Pulley size vs. rpm

[Funkybird]

What would be better, run a small pulley and shift sooner or a slightly larger pulley and shift later, assuming both setups would be near max impeller speed?
On my 355/f1a combo I run a 7.65 crank pulley and a 3.40 drive pulley. Max impeller speed is reached at roughly 6200 rpm and I see about 18psi when shifting at 6100 rpm.
Would it be beneficial, for instance, to run a 3.70 drive pulley and shift around 6600 while max impeller speed is roughly 6750 rpm?
Would this maybe give me a longer powerband?
On a side note, how bad is it to overspin a centrifugal?

[Fire67]

I'd think that it would make the same peak power, just at a higher rpm because your building up to the same cfm just later. This means that it would build power later in the rpm ranges, or suffer down low. But if you have an over abundance of power down low it may help traction issues.

Why are you wanting to spin more rpm's in the first place? Will your cam/valvetrain handle the extra rpms?

[Funkybird]

I'm more or less thinking out loud and have been curious about this for awhile. My valvetrain is fine for a bit more rpms, just wondered if it would make my powerband wider without effecting low end alot. The only traction problems I have are above 4 grand when the boost really comes on hard.

[Fire67]

It's going to suffer some at lower rpms for sure. How much I couldnt tell you. Maybe your traction issues wont happen till 4500rpm?

My guess goes like this:
The blow moves air linearly in comparison to rpm. If it takes more rpm's to max the airflow out you will certainly see your peak hp at a later rpm. Since the flow of air is linear, I'd expect the power down low to come at the same numbers but later in the range. Probably equal to the delay to peak power.

But I've never seen a comparison like this done on a dyno, so this is just my guess. Id think that you will also see a slightly lower boost pressure. Since the engine will be spinning faster per cfm of air moved by the blower.

[BLOWNDFIZ]

I'm not a fan of huge RPM so in my case I want a pulley combo that will make max boost at 58-6200RPM or earlier and I shift @ 6200 or so.

Honestly the only real way to see what your car likes would be to put it on the dyno. In my opinion the larger the blower pulley used the less potential for belt slip you have (I'm only running an 8 rib set up). If you are cogged then belt slip is obviously not an issue. I think that your engine will also play a big role in what would make the best combo. It all depends on where peak tq and HP occurs.

So to answer your question I like the small pulley shift sooner scenario. Unless your car is nothing but a race car the smaller puley will create more boost earlier and make it more fun on the street.

[Funkybird]

Thanks for the comments guys. My car is deffinately not a race car or close to it, just a fun cruiser that see's the track maybe a couple times a year.
I am running a 12 rib setup so belt slip hasn't been a problem as of yet.
I think i'll stick with this setup and iron out the fueling and tuning issues and hopefully get closer to my goal of 800whp :)

[Grr]

you can overspin most blowers including the F1 about 10% with no problems. I would leave it with the 3.4 and spin it to 6600.