How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

General engine tech -- Drag Racing to Circle Track

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How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by nicholastanguma »

Several years ago there were a few sets of twin port exhaust cylinder heads floating around in the vintage Royal Enfield community. The fellow responsible for creating them stated more than once they were an experiment, not necessarily a performance option. The end result was that they looked very pretty indeed, and afforded a unique exhaust sound, but were definitely NOT a performance step up. Also, the second exhaust pipe and silencer added about 30 lbs to the bike overall, so clearly that didn't help.

But could twin port exhaust be engineered into an actual performance upgrade on a vintage single cylinder's 2 valve head? Hugely downsize the header diameters to the point they provide proper gas velocity, match port flow to a hot carb, hot cam, and big valves, etc? Let's say for the sake of simplicity in this hypothetical scenario that a second exhaust pipe would magically provide no extra weight penalty.

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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by englertracing »

Area is Pi R squared.

There will be much more resistance to flow in two pipes than one of equal area because the dual pipe setup has alot more surface area in contact with the exhaust gas
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by miniv8 »

Supply and demand... Supercharge it..
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by Tuner »

Consider the possibility of a wider power band if the pipes are different length.
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by mag2555 »

Can you post up a picture of the chamber side?
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by BLSTIC »

I don't it's a viable option. There would be a lot more surface area in the head (increasing heat absorption) and the only "gain" you'd get would be from having multiple tuned lengths. That's also not really a gain because all it means is that you're only half right at each tuning point.

However the psychological part of it, with different tuned lengths you can make an engine that sounds like a big single at idle and a twin at high rpm, would be pretty cool in a race.

The concept of a collector on a single cylinder though, is intriguing...
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by Tuner »

BLSTIC wrote: Fri Apr 16, 2021 6:51 pm The concept of a collector on a single cylinder though, is intriguing...
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by nicholastanguma »

mag2555 wrote: Fri Apr 16, 2021 6:47 pm Can you post up a picture of the chamber side?

Sorry, no; I simply haven't any more photos of this setup, and haven't located any more online, either.
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by dhidaka »

Honda uses dual exhaust ports on single cylinder XR500 and XR650 4-valve engines. Never figured out why other than to fit on the bike frame.
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by dannobee »

They likely couldn't keep it cool. That's why Porsche never went to 4 valves on the air cooled engines. And the early 4 valve air cooled bikes had similar cooling problems.
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by nicholastanguma »

dhidaka wrote: Fri Apr 16, 2021 8:46 pm Honda uses dual exhaust ports on single cylinder XR500 and XR650 4-valve engines. Never figured out why other than to fit on the bike frame.

They've been doing that for decades, with displacements ranging from 250 - 350- 400 cubic centimeters in addition to the big singles. Yamaha did it with a few models, too. I've always liked the aesthetic for sure.
Last edited by nicholastanguma on Fri Apr 16, 2021 11:02 pm, edited 2 times in total.
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by nicholastanguma »

dannobee wrote: Fri Apr 16, 2021 9:28 pm They likely couldn't keep it cool. That's why Porsche never went to 4 valves on the air cooled engines. And the early 4 valve air cooled bikes had similar cooling problems.,

That seems to make sense. However, what doesn't seem to make sense is that other air cooled four valve designs don't suffer any overheating problems. For instance, the Suzuki DR650 and 800, the Suzuki LS650, the Yamaha XT600 - 660, the new Harley-Davidson Milwaukee-Eight Vtwin, and the Polopolous Porsche flat 4 are all in continuous use without blowing up or melting down. I could go on with many other examples. What's going one here?
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by modok »

It is usually best to keep the exhaust ports as short as possible on aircooled heads. Why? the exhaust port getting hot can warp the head. The header getting hot does not warp the head.
In the case of the honda xl500, or especially the later radial valve arrangement, combining the ports IN the head would result in a longer/larger exhaust port.
Two exhaust ports might be less thermally efficient, but two smaller overheated areas is less head warping problems than one bigger one.
The two pipes look cool, and that's good, If you put in two exhaust valves it would run even better.
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by Momus »

modok wrote: Sat Apr 17, 2021 12:05 am It is usually best to keep the exhaust ports as short as possible on aircooled heads. Why? the exhaust port getting hot can warp the head. The header getting hot does not warp the head.
In the case of the honda xl500, or especially the later radial valve arrangement, combining the ports IN the head would result in a longer/larger exhaust port.
Two exhaust ports might be less thermally efficient, but two smaller overheated areas is less head warping problems than one bigger one.
The two pipes look cool, and that's good, If you put in two exhaust valves it would run even better.
Two pipes look cool?

To me they look like unnecessary duplication ie an engineering fail for a a host of reasons; many out lined above.

Those Enfields barely make enough power to boil an egg. That 2 port set up would rank as one of the silliest things I've seen given there is a potential market for a good modernised head.

I'd be looking at making one efficient port and pipe and having the second as a wanka.
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Re: How Can Dual Exhaust Ports Be Made Into A Genuine Performance Option For 2 Valve Heads?

Post by modok »

biological organisms prefer symmetry.
It may come as a shock to you, that you are one :lol:
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