I have some weird questions about cars...

[Auto_newb]

What makes good brakes? As long as you can lock up the brakes, how can you get any better than it already is?

What's a floating disc?How is it different from others?

I heard some people say that high revving engines wear out faster, why? (unless it isn't true)

What's the point having high revving engines over high torque engines? They both use the about the same amount of gas, weren't the muscle cars back then low revving, but had lots of torque and horsepower?

On those dyno charts, how come the horsepower starts dropping at HIGHER RPMS? Doesn't it peak out at the high rpms? (I thought the bigger the explosion, the more HP/Torque).

How does retarding/advancing ignition timing help prevent detonation? Doesn't the engine detonate regardless of when the spark plug fires? I thought the fuel/air mixture detonates because as the piston compresses the mix, the air will get hot enough to ignite it?

All explanations are appreciated

[burly]

I'll see if I can help you out a bit here. I do know that whatever I say, there will be some distibuting the facts with other members of the forum, since you are talking about a fairly broad range of topics, and you can get really technical about them. However, hopefully I can provide you with a basic understanding.

Let's start with brakes. It takes several hundred horsepower to get your vehicle to 60 MPH in 120 feet, yet those little brakes can stop you in as short a distance. So, what makes good brakes? Well, first of all, area. The larger the "swept" area of the brakes, the more contact there is between the pads and the rotors/drums. The more contact area for a given abrasiveness (or bite) of a pad, the more stopping power. Now, while you cannot put anymore braking power to the ground then your tires have traction, it isn't quite as simple as "as long as you can lock them up, they are as good as they can be". One big factor, is brake fade. Fade is caused when the rotor/drum and the pads heat up so much that a glazing occurs on the pads, drastically limiting the stopping power. Larger brakes have more swept area and thus more surface area and mass for heat dissipation. It has a double effect, since there is more swept area and thus stopping power, it requires a lower application of force to apply the same amount of stopping power as a smaller brake system, and with less pressure per square inch, less heat is generated. So while the stock brakes on your vehicle may be able to lock up the wheels on the highway to stop you in an emergency, they would not be good for racing. After such a hard stop, if you were to have to do it again soon afterwards, your stopping distance would likely be much greater. At the track, this translates into massive brake fade and pad wear when driven hard.

On to engines. High revving engines, everything else equal, do generally wear out faster (other things like torque, application, and engine type play roles as well amonst other things). This is because at increased engine speeds, there is increased heat production, increased valvetrain stress, the crank is spinning faster and harder, and all and all, more parts are closer to their limit. Heat is probably the number one issue. At higher speeds, the friction on the parts increases (its like rubbing your hands together - the faster you move them together, the hotter they get). This has a dual effect. As the parts heat up, they generally expand, lowering the tolerances in many key areas, and thus introducing even more friction ( this is like pushing your hands together harder as you rub them together).. There are many other more technical reasons, depending on application and engine type, but basically, increased heat and friction along with increase strain due to acceleration levels on various parts (like the valve springs and pistons) is a major concern in all engines.

High revving versus high torque engines is going to be a hot topic area. There are many other variables going on here but I'll try and keep it as basic and untechnical as possible. First of all, they don't necessarily use the same amount of gas. That is dependant on the type of engine and its efficiency. However, there is only a certain amount of energy in a given volume of fuel, and there is an ideal air/fuel ratio, yes. High revving engines do allow you to stay in one gear for a longer time, allow more acceleration and speed, as any time the transmission is not in gear, the engine is not working to move the vehicle. Another issue is horspower. It is easier to keep an engine in its powerband if that powerband is wider and the time it takes to shift into another gear won't drop the engine out of its powerband. This is also why close ratio transmissions with more gears (6 or 7) are used in performance applications - to keep the engine in its power band while maintaining both acceleration and top end. High torque low revving engines can move a car quickly also, but since the high torque motors usually have larger displacements and therefore more massive moving mass, the engine cannot build up speed as quickly has a high revving engine, since the high revving engine usually has lighter moving parts and revs quicker.
Hopefully, someone will clear that last part up a bit. I don't think I put that very well.

On a dyno chart you usually have two graphs, torque and horsepower. Horsepower is a mathematical product of RPM and torque. Therefore, if torque numbers begin falling off faster than RPMs are climbing, horsepower begins falling as well. The reason you get more horsepower at more RPMs is because of this mathematical relatation between RPM and torque to horsepower. Therefore, max HP is not simply at max RPM.

TIming has a lot to due with efficiency of the engine. By advancing the timing, you allow more of the power of the combustion to be transfer into the mechanical system. However, you also run a greater risk of detonation or preignition - a topic all of its own. This is why if your engine detects "knocking" - usually due to poor octane gas - it retards the timing to prevent damage to the engine. Since less power is transferred to the engine less stress is placed on it. The timing, btw, has to due with the relation of the spark to the location of the piston in the cylinder. Since this is a distance (actually, its measured in degrees, but it translates into a distance for a given engine) as RPM s increase, the time it takes the piston to traverse the distance decreases. This gives the combustion less time to take place before the piston head reaches it. By giving the timing an advance, you ignite it later in time and therefore when the piston is closer to Top Dead Center. At slower engine speeds this increases torque. However, as engine RPMs build, power is lost since the combustion cannot happen quick enough to transfer power to the engine. The problem with advancing the timing too far, is that if the octane rating on the gasoline is too low, the heat from the compression of the air/fuel mixture will become sufficient to ignite it without the spark at all, and this could lead to detonation and can melt the pistons. In a gasoline engine, this ignition due to compression is not desired at all. However, in diesel engines, thats exactly how they work. They use the compression to ignite the air/fuel mixture. Diesel burns cooler and the engines are built to withstand higher compressions, so they can withstand this. They generally produce more torque, however due in part to the larger moving mass required to make them strong enough, they are limited in RPM ranges.


I hope this starts you off well. THere are a lot of online resources that can help you understand each of these topics better in depth. One of them is this forum! Many of these topics you've brought up here have pages and pages, threads and threads of conversation on the topic in this forum. Searching through them is a good place to start. Someone else jump in and clean up my mess please.

[Auto_newb]

Thanks for the explanation, although I don't really understand the last 2 about the dyno chart and the detonation, but thanks anyway

[burly]

For more explainations on detonation there is one in this forum pertaining to detonation, Honda preludes and premium fuel. That would be a good place to start.

As for the dyno chart, basically the reason why horsepower can begin to fall off at higher RPMs is that horsepower is not measured directly. Horsepower is a calculation. It is a measure of power yes, but it is actually calculated. The two variables in the equation are Torque and RPMs. The reason horsepower numbers can fall at higher numbers is that for a given increase in RPMs - one of the variables in the horsepower equation - the torque may decrease by a larger amount. For example, for a given interval in engine RPMs, the RPMs may increase by 10% but Torque may fall by 15%. This decrease in torque is more than the increase in RPM can make up for. Thus the overall number is lower. Lemme know if a numerical example might help, I'd be glad to give one.

[MagicRat]

That is an excellent, practical explaination of some very complicated topics.

[Oldengineer]

The discussion on brakes didn't mention one major factor that can causing fading. Its possible to get brakes so hot that the brake fluid actually boils. If this happens, when you hit the brake pedal, it'll go to the floor and you've got little or no brakes. Had this happen to me on mountain roads on a 1995 Chrysler Cirrus a few years back. The fix - flush the brake system and put fluid in with a higher boiling point.

Regarding the floating disc question - that refers to the brake caliper design used on the car. Most cars today use a single piston floating design. That means that the caliper "floats" on its mounts. When you apply the brake, the inboard shoe (next to the piston) hits the rotor, the entire caliper slides on its mount, and the outboard shoe is pulled in to the rotor as well. Some high performance cars used a non-floating design that have 1 or 2 pistons on both sides of the brake caliper to force the shoes to contact the rotor when the brakes are applied.

Regards
Oldengineer

[SaabJohan]

Power can also be measured and torque calculated from that, it is however common practice to measure torque and calculated power Power can for example be measured using friction heat released per time unit.

[curtis73]

What makes good brakes? As long as you can lock up the brakes, how can you get any better than it already is?

There are a few things I would like to add here about your lockup question. Since brake size plays a big role in this. If you think about it, the size of the brake disc or drum plays a big role. If you double the diameter of the disc for instance, you technically need half the friction to make the same amount of brake torque since the distance from the friction to the hub is greater. An analogy here is try to stop a spinning wheel by grabbing the edge of the rim or by gripping the hub. It takes much more squeeze to stop it at the hub. That example assumes a lot of variables, but for the sake of demonstration it works for now. Brake lockup would require the same amount of brake torque in both situations (assuming the same tire/road friction), but the small rotor is going to be more "grabby" meaning that it will be harder to moderate friction between lockup and almost-lockup. This means that you'll have a tough time using the 80%-99% range of your braking before lockup. Using a larger disc (or drum) allows easier moderation in this range for improved braking. An important assumption here is that maximum braking does not occur at lockup. Rolling tires have more grip than skidding tires. Therefore your maximum braking takes place in that hard-to-moderate area before lockup. All other things equal, larger diameter brakes will perform better than small diameter brakes. Other side-effects happen here, too, less heat generated per square inch of disc/drum and therefore greater durability via reduced thermal stress.

What's a floating disc?How is it different from others?

Technically, a floating disc is one that just has holes and slips over the lugs sandwiched between the hub and wheel. It doesn't float since its bolted tight. Fixed discs are where the disc is part of the hub. The center bore of the disc holds the bearings and the weight. In this case, usually the lugs are press-fit in the "hat" of the disc and the wheel bolts directly to the disc. Pretty much any american RWD car with a solid axle and 4 wheel discs (Mercury Marauder, Impala SS, RWD caddys, etc) you'll find fixed discs front and floating rear.

Floating calipers mount on pins. Fluid pressure is applied to one side which pushes the piston against the pad, pulling the caliper tight against the rotor and thereby applying equal pressure on both sides just like someone said. Fixed calipers add fluid to both sides, each having a number of pistons. The more pistons you have, the more equal the pressure is applied and helps braking. The single piston floating caliper is quite adequate though and offers supreme simplicity. The pinnacle of race technology brakes is the 8-piston fixed caliper, but that is 8 times the number of things which can fail.

What's the point having high revving engines over high torque engines? They both use the about the same amount of gas, weren't the muscle cars back then low revving, but had lots of torque and horsepower?

In my opinion, the answer to this question is two-fold. The manufacturers have made HP a buzzword. A high revving engine is basically making chicken salad from chicken s#!t. Import buzz-bomb kids take their 2.0L engines and make 500 hp. Impressive, yes, but they've destroyed low end torque and have to rev to 8000 rpms to get their power. Then they wonder why a stock 260-hp, 4300-lb Impala SS like mine destroys them at the drags. A litte bit more on why in the next part: The other reason is that truth in advertising traditionally didn't apply to HP. Depending on the car, hp was reported very inaccurately based on insurance, safety, and target audience. A '70 LS6 454 Chevelle SS was rated at something like 450 hp, when in actuality it made more like 525. Even rated at 450, insurance companies didn't want to touch it. Conversely, cars like my 140-hp Cutlass Salon actually made 125 hp. They wanted it to sound better on paper.

On those dyno charts, how come the horsepower starts dropping at HIGHER RPMS? Doesn't it peak out at the high rpms? (I thought the bigger the explosion, the more HP/Torque).

Engines are air pumps and as a by-product they make power. The RPM where they are the most efficient is where the torque peaks. If you run an engine on the dyno and measure its torque curve, you'll see that it peaks at a certain RPM (lets say 2500 RPM for a street engine). The efficiency to which I'm referring is VE or volumetric efficiency. This can be demonstrated by this example. If your cylinder's swept volume is 50 cu. in., on the intake stroke it theoretically takes in 50 ci of air/fuel. In actuality it takes less. At a really slow speed like 20 rpms it would operate at 100% VE since there is nothing really standing in the way of air intake. At high speeds, the valves aren't open long enough for air to rush in and fill completely. In an operating engine, VE increases as intake velocity increases when you rev. At a certain point, velocity peaks near the 2500 rpm example we're using. At this peak point, the restriction of the intake tract starts hindering how much air makes it in the cylinder. For this reason, higher performance intakes have bigger cross section intake parts. The velocity of those intakes peaks at a higher RPM and their torque peaks higher. Since engine size plays an important role in air demand, you can also see how the same intake on a smaller engine will have a higher peak RPM. That's a lot of analogies and tech, but let me know if you don't understand.

Now, HP is calculated by this formula: HP equals TQ times RPM divided by 5252. If you measure the torque at 2500 rpms as being 300 the formula shows us that at 2500 rpms, that engine makes 143 hp. Seems wimpy, right? Lets say our engine makes 250 TQ by 5252 rpms. By the formula, its making 250 hp. You'll also notice that every dyno chart you see (because of this formula) at 5252 rpms, the torque and hp curves cross.

Now picture that we take our engine and shift the torque curve up in RPMs, but keep the same peak of 300. We have just gained significant HP by this formula. It is an inescapable mathematical trade off. What we can do, however is develop parts that flatten the torque curve. Not only does flattening it add high horsepower, it boosts low end torque. Properly matching components helps to flatten torque curves since they will work together at more rpm points instead of only at one point.

How does retarding/advancing ignition timing help prevent detonation? Doesn't the engine detonate regardless of when the spark plug fires? I thought the fuel/air mixture detonates because as the piston compresses the mix, the air will get hot enough to ignite it?

two reasons. 1) if the engine is detonation because the ignition timing is causing it, then its too early and its technically pinging. The flame front is meeting the piston before top dead center and smacking it. 2) retarding timing reduces net combustion temperature. The lower temps reduce hot spots and pre-ignition.

These terms are so often misused so I'll clear them up. Pre-ignition is when a hot spot in the combustion space ignites the mixture during the compression stroke before the spark would have. Ping (sometimes incorrectly called knock) is when the flame front smacks the piston before TDC. It is the physical noise caused by the spiking pressures and is not an ignition term. Ping can be caused by pre-ignition or just advancing the timing too far, or the last term; Detonation. This is when separate flame fronts collide in the middle and cause ping. There are several methods by which this can happen. Spark and pre-ignition can each make their own flame and they meet in the middle. Also, as normal spark-flame ignition progresses, pressures rise rapidly. That rise in pressure can cause compression ignition somewhere else in the space and those two fronts collide. Those are the ASE definitions. Either way there are some important things to know. Ping is bad since its like smacking your pistons with a hammer. It does not have to be heard to be bad. Especially in quieter cars its almost impossible to hear and very damaging ping can be taking place without ever hearing it.

[beef_bourito]

the equation for measuring horsepower is:
torque x RPM/5252.

[Auto_newb]

So a floating disc is a disc made up of several parts, and a fixed disc is a one-piece disc?

Is it because of VE that the HP and Torque drops? If the VE was always 100, theoretically the graph would go constantly up right?

When you say "flame front" are you referring to "explosion"?

[curtis73]

Originally Posted by Auto_newb

So a floating disc is a disc made up of several parts, and a fixed disc is a one-piece disc?

Not necessarily. *pause while finding images from the 'net*... Here is a photo of a fixed rotor. It is fixed because it is the disc, hub, bearing housing, and basically everything that turns. You'll see in the middle is where the nut holds on the bearings. The lugs are press-fit into the center and the wheel bolts to it.


Here is a good photo of a floating disc. Like I said, though, it doesn't really float since its pinched between the wheel and hub. This particular example is a two piece design where the disc bolts to the "hat" but they need not be. They can be one singular casting. A one-piece cast floating example would look like the first picture but have an empty hole in the middle and empty holes for the lugs like the second picture.


A good example of where this type of floating disc would be used is on rear axles where the axle ends look like this picture. Here, the axle shaft is one piece with lug studs pressed in the end. The "hat" of the floating disc just slides on over the studs and you bolt the wheel on over it.

Is it because of VE that the HP and Torque drops? If the VE was always 100, theoretically the graph would go constantly up right?

Exactly. If VE remained 100%, the engine would have an infinitely flat torque curve through all RPMs and HP (by the equation) would continue to rise. Manufacturers attempt to stretch the peak VE over more RPMS by having variable volume intake runners and some have dual path intakes where there are long thin runners used for low rpm and then a valve switches to short fat runners for high rpms. It is important to also note that all of the components work together. I've been using intake tuning as the example, but it involves cams, ignition timing, exhaust tuning, etc. The reason is that the dynamics of gasses like the air ingested by the engine are surprisingly inflexible, while the demands the engine places on air are extremely variable. An engine is like your mouth sucking a milkshake through a straw. If you suck slowly you get a little. Suck harder and you get more, but you reach a point where no matter how much harder you suck, you're not getting any more milkshake. At this point the friction in the straw and viscosity of the shake reach their limit and won't flow any more. If you increase the size of your straw, that point of diminishing return is at a harder suck-point and higher volume. This analogy is good to demonstrate another point. If you are sucking shake at a constant volume, the velocity of the liquid would be slower in the larger straw. The inertia in the larger straw is therefore lower than with the smaller straw. Applying this back to intake design (and exhaust design for that matter), Peak VE occurs at this max velocity because it takes advantage of that inertia. With the intake valve open, the falling piston starts the air moving. The high velocity air generates inertia that keeps filling the cylinder even after the piston stops its downward motion. Here's where tuning everything together is key. In this example a properly chosen cam would close the valve after the maximum extra air has been packed in, but before it starts to flow back out. (reversion). I know I'm getting pretty deep, but this is the exact reason why really hot cams have lopey idles. They hold the valve open longer (great for high RPMs when the air can't keep up with the faster piston) but at idle, they're open long enough that stuff gets reverted back into the intake. It bleeds off pressure that would have been trapped in the cylinder had the cam closed eariler. A common way of boosting low end torque with bigger cams is to advance the cam so the valve closes earlier in this idle situation, therefore trapping more charge in the cylinder at low rpms.

In practice, typical street engines peak at 75=85% VE. A really well-tuned race engine may see over 100% VE if it takes enough advantage of the intertia of the incoming air harmonics. A real easy way to overcome the VE problem is to use a turbo or supercharger.

When you say "flame front" are you referring to "explosion"?

Yep. The flame kernel starts at its origin (hopefully the spark plug) and expands in all directions. The reason its treated as a flame front is because although it seems like an instantaneous flash, compared to the speeds of the piston and the crankshaft its pretty slow. That's why ignition timing settings are before TDC. The flame needs time to get to the piston so it starts early. If it sparked at TDC, by the time the flame made it to the piston, it would already be headed away. How fast it burns is dependent on several factors like compression, combustion chamber design, air/fuel mix, and how evenly distrubuted the mixture is. I have a great picture of flame front expansion stop-photos in a jag engine but they're hard copies and my scanner isn't working. You might hear terms like "squish" or "quench." These terms apply to the shape of the piston/combustion chamber having areas which are geometrically designed to swirl the mixture as the piston races up. Turbulent charges stay more evenly mixed and therefore burn cleaner, more completely, and more predictably over a wider range of parameters like engine temp, and fuel quality. The net result of the newer quench technology is more power, better efficiency, and fewer emissions. Its not a cure-all, but a step in the right direction.

[SaabJohan]

The "real" formula for calculating power from torque:
P=M*angular velocity
This can then be simplified for different units, as a standard the units are P (W), M (Nm) and for angular velocity rad/s.

VE, some values:
- A stock 170 hp Volvo engine peaks at 95% volumetric efficiency
- A modern F1 engine have a VE of about 118% at maximum power
- pro stock engines can have VE:s of about 130%

Some of the highest numbers for NA 4 stroke engines is likely to be found on the old supertouring car engines like BTCC, this since they were limited to 2 liters and 8500 rpm so VE and efficiency were they few ways to increase power output.

When the spark ignites the mixture we will get a flame speed of about 0.5 m/s, as it burns the burned gases will have a lower density and therefore push flame in front of it, this turbulent flame front can burn with a speed of about 50 m/s.

Note that there are no explosions in an engine, the flame speed is too low to call it an explosion.