Why is the XS11 Redline So Low?

[IanDMacDonald]

Compared to some of my other bikes (Hondas), why is the redline so low? I have a '77 CB400F Super Sport that has a 10,500 redline. Just curious.

[jimbyjimb]

No shame in asking, but some engine 101:

Many things have to do with RPM in an engine, the foremost being what it was engineered for. The best answer is the XS just doesn't need to turn more RPM. It's not a race bike.

The reason your 400 turns more RPM is two-fold. Small displacement engines by nature will tend to turn more RPM due to shorter travelling distances of the rotating mass (shorter stroke), and many times because of less rotating mass. Also, obviously a 400cc engine will likely not make the same horsepower to a technologically similar 1000cc engine. Therefore, additional gearing will be needed to make the weight move (the weight being the motorcycle). Having extra RPM available at the top allows for lower gearing to make up for lack of low-end torque without sacrifice to top speed capabilities.

High-RPM engines are generally higher performance per size, and can generally be expected to love shorter lives as well. Huge deisel engines make tremendous power, but turn very low RPM with your average 14 liter-truck engine turing under 2k as a redline.

Can any engine be built to turn higher RPM? Yes. There are HUGE engines that turn high RPM for their sizes, like racing semi trucks, funny cars/top fuel. Keep in mind those are VERY expensive engines with superior metallurgy in the rotating assemblies to handle all the stress of that weight turning so fast. Not only that, but they have extremely short lifespans. Like, super short.

Anyway, there are a ton of factors at play with size/RPM. Quality, materials, technology, engineering, purpose, lifespan, etc etc...

[DGXSER]

Yep, Redline is simply a sticker on the tach. It represents the RPM limit determined by the mfg that will give the desired performance with limited sacrifice to the engines longevity.

My Katana 600 loves to live in the 4-8k RPM range, redline is 12k IIRC. That engine sounds like it is about to implode at those RPMs. And I am sure it will not live a long happy life if I consistently play in that range. But that 600 cc engine needs those revs to develop the HP to propel my butt at highway speeds.

Take it to the next level and my high HP 502 engine in my Cutlass redlines at 5300 RPM. Very low for a bike, but higher than I ever took my F150s 5.4L V8.

Now you can get into all the details of stroke versus bore, cam lobe design, valve sizing, weight of engine components....blah blah blah...yadda yadda yadda. But that would require an entire book to cover in appropriate detail to get to where redline is established.

[jimbyjimb]

The general rule is; All things being equal, small engines make more RPM, big engines make more torque down low. All things being equal.

[clcorbin]

The limiting factor on any engine is piston velocity. Piston velocity varies linearly with RPM and stroke. Your little 400 4 banger has a much shorter stroke than these 1100s do which means given the same piston velocity, they are turning a much higher rpm. Therefore, they can have a higher red line on the tach but have the same piston velocity as the bigger, slower turning XS11.

Piston velocity is also the main reason why modern sport bikes have such over square bore/stroke ratios. They get the larger displacement via the large bore, but keep a short stroke that keeps the piston velocity down which allows them to run VERY high rpms to make lots of power (remember, hp is a calculated value from torque and rpm. And torque is pretty much a function of displacement, so you have to spin them faster and faster to keep pushing the power levels up with the same displacement).

[Tat2demon]

Co-worker brought in his Ninja 600 today and I noticed the redline on that was 16.5.

[crazy steve]

While piston speed enters into it, the limiting factor on most motors is the valve train. Controlling the valves at high rpm gets hard, all the more so with larger sized valves. The XS motor is only a two-valve, most modern motors are four-valve. Four small valves can flow the same or more air as two, but can weigh considerably less per valve. Less weight, less inertia, so the valve can react faster with a smaller, lighter spring.

Ducati addressed this with their 'desmodromic' valve train (http://www.ducatiusa.com/history/edi...iming/index.do) that allows their bikes to out-rev similar bikes, but it's complexity has limited it's application and Ducati's aren't known for their reliability....

[donebysunday]

17500

Co-worker brought in his Ninja 600 today and I noticed the redline on that was 16.5.

Was it Yamaha that was going to build a bike that would red line at 17500, read something in a bike rag a year or so ago.
Just recently read about a bike red line of 18000, think it was Honda ?
If one of these let loose you might as well kiss your arse and the rest of you goodbye too.

[clcorbin]

I THINK the Japanese market CRB400RR had a 19,000 rpm redline. It was stupid high at any rate!

[MPittma100]

Two Things

RPM is engineered into an engine from the beginning and the max RPM is determined mainly by the weight of the rotating assembly (pistons, rods, crank). RPM is governed by the camshaft(s). All camshafts have a basic operating range - an RPM where they start making horsepower up to where they stop making horsepower and then an RPM limit or max (AKA valve float).

Maximum horsepower in any engine is made well before max RPM. Maximum torque in any engine is achieved well before maximum horsepower is reached. Therefore, shift points for the fastest ET or 0 to 60 MPH times are made somewhere between max torque and max HP. This information is found from: a) dyno test or b) camshaft manufacturer specs. A run down the track by an engine shifted at the peak hp/torque window will post a quicker time than one shifted at max RPM. Running an engine to max RPM is just wearing it out faster.

Good example is a diesel engine. They are on average at 17:1 compression ratio with heavy rotating assemblies. Their max RPM is quite low - anywhere from 2250 (larger engines) to 3400 for the smaller engines. Spinning this weight any faster will stretch the rods and contact of the piston and cylinder head occurs. Use your imagination as to what follows.

8500 max RPM for the XS1100 is spot-on.

MP

[crazy steve]

...RPM is governed by the camshaft(s)...

Well, yes and no. While the cam profile can have a big say in where in the RPM range the motor makes power, there's much more to it than that. Valve size, port flow, runner length, and exhaust backpressure are just a few things that will effect RPM capabilities. It's entirely possible to have two motors, identical in every respect except for flow though the head, and have one max out at much lower RPM simply because it 'runs out of breath'.

Manufacturers put red lines on motors for a variety of reasons; in most cases it's because of valve float (see my first post) as that's nearly always the weakest link. Let a valve 'kiss' a piston a few too many times and break the head off and how good the rest of the rotating assembly is quickly become moot. Some motors are capable of much higher RPMS but for 'other' reasons have a lower limit (the SHO Taurus comes to mind; these were rev-limited not to save the motor, but the accessories like the alternator and power steering pump). There's always a bit of 'fudge factor' (to allow for missed shifts) but there's rarely any more power to be had by spinning them faster....

[Guest]

You can change it simply by using a Red Sharpie Marker.

I also changed my exhaust to Jardine 4 into two headers and new carb jets.

[fredintoon]

Compared to some of my other bikes (Hondas), why is the redline so low? I have a '77 CB400F Super Sport that has a 10,500 redline. Just curious.

Hi Ian,
better ask how is it so high? Assuming that piston speed is the limiting factor your 400cc bike should go to 17,000.

[xswilly]

hey wickedgood, where abouts in maine are you,
anywhere near hollis