Tuning the C20LET - RAM air history and facts

Ram-air theory has been heavily researched in manufacturers' R&D departments. Just think where F1 cars have their air intake !

In the motorcycle world, Kawasaki was the first manufacturer to 'reinvent' the RAM-air setup, with the 1990 ZZR1100. The bike was rated at 145bhp on a dyno, and rumours had it producing 150-170bhp at full speed (175+mph on the clocks). The growling from the intake and the ease of revving (compared to it's predecessors the ZX10 and the 1000RX) made it FEEL like it was kinda supercharged.

Many tried to measure the 'supercharge effect' within the pressurised airbox, all were unsuccessful, myself included. It's hard to rig a sensitive manometer on a bike doing 150mhp+, it really is!
Others tried it on a dyno, with fans blowing through the intake. The power differences were small, and the pressurisation not worth mentioning (if at ALL measurable!)

So what's the story, was RAM-air a gimmick, or a black art?

The next version of the ZZR1100 had a twin-RAMair setup, and despite this the power quotes remained unchanged. Weird.
Other manufacturers followed, Honda with the CBR600, then Suzuki. Kawasaki introduced RAMair in all it's latest sporty models and still no-one (outside the factory) really knew what sort of gains were being achieved. Typical RamAir systems nowadays are below (or around) the headlights and they look like this:

Then 3-4 years ago Kawasaki announced the ZX-12, a long-awaited monster that was rumoured to be gaining at least 20bhp from an optimised RAM-air setup, the best ever. The intake snout protruded quite a bit at the front, making it ugly by many people's standards (and losing sales in the process). Kawasaki admitted that the aircraft engineers that designed the system wanted the snout to protrude more than an extra inch, but the marketing department wanted it flush with the bodywork.
A compromise was reached, but it was sub-optimal none the less. The airbox was right behind the intake, forcing the frame to be sculpted around it(!). You can see the intake protruding right underneath the headlights:

Also the whole engine is tilted so that the intake ports are straight behind the 'intake line'.
Despite all this 'natural supercharging' etc. the bike could not beat it's main rival, the 1300cc Hayabusa, so the myth of RAM-air deflated a bit. To it's credit, the Hayabusa has 100cc more, AND a ramair intake, though it's nowhere near as efficient as the Kawasaki's.

By now private research had come up with figures, quite unimpressive for some:
No OEM pressurised airbox could manage a single psi of 'boost' even at top speed.
The best ones would max out at less than 0.5 psi, with 0.2 psi more common at around 100mph.

So is THAT it? 0.2psi? big deal

But look carefully and let's see what that is compared to:
A non-ramair airbox breathes from the side or a low-pressure area, which is probably also affected by engine heat.
A ramair box is stuffed with fresh air exactly at ambient temperature. So what's the difference then?

As a rule of thumb, for every 4C that air temp goes up, 1% of power is lost. It's easy to have the airbox temp go up by 4C, sometimes a lot more. A ram-air box is always cooler.
More importantly, a non-pressurised airbox is typically under vacuum, and under load at high revs that is several psi worth of vacuum.
People think that it's atmospheric pressure in there, but it isn't. Just calculate how many liters of air are demanded to flow when the throttle is wide open, and you'll realise that the airbox is empty within milliseconds. It takes time to replenish these reserves, typically almost a second (with the throttle closed). If the throttle is kept wide open, the 'replenishment' rate is far lower than the engine's air consumption, so the airbox remains in deep vacuum all through the acceleration process. The higher the engine speed, the deeper the vacuum.

That's where a RAMair intake comes in. It doesn't pressurise more than a few tenths of a psi during cruise conditions, but when the throttle is whacked open, the vacuum appears a bit later - maybe a second later. That feels like better throttle response, because that's what it is. Even then the vacuum is always lower, because now the replenishment rate is much closer to the engine's consumption rate.

It's under these dynamic conditions that RAMair shows it's teeth, but that's near impossible to replicate on a dyno. Natural 150mph airflow cannot be replicated by fans, even big ducted ones. Natural airflow isn't preheated, either, unlike fans.

When this replenishment rate is combined with the lowest possible intake temperatures, 5% power increases are typical, with over 10% in some cases. The smaller the airbox (compared to the engine's bhp) the easier it is to feel these gains, and at 100mph part-throttle response can be much improved because the airbox won't be in deep vacuum at all.

Readers' feedback

Yes, there are people who actually read this stuff and take the time to write back! Here is the view of Spyros Moraitis, an automotive engineer who knows a couple of things on the subject:

The airbox does not contribute to pressure drops other than it increases friction area and it transforms kinetic energy into piezometric energy and back. That requires some work done. This work will be done by the engine but the energy losses can be gained back, as i already described, by the kinetic energy of the air if the induction system gets a flow with an increased velocity.
The pressure in the airbox never drops as long as its inlet has sufficient cross sectional area to allow for a sufficient flow rate.
In summary, the airbox contributes to losses by : 1) Transforming energy 2) Friction/Viscous losses.

I think that Spyros is correct, but he also discards pressure losses because theoretically they shouldn't be there. But theoretically the volume of the LET airbox should be around 20litres, which I assure you it isn't.
So these losses will add to those by energy transformation, friction and viscosity. Yet more reasons to strive for a decent RAM-air intake.

 

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RamAir measurements and ideas from PlanetSoarer Local copy here (page1 page2 page3 page4 page5 page6 page7)