I have had my stock s13 sr20det in my 240 since february and I have walboro fuel pump greddy rs bov and just recently like yesturday I started noticing it was making a popping sound out the exhaust when I would down shift the car to slow down, When it sits and idles it runs perfect it only does it on down shifts and pops once when I change to each gear. The rpms would only get up to around 3000 then go down from there and it was like it was back firing. about 3 months ago I installed a tuned ECU with the fuel turned up so I could run higher boost levels. and I also have my stock catalyic converter on there. as for my spark plugs the are NGK BKR7 not irdium or anything special but I have been running them for 4 months. I just want to know if this is normal and if my car is going to be alright. I have never meet another person with an SR to compare my car to so any answers would help.