Join Date: Jan 2019
Location: Sedgwick, Kansas
Just to be clear, it doesn't sound like your engine is hurting for power, as it is. I am surprised you're getting 20 mpg on the highway, but honestly , those AFR heads are amazing.
It does sound like you're looking for more low-end grunt and streetability. The changes I suggested would offer that. I'm not saying to make it a tractor!
"big heads, mild cam" is actually a thing, and it's a recipe for a really hot street engine. It would also ruin your rumpity idle, though. Engineering's always about compromises, so finding the set of characteristics that's best for you is important. You made a lot of awesome choices for your motor, and as a package, it sounds really good.
Jdub's recipe for the longtubes WOULD add more power... on the top end. Long-tube headers have only one harmonic where they start helping your exhaust, and that's pretty high - usually around 5500-6500 RPMs.
Try-Y headers have two frequency ranges in which they improve scavenging. One of them in the midrange, and another on the top, around 5500 RPMs. Which is great for a street motor! All the other exhaust setups basically do nothing except get the burnt gases out. On the top end, your engine is making enough power that the 2 1/2" exhaust is well-matched; it's not holding back anything, and probably helps peak HP. So - simply said - Longtubes = big boost, but only at the very top. Tri-Y headers = modest boost at mid AND top end. And shorties/cast iron manifolds? Nothing.
However, on the bottom end, it's also big enough that you get very little scavenging at low RPMs - that requires velocity, and the bigger pipes don't help with that. Smaller pipes create more of an effect, despite being 'too small' to make max power on the top end. Again, this is another tradeoff. This is not something I'd change, since it's already done. Your exhaust, as-is, is probably pretty close. I don't think you'd see big power gains OR losses if you mess with it. We're talking about 5 horsepower for hundreds of bucks.
Where I think you WOULD see pretty dramatic changes is cam. With your setup, I would not be surprised to see 400 lb-ft of torque from 2000 rpms all the way to 6500, with a more modest cam selection. Right now, I think your powerband is probably a little narrower and peakier, kicking in more around 3000 and doing great right until around 6500 where you'd be valve-limited by your (presumably hydraulic) setup. If you pulled that down and widened it out a little, you would lose some on the top end, but gain a LOT throughout the rest of your powerband. Again, this would be dumb if you were racing your car, but for something you just want to drive, it would make a lot of sense.