[nicholastanguma]

Set aside the packaging problems normally associated with having to mount the carb and air filter so high up in a downdraft head configuration, and instead just focus on the performance aspect.

In the 80s and 90s, when the Sound of Singles racing series was in its glorious air cooled, carbureted heyday, as far as I know the hottest engines were the Rotax and Honda 4 valvers using custom downdraft heads (sometimes dual carbed, no less).

Not necessarily downdraft carbs, mind you, but downdraft heads.

Just how much better for performance is a downdraught head vs a standard horizontal head?


350cc Rotax with a single carb downdraught head




675cc Rotax with a dual carb downdraught head




VS


Standard horizontal sidedraft head

[AzCal Retred]

Here's a downdraft design by the tuning fork folks. The boys from Hamamatsu used near vertical intake tracts and a "laid down" cylinder configuration to make very nice power and achieve a low CG engine design. A swiss-cheese 5-valve head moved a lot of air in and out quickly, short stroke wide bore cylinders allowed for generous RPM range. ANd check out the snow machine hardware too.

https://en.wikipedia.org/wiki/Yamaha_Genesis_engine
The DOHC Genesis engine has five valves per cylinder and downdraft carburetors. Yamaha adopted the 5-valve concept, using three intake valves and two exhaust valves per cylinder, because it allowed both excellent volumetric efficiency and high rpm. As designers began to appreciate the benefits of multi-valve layout, 4-valves-per-cylinder (and even 3-valves) became common. In a multivalve engine, maximum rpm is limited by the size of the largest valves: in a 3-valver, the exhaust valve is larger, and in a 4-valver, the inlet valves are larger. The significance of the five-valve engine compared to other multivalve configurations engines is that (like a 3-valver) there were more intake valves than exhaust ones; yet all valves were small and of very similar sizes and weights.

Downdraft carburetors were adopted for improved gas flow, but, for motorcycles, a disadvantage was that the intake airbox was above the engine, taking up space from the fuel tank, rather than being sited conventionally behind the engine.

The Genesis engine first appeared on the 1984 Yamaha FZ750 and was later used on a variety of motorcycles for different kinds of markets. The design was used for parallel-twins (such as the TDM and the TRX) as well as for four-cylinder models. Both fours and twins had the cylinder block slanted forwards 45 degrees. The Genesis engine was also used in the Yamaha YZF-R1, FZX700, FZ750, and the USA-only FZ700.

Other applications ranged from the Supersport YZF-R6 and YZF-R1 models using electronic fuel injection with YCCT and YCCI to the less extreme but still powerful Yamaha FZ6 (4 valve per cylinder) and FZ1 Fazer line which had a simpler fuel injection designs but was essentially the same basic engine design. The most significant changes made were the 4-valve per cylinder configuration and the crossplane crankshaft found on the YZF-R1 2009, along with the change of cylinder sleeves to steel for high precision and high temperature durability. Throughout its long period of production, the Genesis range has successfully provided performance and reliability.

Because of its high performance capacity (usually from 100 BHP - 350 BHP), the engine has also been employed in various other uses such as gyroplanes and high power marine engines, making it an affordable solution for a high performance applications. The engine is also used in a racecar championship in Norway called SevenRacing. It is a copy of Lotus Super Seven, and is produced by MK and Stuart Taylor.


https://global.yamaha-motor.com/stories/history/stories/0022.html

https://global.yamaha-motor.com/stories/history/stories/0004.html

https://global.yamaha-motor.com/business/4st-snowmobiles-10th-anniv/edition3/feature_002/
https://en.wikipedia.org/wiki/Yamaha_Phazer
Yamaha released a bold new snowmobile under the Phazer name-plate for the 2007 model year. [1] The snowmobile featured a radical new design which is inspired from the YZ250F motocross bike. The new Phazer is powered by a new 80hp 499cc fuel-injected liquid-cooled four-stroke twin which makes its peak power at 11,000RPM. The engine is based on Yamaha's highly successful and dependable YZ250F dirt bike engine. With a 487 pound estimated dry weight, it is one of the lightest production four-strokes.

[nicholastanguma]

Great post, thanks, thumbs up!

[ace.cafe]

Downdraft is generally better.
It typically gives a straighter port, which should flow more air.
It also has the advantage that any poorly atomized fuel droplets(if any) will drop right into the cylinder, not on the port floor.

Even on sidedraft ports, it is common practice in performance headwork to raise the port as high as possible, even if only 1/8" higher. Any higher port angle will typically yield benefits if everything else is done to suit it.

[nicholastanguma]

Downdraft is generally better.
It typically gives a straighter port, which should flow more air.
It also has the advantage that any poorly atomized fuel droplets(if any) will drop right into the cylinder, not on the port floor.

Even on sidedraft ports, it is common practice in performance headwork to raise the port as high as possible, even if only 1/8" higher. Any higher port angle will typically yield benefits if everything else is done to suit it.

Exactly the kind of info I was looking for.  Thanks, Tom.

[Richard230]

My 2002 Yamaha FZ1 uses a 5-valve head, but with horizontal-intake carburetors. I checked the valve adjustments at 28K miles and they were all within spec, although the really little 5th-column intake valves were just barely.  I left them alone as they looked pretty much impossible to access to replace their shims without taking the engine apart. The 5-valve Yamaha engine really runs well. It pulls smoothly from 2K rpm and gets really fast above 8K rpm up to its conservative 11.5K redline. A really great engine, but I don't think Yamaha uses that design any more. Likely due to cost.

[gizzo]

My 2002 Yamaha FZ1 uses a 5-valve head, but with horizontal-intake carburetors. I checked the valve adjustments at 28K miles and they were all within spec, although the really little 5th-column intake valves were just barely.  I left them alone as they looked pretty much impossible to access to replace their shims without taking the engine apart. The 5-valve Yamaha engine really runs well. It pulls smoothly from 2K rpm and gets really fast above 8K rpm up to its conservative 11.5K redline. A really great engine, but I don't think Yamaha uses that design any more. Likely due to cost.

My 1998 TRX850, also. 5valve P twin,  only revved to about 8k, not 11.5 like your 4cyl. Damn that was a good bike.

[TrianglePete]

Anything to straighten the short side

and keep high velocity.

Higher velocity  lower the pressure

Better vaporization   cleaner burn   Higher  HP

[NVDucati]

Downdraft is generally better.
It typically gives a straighter port, which should flow more air.
It also has the advantage that any poorly atomized fuel droplets(if any) will drop right into the cylinder, not on the port floor.

Even on sidedraft ports, it is common practice in performance headwork to raise the port as high as possible, even if only 1/8" higher. Any higher port angle will typically yield benefits if everything else is done to suit it.

Tom, have you ever played with Coanda theory operated air entrainment (amplifiers)? I have been fascinated with the notion for a long time. They are used in industrial factories and medical compressed air systems. I considered while converting a small aircraft engine to propane but the FAA certification was out of reach for that project.
   There is a Canadian company which sells a fixed Coanda body in small sizes. They are dealing mostly with compressed air but the principle is the same for vacuum. There is a video part way down the page for folks who are curious.
https://www.nexflow.com/products/energy-efficient-blow-off-products/air-amplifiers/standard-fixed-air-amplifiers/

[ace.cafe]

Tom, have you ever played with Coanda theory operated air entrainment (amplifiers)? I have been fascinated with the notion for a long time. They are used in industrial factories and medical compressed air systems. I considered while converting a small aircraft engine to propane but the FAA certification was out of reach for that project.
   There is a Canadian company which sells a fixed Coanda body in small sizes. They are dealing mostly with compressed air but the principle is the same for vacuum. There is a video part way down the page for folks who are curious.
https://www.nexflow.com/products/energy-efficient-blow-off-products/air-amplifiers/standard-fixed-air-amplifiers/

I am familiar with the coanda effect, but I don't know of anyone using it in any automotive application. It is interesting, though. Basically the same idea Dyson fan uses.

[Arschloch]

Looks like a venturi.