If that was for me, then yes. I am using a keihin clone with a stock exhaust flange cut off and used as a shorty intake - it is 40mm bolt spacing and 19.5mm ID vs restrictive stock intake which tapers way down to 14.5mm ID, so works well and is something most of us would have lying around.How about your intake manifold my friend, is that ID bigger than stock alum intake?
Stinger I.D. is the most important part of tuning the pipe not for where it hit's but for how much it will over rev after the hit,smaller I.D runs on past the peak better than a large one but the large one produces more torque below the hit so again you have to find the happy medium.12 x the stingers I.D. for length is correct as a starting point but the stingers I.D. should be around 40% of the port flange or head pipe dia,so where most of these have less than a 23mm port flange dia 40% of that is only 13.8mm so 14mm is really the starting point and as port dia and or head tube dia increase so does the stingers I.D. . The length of a 16mm stinger should be 7 and a 1/2 inches but 8 will work fine since as stated above the length has little effect on power output, but rather in DB output and by putting the stinger further toward the belly reduces DB levels quite dramatically and right in the belly almost in half with very little effect on power.Old thread and I cant remember if I posted in here yet but I have tested multiple stinger lengths from 14" down to 6" and inside diameters from 15mm to 22mm on the same bike and I really like 16mm ID with 8" length as a starting point. I chose that ID because based on how ported the exhaust is on a chinagirl your stinger should be 14-17mm ID with a mildly ported jug being close to 16mm. As luck would have it, 3/8" ID is 15.8mm which is close enough and very cheap and common tubing to source locally. The starting length of 8" comes from the generic recommendation of 12x stinger ID which is just under 8", which works out because that puts the stinger just slightly past the seat on many frames which looks good too. You can change where the pipe hits by playing with the ID and length of the stinger, but the ID has a MUCH more dramatic and noticeable effect than the length does unless you get crazy with the length changes.