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powerband

bone

"bangle for president"
Joined
Jan 14, 2004
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16,814
Location
belgium!!
Car(s)
Volvo V40 & Yamaha Banshee
there is sth i can't figure out, probably sth stupid my mind can't get around, but here it goes

hp = (torque * RPM)/5252

now, if you use lightweight pistons, flywheel, ... it climbs in revs much faster (look at the BMI test of the mines skyline, can't go any faster)

resulting in a smaller, less useable powerband...
on the dyno this would show as a very small graph, but torque is the same and rpm is the same, so hp will be the same as well

so what's the use of those lightweight parts, if you use my reasoning, you would use parts as heavy as possible so the powerband is wider, more useable, caus hp and rpm stay the same

^I KNOW THIS IS COMPLETELY WRONG^
so, can anyone explain correctly?
 
The powerband doesn't change, what does change is the engine's ability to rev and respond. The lighter the internals the less energy their movement will generate, so there's less momentum & inertia from the moving pistons, conrods, etc that has to be overcome as the engine tries to build revs

That's a fairly average explanation though.

In short - lighter components won't change the power or torque, what they will change is the responsiveness of the motor. The only way they can change the power is by allowing the engine to rev higher, so you could end up with a wider powerband. Of course if the lighter components are a different specification then power & torque can be affected.

What you're seeing with a tuned motor could be two things - it revs faster because it can due to the lighter components, so your acceleration is better (powerband is the same, the engine is just able to rev across that rpm range faster), or there's also been an increase in turbo boost, which will also make the engine rev faster (when the turbo has spooled up).
 
with heavier components, you create problems.

1, power of the engine is diverted from powering the motion of the car to moving the pistons up and down - thereby reducing engine efficiency and causing more vibrations, thereby requiring more weight in balancing the engine, thereby increasing the weight further meaning the idle to redline times are increased.

2, engine breaking effect is reduced, while this is desirable for cruising in some respects, it means that going downhill more braking is required, plus the brakes will also have to work "more against" the engine - I know they actually don't work against the engine in this case, but the higher inertia of the components don't slow down as fast as lighter components meaning less force is applied to slowing the car down. (no I don't understand that paragraph either)

3. The torque (which thereby effects the power) is affected by things like:
- stroke length (longer for more torque [uses more of the combustion], shorter for higher revs)
- cylinder bore (the bigger it is, the more torque)
- compression ratio of the engine (how much the air is compressed from the cylinder at the end of the induction stroke, to the end of the induction stroke (I think, but feel free to correct me someone) - the bigger this is, the greater the torque)
-number of cylinders (because the torque is delivered in bursts, more cylinders allow for a smoother delivery of the torque, and for a higher mean torque across the full 720? cycle of the engine)
-the fuel-air mixture, this is the biggest variable between two engines that have the same characteristics as above, by altering it in some witch-craft way, greater low rev torque can be created and so on and so forth. Variable valve timing does something to alter the fuel air mixture.

(sorry if any of the above is patronising and or plainly wrong - corrections welcome)
 
Blythy said:
3. The torque (which thereby effects the power) is affected by things like:
- stroke length (longer for more torque [uses more of the combustion], shorter for higher revs)
- cylinder bore (the bigger it is, the more torque)
bore = torque - bore the cylinder out, you get more torque.
stroke = horsepower - add stroke to the piston, you get more horsepower.
Blythy said:
-the fuel-air mixture, this is the biggest variable between two engines that have the same characteristics as above, by altering it in some witch-craft way, greater low rev torque can be created and so on and so forth. Variable valve timing does something to alter the fuel air mixture.
the timing does that more than the fuel-air mixture. the fuel-air mixture would vary the horsepower.

so far, both of you are correct.
 
Ok, lemme see if we're on the right wavelengths here

if the stroke is shorter there's a hell of a lot less torque (that's why F1 engines idle at 4000 rpm), but that allows you to get higher revs therefore more horsepower.

If the cylinder has a greater bore, then it will have greater torque because there's more surface area for the combustion to act on.

I fail to see how adding stroke to the engine gives more horsepower (expcept in the case of increasing mean torque by allowing the torque from combustion to act over a longer period.)
 
correct. if you are wanting more torque, the added fuel-air combustion from added bore increases the torque on the crank.

adding stroke reduces the torque because the combustion does not act on the piston for very long because of the long travel of the piston, increasing horsepower. horsepower is increased because the power is sustained longer through the lengthened piston rod, which makes the compression ratio higher. for instance, many chevy 350s are stroked to 383s, giving, depending upon intake, carb, about 400HP. it may increase torque a little, but not as much as boring it out. you also increase horsepower a little from boring too.

take for example our '65 Ford Falcon NHRA dragster. we have a Ford 460 bored out to 605cid, but the stroke isn't lengthen by much, if at all. we recently broke the engine dyno of our engine guy. doing rough calculations, we figured 1000HP and about the same or more of torque running methanol. the engine started with maybe 500HP and 600ft-lb on methanol before boring. boring makes a big difference.

torque is the medium that gets you off the line or accelerates you. horsepower is the medium that sustains your speed. does this explanation help, or make it worse?
 
burnsy said:
torque is the medium that gets you off the line or accelerates you. horsepower is the medium that sustains your speed. does this explanation help, or make it worse?


That's how I normally explain it to people - Torque gives acceleration, Horsepower gives top speed.

Or, as Carroll Shelby said, Torque wins races and Horsepower sells cars. ;)
 
bore = torque - bore the cylinder out, you get more torque.
stroke = horsepower - add stroke to the piston, you get more horsepower.


Thats a bit misleading, the 383 you mentioned gets more power because of increases displacement and compression ratio, not only because the stroke is increased.
Keeping the displacement the same on 2 engines, one having a long stroke, one having a short stroke, the one with the the shorter stroke will be able to rev higher because the time it takes the piston to go around is shorter. Higher revs = higher horsepower. Thats why F1 engines can rev to 18000RPM, low displacement V-10's (or now the new 2.4L V-8's) that have very short strokes. They also make very little torque which is why they have to rev to 18000 to make power.
 
wrong, the one with the shorter stroke now has lower compression, so lower horsepower. F1 engines are still able to get huge horsepower because they control the compression and have the intakes flowed perfectly. horsepower is heavily influenced by how well your intake flows. take a look at the specs of one and you'll see the compression is high. normal for a passenger car is about 9 - 9.5:1 ratio. you are right that they don't make torque.

sure it could rev higher because of shorter stroke, but that doesn't mean that you get more horsepower. take the same engine with the same everything in it and you just shorten the piston rod, you will loose horsepower.

the 383 is not bored out much, maybe .030 over, but the big change in displacement is from the new stroke.

make sure you do your own research before challenging others.
 
can't find any websites that have the specs of compression ratio in the new V8 F1 engines. probably it's top secret
 
i'll check in my superchargers book tomorrow to definitively make sure about bore vs stroke. i hope that we aren't confusing bone with all this.
 
burnsy said:
can't find any websites that have the specs of compression ratio in the new V8 F1 engines. probably it's top secret

It should be, since none of the new V8 engines have seen the race tracks yet. They are still relatively early development versions.

But that's offtopic.
 
Blythy said:
I fail to see how adding stroke to the engine gives more horsepower (expcept in the case of increasing mean torque by allowing the torque from combustion to act over a longer period.)

that's exactly what it is, you have a longer stroke, so the power is used for a longer period of time, giving more power
but if you increase stroke, the amount of RPM will decrease, so i'm not realy sure you actually would get more hp

but back to the original question
take 2 cars, with the same amount of torque and horsepower, but 1 with lightweight parts and 1 with normal parts.
if you have a dragrace between them, the one with the lighter parts will win for sure, although on the dynograph, they both have the same amount of torque and hp

so what i'm apparently trying to say is that a dynograph tells shit, you don't know anything with it, there's a criteria missing, the speed in which the revs increase (aka the length of the X-axis)
 
bone said:
but back to the original question
take 2 cars, with the same amount of torque and horsepower, but 1 with lightweight parts and 1 with normal parts.
if you have a dragrace between them, the one with the lighter parts will win for sure, although on the dynograph, they both have the same amount of torque and hp

so what i'm apparently trying to say is that a dynograph tells shit, you don't know anything with it, there's a criteria missing, the speed in which the revs increase (aka the length of the X-axis)

Correct - though what a round-about way of saying it :p

A dyno chart will only tell you the motor's output across the rpm range, and how progressively those outputs build or fall, so it's just one factor in the car's performance (gearing, weight, grip etc)
 
burnsy said:
wrong, the one with the shorter stroke now has lower compression, so lower horsepower.

Wrong.
Compression ratio has nothing to do with bore and stroke.
Back to the other point:
Let?s say we take an 1.6 liter angine, 4 cylinders, in line.
Now imagine that said engine is square (bore=stroke) and it?s producing 100hp at 6.000rpm and 15Kgfm of torque at 3.000rpm.
If we make it oversquare (bore>stroke) it will rev higher, but it will probably produce the same 100hp@6.000rpm and 15Kgfm@3.000, having more power over 6.000rpm and more torque over 3.000rpm.
make this engine undersquare (bore<stroke) and the opposite happens:
more torque in lower revs and a "loss" in horsepower, because the hp and torque curves change.

Oh, and that?s not a nice way to answer a post:
burnsy said:
make sure you do your own research before challenging others.

As far as "research" goes, FIAT launched a few years ago a "Corsa Lunga" (long-stroke) version of the Brava. The engine was exactly the same, but with longer stroke and smaller bore. The result: more torque in the lower rev range... ;)
 
I've just read it, and I don't see anything that supports your arguement.

The 383 example you used works so well because, well, most V8's in the past were crap. Rasing the stroke raised compression and displacement which rasied torque and horsepower.
 
thanks. that's what i've been saying. however, quoting the book:
How to successfully change P is the crux of this book. Two ways exist: raising the compression ratio or using forced induction.
as i said before, increasing stroke will increase compression unless you use a different head.

Redliner: the Corsa Lunga got the massive torque because of a well flowed intake manifold and probably 4 valves/cylinder.
 
i give up. not a single one of you actually know what you are talking about. i actually work on engines. we'll be soon in Top Alcohol in NHRA with a 3000HP engine.

now i see why Haz left the forum.
 
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