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Discussion Starter · #1 ·
I read all the auto magazines and have been wondering for years. Most high performance cars have two things that I read about, but really don't understand.

1. What does the rear diffuser actually do?

2. What is a dry sump lubrication system?

Thanks!
 

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I read all the auto magazines and have been wondering for years. Most high performance cars have two things that I read about, but really don't understand.

1. What does the rear diffuser actually do?

2. What is a dry sump lubrication system?

Thanks!

To be overly simplistic..

1) think of an airplane wing. the longer distance on top results in a different air speed ( higher above the wing) and hense less dense air above resultng in lift. For a car, if you can extend the underbody you can adjust the air speed, and downforce generated. If you look at a car like the zonda with a huge rear diffuser, they are extending the underbody to have more surface area than the top of the car resulting in a car that can generate substantial downforce.

2) Basically a remote oil lubrication system. You scavange the oil that falls to the bottom and send it to another remote location. Then this is pumped back into the system. Advantages are that the drivetrain can be lowered, That you can have a very large oil capacity, That you can place this extra oil and weight in a sutable location for weight distrabution and heat exchange, and you can prevent G force induced oil pressure loss by having a sperate res. in which the oil cannot slosh around and miss the oil pickup.

LEt me know if you have any more specific questions, as I tried to answer this as generally as I could.
 

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I read all the auto magazines and have been wondering for years. Most high performance cars have two things that I read about, but really don't understand.

1. What does the rear diffuser actually do?

2. What is a dry sump lubrication system?

Thanks!
1. It provides down force and proper air circulation from the back of the car. As you speed up the car pushes a lot of air under it and if it is not exiting the car in a proper and controlled manner it can cause the back end of the car to hydroplane but on air. So a good diffuser can provide a lot of stability for a high performing car. The M's and most performance car have a built in diffuser, if you look at the cut and curves of the rear bumper as it folds under the car you can clearly see the bumper taking a different shape as it is curving.

or better explained: by race tech 110:
"The Diffuser

The purpose of the diffuser is to allow the air that has been accelerated (so that its higher speed can produce a lower pressure) under the car to decelerate back to close to the same speed and pressure it was before the car ran into it. The better the diffuser works, the less drag the underwing produces.

You also see lateral fences or strakes in the diffuser area at the rear of the car. These have been banned from Indy cars (that means they worked), but Indy Lights and F1 cars still use them. The strakes probably do several things, but one of the most obvious is that they prevent low-speed, high-pressure air getting in along the sides of the car from disturbing the flow in the central area of the diffuser."


2.its best explained by: "In a wet sump, the oil that you put into the engine is stored beneath the crankshaft in the oil pan. This pan has to be large and deep enough to hold four to six quarts of oil -- think about two 3-liter bottles of soda and you can see that this storage area is pretty big.

In a wet sump, the oil pump sucks oil from the bottom of the oil pan through a tube, and then pumps it to the rest of the engine.

In a dry sump, extra oil is stored in a tank outside the engine rather than in the oil pan. There are at least two oil pumps in a dry sump -- one pulls oil from the sump and sends it to the tank, and the other takes oil from the tank and sends it to lubricate the engine. The minimum amount of oil possible remains in the engine.

Dry sump systems have several important advantages over wet sumps:

Because a dry sump does not need to have an oil pan big enough to hold the oil under the engine, the main mass of the engine can be placed lower in the vehicle. This helps lower the center of gravity and can also help aerodynamics
The oil capacity of a dry sump can be as big as you want. The tank holding the oil can be placed anywhere on the vehicle.
In a wet sump, turning, braking and acceleration can cause the oil to pool on one side of the engine. This sloshing can dip the crankshaft into the oil as it turns or uncover the pump's pick-up tube.
Excess oil around the crankshaft in a wet sump can get on the shaft and cut horsepower. Some people claim improvements of as much as 15 horsepower by switching to a dry sump." From "how stuff works"
 

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Discussion Starter · #4 ·
Thanks! Both answered the question. Have a little experience with wings (diffuser).:cheers:
 

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But let's not forget that the diffuser of the E39 M5 is about 3 miles off the ground, so it makes very little difference. In fact, the design of the E39 M5 diffuser actually catches some air. Furthermore, it's not even technically called "diffuser" but "valance"
 

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But let's not forget that the diffuser of the E39 M5 is about 3 miles off the ground, so it makes very little difference. In fact, the design of the E39 M5 diffuser actually catches some air. Furthermore, it's not even technically called "diffuser" but "valance"

That is exactly what I was going to say. Although the word diffuser has been very well defined above, the E39 M5's Plastic valance is in no way a technical diffuser. I find it amusing when people call it that. It is too ill-designed, too high off the ground to act as a proper diffuser.
Given that the car was sold 9-11 years ago and the diffuser designed even before that, it is understandable.
I just wish that BMW offered a factory proper diffuser option on a car that can hit 180+ MPH, at those speeds things like the diffuser do help.
 

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That is exactly what I was going to say. Although the word diffuser has been very well defined above, the E39 M5's Plastic valance is in no way a technical diffuser. I find it amusing when people call it that. It is too ill-designed, too high off the ground to act as a proper diffuser.
Given that the car was sold 9-11 years ago and the diffuser designed even before that, it is understandable.
I just wish that BMW offered a factory proper diffuser option on a car that can hit 180+ MPH, at those speeds things like the diffuser do help.
Surprisingly, however, the tiny spoiler does actually have an effect. But people who regularly drive the M5 well into the triple digits don't complain about stability because of a crappy diffuser (if we can even call it that).
 

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Diffusers are meant to reduce drag. The majority of a car's drag is caused by the vacuum the car makes as it passes through the air. They channel air up into that vacuum from under the car (and produce some extra air velocity that can help make some downforce) and allow a car to accelerate more efficiently at high speeds. When combined with a good spoiler, the effect can be multiplied. The diffuser on our cars dont really work very well because a) it doesnt start far enough under the car to catch any airflow and b) because the car isnt particularly low to the ground.
 
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