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my 671 isn't making boost?

8K views 10 replies 8 participants last post by  rags70ss 
#1 ·
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.
 
#3 ·
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?
 
#4 ·
68NovaSS said:
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.
 
#6 · (Edited)
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.
 
#7 ·
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.
 
#8 ·
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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