I am getting about 6 psi of boost at wot with my b&m 144 blower using the 3.23" dia. top pulley. I am considering getting the 2.5" dia. which will increase boost, but how do I know if the boost is just building up behind the intake valve in the intake mainfold? I have a pretty aggresive blower cam and it just feels like the engine wants more boost. I have all the forged components that are ready for high boost pressures. This is a 350 that was built for a supercharger.

 

That's all that boost is. It is the restriction of the compressed air trying to enter the engine. Just go safe on the af ratio and you'll be fine....

 

yeah but you CAN get "false boost" when the air simply cnt move fast enough because of a restriction somwhere (usually exhaust or cam intake duration) the boost WILL build up and give you a false boost reading (# in the manifold does not = # in the cylinder, in PSI) especially when things heat up. the 3.X+" Pulley should be good for about 5 PSI according to holley, although I had a 2" pulley and was getting only 4 PSI out of my stout 355. it also depends on how old the blower is. mine was old.

putting a 144 on a stock 305 L03 motor WILL yield false boost readings. it may say up to 12 PSI in the intake, when your only getting 18-19PSI worth of atmospheric pressure in the cylinders.

 

¿Que?

All boost is, is when manifold pressure goes over 0psig inside the manifold because more air is being forced into the engine then it can flow on it’s own… so to get boost you HAVE to have air ‘back up’ in the manifold. The pressure seen in the cylinder is never what is in the intake, if it was you wouldn’t have any dynamic compression at idle/part throttle since at those points there is less then 0psig in the intake

 

Hey how fast is the car with that blower? I am looking at a blower like that within the year and was wondering what kind of times or power numbers that setup is putting out. I think if I shell out the cash though Im going to the 177. Ive got a spare hood to cut up.

 

Crossfire i worded it wrong. What im referring to is somthing in WEIAND and Holley's book on superchargering, as well as "street supercharging" book i read. its a false boost reading. its having so much that you get abnormally high #'s like i said earlier, but its not actually whats making its way through the heads. I guess the way it was put to me, was the blower puts out X amount of air... then the intake automatically gets that air right? from there the pressure increase forces air down to the cylinders... through the heads and valves... etc...

but when you get to a certain point... say 5 PSI on a stock 305... any extra air just doesnt make its way past the heads, simply due to restriction. so the extra PSI, say from going to a smaller pulley, is only seen in the INTAKE... where the heads and cylinders have little or no change... but since boost readings are taken from the intake... you see 9-11 PSI of boost by stepping up to a 6 PSI pulley from a 5 PSI pulley... the extra boost just doesnt make its way down...

Im probably not explaining it correctly but thats sort of how i remember it. its in holley's manual... somwhere.... they call it "fake boost" or somthing rediculous. it happened to me on my 305, i was running a 7 PSI pulley (for a 350) and was seeing 12 PSI in my intake on my 305. stock. 12 PSI on a stock 305 would normally blow it sky high, but i did it... with some timing retard but no race fuel. i dropped to a 5 PSI pulley because i was afraid of that 12 number, and my boost dropped down to 7 PSI. weird huh?

 

To be kinda blunt, I can’t make any sense of what you’re trying to describe. My best guess is that you’re getting at the pulses a positive displacement blower can cause in the intake from the rotor lobes only actually pumping air at certain parts of the revolution. In some cases (like datalogging) you might see something like 12psi one frame and 0.2 the next, and the actual boost is someplace in between. If you have your boost gauge hooked up close to the blower outlet you will see a higher then actual reading unless you install a restriction and a bleed in the line to the boost gauge.

 

What king is talking about is exactly what i asking. This is a quote from holley's instruction book for the blower So i think i may have answered my own question because i have a good amount of duration and it seems that you only run into problems when using a small duration cam.

 

thats what i was looking for! heh.

well, yes its good to have slightly longer duration than stock, or slighty more than slightly (eh?) but:

DO NOT OVERCAM a blower motor. you will be sorry.... especially with one of those cool small blowers... oh boy does it rob your TORQUE even with BOOST!

example:
355 chevy, 144 blower, 6 PSI of boost, 410~ RWHP @ 5900 RPMS:
234/244 @ .050, 112 LSA....
With 2400 Stall and 2.77 gearings:

Could not spin the tires by powerbraking, even under boost! Talk about lack of low end torque!
A simple gear change solved all that though, 3.42's woke it up BIG time. but theres still that crapy fuel economy...

point is, make sure if its a street car, that its CAMMED like one.

 

heh i had 3.08 gears and i never could put it in overdrive with a .68 5 gear. It wanted to drop it down to 1300-1400 rpm @55-60mph. My motor didnt like that much. It perfers rpms around 2000 or above so i just installed a new carrier and 4.11s which should put me around 2000 rpm in overdrive at 60. havent tested it out yet because i didnt have any gear lube and its xmas doh! I call it a street car buts its a little more than that hehe!!

 

Yea, but that’s not saying anything along the lines of ‘false boost…’ it’s saying basically when the engine can’t use as much air you’ll get a higher boost reading and lower HP, and when the engine can use more air you get a lower boost reading and higher HP.

As I’ve said before, it’s only about 1 thing, the amount of air you force into the cylinders. Whether you do it at 5psi, 20psi or NA it doesn’t matter, but the mass of air that actually makes it in determines the power produced. If the engine won’t allow more air in at the current pressure while the blower is trying to pump more air in then the pressure will go up till the blower looses efficiency and stops pumping as much air or till the boost is enough to force that volume of air in.

 

and your exactly right! but that doesnt answer the poor guys question...

i think he mentioned somthing about "more boost" and "agressive blower cam". which makes sense.

An aggressive blower cam is goina use the boost to shove the exhaust out faster, and its goina shove alot of intake boost right out the intake anyways, because of the late intake valve closing point. in other words, i dont care if you have 20 PSI of boost, until you really get into the cam's Powerband, its goina feel sluggish and like crap.

Just how aggressive is the blower cam? if it has a distinctive lope, its prolly too aggressive for your blower.