Another dumb question from a newbie to turbocharging. Why do most people use a blow through system instead of a draw through system?

Nah, its a good question...
Blow through allows you to use an intercooler (you are only cooling compressed air coming out of the turbo and no fuel is in suspension yet).
It is not advised to use the an intercooler with a draw through since the fuel would fall out of suspension as it passes through the intercooler making for a not so good mix of air/fuel.

Leaving the carburetor on the manifold retains the original fuel distribution which means, in most cases, better idle, throttle response and emissions when compared to a draw through system. Draw through systems also require a dynamic seal on the compressor to handle manifold vacuum. This is uncommon on most turbochargers as neither diesel nor most EFI engines subject the compressor to vacuum.

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Joshua

On the other hand a drawthru system doesn't require an expensive fuel system to maintain the fuel delivery to the carburetor. Nor expensive modifications to withstand boost. With the carb in front of the turbo, the mixture is homogenized for even distribution. Intercooling can be done using water and/or methanol injection and is probably not needed under 10psi of boost, contrary to the tuner crowds opinion. A carbon seal can be had for most turbos and can be installed when you do a rebuild.

The McInnes turbo book says there is no advantage to one or the other when you are under 10psi of boost. Above that, there have been successful systems built using either setup.

And this can go on and on, depending of ones viewpoint. And everybody knows that you are just wasting your time with carbs. You need to spend thousands of dollars on EFI to make it all work correctly...

Thanks guys, all your answers have been very informative. I am such a newbie I never even thought of the effects of manifold vacuum on the compressor.

Josh, I understand about the Diesels, because they don't have a throttle (butterfly) to close and therefore cannot create vacuum. But I am wondering how (gasoline) EFI can get away with no vacuum, or did I mis-understand something...

Just because the fuel is entering the airstream on its way to the cylinder, by injection rather than a vacuum signal, I fail to see how volume of that air/gas mixture could be managed to create, say, an idle situation, without a (vacuum-causing) throttle.

Can you explain that to me in a way I can understand it?

I am struggling here...

Bill