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Does anyone know if they make under drive pulleys for this motor. Or will the 5.7 kit for on this motor? Also looking for a set of valve covers.
 

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Externally most everything for a given year is the same or interchangeable between your L99 and the LT1
That said, do your research on reduction pulleys for these cars.
Because of the cam driven water pump, the gains are less than other engines see.
Many with stock engines with stock idle speeds have had to resort to smaller alt pullies to speed the alternators back up.
End result, you would just be slowing the PS at that point.
Great for road coarse duty, ,,,,

March pulleys space the damper way forward, arguably not the best deal but many have used them.

Never seen or used this damper JEGS 51606: Chevy LT1 Harmonic Balancer/Damper 10% Underdrive Pulley without Hub | JEGS

ATI 917327 is good but costly.
 

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OverDriven Alternator + UnderDriven CrankShaft = UnderDriven PowerSteering & Air Conditioning
If you don't overdrive the alternator, you might regret it. You can get away with underdriving the power steering & AC.
Okay so anything from the LT1 should fit my motor? And thank you.
Nearly every mod that fits on an LT1 fits on an L99, but the more expensive the mod is - Tri-Y headers, for example - the more you'll be disappointed.

3.42 or 3.73 axle gears & reprogramming, do that schitt before anything else! No one ever regrets those.

An L99 sedan with EVERY budget mod & 3.73, vs an LT1 wagon or Fleetwood with 2.93 with no mods …
… is a bit like the speshal olimbix.

Long story short: THE best bang-for-your-buck mod for an L99 car is an LT1 swap.
 

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Okay so anything from the lt1 should fit my motor?
That statement is too general, NO.

For example the injectors have a different rating.

To prevent expensive disappointment research and ask about each part.

To me a 3.73 would be the best use of money as it is fairly low tech.(you still need a reluctor and reprogram) It is 2021 and good used LT1 will be harder to find and rebuilds are not cheap.
 

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While you're removing the crankshaft harmonic damper - check that rubber ring in it real good. It's likely cracked and shot at this pointt. New stock design ones are cheap.
 

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