Draw-thru turbos were the cutting edge of big HP gains on existing engines from VW's to Big Blocks'.
With the Draw-Thru (sp) setup, high vacuum at the carb will keep PV closed at all throttle positions. Carb'd draw-thru turbo's typically have a way to get the Power Valve Circuit to OPEN when the Intake manifold transitions to positive boost pressure .
GM-BOP V6's and V8' draw-thru turbo's used a specifically built Quadrajet that simply ran the PV vacuum circuit
externally and used a proprietary vacuum switch mounted on the intake manifold to 'switch' the carbs PV circuit to open under boost.
ONLY the BOP draw-thru Quadrajet has the external port. I used a generic vacuum-dependent small engine safety fuel line shutoff to 'switch' the Qjet's PV circuit on my Draw-Thru BOP setup.
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Draw-thru setups require huge CFM increase as boost-RPM increases. the proprietary
external PV ported Quadrajet , the BOP system uses is rated at 900 CFM at WOT !. This is the reason for the 'Crutch" or supplemental fuel supplies tied to boost that were often built.
Blow-thru systems actually often need smaller CFM requirementsto maintain optimal AFR ratios. My Bow-thru project had a 2300-500CFM initially but hits better AFR's with a 350CFM and smaller jets. The popular 2300 series 2BBl can be boost modified as per 'hangar 18' mods fairly simply.
I added an external PV port to this blow-thru carb 2300:
With normal NA operation, power valve 'opens' / adds more fuel when vacuum/suction drops as you press the accelerator and the carb throttle plate opens to atmosphere. The PV's are rated in vacuum typically @ 2 to 9 hg. Obviously that is the opposite of what you need when you pressurize / blow-thru a carb.
A new option for
Blow-thru forced induction is a proprietary Power Valve (Holley type) made to Open under Boost which basically operates backward from a normal PV. It stays closed until bost and is adjustable. This requires sealing the PV vacuum port and letting fuel bowl pressure (Boost) operate the PV.
have (boosted) fun ...