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 Blow-thru Quadrajet - Can it be done? 
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Joined: Sat May 09, 2015 10:08 pm
Posts: 2
Hi all,

My name is Enrique, some of you might know me from other forums such as Team Chevelle.

My inquiry is purely hypothetical. I don't currently have a turbo build in progress, I just spend too much time daydreaming about stuff.

It seems like the general consensus among the fans of turbocharging is that there is no point in attempting to use a Qjet in a blow-thru application. Typically the reasoning is that the single inlet will not allow you to produce enough power to make turbocharging worthwhile. Along with the general "why bother" attitude, there seems to be very little information on how to reconfigure a Quadrajet for blow-thru duty.

I can agree that this would be the case if you're running a big v8 with a big turbo(s) combo that you expect to make big horsepower numbers. But what if you're just the type of guy that wants to strap a turbo onto a small to medium displacement street engine and make say 500-600hp? Or what if you happen to have a dual quad intake or tunnel ram, two Qjets, and enough time and/or skill to fabricate the plumbing to feed boost to both carbs. It seems like having dual Qjets should allow a guy to make some pretty impressive power numbers.

So if your power goals are within the limitations of the single inlet and small fuel bowl capacity of the Quadrajet, what would it take to make one work in a blow-thru application?

I guess I'm just trying to understand why Quadrajets are pretty much never used in blow-thru configuration. Is the process much more difficult than it is is for a Holley? Does it require expensive equipment?

Maybe I'm the only one interested in trying this?

Thanks,
Enrique.


Sat May 09, 2015 11:02 pm
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Joined: Fri Feb 08, 2013 2:37 pm
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I think it was in the Turbo Forum I saw a link to a website with information about a GM engineering exercise in '71 or '72 with a turbo blow-through QJet on a 350 Pontiac in a Ventura or Firebird. Been several years ago I read about it so the fog of time may be ..... I think it was only 8 or 10 psi. Perhaps Google will find it. http://www.theturboforums.com/forum.php Use the search feature and enter blowthrough QJet Pontiac or some similar words.


Sun May 10, 2015 3:53 am
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Joined: Mon Dec 29, 2014 1:26 am
Posts: 433
The Holley's power valve is easier to tweak for huge amounts of fuel only when needed without affecting the off-boost running. Q-jet (and Carterbrock) can work OK up to a point but are more likely to have the part throttle/off-boost manners compromised.

A turbo engine is not making anywhere near max power at ~5"hg so the enrichment can be delayed; a N/A engine nearing zero vacuum is at the top end of its range whereas a turbo engine nearing zero vacuum is just about ready to start doing something.

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Sun May 10, 2015 1:24 pm
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Dual carbs with boost is the opposite of what you need. Pressurized air is more dense, so more air molecules will fit through the same size hole (your carb venturis) as pressure (and density) increases. A 600CFM carb that's too small for a 600hp N/A engine will not be too small for a 600hp turbo engine because of that air density thing.

And a blow-through Holley doesn't need to be anything fancy or expensive or be storebought and have 'blow-through' printed on the very expensive box. I know several folks who have made over 1000hp at the tire with a plain $100 swap meet Holley 650DP (with some internal tweaks, of course).

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Sun May 10, 2015 1:49 pm
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Joined: Sat May 09, 2015 10:08 pm
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Thanks for the leads and replies. If I can dig up enough info about how to convert one for blow-thru, I may just attempt it.

I already have one Holley core that is destined to be converted since those seem to be fairly simple to do. But I still think it would be cool to make a blow-thru Qjet. Mostly because I like the idea of taking something like an L48 or L82 type engine and putting some boost through it as a "vintage" version of the ever-popular "Stock+Turbo" Gen III/IV builds. Doing it with a Qjet wouldn't even require a new intake manifold or square/spread adapter. But then again we'd be pushing boost through a dual plane intake.

I'm starting to understand why at least a few others have contemplated doing the same, but most (all?) just end up as pipe dreams.


Mon May 11, 2015 10:55 am
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Joined: Mon Dec 29, 2014 1:26 am
Posts: 433
Is there a seal available for the Qjet accelerator pump shaft? I know the marine rebuild kit for the Carterbrock comes with one, I've never seen one for a Qjet (doesn't mean they don't exist though).

There's really nothing in any carb to convert that turns it into a blow-through carb. Brass floats have to be swapped for Nitrophyl, and it has to be tuned with jets & bleeds to give enough fuel for the new higher power level, that's all. Ignore all that mess about 'air sealing' throttle shafts, I've never seen any of them leak. If the shafts are loose because they're worn out they need to be bushed or the baseplate replaced, and that goes for whether it's blow-through or N/A.

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Mon May 11, 2015 3:24 pm
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