The Basics Of The Draw Through Turbo

Lets get started off talking about the two main choices when it comes to the types of turbo setup for your VW engine.

The first one is called a draw through (or draw thru) style. Basically, what this refers to is the carburetors relationship to the turbo charger. As you can see here, the air and fuel are drawn through the turbo charger and then delivered to the engine.

Seems simple enough right? Just hook a turbo to your exhaust, slap a carb on the turbo and your off. Well, no, I mean, you could do it like that, you also could heat your home by setting your furniture on fire but I highly doubt either will have the end result you want.

The picture above of the red sedan is really about as good of a setup as you are going to get when running a draw through. Let me be perfectly frank here, to me, there is no such thing as a good draw through setup. I refer you back to one of my earlier posts about having power but also having the car behave the exact same way it did in stock form when just putting around. You just are NOT going to get that with a draw through setup. Oh, believe me, I can already hear the outrage from some of you that have a draw through, who think it runs just perfect. Well, it doesn’t, you just have low standards or don’t know any better.

You will notice that I made no reference to how well a draw through turbo setup runs at the drag strip or pretty much any application where heavy throttle is used. That is because a draw through set up can flat out haul ass and make some huge power numbers and that’s just a fact. Can they make as much power as a PROPERLY tuned blow through? (more on blow through later) the answer is no, no they won’t but still, a draw through can be crazy fast and powerful.

So what’s my hang up with the draw through then you may wonder. It’s because I drive my cars at less than full throttle most of the time and because in my old age, I am VERY picky about how a car performs in stop and go traffic, at light cruise, taking off on a steep hill and so forth. I care about this as much as I do about its power when I have my foot in it. This was NOT always the case with me, I used to drive some of the most ill mannered shit on the street that you can imagine. As long as it pinned me in the seat when I stood on it and would sort of keep running at part throttle, all was fine.

In my youth, I once put together an engine with such a huge camshaft that the engine would not even build enough vacuum for the power brakes to work. Who cares, stopping is for wimps anyway, this thing really starts moving when I hit 6000 rpm.

Point being here, you might be young enough or young at heart enough to put up with an ill mannered engine as long as it really honks it on when you want and that’s cool, I get it, I was once there myself. If that’s you perhaps you will choose a draw through setup. After I explain how to set up a blow through though, I can’t really imagine why you would want a draw through.

With a draw through, basic physics is what’s giving you the stiff one eye when it comes to street manners. Gasoline (liquid) is heavier that air. When the gasoline and air mixture get drawn from the carb, through the fins of the turbo, go around the snail shaped turbo housing, down a pipe, then make a 90 degree turn, followed by another 90 degree turn and then into the head, guess what happens to the airborne fuel droplets that are heavier than the air carrying it. Right, they drop out of suspension, line the turbo and intake with puddles of raw fuel.

Keep in mind, the fuel droplets won’t fall out of suspension when at full throttle because there is so much air rushing through it, that high speed air has no issue packing along fuel droplets until the end of the ride at the cylinder. What happens when at light throttle though, that is what my problem is. The fuel coats everything inside and just sits there, it sits there until you give it some hard throttle, then it all comes rushing forward into your engine all at once. A sputter and a large cloud of black smoke boil from your exhaust, then you’re off like a rocket.

I don’t mean to piss all over the parade you’re having with your draw through turbo, but if you ever went for a ride in one of my blow through turbo cars, it would do just that.

What I really want is someone to prove me wrong on this, I want to see a draw through bug that will match the same manners as a good stocker. I’m not going to be happy with close enough, even one hiccup you can’t get rid of counts.

I mentioned the red sedan above being about the best draw through setup and here is why. He has done his very best to get rid of the issues faced with a draw through. He is using a side draft carburetor, most of these come from Dellorto or Weber and they are pretty darn tunable, about as tunable as you can get in a carburetor. That’s one step in the right direction. The next step he took was to make the majority of the air/fuel mixture go downhill. He is using gravity to his advantage here by giving the fuel less spots to stick to and puddle up. He most likely has some issue with it, but it may be so slight that he would not even consider it an issue.

However, his throttle response (like most all draw through setups) will be lazy. Think about a hot NA (naturally aspirated, a.k.a no turbo) engine at idle, then when the throttle is blipped quick, it really does not sound like a rev, but really more like the crack of a whip, WHAM, it hits hard. Yeah, you’re not going to get that with a draw through.

As hard as it may be to believe, I am not directly attempting to piss anyone off here. I just live my life in a very fact based way. In other words, I am very open minded, I don’t decide something is true or great, then cover my ears and make humming noises if someone has a different opinion. I want to hear it, but more importantly, I want to hear the facts from their side of things so I can put my own side to the test. I learn new things all the time, I learn new things about stuff I have been doing for years. I don’t know everything and I will be the first to admit that. It’s when you think you already know everything that you stop learning and that’s a mistake in pretty much any part of life.

My next entry will be on the blow through turbo configuration, it will be in fine detail of what you need to accomplish your goals and not just a short summery like this. Prepare yourself for some info coming soon.