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What is blow-through turbocharging?

What is blow-through turbocharging?

Turbocharging is almost like free performance. After all, it uses the gases your car already produces to spin a turbine and force more air back into the engine. And more power usually equals faster. That's always good.

At first, it was a true performance thing, with cars such as the Saab 99 Turbo and Ford Escort RS Turbo flying the flag for all things powerful. Then, manufacturers recognised that you could use turbocharging to boost the power of a small engine, which meant they could use tiny engines in their cars, increasing fuel economy and lowering emissions. In theory at least. Turbocharging was commonplace.

However, to have a turbocharging system work as efficiently as possible, it really needs to be paired with computer-controlled fuel injection, which can meter the amount of fuel precisely as required so the engine never uses more fuel than it needs.

But don’t fret if you’re running an older, carburettor-engined car. There is an option that’s open to you. Two, actually. You could go for a blow-through setup, or even a draw-through set up. 

Without getting into the real nitty-gritty, either system will require modification to the carb in order to ensure fuel delivery and pressure regulation. Neither a blow-through or draw-through system is a quick bolt-on road to big power. However, both do open up the doors of forced induction to older cars. 

With a blow-through set up, the turbo feeds the carburettor. It’s the usual state of affairs as the exhaust gasses spool the turbo, but then rather than the turbo’s compressor side feeding into the cylinders directly, it instead blows through the carburettor (hence the name).

This is why you need to have your carb set up properly. Subject a standard carb to this, and it simply won’t be able to deliver enough fuel to match the compressed air coming from the turbo. If the engine runs at all, it won’t run well. 

With a draw-through set-up, the turbo sits after the carburettor, so rather than the carb feeding the engine, it’s actually feeding the turbo.

The turbo sucks in the cold air and fuel, and then delivers it to the engine. This is an effective method of getting power, but the addition of fuel within the turbo can shorten its life dramatically. 

Also, given that the carburettor is atmospheric by nature, you will need to ensure it's in an environment in which it can match the pressure of the turbo. As such, it may be necessary to create a shroud or cover for the carb, in which the pressure can be stabilised.

Both a draw-though and blow-through system require a lot of maths, a lot of fabrication and a lot of tuning. However, done right, they can be a fantastically old-school way of giving your older car some much-needed boost.