Blowthru Vs Drawthru turbo/supercharge - The Clueless Brit is here! Options

[fall-apart-dave]

Got a 914 twin carb setup. Want to turbo it. Have a turbo. Selling turbo after advice. Advice is contested, I get confused, want to cry, need answers!

AAAAAAAAAAAAALLLLLl-Righty then!

Here goes!

I want to turbo/supercharge my 914 motor in my Baja. I found some good advice, use a single carb and a pullthrough because blowthrough doesn't work so good. I asked about twin turbo's on pullthrough, don't even GO down that road, I was told! Pulses on exhaust wouldn't be right without a complicated 4 to 1 to two to one setup (turbo would get pulse, pluse, nothing, nothing with cylinders firing unless header was merged, split, then merged again). So, twin superchargers on pullthrough was my next idea. Too big, too heavy, not worth it. I'd agree. But what of blowthrough? I was told that a blowthrough turbo doesn't run very well, is unsteady, carbs don't like it, nightmare to get it to run right, etc etc. Something about venturi effect not happening with blowthrough, too. The advice was, ditch the carbs, use a single sidedraft or better yet use injection. The latter idea, about injection, I would be inclined to also agree with. Old VW Golf injectors would work, so I am told though I am sceptical about that and actually, I'd kinda like to keep carbs for the challenge.
BUT, does it work? I don't know. SO, the arguement is there, and I pass it over to you guys. Pullthru, or blowthru charging?

Yes, the cluless Brit is back asking stupid questions and providing a giggle to those who can be arsed reading me!

[ArtechnikA]

Pullthru, or blowthru charging?

In General - the suckthrough system is easier to make work if you're trying to engineer the system yourself from scratch. you do need a turbo with a positive piston ring type seal to keep manifold vacuum from sucking oil past the turbo seal. virtually all turbochargers available nowadays incorporate this kind of seal.

but since you're trying to distribute fuel mixture instead of just air, there are issues with puddling and uneven distribution with changes of direction, as there must be with a flat engine. (this is why carbureted VW T-I engines have manifold heat riser tubes...)

but the carb only sees the standard atmosphere it was designed for.

When you presurise a carb, you need to make sure that *all* the pressure references are seeing boost pressure. and that all the components are happy being pressurised - things like floats. but the upside is that fuel distribution is not a factor, because you retain a single straight shot through the throttle body. You also need to make sure you have a way of referencing the fuel pressure to the boost pressure.

Step One is to read up on this stuff. once upon a time the High MacInness book "Turbochargers" was a standard reference but it's quite dated now. Have a look at "Maximum Boost" by Corky Bell. CB Performance also has a turbocharging book that's got good reviews but I haven't seen it so I can't comment - but if I were seriously looking at turbo'ing a car, it'd be on my reading short list.