Superchargers ratios and pulley design

[Jase]

similarly enough carlos should have my manifold finished end of the month,

i plan on running a 3.8 buick with a 3/53 blower,

max revs on the engine is 5500 at 232 cubes,

blower would seem to be good for 159 cubes but will over drive it to pick up the slack,

I'm aiming for 8-9 psi running twin throttle bodies injection and a remapped ecu or squirt.

Although the blowers a little smaller its fully rebuilt and should spin hard enough without losing its efficiency due to the lower rev limit of the engine.

Still taking into consideration 5500 is probably double what the old diesel would do.

[Yowzer]

http://www.megaupload.com/?d=W8ENFDWM

Enjoy

[Testament]

Dont take the whole 426cu thing as gospel. It's not quite that. (Don't ask me what it actually displaces)

My old man is planning to fit a 4/71 to a 265 hemi down the track. So your numbers are quite relevent to his build.

I run an S/C14 (Allegedly 1.4 litre) on a 1500cc motor. @ 1:1 ratio. Makes 10-11psi.

giving that a 4 stroke engine only goes through half its displacement per rev you are trying to shove 1.4 litres in to what would have been 0.75 litres - by the numbers that would be 12.74psi so taking into account losses for leakage and heat etc. that sounds pretty much right.

so R100 has a 262 cube engine and wants about 7psi - call it 0.5 bar

so 0.5 bar = pressure ratio of 1.5

1.5 x 131cubes = 196.5cubes per rev

now with a little research I found this chart below which shows there are 2 sizes of 4/71 blower - if you can measure the rotor length we can work out which you have and what pulley ratio is needed. In either case an underdrive is needed.

[KKtrips]

I just realised that the 4/71 superchargers are designed for detroit diesels which are 2-stroke so they will inhale twice the amount of air I estimated - hazy sunday afternoon brain is struggling to figure out if this is relevant or not..

[Testament]

not really relevant once you have the actual blower displacement. on the detroit diesels the blowers were more for scavenging than boosting, to blow the spent exhaust gas out and supply clean air in.

[RUNAMUCK]

225cubes per rev is the bit we need to know. All things being equal if one was run on a 225 slant six at 1:1, it would make one bar.

I had heard somewhere that they didn't displace the full 426 cubes per rev.

So 265cui X16.39= 4343cc

X5500=23888425

/2=11944212.5cc of air.

7psi+14.7psi/14.7psi=1.476 (Boost ratio)

11944212.5 X 1.476=17631932.74 (CC's of air required to create 7psi boost)

Blower displacement:

225 X 16.39=3687.75CC's per rev.

17631932.74(CC's of air to make 7psi)/3687.75 (CC's per rev of the blower) =4781.21 (Blower rpms)

4781.21/5500= .8693126940:1 Blower drive ratio required to make 7psi on a 265 cubic inch motor.

Now In my experience, that actual ration wouldnt quite make 7psi. It might come close if the blower was freshly overhauled, and clearance'd nice and tight. Heat isn't as much of a factor in losing boost. It just tends to reduce the density of the charge. (And therefore reduce power)

I did once read some where on the www that if a roots blower is turned slowly, there is more time for the air to leak past the rotors. And it was better to spin a small blower fast, than to spin a big blower slowly. (Just the opinion of that person btw, not what I myself think) Also blower speed isn't always the greatest contributer to heat generation. (Although obviously if you put an S/C12 on a 440 big block it would damn near catch fire if spun fast enough to make boost)

Depending on how well designed a given blower is, it's the actual compression of the air which heats it up. Some blowers heat the air more than others. I would personally not bother trying to get more than 12psi from a roots blower. Although on the right kind of setup (Intercooler etc) more might be do-able

[RUNAMUCK]

PS,

Feel free to check my maths above. I aint no rocket scientist. And it's quite possible I fucked it up.

[Testament]

yeah seems right, although you didn't need to convert to cc's when the engine size and blower size were both in cu anyway

basically

( pressure ratio X engine displacement x 0.5 ) / blower displacement = drive ratio

with both engine and blower displacement being in the same units.

I can't see that sealing issue being real - that would only come into play if the blower was spinning exceedingly slowly.

[Yowzer]

Them old blowers probably didn't have that great tolerances though

[Testament]

Them old blowers probably didn't have that great tolerances though

Sure they did. why wouldn't they? they may have a little more clearance than the very latest teflon coated units but the tolerances and internal clearances are still very small. Basically, micrometers are not new technology - and them old dudes knew how to use em.

[Mr. Mainstream]

If anyone is playing with or thinking bout using a L67 from a 3800 commy.

this site is was quite helpful when i was overboosting my Holden.

Has many maps and charts for the blower on there.

Gotta Log in thou...

http://l67torque.com/

[KKtrips]

Nothing to do with facts but fuck it shows that superchargers are BOSS

BKXErw5D0ak

[R100]

Ripped my blower apart today and dropped it off at the metal polishers. In the picture below youl see the rotors from my 471 and the blower in the back ground is a 371. To the left is my "blower drive services" 471 to chev V6 manifold. A fricken rare and hard to find manifold.

Once i get the blower back ill chuck in new bearings and seals.

Got to get a blower snout and pulleys etc from Als blower drives.

Will make a build thread for my Chevy Vega in a week or so.

[sentra]

bart mega math

how much of effect does the cam have on the theoretical number in practice?

all i figured when i did mine was how many rpm the unit worked efficently/diddnt explode at

and blindly geared it to suit,

ignoring blower longetivity, for max power im guessing we just chase the ratio untill the

iat becomes unmanagable/seat of the pants math?

[Testament]

cam will have a reasonable effect - bigger inlet cam will lower boost in the manifold a small amount as the air will get into the cylinder easier. longer durarion exahust cam/more overlap will lose a little boost but will mean no reversion (clean air/fuel mix) which is a good thing. but you will blow some fuel out the exhaust so economy will be poo, but maybe flames out the exhaust also so totally worth it.

[RUNAMUCK]

As above.

My 1564cc datsun makes 10-11psi with a large cam. I ran my indentical set up on another A series motor (1397cc's) with a stock cam and made around one bar of boost. That very same motor (In another life) made around 11psi with a big (N/A) cam.

If you're going to all the trouble of forcing, then fitting a nice big cam is a no brainer. Blower grinds run a wider LSA. This reduces the overlap, reducing charge going out the tail pipe. And opens the ex valve sooner. (sooner before BDC) This bleeds off a little bit of power, but uses some of the energy of the still expanding gas to evacuate the cylinder. Since the in side is being forced in, it helps the motor expel the used gas.

Some forced cams run a bit of extra duration on the exhuast too.

I know one bloke who has run a few 350's on nitrous. He reckoned sometimes he'd fit higher ratio rocker arms on the exhaust valves to help with those duties.

You asking for your boat Sentra?

The other two upsides to running a larger cam is, lower boost figure means denser charger. And bigger intake vent means more charge gets in, and more power is made. win-win. (Although your sweet W2A intercooler must really get the density up)