I have a STS D21B that has well over 4 gallons on it and still keeps up with more expensive engines no problem. I just got and all most done breaking in a STS D21B RBmod and will be racing it in round 3 of the Georgia Championship Series this weekend. Will keep the stock one as back up. I all so have a STS 28 for my Savage but with racing just about every weekend have not had time to install it. Mark at STS Micro Engines is a GREAT guy to deal with and stocks more parts for these engines then any I have seen in the US.
http://www.stsmicroengines.com/
There is all so a help forum on his site. I live on the east coast and Mark is on the west coast. I get my stuff in 3 business days.