high flow exhaust = 3-5% mpg gain?

[tjts1]

If a performance exhaust helps improve FE, a simple electric cut out would accomplish the same thing at much lower cost and you could close it at low load for a quiet idle.

[comptiger5000]

^ Not necessarily. That would reduce backpressure, but it won't do much as far as controlling what RPM the exhaust is tuned for.

[Varn]

Hi incautious
Too much free flow can suck some of the unburnt fuel out of the exhaust ports. At low rpm the valves are open longer. Guess that I am assuming that you are trying to keep the engine at low rpm. Arn't Borla exhausts something over a thousand dollars? That is about the value of 2 of my 3 cars.

Good luck

[jtbo]

Modern vehicles have variable intake runner length, that gives at least two rpm where intake is tuned for, but I guess it has not been made for exhaust yet?

It would require quite bit of work to get such system made so that it would work for few hundred of thousand miles, but that would offer again bit more efficiency when there is less compromise needed

But exhaust manifold on many cars is quite horrible, all exhaust ports can have different length of manifold pipe attached to it, while those pipes should be equal length, at some cars exhaust manifold is log where exhaust ports are at one side and gases exit from one end of log, quite horrible.

Replacing exhaust manifold and adjusting motor to change should give a nice improvement, especially when both intake and exhaust manifolds will be tuned for low rpm which is used mostly when driving.

[tjts1]

Originally Posted by comptiger5000

^ Not necessarily. That would reduce backpressure, but it won't do much as far as controlling what RPM the exhaust is tuned for.

I have ways. You don't want to know about them but I have ways.

Speaking of tuned exhaust, check out the manifold on the 2.0 sky-whatever motor on the 2012 mazda3. High FE is goes from 33 to 40.




Who says exhaust work can't yield FE?

[dcb]

Who says the exhaust is solely (or even partly) responsible?

[Phantom]

Yes but that is a manifold that is tuned to have the pressure waves from two opposite firing pistons create exhaust scavenging from the pulses passing the crossover point. Then those two do the same when they join the other two on the same bank. Each junction is tuned for running at a set RPM range.

Also that engine does not get that MPG just from the exhaust but it runs a 14:1 compression ratio, has the long tube equal length headers, direct fuel injection, special dished pistons, and dual electronically variable valve timing; on top of that it has a 6-speed transmission.

[tjts1]

Originally Posted by dcb

Who says the exhaust is solely (or even partly) responsible?

Mazda says.

[jtbo]

Modern cars take heat from exhaust and use that to heat cabin, at least some European cars have such system, but that does of course increase efficiency too.

That Mazda exhaust looks quite complex, would be interesting to have some tech analyse from it as I would like to know how it works, that is what I would like to know from everything actually

[dcb]

Originally Posted by tjts1

Mazda says.

Please cite your sources, or admit your mistake.