My bro just got a 2005 SS supercharged. He says he wants to put on a pulley soon.

Can someone explain to me the basics on how a smaller supercharger pulley adds more hp???

What is knock retard, and how it pertains to pulley swaps?

Are supporting mods needed for a pulley swap (spark plugs, tune)?

What kind of hp can be gained on just a pulley swap?

 

This should help ya out!
https://montecarloforum.com/forum/sh...ad.php?t=10942

Start at post Number 5

 

Hey thanks Captain, that's just what I was looking for!

 

Think about the supercharger- it pushes out a certain amount of air each time it spins around. Putting on a smaller pulley than stock (given the same size crank pulley) will make it spin faster. So say with the stock pulley size (3.8"), it spins 11,000 rpm when the engine is at redline- with a smaller 3.4", it would be spinning 14,000 rpm (because the smaller pulley has to spin more times per turn of the driving pulley on the crank shaft). Those rpm numbers are just made up- but you get the idea.

Obviously what happens if you spin a pump faster? More stuff comes out- air in this case. More air shoved into the engine + more fuel from the fuel injectors (the computer measures how much air is actually going into the engine) = more power - ugh ugh ugh!

Knock retard is the computer's response to sensing actual knock in the engine. It removes spark timing to keep the engine from 'knocking'. In general, more spark timing = more power, but you can only run so much timing for a certain combination before it 'knocks'- this is much more relevant to L67's because the knock (and resulting KR) limit what pulley size you can run.

Take a stock 3.8" pulley on a stock engine- and it probably has a couple degrees of KR when you floor it. Add a much smaller pulley- say a 3.4", and it spins the supercharger faster- and forces more air in, which makes the explosion more powerful- however, it also makes more heat, and more heat means more knock- so if it already had some KR stock, it now has a lot more KR (assuming the person didn't do other KR reducing mods, like an intake, downpipe, headers, rockers, etc). So even though the person has a smaller pulley- which is forcing more air in, the extra heat and pressure because the person didn't do supporting mods causes more knock (and KR as a result)- which means less timing, and less power- usually less power than with the stock pulley. The engine has a certain timing range that makes the most power- and if through KR you're dropping below that by a good bit (ie usually more than a couple degrees of KR at wide open throttle), your engine power drops off drastically. I have seen a 3.25" pulley car with few mods get beaten pretty badly by a stock 3.8" pulley car with no other mods. The key to making the most power on the 3800's, is to keep knock down to virtually 0 (a blip here and there is no big deal).

You won't actually hear knock on one of these engines (because it only occurs a few times before the computer pulls enough timing through KR to keep the engine from knocking), the only way to see it is to use a scan tool plugged into the OBD2 port to scan for KR. However, with a smaller pulley- the knock that occurs can be much more violent than stock- and MANY people have chipped pistons from that (doing a smaller pulley without scanning for KR first- and without doing the proper supporting mods to run the pulley size)- the pistons chip off pieces of the top, and the engine still runs- but just worse than normal (especially as more pieces break off).

Also, you have to keep in mind that you'll only get knock when the engine is under SEVERE load- heavy throttle in high gears with low rpms (pretty much impossible in an automatic transmission), full throttle, heavy throttle going up a steep incline, etc. The engine won't have knock (or KR) just driving around town in normal driving.

Yes, and they vary based on what size pulley. Something simple like a 3.4" (stock is 3.8")- only takes minor mods- cold air intake, downpipe & powerlog or headers, pcm tune, some cars need modded rockers, etc. Something smaller like a 3.0" needs either an aftermarket camshaft or an intercooler on top of the basic mods. Something much smaller like a 2.8 or 2.55" (the smallest I've ever seen) requires all the available engine airflow mods- cam and intercooler, ported heads, ported blower, headers, etc etc.

The thing you have to keep in mind is the further you go down on pulley size, the further the blower gets from its efficient range- so heat output starts to go up a lot with the smaller pulleys- so it tends to take more and more mods and more money to drop each pulley size without knock/KR.

Sticking on a 3.4" (the next common step down other than a 3.5") in place of a stock 3.8" without doing any other mods will likely yield very little to no power gain (assuming the car had some KR stock as much do). The car will make more boost, run less timing- and end up a wash, or even maybe a little worse off for power. With supporting mods and a pulley swap- its hard to say- because it depends on which mods you do- but most basic bolt on 3.4" pulley setups seem to be up around 300 crank horsepower (up from 240 stock crank horsepower)- maybe a little under depending what is done.


Hope that cleared it up a little, I tried not to get too deep into stuff.