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Run_Away
Run_Away Dork
11/28/21 5:13 p.m.

In reply to CrustyRedXpress :

You're exactly right in what happens. When a high pressure pulse of exhaust gas from one cylinder passes through the pipes, there's a low pressure area right behind it. This can suck in atmospheric air behind it, like a venturi effect.

This is how aftermarket headers can make a bunch of power by tuning the diameter and length of the primaries before the collector so this low pressure area will actually draw out the exhaust from it's neighbouring pipe.  

malibuguy
malibuguy GRM+ Memberand HalfDork
11/29/21 7:53 a.m.
CrustyRedXpress said:

In reply to AngryCorvair (Forum Supporter) :

But if the exhaust gas is under positive pressure and escaping to the atmosphere, how would that change the 02 readings?

Is there some part of the combustion cycle where air could get sucked into the exhaust system before the 02 sensor?

Its very common on old v8s to find exhaust leaks.  I deal with this very often, you its alarming how much E36 M3 I see that people pay to have done or have a "buddy" do and be absolute trash and be riddled with leaks.

In terms of the 02...most of the time they get put into the collector or just after.  Well if they have leaks at the head or at the collector it skews what the O2 sees.  Anything about 18" past the o2 is ok.

This is primarily why I preach RemFlex gaskets especially on old SBCs.  Fantastic gasket.  Ive tried them all and these are the only ones that dont leak even in sub optimal situations.  Fun fact my dads Monte Carlo has one broken off bolt in the head from when the orignal manifolds crumbled into pieces (ive never seen that before) and I could not extract it without pulling the head.  Remflexs went on anyways with the shorty headers.  3 years later still tighter then Folsom Prison

NOHOME
NOHOME MegaDork
11/29/21 9:21 a.m.

In reply to malibuguy :

Times 100 on the remflex header gaskets AND ramplock header bolts.

And just for fun, check the header flanges with a flat surface to see if they are flat. Dont car if they are new or who made them, it is scary how many are warped from the get-go.

By going back to a carb, I think I have found all the drivetrain issues that were undermining the Fitech.  That said, with the car driveable as is, I have not found the motivation to take the Fitech off the shelf and give it another try. 

SkinnyG (Forum Supporter)
SkinnyG (Forum Supporter) UberDork
11/29/21 9:56 a.m.

Also a fan of Remflex gaskets.

And positioning the 3" collector with one of the 3 bolts at the bottom.  Cuts into ground clearance, but saves the flange gasket when I scrape over a berms and speedbumps in parkinglots.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
1/11/22 8:22 p.m.
CrustyRedXpress said:

In reply to AngryCorvair (Forum Supporter) :

But if the exhaust gas is under positive pressure and escaping to the atmosphere, how would that change the 02 readings?

Is there some part of the combustion cycle where air could get sucked into the exhaust system before the 02 sensor?

Bernoulli's law.  High velocity = low pressure.  It's how wings work, how carburetors work, and how exhaust flange leaks pull air IN to the exhaust system.

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