Is a 6-71 to small?

[Jim W]

Wondering if a 6-71 BDS alchy blower is to small for a BBC 468 12-1/2-1 on alcohol turning 7,500 RPM's? Looking to make 15-20 PSI.
Thanks in advance.
Be good, Jim

[PE826]

Jim,
I think it would be a little small, it might make the boost your after if you spin it fast enough but you will lose alot of your efficiency in the process. I have been running a littlefield 8-71 that's stripped and I'm making 17-20psi at 45-60% over depending on the air. It sounds awesome but a 14-71 would make the same amout of boost at 19% over without heating the air as much.

[Jim W]

PE,
what size engine are you running?
Be good, Jim

[PE826]

I run a 496, I just now realized you are running a 468. Even though you are running less ci. I think you benifit from a larger blower. My racing buddy runs a 468 with simular heads and compression to mine; he made the switch from a identical 8-71 to mine to a mag 14-71 and it made a huge difference. He is able to compensate for bad air conditions where I can't due to the drive difference. What stage is the BDS blower you have?

[Jim W]

I have a chance to trade a B&M blower I now have for a stage 4 or 4.5, 6-71.
Proably should just get a used 14-71 but thought I would ask the question about the 6-71. Figured it was a bit small.
Be good, Jim

[84 cougar]

All good advice above, but keep in mind THE BIGGER THE BLOWER, THE MORE POWER IT TAKES TO TURN IT. Your engine size, A 6-71 will work, a 8-71 will work better in my opinion. A 10-71 would be as big as I think I would go, however they will all work. It also depends on the style, and stage of the blower. I had a worn out 6-71 on a 491 BBC that used to make 15 lbs at 7500 Rpm on gas.Good Luck. Jocko

[DetroitJim]

Keep in mind, boost is not equal to horsepower. Most of the psi reading is just heat, not increased air density. It is also the amount of air that is NOT getting into the cylinder past the valves. The faster the blower turns, and the more resistance to airflow it sees, efficiency goes way down. A Rootes blower is about 45% efficient on a good setup. The best way to go is a slow turning blower and high airflow through the entire intake and exhaust. An 8-71 or 10-71 would be about right for you, and don't skimp on cam size!
DJ

[adjones419]

Jim,
I think it would be a little small, it might make the boost your after if you spin it fast enough but you will lose alot of your efficiency in the process. I have been running a littlefield 8-71 that's stripped and I'm making 17-20psi at 45-60% over depending on the air. It sounds awesome but a 14-71 would make the same amout of boost at 19% over without heating the air as much.
What condition is your 8-71 in? 60% over seems like an awful lot to only get 20lbs of boost.

[PE826]

It needs to be stripped, they have about 2 seasons on them. I have been planning on upgrading to a 14 so I have held off. I was running in horriable air when it made 20@60, I too thought it was a little low and I ended up pulling the manifold to check the intake gaskets. It's a nice blower, double stripped and in good shape, however last season I could make over 20#'s at 45%. Over 50% does not net much gain on my setup, it never has.

[Jim W]

The best way to go is a slow turning blower and high airflow through the entire intake and exhaust. An 8-71 or 10-71 would be about right for you, and don't skimp on cam size!
DJ
My problem in decision making is....
when you step up to a 10-71 (which I think would be ideal in my situation)they sometimes are more money than a 14-71.
The 14-71 is over kill but seems to be cost effective.
Be good, Jim

[jimsplace]

Bigger blowers do not turn harder than little blowers for the same boost. Go with the larger 14-71 blower. Most of the competition manifolds are tall enough to clear some of the shorter distributors. I run a 14-71 with a crank trigger and the MSD Crab distributor and still have 4 1/2" clearance. Some/most distributors will be too tall with a 14-71 and an off-set is required (pricey).
Be sure to check the manifold height for clearance. Also, some of the non competition manifolds have the pop-off located on the distributor end of the manifold and may interfere with the distributor and wires.