K31 or B2 (S400)
 

which turbo is better to use as a bottom charger in a twin setup? i know the K31 is the bases for the A5K but with a smaller exhaust housing, but how about the unmodified form? can the unmodified K31 be used in a single setup or only in the twin setup? or is it better to go with something even bigger, along the lines of the big brother, for a consistent street truck?

a Big Brother is going to need some serious fuel to be used on the bottom of a twin setup.

might want to call ATS and see what they say, although they arent going to give away all their secrets on the Aurora 5k

hope this helps

For towing a K31 with wastegate will do great...The s400 non wastegated takes a lot of fuel to spool...once rolling it is good but it would be a tough job to tow with unless you are wanting to keep the revs up with a 6speed....Of course a small enough top turbo and you'd be ok. I ran a super B/ s400 non wastegated for 4 months and empty it was a blast but my mileage was well poooooorrrr. I just couldn't keep my foot out of it....towing EGT's would rocket from a standstill but then were ok. ks

If I remember right, a K31 has a 71mm inducer and an S400 is 74mm. The K31 will spool alittle faster, but they are both great in twin setups. I have towed with my S300/S400 twins and as long as I kept the rpm's over 1700, they would spool now and I could't get over 1200EGT's and that was at about 430hp.

K31 as we know it on this board has a 71mm cold wheel. The S400 can be had in a variety of flavors though with many different cold wheels (maybe more). The S400 is a little bit heavier of a platform than the k31 but will also take a little bit more fuel to spool.
The advantage of the K31 is that it is wastegated and it is a performance charger out of the box.
The S400 series advantages are price, different wheel and housing sizes.
If it was just a street truck running twins then a K31 on the bottom would be great...If you want to go up into more power then of course the S400 is probably the way to go.
The turbos in single form will need smaller turbine housings and are usually reserved for the sledpuller types or all out competitive trucks.

here is a list I found of the many setups that an S400 can come in.

http://www.bullseyepower.com/S400.asp