KB 2.8H boost question

[brianstrawberry]

I recently added a 2.8H to my cobra and am a little confused about the boost situation...

Supporting mods include

JLT RAI
Accufab TB
60# injectors
stg 2 FPDM
twin GT pumps
SCT3000 MAF
BF Idlers
O/R Prochamber
Borla Stingers
SCT livewire

I am running the 3.75 upper pulley, which is rated at 18.5lbs of boost and am only seeing 16.5lbs... My tuner suggested adding a lower pulley to get more boost and more power. (Knowing that the 2.8 doesn't really start working till you near or pass 20lbs)

My first guess is that the boost is off from the cattless setup, but two pounds seems like a lot.

I am curious about what the pros have to say about this?

 

[jumperjack]

The formulas that KB gives you are just an approximate #. My 2.8H varied by 2 lbs. just from 50* to 85* temps. I ran the 3.25 upper stock lower and saw 21.8 lbs in cooler weather and 19.5-19.8 in warmer weather. Your tuner is right about spinning the blower but IMHO with a 2.8H I would just switch out the upper unless you are looking for more than 20-22lbs. I think a 3.50 would get you in the 17-19 lb. range. I ran 100 unleaded in mine but if you go with the 3.5 you can run 93 as long as you keep the timing around 18* or less.

 

[sprstr1000]

does KB put out diff amounts of psi compared with the whipple
ive got a 3.50 upper stock lower and im gettin 13psi
i ordered a 3.00 which should bring me to 17psi

 

[exdeath]

Actual boost is always going to be less than the theoretical boost derived from pulley size and rotor RPM. Volumetric efficiency of course has a huge effect since boost is a measure of airflow restriction.

 

[GodStang]

does KB put out diff amounts of psi compared with the whipple
ive got a 3.50 upper stock lower and im gettin 13psi
i ordered a 3.00 which should bring me to 17psi

Yes they are different. Compressor size and gear ratio change what the pulleys give you for boost. Also all your other mods add subtract from boost. I have a 3.75 upper and 6lb lower and only make 18.5psi

 

[brianstrawberry]

ok, starting to come together I think. My boost is typically 15lbs and have seen as high as 16.5 once or twice. Since I already have the lower kit I am considering running a 3# ring which should get me to 18lbs for the most part with some cases where I would be seeing 19.5lbs.

Sound like a good plan?

 

[jumperjack]

ok, starting to come together I think. My boost is typically 15lbs and have seen as high as 16.5 once or twice. Since I already have the lower kit I am considering running a 3# ring which should get me to 18lbs for the most part with some cases where I would be seeing 19.5lbs.

Sound like a good plan?

Just my thought but I would definitely go with a smaller upper before I would I would take the stock lower off. You can easily accomplish your goals with a $69.00 pulley change. I ran the 3.25 KB pulley w/NAPA belt and had no belt slip. I would keep the cage on the lower but that's just me. It is always easier to change the SC pulley then the lower.