Okay so since I've had my ottp stage 1 kit, I've seen 15 psi at WOT but today after a long time of not bothering to look, the most i could pull was 12.5 psi at 7,000 rpm. any ideas what the issue is?

also, i plan on moving up to ottp stage 2 soon. what's the average max psi at 7k with a 2.8 pulley?

 

Possible belt slip? I've got an OTTP stage 2 and see 15psi. You're boost won't go up with a 2.8. It will just pull harder

 

it won't go up with a shorter pulley? this is probably a noobish thing to say, but i thought that was the point of a shorter SC pulley...

 

2.8 Max boost is anywhere between 15-and 17.5 depending on exhaust mods.

 

do you think my exhaust is the reason for going down 2-3 psi? when i was hitting 15 psi i had stock exhaust, now i run full 3" exhaust.

 

You xould easily lose 2-3 pounds with a highflow cat and header but i think the issue your having sounds more like belt slip

 

well, i did this 2 or 3 times to see if it was consistant and it was. i'm going to say that if you can lose boost from exhaust then that's it since i've only had my header and dp for only 3 or 4 weeks now.

i should still have the same power and even a hair more with the exhaust though, right? i mean it isn't robbing me of anything. just more power, less psi?

 

Thats probably it. You're exhaust is more free flowing thus the charger not having to work as hard to make boost. More boost doesnt always mean more power. So many people think it does. You will definatly have more HP with full exhaust even though you're running less boost. Like i said the charger just isn't having to work as hard

 

that's why i asked. i felt sceptic of saying that less boost as a result of a mod that's supposed to add a little over 10 hp/tq couldn't be hurting just because my needle says i'm running less boost. thanks.

 

Here check this out. A guy does a comparison between a 3.0 and 2.8 pulley. The 2.8 builds boost faster but both top out at 15.5psi in the end.

https://www.cobaltss.net/forums/2-0l-lsj-performance-tech-47/2-8in-vs-3in-pulley-boost-charted-214128/

 

i only hit 15 psi with a 2.7 pulley and 3 in exhaust so 12 sounds about right

and no matter what if you go to a smaller pulley without changing your exhaust you boost will got up

 

Stock exhaust, 2.8 pulley, dual pass, option B = 17 psi

 

My buddy has a GM stage 2, stock exhaust like me and with my 2.8 we both make 15psi

 

2.9 hitting 17 at 7k

 

then there is something wrong with your car i used to make 17psi on a 2.8 before i went to a bigger exhaust

 

Elevation plays a part in boost as well. And a clogged cat. Cooling mods wont make you're boost go up. The guy did back to back pulls with different pulleys. The chart dont lie

 

well i do have full exhaust(2.5) and headers, cai and the 7k redline helps get it there

 

i have the same set up when i made 17 on a 2.8 but with full 3in exhaust on a 2.7 i only make 15

 

2.5 will make more boost btw plus it all depends on the tune

 

Yeah there are so many factors that come into play that everyone will have a different story. But that chart is back to back with a 3.0" and a 2.8" pulley. Nothing changed inbetween. Both maxed out at 15.5psi in the end. Even tho the 3.0" spiked to 15.5psi in the very end which doesnt help you much

 

seems like those of us who are running less boost on smaller pulleys are running full 3" exhaust, not 2.5". at the end of the day though, if you can get more horsepower with less psi then that's just that much less pressure in the engine, so it sounds good to me. though i will admit, seeing that boost gauge spike does give me a woody. j/k

 

ok, so, the technical answer, here goes:

boost doesn't determine airflow. boost is pressure, right? My car: stock exhaust with OTTP Stage 2 2.8 pulley, OTTP heat exchanger. I hit 17psi.

more exhaust flow means less restriction... but not just in the exhaust! the car breathes better as a whole. the SC is pressurizing you're cylinders, but in every car you have a period in the cumbustion cycle where the intake AND the exhaust are both partially open, called "valve overlap". before the power stroke completes, the exhaust valve starts opening so the presurized burnt air/gas mixture starts to flow out under pressure. as the exhaust valve is starting to close towards the end of the exaust stroke, the intake starts to open so that it doesn't JUST start opening on the intake stroke. Of course it's timed perfectly so it won't open and smash into your piston :haha:

so that sliiight overlap is where the high flow exhaust helps. the SC is under less presure when it helps push out a bit of exhaust, called "scavenging" the exhaust. It is this same principle of scavenging the exhaust that helped to bring the supercharger into the automotive scene for old skool 2-stroke diesel engines, where scavenging is necessary. anywho...

With the better breathing the car has less restriction. Boost IS restriction, it's pressure. Less restriction? less pressure. Blow into a balloon like the ones used for balloon animals the skinny ones, you can fill it up just fine. You displace the air into the balloon. Blow up a regular balloon, it's easier to blow up. You used the same amount of air both times. Which has less restriction? the bigger balloon.

Ya got the same airflow, don't think that 12.5 PSI is bad. Infact, it's better for your IAT2's