Determining Redline

[Skidmarx]

How does and engine manufacturer determine the redline for an engine? Why is it okay to rev one engine to 14K and another to only 9.5K?

 

[Sidewalk]

Very long list of reasons. The shortest is piston speed. The rods can only handle so much before giving out.

 

[afm199]

Many reasons. A typical piston ring loses ability to wipe oil when it reaches 90 fps, give or take 10 or 15 fps. So the old LONGGGG stroke motors with a four inch stroke had relatively low rpm limits. take the same CCs and make the piston wider and the stroke shorter, and rpms increase. Ring wiping ability is a big limit. Others are cam timing... the cam will be set up to deliver power in a certain way, and may allocate some top end power to midrange, meaning as the motor reaches near redline, the power drops off. That's why aftermarket cams and ecu's raise rev limits. Rods also stretch and valves float, creating a double bind as a valve stops closing all the way ( the spring is not strong enough to slam it shut in the very brief period it is open). Rod stretches, valve does not close and it contact the piston or another valve. Cheap motor with a poorly balanced crank keeps rpm down. When valves and components are made of Titanium valve train weighs less and can sustain higher rpms.

But the big deal is piston speed, when the piston goes much faster than 60 mph problems start to happen. My old Norton at 6800 rpm had the same piston speed as my SV650 at 10,500.

 

[reckon]

Many reasons. A typical piston ring loses ability to wipe oil when it reaches 90 fps, give or take 10 or 15 fps. So the old LONGGGG stroke motors with a four inch stroke had relatively low rpm limits. take the same CCs and make the piston wider and the stroke shorter, and rpms increase. Ring wiping ability is a big limit. Others are cam timing... the cam will be set up to deliver power in a certain way, and may allocate some top end power to midrange, meaning as the motor reaches near redline, the power drops off. That's why aftermarket cams and ecu's raise rev limits. Rods also stretch and valves float, creating a double bind as a valve stops closing all the way ( the spring is not strong enough to slam it shut in the very brief period it is open). Rod stretches, valve does not close and it contact the piston or another valve. Cheap motor with a poorly balanced crank keeps rpm down. When valves and components are made of Titanium valve train weighs less and can sustain higher rpms.

But the big deal is piston speed, when the piston goes much faster than 60 mph problems start to happen. My old Norton at 6800 rpm had the same piston speed as my SV650 at 10,500.

I always thought it was valve float, or crank balance,,....

thanks for that info, good stuffs

 

[Skidmarx]

Clearly I'm out of my element here. Forgive me if this is a totally stupid question, but what effect does a long or short piston travel have on engine performance?

Does a longer stroke have any correlation to higher torque outputs? Is this why high rpm engines can be a little anemic at the low end? Is it that their short stroke doesn't produce significant torque until the engine winds up?

Yeah, I have no idea what I'm talking about. Thanks for being patient. I'm trying.

 

[joespeedfast]

As far as production engines go, it has far more to do with intended usage, desired peak output and delivery character more than design or material limits.
That modern 1250 bandit engine revs lower, has less peak power and a lesser torque curve than a 99(96 engine design) 1300 Busa engine. Even though that engine is 10+ years newer, it's intended use and application are different.
Also design limits only come into play when all the good materials are used(lightest, stongest). Material limits go hand in hand with intended use and output as delivered stock. Also cost determines parts qaulity(pistons-rods). arty

 

[Sidewalk]

Clearly I'm out of my element here. Forgive me if this is a totally stupid question, but what effect does a long or short piston travel have on engine performance?

Yes, it limits engine RPM. Longer stroke means more FPM which limits RPM.

Does a longer stroke have any correlation to higher torque outputs? Is this why high rpm engines can be a little anemic at the low end? Is it that their short stroke doesn't produce significant torque until the engine winds up?

No appreciable difference. In theory a longer stroke has more leverage at the crank. In practice it has far more to do with valve timing, intake runners and plenum volume.

The reason high RPM engines feel weak at low RPM's is because the manufacturer designed them to run at high RPM's. If you had all those RPM's to play with an then didn't use them, it would be a huge waste of available power. You would have more torque down low (like the FZ1 with the R1 engine), but it wouldn't have the HP of the R1 because it isn't making full use of the available RPM's.

HP is a calculation of torque and RPM. The higher each goes, the more HP.

 

[ST Guy]

It's not the speed of the piston, it's the forces that it and the rod and other reciprocating parts see when they change direction. If the piston never changed direction and always went one way, it wouldn't really matter how fast it went as long as friction and therefore heat was controlled. (Oil control is a different issue, though related.)

When a piston (and rod) reaches the bottom or top of the stroke, it changes direction extremely rapidly and tremendous forces are imparted into the piston and rod. At a given RPM, the heavier the parts the greater these forces and the lower the RPM limit is. Conversely, the lighter the parts, the lower the forces and the higher the RPM limit. That's one reason that manufacturers are always looking to make those parts lighter. Increasing the strength of the base material is another way to allow greater forces.

Of course, there are other factors as well. For instance, I know that my ST1100 can reve way beyond it's 8000 RPM limit. But because of the way the motor was designed (broad, low to mid range torque curve), it doesn't breath very well once the revs get above redline. Bottom line is that there are many factors to setting redlines besides whether the parts can take the loads or not.

Hope that helps.

 

[Skidmarx]

Wow, that's great information...and it makes complete sense!

Redline is a function of the design of the engine for its intended use. Factors coming into play are: quality of materials, cam timing, stroke length, and I assume, the ability of the all the parts to get enough lubrication.

Pretty cool!

By the way. What's "fps"? Apparently rings loose the ability to seal the oil or clear the oil from a cylinder wall at above 90 fps? Is that right?

What are 90 fps?

Classes are now in session at UOB...University of BARF!

 

[07chuck]

Wow, that's great information...and it makes complete sense!

Redline is a function of the design of the engine for its intended use. Factors coming into play are: quality of materials, cam timing, stroke length, and I assume, the ability of the all the parts to get enough lubrication.

Pretty cool!

By the way. What's "fps"? Apparently rings loose the ability to seal the oil or clear the oil from a cylinder wall at above 90 fps? Is that right?

What are 90 fps?

Classes are now in session at UOB...University of BARF!

Feet per second.

What happens with the rings is called flutter, to allow for heat expansion the groove that the rings sit in is larger than the ring itself(and there's always a little extra on production bikes due to manufacturing processes), at higher piston speeds the ring can't recover from the change of direction so doesn't complete the seal.

Besides weight it is one of the main limiting factors on piston speed.

 

[OldYam]

fps = feet per second.
Think about this sometime when your engine is turning at 6,000 rpm (not very fast for today's motorcycles). 60 seconds in a minute means that the poor little piston has to go from a complete stop at the bottom of the stroke, accelerate up the cylinder, then decelerate and come to a complete stop at the top of the stroke, then reverse the motion and jerk down again 100 TIMES EVERY SECOND. Imagine the stress on the wrist pin. It's a wonder everything don't just fly apart.

 

[afm199]

fps = feet per second.
Think about this sometime when your engine is turning at 6,000 rpm (not very fast for today's motorcycles). 60 seconds in a minute means that the poor little piston has to go from a complete stop at the bottom of the stroke, accelerate up the cylinder, then decelerate and come to a complete stop at the top of the stroke, then reverse the motion and jerk down again 100 TIMES EVERY SECOND. Imagine the stress on the wrist pin. It's a wonder everything don't just fly apart.

Exactly, now look at a motor turning at 18,000 rpm, and the piston is bobbing up and down 300 TIMES A SECOND!! Now remember that piston is probably not actually traveling much more than a 1970 Norton in terms of piston speed, as the 90 fps ( sixty mile an hour ) piston speed is the limit on both. The 18,000 rpm motor has a very very short stroke, the 6000 rpm motor a very long one.
There are motors that exceed 90 fps, but it seems to be a rough limit.

 

[Skidmarx]

That's just crazy to think about. It doesn't seem possible that engine components can take that kind of abuse for tens of thousands of miles!

Nor does it seem possible that you can get 100K on a motorcycle engine, but I've heard of people that do. Must be significantly worn by that time...

I'd type more, but I've got to go to my garage. I'm going to have a group hug with my bikes. I need to be kinder to them. They've been so unappreciated.

 

[gixxerboy55]

Most four cylinder bikes will easily do 100 k as most of the time there is very little load on the engine compared to a one cylinder motor. As for piston piston speed a gsxr 600 at redline 15,500 rpm, has less piston speed then a gsxr 1000 at redline, if i remember about 13,000 rpm. The 1000 is based on the 750 and is basically a stroker motor so it has a fairly high piston speed.

 

[afm199]

That's just crazy to think about. It doesn't seem possible that engine components can take that kind of abuse for tens of thousands of miles!

Nor does it seem possible that you can get 100K on a motorcycle engine, but I've heard of people that do. Must be significantly worn by that time...

I'd type more, but I've got to go to my garage. I'm going to have a group hug with my bikes. I need to be kinder to them. They've been so unappreciated.

The amazing thing is the ancient shell bearing/oil pressure system that has been in use for probably a century and still allows amazing longevity and rpm. The little shells hold oil under pressure between the crank pin and the shell, whether rod or main, and theoretically the pin never touches the shell, the oil film of a couple thousandths of thickness keeps parts from touching. In practice, motors like singles and twins get some serious bearing shining at high rpms with high comp motors. My race motor with 2k on it had about 5/10,000 of an inch wear, which is a lot for an SV, but it has been absolutely flogged and bounced off the rev limiter fifty times this last season.

 

[Skidmarx]

Most four cylinder bikes will easily do 100 k as most of the time there is very little load on the engine compared to a one cylinder motor. As for piston piston speed a gsxr 600 at redline 15,500 rpm, has less piston speed then a gsxr 1000 at redline, if i remember about 13,000 rpm. The 1000 is based on the 750 and is basically a stroker motor so it has a fairly high piston speed.

If I'm understanding this correctly, piston speed is dependent on stroke length, not just over all rpm. A bike at 6k rpm might have the same piston speed at a bike at 14k rpm because of the distance (ie stroke) the piston has to travel.

Motors are limited to a piston speed of roughly 90 feet per second, regardless of the rpm. Some motors achieve that speed at at a lower rpm (long stroke) and some at higher rpm (short stroke).

Am I getting this?

 

[gixxerboy55]

You got it easy as pie.

 

[jake28]

When a piston (and rod) reaches the bottom or top of the stroke, it changes direction extremely rapidly and tremendous forces are imparted into the piston and rod. At a given RPM, the heavier the parts the greater these forces and the lower the RPM limit is. Conversely, the lighter the parts, the lower the forces and the higher the RPM limit. That's one reason that manufacturers are always looking to make those parts lighter. Increasing the strength of the base material is another way to allow greater forces.

Newton's Second Law: F=ma

 

[Sidewalk]

Motors are limited to a piston speed of roughly 90 feet per second, regardless of the rpm. Some motors achieve that speed at at a lower rpm (long stroke) and some at higher rpm (short stroke).

Am I getting this?

Lets through in another variable then: rod length. A short rod is forced to accelerate faster then a longer rod. Increasing the ratio allows for slower acceleration, which increases the RPM limit. Of course, you have limits to the ratio. And, when you increase rod length, you increase mass.

And a bonus-the rod is put under the most stress at the top of the exhaust stroke.

 

[afm199]

and a last thing, as stroke has radically decreased, pistons have gone from looking like water glasses to just enough round piston to hold the rings and a short skirt barely long enough for the piston pin bosses. My old Norton pistons weighed a ton and were longgg, the SV pistons ( motor is close to the same size) are much wider and much shorter.