I'd be more worried about burning up valves from losing the scavenging effect of the backpressure. Correct me if I'm wrong, (which is more often than I like
), but the scavenging effect of the backpressure helps cool the exhaust valve.
The primary cooling of the valves takes place through the valve seat. You actually
don't want the valve to be cooled too much - you could end up warping the valve head! Then, you're really boned...
That's why even racing cars (all-balls-out maximum-effort jobs in particular...) will run either open headers or individual pipes - to keep the valve from getting cooled off too quickly and warping. That's why you can get "header mufflers" and often "header cats" - because you're going to have to run
something off of your exhaust port.
Backpressure actually runs counter to scavenging. "Scavenging" is the effects that remove exhaust gases from the combustion chamber, and encourage filling with the fuel/air intake mix. "Backpressure" refers to restrictions in the exhaust system that end up "creating" system pressure (changes in section, changes in flow direction, ...) and cause a mild reversion in exhaust gas flow. Backpressure is something you do not want, scavenging is something you want to encourage.
Scavenging can be increased (and backpressure minimised) by tuning the diameter of the exhaust tubing to minimise temperature variations in the exhaust pulses (there is a formula to determine optimax ID of the tubing, but I don't recall what it is offhand.) The optimax will give you maximum flow by both having
enough internal cross-sectional area to accommodate effective flow of the pulse volume but
not too much internal cross-sectional area to cause the outer edges to cool and cause internal turbulence in the exhaust pulse (which can also contribute to backpressure.)
I seem to want to recall the stable optimax pipe size for our 242ci engines being somewhere around 2-3/8" or so, meaning you can use either a 2-1/4" or 2-1/2" pipe with minimal ill effects.
Granted, the foregoing assumes you do
not have a turbocharger in the system. If you do, the turbocharger uses most of the heat energy available in the exhaust gas pulse, and you just want to evacuate it as quickly as possible (since the exhaust turbine breaks up the "pulses" of exhaust gas anyhow.) This is why turbocharged vehicle exhaust systems tend to resemble mid-size Diesel exhausts, and why Diesel exhausts tend to be made with tubing with a 4-5" ID...
From where we'd end up wandering farther into theory than we probably want to do here...
condones the removal of Federally Mandated emissions equipment on vehicles.
