There is a significant difference between the diffuser on the DBS and the Camaro. The Aston Martin's diffuser is preceeded by a very flat and smooth undertray, and the diffuser -- while not quite as severe or as large as that on the 430 Scuderia, for example -- does actually help smooth the airflow transition.
Hm. Now, the Scuderia, I couldn't comment on, I've not got under the back of one. Come to think of it, I'm (we're) making huge assumptions about the Camaro.
However, the DBS, I can comment on. It does have undertrays, yes. I'm not entirely certain this is relevant. The rear valance is not a particularly effective diffusing element, and nor am I aware that it was designed to be one. In order to achieve what a racecar diffuser achieves, there has to be a very carefully calculated (and computationally modelled) form which allows the air to slow down without separation, thus lowering pressure under the rear of the car and creating a net downforce. There's a big gap after the flat undertray where the rear subframe is, which has a polypropylene cover which lots of venting in it to attempt to manage the hot air above it created by the transmission and air passing backwards down the 'spine' of the car from engine bay and the closely channelled exhaust system. Part of the rear near the rear valance (I'm fairly certain) has a big void and some tunnels for the transmission cooler, but I'll check tomorrow.
However, since you're interested, I'll ask the guys (uh, actually, might be girl) in aero about the aerodyanamic nature of the valance and see what she/they say.
The Camaro, on the other hand, has no such undertray, which means the 'diffuser' obviously has nothing to attach to. It ends up protruding into the airflow below the car. A bumper that does this slows air speed, increasing dynamic pressure, which in turn increases lift from under the car. This results in true irony: the element designed to imply positive rear downforce ends up creating its opposite.
At risk of repeating myself, no, seriously:
Iincreasing drag? I'd like to see the evidence.
Um, common sense? A keen eye? A little knowledge of basic aerodynamics?
I don't think we need full CFD to think this one through.
I beg to differ. McLaren's 1976 F1 car (the MP19, I think) turned from a perfectly balanced race car into a complete pig midway through the season. No-one had any idea why until someone realised that as part of the engine cooling development, they had repositioned some oil coolers, moving them by 6 inches. This was enough to wreck the aerodynamic balance of the car. They moved them to their original position, and the car's sweet handling was restored.
Airflow is nine parts scientific study and one part black magic. You have to do the modelling to see what happens, and very often what you see in the tunnel isn't what you get in the field.
In the early- to mid-sixties, GM were in a lawsuit nightmare brought about by Ralph Nader and the Corvair. GM made it their goal to become the best chassis, suspension, tyre dynamics and aerodynamics people in the business, and gave a small team of engineers and 'car-guy' lawyers a blank chequebook and as much experimental time and Milford Proving Ground resource as they needed. Aside from completely exonerating the Corvair in 'normal driving circumstances' (and an admittance that under certain 'emergency' manoeuvers, the car could be made to 'tuck-under') one of the corollaries of this was, coincidentally, the original 1967 Camaro development including some of the earliest 'modern' computational dynamics models, for fatigue stress analysis, suspension design, and aerodynamics. There wasn't much they could do to change the outcome, as the development programme was so rushed to chase down the Mustang, but GM poured cash into the analysis anyway - they didn't want a total pup on their hands. As it happens, the shape was remarkably balanced and aerodyanmically sound - the result a happy success. (They never really fixed the axle tramp though, that's one the computers didn't spot.
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Development on aerodynamic parts for the Camaro during this time also helped the Penske Team beat the Mustangs in the TransAm championships - due to an understanding of applied aerodynamics, suspenion ride & handling and tyre research. As well as a bit of luck and some vulpine cunning...
Another corollary was the extension of thie aerodynamic knowledge to the forfront of racing - something else happening within the umbrella of GM at the time was a Gran Turismo friend of ours, the Chaparral project. These cars were absolutely pioneering in the shaping of modern Le Mans prototype sportscars and application of aerodynamic aids. Not even Formula One were up to their level, and they watched the Chaparrals running and racing with awe and disregard for the science and knowledge - hence some pretty shoddy copies which subsequently got aero aids banned in their series. Chaparral's experimental nature, some trial and error along with some very innovative yet simple data gathering techniques, meant they were years ahead of the competition - and this knowledge found its way back to GM via Corvette and other more prosaic everyday cars including the Camaro. This team assisted in the Corvair investigation, making the language of 'oversteer' and 'understeer' so simple, even the US Government understood by the end of it.
We've digressed a tiny bit here. But my point is this: aerodynamics (or any vehicle dynamics, come to think of it) can't be done by armchair observation.
So, where is the evidence backing your original claim?
) is the 420RE, which actually has 430hp and 385 ft-lbs torque (thanks to a Roush roots-type blower) but no IRS, just a stiffened LRA, not to mention a Tremec TR3650 5-speed gearbox with a Roush quick shift.
