Topic: Higher or lower compression for bottom end power?

[Soos]

I know high compression motors can pour out the power...
But with high CR pistons, most people go with bigger pistons anyways.


In the example of a bike with a stock cam, mild porting, new valve job, and lets say 67mm pistons.(haha, i wish...)
Would there be simply a power LOSS by having lower compression pistons?
Or would it simply shift where the power is produced in the RPM band compared to the same pistons with a higher compression ratio?


While i have your ears....(eyes?)
Is there really a big difference in oil temps with higher and lower compression motors?


l8r

[MRieck]

Higher compression will improve you "low" end. You will make more torque. High compression pistons will elevate oil teps but depends on the size of the bore and heat shedding ability. My FJ is 1314cc with almost 11 to 1. It has about 95lbs of torque but I have seen oil temps well over 300 F after being stuck in traffic for 20 minutes. That's with the largest Earl's cooler I could fit

[Rod]

Higher compression will improve you "low" end. You will make more torque. High compression pistons will elevate oil teps but depends on the size of the bore and heat shedding ability. My FJ is 1314cc with almost 11 to 1. It has about 95lbs of torque but I have seen oil temps well over 300 F after being stuck in traffic for 20 minutes. That's with the largest Earl's cooler I could fit

Thats my understanding too, torque = acceleration, bhp = top speed. If you want a short circuit racer that really pulls you out of turns then go for high c/r and get the extra torque. If you're racing at Le Mans with the 4 mile mulsanne straight, back off the c/r a little and all other things being equal you will pull a higher top end. I run 12.5/1 on my 500 race engine, the torque builds so strongly from 7k that you'd think its the bhp curve until you get another kick in the pants at 8k.

[TwoTired]

Higher compression makes more power anywhere in the RPM band, other factors not considered.
But, factors such as cam grind are pretty large considerations.

The bike's air cooling system is factory designed for the power made by the stock engine.  It's those cooling fins you'll notice festooned around the engine.
Any field modification that increases the power output of the engine, also increases the amount of waste heat the those cooling fins must dissipate.

Unless you add more cooling fin area, increase the air flow over the existing fins, or increase the air temp differential between fins and ambient surrounding air,  an engine power boost over stock will also make the engine run hotter.  (Laws of thermo dynamics, you know.)

When the engine is actually making more power than what was designed to operate at, a hotter engine will naturally make the oil inside hotter as well.
Also, the oil under increased pressure in the journal heats more from the higher compressive loads.  Simply squeezing oil, even without the combustion chamber heat, generates heat within the oil.

Cheers,

[Rod]

Higher compression makes more power anywhere in the RPM band, other factors not considered.
But, factors such as cam grind are pretty large considerations.

TwoTired - could you be more specific, by power do you mean torque or bhp? could you tell me how increased c/r effects torque and bhp separately, as a racer we look at them individually as engine characteristics so it would be really useful to know. The term 'power' is too vague.
Thanks

[tsflstb]

I'll take a crack at it and let TwoTired grade my work.  Go easy on me.

Torque is a force that can be measured – like on a dynamometer.
Horsepower is a unit of power (force per unit of time) some guy defined based on the amount of work a horse was observed doing (moving a 550 pound weight one foot in one second).  HP is calculated from torque.

HP = (Torque*RPM)/5252

Torque is force X distance.  The geometry of the engine (distance) is constant, so the combustion pressure on the piston (the force) is the only thing that changes torque.  A higher compression ratio will give you a higher mean effective pressure for each combustion cycle and thus more torque at any RPM. 

The torque varies with RPM, and your bike will accelerate hardest at its torque peak.  Where that torque peak lies depends on a lot of things including the flow characteristics of the engine.  Guys much smarter than me make a good living porting heads, developing cam grinds and figuring out that part of the equation.
 

[mlinder]

exactly.

Torque and HP are not separate beasts.
Torque is merely force (or work).
HP is merely a measuring of force (work) over time.

They are the same thing.

[TwoTired]

Higher compression makes more power anywhere in the RPM band, other factors not considered.
But, factors such as cam grind are pretty large considerations.

TwoTired - could you be more specific, by power do you mean torque or bhp? could you tell me how increased c/r effects torque and bhp separately, as a racer we look at them individually as engine characteristics so it would be really useful to know. The term 'power' is too vague.
Thanks

I began to write something up.  But, I have 3 sets of carbs to rebuild, three fork sets to refurbish, a sidecar to mount, lawn to mow, and...you don't really want to know the whole list.
So, I did a quick search.
http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/index.html

However, torque and HP are different, but related in IC engines.  Motors CAN develop torque without producing any work.  Electric ones come to mind.  HP involves work over time.

Maybe I can add more later tonight...

Cheers,

[mlinder]

Didn't know we were going to be talking about the two outside of how they relate to each other in our internal combustion engines.
I'll be more precise, then:

Torque and HP are not separate beasts. in the context of the internal combustion engines in our motorcycles that we are talking about
Torque is merely force (or work).
HP is merely a measuring of force (work) over time. in the context of the internal combustion engines in our motorcycles that we are talking about


 

[TwoTired]

Didn't know we were going to be talking about the two outside of how they relate to each other in our internal combustion engines.
I'll be more precise, then:

Torque and HP are not separate beasts. in the context of the internal combustion engines in our motorcycles that we are talking about
Torque is merely force (or work).
HP is merely a measuring of force (work) over time. in the context of the internal combustion engines in our motorcycles that we are talking about


 

By your own presented definitions, they are different.  Torque does not include time as a factor, HP does.  How can you maintain they are the same when you describe/define them differently?  While it is true that you get both these factors at the same time in an IC engine (for non-zero values), it doesn't mean the two factors are interchangeable or mean the same thing.

Further, torque is not the same as work as for as physics terms go.  Torque is the application of energy, work is when the energy is actually transferred.  Beyond that, the availability of torque does not imply delivery of said force.

I don't see how this is helping other posters, though. 

Back to the original point...
Higher compression increases the energy density at combustion time, allowing the engine to make more Hp AND torque.  If you wish more of one than the other, then address fuel burn timing and duration in relation to crank position.  If your burn pressures are concentrated while the crank rod journal is positioned near or around the 90 degree point, you increase torque, as that is where the lever arm has the most mechanical advantage for the twisting force.  If you make the burn and duration timing more in line with the entire power stroke, you increase horsepower, as that maximizes the power duration.  And yes, you can do both, but not to maximum advantage on both factors simultaneously.

Increased compression is an enabler for more power. Where, exactly, you maximize this power or force is to a greater extent dependent on engine timing factors, of ignition, cam, piston stroke, and breathing characteristics.  But, it is more complex than that.  Air has mass, and inertia.  Velocity introduces other factors to charge density and burn timing.

Finally, light machines or machines that have low resistance to inertial change perform better with horsepower increases.  Conversely, machines with high inertial resistance (e.g.) more weight) perform better with torque increases.

That's how I've come to understand it.

Feel free to argue.

Cheers,

[754]

If you think torque & horsepower  are not seperate beasts, then please explain why a single gear Sportster can do an 8 sec 1/4 mile, and most 7 sec Kawis or Suzis cannot.

 In the end it is a combination of both.. that is why a 100 hp Sporty could eat a 115 hp 4 in the 1/4 mile..

 At the end of the day unless you spend most of your day at or near redline, torque is what you use most on the street.

 That being said, I will say this, I have a motor that put out an amazing amount of torque for a 4 cylinder.. and it has been to put it mildly a lot of fun.. being able to short shift and still pull like crazy at 4 or 5 K.. and still rev to 11k if you feel like it.

 The main factor here is my combination, and I think the Webers play a large part.. Everyone that ever rode the bikes, even on stock displacement and cam was surprised at the response and low end and midrange power.

 But you know if you want max torque, I would seriously think of a single Weber onto a plenum, leading to the 4 runners..

[754]

Hi comp motors make HP & heat..

 How much heat you are willing to put up with & what fuel you are willing to run are major considerations..for street..

 Good news though, most hi-comp pistons can be cut down to lower compression..

[mlinder]

Fraid not, 754, and TT, if you are arguing semantics. At the end of the day, the formula is the same. HP is torque over time. Which makes HP the same as torque, just over time. The function of something is its defining factor. rolling an elephant down the street a hundred yards doesnt change it from being an elephant. It's just a hundred yards down the line.
Going to bed, but happy to argue using proper semantics tomorrow.

[Soos]

WOW......

Quite an informed bunch of replies.

I was under the assumption HP was torque over time as well...
l8r

[mlinder]

I'll start with 754's statement.

754, you left out some extremely important information regarding 100hp sporty vs 115hp sportbike.
Namely, at what RPM are these bikes making peak HP? And at what RPM do the engines start to make torque within 25% or so of their peak torque?

As I said, HP is torque (or to make TT happy, HP is a direct FUNCTION of torque.)

Everyone knows that, all other things being equal, if both bikes could launch at peak HP and the tires held, the 115hp bike would be fastest down the 1/4 mile.

But that doesn't happen.
Your 100hp sporty is likely making closer to peak torque (and thus, horsepower) over a longer (larger) rev range, and sooner, hence the faster 1/4 mile.

What does this mean? It means the sporty is making MORE horsepower for a longer period of time than the sportbike in the confines of a 1/4 mile track. Put those bikes on a half-mile straightaway, and the situation changes rapidly.

HP is a function of torque. remember, if you make 60 lbft at 5252 RPM, you will always make 60hp at 5252 RPM.
If you make 70lbft at 7500 RPM, you will always make 100hp at 7500 RPM. With any engine. There are no variables. Period. Thus Torque IS HP in the application at hand (IC engines).

Your desired application of said HP is important in determining how you want to build your motor. Peak HP, or length of peak HP curve.
You have to pick one given constraints of an engines size and it's design.

[Rod]

WOW......

Quite an informed bunch of replies.

Soos I'd second that, some impressive knowledge - One thing that interests me in this exchange of knowledge/views is understanding how the theory informs application. I'm a research scientist but not an engineer, and I know that many of the things I 'know' dont necessarily benefit people in the applied field. So whilst its good to know what the definitions of torque and bhp are, as a bike rider/racer my real need is to understand which line on my dyno readout is the one that tells me what I'm going to experience through the seat of my pants, whether I can now stick with the guy who blitzed me last time out etc. I used to race a 500/4 in a F2 category against RD(RZ)350's most of whom ran TZ internals, so I was definitely down on bhp but could out accelerate them from the turns (as could the 600 Dukes that ran in that class), only to get passed on the straights. So thats the real-world experience, can it be explained in the theory of bhp & torque if they are essentially the same thing?

[mlinder]

WOW......

Quite an informed bunch of replies.

Soos I'd second that, some impressive knowledge - One thing that interests me in this exchange of knowledge/views is understanding how the theory informs application. I'm a research scientist but not an engineer, and I know that many of the things I 'know' dont necessarily benefit people in the applied field. So whilst its good to know what the definitions of torque and bhp are, as a bike rider/racer my real need is to understand which line on my dyno readout is the one that tells me what I'm going to experience through the seat of my pants, whether I can now stick with the guy who blitzed me last time out etc. I used to race a 500/4 in a F2 category against RD(RZ)350's most of whom ran TZ internals, so I was definitely down on bhp but could out accelerate them from the turns (as could the 600 Dukes that ran in that class), only to get passed on the straights. So thats the real-world experience, can it be explained in the theory of bhp & torque if they are essentially the same thing?

Please read my last post.

It can be explained, HP being the function of torque at a given rpm. The rz350's may only make 25lbft ~ at 14000rpm... which is 66hp.
Your 500/4 may make 35lbft, but at 9k rpm, with torque dropping significantly after 9k...... giving you something less than 60hp.
With proper gearing, I would assume you are both at those kinds of RPM in similar places. The rz's are lighter, and making more power, in the same spot.
RZ is obviously going to get down the straightaway faster.

As you can see, the rz makes significantly less torque at EQUAL rev ranges, but, since the rz is built to carry higher RPM, it makes more horsepower at the end of it's rpm range.

[mlinder]

By your own presented definitions, they are different.  Torque does not include time as a factor, HP does.  How can you maintain they are the same when you describe/define them differently?  While it is true that you get both these factors at the same time in an IC engine (for non-zero values), it doesn't mean the two factors are interchangeable or mean the same thing.

Further, torque is not the same as work as for as physics terms go.  Torque is the application of energy, work is when the energy is actually transferred.  Beyond that, the availability of torque does not imply delivery of said force.

I don't see how this is helping other posters, though. 

Back to the original point...
Higher compression increases the energy density at combustion time, allowing the engine to make more Hp AND torque.  If you wish more of one than the other, then address fuel burn timing and duration in relation to crank position.  If your burn pressures are concentrated while the crank rod journal is positioned near or around the 90 degree point, you increase torque, as that is where the lever arm has the most mechanical advantage for the twisting force.  If you make the burn and duration timing more in line with the entire power stroke, you increase horsepower, as that maximizes the power duration.  And yes, you can do both, but not to maximum advantage on both factors simultaneously.

Increased compression is an enabler for more power. Where, exactly, you maximize this power or force is to a greater extent dependent on engine timing factors, of ignition, cam, piston stroke, and breathing characteristics.  But, it is more complex than that.  Air has mass, and inertia.  Velocity introduces other factors to charge density and burn timing.

Finally, light machines or machines that have low resistance to inertial change perform better with horsepower increases.  Conversely, machines with high inertial resistance (e.g.) more weight) perform better with torque increases.

That's how I've come to understand it.

Feel free to argue.

Cheers,

You can't make more torque without making more horsepower.
The math is always the same. If an engine makes a given amount of torque at any given RPM, it is making an amount of horsepower that is inflexible. It is merely expressing the torque in a different manner.
 
What you are implying is that if I have a cubic foot container of water, and funnel it through a tube to a container that measures it's weight instead of volume, that I have somehow changed its properties.
I haven't. I'm merely measuring it in a different way. The water remains the same in either the cubic foot container or the weighing container. Passing it through a hose did not change it in any way.
The weight of the water and the volume of the water is the same thing, and we are merely expressing it's properties in different fashions.

/edited for redundancy. I tend to do that, sorry.

[754]

all dictionary definitions aside, I am still disagreeing.

 Do you have any idea of how much you can increase torque in a V-twin, without a corresponding increase in HP?

 or shall I say, increase torque at the expense of horsepower..


 funny it has not been mentioned but torque is often described as ability to do work..

[mlinder]

No you can't.

You can increase torque in one area without increasing HP in another area, but by definition, if you are creating more torque, you are creating more HP.

Again, HP = (tq*rpm)/5252
Quite simply, if your sporty is making 80lbft of torque at 4k rpm, it is making 60.9 hp at the same RPM. If you increase torque to 90lbft at 4k rpm, it is making 68.64 hp at the same RPM. You have increased horsepower by increasing torque. You cannot argue this. This is like arguing against the effects of gravity.
What you are referring to is increasing torque in one rpm range while not increasing it in another. If you increase torque at 4k rpm, but do not increase it at 6k rpm (likely the beginning of the engines peak hp), you have increased hp in the 4k range while leaving it alone in the higher rev ranges.
This does not mean you haven't increased horsepower, it means only you haven't increased peak horsepower.
This isn't something up for argument. It's simply the way things work.

[754]

The books have it all wrong

Here is the definition of torque, as explained by a local BritBike rider, named Hylton..

And I thoroughly respect his opinion..

 It goes like this,

When you wake up in the morning with a p!ss hardon, you go to take a leak.

While bearing down on the er "member", to perform said task, your heels come up off the ground a few inches..

"Now THAT is TORQUE!!"


Most of us in these parts respect his opinion. If I did not respect him I would tell the story about how I had to haul his Norton back from the Drags, or how I had to ask the girl at the gas station, to help pushstart, his B50.. but we really like the guy so our lips are zipped...

[Joksa]

Higher compression makes more power anywhere in the RPM band, other factors not considered.

Do you have any idea of how much you can increase torque in a V-twin, without a corresponding increase in HP?

If peak power was already restricted by the airflow, you might not gain peak power.

[bistromath]

Speaking scientifically, mlinder is dead right. HP = TQ * RPM. Can't change that, same way you can't change the sun rising in the east. It's just how it is.

But (there's always a but)...

When people speak of a "torquey" engine, they mean one that develops a lot of power in low RPM ranges, like a V-twin or a Chevy V8. Can't argue that Harleys pull like hell right off the line. But while their peak torque is greater than an inline 4, they don't rev as high -- and so that RZ can take them in the straights. You can spin a smaller displacement engine to light speed, and while its peak torque is never very large, its HP can be just because it is spinning so fast. That doesn't mean it isn't pulling your arms out of your sockets, just that it didn't pull your arms out of their sockets off the line. You'll never get that low-end grunt like you will with a bigger motor.

You can definitely tune a motor to achieve greater torque at the expense of peak HP: just put in a cam with less duration. Your bottom-end torque will increase, while your top-end HP will suffer because the cam just won't breathe up there. Every Japanese inline four compact car is tuned like that, to give decent torque off the line from a small motor.

Speaking of "HP" and "torque" without their corresponding power curves is basically useless. Neither spec tells you where that motor is making its power, or how usable that power is. I can show you a motor with ludicrous HP that will fall flat on its face outside the narrow RPM band for which it was tuned. And I can show you a Ducati 916 which, while not the most powerful bike on earth, won a crapload of races because its legendary powerband was so wide that racers could depend on having the power no matter where in the RPM range they were.

The reason greater torque generally corresponds to "more grunt" in most people's minds is that the only time most people demand that sort of acceleration is from a stoplight, not when you're already doing sixty on the highway -- and at a stoplight, it's torque, not peak HP, that matters!

[mlinder]

Well, yes and no, bistro.

Correct in the fact, of course, that talking abut power without reference to the power curve is pretty much a useless endeavour, but you are still referring to torque and horsepower as separate entities.
I honestly have to wonder how the whole misconception about the two being somehow different started. I used to think they were different, until that magic formula for calculating HP was shown to me. At that point I relaised that HP is merely a different way of expressing torque.

There really is no 'but' involved (perhaps a butt, for not giving up on this topic, but hey), the power you feel at a stop light and the power you feel accelerating from 100 to 130mph is the same power.
It doesn't, at 5252 RPM 'magically' switch over to 'horsepower', some 'different kind' of power that is somehow disconnected form 'torque'. It's the same damned power, and each measurement and expression of this power coresponds directly to the other without variation, in every internal combustion engine that has ever existed and will ever exist.

[mlinder]

754, as Bistro said, and I said earlier, you are confusing 'peak horsepower' with 'horsepower'.

Tell you what. I'll bet you or any of your engine building friends that disagree with me that raising torque raises horsepower without exception (being they are the same thing, afterall..) 1000 dollars that if they increase an engines torque by 10 at 4krpm, it will increase hp by some 7.7.
Or pick another RPM, wherever. Pick any torque increase, anything you want. I will calculate the horsepower increase on the fly, because there are no other variables other than torque and rpm. Since there are no variables, I can tell you exactly how much horsepower will increase at the same RPM a torque increase occurs. There's no guess work. I'm really not sure how to make this more plain to you.

In any case, ask your engine building friends if they would like to go to bat for you in putting up a wager against me. None of them will take it. And if they do, please, please, let them. I could always use an extra grand.