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.
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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.