understanding blower percent driven .. over/under

[noslin]

Ok, i dont have a clue the percentage means and how it relates to boost. i can understand it will be different for each size/type of blower. but lets take 6-71 for discussion purposes.  if you want to get technical then just choose something and state what it is.

for first questions, if there is over and under driven.. what is zero driven.  how many lbs of boost is zero driven or nuetral (however it is refered too).  im guessing over driven will make more boost and visa versa for under driven.  so, what is 10 percent over and under. 

i guess the question would be what is neutral really.  then can go from there i guess.   is neutral atmosphere or one atmosphere? if its one atmosphere then 10% under would be 1.5lbs less.. so 14.7lbs is 1 amto then 10% would be 1.5 less then that?

ty
dean

[ricardo1967]

Blower overdrive/underdrive refers to the drive ratio between crankshaft and blower, just that. Manifold boost will be a consequence of that drive ratio (among many other factors).

Not such a thing as 'zero driven' If drive ratio is 1:1, it could be called direct-drive.

[dreracecar]

Boost is not increase due to changing drive numbers  the overdrive determines how soon the blower comes up to full boost before it flattens out

[noslin]

If a 1:1 ratio, then in a typical blower setup, is the blower itself turning same rpm as motor or is it such like the cam.. 1/2 rpm

Are all blowers rated the same way, industry standard. So to compare same size blowers.

So a person tells me on his setup for 8-71 thats 35% over driven. What does that corelate too?  Just means ramp is faster to peak boost?

[Frontenginedragsters]

It means the blower is spinning 35% faster than the crankshaft.
Pressure is higher at a lower engine speed than if it was 1 to 1 ratio.
You also have to know how much is too much.

Matt

[dreracecar]

If a 1:1 ratio, then in a typical blower setup, is the blower itself turning same rpm as motor or is it such like the cam.. 1/2 rpm

Are all blowers rated the same way, industry standard. So to compare same size blowers.

So a person tells me on his setup for 8-71 thats 35% over driven. What does that corelate too?  Just means ramp is faster to peak boost?

Cam is 2 to 1 because the cam gear has twice the teeth count as the crank gear- it turns 1/2 speed. equal tooth count has them both turning the same.

Im theory yes, but we dont race theorys. Different makes have their own rotor cuts and tricks so it is possable  to have a 6/71 out perform an 8/71.

Its about filling the engine with enough air givem the CI.  Say the engine is a 562ci and needs more volume at the hit thru the first 60' vs a 350ci running a 6/71 at 20% over to acheive the same 60' time.

Everything is about matching parts. you may think that putting a 12/71 at a lower % would be better, but the 12/71 takes more HP to spin that the net gain over spinning a 6 or an 8 at a higher % is basicly "0" but at the higher expense/cost of the 12/71

[noslin]

I found wiend chart for 671 to reference. Its interesting the percent driven is cubic inch based too. Smaller cubes bigger percentage.  Eg; 327 @30%=27.1 lbs and 540 is 10.x lbs psi.

Why would boost be less for different cubic inch displacements .  The amount of air bieng forced into the engine doesnt change.  i can see volume wise the blower will have a lesser affect (gain) on bigger displacement but the amount of air (and weight of air) being induced doesnt change given same percent driven right.

http://www.onallcylinders.com/wp-content/uploads/2013/02/Drive-Ratios1.jpg

Ty
Dean

[wideopen231]

Boost is basically pressure built up because of restriction of air getting into cylinder. Bigger cylinder lets more air in and that means less pressure in intake(boost). If you have more pressure you can force more air into cylinder. That means more oxygen and you will be able to burn more fuel and generate more power. Also more stress on parts like crank and rods. Engine size , head flow and cam design can all effect boost given a blower that moves same amount of air.

 Different design blowers can make more air with less overdrive, That means less power needed  to spin blower. More power to spin the tires. Example  a 14/71 Hi Helix(more rotor twist) at 50% overdrive takes 300 HP to spin it. A screw style blower takes almost nothing power wise to make same  or more boost. Even at a lot higher over drive like 120% over engine speed. Motor at 10,000 and blower at 22,000 rpm rotor speed.

[dreracecar]

The bigger engine eats more than the blower can feed it , and has a lot to do with plenum size.
You can run a small shop air compresser  to blow chips away, but hookup a air grinder and it will stall given time and with the pump running

[JoydayVox]

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