Dirty car reveals aero secrets, holes in hood not just there to lure c*cks to the M3!

[Richbot]

I was getting ready to clean my car when I thought, hmm, that's weird, lots of dirt collected on top of the hood bulge. Then I looked closer and noticed that there were little trails where condensation from being parked outside all night at work had run across the car on the way home. Some of them led to the engine air intake on the hood. But wait, the "dead" hole on the other side has no trails leading to it. Could it be? Does the hood intake actually get a little positive pressure (or maybe just suction from the engine) at speed? Could the ///M in ///M3 still mean something? It appears so. Kinda neat to see this thing actually works. Also, it looks like there might be a cat hanging around my parking lot at work, awesome.

[Krozi]

Wait.....what? You do know that, what would be the driver side foglight, is a big air ram right?

[Dengel]

I feel like those trails are coming FROM the hole, not TO it. The front air inlet is going to pass a lot of air through which is why water always sprays from there after washing the car.

[Richbot]

Originally Posted by Dengel

I feel like those trails are coming FROM the hole, not TO it. The front air inlet is going to pass a lot of air through which is why water always sprays from there after washing the car.

No condensation started out anywhere near the holes, it was all on the middle of the hood. I'd buy it if there was water in there when I started out, but there wasn't, and I wasn't stopped for any length of time. Everywhere else on the hood, the trail starts near the leading edge of the hood stops once it gets to the edge of hood bulge near the windshield, except by the hood vent, where the trails bend 180 degrees toward the intake. I suppose some could have followed the outer edge of the hood vent trim until it got to the end, then run out from there toward the intake, but that doesn't explain why 2/3 of the trailing edge of the vent trim is clean.

The only thing that explains it is a pressure differential caused by the intake. I guess somebody (besides BMW) needs to get this thing in a tunnel with a smoke wand next to the intake and see what happens.

The front intake has to make a 90 degree turn before it gets to the airbox. Which might explain why the high pressure zone where the hood meets the windshield was worth exploiting.

[mdosu]

Yeah, right side is intake, left side is a vent

look on the underside, they're also pointed different directions. looks like it gets air circulation under the hood and the right side goes straight into the airbox or not?

[Chest Rockwell]

Holy shibb!! Is everybody's engine compartment painted? Mine is all primer black, even the underside of the hood.

[Dengel]

Originally Posted by Chest Rockwell

Holy shibb!! Is everybody's engine compartment painted? Mine is all primer black, even the underside of the hood.

Is your car black? the inside of the hood is more of a matte/flat finish

[AUS1969]

Extended summer driving on I-90 yielded a bug collection on the inside of the driver's side vent. I don't see how that could be unless air goes out of the hole, so it's not an intake.

[Dannys M3]

I would be more worried about killing the cat that is walking on the hood and scratching your paint.

[skierman64]

Originally Posted by AUS1969

Extended summer driving on I-90 yielded a bug collection on the inside of the driver's side vent. I don't see how that could be unless air goes out of the hole, so it's not an intake.

I think at lower rpm highway type driving air passes through the airbox and the excess goes out the hood vent but at higher rpm and full throttle operation the engine needs more air and actually draws air in though the hood vent.

[MysticBlue]

There is no air flow through the passenger side. Slits in the underside rubber let water drain out. That's it. It's purely cosmetic. The driver's side is mostly to let air flow through the air box so it doesn't get hot when the throttle is closed. If it wasn't there, while you were braking or otherwise not on the throttle, the air going into the intake at the lower left side and the grill intake would be dead-headed and the air in the air box would get heat soaked form the engine compartment heat. With the hood vent, there is constant flow through the box to keep it cool so when you do get back on the throttle, it's not hot air. As noted by someone already, bugs and debri sometimes collect on the inside of this vent indicating flow is mostly going out. When raining, or after washing, at highway speeds water comes out of it. It does not flow into it. From a standing start, it probably sucks air into it at full throttle, but I don't think that is it's main purpose. It's main purpose is to allow flow through to keep the air in the air box from getting hot.

[spyderco10]

Originally Posted by MysticBlue

There is no air flow through the passenger side. Slits in the underside rubber let water drain out. That's it. It's purely cosmetic. The driver's side is mostly to let air flow through the air box so it doesn't get hot when the throttle is closed. If it wasn't there, while you were braking or otherwise not on the throttle, the air going into the intake at the lower left side and the grill intake would be dead-headed and the air in the air box would get heat soaked form the engine compartment heat. With the hood vent, there is constant flow through the box to keep it cool so when you do get back on the throttle, it's not hot air. As noted by someone already, bugs and debri sometimes collect on the inside of this vent indicating flow is mostly going out. When raining, or after washing, at highway speeds water comes out of it. It does not flow into it. From a standing start, it probably sucks air into it at full throttle, but I don't think that is it's main purpose. It's main purpose is to allow flow through to keep the air in the air box from getting hot.

good explanation!

[Leonardo629]

The hood vent still sucks in air from the outside, check out the picture. S65 comes with a cold air intake from the factory (#3). That's why the stock intake is so efficient that an aft. market intake kit usually does not yield enough gains to justify the price tag.

The dead-air theory above is not correct since even when you're on the brakes with the throttle closed....the engine is still ingesting air (or the motor would bog and bog then shuts off). So the air in the airbox is never constant once the motor is running.

The picture also highlights probably the weakest link of the intake...which is the air filter itself. Then it is not a surprise that we've been seeing most gains when we replace the factory unit with BMC, K&N, MS...etc..etc..

http://www.e90post.com/forums/showpo...66&postcount=1

[e46e92love]

Originally Posted by Leonardo629

The hood vent still sucks in air from the outside, check out the picture. S65 comes with a cold air intake from the factory (#3). That's why the stock intake is so efficient that an aft. market intake kit usually does not yield enough gains to justify the price tag.

The dead-air theory above is not correct since even when you're on the brakes with the throttle closed....the engine is still ingesting air (or the motor would bog and bog then shuts off). So the air in the airbox is never constant once the motor is running.

The picture also highlights probably the weakest link of the intake...which is the air filter itself. Then it is not a surprise that we've been seeing most gains when we replace the factory unit with BMC, K&N, MS...etc..etc..

http://www.e90post.com/forums/showpo...66&postcount=1

Bingo....for those of on here since before the ///M was even released you will remember the long discussion threads involving people such as SWAMP, PG, Lucid and others about how many if not all of the after market intakes do nothing, that the stock intake kicks arse and that the only real shortcoming as far as air flow is concerned is the filter.

Additionally, it has been well documented but I'm too lazy to find the threads that discuss this that in fact the driver's side air vent is in fact VERY active and serves the purpose of getting air into that engine at higher rpms when she gets really thirsty for air (see, and you just thought she sucked gas). I can't remember where and I have searched but somewhere its listed the ridiculous amount of air it sucks in at 8400 rpms.

Cheers,
e46e92

[Richbot]

Originally Posted by e46e92love

Additionally, it has been well documented but I'm too lazy to find the threads that discuss this that in fact the driver's side air vent is in fact VERY active and serves the purpose of getting air into that engine at higher rpms when she gets really thirsty for air (see, and you just thought she sucked gas). I can't remember where and I have searched but somewhere its listed the ridiculous amount of air it sucks in at 8400 rpms.

Cheers,
e46e92

4.0l * 8000/2 = 16,000 liters per minute at 8000 rpm

To put that in perspective in 'murrican units, that's over 70 gallons per second, or enough to suck all the air out of your typical 2000 square foot house in about half an hour.

There may have been little bit of extralegal high rpm activity on the drive that generated these photos.

[Serious]

If the intake is for "sucking in" air then why did they choose to put it in a notorious low pressure zone?

[sor]

Someone put a piece of paper on it and then rev the engine, see if it blows away or sticks to the hood at various rpm.

[Richbot]

The cowl is a high pressure zone...

[dexx]

Originally Posted by Richbot

4.0l * 8000/2 = 16,000 liters per minute at 8000 rpm

To put that in perspective in 'murrican units, that's over 70 gallons per second, or enough to suck all the air out of your typical 2000 square foot house in about half an hour.

There may have been little bit of extralegal high rpm activity on the drive that generated these photos.

That calculation has no basis in reality. Assuming the intake and exhaust is in tune at that frequency, efficiency will be over 1.0, there will be more air in the cylinder than would fit at atmospheric pressure at full displacement.

I need to find a datalogger, but the MAF reading can be measured directly.

[Richbot]

Originally Posted by dexx

That calculation has no basis in reality. Assuming the intake and exhaust is in tune at that frequency, efficiency will be over 1.0, there will be more air in the cylinder than would fit at atmospheric pressure at full displacement.

I need to find a datalogger, but the MAF reading can be measured directly.

It's just a WAG ballpark figure, ferchrist's sake. But if you really must quibble, try to get your facts straight. There's no MAF on these M3's.

[Serious]

Originally Posted by Richbot

The cowl is a high pressure zone...

Yes but the intake isn't located in the cowl. It's about 3/4 up the hood.

I should've said near ambient, not low pressure.


(granted this isn't a model of an m3 but it's close enough for general observation.)

[kitw]

Originally Posted by Richbot

There's no MAF on an M3 and there hasn't been for about 10 years...

haha, this is a further stupid quibble, but the US spec 2006 E46 M3 still had a MAF.