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Carb. 300 Turbo Help Please!!

Moderator: Mod Squad

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Posts: 11
Joined: Fri Aug 19, 2016 6:43 am
Location: Mesa, Az

Carb. 300 Turbo Help Please!!

Post #1 by Weldermike92 » Sun Aug 05, 2018 5:21 am

Ok, I'm apologizing now cause I'm sure this has been covered a bunch of times and don't really have the time to skim threw 1200+ posts..... And for this ungodly long post....

Ok, so I am getting ready to turbo my 78 e100, The 300 has been freshly rebuilt some time last year, its been bored .20 over, right now I'm doing the E.F.I exhaust manifolds, using stock log intake that I've ported very heavily with 2 barrel adapter with holley 500 cfm most likely go in to go with the chevy rocker arm swap as well, has the dura spark 2 vacuum advance ignition and going to do a recurve on distributor... I believe that covers all been done to it.. So basically stockish..

I use this van as weekend cruiser and off-road it quite a bit.. No towing or hauling for work.. Not trying to 500+HP or and thing like that.. Goal is 5-8 PSI max for now.. I know and understand I should get a 4 barrel intake and get the head built and use ARP studs, but it isn't in the budget just yet for all these awesome things... I understand this isn't gonna be producing the best results, but the best results isn't my goal as of this time...

Here's my questions, (appreciate you for hanging in this long)..

I been spending a lot of time researching about the "draw through" set up, looks easy & simple to set up like the idea of not having to buy intercooler, Really wanna use a Holset HX35 turbo off a Cummins or some type of quick spool turbo, but hitting a wall with needing carbon seals to do a "Draw through" set up..

What can I do to help with the vacuum problem with this set up and what do I need to do to timing, fuel ECT... How bad will it smoke with not running carbon seals? Is there anything to help with the vacuum issue, like making a controlled vacuum leak or restricting the oil flow or making some kind of blow off valve when letting off the throttle?

I'm 95% mind set on "draw through" set but still curious about the "blow through" setup tho, it fixes the whole carbon seals problem, but I know there is a laundry list of mods to do to everything to set it up..

What are the things to do if I go the "blow through" route? Like things to carb, fuel, timing etc.. (If your still here reading my novel your a champ and momma definitely didn't raise no quiter and I respect you for it..)

100% committed with turbo build, spent the last week, dedicating the entire passenger side of engine bay to do, relocated battery, wiring harnesses, voltage regulator ECT.. Pulled all the a.c. system out.. Everything from under dash and fire wall everything, I live in AZ so who needs that crap when you got wing windows...

So thank you, every and any tips knowledge or wise wisdom will be very appreciated and helpful while I'm doing this project..

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Re: Carb. 300 Turbo Help Please!!

Post #2 by pmuller9 » Mon Aug 06, 2018 11:37 am

For a draw-thru:
The chamber in the carb for the power valve needs to be modified so it gets signal directly from the intake manifold rather than from under the carburetor.
This is because on a draw through set-up the carburetor always sees high vacuum and the power valve would never open to richen the fuel mixture.
The power valve fuel passage may need to be drilled larger to allow more fuel to the main circuit.

Because the carburetor creates a pressure drop before the turbocharger compressor, the turbocharger has to operate at a higher pressure ratio for a given boost level. Less efficient set-up.

Then you still have the oil seal problem. Any type of vacuum blow-off valve would create a lean condition when open or leaking which is not good.

For a Blow-thru:
The carburetor power valve circuit may also need to be enlarged for additional fuel.
The fuel pressure needs to be boost referenced.
The fuel pump needs to be able to supply around 20psi for your application.

The carburetor is smaller than with the draw-thru

The Draw-thru doesn't use an intercooler because it is dangerous to have that much fuel mixture to explode.

The Blow-thru doesn't have to be used with an intercooler but it will make more power with one.

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