Hi guys just wanted to know if anyone can explain why running an engine with a restrictive exhaust gives better econ stats then one that lets the engine make full power.
Nope, doesn’t help
[size=85]Killrob Edit: not much at least. 
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Well, yes it does help! If you are running a naturally aspirated system, having the exhaust optimized for cruise air-flow means that you will be limited at higher RPM at max throttle, but you do get slightly better economy ratings in cruise.
Wait, now I’m confused, the thread title says emissions but the post says economy 
Hehe yes, but then again… if you are slightly more efficient, shouldn’t emissions drop too as they are directly proportional to each other if burn time and temperature doesn’t change?
Hm, I don’t know, you’re still burning the same amount of fuel and you just get more power out of it.
good point, yes… but only at full throttle?
So, more of it is burned thus giving off less emissions over dumping the larger amount of unburnt fuel into the exaust system.
I may be dead wrong.
Considering this thread is about exhaust diameter: The exhaust setup determines, in which RPM range resonance effects in the exhaust system help to extract the exhaust gas out of the cylinder (so the engine does not have to push that hard). This makes it in that specific RPM range more efficient (-> better economy in that RPM range). The exhaust has no influence on the percentage of the injected fuel which is burnt. This is influenced by how the fuel is injected and how the air-fuel mixture is distributed in the combustion chamber.
Part of the emissions come from unburnt fuel (HC). Another part, NO, can be reduced with exhaust gas still being in the combustion chamber because this decreases the burning temperature (-> exhaust gas recirculation). You could possibly say that a exhaust, which is helping to pull out the exhaust gas, also extracts the HC out of the cylinder, preventing them from getting burned in the next cycle and thus increasing emissions. But as I said, the helping effect only exists in a specific RPM range and I guess over the whole range the effect should level itself out again (at least so much that Automation shouldn’t care about that).