GT,
Good post (funny that in another thread people were calling you a troll ). One point, the oil scavenging ring actually does work better in one direction vs the other.

As for the follow up question, what affects engine braking, there are a number of things. The short version is engine braking works out to be how much torque does it take to turn over the engine and how fast are you turning it.

How fast it turns is easy, what's the overall gear ratio between the wheels and the engine. We all (hopefully) know that we can downshift to slow our selves when driving down a long hill. We down shift because it forces the engine to spin faster thus provides more braking.

The second part is how much force does it take to turn over the engine. Note that we aren't talking about pumping losses as in this case we have the throttle closed. We also aren't talking about mechanical efficiency. When the engine is slowing us down it technically is less than 0% efficient. We put gas in yet it slows the car rather than makes it go faster.

Higher compression ratio makes it harder to turn the engine over. We have to compress the air that gets into the cylinders that much more. Displacement is a big factor. Generally speaking a bigger engine will take more torque to turn over than a small one. Keep in mind that gear ratios come into play when we do this in a production car. A VW 1.8T would likely have less engine braking than a 1.6L Miata because the 1.8T is geared to spin slower than the Miata. At the same time I would expect a 3L V6 to have more engine braking than a 2.4L I4 if they have the same gear ratios.

V vs I vs H blocks would have little affect on the amount of engine braking.

 

Now you're making me think

A simple mod on 99-04 V8 Mustang's is a restrictor plate on the IAC (intake idle contol) valve for increased engine braking. I've tried it and it does indeed work. The valve will try to increase the air going to the engine to keep rpms from falling when you lift the throttle, presumably in an effort to make the car drive "smoother," but the restrictor plate effectively defeats the valve (it allows the minimum volume of air to keep the engine from stalling, but no more.)

I surmise a small engine in a heavy car will have less braking effect than a large engine in a lighter car. If you can restrict the air bypassing the throttle body to a minimum required to maintain combustion, you could maximize engine braking.

In my experience, my old 1.6L Civic exhibited much less engine braking than either a 5.0 fox body Mustang or a 99+ 4.6L Mustang. I'm not sure of the reasons. A number of years ago I read an article that discussed a Japanese car maker that programmed the ECU to shut off fuel during engine braking in an effort to realize economy savings, only to have customers complain that engine braking was non-existent. The company revised the ECU in later models to restore fuel delivery. I don't remember what company or car, and I'm not sure I understand how fuel cut-off can affect engine braking, so take that as very anecdotal data.

 

ah, rockville to the rescue

(I was writing my post above while you were posting - it isn't a response to yours)

 

It's called "copy 'n paste".

 

quote:


"There are a number of reasons you might want to engage in engine braking. The first is that it puts little or no more load on your engine than it would otherwise bear, but saves considerable wear on your brakes. This leads into the next reason, primarily in the case of racing, but also when going down long hills; Frequent use of brakes not only wears out the pads but also heats them up. This leads to brake fade (most brake pads do not grip well when hot) and to boiling of the brake fluid, which does not work properly once it has been boiled. Brake fade is only a problem during overuse of brakes, such as going down long, steep hills, or during "aggressive" driving. The third reason, also primarily significant in sports driving, is that you want to decelerate into turns and accelerate out of them. Downshifting increases the effectiveness of engine braking, and increases the number of RPMs at a given speed, which tends to assist in acceleration."

 

I like to think of it this way:

brake pads are much easier to replace than an engine, so I brake with the brakes.....

 

[QUOTE=rockville,Feb 24 2007, 06:06 PM] As for the follow up question, what affects engine braking, there are a number of things.

 

i heard it straight from the mouth of several professional engine builders.

engine braking is bad. it does in fact create pressure which sucks oil into the combustion chamber past the rings. additionally, it puts additional strain on the wrist pins.

 

+100000000000

 

I highly doubt that engine braking causing ANY problems... there have been many threads debating this issue for the past few years. xviper summed it up nicely once, saying that since your engine has the power to bring you car up to speed, why would it have a problem slowing it down? Even the manual recommends engine braking! I call on the majority of these posts