TOPIC:

Friend and i are betting on the top speed of a kz900 with a header and pods? He says the bike will go maybe 115mph max. I say 130ish? Who is closer?

Flip a nickle; there's too many individual variables to that question to give a pat answer to...

Running right, jetted for the pods and header and otherwise stock including gearing, the config you're talking about might get a peek at 130 on the clock. Maybe.

Given the usual state of maintenence - major miles, well-worn top end, it'd be lucky to see 115.

There was a major thread on the topic a couple of months ago...

Pterosaur, that's an intelligent well reasoned answer, and the first answer in the thread as well.
I predict that someone will disagree with it, but it won't be me.

steell wrote:

Pterosaur, that's an intelligent well reasoned answer, and the first answer in the thread as well.

Thanks.

Funny, I went searching for that previous thread to put up a link, and I can't find the bloody thing...

I predict that someone will disagree with it, but it won't be me.

(major laugh)

Yeah, I thought about that before even answering - and I'm sure that a few will come along to swear up and down on a stack of bibles that they've seen 145 or some such. The reply to a statement like that is that either the machine wasn't stock or they weren't there - speedo error, wind flapping eyelids, or they were on a KZ Bar Stool when they did it...

Put a 33 or 34 tooth rear sprocket on a stock KZ and it'll easily do 130+ - it'll just take half an hour to get there!

Post edited by: Pterosaur, at: 2006/07/05 01:35

yeah i looked at all the posts and couldnt find the topic either. My old ltd topped out at 102mph on a radar gun, and i weigh about 260

kzmarvo wrote:

yeah i looked at all the posts and couldnt find the topic either. My old ltd topped out at 102mph on a radar gun, and i weigh about 260

Yeah, there's a reason that jockeys are little fellers with bulimia...

I went looking for that thread again myself - either the header title got changed or it just got wiped somehow...

A shame, that. Out of curiosity, I took my box-stock, fresh top end '78 C1A out and did a measured mile with a 600 CBR and came up with an all-out top end of 110mph pushing a cop windshield and all the goodies.

The old Cycle World road tests give a '78 KZ a 123mph top end, and that sounds about right on a fresh machine, give or take a few ticks.

Back during WWII, the Brits took a bunch of planes off the assembly line - Spitfires and Hurricanes - and tested top speeds for supposedly identical machines and came up with a 35-40mph high-low discrepancy. While the spread wouldn't be that great, it's a fair assumption that the same general rule would apply to mosickles.

And also as noted, the old Kaw mechanical speedos were well known to be up to and better than 10mph optimistic as a rule of thumb.

And really, it's all academic anyways; unless you're Lorcan, it's a whole bunch more practical to have a bike that'll do 80 all day long than to have one that'll do 130 for 10 minutes and then need a grand's worth of motor work...

Hey - lookee what I found:

I recalled that I'd written up the top end post in Notepad, and sure enough, I had a copy of the post I made on the topic lying around:

**************

Just in the interest of psuedo-scientific mis-information, I decided to conduct as reliable a top end test as I could muster short of finding a state trooper, flipping him off and playing a game of "catch me if you can" yesterday afternoon.

The setup:

1. Checked valve adjustment.

2. Did a compression check. 1/163, 2/157, 3/168, 4/161.

3. Installed a new set of points and condenser, set timing.

4 Adjusted cam chain tension.

5. Changed oil & filter.

6. Greased swingarm.

7. Checked tire inflation, set at 38psi.

8. Filled the tank to the *Idiot Level* with Sunoco 94.

The Bike:

1978 KZ1000C1A, 29,468 original miles, 1 year on fresh OEM rings, valve job, guides, seals, and entirely new cam chain/idlers/sprockets and guides. 15 tooth countershaft and 39 tooth final sprockets - stock gearing.

When the jugs were inspected at roughly 28,000, the bores were so clean and close to spec that we decided to go with a two-step bar and then ball hone, keeping the original pistons. The "acceptable" compression range is 155 to 185, and I figure that the ring-only installation is why the compression check readings were at the lower and of the scale.

The bike is equipped with the standard flat windshield, floorboards, saddlebags, crash bar and Vetter trunk box. The bags held about 30lbs of assorted tools among other things, The Vetter box held about 15lbs of assorted stuff; chaps, jacket, etc.

The bike is as plain box stock as can be found these days aside from the cop add-ons.

Me, 5"10', 225, depending on breakfast.

The course:

Two measured miles on a recently re-topped rural interstate - one to accelerate, one to measure. The end of the second mile shortly afterward lets off into a quickie off ramp - good for the possibility of getting lost were one to attract the wrong audience.

I've got a set of (originally) calibrated twin speedos that municipal judges thought were fair enough back in the day. One of the neat features is that inside the right speedometer is a solenoid that traps the needle against the glass when a switch on the right block is thrown - before mobile radar, a motor cop would follow some scofflaw, then flip the switch and lock the right speedo.

I also have one of those little digital clocks with an elapsed second readout stuck to a piece of velcro on the windshield frame. Using the 3600D/T = S (3600 (seconds per hour) multiplied by number of miles (1) divided by number of elapsed seconds T = speed S gives the timed mph.

Then there's the odd bit of serendipity; while topping up at the Sunoco, what happens along but 2 guys, one on a CBR600, another on a 2005 ZX-6R. They pulled up alongside the next pump and exchanged pleasantries.

"Don't see many of them. Is it fast?" ZX asked.

"Funny you mention that - I'm about to find out."

That got their attention. In the course of a short conversation, they decided to get in on the fun. The plan was modified such:

They were equipped with walkie-talkie cells, so CBR would get out ahead on the overpass on the exit and keep an eye out for troopers while ZX kept pace with me on his digital speedo. We'd do a last check and then start the run. If any Barneys showed after the run started and we made the exit ramp, scatter in 3 directions became the plan.

Out on the berm at the start, we did a final check with CBR. No Barneys. About 2 minutes later, ZX-6R and I pulled up on the ramp to meet CBR600.

My right speedo needle was stuck on 112 when I flicked the switch. 6R said he'd been reading between 100 and 114 during the run. I clocked 33 seconds, which by the formula comes out to 109.9. The truth is right in there, somewhere.

So how does that compare to a street stock KZ900?

Well, without getting NASA involved, we can come relatively close using some engineering approximations.

The biggest single source of drag on a "C" model cop bike is that big 'ole barn door of a windshield. The bags add some, but less than you think because they exist in what's called "turbulent flow" at the rear of the bike. The Vetter box may in fact offer an aerodynamic advantage because it extends the faired area well behind the rider, delaying the formation of a low-pressure area in the slipstream. If you've ever watched "Mythbusters", there's an episode where they found that fuel efficiency was actually improved by leaving a pickup truck's tail gate in the *raised* position. Same principle applies here.

In order to get a number for the horsepower penalty imposed by the windshield, we have to find a number known as its *co-efficient of drag*, or CD. It's roughly 3 feet by 2 feet, or 6 square feet in area.

The Cambridge University Fluid Dynamics lab has a neat little program that will approximate the windshield CD by substituting a cylinder 2.5 feet in diameter for roughly the same surface area. In air, at 160.23 feet per second (109mph), an aerodynamic drag value 32.893 pounds per foot of length is generated at a CD value of .04102.

Plug those numbers into an Aero Drag calulator and 6 square feet of frontal area at a CD of .4102 at 109 mph, and the horsepower penalty of the windshield comes out at 21.2.

**************

Shuckey darns. The last few paragraphs are missing.

The upshot of the missing part was that because *"...dynamic air pressure is proportional to the SQUARE of speed, the horsepower to attain that speed goes up as the CUBE of that speed change."
In other words, if a stock KZ puts out its rated 82 horsepower at 123mph, if all other factors are equal, it'd have to be putting out in the neighborhood of 110hp to wind her up to 145mph.

That bit of mental arithmetic bears out when you read Lorcan's posted numbers - last I saw, he was seeing 198mph at 230hp. The ratio of increase in speed to horsepower gain isn't linear - it's logarithmic.

And there you have it...

*A concise discussion of horsepower and drag measurements:

pweb.jps.net/~snowbum/hpcalc.html

Post edited by: Pterosaur, at: 2006/07/05 03:53

oh boy here we go again! like they said it comes down to state of tune also depends on how the rest of bike is set up.top speed charges with an ill handling bike are no fun! it also depends on how lucky the rider is feeling:evil: anytime you get over 100mph you are flying after that you just pick a direction and hope for best! hahaha

Well I took My old green machine out this weekend and hit about 120ish on it. I cant see it doing much more than that with out some bigger carbs and cams; sprocket too.

I've gone 122 in the quarter with stock pistons, 29mm smoothbores and sidewinder pipe. Can't recall gearing but it was set for strip. With stock gearing was not getting into 5th gear before end of track. I could see going 130ish with stock gear and maybe 1/2 a mile of road. My KZ650 did an indicated 200kph (120mph), but I agree speedos can be optimistic. I got my brothers pure stock 84 1100LTD up to 245 kph indicated. That bike has 4mm bigger valves (I think), more cc's but is a lumpy cruiser shape with shaft drive.

wireman wrote:

...anytime you get over 100mph you are flying after that you just pick a direction and hope for best! hahaha

Post edited by: Pterosaur, at: 2006/07/05 15:58

KZ900 no pods, small carbs (26mm) but pipe - an indicated 125 mph was easy, but who knows what this equates to after speedo error correction - maybe 110-115mph?[/quote]. Always feels like it could go faster though, if only...

Z1 with 1260 high compression kit, pods, pipe, etc at altitude (Denver, CO) got 137mph at the end of the quarter mile.

kzmarvo wrote:

Friend and i are betting on the top speed of a kz900 with a header and pods? He says the bike will go maybe 115mph max. I say 130ish? Who is closer?