Well you might ask, why would you want to run boost down an old piece of technology that was never designed for it? Why not go injection and tune your car with a computer?
Well, if you know how to modify a carburettor for boost, It can be fun, cost effective, keeps the mechanic feel of an old car and make considerable power gains.
Now I’m specifically talking about blow through turbo setups like my Corolla. Whilst they aren’t as easy to tune as a fuel injected car, after a bit of work tinkering, you can get these set ups working reliably. Check out my post How to Build a Blow Through Turbo Setup.
Can you use your stock carburettor for boost?
The answer is it’s not recommended but you can make it work considering what carb your factory one is. If you have a Weber or a Holley as standard, there is a good chance you can make it work. There are probably others that may work but most will give you headaches because:
- Most factory carbs either have too much pollution gear on them
- They won’t seal properly to hold boost
- Aren’t strong enough to hold boost
- Don’t have a big enough needle and seat to hold the higher fuel pressure
- Not easy to modify
- Not many options for aftermarket fuel jets and other parts
If you want to use your stock carb, ensure you can seal your boost plenum on tight and only run low boost, maybe up to 5-7psi max. You really want to ensure that it can hold the extra fuel pressure required for boost and there are no boost leaks from any other part of the carb.
When I first boosted the Corolla, I used the stock Aisan carb. Once I got the plenum to sealed and worked out what fuel pressure I could run, it was fun until I blow the rubber plunger seal.
Once I changed over the 32/36 Weber, it was night and day difference. More boost, more fuel and more power.
Modify a carburettor for boost
Why a Weber and Holley carb is preferred is that they are strong and have a great range of aftermarket parts. Here are list of mods to make your carb boost friendly:
Remove the choke – Not only is the choke going to hindering the boost flowing down your venturi, once you have a sealed boost plenum on your car, it acts like a choke for those cold stars.
Seal up the carb – seal up any areas that will leak boost. In the case of my 32/36 weber, I’ve had to seal up where the choke linkages ran through. I’ve used a metal filler.
Run a bigger needle and seat – If you have the option of running a stronger needle and seat, do it. This will help hold the higher fuel pressure required. Where your stock carb will likely run 3-4psi fuel pressure, your boosted application will run 4-7psi base fuel pressure. The bigger or stronger needle and seat can hold these higher base pressures.
Change your fuel float to plastic – if you carb runs brass floats, these can distort and even crush under high boost pressure. Running plastic floats ensure they can cope with the boost.
Change your fuel jets – As you add more air, you need to add more fuel. Using an air fuel metre to gauge what size of jets you need.
Add a boost reference power valve – Change your power valve to a boost reference unit. A Boost Referenced Power Valve Actuator converts the standard power valve from vacuum operation to boost referenced, using a modified power valve actuator and custom wound compression spring. Gives boost fuel enrichment using the standard power valve circuit. The spring specification gives a trigger point of x amount of psi boost.
The next evolution of blow through carburettors
A lot of EFI companies are now making fuel injection carburettor replacements. Holley Super Sniper is one that is specifically designed to work with boosted applications. They come in 2 Barrel and 4 barrell versions.
The major benefits of using these types of system is that they are easy to tune or have built in self tuning abilities. For about $2000AUD, you can have a new age carburetor that starts easy on cold mornings, never needs maintenance and always gives the right air fuel mixture to your engine no matter how much boost your running (Super Sniper maxes out at 28psi).
They use multi injectors, wide band sensor and a computer to regulate how your engine runs. These are great option for those that don’t want to go the full hog with an EFI setup but take away the difficulty in tuning a carburettor.
Happy Boosting!
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