Time to look under the cover of tuning a Big Air system. Beginning of the year 2009, shortely after the 2009 VMax had arrived we tested various kinds of Big Air configurations to find the one worthy for the VMax. 

Now that we have access to the new leader in exhausts: Akrapovic - we re-ran a handful of tests - some 150 runs - to re-validate if the exhaust would have a significant impact on the Big Air Design. Long story short - The Monster Big Air still trumps all other designs we did by a significant margin. 

Why do a Big Air to start with? For racing purposes we want to increase the volumetric efficiency of an engine to the point where maximum power is produced in the power band of choice. In our case we are looking for peak power between 8,000 and 10,000 rpm - without giving up too much in other areas.

To start with, we made various velocity stack designs - very traditional design with the stack length tuned to a certain rpm for peak torque. In order to do so we picked the largest possible individual filters from K&N we could incorporate into our design, so all variations could use identical filters. Of some 20 versions made all work with the same KN 3710s and once he filters are on, you cannot tell the difference in terms of looks. It all happens inside the filters.

The velocity stack design with tube lengths varying from extremely short to very long, allow for tuned applications at various engine speeds. We have hundreds of charts in our library that detail changes cause by varying dimensions.

In addition we also tested different size tubes front and rear as we expected that the shorter rear exhaust headers might work well with different length tubes. For the velocity stack design we found that this worked best in most cases. The rear tubes being ~25mm shorter than the front tubes produced more power and a broader torque band than equal length velocity tubes. 

On all the charts the blue line represents the final Monster Big Air design with Filters. The dotted red line is a stock VMax with TopSpeed so it revs to 10,000 rpm in 4th and 5th gear.

The longest tubes we made clearly made the most torque around 6,000 rpm, with some significant bumps over stock. However no matter how short we went with the velocity tube design,  all of them fell off at higher rpms. Riding the bike like that made for some fun torque in the middle of the rpm band, but accelerating hard till redline did not feel nearly as strong. Main reason: Torque falling off too rapidly, making runs past 8,000 not as crisp as we liked them.

In addition to the velocity stack design we also made a couple sets of venturi stacks - the ones used for the Monster Big Air. The biggest difference between traditional velocity stacks is the more complex venturi design of the stack, that promotes increase acceleration of air.

While the venturi stacks did not deliver as must mid range boost, they clearly dominate the high end power delivery. While the velocity stacks maxed out at 190-191 rwhps, the venturi stacks went past 195 SAE or 200 STD at the wheel. 

Also very interesting to note: Below 5,000 rpm all designs perform virtually identical, only the stock bike makes a touch more at just 3,000 rpm but gets left in the dust +-250 rpm from there. The reason might not be obvious but its there: YCCT - Yamaha's throttle by wire system. It limits the throttle opening to 34% and climbing at lower rpms. It goes wide open at just under 6,000 rpm. Without that YCCI - variable intake stacks, that creates a small little dip on most curves around 6,500 rpm. It can be lifted by more restrictive velocity stacks at the cost of top end power - or you wait a little more for an electronic solution for this dip and more torque overall - we will be starting testing in the upcoming weeks.

All in all we found that the venturi stack design gained 4-5 rwhps over any velocity stack combination we tested. 

What surprised us a little was the fact that the venturi design worked better with filters installed than without. While topend power is virtually identical, the Big Air improves torque by using the recommended RU3710 filters compared to no filters. Part of that is the design of the stacks that leverage the radiused inside of the filters combined with the design of the symmetrical round filters that promote better symmetrical airflow.

And more charts in SAE:

As well as raw numbers for those that want to know what is going on under the cover:

All runs graphed here have been performed on a 2009 VMax, with Stock rear tire, Fully Synthetic Yamalube oil, TopSpeed and EXUP eliminator, AIS blocked off, 91 Octane pump gas and on the same day, with very constant conditions.

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