When does the blower start working? I know it's not at idle, part throttle, and all that. I have a 6-71 on my Hemi, with 2x4's and I'm wondering. If I floor it, does it start as soon as both carbs are opened up, or does it need to get up to a certain RPM? Never drove a supercharged car before. Thanks, guys...
It has to do with engine RPM and the overdrive / underdrive ratio on the blower belts. The blower has to pump enough air to overcome engine vacuum, then add to it with pressurized fuel/air mix. So figure boost starts at 2000-3000 RPM as a rough ballpark - get a boost/vacuum gauge and you will know for sure.
great looking avatar, and as another blower newbie, thanks for the info. I picked up the book " A do-it-yourself guide to Street Supercharging, how to install and tune blowers" by Pat Ganahl, from the advice of people here on the H.A.M.B. I thumbed thru it, I really need to sit down and read it all, it's a good detailed "Superchargers for dummies" type book, from what I can tell.
A supercharger (blower) doesn't have to "spool-up" like a turbocharger does, which is an advantage at initial accelleration from lower RPMs. A blower's positive pressure at a given RPM depends on the gearing and displacement of the blower.
As soon as you open the throttle enough to let in enough air, it will make boost. Usually around 1/4 throttle. Yeah, if you floor it you'll notice.
You can't give any specific RPM that a blower starts to make boost. An engine with a supercharger can run at 2000-3000 RPM and still have vacuum if it is not being run under a load. You have to consider how much air the carbs are letting through. If the throttle plates allow more air through than the engine actually can use if it were normally aspirated (based on RPM and load) then there will be boost. You can have a supercharged engine that is idling, have no boost, then push the throttle open just a slight increment very quickly, and for a short instant there will be boost, since the throttle/carbs are now letting more air in than the engine can use, so the supercharger can exceed the volume that normal engine vacuum would require. But this only lasts for a second. As soon as the engine "catches up", boost will fade to vacuum. And the position of throttle plates. Unless you're talking about a diesel engine, which has no throttle blades controlling the amount of air that the supercharger can pull.
Unless some special rotors or a special helix to the rotors are used, 426ci. The old way of identifying displacement was was 6 71 cubic in cylinders. 6 X 71= 426.
In the original design of the GMC Supercharger, a 6-71 blower was designed to scavenge six cylinders of 71 cubic inches each and would be used on a two-stroke diesel of 426 cubic inches, which is designated a 6-71; the blower takes this same designation. However, because 6-71 is actually the engine's designation, the actual displacement is less than the simple multiplication would suggest. A 6-71 actually pumps 339 cubic inches per revolution It is generally accepted in the street car world that a Roots blower is going to start making boost around 2500-3000 rpm and because they are a positive displacement supercharger, boost is pretty much all in shortly thereafter.
Blowers work differently from turboes. Yes you'll have some "boost lag" from a turbo where it has to spool up as opposed to a supercharger. The nice thing about a turbo is that is doesn't run off a belt and so has no friction/drag on the engine. But you will feel the blowers hit almost instantaneously, unlike a turbo.
I think that affects the amount of boost it is capable of, by how much air it is moving, more than how soon. (although I could be off on that) Remember that Roots blowers are positive displacement and do not actually compress air, they just move it.
Stick a boost guage on it and find out! Boost starts when the output of the blower is greater than what the engine can draw in on it's own. You can vary the output of the blower by changing the speed you drive it at, reletive to the crankshaft, by changing the either, or both the crank and blower pulley sizes.. How come you're running your blower without a boost guage anyway??? If you don't know how much boost you're making and at what rpm, you have no idea what the blower is doing and you risk overboosting and destroying your engine...or underboosting and having a dog. Mart3406 ================================
There are two different size 671 supercharger cases, a large and a small bore. The power seems like its all there all the time. Whatching the boost gauge when the engine was on the dyno I noticed the blower made 80% of its pressure down low and then as the rpm increased towards redline it got the last 20 percent. We were starting the pulls at 4000 rpm so it probably isn't relevant to what you're asking about the seat of the pants. It is hard to watch the boost gauge when you are driving You just need to finish and drive that sucker! I can't wait. overboosting!!! I like it.
I ordered it on-line,...from Amazon. this is the one I ordered: http://www.amazon.com/Street-Superc...=sr_1_8?ie=UTF8&s=books&qid=1269890425&sr=1-8 Looks like there is another, by the same author: http://www.amazon.com/Street-Superc...=sr_1_5?ie=UTF8&s=books&qid=1269890425&sr=1-5
------------------------------ What the hell.....if your gonna' suggest he run it injected on nitro instead, than just for a few more grins,, D ), I think he may as well go all out and go for 'broke' (lliterally!) and add a little hydrazine to the mix, too!!! Mart3406 ==============================
In theory, I'd suspect it'd affect BOTH. If the blower's loose enough, you'll have to build more speed in the blower before it moves enough fuel/air for improved performance (boost), causing some what of a delay
As others have stated the point that boost comes on is heavily influenced by the drive ratio of the blower to the crank. My 671 is 19% underdriven with 7.5 :1 blower pistons. Comes on about 3200 RPM. Makes for a nice low compression engine the rest of the time.
That made it look like Mad Max was using a magnetic clutch on the blower drive and the rotors were not turning until the clutch was engaged. I have run my supercharged engine with the belt removed. They will run, but not very well. Mad Max was using "Movie Magic."
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