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I just put a 6-71 blower on my newly redone 383. All internal parts are high quality and forged. My compression ratio is about 8.1:1. My blower is giving me almost 5 lbs of boost when i floor it. Its fast enough for me and I'm more than satisfied with the power, but what do you think the "safe" amount of boost it would be able to handle for a daily driver? And also what is the absolute "safe" limit of boost would be to run it a couple of times at the track just to see what it can do? And also is there any other way to adjust the boost it delivers besides swapping pulleys? There is a valve (my engine guy calls it a pop-off valve, I've only heard blow-off valve but im sure the purpose is the same and maybe they are the same thing but hes a bit older and maybe thats just what hes used to calling it? i dont know) on the back of the manifold. I'm sure that could be adjusted?? I'm young and still learning so help me out!! thanks!!
 

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The "Pop-off" valve is there in case the engine back-fires.:yes: It's not there to adjust boost.:no: Remember, because of it's location in the intake system, if it leaks at all (it's NOT supposed to) it will also be leaking FUEL!!!:eek:

A "Blow-off" valve (or BOV) is used either in centrifugal supercharged applications or turbo applications to release boost when the throttle is lifted in order to make life easier on the impellers. Because they are installed in the intake piping well before the injectors/carb, what they release is nothing but compressed air.:yes:
 

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hmm

i was at my buddys shop the other day ,, he just did a blown engine , 355 with a 8-71 ( hey its what it had on it when it came in , the guy told him to make it work ) ,,,, this thing was making 14 lbs at around 5000. The customer wanted it to sound good too , so he put a hyd roller supercharger grind in it with 240 @ .050

Best pull was 500 hp around 5300 using pump gas still going as high as 14 lbs . Power was taking a big dip above this and it was using fuel like crazy. I think it was pulling too much air via too much boost and then the pop off valve was lifting . This engine also had 8-1 . I think the blower should have been slowed down where it made its max pressure where they wanted it to be the top rpm. Or at least observe what it was doing at higher RPM's. He didn't want to push it past 12-14 lbs so he wouldn't push the RPM's past this.

I don't think they got near what it was capable of HP wise. He called the guy and told him he'd have to charge him for another dyno day , and to get some different pulleys , but the guy opted out, said 500 hp was plenty ( show car ) . I think its a shame it wasn't set up for its total capacity.
 

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Discussion Starter #6
The "Pop-off" valve is there in case the engine back-fires.:yes: It's not there to adjust boost.:no: Remember, because of it's location in the intake system, if it leaks at all (it's NOT supposed to) it will also be leaking FUEL!!!:eek:

A "Blow-off" valve (or BOV) is used either in centrifugal supercharged applications or turbo applications to release boost when the throttle is lifted in order to make life easier on the impellers. Because they are installed in the intake piping well before the injectors/carb, what they release is nothing but compressed air.:yes:
That makes sense.. haha.. i didnt even think about the fact that there would be fuel in that mixture of compressed air.. thank you very much sir..
 

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Blower motors and boost can be a confusing thing, looking from outside. One thing to keep in mind is that boost is a reference number of what's not going into the cylinder. It's why when you look at a pulley/boost reference chart, comparing apples to apples, the same pulley configuration will make less boost in a larger engine. There's bigger holes to fill. The only way to know if you can run more boost w/a given combination is to read the plugs and see if the motor is happy with it. Heat also plays a HUGE roll in a roots style (or any for that matter) engine. The longer that thing runs, the hotter it gets. Want to see a roots blown engine lose 50hp real quick? Let me make three consecutive pulls on the dyno. The other by product of heat is detonation. It's why i see guys that have stuff that run ok on pump gas on the dyno, then wreck themselves in the car. Load is different on the engine and heat becomes a factor. The best way to avoid heat is put the biggest blower on it you can. :D The slower you can spin the blower to produce the power level you want, the less heat your going to produce. Given a choice, i don't put anything smaller than an 8-71 on anything that the customer wants to make over 600hp with. My general rule of thumb is that you can double your hp on a decent built engine with 15lbs of boost. So if you have a realistic 400hp small block n/a, and make about 7-8lbs of boost, you should make around 600hp. I just did a 355" engine w/a 8-71 that made 720ish hp with about 10lbs of boost. I think we could have made some more, but it's a stock block deal and it's already waaaaaayyyyy past it's limitation.
i was at my buddys shop the other day ,, he just did a blown engine , 355 with a 8-71 ( hey its what it had on it when it came in , the guy told him to make it work ) ,,,, this thing was making 14 lbs at around 5000. The customer wanted it to sound good too , so he put a hyd roller supercharger grind in it with 240 @ .050

Best pull was 500 hp around 5300 using pump gas still going as high as 14 lbs . Power was taking a big dip above this and it was using fuel like crazy. I think it was pulling too much air via too much boost and then the pop off valve was lifting . This engine also had 8-1 . I think the blower should have been slowed down where it made its max pressure where they wanted it to be the top rpm. Or at least observe what it was doing at higher RPM's. He didn't want to push it past 12-14 lbs so he wouldn't push the RPM's past this.

I don't think they got near what it was capable of HP wise. He called the guy and told him he'd have to charge him for another dyno day , and to get some different pulleys , but the guy opted out, said 500 hp was plenty ( show car ) . I think its a shame it wasn't set up for its total capacity.
My guess would be that they were shy on timing. Blower engines love timing. When it's retarded you'll see the fuel go way up and the power go the other direction. I consulted on a dyno session for a guy with his centrifugal Procharger engine. We tuned 140hp in it in about 15min playing with the timing curve. If you haven't been there before though, it gets a little spooky 'cause you can separate the rods from the crank in hurry. :)
shawn
 
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