Well more about A/R's I guess. I am looking into piecing together a turbo kit and have gotten stumped on the question of what A/R size to use. When I look in *Maximum Boost* it says that a small quick spooling turbo is anything below .7 but from the searches I have performed around here it looks like most people are using their turbo's in the .6 range. I'm sure other things affect this as well such as compressor urbine size etc, but I'm confused as to what the A/R should be. I would probably never make more than 250 WHP, probably shoot for 175 to start off with. But I would like something that allows me to smoothly work up to 250 replacing internals etc along the way. What size would be correct for this?

 

If you don't want to go over 250 WHP, then a .62 or .65 would work fine. The lower AR will develop boost sooner, but at the expense of thermal efficency at high RPM/Flow. A higher ar will spool later, but be more efficent at high RPM/Flow. For a daily driver, I'd reccomend going for the quick spool.

Red is considering a .82 for his MADD BOOST & JUICE motor.

I'm planning on a .76 or a .82 for my ALL MADD BOOST motor.

 

I have seen used T3/T4 hybrids around here that have rather small A/R's. Somewhere in the high .4's and low .5's, would these be acceptable? Or would that be way to small? Thanks for the help Random.

 

it all depends on what you want from your car! Are you looking for a drag monster or a street car? If you find a t3/t4 for a good price then don't worry about it if the a/r ratio is 42 45 or 56 what ever the turbo will work just fine don't worry so much about it! Just remember!!! The smaller the A/R the faster it spools up The larger the A/R the longer it takes! Thats about it! Anywhere from 4 to 6 is ideal for a street car!!!!!

 

Thanks you guys. I appreciate the info.

 

the problem with an AR of say 45 on a T25 is that it would spool so fast as to be outside of it's efficency range above about 3000 rpm, by 6000-6500 RPM redline, you'd be WAY outside the efficency range. In theory, you could build boost fast enough to hit the surge line/point.

A turbo that is too small can be worse than one that is too large.

I suggest you look up the compressor maps for different ar's, and then set your sights on a *range* (say .52 to .65) or something of that sort. If you're not sure how to read a compressor map, or how to plot your engine on it, pick up Maximum boost by corkey bell, he covers it rather well.

 

My turbine A/R is 0.49 on my T25. That's exactly what the online calculators at http://www.turbofast.com.au/javacalc.html said I should use (1.8L, 5 to 7 psi boost, intercooled).

My car is just going to be a fast daily driver . . . for now.

ac

 

That's what I was worried about Random. BTW I have both the maximum boost book plus the supercharged book by Corky Bell. But I cuoldn't get a straight answer from them so I though I would check here.

Is there a difference in having a .49 A/R on a T25 and having one on a T3/T4. I'm assuming the difference is that the turbine and compressor housing are larger. If that's the case wouldn't a .4x on a T3 be larger than a .4x on a T25. Or am I way off on this one?