How much Boost does it put out?

[rockford0384]

I wondering how much boost a 3.2 or 3.25 pulley will put into your engine. Without a bigger downpipe. I currently have a 3.4 pulley and runs about 9-10.5 pounds, without a CAI. Im buying a 99 GTP and thinking about putting a next size smaller pulley on it when i get it. If I do put a smaller pulley like the 3.25, would or should i put a bigger downpipe on it? Only reason I ask I dont know if there would be to much boost goming in and not air flow out of the engine... so I want to know if its going to hurt the engine or not. thanks

[kilroypr]

You are correct I have a 3.4" pulley and the most PSI I have shoveled down the Intake Manifold is 11 PSI's. Based on suggestions from Ripn12s and others I learned the KR on my car with the OEM temp setting was 6.6deg(yours could be different and you have to check this out before any further moding is done to the intake PSI's), that is not good. So before you throw in a smaller pulley make sure the KR is on the aceptable 3deg range at WOT. If it is above 7deg then the smaller pulley will harm you more than help you. The Knocking would be so great that the timing would have to retard too much to not distroy rings, piston heads or other hardware.
In order to reduce knocking you should use higher octane fuel, cooler plugs, cooler tstat(that is for starters).
Then install headers, intercooler(for an even smaller pulley). I have a 2.8" thrasher pulley that I have not tried yet because the KR with the 3.4" is 2.3 on 93 octane gas, thus, if I install this smaller pulley I definetly would get a KR bigger than 7degs and that is not good.
I will install an intercooler and when I do get the money for that then I will try the smaller pulley to see what happens. So in other words, get a scanner, check the KR, is on the 3degs accepatable range, cool it a little with plugs and tstat. Then try the 3.25" and let us know what you get from it

[rockford0384]

but how much boost does it put into the engine w/out a CAI? anybody

[kilroypr]

Well, let's do some math here. If the Std pulley wich is 3.8 pumps 6psi, 3.4 pumps 11psi then theoretically speaking 3.2 should be pumping around 20psi's. HEre is the math behind this, 3.8/6=.63, 3.4/11=.31. These are the facts. As you can see dividing the pulley size against the top PSI it generates on an eaton M90 OEM SC then would make sense that 3.2/X=.16, X=PSI's. Since on the previous divisions the ratio almost got cut by a half of the previous 1 then the divisor for the 3.2" pulley should be around half of the divisor of the 3.4" pulley. So X=3.2/.16, thus, 3.2/.16=20 PSI's Max for the 3.2. Since you might not be driving the engine at the high RPM range all the time I will assume that 18 psi's is a good estimate for a loaded 3800 sc with a 3.2" pulley.

[Ripn12s]

With a 3.25/intake/exhuast/

My car saw around 12-13 lbs of boost

With a Cam/3.1/intake/exhuast/DP/Ported Manifold/

My car sees 13 lbs of boost

[JoeJoe231455]

Yeah, you should see 13-14 psi without any serious exhaust work. Too much for a non intercooled engine. Get some more air flow through the motor and the 3.2 or 3.25 will make the standard 11-12psi. Headers are the key to smaller pulleys. Stuff like rockers and a cam will only drop boost significantly if it has an open exhaust system to ditch the increased gas output. Blower outlet porting is another good way to increase airflow and drop boost.

[kilroypr]

Thanks guys, nothing compare to empirical data.

[KnightRider2]

Im only getting 11 PSI with a 3.0 pulley. 11 PSI is a lot of boost if you don't have some other mods like headers, intercooler, rockers.