Turbo systems, over-boost & plumbing for dummies!

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Induction lesson Requested

Postby epipete » Thu 22.10.2015, 06:39

Hi, looking at the recent thread regarding BOV and Dump Valves had me thinking about the Induction plumbing and my ignorance on the subject. It was a great many years after I left school that I realised that sitting in class or at a meeting being to embarrassed to put your hand up and say 'Pleae Sir, I don't understand' was a dumb thing to do and so.....

Can someone, please, confirm my understanding:


1. Ambient air is drawn in via Hose #9 to the Underside of the Airbox where it passes through the filter to
2. The Turbo via the Airbox to Turbocharger Hose, where it is compressed & becomes very hot and thus reducing its density
3. The hot pressurised air leaves the turbo at great speed via Hose #36 enroute to the Charge Cooler where it is cooled prior to being fed to the Plenum via Hose #53 & #64 et al.
4. The air is fed to the engine where it is burned with fuel and the waste is sent out via the exhaust
5. Continuously repeat the above.

I know that that is simplistic but in essence is it right?

The BOV or Dump Valve is inserted between the Hose #53 and the Airbox and thus receives cold, fast dense air which is 'dumped into the Airbox at the control of a vacuum from the plenum. Why / what on earth does this do???

Sorry, Sir, for being dumb, but I don't understand!
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Re: Induction lesson Requested

Postby Fredjohn » Thu 22.10.2015, 07:33

Correct points 1-5, put simply.

Now the dump valve: when you shut the throttle a mass of high pressure air hits a closed throttle valve. Nowhere to go, so it backs up and slows down the spinning turbo. So plumb in the DV to the pipe prior to the plenum, and on closing the throttle (when say changing gear) it opens the DV (by use of the small vacuum pipe) to the atmosphere and dumps all the high pressure air to the outside (whoosh), or back into the air filter box (quiet) depending on the type you have. Hence there is no back pressure to slow down the turbo too quickly and it keeps spinning faster. So when you then floor the throttle after a gearchange the turbo is still spinning quite quickly and picks up the boost more quickly than without the DV. So you get better performance with less turbo lag and less wear on turbo as it is not slowing/accelerating so much.

Well that's the theory as to why you fit one. Whether it actually makes any difference is open to debate.

This was a Simple Man's Guide production.

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Re: Turbo systems, over-boost & plumbing for dummies!

Postby Enright » Thu 22.10.2015, 08:38

Pete I thought that your above question might compliment this thread that I created some 5 years ago, so I took the liberty of merging them - I hope you find the info useful.

It could be my interpretation, but I think that your point 2 is slightly off the mark. The turbo, in pressurising the air INcreases it's density, but due to the fact that it's bolted to a hot engine and with the friction of all those air molecules rubbing together (like rubbing your hands together) as a by-product it does generates some heat in the pressurised air.
The chargecooler is a useful addition to the system which ensures that the air ultimately fed into the fuel injection system is as cool as possible, and therefore as high density as possible.
Striking fear into the hearts of duplicate posts everywhere...! :P

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Re: Turbo systems, over-boost & plumbing for dummies!

Postby epipete » Thu 22.10.2015, 10:06

Gotcha Guys, thank you!
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Re: Turbo systems, over-boost & plumbing for dummies!

Postby rip » Thu 22.10.2015, 12:12

The turbo also gets hot because it it hot exhaust driving it & the entire unit is kept as small as possible.
This was the standard design for turbos but may change after recent developments as implemented in Mercedes' 2014 F1 power unit, where the turbine & compressor have been separated.
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