Principles of exhaust design!

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NissanJon
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Principles of exhaust design!

Post by NissanJon »

Greetings gearheads,

I have been in correspondence with Calvin Elston in the not so distance past to gain a deeper understanding of exhaust design. He pointed almost confusing details of exhaust behavior and considerations.

I wanted to poll the group about an general 'Ideal' design. What are the primary goals in making a header and pipe design? 🤔 How would you word the parameters? As short as possible? Balance the ratio between size and length? The choke must be 15% smaller than the port? I'm making those up because I honestly don't know. When I look at designs I can't tell the difference between a poor design and a great design. If I want to improve my own exhaust I wouldn't even know where to start! Imagine if I were 10 years old, how would you communicate to me the ideal design?!😄
rebelrouser
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Re: Principles of exhaust design!

Post by rebelrouser »

I am not an expert, just built a few sets of headers. Mainly because I want to be able to change spark plugs and the starter with out pulling the engine half way out, so I built my own.
Not getting into physics, but buy a copy of pipemax, and let that software do the work for you. I usually start with a tube at the flange approximately the same same diameter of the port coming out of the head and from what little testing I have done a step header will out perform a straight diameter header, and on the cars I have drag raced a tri-Y type collector seems to work better as well. Again most of my drag cars ran mid 9's to low 10's and weighed around 3,600 to 3,200 lbs.
But the pipemax program will calculate tube lengths and step lengths and diameters, and give you resonance dimensions. The other thing is the skill in cutting bends and welding so you have long sweeping bends and smooth inside welds. As well as getting all that packed into a engine compartment.
Never did a bunch of back to back testing, but almost everyone comments on how good the headers I built sound, and the couple cars that I did where the headers were the major change, they went a little faster. And the exhaust system from my experience is small gains, not huge ones anyway.
David Redszus
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Re: Principles of exhaust design!

Post by David Redszus »

NissanJon wrote: Tue Jul 27, 2021 4:55 pm Greetings gearheads,

I have been in correspondence with Calvin Elston in the not so distance past to gain a deeper understanding of exhaust design. He pointed almost confusing details of exhaust behavior and considerations.

I wanted to poll the group about an general 'Ideal' design. What are the primary goals in making a header and pipe design? 🤔 How would you word the parameters? As short as possible? Balance the ratio between size and length? The choke must be 15% smaller than the port? I'm making those up because I honestly don't know. When I look at designs I can't tell the difference between a poor design and a great design. If I want to improve my own exhaust I wouldn't even know where to start! Imagine if I were 10 years old, how would you communicate to me the ideal design?!😄
Start by reading Dr. Gordon Blair's book on four stroke engines. Try to understand the principles of gas dynamics first.
Everything else follows from that starting point.
BLSTIC
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Re: Principles of exhaust design!

Post by BLSTIC »

Also a complete amateur but with lots of theory.

A lot of exhaust tuning comes down to "do no harm".

Keeping joining pipes at least [exhaust duration] degrees apart avoids blowdown interference (blowdown pulse messing with overlap when exhaust valve is still open) but on a cross-plane V8 that's kinda hard and I'm pretty sure that problem hasn't actually been solved yet in a header that fits in a street car. Ideal is to keep them grouped apart and joined by pipes long enough that the pulse can't make it in time at the desired rpm band. That math is a little past me at this point. But pipemax will probably do it for you (I don't have pipemax, but I hear it's *really good*).

Also remember that any pipe length you build is ideal for one rpm and exactly wrong for another. For a narrow powerband this is fine, make them the best 'right' length that will fit (there are multiple). For a wide powerband, you can't get optimal everywhere on all cylinders.

Regarding diameters, remember that velocity is a thing and will fight reversion and help with the blowdown pulses (if the gas speed is high any pulse coming back up the pipe moves that much slower), but a pipe too small will become restrictive and leave you with pressure in the exhaust port on overlap. My understanding is that gas expands as it leaves the cylinder and in order to keep the velocity down (to keep wall friction to a sane level and also to avoid putting too much energy into the gas for no gain, because energy is MV^2 and all that) the pipe needs to expand, which is why I think stepped headers work.
MetricMuscle
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Re: Principles of exhaust design!

Post by MetricMuscle »

NissanJon wrote: Tue Jul 27, 2021 4:55 pm Greetings gearheads,

I have been in correspondence with Calvin Elston in the not so distance past to gain a deeper understanding of exhaust design. He pointed almost confusing details of exhaust behavior and considerations.

I wanted to poll the group about an general 'Ideal' design. What are the primary goals in making a header and pipe design? 🤔 How would you word the parameters? As short as possible? Balance the ratio between size and length? The choke must be 15% smaller than the port? I'm making those up because I honestly don't know. When I look at designs I can't tell the difference between a poor design and a great design. If I want to improve my own exhaust I wouldn't even know where to start! Imagine if I were 10 years old, how would you communicate to me the ideal design?!😄
Calvin Elston is the Chuck Norris of exhaust systems! HIs experience with regard to what is most important is fantastic, probably what has influenced how Exhaust Design programs calculate data. We should pay attention to the details he worries about.

The more I research and dabble with exhaust design and fabrication, the more I realize the limitations, mostly determined by engine and vehicle design but also my willingness. What I have had to do lately is prioritize the design aspects which are most important if all are not attainable. Quite often I'll read a post where someone is trying to compensate for a particular issue an engine has which isn't something I need to worry about. From what I've gathered which pertains to all IC engines:

-Collector length, Primary length, Collector diameter, Primary diameter is the order of importance but it is best to keep primary diameter small to start with, sometimes even smaller than the port diameter. Resist the urge to go larger than your engine needs, no gains to be had.

-Stepping the primary diameter up at the proper lengths.

So, NissanJon, what kinda vehicle/engine are you working on?

I have an INFINITI FX50S with the VK50VE engine. The exhaust manifolds are painful to look at but they may work better than they appear. The rest of the system is pretty good but I'm working on a cutout so I can enjoy the best of both worlds. Headers will come later.
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