how to determine blower gear drive ratio?
 

engine is a 355 smallblock chevy
9:1 compression
im assuming that the blower needs to be under driven or a 1 to 1 ratio
how do i calculate to get the proper cog gears?

Divide the top cog into the bottom cog



20 top divided by 40 bottom = 2 to 1
or

40 top divided by 40 bottom = 1 to 1

Your pully cogs will vary this is just an example

Maybe Zip can share with us,he is a blower guy :P This is just a guess from me :P :P :P

Tom is correct by the calculations. there is alot more invloved in determining how much boost is being generated by the blower....

many things effect the amount of boost a blower is putting out....

* type of blower,teflon stripped or not, tolerances,how old the blower is.
* the way the engine is built, heads,cam etc....
* and of course the gear drive ratio.

when you speak with a Blower company like BDS etc...they will give you a ball park of what a certain drive ratio will make on a certain type of their blowers ..when they came up with these #'s they used a base line small block/Big block with their recommended blower cam with un-ported head and ported heads...and they designed their boost charts..(slide chart)

example:" when i ran my Alochol blown injected small block.with a BDS stage 4 competition blower...BDS said the drive ratio i had should make between 14-15lbs of boost....well i had extensive work done on my motor and it made a tad over 17lbs of boost on the dyno...

so you cannot know for sure what a drive ratio will make..they can only get you in the ball park with their Slide chart..


by re-reading your intitial post..you need to INVERT your question and ask yourself how much boost you wanna make ( taking everything above into consideration) then you work toward your drive ratio..

i get the feeling you have a basic small block..next to stock.....without getting into a nuch of other stuff a blower motor musy have ( cam,heads,headgaskets, etc...)...i will say no more than 7 to 9lbs of boost tops on a stock type motor

i hope this helps you...Zip you have any input here


Brian

just to add here....
boost is nothing more than the measure of pressure built up in the intake.

most companies that sell street blower kits that go onto mildly modified motors set them up to underdrive them (top pully larger than bottom) maybe 10 to 15 % underdriven, this gives anywere from 5 to 8 lbs of boost, depending on engine. Which is usually pleanty for a performance street car/showcar/ occasional track car, and is usually all a stock style or mildly moldified motor can handle.

since the whole idea of any race/performance motor is to get as much air into and out of the motor as quickly and as efficently as possible, supercharged motors really work better with as little restriction as possible.
throw away all the ideas of head ports and intake runners, and header primaries all selected to build mid range and low end tork.
supercharged motors love "big" big ports, big headers, big cid, big cams. But of course these things still have to be sized accordingly.

if you have a mild small block lets say, with a 6-71 kit builds 8 lbs of boost, later you put a big block, with good heads, cam , now you only see 4 lbls of boost, this means that the big block is more efficent at getting the air through the motor and does not have as much pressure in the intake.
or say you go from a 6-71 to 8-71, on the same motor, same percent of drive, you will see a gain of boost.
you cannot go wrong asking bds for advice,(there are others too) they got a catalog with charts and things to help you along.

thks for the info guys.
a bit more on the motor
tall deck world block
Scat 4340 Pro Comp Lightwt. Crank 3.8 stroke
titanium con rods
wisco forged pistons
AFR 227 cnc race heads
9:1 comp ratio (static)
isky roller blower cam (dont have the specs)
AFR 1:6 ratio roller rockers (intake)
AFR 1:5 ratio roller rockers (exhaust)
AFR stud girdle
rev kit (crane)
weiand 6-71 polished blower
dual 600cfm demon carbs
unilite dist. with blower cap
MSD 6 BTM
blaster SS coil
Ill get more specs from him if needed
helping him out cuz its our first blower install and wanna make sure no expensive mistakes happen.

You've got good parts, but you didn't say is this going to be street driven?
If so the biggest limiting factor you have is the gas you are going to run. With 9.0 compression 5Lbs of boost would bring your dynamic compression ratio up to 11.86 to 1, which is pretty high for pump gas at 93 octane.
In your original post on the 27th you stated 7.0 to 1 compression, if that were the case you could run up to 8 lbs. boost.
As to what pulleys you need to run, i assume when you bought the blower it came with pulleys. The normal kits say from Jegs come with 10 1/2 percent under drive at least on the 8:71 blowers.
You are not going to know what pulleys you need until you have the engine running and know what kind of boost the blower is going to make with your setup. The heads you have will flow good but also as mentioned by another poster you have to get the exhaust out also.
We just went thru this with a 383 and 8:71, with -26CC pistions and .043 gaskets. This created an 8.7 to one static. we're in the process of changing the pistons to extreme duty JE's with a -28CC, and going to Cosmedic .074 tk. gaskets. this will drop the static to 8.27, dropping the blower pulley down to 14 1/2 underdrive so it should put out 6lbs boost and will keep the dynamic to about 11.5 to 1, which with octane booster it should be streetable.
Whew!!, did i say all that.

Zip.

PS, how did you come up with 355" with a 3.8 stroke crank? what is the bore size on the blk.

opps :shock:
4 inch bore on tall deck block slightly more cubic inches
motor is my brothers he told me 7:1 but looking at the pistons (flattops) and the build sheet machinist figure say 9:1
btw the primary headers are 2" so it got enough for breathing room.
he will be mostly using it on the street but tends to drive em hard.
he will be running off Sunoco Supreme NOS 117 but im sure the ocasional 93 octane aswell.
im thinking 5 pounds max for that motor
sorry for the confusion

Not knowing the cam timing, and useing the flattops, also not including the valve relief, because i don't know how deep those are i assume you have 6.2 rods because of the tall deck i also used 64CC heads. The CU.IN. is 382"s and the static compression ratio is 11.74 with .043 gaskets.
Using the blower boost of 5 LBS and 1000ft elevation with this combo Your dynamic compression ratio would be 15.47 to 1, it will need good gas for sure.
You're going to have to have .030/.035 end gap on the top ring also because it will get pretty hot.
I built my first blower motor 43 yrs. ago and it seems like yesterday and the first time it fired i think the edges of my mouth almost touched my ears. It was a 365", 6:71 with hilborn 2 port injection on gas. 2 yrs later i set a world record in A/Mod sports in AHRA with the car and same motor.@ 8.00lbs per inch/ same as B/GS in those days.

Zip.

Zip is right on the compression with over 15 to 1 compression the 117 Sunoco will be a Must...if he were stripping it only it would be very good with Alcohol and a bird catcher..would really make things interesting.

i saw where the msd 6 btm is being used...good choice...

i personally do not like the uni-light distributers. never had much luck keepoing the modules from burning out..when they go the car is dead..the blow pretty often ..at least the older ones did...

sounds like its going to be a beast for the street..CONGRATS...

Brian

ah, good catch Brian, I have to agree , ditch the unilite, I burnt out 2 moduels years ago, and they are not cheap to replace.
I knew a guy years ago, 70 big block tubbed camaro, would drive around and when things got warmed up enough, the engine would die. Let it cool off for a couple of hours and it would run again...oh yes a unilite there too.
A blower on gas throws off a lot of heat, not good for a unilite sitting right behind it...
your compression will be too high for gas, it will detonate itself to death. and you really dont want to run expensive race gas on the street, takes all the fun out of it.
If you really want all that compression, then you will have to underdrive it lots and make very little boost on the street.
At the track , take the carbs off and put a hat on with alky, and fly.....