Hi all. This question goes out to those of you who have designed and flow tested heads/intakes/exhausts. I'm currently a member of FSAE at University of Central Florida. We design and build a single seat, open wheeled racecar for competition against other Universities. Engine size is limited to 610cc and we are using a 600cc Honda f4i bike engine. My question involves volume and flow of the intake. The rules dictate we must use a 20mm restrictor. Forced induction is allowed but this year we are keeping it NA. There are a few of us considering doing a senior design project on the intake... from throttle plate to engine. When we model and flow test it, what is more important, flow velocity or volume? Would a high-volume, low-flow plenum cause throttle response problems? Is there a point where increasing velocity hurts performance? Thanks.

 

pumping losses from restrictor plates usually make high velocity designs not "cost effective" because you spend a lot of energy increasing the velocity after the restriction and the return isn't worth the energy expended.

I'd concentrate on reducing the turbulence created by the restriction and then optimize for quality flow in your targeted powerband.

 

Thanks for the tip. We're definitely going to maximize laminar flow through the intake. The restrictor we've been using for the past few years has left a lot to be desired. The 'steps' from the cnc are easily visible and the sharp edges are not doing airflow any good.

 

To oversimplify, for all out power, high volume flow is what matters. For low end, flow velocity is what matters.

That being said, I'd put more emphasis on flow velocity, given the characteristics of the engine you are using and the type of course that you will be running on. The 4 cylinder Honda motors are certainly not lacking in power, even when restricted. What they do lack is torque. You can help this somewhat by gearing the car up (ours was geared to top out at ~110 in 6th gear). However, if you're gonna be shifting a lot, and good throttle response is crucial (so that it doesn't bog on shifts), not to mention, the wider you can make the power band, the better. You end up having to shift a LOT in these cars, given how close the ratios are (even more so if you gear it up like ours was), which can cost you time on an autocross course. IF you focus on flow velocity, get a wider powerband as a consequence, and can cut out a couple shifts, you'll keep the car more stable in the turns (which is really where you make your time on the autocross/endurance) and be on the gas for more of the course. I know we ended up dropping out of our best powerband a lot on the autocross course, just because its so much to process at once. To shift at the ideal points means SO many shifts, and its just too much input for the driver to handle. You might give up a little bit of time in the acceleration event, but given that the endurance event and autocross make up such a bigger part of the event, this is to your advantage as far as overall points.