chaase

Water hitting the engine will have no effect on it. The engines on most cars gets pummeled by water constantly on rainy days. It isn't going to harm a thing.

Solofast

First of all a couple of misconceptions need to be addressed here.

One is that ET is absolutey wrong when he uses his bucket analogy in saying that there is no advantage of an engine inlet ram pressure recovery. You absolutely do get benefit from ram air at the speeds we are talking about here, and, the amount you get can be easily quantified. A ram air inlet that is forward facing and captures the total pressure (what you feel with your hand out the window) created by the car moving thru the air at 150 mph will have a total pressure that is 1.034 times that of the air that would be sucked from an inlet that was not forward facing. If you assume that you have an normal atmosphere (14.7 psia) the inlet pressure goes up to 15.053 psi. That is not a lot of pressure increase, but it nets (when other factors are thrown in that I am not bothering with here) about a 10 hp increase (assuming a 400hp engine). You can look it up in the One Dimensional Isentropic Compressible Flow Function for a mach number of 0.22, which is about 150 mph and see the numbers if you are so inclined. No it is not a lot of power, but it is free horsepower. If you don't caputure the pressure you don't get it. As an aside at 300 mph the pressure increase is 14.2% and the air density gain is 10%. Not bad for a freebie, easy to see why most all normally aspriated racing engines use a forward facing scoop.

Another very common misconception is that venting the hood reduces drag. In most cases it will increase drag. What venting the hood does is lower the pressure under the hood (which is good for more downforce too) and thereby increases the flow thru the radiator. Both of these are good things if you are on a road course and want downforce and more cooling. If you are running for top speed however, it is more beneficial to have the hood not vented, since all of the air going thru the radiator creates drag if you don't reaccelerate it at the end of the passage (which we don't do, we dump it under the car). The more air passing thru the radiator the more drag you get. The racing cars are using a totally different concept and are dumping the air out the top, but because of our radiator location and configuration, just cutting a hole in the hood isn't going to help the drag issue at all.

I have yet to hear about any hoods blowing off at speed, so the buldging is really not much of an issue, other than it being disconcerting to look at.

comp

where did those come from ??

MattB

From what I've read around here, the cowl is always higher pressure than whats under the hood. Speed does not really matter, both pressures will go up. So, air will always go in to the engine bay from the cowel. Some cars had cowel induction to take advantage of it.

Cobra4B

I have a DRM vented hood too... no issues w/ rain, the front openings allow water to fall between the radiator and motor and the rears have rain traps. Furthermore, I sealed my knock sensors when I did my cam swap so I can spray a hose on my motor all day long.

OKsweetrides

What has the pressure come to be measured at underhood at speed?

tiojames

Two good reasons. One, the air that comes in the front of our cars has no place to exit except under the car and if it is lowered like most of ours are the air exits slower than it is taken in causing build up which inhibits the cooling process and tends, two, to lift the front of the car at high speeds. My car runs cooler and the front is now glued to the road( had a short 150 mph run to test it) since I installed the VIS Extractor hood. Letting the air pressure build up actually reduces the ram air effect as it keeps the air from entering as fast as it could.

davidfarmer

Those are JCWhitney Louvers, made my Ventshade I think. They are plastic, but haven't melted through yet.

They teams that tested at the GM Wind Tunnel say the cowl is higher pressure.....I have no reason not to believe them. Opening it just pumps more air into the engine bay, which will spill out under the car causing lift.

Solofast

Lowering too much causes the front end to lift, but not for the reason you are suggesting.

Lowering a C5 too much in the front causes the front end to lift, not because of buildup of air under the hood, but because of a buildup of pressure under the overhung nose of the car. There was an exhaustive (pardon the pun) thread on the Autocross and Roadracing page of the forum that went into the issue of vented hoods in great detail. Take a look at this link.

http://forums.corvetteforum.com/show....php?t=1694809

There is a specific post on front end lift and why and wherefore of it...There is a lot of technical stuff in there, but note that KBaerobiscut is an aerodynamicist by trade as well as having a lot of wind tunnel experience WITH THE C5... HE KNOWS HIS STUFF, listen to him....

Secondly this comment

"Letting the air pressure build up actually reduces the ram air effect as it keeps the air from entering as fast as it could"

is not true. As you decelerate the air the static pressure builds up, essentially trading velocity for pressure (actually the pressure change is a square of the velocity change, but you get the idea) So you don't lose the effect of ram air by slowing down the flow, the energy is just transformed to pressure....

sxeC7

As I see it, the only way to prove or disprove the ram effect on engine performance is to set up a wind tunnel test with the car on a dynometer in the wind tunnel and have readings made with the wind source on and off.

I think there is some confusion between aerodynamic drag and air pressure. Instead of a coffee cup, hold a coffee can that is open on both ends out of the window. There is little or no resistance because there is only a very thin area to create drag. If you look at it from the side, the air is basically stationary and the can is moving through it. Close the aft end of the can and drag is created, but the air is still stationary and just flows around the can. A point to ponder, if ram air is so important at high speeds, why don't NASCAR racers have large scoops piped into the carb?

I suggest that the C5 hood is not raised by air pressure within the engine bay, but aerodynamic lift from the air passing over it and that extractor hoods decrease that lift through their vents.


By the way, it's damn good to see E-T back in the fray again.

555ss

[QUOTE=sxe60;1563881609]As I see it, the only way to prove or disprove the ram effect on engine performance is to set up a wind tunnel test with the car on a dynometer in the wind tunnel and have readings made with the wind source on and off.

I think there is some confusion between aerodynamic drag and air pressure. Instead of a coffee cup, hold a coffee can that is open on both ends out of the window. There is little or no resistance because there is only a very thin area to create drag. If you look at it from the side, the air is basically stationary and the can is moving through it. Close the aft end of the can and drag is created, but the air is still stationary and just flows around the can. A point to ponder, if ram air is so important at high speeds, why don't NASCAR racers have large scoops piped into the carb?



QUOTE]

NASCAR will not let them use a hood scoop so they use a cowl type inlet taking advantage of the high pressure area at the windshield base ( ever see them spray starting fluid at the bottom of the windshield when the driver stalls int the pits) my drag car responded with a .1 increase when the 6 inch cowl hood(forward facing scoop is not allowed) was ducted to the carb due in part to the high pressure at the base of the windshield.

AU N EGL

and have ppl noticed how much speed tape, duct tape or 200 mph tape is on the front of the Cup car? Too close off as many openings as possible for more aerodynamics.

Then again Cup cars dont have inner wheel wheels or close bottom cars.

davidfarmer

I agree, closing off the front and bottom/front of a car will definitely decrease drag and lift, however then you have no cooling. Purpose built racecars take advantage of this and seal cooling airflow so that it does in the front (high pressure) and out the top (creating downforce, like a wing effect).

The C4/C5 is especially bad liftwise because it is a bottom breather. The entire premise of pressurising the radiator by cramming air UNDER the nose of the car is bad for aero. Even the C6 and C6Z06 are only slight improvements, as the bottom is still open (C6) and has an airdam (C6Z06), again creating pressure under the nose.

This is all off of the original topic, but fun to talk about.

sxeC7

That's the point, a large forward facing scoop would produce lots of drag. Where as, taking air from a known existing pressure area is free HP.

Twospeed

I for one, am enjoying this post so far. Very informative

97C5owner

Me too. Lots of useful knowledge. So the Var Ram system uses an existing opening. Would this contribute to drag, or simply take advantage of an open spot. Will it take advantage of the effect solofast described?

1fastdog

I run an Advanced Composite Products hood on my Z06 and it works. They do race body Corvettes bodies for World Challenge and GrandAm corvettes. All wind tunnel tested at GM's wind tunnel.

Real deal pieces.

My hood:

Solofast

Well, it depends...

First, all openings that admit air to the car contribute to drag. Be it a separate scoop or air captured from a high pressure area. Any air that comes "on board" the car is essentially slowed down from the free stream velocity and carries with it a momentum loss. That's why the NASCAR guys are out there making the grille openings smaller. Less air coming into the car results in lower drag. In aviation circles this is called "cooling drag". You can do a search on it, but basically if you take air into the car, and then don't reaccelerate it coming out of car, there is a momentum penalty for it and that's what you pay. Part of doing business... A scoop is usually worse than taking air from a high pressure area, but there isn't that much difference, it depends on wake effects and where the scoop is and how it is shaped.

The forward facing opening needs to be sealed so that all the air going into it goes into the inlet and not leak past and into the hood area. All of the air that doesn't go thru the radiator or into the engine is (for aerodynamic purposes) wasted. If you have openings that leak air to the area behind the radiator if they are small enough (compared to the inlet area) you can still get most or essentially all of the pressure rise. If the air can sneak past the inlets, the pressure in the area won't be as high, and you will likely not get as much of a pressure rise.

555ss

ohhhh, pardon me I can be a bit thick I thought You meant the scoop would not help.

comp