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Engine/motor geeks.. Question..

F

FREAKYSTLYEY

Inactive
Jan 14, 2007
56
0
#1
I'm pretty knowledgeable a out engines and the such but I cant for the life of me figure this out.

Why does an inline 4 on a sport bike rev higher and make has it's power higher up than a v or parallel twin. BUT a inline rev revs higher than a v6 and that revs higher than a v8.

Please help me
Understand.
 
#3
Smaller lighter parts that have less inertia. You compare a motorcycle I-4 to an automotive V-8? Think if you took two banks of a 600cc motorcycle engine and made a V8. Have you ever heard of a 1200cc V-8? No, V-8s are much larger, 4700cc or larger. Those parts are big and heavy. Big heavy things are hard to accelerate, and more likely to auto-disassemble if you try. That's why V-8 redlined are so low.

Why does a twin rev slower than a 4?
Same story. A 1000cc twin has two 500cc pistons and big valves. A 1000cc I-4 has four 250cc pistons and tiny valves.

So, since the engine with small parts can only take small gulps of air/fuel, and it can spin fast without breaking, the engineers optimize the engine around making horsepower by spinning faster, taking more gulps in a given time.

The engine with the big parts can't spin fast, but it can take huge gulps of air/fuel. So the engineers optimize the engine around making horse power with torque at a slower RPM.

Big cylinders hot hard, but not fast.
Little cylinders hit lightly, but rapidly.
 
#5
A 250 ninja is a cheap little bike built with economy on mind and 1987 technology. You are probably comparing it to cutting edge 600cc sportbitkes that competition causes serious re-designs every few years.

If customers were lining up to pay 600cc sportbike prices for 250's and all manufacturers were making 250cc twin sportbikes for sale in the USA, competition would give us one heck of a fast and high revving Ninja 250.
 
#6
I'm too lazy to look it up. What is the redline of a 250 Ninja compared to a '85 600 Ninja. The 250 Ninja is a living fossil. Those two dinosaurs might give a fair comparison.

Overall the issue is lightness of parts. Then cheapness of design/manufacturing tweeks the details.

If you built a 2000cc engine as a single, twin, four, six, and eight, all with the same purpose in mind and used the same technology/design/manufacturing. The single would have big heavy parts that rev slow and make big torque. And the eight would have tiny light parts, and spin fast.

BUT...
Nobody is doing this. Different engines are built for different purposes, different displacements, with different profit margins, and different reliabilities/disposabilaties. It is unreasonable to compare a racing motorcycles engine to the engine in a Chevy 4-wheel drive on a farm, and then toss in a econo-bike in a high school parking lot.

Many small light parts will spin faster than few large heavy ones. But this rule gets mucked up a bit because engines designed for reliability won't be designed around high revving power, and economical engines will have cheaper design and material that won't support higher revs.
 
#7
Lightness of parts, length of stroke, camshaft profile and cylinder head design all play a role in how high an engine will rev, and still make power.
 
#9
Mike N said:
Lightness of parts, length of stroke, camshaft profile and cylinder head design all play a role in how high an engine will rev, and still make power.
Click to expand...

Werd. In addition to that, engine balance is a crucial factor for higher revs and certain engine configurations are naturally more out-of-balance (or well-balanced, however you choose to look at it) than others.

Peace,
Greg
 
#10
FREAKYSTLYEY said:
I'm pretty knowledgeable a out engines and the such but I cant for the life of me figure this out.

Why does an inline 4 on a sport bike rev higher and make has it's power higher up than a v or parallel twin. BUT a inline rev revs higher than a v6 and that revs higher than a v8.

Please help me
Understand.
Click to expand...

The connecting rods on a V-twin are on the same pin, unlike an inline, which makes the engine inherently run rougher and makes it need a very heavy flywheel. More mass means it takes more energy to cause it to rotate faster and more time to slow it down. That reduces the top RPM, too. It has lots of low end torque, but you need to wait for it to accelerate.
 
#11
the stresses on a vtwin are alot different then an inline 4. Pistons traveling at angles put force on the cranks and big end. You rev them high they want to pull the motor apart, keep the revs down they develop torque better then a inline 4. A cheaper vtwin like a harley wont rev anywhere near what a duke would, twins can be made to rev high but it costs more in stonger materials to hold them together....
 
#12
FREAKYSTLYEY said:
I'm pretty knowledgeable a out engines and the such but I cant for the life of me figure this out.

Why does an inline 4 on a sport bike rev higher and make has it's power higher up than a v or parallel twin. BUT a inline rev revs higher than a v6 and that revs higher than a v8.

Please help me
Understand.
Click to expand...
The sportbikes' engines are made with lighter retainers, lighter valves, and stronger valve springs. That's just part of the equation to high RPM capabilities. Your run of the mill V-twin, V6 or V8 "can" rotate at RPMs above redline but will stop operating properly since the intake valve stays open during the compression stroke or the head gasket gets blown. What happens is that the OEM valve spring is not strong enough to push the valve closed in time before the next rotation of the cam (or cams) so that the valve stays partially open through all 4-strokes (air/fuel intake, compression, combustion, exhaust). Lighter pistons, piston arms, cranks, and flywheel also aid in reducing mass/weight.

NASCAR engines, I think, can run in the high 8000s and Formula 1 can run into the 11000s so it is doable for V8 and V6 engines but it's all with using the same concepts as high revving sport bike engines.
 
#13
kanonfodr said:
Werd. In addition to that, engine balance is a crucial factor for higher revs and certain engine configurations are naturally more out-of-balance (or well-balanced, however you choose to look at it) than others.

Peace,
Greg
Click to expand...

4 cylinders or V8's tend to be well balanced naturally, a 90 degree V6 is not.
 
#14
placedesjardins said:
The sportbikes' engines are made with lighter retainers, lighter valves, and stronger valve springs. That's just part of the equation to high RPM capabilities. Your run of the mill V-twin, V6 or V8 "can" rotate at RPMs above redline but will stop operating properly since the intake valve stays open during the compression stroke or the head gasket gets blown. What happens is that the OEM valve spring is not strong enough to push the valve closed in time before the next rotation of the cam (or cams) so that the valve stays partially open through all 4-strokes (air/fuel intake, compression, combustion, exhaust). Lighter pistons, piston arms, cranks, and flywheel also aid in reducing mass/weight.

NASCAR engines, I think, can run in the high 8000s and Formula 1 can run into the 11000s so it is doable for V8 and V6 engines but it's all with using the same concepts as high revving sport bike engines.
Click to expand...

Valve float is an rpm killer. I've got titanium retainers and double springs in my Camaro and can spin 7500 rpm without them floating.
 
#16
stroke length also plays an important factor. a longer stroke means the piston has to speed up to travel farther, in the same amount of time. More piston travel, means more friction and more heat

crankshaft distance from the bottom of the cylinders is also crucial. if the crank is higher up in the block of a V engine, the pistons sideload at the bottom of the stroke



the higher RPMs in sport bikes do decrease longevity. The big V10 in my pickup was still running at 300,000. I decided that was enough and bought a new engine. There is no way a sport bike will go that many miles, without ever being torn down
 
#17
Think it has been covered well, but yah stroke length.
or the length of the connecting rod from piston to crank.

in a general sense long stroke equals more torque.
think of a long breaker bar or torque wrench. it allows you to apply more force or torque but would take longer to rotate completly around because or the longer length.

short stroke allows you more rpm, but the trade off is less torque.

think of a shorty ratchet which you could spin around quickly with just your finger.

also the mass of the whole assembly including the pistons connecting rods and crank. the lighter they are the faster they can spin. in a very simplified way of thinking much more complicated than that.

then as mentioned also once you have a piston and crank that can spin faster. the next restriction would be the valve train.

the older cam, lifter, pushrod design has its limits since the higher the rpm the faster it has to open and close. the weakest link is the valve springs which hold the valve closed. and the actual weight and friction of the mechanical method used to open and close the valve(push rod and rocker arms). this is why the overhead cam has pretty much taking over, removing the pushrod and rocker eliminates extra friction and moving parts.

larger valves allow more air fuel to enter, the problem is at higher rpm's it takes more spring pressure to keep a larger valve closed. eventually the valve will "float" because it opens and closes to fast and the spring pressure cannot hold it closed.

the problem with increased spring pressure is it causes even more friction on the valve train. works great to keep em closed, but now the engine has to fight it when it tries to open.

which is why multiple smaller valves are used, still allows more fuel and air to enter, but requires less spring pressure to hold them closed.
 
#18
placedesjardins said:
That's cool. What else have you done to the drivetrain to be running in the 10s?
Click to expand...

Naturally aspirated big block, 13 to 1 compression, single four barrel, TH400, narrowed 12 bolt with 4.10 gears. Car is backhalfed with ladder bars and coil overs. Runs 10's thru the mufflers on street tires and weighs 3600 lbs with me in it.
 
#19
Rememguys I was really speaking if theoretically not actual motors.

But this far I learned..

Longer stroke same displacement = torque LOW REV
Shorter stroke same displacement HP HIGH REV

An engine like a v6 that has 6 combinations per revolution will have more power than a 4 cyl with same displacements.
 

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