Topic: RPM'S and ease of riding and motor damage.

[dramsell]

Hey everyone.

I recently talked to a moto mechanic who told me that my 1978 CB 750K that rpm's at 4500 when going 65 mph was in his opinion really hard to endure on the body, AND it was not made to go those rpm's for highway extended travel causing eventual motor damage).

Is he correct?  I have ridden for a long time (though by no means a mechanic), and have seen many 750 sohc with well over 50K on them. 

Was this bike not built to do highway speeds?

Dave R

[MRieck]

 You got some bad info

[Gordon]

That "mechanic" knows absolutely nothing about 4 cylinder motorcycles. 

[Bodi]

Many American motorcycle mechanics only know about American V-twin engines. Because of the basic design of these engines with the single crank pin and forked roller bearing connecting rods, the maximum RPM is quite low. They are long stroke designs with relatively small bores.
Japanese sport bike engines as a rule have a totally different design philosophy. The SOHC4 engines are basically "square", with stroke and bore equal. This essentially gives more room in the head for valves with a given cylinder displacement, letting the engine breath better. The peak torque available is lower - for equal bore cylinders with equal combustion pressure the reduced stroke means that the con rod force acts on a shorter lever arm to the crankshaft for less crankshaft torque. The higher RPM potential from the improved breathing lets you make lots of horsepower, though; HP = RPM X Torque. So the engine is designed for higher RPM. Plain bearings are used in the rod bearings as needle bearings are unsuitable for high RPM work. Five main bearings are used to control crank flex. All bearings have pressure fed oil lubrication. The engine will happily spin at well over a V-twin's redline for millions of revolutions.
Lugging a SOHC4 engine trying to make it sound like a Hogley Furgeson does much more damage than running it the way it was designed to.

[Jonesy]

Aside from all the correct engineering facts already presented, take comfort in this info: Honda beat the hell out of these engines in pre-production testing to prove the durability.

Before the release of the CB750, the prototypes had a redline of 9,000 RPM and the test riders were told to keep the engines in the red zone of the tach at all times- if the RPM's dropped, they were to downshift and get them back in the red. They put thousands of miles on them and never had a major engine failure.

Also, there was an engine bench test where Honda engineers ran the engine wide-open for 24 hours non-stop.

In short, these engines are designed for high revs, and were tested accordingly.

[Irishguy]

You know...  It's bad when someone like a mechanic knows so little about his chosen profession.  It's even worse when he misinforms potential clients based on that inaccurate knowlege.

Many, many engines from cars to motorcycles are engineered to run at sustained high RPM's for extended periods of time and last for hundreds of thousands of miles with no ill effects.

I own 2 Miata's with over 150k miles on them.  They both have 4:30 rear end gears.  They both cruise at over 4000 rpms.  They both love it.  They both still run like new.  This mechanic is full of crap.

[upperlake04]

Another fine post Bodi

[RRRToolSolutions]

Heavy cranks, pins, cylinders, ect really have little to do with limits. It's piston speed. The long-established, published limits of "stock" components has been 3,400 -3,500 feet per minute. There are a few exceptions, but this normally applies to both 4 and 2 strokes.

Here is a calculator -

http://www.csgnetwork.com/pistonspeedcalc.html

My 1500 Nomad has a 90mm stroke, the CB750 has 63mm, and the ZRX has 58mm. Here is how they look -

90mm @ 4,500 rpm = 2657 fpm
63mm @ 4,500 rpm = 1860 fpm
58mm @ 4,500 rpm = 1712 fpm

90mm @ 6,000 rpm = 3543 fpm
63mm @ 6,000 rpm = 2480 fpm
58mm @ 6,000 rpm = 2283 fpm

And just for giggles -
90mm @ 8,500 rpm = 5019 fpm
63mm @ 8,500 rpm = 3513 fpm
58mm @ 8,500 rpm = 3234 fpm

I'd agree, he doesn't know what he's talking about.

Regards,
Gordon
 

[dramsell]

OK then.

Could someone convert this into english for a non-mechanic?

Thanks!

[Bob Wessner]

They run fine, and virtually forever, as long as you don't exceed the redline and don't lug the engine.

[mcpuffett]

there built to last ,    cheers mick.

[s3d1t0r]

I have to say that this is a prime example of why I believe that these SOHC4 forums are an invaluable tool.  Some of us don't have anywhere else to get a second opinion when we get information from random places outside of these forums.

BTW, I've always heard that as long as you keep the oil and oil filter clean in a Honda MC (including proper maintenance of course), it will keep going virtually forever.  I've never heard anybody say that about any other motorcycle brand, at least not yet.

[BobbyR]

Many American motorcycle mechanics only know about American V-twin engines. Because of the basic design of these engines with the single crank pin and forked roller bearing connecting rods, the maximum RPM is quite low. They are long stroke designs with relatively small bores.
Japanese sport bike engines as a rule have a totally different design philosophy. The SOHC4 engines are basically "square", with stroke and bore equal. This essentially gives more room in the head for valves with a given cylinder displacement, letting the engine breath better. The peak torque available is lower - for equal bore cylinders with equal combustion pressure the reduced stroke means that the con rod force acts on a shorter lever arm to the crankshaft for less crankshaft torque. The higher RPM potential from the improved breathing lets you make lots of horsepower, though; HP = RPM X Torque. So the engine is designed for higher RPM. Plain bearings are used in the rod bearings as needle bearings are unsuitable for high RPM work. Five main bearings are used to control crank flex. All bearings have pressure fed oil lubrication. The engine will happily spin at well over a V-twin's redline for millions of revolutions.
Lugging a SOHC4 engine trying to make it sound like a Hogley Furgeson does much more damage than running it the way it was designed to.

I think there is another in depth analysis post that expalined that the SOHC is an out of square engine that had beenn engineered to run fast. Out of square engines are usually slow reving high torque motors used for industrial application and getting them to rev was the magic which gives us a torque curve that an American twin could only dream of. I will try to dig up that post since it also goes into how the carbs were mounted to take advatage of the ram jet effect and some other neat advances.
To answer the question, you mechanic is wrong. These things love to rev and I have seen them with high mileage and no tear downs. I follwed a guy in the most rusted and dirty bike I have ever seen ridden. It was a Cb500 or and I followed him from Hartford Ct to White Plains NY at 75 MPh. When he stooped for gas i had to get a better look. He told me he rides it year round, and he changes the oil and lubes the chain every two months. Never had the head off, or any break downs and he is the 3rd owner and the odometer flipped sometime in the past.

[KB02]

OK then.
Could someone convert this into english for a non-mechanic?
Thanks!

They run fine, and virtually forever, as long as you don't exceed the redline and don't lug the engine.

they're built to last ,    cheers mick.

Done and done.   

Yeah, I think that guy was thnking about the life expectancy of an American V-Twin engine. Two different beasts. He's talking Oranges and you're riding apples.

[Tower]

@Ilbikes, the calculator you've pointed to, uses an equation that calculates average piston speed across 180o of crank movement.  As you know, the pistons move fastest at 90o TDC and slowest at 0o TDC and 180oTDC (BDC), Calculating piston speed at 90o TDC therefore gets a completely different result.

Here's what I mean using one of the examples you provided:
58mm Stroke @ 8,500 crank rpm = 3,234 piston fpm (this piston fpm is an average measured between 0oTDC and 180oTDC, i.e 180o of crank movement)

But the piston's actual speed is 0 fpm at TDC and 180oTDC, but at 90oTDC its moving at 5,080 fpm @ 8,500 crank rpm

Why the theoretical maximum should be 5080 or 3500 or whatever, isn't very clear.  In the jet engine business, the theoretical maximum has to do with the tip speed of the turbine blades and their desire not to exceed the speed of sound (otherwise the turbine prop tips would create a sonic boom that would move along the blade body with rather catastrophic effects). 

Also recall the Kawasakis that can spin up to 16k rpm.  By these limitations it would mean that the Kawasaki engine would have a stroke of less than 33mm.  This is not the case - even my 50cc moped has a larger stroke.

Since 5080 or 3500 fpm is a long way away from the speed of sound, and it doesn't really work for many newer engines, some other reason must exist for using it.  We should take the csgnetwork website's calculations as mere theory and with a grain of salt.

[RRRToolSolutions]

Wow Tower, gee, I don't know...This piston speed idea is not mine and has been used for over 30 years that I know of. Gordon Jennings, Graham Bell, David Vazard, and many, many others have written of in books, magazines, and speed articles for ages. It is in fact why and how many companies calculated their rpm "red zones". Again, that chart is not the brilliant work of myself. 

Now back to your point - yes there are exceptions. My Honda CB750 is one of them. With a balanced rotating assembly, Carillo rods, and forged pistons I'm able to turn it to 10,500 - well past a 3,500 fpm piston speed. But, it did not come that way. The key word when using the guide was "stock". Did you notice that the 8,500 rpm for the 63mm Honda is exactly where the CB750 redlines at? It's that 3500 fpm thing that the Honda engineers were using back then.
The newer R6's, ZX6's, CBR600's ect all have rpm ranges above 15,000. Why? They are designed for those speeds through use of beefier components and lighter weight. I've also spun my stockers through the years at much higher speeds than those - the term "reliability" was used as well. It's not to say you can't spin a big 90mm past it's 6,000 rpm and 3,500 fpm and it will go "boom", but it does mean that it will not live long there. Oh, and the Kawasaki factory installed/programed rev limiter comes on at 6,000 in this bike. Agan the factory chose to observe that 3,500 fpm limit - not me.

Gordon 

[RRRToolSolutions]

It is the acceleration and stopping at each end of that 0 and 180 you speak of that generates the limiting forces. Speed is merely a simplified way of calculating those violent events with distance over time (fpm). The longer the stroke, the more those speeds are at the beginning and end of every revolution of the crank at a given rpm.

Look through and you'll see almost every single recipicating engine uses this "stock" guide. There have been and still are exceptions built which defy, but those are just that - exceptions, not our before mentioned CB750.

I'm just trying to be helpful, not an a$$.

Regards,
Gordon

[MRieck]

The point of greatest stress (on the rod or more importantly the rod bolts) is the last upward stroke on the exhaust stroke. As a side note Honda, studied flame propagation on engines in the late 50's early 60's and spun them to over 20,000 RPM.

[cleveland]

My 550 was at around 5.5K all the way to work this morning.  Only the second time I've been on the freeway with her, but she sounded great!

[TwoTired]

The CB750F circa 1977-78 all had tachs with 9500 red lines.

Did Honda bend their own rules on piston speed limits?  Did they change something to preserve longevity?  Or, were they just getting desperate to boost sales figures?

 

Cheers,

[LoopsAndLogic]

I went for an Hour and an half ride today with nothing but 50 mph all day long on my 400f.

She sounded sweet and peppy with some surges up to 5k rpm.

 You may make fun of me, but I've never rev'd a bike past 6.5k rpm 

 

LL

[paulages]

my bike (cb550F) absolutely loves 5-8K RPM, especially in a gear which allows for low throttle range. it never feels abused, only loved. 

[Tower]

@Ilbikes, your post is quite valid and I don't disagree with any of its content.  Sorry it sounded that way.  What I wanted to impart was that these "standards" are just as you say, 30+ years old and no longer really apply.  They became standards through observation and the use of 30+ year old metallurgical and other technologies.  Our Hondas are great examples of bikes that first broke through those standards, and today's machines far exceed those standards as well.  Perhaps "exceptions" is no longer what you might call them. I'd say there are new standards in place.

My second point was that the 3500 fpm number, although useful, is not as useful as the 5080 fpm number (and these are fairly simple trig calculations, btw). The reason is again just as the posts imply - the stress caused on engine parts can be easily calculated if you know that the piston is traveling at 5080 fpm and must slow down to 0 fpm within 1/2 stroke.  In this example acceleration (d/ time squared) is about 9,300 m/sec/sec (or somewhere around 500 X gravity. Its actually higher when the moment of inertia is used to calculate force).   Add piston weight to that calc and you have the force placed on the rod, transferred to the crank, then the head bolts, etc, etc.

Here is an example of where using the 3500 fpm standard may not be so great of an idea...  I thought I read on this forum, the discussion of which aftermarket rods were better and how those rods actually stretch less over time.  That discussion was really about which modern materials can best withstand the forces of a high revving CB750 engine (may have been an overbore discussion). Tensile and sheer strength of various metals and alloys is well known. The makers of the poor quality products being discussed in those posts, probably used those old 3500 standards, when perhaps they should have used newer standards.

We're on the same team, as it were, @Ilbikes.  I'm just highlighting a little more "out-of-the-box" thinking.  So, what do you say we make the new standard 6,000 fpm?  Now our Hondas will seem like real old school.

[Gordon]

 You may make fun of me, but I've never rev'd a bike past 6.5k rpm 

 

LL

Well, I won't make fun of you , but I will tell you you've never made good use of that 400F's high-revving power.  It has a 10,000rpm redline for a reason.

[BobbyR]

The CB750F circa 1977-78 all had tachs with 9500 red lines.

Did Honda bend their own rules on piston speed limits?  Did they change something to preserve longevity?  Or, were they just getting desperate to boost sales figures?

 

Cheers,

I would say the sales figures observation is correct. I also have a 140 Mph speedo which is a bit optimistic to say the least.

[Bodain]

I wonder what he would have to say about my 85 Yamaha RZ... <Grin>
I could only speculate, but this statement is about the silliest thing I ever heard.

Well there was a statement recently made by a guy on our local Craigslist.. The guy was selling 78 550k. The bike actually looked quit good. He was going on about the minor flaws in the bike, one of which was the fact that the electric starter didn't work.

I quote " A mechanic told him these old bikes were better off not using the electric starter. They were not designed for it"

Where do these people come from?

[LoopsAndLogic]

Thank you Gordon

 I stopped using my electric starter cause it really drains the battery. And if you drive mostly city, it won't get a good recharge as when you would be driving on backroads or freeways.

I've personally experienced this

LL

[heffay]

great info here from all.

my 350f loves to be at 7k rpm but, do i ride it there regularly?  no, i keep it around 4k.  but damn... above 5k is just where the power kicks in. 

my 92 kawi zx7r redlines at 11,500rpm.  i've even seen 13,000rpm in 6th gear.  it's a dog below 4.5k and cruises happily any given day and all day at 5-7k rpm.  oh yeah... it is reading 43,000miles on the odometer too.  (that's quite a lot for a solo seat race rep)

inline 4's rev HIGH... it took me a number of years to get used to revving them to the moon, but once i did... ahhhh, heaven.

[Soos]

great info here from all.

inline 4's rev HIGH... it took me a number of years to get used to revving them to the moon, but once i did... ahhhh, heaven.

I agree a bit of an eye opener on how far and fast you can go with these motors....

And I now no longer feel i'm abusing my bike at 9000rpm.   
The more I ride my 650, the more I REALLY like the upper ranges anyway.
The aceleration is a kick in the pants, and the harleys have a time keeping up....And my motor is about 1/2 the cc's of theirs.

A buddy of mina has a '06 harley(1100cc ?sportster?) motor in a (?)paco frame(chopper anyways) and he gets me off the line, but as soon as he shifts.... I pass him ... for a second.
'cause about then, i'm at 9 grand in second....

not too bad for a $1500 bike, and being nearly 30 years old, and he spent about 16,000 on his..... HAHAHAHhahha......




l8r

[angeldeville]

Ride it like a two-stroke! I redline mine daily!

The CB900 customs with the low range tranny was geared for morons who wanted to lug a 4 like a Hardly, not many of those used in the low range of the tranny are around today.

[mlinder]

I run the 350f at 2k ~ 5k around town, but anywhere else, I'm between 6.5k and 10k.
The bike loves being over 6.5k. It was designed to run and make power at these speeds.
You are cheeting it of its potential if you never hit 6.5k+

The 750 is 3k-5k around town, and 9k+ when I'm goosing it.
Lemme tell you, 3rd gear at 8.5k is a beautiful thing. Everything get's smooth and precise, everything sounds beautiful. It's not struggling, it's not straining, it actually runs best RIGHT THERE at 8.5.
Yesterday, I was between 7k and 9k for pretty much 2 hours. Never hiccupped, and was smooth as butter. 750 is built for those rev ranges. Don''t abuse your poor bike by never letting it run at it's optimal speeds.

[paulages]

mark, i almost left a similar reply after our ride yesterday. especially after my muffler fell off  , i can really hear and feel when it's in that sweet spot, and it's usually over 6K. even when i was completely pushing it, it never felt strained. like i said previously, mine loves high RPM, low throttle range. low lugging feels like abuse to these motors.

[hymodyne]

70mph, 5th gear @5krpm... runs fine and is almost silent...

hym