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Anyone have any experience or knowledge of what happens if you use an exhaust that is too big, or is too free flowing for the engine size etc ?

Been reading about back pressures and exhaust gases getting slowed down on combustion strokes.....Gases not fully exiting the cylinder etc.


Cheers
Chris

I believe that the understanding is that you lose power if it's too big. A certain amount of back pressure is necessary.

Why, what are you planning Chris?

I think it's quite specific to your engine?

I havent watched this but might be of help

https://youtu.be/pH9JbvuzS3w?si=r70LtQrxeAo0ta-a

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neilbardsley wrote: ↑Wed Sep 24, 2025 1:35 pm I think it's quite specific to your engine?

I havent watched this but might be of help

https://youtu.be/pH9JbvuzS3w?si=r70LtQrxeAo0ta-a

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That’s just a sales pitch!

This is a bit more informative, and the physics of how combustion and back pressure works in an exhaust system.

https://youtu.be/jjPeP_Nn2B4

Basically the lower the back pressure the better...............


Mass flow rate is constant no matter what the pipe diameter is. Velocity changes, but mass flow rate (or volumetric flow rate) does not. Mass is a conserved quantity. Therefore, if one only considers back pressure, the biggest pipe possible is ideal. The edge case being no exhaust system at all has the best performance.

The second point he makes is that back pressure is almost irrelevant to designing a good exhaust. All things considered, you want to minimize it as much as possible. However, like any engineering problem, if it was as easy as putting a big pipe on, you wouldn’t need an engineer. Instead, an exhaust system must be designed to minimize wave interference of the exhaust pressure pulses.

Because the speed of sound is a function of pressure (I.e. it’s a function of density, and density is primarily a function of pressure as gas composition isn’t changing much), and pressure is primarily a function of pipe diameter, the diameter is extremely important. Then, the length of the pipe can be fine tuned to minimize interference.

The takeaway is that back pressure should be as close to zero as possible, but it’s effect on performance is minimal compared to good exhaust design.

edit I’ll add that, if someone is simply replacing a cat-back exhaust system, there is no reason to not use a big pipe. But at some point, there is no measurable gain from doing so. So, you should simply install whatever your preference is for looks and sound.

Back pressure = bad (less than ideal, at best).

Too big exhaust (primaries) = bad

Ideally (no packaging constraints etc), you want primary headers sized for the maximum velocity but no restriction, into a merged collector. The kinetic energy of the gas E =mv2 (mass x velocity squared) shows that the velocity is a lot more important than the mass. That kinetic energy creates the suction between pulses that sucks out exhaust & pulls in intake during the camshaft overlap around TDC. If the pipe is excessively big, the exhaust tumbles & velocity falls away. The merged collector enables each cylinder pulse to be shared with the complementary cylinder. For a street usable exhaust….After the collector you want a short length (engine specific) to a resonance box (effectively an empty silencer) which acts as the reflector for the pulse waves. Then the secondary pipe goes to the actual silencer(s) which should have minimal restriction.

In practice, there are numerous compromises due to cost / packaging/ noise levels/ cats/ emissions etc.

Ian Comerford wrote: ↑Wed Sep 24, 2025 12:21 pm Why, what are you planning Chris?

I drove a friends car this weekend...2.2e, Weber 40 IDA, MSD etc, essentially the same spec as my grey car, but his is fitted with a Dansk sport exhaust with smaller 50mm outlets. It sounds is a lot raspier (not that I am bothered about the noise), but it also felt a lot peppier at lower revs. I have been chasing an issue of bogging down at lower revs for awhile....almost stalling if flooring it from idle. My car is fitted with a Turbo Thomas 60mm dual outlet, "R-style" wide spacing. It looks and sounds great, but after chasing the bogging issue for awhile, carb adjustments, jets, emulsion tubes, fuel pressure, ignition, timing etc etc etc. I am wondering if the exhaust is too big or could be too open for the engine, or in reality would this not make any difference to the bogging down symptoms ? I updated the manifolds to Dansk afew years ago, from memory these a correct "smaller" size for the engine.


Cheers
Chris

Nine One One wrote:

neilbardsley wrote: ↑Wed Sep 24, 2025 1:35 pm I think it's quite specific to your engine?

I havent watched this but might be of help

https://youtu.be/pH9JbvuzS3w?si=r70LtQrxeAo0ta-a

Sent from my 22011119UY using Tapatalk

That’s just a sales pitch!

This is a bit more informative, and the physics of how combustion and back pressure works in an exhaust system.

https://youtu.be/jjPeP_Nn2B4

Basically the lower the back pressure the better...............


Mass flow rate is constant no matter what the pipe diameter is. Velocity changes, but mass flow rate (or volumetric flow rate) does not. Mass is a conserved quantity. Therefore, if one only considers back pressure, the biggest pipe possible is ideal. The edge case being no exhaust system at all has the best performance.

The second point he makes is that back pressure is almost irrelevant to designing a good exhaust. All things considered, you want to minimize it as much as possible. However, like any engineering problem, if it was as easy as putting a big pipe on, you wouldn’t need an engineer. Instead, an exhaust system must be designed to minimize wave interference of the exhaust pressure pulses.

Because the speed of sound is a function of pressure (I.e. it’s a function of density, and density is primarily a function of pressure as gas composition isn’t changing much), and pressure is primarily a function of pipe diameter, the diameter is extremely important. Then, the length of the pipe can be fine tuned to minimize interference.

The takeaway is that back pressure should be as close to zero as possible, but it’s effect on performance is minimal compared to good exhaust design.

edit I’ll add that, if someone is simply replacing a cat-back exhaust system, there is no reason to not use a big pipe. But at some point, there is no measurable gain from doing so. So, you should simply install whatever your preference is for looks and sound.

Yes. I tried to watch it and didn't finish.

I think you are saying that pipe length should be tuned to the engine? That is why most performance exhausts try to have equal length pipes into a single pipe?

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