Ford Focus ST Forum banner

1 - 4 of 4 Posts

·
Registered
Joined
·
5 Posts
Discussion Starter · #1 ·
I've owned my st for 1.5 years now and I recently got the Cobb stage 1 accessport. Now I'm planning stage 3 with a big turbo 2867R but undecided on how big .64 or .86. I would like to get between 350 and 400. I also plan on installing lsd. I have some friends that say the .86 would spool up faster. Its also my daily.
 

·
Registered
Joined
·
463 Posts
You need to not listen to your friends.

GTX2867 .86 A/R vs .64 A/R Turbine Dyno

a/r of exhaust housing is the internal size ratio in the hot exhaust side of the turbo that directly relates to spool up time and exhaust size, which can limit overall power. The larger the housing, the more area of air is being utilized to sprin the turbine as the air pushes past the impeller, slightly less force with the larger a/r, slowing spool, but allowing for more flow for, usually, higher top end numbers.

Look at the graph in that link. Spool is delayed 200-300~ throughout the RPM band, you lose midrange area under the curve, and gain minor numbers up top(at 4psi higher boost levels too!) from the larger exhaust housing, and that's on a much better flowing exhaust port configuration than the ST's headifold, I'm sure.

Go .64, enjoy the faster spooling street manners, and turn up the boost a touch if you want that extra squeeze you probably would never notice on the .86 housing. I guarantee you'll meet your power goals on either housing.
 

·
Registered
Joined
·
344 Posts
I went with .64 a/r and am very happy I did so. The additional lag from the .86 would not be worth it for the 28 series turbos. The .64 gtx2867r is the 400whp turbo. It gives the most area under the curve.
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #4 ·
Thanks for your info and advice. After researching some more I'm actually looking at the GT 2860 RS turbo with 3 " inch downpipe and exhaust. I want to get as close to 350 whp safely without having to worry about internals as much.
 
1 - 4 of 4 Posts
Top