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Turbo back pressure

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Old 27-09-2018, 05:39 PM
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Caddyshack
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Default Turbo back pressure

Maybe one for Mark Shead but can anyone explain in simple terms about turbo back pressure.

a few people have commented that my 7064 Borg Warner may not be big enough for my stroker yb and that if too small it will have too much back pressure at higher revs / power. Where does this back pressure come from, surely the turbo is sucking new air through the inlet side, if the engine is sucking more air than the pressure side can produce where is the back pressure?

‘’What am I missing?

is it just pressure post turbo in the exhaust manifold where the gasses cannot leave the cylinders quickly enough as well as normal exhaust post engine back pressure

Last edited by Caddyshack; 27-09-2018 at 05:42 PM.
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Old 27-09-2018, 08:07 PM
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Back pressure is a build up of flow against a restriction.
If you have too high a turbo inlet pressure then the turbine or exhaust system is the restriction.
In development of my car I have measured post turbine and TIP against boost pressure.
All related. Too little pressure is also bad at TI.
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Old 29-09-2018, 07:08 PM
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Why not ask those making the claims, to support those claims with evidence ? Presumably they have done lots of testing ?

But generally it refers to pre-turbine pressure, not post turbine, but of course all play a role and in general the lower the better.

But if you're running say 30psi boost, it would be entirely reasonable to expect 30-50psi pre-turbine pressure under high load/rpm depending on setup. In some cases you can get pressure ratio below 1:1 but for most part it would be higher.

I'd imagine many OEM turbo setups run pretty high PR's, but they'll use small turbines that spool very fast, so there is always a tradeoff somewhere.
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Old 19-10-2018, 04:39 PM
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Sorry, I missed these replies. Thanks for the replies.

it seems a bit odd that my turbo versus the next one up have the same frame / housing as in they are the same outside dimensions but my 7064 would run high back pressure but from your replies I guess it means my turbo would be struggling to supply enough air to feed the strokes Engine at say 2 bar whereas the "bigger" turbo could flow more and therefore have less back pressure.
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