FAQ: Smaller SC pulley? What Size?

I don't have a KR scanning tool and I've gone much further than is recommended.

If you're going to drop the pulley size, you won't notice it as much until you open up the exhaust and intake.

Do all the supporting mods: plugs, 180 t-stat, FWI, 3" downpipe, 93 octane gas.

THEN drop the pulley. And if you can't scan for KR like me... well, it's always a big risk. I haven't even gone WOT yet with my 3.4, I'm just scared to do it until I at least get the front p-log.

I just don't care, so I put the shit on it and run it. That's not good advice though. Good advice is doing all the supporting mods, then never dropping the pulley.

But that's not very fun, is it?

Buapo wrote:

But that's not very fun, is it?

Nope, but neither is the sound when an engine blows. Good advice about doing the right thing, and we all know there's more than one of us out there that took the chance with our motor and got lucky.

Knowing more now....I see how I was probably getting much worse performance than if I had stayed status quo at many points. Scanning is really a key to knowing what is happening versus thinking you feel it.

Agree with Bill about knowing vs. feeling. You really can't feel knock until it's too late. Here's a study I did last year on the consequences of KR:

https://rivperformance.editboard.com/series-ii-scans-tuning-pcm-f5/the-consequence-of-kr-t1548.htm

Interesting study. It's no secret I'm not on my original motor due to a mistake and scoring the crank slightly. Well that original motor went back in with a polished journal. Leaving work one day, got up to highway speeds and saw 17* of KR. I didn't hear a thing. The PCM was giving me 0* of timing advance by pulling 17* of KR. Being at around 2k rpms the oil pressure looked reasonable, but slightly lower than it shoud have been.

The moral of this story is the bearing was going out and the sensors knew it way before I did.

okay...you guys scared the shit out of me...I was gonna drop to a 3.4 with my current mods, and wait on the 3" DP and exhaust, but now I just ordered a 3.5 and a 3.8 from ZZP...just in case.

Me and matt ( Mr. Riviera) have no knock with a 3.4 and 3" downpipe + cooler tstat and plugs. ( with stock shift points/timing) But thats pretty unusual.

but to be safe i'd go with a 3.5 if i were you. Its probably not a noticeable difference between it and the 3.4.

I'm just gonna wait Tony...I'll put my plugs in this weekend drop to a 3.5 and wait for my aeroforce gauge before I do anything else...hell...I just might keep the 3.8 on until I get my gauge...

I'm running on 93 octane so I should be okay (knocks retards on wood) with the 3.5

I want to put a smaller pulley on my brother's 1997 Riv. I have done some reading in the FAQ and it seems that a 3.6" would be a good choice for a daily driver with no mods and Autolite 605 plugs, but I was wondering if I had to use a 180* thermostat? i am new to supercharged engines so please bear with me.

No pulley size drop can be considered "safe" without first scanning for Knock Retard. Even a totally stock engine can have KR. Look at my mods list in my sig. Even with all those supporting mods, I'm getting KR as high as 8 degrees on hard acceleration, while running a STOCK size pulley!

1998 Riv wrote:

No pulley size drop can be considered "safe" without first scanning for Knock Retard. Even a totally stock engine can have KR. Look at my mods list in my sig. Even with all those supporting mods, I'm getting KR as high as 8 degrees on hard acceleration, while running a STOCK size pulley!

your engine must be wierd or be a factory "FREAK" as in FREAK and messed up.........

AA wrote:

-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 - can keep stock tensioner pulley, or change to 3.5" - keep idler pulley stock.

-OEM SC belt works with 3.3" SC pulley - must change tensioner pulley to 3.5" - can keep stock idler pulley, or change to 3.5".

-OEM SC belt works with 3.2" SC pulley - must change both tensioner & idler pulleys to 3.5".

-OEM SC belt works with 3.1" SC pulley - must change both tensioner & idler pulleys to 3.5".

Tomorrow I am installing my MPS. I bought 3.8, 3.5 & 3.4" S/C pulleys. I'm going to install the 3.5 first using the stock tensioner pulley. I have a 3.5" tensioner pulley on the way to me and will install that when it arrives. My question is, during the winter I will probably put the 3.8 S/C pulley on...will that work with the 3.5" tensioner with stock S/C belt?

Well I figure there are some reasonable explanations. Could be the injectors are bad, or simply plugging up, causing it to be run lean. I have a cleaned/flowed set from Bill that I need to get swapped in sometime. Also, the timing is advanced 3-4 degrees, I believe, with the DHP 1.5.

[quote="Chicken"]

AA wrote:

Tomorrow I am installing my MPS. I bought 3.8, 3.5 & 3.4" S/C pulleys. I'm going to install the 3.5 first using the stock tensioner pulley. I have a 3.5" tensioner pulley on the way to me and will install that when it arrives. My question is, during the winter I will probably put the 3.8 S/C pulley on...will that work with the 3.5" tensioner with stock S/C belt?

Mark, with cold winter temps, it is actually safer to run a smaller pulley than in summer. I'd leave the 3.5 on in winter, if you are running ok with it this summer.

Dave do you have a thread going about your KR? You should check out all your motor mounts very closely, make sure they are in good condition.

[quote="1998 Riv"]

Chicken wrote:

AA wrote:

Tomorrow I am installing my MPS. I bought 3.8, 3.5 & 3.4" S/C pulleys. I'm going to install the 3.5 first using the stock tensioner pulley. I have a 3.5" tensioner pulley on the way to me and will install that when it arrives. My question is, during the winter I will probably put the 3.8 S/C pulley on...will that work with the 3.5" tensioner with stock S/C belt?

Mark, with cold winter temps, it is actually safer to run a smaller pulley than in summer. I'd leave the 3.5 on in winter, if you are running ok with it this summer.

that makes complete sense....thanks. So am I okay with the stock belt running 3.5S/C and 3.5 Tensioner?

I believe on the Riv's layout, you can use stock length belt down to a 3.4 SC pulley, even w/o changing the idler to a 3.5. This info is posted, didn't I see it higher up in this thread also?

Dave,

yeah, I read the info posted and it says it okay to use the stock belt with the 3.4 and stock tensioner or you can go to a 3.5 tensioner...

My original plan was to drop to a 3.4 pulley and change to a 3.5 tensioner so I would get more belt wrap, I already bought a 3.5" tensioner - but you guys scared the hell out of me with 3.4" horror stories if your car is only slightly modded... So I'm going to drop to a 3.5 pulley, but still want to use the new 3.5 tensioner because my stock tensioner has over 120k miles on it....

...so will a stock belt work with a 3.5 s/c and a 3.5 tensioner?

Probably not. I think you'll find the 3.5" tensioner + stock 3.8" SC pulley will make the overall belt tension very tight. I think 3.4" SC pulley is the biggest you can go when using the 3.5" tensioner, on my car anyway.

Be our guinea pig. Try them and see!

...at the risk of seizing my s/c bearing and destroying all my pullies....okay....I'll try it...

No don't worry , that kind of thing only happens to me.

Nah... I meant try getting the belt to slip over both the 3.8" SC and 3.5" tensioner. Bet you can't. I can barely get it to work with the 3.4" SC.

I'll bet Aaron's right, with the 3.5 idler, the larger SC pulleys will be a tough fit.

When the 3.5 idler shows up I'll try it and let you know....

I used the stock setup down to 3.4. At 3.4 I had a very loose tensioner, and since I now swap between a 3.4 and a 3.2 I got a 1" shorter belt. (At the time I was unaware of the larger idler pulley fix...) It works really well for me.