250 vs 420 blower
10-30-2013, 02:29 AM
#3
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I ran a B&M 250 blower on my 502 last season. At WOT the blower was cold to the touch. However, at midrange the blower was hot. I was always concerned about running the 250 on my 502 because I was told by several people that it was a bit to small and would create a hot intake charge. I believe that they were correct from what I found. I switched to a M3 procharger last winter for that very reason......and its more efficient. I would say go with a 420 for 3 main reasons.....(1.) more power, (2.) the 420 will have a cooler intake charge than the 250 will and (3.) the 420 is more efficient than the 250. I have read a lot about the B&M 250 and 420 blowers on here and have found that they are both close in cost! The 420 is also close to the size of a 6-71. The 420 also uses a better mounting system for the crank pulley assem. than the 250 does IMO. I didnt answer your torque question but gave you some good info on the pros and/or cons. Good luck and I hope that helps! Mark
10-30-2013, 09:59 AM
#4
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I had a 250 blower and a 420 blower. The 250 blower at 83% overdriven, made 7psi. The 420 blower at 7% under, is making 6psi. At 5500RPM, my 250 blower was turning 10,000 RPM. The 420 is turning 5100RPM. No question there the 420 is making a bunch less heat. The harder you spin a roots blower, the more power it takes to turn it.
As for comparison size wise, the 420 is 420CID. The 6-71 Small case 331CID, Large Case 411CID. An 8-71 is 436CID. The 420 is sized like a 7-71, if they ever made one. Bigger than a 6, smaller than an 8.
The only place the 250 might make a little more power, is down at low rpm. At 5000+, the 420 will walk away. I have a B&M technical book from the early 90's, when they were pumping out more superchargers than just about any aftermarket supercharger company. They specifically state in it, that on a engine that will sustain longer "in boost" times (marine), the larger 420 is the choice, over the smaller 250/174 offerings. The 174 was a little too small at the time for a street big block, and the 420 was a little too large for the average street 454. The 250 was developed for the street cars wanting a larger blower than a 174, but still short enough to fit under a hood. The 420 required most cars to have to run a big scoop or hood cutout.
As for comparison size wise, the 420 is 420CID. The 6-71 Small case 331CID, Large Case 411CID. An 8-71 is 436CID. The 420 is sized like a 7-71, if they ever made one. Bigger than a 6, smaller than an 8.
The only place the 250 might make a little more power, is down at low rpm. At 5000+, the 420 will walk away. I have a B&M technical book from the early 90's, when they were pumping out more superchargers than just about any aftermarket supercharger company. They specifically state in it, that on a engine that will sustain longer "in boost" times (marine), the larger 420 is the choice, over the smaller 250/174 offerings. The 174 was a little too small at the time for a street big block, and the 420 was a little too large for the average street 454. The 250 was developed for the street cars wanting a larger blower than a 174, but still short enough to fit under a hood. The 420 required most cars to have to run a big scoop or hood cutout.
11-11-2013, 06:34 PM
#9
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Not hard if setup properly. I have several buddies , including myself, running big carbed roots blowers. All idle and run well. Personally I think my 800hp blower engines idle better than my buddies 700hp NA 540's. the blower can "tame" a camshaft at idle to some extent. Whipples and efi, your golden.
11-11-2013, 07:33 PM
#10
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What is the key to set up? What kind of hp will I gain with 250's on a 425 hp 454 with 8.75:1 comp? I would like to end up around 650hp someday will I be close?
