On gasoline engines, you restrict them using the throttle. If you open up the exhaust, the engine will pump more air/fuel mixture at any given throttle position. But when cruising (ie not at WOT) you will just compensate by opening the throttle less than before. The throttle is called a restrictor plate for this reason. There's generally no efficiency gain to intake/exhaust mods because of this. Power gains, though, are obvious.



Diesel engines, that is not the case.

The efficiency gains will therefore only really present at wot. The gain will therefore go more toward faster 0-60 than farther range. Power is energy/time. If you don't use the energy quickly (higher power) it will mean more time to consume or higher mpg

Power is energy per unit time. Efficiency is energy out divided by energy in. Higher power does not mean higher efficiency. It's true that you might see higher efficiency at WOT, but nobody drives at WOT all the time, so it won't make a difference.


Meanwhile the rest of Xoutpost thinks "nobody drives at WOT all the time? Challenge accepted..."

I agree. I run hard almost all the time and my daily routine usually involves hitting 200kph (until I get hit by a flashy cam)[emoji38]

Sent from my SM-A730F using Tapatalk

The trip computer is VERY accurate. Afterall, your ecu knows EXACTLY how much fuel it is injecting into the engine at any given time and how many miles since reset. By the trip computer, I could never do better than just shy of 21 mpg on highway trips, and now I'm getting just shy of 24 mpg on the same routes.

Intake and exhaust mods can very well change the efficiency. Ever hear of pumping losses? Think maintaining 600 deg temperatures in a catalytic converter is done without expending any energy? You often hear that an inefficient exhaust system only costs 2 or 3 hp or such and such, but when the vehicle only requires 20 to 30 hp to maintain it's speed on level ground, there's your 10%.

Like I said above, pumping losses are negated by the fact that gasoline engines are restricted. You take them away from the exhaust and then add them back in with the throttle (which is literally a "variable pumping losses" valve). That is just how gasoline engines work. "You often hear" a lot of things repeated on the internet. And converters are heated by the capitalization of leftover fuel and oxygen (literally burning the leftover fuel). They are not heated by friction or pumping losses.



Trip computers are flawed measuring devices when new, and more so when old. We all have met the C5 Corvette owner who thinks their cars get 30mpg everywhere they go (that's an extreme case, but the C5 owner could argue the same thing you are arguing). The only way to prove it is fuel logs. Even modern cars are inaccurate due to many different factors, and OEMs even program inaccuracies into them. I can prove this with the many years of fuel logs I have for my own cars. I have zero problem believing everything you say if you back it up with fillup logs.



Great work on the headers, and thanks for posting all your work on the forum for all to benefit from.

My wife and my near identical cars get wildly different mpg. She gets 23/24 I get 20/21 highway. Head on nail for energy is energy. If it's expended heating something that energy didn't push the car forward.


Sent from my iPhone using Tapatalk Pro

The gas that heats the catalytic converter would just be dumped out the tailpipe into the atmosphere. That's what cats do. They clean up the unburned gasoline in the exhaust, as well as other reactions. There's no less gasoline going into the exhaust by removing the cats.



The reason they heat up is because it's literally equivalent to a flameless burning of that gas with air. The energy in the unburned gas has to go somewhere. Removing the cat doesnt make the engine put any less fuel in. It just dumps it out into the atmosphere in a form that's more polluting.

I disagree. The trip computer is not accurate in my experience. I log all of my fuel ups and track my mileage. My OBD consistently over reports mileage by .4 miles per gallon. The ECU does know exactly how much fuel it's putting in the engine, but I don't think that is how the OBD calculates MPG. In fact, my scanner has a function that allows you to adjust the reporting accuracy of the OBD. Unfortunately, it only works in liters not gallons. I haven't been able to figure out how to make the adjustment correctly. The adjustments I tried only made it worse, so I went back to "neutral".

Very true that the mpg is not necessarily that accurate but the change should be reasonably accurate. That said, I've seen big changes in mpg after changing some component like the O2 sensors but after 100 miles back to normal.


Sent from my iPhone using Tapatalk Pro