<VV> Draw through Turbo's

[Hugo Miller]

That's very interesting. I know of only one car with a turbo and a 
carburettor and that is the MG Metro. And that is a blow through with a 
pressurised float chamber and special seals on the throttle butterfly 
spindle. I shall have to look up what others there were, but I can't 
think of a single one on the British market. Superchargers were common 
at one time, and they always sucked through the carburettor, which 
sometimes resulted in ridiculously long inlet tracts.
I know all about leaking turbo seals - if you've ever seen a runaway 
diesel drinking it's own oil via the turbo it's pretty spectacular - 
they generally explode unless you can stuff something in the air intake. 
But if you stick a rag in there it will just eat it. A plank of wood is 
about the only thing that will stop it - but it would be a foolish man 
who would get close enough to a 15 litre Caterpillar diesel that is 
revving at about 10,000 rpm threatening to blow up any second. The best 
course of action is to run as fast as your legs will carry you and wait 
for the bang!


On 2020-03-22 18:31, kevin nash via VirtualVairs wrote:
> Message: 4
>
>  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
>
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