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What is blow-through turbocharging?

What is blow-through turbocharging?

About the author

Chris Pollitt is an award-winning automotive word-wrangler, editor of the website Not 2 Grand and a keen collector of crappy old cars.

Who doesn’t love a bit of boost? Forced induction is the best way to get more power out of your car, and it’s something we’ve been celebrating for years.

At first, it was a true performance thing, with cars like the Escort RS Turbo flying the flag for all things powerful. Then, as time moved on it became more commonplace.

Turbocharging is now at the very cutting-edge of engine technology, and not only does it provide power, it also has massive economical benefits. Turbocharging is good, it’s a relatively modern technology that has changed the motoring landscape. 

The thing is, is 99% of applications, turbocharging works by compressing air and then forcing it into the cylinder where it’s mixed with fuel, ignited and then boom, power happens. To get the most bang, you really need fuel injection.

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. 

We’re not going to get into the real nitty gritty here. 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 in the exhaust gasses spool the turbo, but then rather than the turbo’s compressor side feeding into the cylinders directly, it instead blows through – hence the name – the carburettor.

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, 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 its 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.

This isn’t a requirement with a more common turbo engine with fuel injection, as the fuel is, as the name says, being injected into the engine at a high speed and a high pressure. 

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.