Thumb through an American speed parts catalog and you will still see pages upon pages of carburetors and related accessories. There are kits that make converting their mastodon-like muscle cars to fuel injection easy, but it seems they like the old fashioned carbs pretty much.
Admittedly, the carb is still a viable fuel management system. It is tunable with a screwdriver and is quite simple in operation. You don’t need a laptop to fill up a fuel injection table. For what is a purely mechanical device, it does pretty well to make power. Of course it is not as precise nor as emissions friendly as an electronic fuel injection system.
But what about putting a turbocharger in a carbureted engine? When you already have an engine that runs okay with a carburetor, with some modifications you can marry forced induction and mechanical fuel management. This is what we are exploring with this article.
What is a carburetor?
A carburetor is basically a mechanical device that mixes fuel and air to create an atomized mixture that would burn inside the engine’s combustion chamber. Carbs operate using Bernoulli’s principle, where air sucked by an engine goes through a tapered opening called a venturi. This increases the speed of the air, atomizing the fuel being squirted at the same time.
Carbs were a mechanical solution to properly mixing air and fuel to the stoichiometric ratio. They were invented and used in cars in a time when computers made in China were abacuses. Now we have electronic controllers managing fuel and ignition.
The old timers (and many motorcyclists) swear by carburetors. They are remarkably simple, can be tweaked with a screwdriver, and make do with a good internal scrubbing once in a while. The world will still run carbs if it weren’t for pesky emissions regulations and rising need for fuel economy.
If you are converting to carburetion
We will not be able to give you a step-by-step guide in de-computerizing your modern powerplant, or controlling fueling needs with forced-induction via carb – that’s not the point of this article. What we can help you with is by showing you some short guidelines and some pegs you can emulate.
Obviously you have to pick the right size carburetor for your application. Too big and you suffer from poor drivability; too small and you choke the motor at high RPM. Best to look at what other people with the same size engine you have are running. Fine tune the fueling via jetting and the adjustment screws on the carb.
If you will be converting from EFI, you will have to fabricate an intake manifold to bolt the carb into. Also, you will have to think how to control your ignition system. If the ignition system is a distributor type setup or is separately controlled, you’re okay. But if not, either figure out how to install a distributor-timed setup or utilize an aftermarket ignition system.
Carburetor turbo layout
If you will be running carburetors with forced induction, you have to plan where to put the carb in relation to the turbo or blower in the intake stream. You can put the carb before the turbo, which is called the draw-through setup. Or you can put the carb behind the power-adder, or the blow-through setup.
- Draw-through carburetor turbo setup
The draw-through setup is easy as you don’t have to modify a carb for boost duty. However, you cannot plumb an intercooler with this setup. Also, you have to make sure the intake piping will be devoid of low points towards the throttle body or inlet ports. Fuel can collect in those pockets, which can result in poor running or even be a fire hazard. The turbo would also need carbon seals on the shaft to avoid fuel contaminating the center cartridge. That may or not be extant in either a surplus turbo or a cheap China one.
Basically, you shouldn’t consider the draw-through setup.
Blow-through carburetor turbo setup
Basically, this setup leaves the carburetor in its standard position and the turbo does its work to the inlet air before the carb. This layout allows you to use an intercooler to increase inlet air density, as well as a blow-off valve to stave off compressor surge upon throttle closing. This also allows for higher boost pressures than the draw-through setup.
The hiccup here is that you need to modify your existing carburetor to tolerate boost pressures rammed down its (venturi) throat. The mods entail minimum cost but are something that a novice hand should probably undertake only if he or she had an extra carburetor to revert to.
Converting a carb for blow through
This section will serve as a guide but do your due diligence before you attempt any advice herein. We are not responsible for your blown motor. And plus, we haven’t yet attempted to do this to our own carb. So we cannot vouch with absolute certainty that all these will work. Caveat lector. Anywho, here are the stuff to do to get your carb ready for boost.
- Get a spare carburetor to revert to stock or for reference
If in case you screw up with the conversion, or you decide to let go of the turbo setup in the future, then at least you have a spare carb on hand for reference or reversal.
We are planning a turbo Toyota 4K engine for a future project. 4K parts are plentiful still, and new or used carbs are reasonably priced in the Philippines. Before we do tear into our Aisin carb, we’ll acquire a spare.
- Relocate manifold vacuum feed of carburetor (power valve) to where it sees boost (pre-carb)
The carburetor must be able to sense additional air to enable it to deliver more fuel. It must see the actual boost pressure before the charge air reaches the carb. Therefore the vacuum feed of the carburator’s power valve should be relocated from the intake manifold (post-carb) to a point before the carb.
You will have new intake piping for your turbo setup. Fill free to install a hose barb at a convenient location between the intercooler and the carb.
- Fill fuel float with fuel-resistant foam or other lightweight filler so it doesn’t crush under boost
Usually the floater inside the carb is hollow. Under boost the entire inside of the carburetor will be pressurized, thus the float may implode. If you are lucky, you can buy a ready made reinforced float that you can just swap in. If you’re like us, you don’t have a store-bought solution available.
You then must fill the hollow cavity with something light but resistant to boost. We would look at expanding foam spray made out of polyurethane as the filling material. They can be bought in decent hardware stores. Drill a small hole into the float, then spray the foam inside until full.
- Seal carburetor from air leaks
The carb must contain all the boost inside. So if there are any obvious holes or unnecessary inlets in the carb, plug them up with epoxy. Another issue that may arise is boost leaking from the throttle shaft. You may have to get seals/o-rings machined to the carb body to seal this up.
- Install boost referenced fuel pressure regulator, better fuel pump
Once the carb can contain the boost, the fuel pressure has to be bumped up to compensate. Fuel pressure should be at least 3psi higher than boost pressure so that the fuel can be sprayed into the venturi even under boost.
A boost-referenced fuel pressure regulator (FPR) is needed to bring about more fuel pressure. Note that an FPR for carb applications is different from one for EFI, as the latter deals with higher pressures. We are studying the Summit Racing G3032 FPR to see if it will work for carb turbo applications.
Once you acquire the right FPR, plumb it as per included instructions. You may need to upgrade your fuel pump to one with more flow at a higher pressure. Achieving that can be through running a carb-specific electric fuel pump, running two of them in parallel, or using an EFI pump running half the voltage.
- Install carburetor hat
You need to source a carburetor hat. It will replace the air cleaner housing and will serve as the conduit between the round charge piping and the carburetor. You may get lucky and find an OEM part that will work. In the case of a 4K, we just have to look for a 5K hat coming from a Liteace. If not, you’ll have to make one or buy one from someplace. Another surplusan solution made aware to us is using a carb hat for the Mitsubishi L300 4G63 gas engine.
- Tune and monitor with a wideband air/fuel ratio meter
Like a programmable EFI box, tuning will make or break your setup, and the best way to achieve the right fuel/air mix is by knowing the A/F ratio in the first place. Our titos used to look at spark plugs and exhaust gas temperatures to gauge with questionable precision the richness of the air/fuel mix in the combustion chamber. Today we can know it with certainty by plumbing a wideband O2 sensor.
If you have access to a dynamometer facility, they would have one. If you will be street tuning, you can install a wideband gauge like the Innovate MTX-L and connect it to a laptop to log results.
- Retard ignition and get colder spark plugs
Too advanced an ignition timing may cause detonation under boost. This may limit the amount of boost you want to run. The budget way to do this is to disable the mechanical advancer in the distributor, and/or set the base timing lower. The better way is to get an ignition box that retards timing as a function of boost pressure. MSD makes boxes like that.
Also, you have to get colder spark plugs to stave off preignition. One or two heat ranges colder would be good. This is something you should do even when turbocharging an N/A engine with EFI.
Old folks tell us that turbos and carbs don’t mix. What they didn’t have back in the day was Google, where people can share and document how they successfully did things to their cars. Of course not everything on the Internet is Gospel truth, so you must take even this article with some skepticism. If you think there is some wrong info in this article, or want to add more to this topic, feel free to comment away or send us an email at firstname.lastname@example.org.