Intercooler question

I have bought a S-13 red top SR20det that I will be putting in a 1985 turbo hatch for Autocross, not drifting. There is not a good shop here so I am waiting until I have everything before I take the car to a shop out of town. I will be upgrading the turbo to a Garrett t28 and I am thinking about running an intercooler. The dimensions on the units on eBay are similar in size but the inlets/outlets vary from 2.75 to 3 inches. Which is better?
Also, the less expensive kit is 2.75 inches and comes with a blowoff valve already welded on while the second kit is about $60.00 more and is a 3-inch Godspeed and does not mention a BOV. Is one better than the other?

Thanks

I'd save the money and get the BOV, a quarter of an inch probably won't make that much of a difference.

Ok I did a little research and this is what I came up with:

Guy on anotehr site on 04:58:34 PM / 13-Sep-10
 Airflow velocity for different intercooler piping diameters (2.0 2.25 2.5 2.75 3.0 inches)
Hi guys,

In the last few weeks I've seen several posts on whether it is ok to use 2.5" or higher piping, or if somebody could use their ITR AEM CAI as a charge pipe. I tried to help all of them best I could, but jus a short while ago I helped another H-T member out with a similar question. I searched online and used a flow calculator to help me figure out the airflow velocity for a given area and cfm. The site I used is THIS ONE. As you guys can see it has the area in length times width instead of PiRsquare which is the area of a circle. I took the area of each pipie diameter and usd the equivalent area of a square and used those as my length and width. I made it simple and just took the area of the circle and divided by 2. The resulting number was usd as my length and 2 was used as my width.

Your actual results may vary (ie when you add an IC) so just use my numbers as sort of a bench mark to compare different piping sizes. One thing you guys will notice is that none of the velocites goes above 304 MPH or 0.4 mach. According to Corky Bell, Maximum Boost pg 61, 304 MPH or 0.4 mach is the point at which airflow meets increased resistance (drag) and flow losses are experienced.

Anyways here are the numbers I came up with. The velocities are in miles per hour and mach, and the flow rates are in cfm. Measurements for the piping are in inches.

0.4 mach = 304 MPH

2" piping
1.57 x 2 = 3.14 sq in
300 cfm = 156 mph = 0.20 mach
400 cfm = 208 mph = 0.27 mach
500 cfm = 261 mph = 0.34 mach
585 cfm max = 304 mph = 0.40 mach

2.25" piping
3.9740625 sq in = 1.98703125 x 2
300 cfm = 123 mph = 0.16 mach
400 cfm = 164 mph = 0.21 mach
500 cfm = 205 mph = 0.26 mach
600 cfm = 247 mph = 0.32 mach
700 cfm = 288 mph = 0.37 mach
740 cfm max = 304 mph = 0.40 mach

2.5" piping
4.90625 sq in = 2.453125 x 2
300 cfm = 100 mph = 0.13 mach
400 cfm = 133 mph = 0.17 mach
500 cfm = 166 mph = 0.21 mach
600 cfm = 200 mph = 0.26 mach
700 cfm = 233 mph = 0.30 mach
800 cfm = 266 mph = 0.34 mach
900 cfm = 300 mph = 0.39 mach
913 cfm max = 304 mph = 0.40 mach

2.75" piping
5.9365625 sq in = 2.96828125 x 2
300 cfm = 82 mph = 0.10 mach
400 cfm = 110 mph = 0.14 mach
500 cfm = 137 mph = 0.17 mach
600 cfm = 165 mph = 0.21 mach
700 cfm = 192 mph = 0.25 mach
800 cfm = 220 mph = 0.28 mach
900 cfm = 248 mph = 0.32 mach
1000 cfm = 275 mph = 0.36 mach
1100 cfm max = 303 mph = 0.40 mach

3.0" piping
7.065 sq in = 3.5325 x 2
300 cfm = 69 mph = 0.09 mach
400 cfm = 92 mph = 0.12 mach
500 cfm = 115 mph = 0.15 mach
600 cfm = 138 mph = 0.18 mach
700 cfm = 162 mph = 0.21 mach
800 cfm = 185 mph = 0.24 mach
900 cfm = 208 mph = 0.27 mach
1000 cfm = 231 mph = 0.30 mach
1100 cfm = 254 cfm = 0.33 mach
1200 cfm = 277 mph = 0.36 mach
1300 cfm max= 301 mph = 0.39 mach

Back when I wrote this thread there were about 3-4 threads about people wanting to use their 3" CAI as a charge pipe. I made this thread to show why it was a bad idea to use pipe that big in dia.

The numbers can be used as sort of a rough guide for selecting piping diameter. The numbers above are the different velocities for the different amount of air going through a given size pipe. The more air or CFM you have going through a given area, the faster it is moving. A smaller pipe will pressurize faster so there is less lag and the response is better. But the faster the air is moving the hotter it will become.

I guess one eay I could use the numbers is to determine what size pipe to go with from turbo to IC inlet. Say I have a GSR and I want to run 18 psi. The esiest way to find my CFM is multiply the CFM of the engine by the pressure ratio. Just for this example lets say the B18C's CFM is 275 with no boost, but under 18 psi of boost it will flow 610 CFM. Normally people use 2" piping from the turbo to the IC inlet, but if you look at the chart you'll see that with 600+ CFM and a 2" pipe, the air is moving so fast that there is a lot of resistance. So you might be better off using 2.25" pipe or even 2.5" pipe.

That's one way to use it...but like I always say, dont get too caught up in all this number crunching. It is always a good idea to get the opinion of somebody who has a lot of experience with turbo cars.

Your limitations are the front end of our cars. Most of the S12's with intercoolers are limited to certain core sizes, look around on here and find Keith and Mod Mastas, and try to match their core sizes if you want one that will fit with only minor modifications.