Cowl side of hood "flutters dramatically" at very high speeds.
 

Hey guys, I've been real busy with personal shite, therefore the lack of visitations to this webshite.

I had a member ask me about anything special tunewise we need to do for extended high speed runnning. As you know I'm a crazy lunatic and have had my car between 120 and 146mph for even 20 miles at a time. I'll write in a second my response to this individual but I noticed on a couple occasions that the COWL side of the hood sort of "flutters" above speeds of 125mph or so. I mean to the point it seems the hood might fly off the car. You may know I've had the entire front clip off of my car so I'm trying to determine if I messed something up, or that this flutter is an aerodynamic anomaly with these cars at high speeds. Anyone else notice this? Anyway I did back the little rubber "bushings" under the hood to their maximum height. This kind of holds the hood against the latch harder when you close the hood. Seems to have helped but have not had a high speed run in a while.

So, has anyone else noticed this phenomenon?



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The other poster's PM read: While reading your posts in the brownies 4th gear log, i noticed you said you would rather have a car that would run for 2 minutes at 125 mph and not knock. I to am looking for that. What have you seen while tuning that I should take into consideration when tuning for long term(freeway racing) high loads? your posts ahve been very helpful also. thanks!

I responded:
Hey... to answer your question, I tune nothing special... just can't tune it to run on the ragged edge of performance. Then it can withstand high speeds for extended periods. I've noticed that on "runs" like this the BATs stay very cool (maybe 10* F above ambient) when running that hard indicating *the stock intercooler is fine for this kind of duty*. I still shift at 6k on the nose and have not been able (either I pussied out or ran out of room) to push 6th gear past 6krpms. In ATR, you'll notice under the Boost tables that in 5th and 6th, Cobb has set the boost targets to 0.95 (in other words 95%) I suppose to leave a little margin by dialing back the boost 1 psi or so. Anyway, nothing special, just run a medium-hot tune (as I call it) and everything will be fine. I would not recommend extended high speed running without the CDFP upgrade however.

I think the very nature of our cars (cam profiles made for low-end torque, K04 breathless above 6krpms, torque dies as rpms rise about 3500rpms, etc) kind of inherently protect the engine from problems with extended high speed running.

Feel free to share this info on the boards... I have been super busy and unable to really browse much often.

Let me know if you notice the cowl edge of your hood (windshield side) starts to "flutter" above 130mph. Mine does. Must be an aerodynamic anomaly, not sure. I did open the hood and extend the little rubber thingies a half turn to try and "hold the hood more firmly against the latch" so to speak. I might make a post about this.

Cheers.

-- End of my response to the PMer.

<--- Not concerned. My whole hood shakes.

To all y'all aerodynamics get real weird at these kinds of speeds on any car. I mean this car is glued to the road but there's gotta be some relationship between 145mph worth of air coming in that hood scoop, slamming through the intercooler, and then probably spreading in 360 degrees of direction as it slams into the camshaft cover. A simple explanation could be that hood scoop at high speed lets in more air than can be expelled through bottom of car, so it exits at cowl regardless of amount of "cowl seal." If you've ever been under these cars you'd know that Mazda put a little bit of thought into the high speed aerodynamics but still this is no Mercedes-Benz, BMW, or Audi. Any thoughts? (This is just a long winded paragraph for a bump lol).

My thoughts are that there is too much pressure coming in through the scoop for air to flow back out, so it tries to escape by any means necessary, hence the shaking hood.

My thoughts exactly. Mazda NOT IN ('M-B', 'Audi', 'BMW')