So im not a d series fan myself but im working on porting both a d16y8 head and d16z6 for a friend and stock the d16z6 definitely outflows the y8 by far. The casting on the y8 is very crude and the exhaust ports are very very small but with some work the y8 has plenty of potential. Im 90% done with the y8 head and more pics and info is coming up. The only advantage of the y8 head stock is the smaller combustion which is also proven to reduce detonation. So i took it further and polished combustion chambers on both heads to advance timing while dyno tuning to squeeze as much as we can for a stock block built head na build.
This is a budget build and parts include..
Ported y8 intake manifold
68mm tb
3" intake with velocity stack
Ported and polished y8 head milled .030 with crower single valve springs
blox stage 3 cam
Stock y8 block tuned on chrome with a p28 on 91 pump

The y8 intake ports are not straight like the z6 but with some work they are very capable of flowing very well. The exhaust side on the y8 is very horrible stock but looking at the z6 for reference gasket matching the y8 is totally necessary. Ill be done porting the y8 exhaust ports very soon with pics to compare to the stock z6 exhaust side.

Normally i dont gasket match but the z6 above the y8 below the ports are much larger i cant believe Honda from the factory did such a bad job. Im assuming they detuned the 96-2000 civic ex to sell the 99-2000 si b series. Imagine a z6 head on a y8 block with the y8 intake mani, 60mm tb, and higher compression with a factory tune intended for the 92 octane that was around. Ive been told the y8 cam is larger than the 3rd gen b16 cam from others on this forum. Power would be close to the civic si.

The y8 head had a bent exhaust valve and 2 intake valves. Valves have been lapped on both heads with combustion chambers polished.

The d16z6 intake ports came out very nice. I talked to John at Delta Camshaft and he has a grind to replicate the Bisimoto 2.4 which makes great power. Im hoping we can do a baseline with a stock y8 engine and the bolt ons in has on it now. Im curious to see what my ported head and the cam can do with tuning. Looking at some d series builds and dynos i believe this engine is limited to the size of the stock valves.

The z6 exhaust ports came out near identical to the y8 which is the bottom pic. If anyone is planing to run a y8 head and has a dremel or quarter inch grinder porting the exhaust side alone would be very beneficial. Like i said before the y8 exhaust side is very restrictive with plenty of material to remove.