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1 " EMT best flow
Topic Started: Nov 22 2013, 12:01 AM (2,630 Views)
mwebb
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FOG

Jordan
Mar 1 2014, 06:47 AM
What are the values on the graph? Might not have seen them, just can't tell what the numbers are actually representing.... Vacuum? PSI?

Are you measuring BP at idle, or on the road driving?

Please keep in mind, sounding skeptical is not my intent, I'm very interested in this. Just have some questions that may come across incorrectly in this medium.
understand that
my goal was / is
not to reduce back pressure BUT to improve system flow
and there fore
to improve volumetric efficiency
at cruise RPM by fussing with the exhaust

an un intended consequence was that
exhaust back pressure was reduced so much that the back pressure was insufficient to open the EGR back pressure transducer
so
EGR no longer functioned

i had not considered this possibility prior to my fussing with the exhaust

since i have also improved flow at heavy load and hi rpm
i suspect that the exhaust pipe id is still too large for optimum flow at cruise

more testing is required
in the final pipe id that i settle on i can measure back pressure using running compression
i measure pressure in the cylinder during EVO
which is
Exhaust valve open
so that there is no restriction between the top of the piston and atmosphere -
what ever the pressure is in the cylinder at the time is
exhaust back pressure
like this
Posted Image
this is running compression
a mid 1980,s MGB with a bad cat and burnt exhaust valve -
pressure in the cylinder measured with a pressure transducer in the spark plug thread
the cylinder should NOT be firing as there is no spark plug and no spark
the hi peaks happen every 720 degrees , they are right at TDC


at about 150 degrees after TDC you have a plateau of pressure a wee bit above ambient barometric pressure which is
the white horizontal line
that spike upward between about 150 degrees and about 360 degrees ATDC
is a back fire , another cylinder has a burnt valve and is firing into the exhaust
but
the pressure spike shows up in this cylinder
between the second pair od TDC pressure peaks you can see the exhaust back pressure
the amount that the pressure is above the white line between about 150 and 360 degrees ATDC
about 2 psi average , which is very high for that load and rpm

the pressure and vacuum in the cylinder gets measured and
recorded - the file is saved -

so
yes i can measure exhaust back pressure
but
that was not what i was doing
Edited by mwebb, Mar 3 2014, 12:18 AM.
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john.
Elite Member
[ *  *  *  *  * ]
any more details from anybody on this?
does going smaller exhaust pipe improve mpg? I imagine there would be a loss of top end performance.
but for strictly fuel mileage?
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dayle1960
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Fastest Hampster EVER

The only person who I've heard of doing this is mwebb. He jumps on GMF from time to time so be patient and he might reply to your query. Also the style of his writing takes time to get used to. You might want to re-read his thread and see if you can glean some additional nuggets.
Edited by dayle1960, Jul 20 2014, 06:18 PM.
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