(Dampened) Underdrive Crank pully
 

I am soon to be working on building a n/a HPDE streetable track car engine for a friend, aiming for 175whp, so far it's getting a ported bpw4 head, big (285/277 Cat cams Tarmac Rally) cams, likely oversize valves and supporting head mods. Looking for other tricks for n/a "power" for the bottom end and was looking into an underdrive setup as this thing will likely idle at 1200-1500rpm anyways. I understand the issues with needing a damper or oil pump goes boom, but all the aftermarket dampers I can find do not have an underdrive option. I think the Supermiata damper may have had some underdrive, but they are no longer in production and some had keyway fitment issues. Due to space constraints around the water pump pully I would rather undersize the crank damper than make the alternator and water pump pullies larger.

Is there any reason, such as size of the damper assembly, or target frequency, torque transferred through the elastomer etc why an underdriven damper isn't used?
If there isn't I will look into the feasibility getting an underdrive damper made (the part that bolts to the timing gear hub, like the fluidamper)

Why not just use an electric water pump to free up even more power?

I believe the original, pre-revision Fluidamper had a 15% waterpump underdrive. I agree with electric water pump though, as you're presumably using a standalone to control it.

I hit 172hp on a BP4W with a worked over head, square top, RB header, 100k mile bottom end, and tomei cams. Don't over think it.

Want to make it a kitten down low and have even more power up high?
Look to an ITB set up.
It will idle like a stocker and have stock drivability UNTIL you reach the target RPM range for the cams, then it will howl like a banshee to redline.
ITBs require big cams and high compression to work properly.
275 degrees is where ITBs start being needed in my book.
With the addition of ITBs you can add another 20+ degrees of duration and more compression and still have it "Streetable".

What compression ratio are you planning on running with those cams?

He's right!
If you don't mind it being snorty and ill tempered down low cheaper can work fine.

Curly, what duration of cams did you use for that engine?
If you were running stock compression on the 100k short block YOU ARE THE MAN!
Was that WHP?

Increasing the compression on 275 degree cam shafts will help everywhere.

As for the original question, I'd run the Fluid Damper (significant range for vibration dampening) and see how it worked first.
I'd make oversized WP and Alt pullies before I made an under drive crank shaft pully.

Running an electric WP could have many positives but it will have pitfalls and complexity that you will have to work out.
Done right it can change coolant direction and if PWM is used you can delete the thermostat and achieve excellent temp control.

It still doesn't solve an overdriven alternator.

Looking into electric water pump options, fantastic idea, compression is going to be in the 11.0-11.5 range (wiseco 11:1 pistons plus decking the block and head slightly)
ITBs are on the list but may not happen right away, because funding, the water pump solution may be similar.
I have a lathe and should be able to produce the oversize alternator pully easily enough, or possibly see if I can just source one that will fit.
looking at basic EWP designs, I would do it the simplest way I can think of, gut and seal the factory water pump, and add the electric one on the frame rail near where the lower radiator hose passes, and setup a pwm in the ecu to run it slow at low temps and slowly ramp it up to max around 190f

Those cams are way way waaayyyyyy wayyyyyyyyyyyy big for your power goals. Like stupid big. Like what the fuck are you thinking big. Like no matter how hard TechnicalNinja masturposts about ITBs, it'll never drive worth a shit.

Noted, and if you have recommendations for cam specs that are best performing in the 4k-7800rpm range I am all ears. I am not really concerned about drivability as long as it can drive to the track, but I don't want to rebuild the engine every year or two.
This is why I am asking questions now, before ordering anything.
now looking for oversize alternator pully, will see what I can find

I've done two "high" horsepower BP builds. One was a BPZ3, the other a BP4W. Both had around 100k miles, both bottom ends weren't touched. Both got dampers, SUBs, billet oil pump gears, square tops, RB headers, and were practically straight piped. They took their heads to the same guy for some minor port/chamber work. I assembled the heads onto the blocks and tuned to 8000rpm. Their limiters were around 7600, neither made power above that.

The BPZ3 got the "mild" 252s, and a .040" head shave, made 165whp. Drove like a stock VVT swap but with power.
The BP4W got the "medium" 256s, and a .120" head shave and made 172whp. This one didn't idle well, but it was also a MSPNP1.

BTW, the .120" shaved car spun backwards on track, immediately reversing the rotation of his engine, shattering the billet pump gears and seizing the crank. He has a K24 now.

I don't plan on shaving the head more than.020, getting the compression from aftermarket pistons, a k-swap is not in the books for now, although for this use it is the better engine.
as far as alternator pullies go I found this one:
https://store.alternatorparts.com/24...130-cs144.aspx
which is the same bore (17mm) and offset (9mm) but has a 66mm belt diameter, v the miata's 55.
It has 2 extra grooves, but it should work.

Small ones. Like Maruha F cams or Tomei 252/256 in that area. 175whp BP should use small cams, have nearly indefinite lifespan, and be easy to drive.