Everything that you ever wanted to know about the d16b5 (98-00 civic gx engine)
I'm typing 95% of this from memory, so get off me if there's a few things wrong

The engine is most closely related to the civic HX engine (d16y5) It is a vtec-e engine.

Bore = 75mm
Stroke = 90mm
Rod Length = 137mm
R/S Ratio = 1.52
Compression Ratio = 12.5 (reason being that CNG is a fuel that is in gas form (not liquid) it doesn't mix very well with atmospheric air. So Honda raised compression to promote a better mixture.)

The engine code for the d16b5 is PDN

Everything is identical to the d16y5 except a few minor differences:
different pistons and rods
different valves
different cam

d16y5/d16b5 uses the same exhaust springs as a d16y8
d16y5/d16b5/d16y8 all use the same retainers


d16b5 has different exhaust valve springs than the d16y5
d16b5 has different intake and exhaust valves than the d16y5

99 gx uses the same intake manifold gasket as a 97 hx
99 gx intake manifold = 17100-PDN-A00 UNIQUE
99 gx spark plugs = 98079-5787T UNIQUE and they're $12.57 a piece!
99 ex spark plugs = 98079-5514G

99 gx uses the same spark plug wires as the 99 ex

the block assembly (sans pistons/rods) for the d16y8/d16y5 and d16b5 are identical.

piston dome on pdn pistons = 4.63cc (this was calculated using basic algebra 12.5, 32.8cc, y8 headgasket. etc.
combustion chamber volume = 32.8cc (same as d16y5/d16y8)

In 99, honda introduced air injection on the civic gasoline powered civics. Considering that the civic gx runs off of CNG, air injection is no option for the 99 GX. Which is why it shares the 96-98 intake manifold gasket as the d16y5 hx engine.

bare head assembly on the d16y5/d16y8 and d16b5 is identical. (SAME SIZED PORTS)
12100-P2M-010 = 98 hx cylinder head assembly
12100-P2M-010 = 98 ex cylinder head assembly
12100-PDN-A00 = 99 gx cylinder head assembly
12100-PDN-A00 = 98 gx cylinder head assembly

hx/gx rocker assembly


d16b5 pdn piston

d16b5 combustion chamber


d16b5 connecting rod


comparing the gx intake manifold to the ex intake manifold (gx on right in all pics)






d16b5 valve train


excerpt from honda service manual about vtec-e


d16b5 intake manifold (assembled)


d16b5 with oil pan off



d16y8 = 96-00 civic ex sohc vtec engine
d16y5 = 96-00 civic hx sohc vtec-e engine
d16b5 = 96-00 civic gx sohc vtec-e engine


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01/19/2006:
I am growing tired of the d16y5 questions that I am seeing arise on honda-tech. So this will be the last time I will try to explain.

If you have a d16y5, it is basically a d16y8. The MAIN difference is the cam + rockers. Swapping from a d16y5 to a d16y8 is more hassle than it's worth. If I wanted more power out of my d16y5, here's what I'd do. Block modifications are going to be identical to the d16y8, so I will not bother covering this. If you boost, use arp head studs, blah blah blah. I'm not here to cover the obvious.

I would mail crower my cam for a regrind, crower has already said that they can do a regrind of this cam. I would then put a d16y8 intake manifold on it to get rid of the egr. I say d16y8 intake manifold because you can find a bare one for like $50 and all your sensors will bolt up to it. Or just keep your y5 intake manifold, it doesn't matter that much. Or get the skunk2 or edelbrock one. you won't be able to find an obd1 equivalent ecu, so don't even bother trying to find it. Nor will you be able to pull the maps off the d16y5 ecu.

Your best bet is going to be to build the head (same as y8 style) and send your cam to crower to get a regrind. You will need a custom tune to do this. So find a tuner that is very experienced with tuning d16y8s. Again, it is basically a y8, just slightly different. Have VTEC engage at like 2800-3000.