blower pulley sizing help
 

So I have found a way to run a cog belt on my 256 blower if I end up running this blower...my question is for pulley sizing...

blower pulley sizes available....

28,30,32,34,50 tooth sizes available

From what I've read on the conversion this allows you to run a standard cog pulley on the crank like a 6-71 would have...

What size crank pulley should correspond to 5-8 lbs of boost or more?? just kinda would like to get in the ballpark if anyone can offer insight so I don't have stacks of unusable pulleys

Thanks!!

Stewart

Not sure why you would want to do that. Cogs are way more expensive and not as readily available as serpentines..
Its all math. Calculate the how much its overdriven now.
If its a 6" lower and 4" upper, then its 50% overdriven (6 divided by 4 = 1.50 or 50% overdriven)

Well...here's my issue...I have v-belt accessories and the 575 sci lower pulley uses serpentine accessories...I have researching this swap more after I started this thread and it may not be as simple as I thought...trying to come up with a lower pulley solution that will net me 5-8ish lbs boost..I was thinking the cog would be an easy conversion but come to find out 256 shafts are splined not keyed like the 177 so the swap will take a little more enginuity than I thought...so maybe lower pulley solutions may be a better way to go??

I had a 525 sc to start with

Stewart

The 256 pulleys are getting tough to find, you may have some luck posting in the wanted part of the swap shop. That being said converting to a cog system wouldnt be a bad way to go. You could take the stock 4" 575 upper pulley and machine it into an adapter for the standard 6 bolt roots pulleys, although there may be some alignment issues to work through. Nothing spacers couldnt solve. That is assuming you have access to a lathe.

I have a 3.25 splined 575SC upper pulley if that'll work $70 shipped.

I'm pretty sure TBS has keyed shafts for the 250's, if you want to go that way also.

Also, you said you said you started with a 525sc. I assume it had 177 blowers on it, and from your comments they had keyed shafts? If so do you have a keyed shaft for the 177 OR any upper splined pulleys for it? If so I could use either in my build, I have a splined shaft and need to swap to keyed shaft & keyed pulley OR get a splined 10 rib pulley in the 3.25" to 3.5" range.

Pm me

Thanks

After rethinking this it may be smart to keep the grooved pulley, the 250/6 blowers are not very stout. If the motor backfires with the cog belt the belt cannot slip and there is the possibility of throwing the blower out of time due to the bevel cut gears. You may be farther ahead finding serpentine pulleys for your accessories. You need one for alternator, power steering and water pump?

I thought you had found a 671 series blower? If you can use that it is a lot more blower than the 256. I think I might have a snout laying around for a 71 series.

Also, the 3.25" pulley would put you at 6 to 7 psi depending on cam and heads.

Mohav...I don't have anything left from the 525..already sold the blower and everything...sorry..I'll let you know on that pulley whats I figure out what I'm gonna do thanks!!

As far as the 6-71..thats ideally what I'd like to do..I have a standard GMC 6-71 that I need to get converted through dyers...unfortunately I don't have the funds for that unless the 256 setup sells which its been listed for awhile with minimal hits

I was thinking however today before bed(I'm on nightshift lol) has anyone tried putting a blowershop or any other snout on a 256?? from pictures it looks like it will bolt up just unsure on the inside couplings and what not??

I was also thinking the same thing about backfires with a cog setup...that's why I was curious about the snout swap and if that's the case I could run blower shop 250 pulleys and be done

Hopefully this thing will sell soon and all of this won't matter but until then I have to prepare and plan to run the 256 as its what I have and its like new

As far as swapping to serpentine accessories...I think I would have a small fortune in that??? I would need all the brackets too I assume to make everything line up right?? I am also running a crossover no water pump so I would have to buy the crossover with the pulley delete on it and that thing is outrageous priced lol

thanks for all the help so far guys!!

As far as heads and cam on my build...I have dart iron eagle heads and the cam is still up in the air...I bought one that came with the heads but not sure if it will work for me..I think I need to start a dedicated thread on that and see what people say...My engine builder said he's gonna check it out but the numbers are ground off as it was a custom grind for the guy I bought the heads from so he suggested just spending the couple hundred bucks and buying one that I know what it is and know it'll work but I have no idea what to get lol

Will do thanks!! need to do wome measuring when I get home from work..i think the blower pulley is smaller than stock to compensate for the chiller..looks like it was bought as a kit

Looking at your for sale ad I would say you already have an undersized pulley on the 256, I think the Teague superchiller kits for 575 came with 3" or 3.25" depending on the power level of the kit.

yea measured it today...its a 3 inch upper so i'll have to base my lower off of that...hoping to net around 6 lbs would be ideal but anywhere 5-8 I would be happy

so if I have a 7 inch lower and 3 inch upper...from what griff said that will be 2.3333....so would that be 100% overdriven?? how would that equate at 5600 RPM and would that be too much??

https://cimg1.ibsrv.net/gimg/www.off...f4cc32dfb2.jpg

according to this chart from weiand...looking at the 3.05 upper a 7" lower puts me at 130% overdriven...where should I be with the 256?? at what point do these things grenade?? I'm running a chiller as well don't know if I mentioned that

Actually 133% overdriven and most likely it would be close to 10# of boost. I think you will only want to be about 50% overdriven.

TBS 250 pulley/boost chart. http://theblowershop.com/wp-content/...-Catalog-7.pdf

So should I look for a larger upper pulley then?? just looking at the chart I posted even a 6" lower still puts me at 97%

The blower shop chart is based off of a 6.4" lower and seems to correspond with the chart I posted I think??

Also should I factor in my heads/cam and chiller into this equation or just base it off of stock heads and no chiller??

I know its hard to guess boost and overdrive levels I just want to get in the ballpark as these pulleys aint cheap hahaha

Thanks!!

Ok so this is what I came up with...

5.5" lower and my current 3" upper puts me in the 88% overdriven realm...close to anyway....Do you guys think the little ole 256 can handle that?? I haven't been able to find a conversion to put that to a boost number but if you figure I plan to run 5800 rpm on the high side and figure overdrive at 90 to make it easy it puts me at 11,200 blower rpm if I did the math correct....is that ok?? too much??

Mind you I have ported dart heads 8.5:1 comp ratio chiller 454 based engine..I assume the chiller and heads will tame down the boost numbers slightly??

Thanks guys!!

Too much maybe based on the chart
Page 10 http://documents.holley.com/techlibr..._tech_info.pdf

But it looks like Mild Thunder got 7# with those sizes https://www.offshoreonly.com/forums/...ost-chart.html

Something doesn't seem right...the stock lower is 6 1/4" roughly and the 3 inch pulley was part of the whipple chiller kit...I don't understand I guess how that's what it was setup stock(well with the whipple kit) but if I go smaller even on the lower which would lower boost and drive speed it is coming out to spinning it too hard?? I'm just trying to understand...should I just try to match the stock lower?? 6-6 1/2 inch lower???

thanks

bump....can anyone comment on using stock pulley sizes?? something doesn't add up to me??

Stock for what?? A 575SCI?? What size are they??
Keep in mind the 575SCI is a 502 ci. The same size pulleys used on a 454 will have 2# or more boost than a 502.

I have the stock 575 lower which is a little over 6..6 1/4ish and the top pulley I know is not stock as its a 3 inch...my question is if they sold this kit for a stock 575 then why couldn't I go with that pulley ratio on my 454?? as far as the blower RPM is concerned?? I understand it would be more boost on a 454 than a 502 I'm just looking at it from a blower speed stand point?? I'm assuming my heads, the chiller, and the boring may put me in the same boost realm as a stock 502...is that safe to say??

thanks

The stock 575sci pulleys for both top and bottom should put your 454 at 6-7# of boost.

Ok that's what I needed to know...do you know the stock size upper?? I need to calculate that drive ratio and shoot for the same on mine then...I cant use the stock lower so I plan to size the lower accordingly to maintain the 3" upper and achieve 6ish lbs of boost

I read this thread multiple times & maybe I'm stupid but I'm lost on what your starting points are.

What do you have for the upper pulley, lower pulley, type of shafts, length, number of splines, 3 belt lower, single belt/serp lower, etc?

Also your running a chiller, if so, it eats up about 2lbs of boost so that need to be calculated in also when your figure your numbers. Plus the charts will be off a few lbs also.

LOL yea I realize this has been a little confusing...it started with me thinking a cog swap of my 256 was going to be easy...jumped the gun on that so its not easy haha

Here's what I have...

256 blower with whipple "kit" I.E. chiller and 3" upper pulley
stock 575 lower pulley which I can't use as I have v-belt accessories so I have to buy a lower pulley to drive the 256
454 based....030 over, dart iron eagle heads ports cleaned not fully ported, cam is similar to stock 525 efi cam with a wider LSA from what the motor shop told me...was half asleep so didn't get all the detail/specs but I will if needed, JE flat tops with head work will net me 8.4ish:1 comp, stock crank, eagle rods
I'd like to see 6ish so 5-8 lbs of boost if possible without killing the small blower

My main question is what size lower can I get to drive the blower with a 3" upper to net roughly in the desired boost range...options that I've found are 5.5,6,6.5,etc up to 8 inch...My thinking is with the stock sized lower that came on the 575(6 1/4" roughly) I should be in the ballpark but maybe not...as griff pointed out the 575 is 502 based and I'm 454 based so my boost numbers will be higher than the 502 based engine...not sure what the 575 was seeing for boost with this kit from whipple on it but if stock is 4 I would have to guess it bumped it some to compensate for the chiller correct?? so if that puts it at 5-6 that should net me 7-8 on my 454 based engine but with my better flowing heads and such will that drop me back down to 5-6 or should I drive it with the 5.5" lower that's available??

I've looked through all the boost and drive ratio charts several times and if I input the pulleys 5.5 lower and 3" upper according to the charts I'm still way higher boost wise than I'd like...however like I said the stock 575 lower is almost an inch bigger than a 5.5 and it was fine?? so will the 454 base make that much of a difference to where a 6" lower will be too much or am I doing something wrong??

I guess the jist of this has turned into lower pulley solutions to drive the 256...my thinking was a 3v standard blower pulley accessory drive and a 16 rib lower blower pulley from whipple that are offered in the above mentioned sizes...I know the 256 is only 10 rib but I figured with a 16 rib lower its wider and I can just run a 10 rib belt and will give me some margin for error as far as blower belt alignment

So that's pretty much where the thread is now hahaha

Sorry for all the confusion!!

how hard would it really be to change out your other pulleys to 575 serpentine stuff? You just need the pulleys

Ok so it sounds like you have a single upper pulley that will work with your 250 blower and it's 3" diameter and you need to buy a 3 v-belt lower pulley which you can get from 5.5 to 8" in 1/2" increments, does this sum it up correctly?

If so,
Drive ratios:
5.5/3 = 1.83
6/3 = 2.0
6.5/3 = 2.17
7/3 = 2.33
7.5/3 = 2.50
8/3 = 2.67

according to the chart:
https://cimg1.ibsrv.net/gimg/www.off...7c4486b054.jpg

Based on your 454 at .030 over I'd estimate your boost would be about 8lbs at 1.63:1 minus your 2 lbs for the chiller reduction so you'd be about 6lbs

https://cimg2.ibsrv.net/gimg/www.off...6f0aaadd6e.jpg

6lbs + 8.5:1 gives you 12.0:1, according to the charts is about pump gas limits.

Base don the books & the numbers I'd shoot for 1.63:1 someone with real world experience might want to chime in and adjust some of this.

So if you're shooting for 1.63:1 it narrows your bottom pulley down to a 5.5" because you can more easily get upper pulleys that put you within adjustment range your shooting for..
5.5/3.5= 1.57
5.5/3.25 = 1.69
5.5/3 = 1.83

If you get a 5.5" lower and my 3.25 upper you'd be at 1.69:1, which is 9.55lbs minus 2 = 7.55lbs at 8.5:1 static, yields you 12.5:1 to 12.75:1



Now if you want to use stock parts off different motors, you need to specify the number of splines on your upper pulley shaft, the length of the shaft & depth of the pulley to make sure there swappable across engine packages. I learned the hard way Merc changed all of this stuff even within the same engine packages over different years and then people mess with them on top of that so be specific on what your set up is unless you like buying parts that won't work on your stuff.

Or why dont you just find a 525sc lower pulley to drive the three V accessories and use the 575 belt on it. There has to be one in the swap shop.

I thought about a serpentine swap however I think I would have to buy more than just pulleys...I think all the brackets are different and I'm running a crossover so I would have to buy the crossover with the pulley on it which is outrageous expensive so that's out lol

I had a 525 sc lower on my motor originally and I sold it with the blower...however it was much longer than the 575 lower so I think I may run into belt alignment issues with that plan

Those are the charts I was referring to and that's what confused me...If you follow the chart for what I have..6 1/4 lower(stock 575) and 3 inch upper...on a 502 that nets around 12 lbs of boost...even with the drop from the chiller that's 10 lbs...does that sound right?? I don't see that chiller kit from whipple adding that much boost??

I'm not saying you're wrong at all just a little confused on these charts and was wanting clarification on it..I agree with what you're saying and I think the 5.5 is the way to go for sure and worst case I can try to find a larger upper

thanks

Since you have the upper 3" already, I'd go with the 5.5 lower and see how it shakes out.
Its about the only option you have to keep the boost as low as possible with the 3" upper pulley.
Charts are charts and they won't be perfect 7 but are often a good reference.
Mild Thunder had real world numbers with the same 5.5" and 3" pulleys and said he saw 7#

For some reason that link to the thread where he talks about that wont work for me anymore...Was he 502 based or 454?? do you recall?

thanks

I would honestly try to use the 671, sell the 575 stuff off and go for it, you would be much happier.

Not sure why the link won't work for you. Here is his post.

​​​​​​​

A stock 575SCI had a 4" upper pulley. They ran about 4.5# of boost. I had a 3.25" on mine and had 6#. I had the rotors stripped and gained 1# of boost. I tried a 3" pulley for a while, but had a lot of belt slip.

Yea have been a little concerned about the belt slip..went through that on my ford lightning I used to have....I think I'm gonna try the 5.5 lower and 3" upper and see what happens....worst case i'll have to go up on the upper and lower if belt slip becomes an issue

thanks for all the help everyone!!

BBYSTWY, what ever ended up happening with this?

Long story short lol....

Never ran the 256...sold it and ran an 8-71 on the punched out 454...turned out the heads and cam I bought on here(the dart iron eagles) were junk and the machine shop never caught it. Didn’t hurt the bottom end at all just had other issues. So that being said...had to pull the motor again to put heads and a cam in...through the process of that I ended up with a 540 with engine quest heads(thanks mild thunder for the help!) with a 14-71 and Holley efi on it. Made 960 with 4 lbs of boost at 6k rpm...got it in the boat last fall and ran out of weather. Have to finish some wiring and some in the water tuning and she’ll be on the water early spring fingers crossed!!