Well I can speak for the fact that going up to 2.5 isnt going to change much. busted up the whole back half of my exhaust, so I cut it off put a 2.5" thrush glass packed on it and a dump. Sounds great (to me, and thats all that matters) and didnt change any of its driving characteristics at all honestly. It just sounds good now

 

Thanks for the information everyone. I'm not really looking for extra HP, though I might do some minor intake improvements in the future as well. I don't mind the loud sound, I like how it sounds at idle with just the cat, but, the drone is too much on the highway and they can't hear me at the drive through window.

Certainly not looking for a V8 sound but I've always been a big fan on inline 6 motors and their unique sound. I like to hear it.

 

I'm no engineer, but I would think 6 primary tubes 1.5" in diameter (roughly) going into a collector only 2" in diameter would provide enough back pressure just because of the "funnel" effect.

But again... I'm no engineer.

 

Search- exhaust pulse or exhaust scavenging. Also consider exhaust tuning.

"Back-pressure" is never good.

 

"Back pressure" is not a factor. What is a factor is exhaust velocity and its effect on scavenging in the cylinders. Regardless, 2.5" is not too big for the 4.0.

 

I didn't present a theory. It's a well known fact from the fairly early 20th century that exhaust can be tuned for desired performance. Your "theory" would allow us to get our 6 in a row 4.0 engines to put out 400 hp just by putting on 8" exhaust pipe. It obviously doesn't work like that, in either direction.

First, I used the term "back pressure" in my reply and that was wrong. It's a real thing, but when talking about tuned exhaust, it's not necessarily the the correct term. What is being tuned is exhaust gas velocity. Exhaust is not constant flow like a fan blowing, it's a series of pulses when each exhaust valve opens. The lower the RPM, the greater the gap between pulses. Each pulse flows through the exhaust pipe and creates a vacuum / low pressure area behind itself. The next pulse moves into the vacuum gap easier than if there was no vacuum gap.

This is where the tuned exhaust comes into play. A large diameter pipe won't allow an exhaust pulse from some engines to create a very good low pressure area behind itself as the pulse moves through the pipe. A pipe too small will slow down the pulse velocity and also keep it from creating a good low pressure area behind itself. That's why the right sized pipe can maximize the low pressure area created behind each pulse at a specific RPM range.

Guess what happens to the next exhaujst pulse when the one in front of it doesn't leave a low pressure gap behind itself? The next pulse has to push its way through a normal pressure area, causing... back pressure.

Exhaust at high RPM begins to mimick constant flow with virtually no low pressure gaps between exhaust pulses. That's when you want nothing affecting exhaust gas velocity. At low RPM, the pulses flowing through a pipe enables the following pulse to draft along behind and there's a positive effect on torque. This is because the piston is not using as much energy pushing the entire volume of burnt gas out of the cylinder. The low pressure gaps develop very close to the exhaust port on the head, allowing the exhaust gas get out of the cylinder easier.

Once again, this all happens in a specific RPM range for each type of reciprocating engine based on cylinder bore, piston stroke, valve diameter, valve opening, valve open duration, etc. And I specifically said reciprocating, because the exhaust behaves completely different in a rotary engine. In a rotary engine, torque and HP curves run parallel and continue to go up as long as RPM is increasing.

So, we can agree that back pressure is no good, but a huge exhaust pipe can actually allow more back pressure to develop at specific low RPM ranges than a right-sized exhaust pipe. Right-sized meaning the correct size for the engine type and the desired torque at a specific RPM range, and "size" includes diameter AND length.

If you don't believe any of this, review the video you posted at the 5:18 and 10:40 marks and pay careful attention to the torque curve at low RPM. The narrator doesn't elaborate much about this "phenomenon" at 5:18, but he does explain a bit more at 10:40 and also mentions "higher torque at 3100 RPM and lower".

You can read up on the function of expansion chambers on 2-cycle engines or the effect of exhaust size on diesels to get way deep into why wide open, unrestricted exhaust is not the best for all circumstances.

For the OP; as I said before, not likely your 1/4" up-sized muffler will change much of anything, and at highway speed RPM, will likely help.

 

Finally someone who understands that these are NOT high HP engines and that they make all their power at lower RPMs.

 

Yes, a 2-smoker is a great example of how exhaust pipes can affect engine performance!

 

If you ain't mixing gas you ain't whoopin ***! Lol

 

 

trutthhhhhh. I miss my 2-smokers. Real bikes. They faded out to these fancy 4-strokes with their camshaft things. Give me reed valves back and ridiculous powerbands lol.