635 CFM on the PE1820 is based I think on the compressor wheel. Typical engine dyno numbers on the EJ20/EJ25 are about 1.42 CFM/BHP, suggesting about 447 BHP. 20G compressor is rated for about 690 CFM but with a TD05H turbine we tend to see them at about 440 BHP (with octane boosters) rather than the expected 486 BHP. Both turbos share the same issue - they are compressor heavy hybrids and the turbines are choked perhaps 10% short of expectation, but they also spool up well. The other issue to avoid with compressor heavy hybrids is avoiding surge during spool up. I'll put a link to a few flow calc spreadsheets I made - play with the numbers in bold and see the results. You can change the compressor maps by scaling and including new ones. Change the engine size and VE and have a play.
http://www.johnbanks.dsl.pipex.com/flowcalcEJ25720Gmax.xls
http://www.johnbanks.dsl.pipex.com/flowcalcEJ257Garrett.xls
A well matched Garrett will make more like what the compressor map CFM or lb/min numbers suggest, but the shafts are heavier, turbine wheels are bigger and the spool up is worse.
The Garrett nomenclature confuses me as different people use different conventions. I try to go by the Garrett catalogue which refers to the CHRA with -10, -12 or -14 which dictates the size of the turbine wheel and shaft. Then you can have various AR turbine housings. On top of that there are then a variety of compressor wheels in different housings. I used a GT30R-12 with 0.82AR turbine housing, and 52lb/min compressor wheel
http://www.atpturbo.com/Merchant2/m...OD&Product_Code=GRT-TBO-005&Category_Code=GRT
On 2457cc with 8.7:1 compression, 79mm stroke and a long exhaust manifold, in a high gear it would reach 1 bar at about 3000 RPM. In 2nd gear with a cold manifold it was 4500 RPM before it really hit. I had a previous GT30R-11 in a P20 housing and that spooled up similarly but made less power. There are not publically available maps for all the wheels.
For a 1.8 to 2.0 I would go for something a little smaller than a 500 BHP capable GT30R-12 0.82AR as I tried on a 2.5 (and found laggier than ideal and an engine breaker). But if you are going to upgrade you want it to be worthwhile. Despite all the map plugging, until you try it on your engine it will be difficult to know.
On the FQ400 they went quite big and then held back the midrange torque seriously. This is a nasty feeling, I had to do similar with my GT30R on my EJ257 to avoid trashing headgaskets. It kills the spool up and midrange power, and a car with 20% less power and a smaller turbo is all over you until you get to 5th or 6th gear!
"Been looking at a stnd 06 wrx 2.5L Impreza John. I reckon a 20g or Pe1820f would be intresting on a 2.5L impreza. Any idea on fitment or possible parts req to make it possible."
STI intercooler, turbo back exhaust, fuel pump, remap and then the turbo should just bolt on. TD05H-20G should do about 400 BHP and 400 lbft with full boost at about 2500 RPM, would need about 1.4 bar mid, 1.2 bar at the top, should rev out to 7500 RPM, although peak power would be earlier. No one knows how strong the new 5 speed is, so if you started with an STI then you'd get the intercooler, better brakes, tyres, suspension, diffs, DCCD-A etc.