Help me understand blower sizes
 

I know 671 and 871 are based on the old Detroit Diesel 671 and v871 engines. The 71 being the cylinder size and the 6,8 or 16 for that matter means the number of cylinders. What about a wieand 177 and 250 or Holley/B&M 174 and 250? Are the related to the 53 series engine or the smaller 71 series. I know DD made them as small as 353 and 271. I have owned these engines as stationary power units.

Thanks for any help.
Mark

The 177 or 250 number is the measurement of air in cubic inches that the blower pushes through in one rotation. A 671 pushes through 426 cubic inches.

Ok, so a 420 mega Blower is the same as a 671.

The numerical designation for the original GM Roots type blowers were based on the engine displacement the blower was designed for.

Example;

6-71 blowers were designed for:
6 (cylinders) 71 (CID per cylinder)
This, however, does not signify the actual displacement per revolution of the supercharger. The actual displacement per revolution is determined by the rotor length and diameter.


The data for GM based Roots blowers are:-

6:71 small diameter
Rotor dia=5.505", length=14.975", displacement per full turn of rotor=339CI.

6:71 big diameter
Rotor dia=5.778", length=14.975", displacement per full turn of rotor=411CI.

8:71
Rotor dia=5.778", length=15.905", displacement per full turn of rotor=436CI.

10:71
Rotor dia=5.778", length=17.000", displacement per full turn of rotor=466CI.

14:71
Rotor dia=5.778", length=19.000", displacement per full turn of rotor=521CI

This is theoretical displacement.

Retro or high helix will change this considerably.

A worn blower will reduce this a little.

A very good tight new blower and a Teflon stripped blower should be about the same.

Bob

So, when you get a Wieand or BDS 671 blower, How do you know if it's the small diameter or large? You are only talking a 1/4" diferance .

Payton,

The .273" difference is relative, as seen in the 72 CFM increase per revolution over the smaller diameter rotor. The Weiand 6-71 is the 5.778" diameter rotor.

Bob

What makes the difference of going to dual carbs from one bigger one? Proper fuel distribution with more air? I see the 250 is availible with a single or dual.
Would you need a dual carb setup with a 671 blower if you are spinning it slow for only 3# of boost? Or is the total cfm of air goingto be to high for a single to do the job.

We know that by changing pulley ratio,boost pressure will change...assuming 5# boost is to be the magic number,that is obtainable with pulley ratio.

However,with regards to overdrive vs. underdrive one must consider available boost delivery.Hence,SIZE of the required supercharger...Some prefer a model that can be underdriven...The underdriven ratio provides max. pressure at about the same engine rpm, or just before, full timing occurs...Thereafter,the pressure gradient is flat through the increasing engine RPM range.

I ran a single Dominator carb worked by Nickerson on a B&M 250 on 468 CID. Worked very well.

rmb, Thanks for the education. I did not know the exact specifics.

What about the 177 and 250 series blowers???? Are they pushing the 177 and 250 ci of air, theoretically????


Yes a B&M 420 is essentially the same size as a 6-71.

Almost everybody, runs a single carb on the 250/256 blowers. The larger blowers need dual carbs for better fuel distribution and for more fuel.

the bigger blowers need 2 carbs because a singel carb cannot flow enough cfm.

one thing to remember about boost is that the number only means something to that specific engine. two 454's with blowers can have the same boost but very different power numbers. why? boost is read in the intake, not the cylinder. if the heads do not flow or the valves are very small then you can have a higher boost and still not get it to the cylinder to make power. a good flowing head, cam and exhaust will make more power with less boost than a bad flowing head, cam and exhaust with lots more boost.

Way to bring up a thread from the graveyard Mike!! Dam Miller 64's will get ya everytime...lol

Well I'm glad he did. As a noob about to dive into rebuilding two BBC's with 250's and dominators straped to the top this info was worth reading. Thanks!!!

no more 64 for me,im on a coors lite kick,lol,bt you are correct,guess i didnt notice it was a old thread.

that's ok....still an interesting thread

Sell the dominator and go with twin carbs, much better setup. If you keep the dominator, watch the front cylinders, they will be considerably leaner than the rears.