FAQ: Smaller SC pulley? What Size?

Smaller SC pulleys spin the blower faster for more boost; this enables the engine to make more power (in the form of low-end torque). Be careful, though. If you go too small right away, the engine will knock, which will rob power and can even do harm to your pistons under certain conditions.

The diameter of the OEM SC pulley is 3.8". It only takes a small drop in pulley size to make a noticeable power increase. Just 0.1" diameter decrease will yield about 1 PSI increase in boost, which is enough to feel in your seat from increased torque.

For Series II SC (L67) engines, aftermarket SC pulleys are available in diameters ranging from 3.6" down to around 2". Start larger and work your way down. 3.6" pulleys can be used with little or no issues on a stock Series II SC Riv, giving up to ~10 HP increase. A 3.5" pulley may give ~15-20 HP and is probably fine if you run cooler plugs and t-stat, but scanning is a good idea. Racing with a 3.5" SC pulley, or running a 3.4" size on the stock engine is not a good idea unless you scan and tune the PCM with a tuner or AFC device.

Average boost numbers on the stock 3.8" SC pulley will max around 6-8 PSI, depending on the car, other mods, and conditions. With the 3.4", you'll see around 9-11 PSI. This extra air charge can add ~25-30 HP to your engine's output, a very noticeable gain in low-end torque. You'll feel the extra pull when launching the car, and on shifts. With this mod, wheel traction can become an issue.

SC pulleys can be had for as low as $80 in the case of the press-on variety, but a modular system (MPS) is highly recommended. If you plan to modify further in the future, or if you want to experiment with different pulley sizes to dial in just the right boost for your car, an MPS lets you swap on a hub (like changing wheels) in under 5 minutes by loosening a few screws. Modular systems go for between $100 and $130, but additional pulleys are only $50-60, and used pulleys are often sold/traded on message boards. Info on SC pulley installation is found here:

https://rivperformance.editboard.com/Supercharged-3800-Tech-c2/Series-II-FAQ-f19/How-do-I-remove-install-a-SC-Pulley-t511.htm

One of the things you'll notice with a smaller SC pulley is louder and higher pitched supercharger whine. Another thing you can expect is increased KR. With more boost comes higher temps inside the intake manifold, which contributes to knock. Smaller pulleys create higher air flow rates than stock. With the PCM adding more fuel, it's a good idea to scan after installation to confirm your air/fuel mixture is where it should be. If scanning indicates that KR has increased, you may need to reprogram the PCM, use an AFC to tune the air/fuel mix, add more modifications to fight knock, or move to a larger SC pulley.

After adding a smaller SC pulley, you may be wondering if a smaller SC belt is needed. In most cases the answer is no, but when running 3.4" or smaller SC pulleys, it's a good idea to increase the diameter of the SC tensioner OR idler pulley to 3.5" to take up slack.

3.5" (90mm) ribbed idler pulley: GM #88909590; Dayco #89004; Gates #38019
3.5" (90mm) smooth tensioner pulley: INTENSE #STP-635; ZZP #ZZ-6R35SM; Gates #38001

Using a 3.3" SC or smaller pulley requires using both 3.5" SC tensioner and 3.5" idler pulleys. This takes up more slack, and ensures the SC does not rub against itself near the tensioner, while retaining OEM belt size. Here are the rules:

-OEM SC belt works with OEM 3.8" SC pulley - keep tensioner and idler pulleys stock.

-OEM SC belt works with 3.6" and 3.5" SC pulleys - keep tensioner and idler pulleys stock.

-OEM SC belt works with 3.4" SC pulley - change one: tensioner pulley OR idler pulley to 3.5" dia. One of these remains stock size.

-OEM SC belt works with 3.3" SC pulley - change tensioner pulley AND idler pulley to 3.5".

-OEM SC belt works with 3.2" SC pulley - change tensioner pulley AND idler pulley to 3.5".

-OEM SC belt works with 3.1" SC pulley - change tensioner pulley AND idler pulley to 3.5".

-using 3.0" or smaller SC pulleys will likely require a smaller SC belt size.

If your car has 85k miles or more, it is a good idea to change all of the idler and tensioner pulleys on both the accessory and SC belts. There are 5 pulleys total that run about $100 for the set. For more info on pulleys/belts:

https://rivperformance.editboard.com/Supercharged-3800-Tech-c2/Series-II-FAQ-f19/-p4782.htm#4782

I am pretty well educated on the fact that a 3.4 inch pulley gets about 25-30 hp and a 3.25 inch pulley will get around 30+ hp... i have a cold air intake and 2.5 inch exhaust straight back from the cat.. no mufflers but i have two 3.5 inch tips they are 18 inches long... i am going to get a 160 degree or 180 degree thermostat?? which pulley shall i go with without gettin knock retard??

Welcome to the board! A good read through the FAQ will answer your questions in more detail.

A 3.25" SC pulley is out of the question for you with your current mods. Even a 3.4" is risky without a scanner to monior KR. The safest way to go would be a 3.6" or 3.5" SC pulley. Any smaller and you'll need to scan.

A 180º t-stat is best for daily driving.

i agree with Aaron, your exhaust mod doesnt really help relieve heat or back pressure, and if the CAI is one of the ebay ones, it's really a hot air intake which can cause more knock when there is heat soak. you can safely drop to a 3.5" IMO if you have a great running and clean engine and some supporting mods like colder plugs. make sure all the maint items have been taken care of before you get the mod bug
oh, and you should introduce yourself in the welcome page along with pics in the showroom. (so we can properly welcome you and see your ride! )
Matthew

thanks i have a home built CAI made out of 3.5 inch stainless steel which is wrapped in heat blocker i sort of went off of what someone else had done with PVC pipe... i will probably go with the 3.4 inch pulley and then see where i am at but yes i will first buy the 180 degree thermostat..... and to Mr. Riviera thats one nice Riv man i really wish i got a pearl white i instead got a black one......

i love the black riv's with some dark tint and chrome rims. dont forget to get some colder plugs along witht he 180 t-stat. and as Aaron mention'd read that FAQ it has a ton of good info.
congrats on the DIY CAI btw.

Mr.Riviera wrote:

i love the black riv's with some dark tint and chrome rims. dont forget to get some colder plugs along witht he 180 t-stat. and as Aaron mention'd read that FAQ it has a ton of good info.
congrats on the DIY CAI btw.

i had already bought some spark plugs they are those NGK or whatever forgot LOL but this may sound silly but i couldnt do the sixth one till i found out i could tilt the engine foward by unbolting the dog bone/!!!!!!! but yes i do need a paint job at least on two panels front left fender and rear right quarter because they had no damage on them i gottem from the junk yard!!! but they were oxidized so im gonna have my buddy paint them for 300 bucks (cost of paint).....lemme kno if thats a good price lol

so yah i will probably be modding my riviera this summer i kno i said i was going to sell it WHICH I AM but i really wanna see the potential so i am going to install my FWI and get a 3.4 inch pulley with a new 180 t stat..... and then i am going to buy one of those AFC 2.0 gadgets to add more fuel <<<idk if thats what it does LMAO but yes then i will see if i can keep up to mr SRT-4

Keep in mind you need a scanner to use the AFC.

AA wrote:

Keep in mind you need a scanner to use the AFC.

http://www.zzperformance.com/grand_prix/products1.php?id=172&catid=107

so to run the AFC i need a scanner??? how come it says "Better than an AFPR, the Mini-AFC allows injector swaps w/o PCM tuning"

98riviera98 wrote:

so yah i will probably be modding my riviera this summer i kno i said i was going to sell it WHICH I AM but i really wanna see the potential so i am going to install my FWI and get a 3.4 inch pulley with a new 180 t stat..... and then i am going to buy one of those AFC 2.0 gadgets to add more fuel <<<idk if thats what it does LMAO but yes then i will see if i can keep up to mr SRT-4

Well if you were my little brother I'd slap you upside the head. I don't understand why you'd want to waste money doing halfassed mods to a high mileage car that you'll be selling just to "beat" a dodge neon. I'm sure you'd feel real dumb if you ended up blowing the engine/tranny and suddenly your 4k riv is worth 1500. Then you end up dumping 2k into the thing to bring the value back to what it was in the first place and you'll end up netting maybe 1k if you're lucky after the fact.

But you're thankfully not my little brother and I'm just playing the "devil's advocate."

AHAHHAHAHA i really just wanna see the potential and actually the buyer of my car MY FRIEND will want mods

Without TUNING - not without SCANNING. You shouldn't be making too many mods or a pulley change without regular scanning. Know your engine, know the paramaters it runs at, know what it means when you have KR and why you have it.

PCM Tuning would take the place of an AFC. Scanning is used to TUNE the AFC. You are seriously changing the parameters from your MAF when you use an AFC. If you don't scan vigorously while TUNING your AFC you are going to do some damage.

You need to scan for MAF Hz, Front O2, MAP, IPW, KR at the very least. With all that info you can start making changes with an AFC.

why do scanners have to be so much $$$$$$$$$$$$$$$$$$$$

Keep your eyes open for a PC based autotap, that's the best. New they run $300. Mine sold used for $200. At that point you might as well spend $400 and get a tuner, that way you don't need an AFC!

i have a laptop and i'm not a ricer but i'd like to have a digital boost gauge and various other info such as KR and AIR FUEL MIXture to be displayed on the lap top is this what the autotap does??

Yep.

Gauges and even better - recordings! The ability to export your scans so others can help you out is the real benefit to an Autotap.

you should sell your chrome centercaps to me and tell your friend they fell off...

Jason wrote:

you should sell your chrome centercaps to me and tell your friend they fell off...

hahaha i can get u four of them for 100$ if u want shipped!!!

check your PM

do i still need the scanner if just swap in a 3.5" pulley?? or should i wait??
thanx again guys..!!!

You SHOULD get a scanner to begin with. If your car has knock now you don't want to be adding more by dropping pulley sizes. You could see if theres anyone near you who has a scanner that will scan for you (riv people or try a grand prix club).

You should also put in some colder plugs (autolite 104s or 605s) and a lower t-stat (180*) just to help out.

How many miles are on the car and what year?

I had negligible KR with my stock 3.8 pulley. In the 1's, max. Then I did a couple supporting mods, like AL605 plugs, 180 t-stat, K&N filter. Bought a 3.6 mps kit, and swapped pulleys. Finally found time to scan with the 3.6, and I spiked at 6.15 deg of KR. You know the moral of the story. Scan first.

I've ported a spare front exhaust manifold, and ordered a 3" DP, hopefully those additional mods will bring things back down to an acceptable KR level. But what I really need is someone to tune it.

Looking to go smaller guys, wheres a good source to get a 3.5 or 3.4 pulley? Also, does anyone have a scanner or know someone with a used scanner so I can tune the pcm?