Guy welding it up at the shop said it will make the motor not run right… is this true?
Not necessarily. Exhaust guys like to say this all the time. But it is the most inaccurate assessment of what is happening with exhaust pressure in an engine. In fact, what they are referring to is exhaust wave tuning but they don't really understand it. And it's going to happen whether you go to a 2.5 or 3 in exhaust. What happens is when gases go through a runner. Whether it's intake or exhaust, it has what's called a positive and a negative wave. I'll try to explain this a bit better in the paragraphs below to make this easier to read. But first here is a very short answer.
The short version is that air in the intake and exhaust doesn't just stop moving when we close a valve. It will keep moving in what is called positive and negative waves. Changing exhaust runners changes this wave. Both in diameter and length. Ultimately we are trying to time these waves with the opening and closing of the valves through the camshaft to maximize the amount of work done on the piston. We do this through a primarily duration of the valves being open, and finding the optimal time to open the intake valve, and then shortly after close the exhaust valve. Below is how this works.
Every time this wave reaches an opening, like an exhaust collector or intake plenum, it actually flips and then heads back up towards the valve. It's kind of an odd but predictable behavior. This is called a negative pressure wave as it reflects the last wave and flips. It then reaches the open exhaust port and does the same thing. It reflects and flips again. Each time it does this, it decreases in pressure. Just like watching an ocean wave crash on the beach, then fall back into the ocean, then comes back up onto the beach in a much smaller wave, then falls back into the ocean again. Ever notice the wave going back into the ocean looks like it's rolling into the sand underneath the water? That is a reflected wave
Now back to engines. Let's look at this cycle from the beginning. Starting at near TDC with the spark plug firing. We have a bunch of air and fuel mixed under compression. So if you are visualizing the piston position, the valves are closed, the piston is nearly at the top, the actual volume of the combustion chamber is small because the piston is on it's way to TDC, but the pressure is pretty high. About to get very, very high because heat from ignition under compression creates a lot of pressure. We ignite the fuel before the piston reaches the top because it takes a short amount of time to ignite. This pressure is what pushes the piston down.
As the piston moves down, the pressure decays. We are now at Bottom Dead Center (BDC) of the exhaust stroke. The exhaust valve is opening. The combustion chamber now has a lot of volume but not as much pressure. As soon as we open that exhaust valve, that pressure drops a little more. The piston moves up and begins to push the exhaust out. Now here is where things get really interesting, and where a lot of misinformation comes from. We just talked about how exhaust leaving the cylidner, creates pressure waves in the exhaust runners. While all this time the intake valve was closed the fresh air in the intake manifold runners are doing the same thing. What were to happen if we can time it just right to where an intake pressure wave reaches the cylinder at just the right time and we open the intake valve?
The interesting part is that as we now start to open the intake valve while the exhaust valve is still partially open, (this is camshaft duration overlap) and the exhaust pressure waves have decayed, and the pressure inside the combustion chamber has decayed, the pressure waves in the intake runners should now have a higher pressure than all the exhaust pressures, and in an ideal time stamp of a few miliseconds, we just opened the intake valve as that pressure wave arrives. The intake air pushes the remaining exhaust air out the exhaust valve. If we time the closing of the exhaust valve just right, we get an efficient scavenging effect. Meaning we are splitting every hair we possibly can to maximize the amount of fresh air from the intake manifold so we can maximize the amount of work (pressure from combustion) onto the piston to create more power. And we are also timing this correctly so that there is no contact between the valves and the piston. The exhaust valve closes, the intake valve continues to open, and the piston moves past TDC again and starts pulling more air into the cylinder.
How long the runners on intake and exhaust depend a lot on where we want the torque in the engine to be the highest. Longer runners improve low end torque. Shorter runners improve high end HP. And it's our job to find a camshaft and intake and exhaust runner length that maximizes this for our needs from the engine.
In summary no, opening up your exhaust to breathe better is not going to negatively effect your engine. But if you want to truly maximize it's effect, call up comp cams, iske racing cams, etc. and see what they would suggest for a camshaft to suit your exhaust and intake manifolds. They can nerd out so much more than I am capable of on this kind of stuff. It's a fascinating feat of engineering in my opinion. It's all happening extremely fast.
If you want to take this even deeper, start researching into valve timing events.
IVO (Intake Valve Opening)
IVC (Intake Valve Closing)
EVO (Exhaust Valve Opening)
EVC (Exhaust Valve Closing)
Where in rotation we time these events plays a major role into how much air we can get into an engine to increase it's power output. I hope this helps!
