to keep the area high enough to not restrict flow. Consider a household garden leafblower blowing into the same 2" PVC pipe and drinking straw.

In both cases, there will be more than enough velocity and flow to spin up the pinwheel - however, the drinking straw will choke off the leaf blower to the point where you can hear the poor motor struggling against the back pressure. The same thing happens when you use a turbo with an A/R that's too small for the amount of aiir your engine can flow. It spools up nice and quick, but because the area for the exhaust gas to flow is so small, the skyrocketing exhaust backpressure chokes your engine and restricts power.

The idea is to pick the smallest A/R that doesn't create too much exhaust back pressure and restrict your power output.

Re: turbine diameters
 

thanks for the above ^^^^ very imformative. i am going to go to a local muffler shop and get a 2.5in turbo back exhaust....with a 42/48 is 2.5 a little small??? because i think i can go 3 inch.... ;D i will be upgrading the turbo in the near future to a t3 super 60.... ;D

Re: turbine diameters
 

There is no such thing as a turbo exhaust that is too big. 2.5" is big enough to not be terribly restrictive, depending on what kind of catalytic converter and muffler you have, but the bigger exhaust will always give you faster turbo spoolup.

Re: turbine diameters
 

i thought turbo needed a little backpressure, i have a hks super dragger exhaust and its a ------- piece. $500 totaly wasted, but i didnt pay for it. anyways it rusted through a couple of places so i am going to ask my buddies at the muffler shop to just cut that ---- off and throw a straight 2.5 pipe all the way back to my muffler wich has no glasspacking ;D ------- hks

Re: turbine diameters
 

the less backpressure the turbo has, the better. The best exhaust is NO exhaust on a turbo car