Landracing Forum
Bonneville Salt Flats Discussion => Bonneville General Chat => Topic started by: frp on December 09, 2013, 08:30:02 PM
-
Back in 2011, I started what turned out to be an epic thread, when I asked about the benefits / liabilities of discharging large volumes of superhot exhaust gasses under the body of a sedan bodied car over 200 mph.
At the risk (or reward) of the same thing happening, I am very interested to know if it is legal to allow under-hood pressure to vent between the firewall and the hood by removing the rubber seal that runs across the back of the engine compartment at the firewall interface with the bottom of the hood?
And, assuming it is legal (since I can't find anywhere in the rule book that addresses this type of seal) What are likely to be the aero benefits, or penalties, involved with part of the hot under-hood air venting at the base of the windshield?
A lot of seemingly well informed folks shared on my previous question in 2011. I am hopping they are still on this forum - thanks. FRP
-
Yes you can remove the seal if you like.....
Lots of information on this site and other places on the effects. I think you'll find aero wise it's not the best path to remove the seal. Some teams go to extream efforts to seal it better.
-
Mike is correct. Removing the seal allows air to enter the engine bay from the windshield base and the drag numbers go up and the car loses down force. This has been proven over and over again in wind tunnel tests.
I think David talks about it in this article: http://www.hotrod.com/techarticles/body/113_0703_car_aerodynamics/
-
Thanks guys. Good info in the HR article.
Since it is expressly illegal to change the grill opening or do any kind of streamlining due to Production body rules, one is left with looking for legal ways of mitigating the drag forces generated by air being forced through the grill openings (and if the hood is well sealed at the rear as it is on my car) and being forced out below the car at the fire wall floor turn area.
The basis of my question is whether relieving some air out and over the car would lessen or increase overall drag. Intuitively I would think that a
car with generous grille openings (at over 250 MPH as in my car) would produce greater pressure in the engine compartment than that which would exist at the base of the wind-shield.
I wonder if someone knows if that intuition is true or false. I wish I were one of them there Aerodynamicists!
-
Airflow under the hood is managed on many modern cars. All of the air that comes through the radiator is subject to drag on its way over and around the engine. Air that passes through radiator baffle openings, such as headlamp bucket cutouts, A/C line openings, even the hood cable release hole...is all air that gets slowed down as it drags over underhood components. Simple engine covers are commonplace, these days, and have multiple benefits for MPG, component protection, and even (claimed) pedestrian safety.
Air pressure is higher at the base of the windshield, of course, but its also very high at those cutouts under the front bumper. That high pressure point is where the engineers have learned to manage the available pressure to aero advantage under the hood, as it lifts into lower pressure over the top of the engine.
Dont forget the openings into the front fender forward quarters. If those are open, inside the nose, more air volume is accepted through the grill opening (or under bumper inlet area). More air entering is more air slowed down by unstreamlined components.
Back in the mid-90s, I set up a demonstration for some visiting engineers, to help them understand why they were seeing so much damage to A/C condensors on a certain new luxury sedan. We had a line of near new vehicles, that were in company use on local freeways, for the demo.
We just took off the front wheels, pulled out the fender liners, and let all the collected stuff fall on the floor. The biggest "oohs and ahhs" came when 1/2 of a plastic Saturn hubcap fell out of the drivers side front fender of one car. That took some big airflow to bend and ram that piece of hubcap through the hole at the end of the radiator baffle.
Failure to manage that "after inlet" airflow was costing a lot of money and some efficiency. This is why motorcycles are such bad aero...too much lumpy stuff in too much airflow (and the older we get, the lumpier we get)!
JimL
PS: What I didnt say....venting the underhood will slow you down because it lets more air flow over more unstreamlined stuff. This is an issue that is carefully addressed on modern, high mpg cars. Go to a dealer, talk to the Service Manager, spend a little time looking at the bottom of the engine bays to see how much money the manufacturers are spending on this area of airflow control.
This is not a new concept...I remember seeing certain models, many years ago, with a large "heavy tar paper" type baffle just below the crank pulley area. I was told it was a lesson in how to keep the leaky front seals from blowing so much oil over the top of the engine and coating the bottom of the hood! Reduce the air blast getting in, and the oil just dripped off onto the ground where the customer didnt look (dont we always tell the wife, "that must have been somebody elses car that leaked where you were parked".)
-
starting with a "sedan" built for high speeds from the factory would be the way to fly.
for instance, mercedes 220 chassis "s" class
or a 211 "e" class, very slick, and has shutters that keep air out of the radiator.
they have a ton of work put towards managing airflow through and out of engine bay making them extremely stable at autobahn driving speeds. (155 gentlemens handshake speed limited)
or bigger bmw, 8 or 5 series.
all these cars are now 10-15 years old, and dirt cheap if they have issues.
-
Thats true about their aero, but the big Mercs, running in Production, are pretty tough to make enough power. First, you have to Time-Sert the blocks, and then the heads come loose anyway!...on a stock engine, no less. :x.
After that is solved, you still have a huge investment to get to the power capability of a good small block....power that can last 5 miles at a time. I sat in a meeting where a Factory effort Production class speed record attempt "idea" was discussed. When we explained how much power, for how long, would be needed...all the other sides engineers pushed themselves away from the table at arms length.
It was a funny sight to see! :lol: ...and I learned how to say, "Oh SH#€" in another language.
Thanks for bringing back that memory...made me smile this morning. :cheers:
Regards, JimL
-
in the 211 the e55amg was a supercharged 5.5liter and made 463hp (and 490ishin the 220 chassis) yes, it is a high dollar setup to start with from an amg standpoint, but production supercharged would work if that engine was used in a cheap high mileage e320.
the two blown engines we took apart at mercedes suffered from broken ring lands well before the heads ever came off, with a massive crank pulley and a deleted emissions tune. i imagine someone ran cheap gas in it without bleeding the air out the intercooler system, resulted in a piston in several pieces.
-
No problem. The C/PS record at Bonneville is 253.428 and the D/PS is 297.663 mph. :-o
-
hmm--- :-o so what you guys seem to be saying is there is no free passage of air 8-)