I believe that the bigger pully on the tensioner gives you more belt wrap around the */c pully to help with belt slippage.

what belt size should i go with the 3.4" pully i right know have a 715k6 on right nowI


if thats ture then could you not go with a smaller belt??

you are fine with the 715.. IF you have a 3.4....

if you have an oversized idler you can run a 3.4-3.2 with a stock belt..

$10 says you have a 3.4

What these guys mean to say is that if you haven't opened up your valvetrain and intake/exhaust ports considerably, then anything smaller than a 3.4" pulley will cause KR very badly. KR is Knock Retard, which means the PCM is pulling your timing, and shutting down spark in an attempt to stop "detonation", or the "knock" sound from the block.

Detonation is when the fuel/air mixture in your cylinder ignites before the piston is ready. Too much of this, even with the PCM pulling your timing, and you will burn/melt/blow your valves, the seats, and possibly even the piston itself, causing your engine to go BOOM in a big way. A 3.0" pulley will very likely result in KR even during normal acceleration, maybe even idle. 3.0" is extreme.

So, as was stated above, if you have a 3.0" pulley, do not drive it anymore, and take it off immediately. If anything, put the stock pulley back on until you get the 3.4".

We're only, honestly, trying to help you keep from harming your car, and potentially yourself. When the engine goes boom, parts come through the block and goes who knows where at the speed of a bullet. Just take care, ok?