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Old 01-01-2011, 11:02 AM
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B&M blower

hi! what is the difference between a B & M 250 and 420 megablower?
I have a B & M blower I find two number: 18284 on top of the blower and the 5394 cast on the side.
is this a 250 or a 420 blower ?
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Old 01-01-2011, 11:13 AM
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I don't understand?
 

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i'm guessing if you post a pic, many guys on here can tell you just by seeing it
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Old 01-01-2011, 12:47 PM
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blower

try to post a picture
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Old 01-01-2011, 12:55 PM
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I say a 250
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Old 01-01-2011, 01:03 PM
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I agree with pepi. A 420 is much "fatter."
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Old 01-01-2011, 05:15 PM
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blower

How This Fits In A Bbc Or Pontiac 400 Engine, Is It Too Small ?
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Old 01-01-2011, 08:41 PM
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Quote:
Originally Posted by rumo
How This Fits In A Bbc Or Pontiac 400 Engine, Is It Too Small ?
No such thing as a blower that's too small. You just keep changin' pulleys until you see boost on your manifold gauge.
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Old 01-01-2011, 11:44 PM
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In terms of "swept volume" the B & M/Weiand 250 is roughly equal to a GMC 4:71 while the B & M/Weiand 420 is roughly a GMC 6:71. If you are intending primarily street use, or a mixture of street/strip use, a 250/4:71 will provide at modest engine rpms more than enough boost than sensibly can be used on the street. If you are instead primarily aiming for drag strip use, then a 420/6:71 would be optimal. To produce the same amount of boost, however, a 250/4:71 must be spun faster than a 420/6:71. That means that the smaller blower will, by comparison, produce more heat, particularly at higher rpms under boost. On the street, at least in my experience, the heat differential in having to spin a 4:71 faster than a 6:71 is not a problem.
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Old 01-02-2011, 05:21 AM
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blower

as I understand, it is 420 as long as 250 but 420 is higher, does anyone have any size on a 420?
is there any other way I can see the difference?
thanks for all the responses I have received
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Old 01-02-2011, 07:56 AM
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All the dimensions and lots of other supercharger specific technical stuff can be downloaded from the Holley/Weiand site.

The 250cid blower is Just right for a 400cid motor but the single carb is going to be the deal breaker. You will never get even cylinder cylinder fuel ratios using a single carb on that blower. It will never work well.
On this blower, two carbs is better.

Get the dual carb top adapter and two compatible carbs.
two 750's is good. Must be "boost referenced" Two blower friendly moded edelbrock 750's work well. two 750 double pumpers with blower friendly mods works great too.

You will spend less time and money modding two carbs than trying to make the one carb work on this blower.
A sucessful blower motor is all in the correct setup.

Starting with two carbs.
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Old 01-02-2011, 12:29 PM
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Following the suggestions of FBird, I found dimensions and diagrams for the particular blowers running through goggle the following: "Weiand Holley 250 dimensions 420 dimensions". Also, including multiple engine dyno runs, I tried to run an 800 cfm atop a 250 blower, first with a carb plate centered in the middle of the blower, then with a "marine" carb plate which positioned the large carb further towards the front of the blower case. Even with the 800 cfm mounted closer to the front of the blower, cylinders 1 and 2 consistently ran much leaner, and much hotter, than cylinders 3-8. Consequently, the engine repeatedly backfired through the carburetor, first requiring replacing a ruptured gasket beneath the "popoff" plate, then having to replace the plate itself because it was bent, and finally, having to rebuild the blower due to bent rotors . Once I went to two 4 fours (2 boost referenced Holley 600s), and locked the timing at 34 degrees (Thanks to FBird), the backfires ceased. In short, although single carb mounts are available for use with the 250 blower, a single carb will not work. I say this after successfully having run a single 800 cfm for several years atop a B&M/Weiand 144 blower. The i44 is, however, much more compact than the much longer 250, and directs the fuel flow more evenly to cylinders.
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Old 01-02-2011, 03:12 PM
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Here, in summary, are the dyno details demonstrating that a single 800 cfm carb will not work well with a B&M/Weiand/Holley 250 blower. I dug into my files and looked over the many dyno runs made when we were trying to get a single carb to work. With a 1.6 pulley, horsepower peaked at 5800 rpm. At that rpm, cylinders 1, 2, and 3 respectively registered median Exhaust Gas Temperatures (EGT) of 1496, 1488, and 1402 degrees while the median EGT for cylinders 4-8 was 1319. For cylinders 1 and 2, that means they are running about 180 degrees hotter than cylinders 4-8. Given how the 250 blower distributes fuel, when fed by a single carb, the air fuel mixture in cylinders 4-8 is much richer, creating a lower EGT for those cylinders, when compared to the lean condition in the first 3 cylinders, and in 1 and 2 in particular, yielding EGT near 1500 degrees.
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Old 01-02-2011, 05:42 PM
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blower

thanks for all the information
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