rzograbian

what does a/r on a single turbo mean and it is better to be higher or lower? what does it measure and what is better if you are going for top end hp..

thanks for all your help,, trying to get some knowledge of what to buy!

RETed

A = area or cross section, R = radial; it's the ratio of the cross section area (of the turbine passage) versus the radial length from center at any particular point (in the turbine housing). Basically, the number dictates how quick the turbine passages closes from entry to the point the exhaust gases hit the exhaust wheel.

To simplify things, the A/R dictates your power band of the turbo. Lower A/R makes for quicker spooling turbo. Higher A/R makes for power later in the RPM band. So why not just run a small A/R for the quick spool? There's a downside that if the A/R is "too small", you'll choke your top end power. So everything is a compromise.

Turbine A/R selection is one of those black magic things that most rotary experts don't like to talk about openly.

Compressor A/R don't mean jack, so ignore any A/R numbers from the compressor side.


-Ted

Fish

Ted, the other problem with small A/R's on rotary's is that they do not get rid of the exhaust gases that well.

I know of one stock 13B engine using a GT30 with the small housing on it, that had carbon build up and went pop.

Always go for the bigger exhaust housing on a single turbo.

Dan

'//icked

So bigger Turbine A/R selection means more power but later on in the RPM's (more turbo lag); while smaller means quick spool but less power up top. Is that correct? I always see P trim and Q trim and numbers such as 1.05 or .58... I am a newb so if someone wanted to take the time to explain those to me in a broken down form like Ted did for the A/R I'd really appreciate it.

little rotor

P and Q trim are two different sized T04 turbine wheels.
P is the most commonly used on a rotary. Q is larger and is normally considered for drag only.
The number you refered to is the A/R ratio of the turbine housing as Ted explained. People use anything from 0.84 -1.32 depending on their goals.
All this information is describing the exhaust side of the turbo only.

Carl Byck

"So bigger Turbine A/R selection means more power but later on in the RPM's (more turbo lag);"

Not necessarily, could be more power unless you are already at the optimum size. Yes, later in the RPM band

" while smaller means quick spool but less power up top."

Quicker spool both in terms of RPM, and response, yes.
Might not be less power if that AR is the optimum size for the exhaust energy you can produce.
This is what Ted is refering to as the Black Magic", in a nutshell, you need to set a goal, "X" HP@ "Y" rpm, or this range of HP within this rpm band. Once you do this, you then select an AR that matches those needs, for most people this means experimenting, or using known results for a given combo. For a select few magicians, There are mathematical equations which take the guess work out. The equations are known(and available), but generally several of the variables are elusive, so for the common man, Other peoples results can be extrapolated to put you in the neighborhood of the results you are looking for.
As for Q, P, R, O, P, trim these refer to different geometries of various wheels, AND the ratio between the largest, and smallest(inducer/exducer, minor/major) dimension of the vanes on the wheel. It is NOT true that "x" trim is for this application, and "y" trim is fo this. For certain ARs (and combos of hot, and cold sides)a particular trim may be better suited even though, as a general rule it should be slower to respond. A perfect example of this is a Q trim T66, It will for a given size AR generally spool as fast, or faster than a P trim, even though the P trim is smaller, and should spool faster. It is just not that simple, that is why it is best to use a known combination, and not strike out on your own. If you are a racer looking for that last 100th, then you better understand the magic, for the rest of us, just tell us the RPM band, and HP target, there are plenty of guys who can tell you how to get there. Hope that helps, Carl