250 vs 420 blower
 

How much more hp and torque will the 420 make on the same 454? Any pros or cons for either?

I say 250 blower because I have 2 to sell :lolhit:

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

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.

How much horse power and torque will I gain with these blowers on a 425 hp 500 lbs of torque 454?

Merc made a 525sc which was a 454 with a blower. You should have no problem adding a 100hp depending on cam and boost.

How hard is it to get a blower motor to run and idle well?

Welfare.......how did your motors run with your 250's on them? What motors were they on and how many hp did you have?

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.

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?