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Old 04-24-2008, 02:38 PM
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my 671 isn't making boost?

i'm not sure whats going on here. I've been doing some reading and it seems like most people go to full boost as soon as they introduce load on the motor? Mine doesnt get to full boost until I'm well into 4000+rpm at WOT...I thought that was normal!

what would cause my blower to not make full boost like that?! Its a weiand 671 blower with dual eddy 600 carbs up top on a 350 with 12% underdriven pullies.

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Old 04-24-2008, 02:42 PM
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6-71

Hi,check your belts,they may be slipping.
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Old 04-24-2008, 02:49 PM
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That's a cogged belt, won't slip. You sure you're not making boost at lower rpm's? Driving down the freeway at steady rpm's you're not going to be into boost anyway, as you said, boost comes in under load. If you hammer it from idle you're going to have boost if the blower is spinning! It's not an old blower with rotor to case clearances out of spec is it?
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Old 04-24-2008, 03:04 PM
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Quote:
Originally Posted by 68NovaSS
That's a cogged belt, won't slip. You sure you're not making boost at lower rpm's? Driving down the freeway at steady rpm's you're not going to be into boost anyway, as you said, boost comes in under load. If you hammer it from idle you're going to have boost if the blower is spinning! It's not an old blower with rotor to case clearances out of spec is it?
it is an older blower, but it doesnt really have that many miles on it. It definately doesnt show any boost right away when there is load on the motor.
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Old 04-24-2008, 03:58 PM
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I'm not a blower guru by any means, just thinking out loud.....
a restriction in the air supply will prevent reaching boost. Have you checked to see that the carbs are opening fully? Have you tried running 1:1? What cam are you using? What header size?
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Old 04-24-2008, 05:19 PM
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Twin 600's are too much for a 350CID, or 383 for that matter. You are most likely over carb'ed, and running too much air/fuel for the street. Switch to twin 500's to cut down on the volume of air and rejet to 107's, with 73-42(part #1455) metering rods with 8"(silver) step-up springs. This combo gets a full boost of 20 earlier with a 1.1 blower drive.

Last edited by carsavvycook; 04-24-2008 at 05:26 PM. Reason: additional wording
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Old 04-24-2008, 08:16 PM
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If you go 1:1 on a 350 it would produce 15.3 #'s of boost. On a 350 with only 8.5:1 static cr - with 14#'s of boost, that's going to raise your effective cr to 16.6:1. 15+ #'s of boost with a roots type blower is a lot for the street, can your motor hold that?

I have an 8-71 on a 383 with two BG 750 BC's on the street, it's very streetable, if not a little brutal. I opened up all eight primary idle air bleeds to help idle quality, and jetted it down two numbers on both sides, other than that, nothing.
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Old 04-24-2008, 08:42 PM
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A roots blower is an air pump, restrict the inlet or restrict the exhaust and it won't produce boost. Your not going to get boost until you open the butterflies and put the motor under load. I'd increase the drive ratio to 1 to 1 even with 8.5 to one compression and make sure I have good gas and a conservative advance in the ignition. The motor will be lots more fun to drive and won't self destruct unless you ignore any detonation signs. Remember this is street driving and your full throttle time will just be 2 or 3 seconds at best.
If you want to check the blower condition take it off and turn it upside down and check the clearance with a feeler gage. Bottom should be about .006 minimum and most street blower should be .008 with .012 needing to be refreshed by a professional. The timing should be .011 between the rotors and look at the condition of the rotor tips also for damage. My guess is you should try and up the drive ratio and give it a try! Gary Roushkolb Superchargers.
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Old 04-25-2008, 12:00 AM
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Old 04-26-2008, 08:23 AM
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darkostoj, I think your questions will be answered if you go to the Holley web site, go to tech service , then holley tech library,and then supercharger tech information. They have a good explanation of boost. Joe k
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Old 07-15-2008, 09:35 AM
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re: my 671 isn't making boost?

My 6-71 blower with a 256 cam will make max boost at 6K the boost will start at about 2 # at 2 K or so and increase with motor RPM. Oh ya its 17 % overdriven 11# boost.
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