Lets start off with a discussion of why it is you should run a turbo as a power upgrade and the mouth breathing boobery that think otherwise.
How many times have you heard the phrase “there is no replacement for displacement?” If you haven’t heard it, consider yourself to keep good company. The saying is technically true. I mean, if you take a 2500cc vw engine, and do the exact same thing to a 1600cc vw engine, then it’s a no brainer, the larger displacement engine will kill it. However, when some window licker says it to defend his choice of building a bigger engine INSTEAD of using a turbo, this guy is a clown, stop listening and back away slowly.
Here is a short list as to why you should not listen to anymore advice this person may offer.
1: He does not even understand the basics of how an engine or a turbo works (more on this later).
2: He made a choice to spend an assload more money on something slower than a turbo engine.
3: To come even remotely close to the power a turbo engine will make, he has to make his run kind of shitty for town driving or just cruising.
Number one is the biggy and requires some explanation, so hang in there and I hope to make this as clear as I possibly can.
On earth, the atmospheric pressure at sea level is 14.7 psi. What does that mean? Okay, lets say you have a ziplock bag at the beach (sea level). When you are still at sea level, you seal the bag air tight. Now get in your car and head for the mountain pass. When you reach the summit, lets just say that it’s 6000 feet above sea level, take a look at the ziplock bag you sealed up when you were in a 14.7 psi environment. The goofy thing has swelled up and almost looks like a balloon, why? Pretty simple really, you sealed up the bag when the air pressure outside of the bag and inside of the bag was the exact same 14.7 psi. You have now traveled to 6000 feet above sea level and the pressure up there is only 11.8 psi. That means there is a difference of 2.9 psi in the bag .
In other words, the pressure in the bag is still 14.7, but now the bag is surrounded by only 11.8 psi pushing on it. This mean the bag has an extra 2.9 psi trying to force it’s way out. Get it?
What the hell does this have to do with my engine? In a word, everything. At its core, an engine is an air pump that is assisted in pumping air by the internal combustion process. If all things are equal, then the bigger the engine is, the more air it can pump and the more power it will make in turn.
But what happens when things are not equal any longer? Take a 1600cc engine, which is actually 1584ccs and put it against a non turbo 2332. The 2332 would kill it right? Of course it would. Remember the part about the bigger air pump making more power? Well, here ya go. Lets say both engines are running at sea level (14.7 psi). This means that both the 1584cc and 2332cc engine have the same 14.7 psi trying to gain access into the cylinders to make power. Right, but what happens if we put a turbo on the 1584 and raise the pressure above the carb that is trying to force it’s way into the engine to 14.7 pounds ABOVE the already existing 14.7 psi around it? Well, now you have a 1584cc engine that is getting twice the amount of air that it was, so your essentially dealing with a 3168cc engine. Now who is your money on, the 2332 or the 3168?
While it is not that cut and dried, I am using it as my example to get across what I am saying. A bigger engine pumps more air and so does a smaller engine that has air forced into it. Thus the term “forced induction engine”.
Now that I have that out of the way, lets get you started with what you are going to need to get this on the way. Lets start with something you may already have, a carb or carbs. Pretty much any carb can be used in a blow through application, but I suggest very strongly that you do yourself a favor and use a weber idf or dellorto drla type carb. These are all over the place for sale and turbo parts that you don’t have to make yourself are also available for them.
This next part is my opinion, but try to keep in mind that this opinion is based on me building MANY blow through carbureted systems and over many years. You will often read that you NEED to seal the throttle shafts of the carb to keep fuel from “spraying out” when under boost. First off, the person who started this rumor just so happens to offer the very service you need to seal up your huge gas leak. If you detected a slight hint of sarcasm in that sentence, good for you. Here is something to ponder, if your throttle shaft is so loose that fuel sprays out of it under boost pressure, don’t you think that might just be a huge vacuum leak even without a turbo setup? I have ran both Weber and Dellorto carbs in blow through applications and MANY of them, none of which had the special super secret squirrel seals machined into the carb body. The very worst I have ever seen was on a sandrail running over 20 pounds of boost through two Weber IDF carbs for years, all you could see was a slight damp spot at the end of one throttle shaft and the only reason you could see it is because a little dirt dust stuck to it. I have seen this very same thing on naturally aspirated engines before. In my opinion, the need to seal the throttle shaft on Weber or Dell carbs is bullshit. But, if you are just clamoring to spend an extra 200 bucks on something you don’t need, have at it, it won’t hurt anything but your wallet.
Lets talk about what you should have. The carb or carbs themselves is a good start. I’ll let you in on a little secret here, after years of testing and tuning carbs with a turbo blowing through them, pretty much all you need is the carb with a turbo hat (pressure hat) on it to plumb the turbo into it. However, running just the carb with no special mods for the turbo is more for the advanced tuner (or the lucky). For most of you, I recommend two things.
These modulator rings or mod rings for short, they go on top of each crab throat like a velocity stack. I will tell you right now, they are not just a velocity stack. I feel it necessary to explain the function of these parts so you get a better understanding of what is going on. There is no super short way to just sum up what these do really, so this might be a little long winded.
A carb actually works the opposite of what most people think, it’s not really sucking in. For this, explanation, you have to go back to where I mentioned the ziplock bag at sea level and then at 6000 feet above sea level. If you open the bag with 14.7 atmosphere sealed in it when you are in an atmosphere of 6000 feet, the atmosphere in the bag will go rushing out into the lower pressure of the 6000 foot elevation. High pressure always rushes to low pressure, much like water wants to run downhill. A simple party balloon full of air, the higher pressure in the balloon is trying to get outside to where the pressure is lower. The balloon going flat rather quickly when you let the air out of it is a perfect example of higher pressure running toward low pressure. If that was not the case, you would untie the bottom of the balloon and it would just stay full of air.
This is the same way the carb on an engine operates, high pressure rushing through the carb to get to the low pressure inside the engine. As your engine is running, it is creating a lower pressure environment inside the intake runners. What happens when you let the air out of the balloon is the same thing, the higher pressure outside the engine is trying to rush into the low pressure inside. The high pressure going through the restriction that is the carburetor pulls fuel from the correctly placed holes in the carb that were put there by the engineers. Think truck passing you on the freeway and its pull. As you can probably tell, that is a massive over simplification, but I need to keep moving, this is a topic that could take up days, I do hope you get the idea, because I put it there to help you better understand this next part.
The modulator ring.
The mod ring acts as a slight restriction at the top of your carb throat. Why the hell would you want a restriction on your carb throat? I’ll tell you. As the turbo creates pressure in the turbo hat on top of your carb, this acts as the higher pressure atmosphere I mentioned before, this makes your engine pump more air and more power. Alright, so lets say you have the main jet of your carb sized correctly for a 1600cc engine and the air the a 1600cc engine will use at sea level. Great, but now you have just massively increased the amount of air going into your engine by adding a turbo into the mix. More air will need more fuel, just as a larger displacement engine need more fuel to mix with its air, you now need the same. The answer would appear to be installing larger main jets right? Sure, that will work, if you drive at full throttle everywhere you go. Other than that, the big main jet is going to give you too much fuel everywhere but wide open throttle. Black smoke, fouled plugs, horrid mileage and so forth. Gee, if there were only a way to make a smaller main jet in a carb act larger when you needed it to and only when you needed it. Yep, that’s what a mod ring is for.
The mod ring creates a slight restriction above the carb throat, this in turn makes the pressure above the carb higher than what is going through the throat of the carb. What this does is since the vent for your float bowl is also on top of the carb, it makes your float bowl (where your fuel is stored in the carb) a higher pressure than the area that the fuel is going to rush into (your carb throat). Remember how high pressure wants to rush into low pressure? What this high pressure in your float bowl does is it makes you main jets operate under higher pressure, thus making more fuel come from the main jet than normally would. This of course only takes place under high boost where your engine needs more fuel, so it works out pretty darn good.
I should mention that there are other ways to make this event happen, but I find the mod rings to be very easy to do. Just put them on like you would a velocity stack, jet accordingly (more on this later) and you are off.
Lets say that you find yourself in a spot where you need even more fuel up on boost, even after the mod ring installation. To me, the usually means your carbs are a little too big, but whatever, we can work with that. If you jet up to a larger main to cover up this lack of fuel, you get yourself back into the same spot of having too big of a main everywhere but under hard throttle. Sure, it may run alright, but it can be a bunch better. This is where some tuning comes in the form of a special emulsion tube (e tube).
To be perfectly honest, I really do not like steering anyone toward CB performance for two reasons. One, they normally try to really bone you the shipping price of small items like jets and two, apparently, they send you the shit you ordered whenever in the fuck they feel like it. I have had some stuff from them arrive pretty quick, other stuff (that they manufacture mind you) take a VERY long time. Really, for Weber and Dell Parts, I would really prefer to send you in the direction of a Dude named Gabriel who sells on ebay under the name alpha1750. Yeah, he is in Italy, but the cat is really on the ball. He always ships the same day, you get charged what it ACTUALLY costs to ship and he is the nicest guy you will ever meet. Yes, that’s right, ordering jets from Italy is not only faster, but it’s cheaper than ordering them from California. Something seems wrong with that, but whatever, I like Gabe and he has always been awesome to deal with.
Without a long and pretty much useless tutorial on the emulsion tube, here is what the idea behind the turbo emulsion tube is. To be able to richen up (more fuel) the higher end of the rpm range and Lean out (less fuel) the lower rpm range like light cruise. Now, I have/had personally used turbo e tubes for years, with pretty decent success. I am going to go ahead and recommend them, but at the same time, tell you they are not 100% needed. As a matter of fact, my current blow through setup does not use turbo e tubes and it is by far one of the best running and driving cars I have ever been in. I am recommending the turbo e tubes, because without them, it takes a shot more know how to get the tune just right.
Speaking of tuning, this is something I feel you NEED. A wideband tuner. This is a device that goes into your exhaust (an o2 sensor) and tells you what your air and fuel mixture is from a gauge on your dash. This takes a bunch of guesswork out of tuning. If you have a bad flat spot at a certain rpm, simply look at the gauge when it happens. It will tell you if it’s from lack of fuel or too much fuel. Also, it will tell you if you need to richen up the jetting when under full boost. Letting an engine go too lean under boost for more than a second or two is a death sentence for it. I have seen engine parts look as if someone took a cutting torch to them because of this, so don’t let it happen and get a wideband tuner.
What about reading plugs you ask? I mean, several guys you know tell you that reading a plug is just as good. Okay, so here is the PROPER way to read spark plugs. Go out to a straight stretch, with the car warmed up, make a full throttle run, when your engine is at max rpm, shut the key off to kill the engine, push in the clutch, come to a stop, pull a plug, get out a fucking cut off wheel, destroy the plug by cutting off all the metal around the insulator until you can see the very base of the insulator, where you will read the little ring at the very base. Back before wideband tuners were available everywhere, I would cut the crimp part of the metal so I could pull out the porcelain, like so.
This will then tell you how your tune is doing at wide open throttle, assuming that you are not running pump gas with ethanol, assuming your engine did not lean out and explode before you could shut it off. Ya know what, just get a fuckin wideband tuner.
If you are choosing a set of carbs for your build and don’t already have some, go small. Seriously, smaller than you think and smaller than most people will tell you. Weber 40s and Dell 36s work absolutely awesome on a blow through turbo application. As a matter of fact, I have used 40 idf carbs on a 2332cc turbo engine. The thing is a dream to drive and has gobs of power. Leave the big stuff for drag race only guys, you will thank me.
I’m sure that some of you have noticed that I have not mentioned fuel injection here. Fuel injection is nothing short of awesome for a turbo application, it’s awesome for all applications for that matter. You will never get the precise fuel metering from a carb that you can get from EFI. I am not covering it for two reasons. One, most VW do it yourselfers are much more comfortable with carbs and I want you to get your feet wet with something you know first. Two, there is really not as much to explain about turbo charging an efi engine. There are no big secrets, you just do it and tune it accordingly.
Now, if you are going to run efi, I am sure you have heard of megasquirt. Use one of their systems, if you buy a ready made kit from CB or Redline, you clearly have more money to set fire to than I do. A mega squirt system will give you more tenability for pennies on the dollar.
This post is going on forever, I need to break this shit up. My next entry will be on the hardware needed.