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Thread: Blower size and amount of boost.

  1. #1

    Blower size and amount of boost.

    I'm starting to collect parts to build a blower motor, and am wondering if it is better to run a larger blower at a slower speed versus a smaller blower at a higher speed to produce the same amount of boost?

    Also, what would be a good starting point as far as boost goes? 238 cu. in. inline six, alcohol injected, the motor will be in a dragster (not street driven). steel crank, decent stock rods w/ good rod bolts, head studs, and head had been o- ringed. I have had the motor @ 15.5-1 carbureted on alcohol, but because of the intake I kept leaning out #1 & #6 cylinder. so I will be going to mechanical injecion. I figure static compression around 11-1, and roughly 12 - 16 psi. not sure where to start.

  2. #2

    Blower size and amount of boost

    To clear up my question. I was looking at a 4-71 which would produce a max of around 22 psi. and I was told I should go with a 6-71. that it had a standard pattern for an injector setup.

    the 4-71 would make around 18 psi @ 20% over, and a 6-71 would make 18 psi @ 20% under.

  3. #3
    Senior Member RACING JUNKIE
    Join Date
    Dec 2004
    austin texas
    your boost calculation seem tooo far off! Are you reffering to a teflon stripped 4-71 and 6-71? So your putting the blower on a straight six, right?

  4. #4
    with bigger blower, along with the advantages of standard injection pattern, you can run lower overdrive for same boost which will give you cooler intake temps. Pending blower, usually wouldn't have to worry bout that until you doing high overdrive percentages. (45-50+%)

  5. #5
    No, that wasn't teflon stripped. it was off some charts I found on Dyers Blowers site. Right now I just want to get a blower so I can start building the intake for it. I am leaning towards the 6-71. I was just worried that bigger might be harder to tune because the motor is so small. But this is totally different than a N/A carbureted set up.

    Yes it's an inline. for some reason years ago I started playing with them, and now cant seem to get away fom them.

  6. #6
    Senior Member RACING JUNKIE
    Join Date
    Jan 2006
    I don't know why or how a blower mfg. can rate a blower at a set boost. With a 350" and a 671 that's not tefloned, it would be impossible to get 23lbs of boost at 1 to 1 ratio if the heads and the exaust had any decent flow rate at all. Here's a couple articles that really tell it like it is. Of course the smaller inche motor could build more boost. i run a 1471 tefloned with a buzzard catcher on a 358" on alcohol, now going to a 383" motor.


    Article #1

    While boost seems to be the way all superchargers are rated, the key to making power is moving air. Whether a supercharger just moves air the way a Roots does or actually compresses the air the way a screw supercharger or a centrifugal does, the boost you read on the gauge is as much a function of the engine as it is the supercharger.

    Let’s stuff a large-by-huge 6-71 supercharger on top of a stock 350 engine that makes a normally aspirated 300 hp. We’ll put headers and a good ignition on it, but nothing else in the way of trick engine parts. Even underdriving the supercharger (where the blower is running slower than engine speed), we could easily make 8 psi of boost. Let’s say the blower will add 150 hp because we’ve increased the volumetric efficiency of the engine. In other words, we stuffed more air into the engine.

    Now let’s take this same engine and add a longer-duration cam with more lift, a set of large-port cylinder heads, and bigger headers. Now we have made it easier for the engine to move more air both in and out. Now bolt the supercharger back on with the same drive ratio as before. Now the engine makes over 600 hp, but it did so at a lower boost level. Why?

    Since the engine can now ingest more air than before, and the supercharger is running at the same speed, the boost will drop because the engine is using more of the air “stacked up” inside the intake manifold. So while boost can be used as some type of reference (10 psi versus 25 psi), it’s really a very nebulous number. This is because each engine will determine the boost level it will attain based on its individual equipment. Boost is like a very inaccurate yardstick. It has some value, but it’s not as precise as you might think.

    Article #2

    With a roots supercharger you will definitely be limited by the maximum boost pressure and maximum Rpm the supercharger can be turned. There will be far more to be gained by reducing the total back pressure after the supercharger, and that includes the cylinder head, induction system, intercooler, camshaft and especially the entire exhaust system.

    Anything that causes chronic supercharger constipation is really bad news for airflow, and trying to improve the blower itself is going about it the wrong way.

    Build yourself a traditional high performance engine, but with suitable cam and compression ratio. You will make more Hp on less boost, and much lower thermal sress on everything. Let the whole thing breathe.

    The greatest initial gains are usually made with the exhaust system. There is roughly a 1:1 relationship between boost pressure and total exhaust back pressure. Reduce exhaust back pressure by 4psi, and boost falls by roughly 4psi.

    You then turn the supercharger faster to restore the original boost pressure. Flow increases, power increases, but induction temperatures and the detonation threshold stay about the same.

  7. #7
    Thanks, I'm going to get a 6-71 and start building. in the mean time I'll keep reading and studying up on this. I appreciate the help and all the info thats on this site. It's been a pleasure reading all the posts. I've been trying to catch up. I've been away from hotrods for about 12 years. I am just realizing how much I missed them. thanks again.

  8. #8
    Just musing.
    I am a Roots fan but Ya know..... that's some serious weight to hang out on an inline 6 manifold. It's going to take some serious support work. It's also going to take some long strange snout and pulley work to line things up. I am kind of wondering if one of the centrifigal chargers would be a better choice overall.

    One more muse if using the Roots.
    Instead of sitting it upright in the normal hot rod manner, hang it vertical like it was on the Detroit diesel engine it originally came off of. Manifold construction and alignment should be a more straight forward undertaking.

    Dave Koehler

  9. #9
    Senior Member EXPERT BUILDER
    Join Date
    Aug 2005
    Tell City, IN
    My friends 540bbc with 14° Solid Big Cheif heads and a 14-71 Kebelco Hi-Helix retro Superman blower (long name lol) it at 20% over made right at 37Lbs of boost.

  10. #10
    I ordered a 6-71 from mooneyham. is should be here in a week or so, soon I'll be forming the material for the intake. from what i figured so far the snout will be around 6" give or take. I think I will be running a bugcatcher on it, my other option was hillborn 3" 2 port. but the enderlie looks to be a little cheaper. i love the looks of both, not sure if one would work better than the other. 5 kids... i'll have plenty of time to decide before i make that purchase lol.

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