I have a stock 8,5:1cr 350 with 882 heads and a b&m 144 blower. With the stock 6" and 3,06" pulleys it only gets about 3,5psi . Carb is a holley 750 with vac secondarys, its boost referenced. Running a hei ignition with a totaltiming of 28*. Blockhugger headers and 2,5 exhaust. Trans is a th350 and rear gears are 3,36.

What could cause the low boost numbers ? I have a 7" lower pulley and a 2,5" and 2,66" (not sure on this one) uppers.

What spring should i run for the secondarys? got the softest one in ther right now ?

 

Since it's hard to tell what size pulleys you are running, it's hard to guess why your boost is low. When you say "stock" engine, do you mean cam also? Different cam profiles will effect boost. I'm also assuming you read the boost at WOT.

How well an engine "breathes" will effect boost - not a bad thing, it just means your engine is using the boost instead of stacking it up in the intake where you can read it. Worn teflon seals on the blower rotors will effect boost (if you have teflon seals). Even the ones without teflon can wear over time between the casing and rotors.

Changing to a smaller upper pulley diameter will spin the blower faster and increase boost - but there's a limit on how fast B&M wants their blowers to spin. They usually have a "minimum" size already figured out for most blowers. Also keep in mind the faster it spins at lower RPM's, the intake charge will most likely be hotter....

 

I am running the stock pulleys , 6" lower and 3,06" upper.
As far as i know the cam is what was standard in a 76 corvette vith L48 engine.
Yes boost was measured at wot.

Blower has few miles since it has been rebuilt by blowershop.

 

Well, I'd try changing to different pulley arrangements and see what you get....go to the 2.66 upper with the 6" lower, then go to the 2.5 upper and if that doesn't get you the boost you want, change to the 7" lower with 3" upper - etc...

 

On a stock or near-stock 350 you should be getting more (at least 5). Most likely reason it's reading low is that you aren't getting to full WOT. Check your throttle cable and make sure that when the pedal is on the carpet that the throttle arm if fully open, resting on the wide-open throttle arm stop. Also look down the throttle bores to make sure that the throttle plates are vertical, not slightly angled. Roots blowers are very sensitive to intake restrictions above the blower.

Since you're running a vac sec carb you need to be sure it's getting the secondaries wide open as well. Assuming it is functioning properly, a lighter spring in the secondary vacuum chamber would be a logical next step as well. And if the primary throttles don't get wide open (see above) the secondaries are never going to get anywhere close to WOT either.

 

Did some testing today, the throttle gets wide open, and the sec also gets wide open, used the lightest spring for the sec.
I think the problem was i did not hold it at wot long enough, think i saw about 5psi on the gauge but i had to let off, i really need to do an wheel alignmet
I will try a stiffer sec spring cause i felt a bog

 

Another question, is the rear housing suposed to have gear oil or grease in it ? It´s a B&M unit

 

I believe they all take some type of oil. If you check on the Weiand website, it may tell you.

 

Rear bearings are sealed (no oil reservoir to fill). Front ones are in an oil bath because they also have to lubricate the sychro gears that keep the 2 lobes in proper mesh with eachother.

 

damon is correct.the rear bearings on the blower are greased and each side has a seal as well.........ive hd my 144 apart once just to see what it contains....the teflon strips were a pita to replace though............

 

you need a blower cam, a blower without a proper cam is a mismatched combo no matter how well your engine breathes. You may be blowing some of that boost out your exhaust. a wide lsa such as 112 to 114 is great, lunati has some good blower grinds for these little blowesr, dont have a part number handy, any nitrous grinds are similar too. They hold the cylinder open longer to trap more boost and close the exhaust faster i believe. Your comp. ratio is perfect to cram in alot of pressure

 

3-5psi sounds right for the big top pulley. I get 8 with a small top pulley. And I would ditch the vacuum secondary Carb in favor of a mechanical. IMO