<VV> Draw through Turbo's

[kevin nash]

Message: 4
Date: Sun, 22 Mar 2020 12:49:20 +0000
From: Hugo Miller <hugo at aruncoaches.co.uk>
To: <virtualvairs at corvair.org>
Subject: Re: <VV> Carb mystery
Message-ID: <418a6ed99c7da56943019cc1b23ec30c at aruncoaches.co.uk>
Content-Type: text/plain; charset=UTF-8; format=flowed

You mean the float chamber isn't always vented to atmosphere? SU's are.
Unless you want to turbo-charge them, then the float has to have the
same pressure as the air intake. As I write these words I've just
realised something - th Spyder SUCKS through the carb, like a
supercharger, doesn't it? I've never seen that before - the only
carburated cars with a turbo I've seen are the MG Metro's, where the
carb is AFTER the turbo (which is why the float chamber need to be
pressurised). Interesting. Turbo-charging didn't really come in until
fuel injection, in the UK at any rate. Does that make the Spyder unique?
On a broader note, your comments about that flap valve thing, and
bending linkages to make them work, point up the difference between
American and European engineering. American cars are, without wishing to
be insulting, crude by coparison. But they work, and work well.
It was the same in WWII - German aircraft were built to perfection by
skilled matser-craftsmen. Consequently they took ages to build, and had
to be sent back to the factory for repair. The U.S., by contrast,
churned out aircraft by the million. They were not perfect, but they
were good enough to do the job. Instead of Master crafstmen, they were
built by 'Rosie the rivetter' on the production line - housewives who
wer trained to do one specific job and nothing more. We all know which
system prevailed.
The Nazis believed that a woman's place was in the home, so, when they
had to send their skilled men to the front, they used slaves from the
concentration camps. When these poor sols weren't actually dropping dead
on the job, they were sabotaging their work.
So American pragmatism triumphed over nazi ideology.


 Hugo- no the Spyder is not unique to use a draw through carburetor set-up!! not even close! In the early days of turbocharging, it was way more common to use a draw through arrangement on carbureted turbo's... the Olds jetfire, the late 70's buick grand national(?)
numerous japanese makes in the early 80's also did it this way. One of the quirks of this set-up is the compressor has to have a special seal on it that will prevent lubricating oil from getting sucked into the intake when the throttle is closed, and because this draw through configuration used to be so common, Garret turbo's in particular, can be had with this type of seal. The advantage of that seal is that it is tough, can be used on anything blow through or not, the disadvantage is that it has a bit more drag.
Although draw throughs would "seem" to not be the best way to turbo an engine, they do have some neat advantages... harder (nearly impossible) to send one into a surge, at least for the same conditions this usually happens on a blow through. Draw throughs tend to run cooler at part throttle/light loads than blow throughs do. It is the easiest, simplest way to get a carb to work, as the carb is always sensing vacuum. Theres no weird airflow changes being caused by blade angle on draw throughs either... I've seen a couple of youtube video's that test airflow variations to individual cylinders at various blade angles, and it is pretty amazing as to how large these variations can be. One test shows some cylinders down 50% on airflow compared to another at part throttle, and basically no variation at wide open throttle!! With blow throughs, if the butterfly is big enough to never be a significant restriction at wide open throttle and full boost, it is also big enough to cause significant drivability problems out of boost, but on a draw though, they can be sized large enough to never be a restriction (if using EFI) and not have any drivability issues at all.
Carbed blow throughs tend to have massive flow losses even when the carb is sized for the
engine... one account that I read about, a guy bothered to test the manifold pressure at full boost before and after the carb... he was getting 28 psi before the carb, and 20psi after!!
Kevin Nash
Draw through EFI daily driver, max boost tested=22psi