Performance: Engine RPM at 60mph

bdento59 · Post by **bdento59** » Mon Sep 08, 2008 6:11 pm

My stock '80 X shows the following RPM's at speeds:

55 mph 4200 rpm
60 mph 4600 rpm
65 mph 5000 rpm, etc...

Everything I've read in the CB-X Gold Portfolio says that the rpm @ 60mph in top gear should be 4200-4300 rpm. Just wondering why mine is running so much higher? I have stock 18/42T sprockets and stock size wheels and tires. Weird. What are others seeing as indicated rpm at 60mph? TIA.

Terry · Post by **Terry** » Mon Sep 08, 2008 7:07 pm

Mine are pretty much like this, considering the clocks are nearly thirty years old, I don't look for ultimate accuracy. My speedo however seems to be spot on when I come across one of those radar trailers you see every now and then telling you the speed limit and YOUR speed.

60=4000
70=5000
80=6000
90=7000

Phil · Post by **Phil** » Mon Sep 08, 2008 7:13 pm

Just a curly hair over 4000 at 60 on my 79.

Don #6141 · Post by **Don #6141** » Mon Sep 08, 2008 7:19 pm

The 'official' road test done on the '79 when it first came out says 4191 RPM @ 60 mph

http://www.cbxclub.com/davespage/cbx-10.html

Mine reads a touch over 4K - I suspect one of your clocks is just off a bit

Don

EMS · Post by **EMS** » Mon Sep 08, 2008 7:59 pm

Bill: Are you sure you have the original tach? Also, your speedo could run behind. Old instruments sometimes do this. Try to ride with a buddy and see if your speedo indicates in the right speed range. For the 1980 CBX Honda spec'd 14.0 mph per 1000 rpm in fifth gear.
4000 rpm = 56 mph, 60 mph = 4285 rpm

bdento59 · Post by **bdento59** » Mon Sep 08, 2008 9:15 pm

EMS wrote:Bill: Are you sure you have the original tach? Also, your speedo could run behind. Old instruments sometimes do this.

Hi EMS,

Not sure, but I think it's the original tacho; it certainly looks original from the yellow plastic case, bezel shape, face, shitty lighting, etc. I guess I'll go look for a "your speed" setup somewhere and give it a couple runs through the traps <g>. Thanks for the feedback in any event. I think I'm going to go down a few teeth in the rear when it comes time for new sprockets, as the bike just seems a bit frenetic on the highway in high gear.

alimey4u2 · Post by **alimey4u2** » Tue Sep 09, 2008 5:55 am

Handheld tachometer on the crankshaft ( about $185.00 at McMaster Carr) or variable frequency strobe is the only accurate way to check....

EMS · Post by **EMS** » Tue Sep 09, 2008 7:42 am

bdento59 wrote: Not sure, but I think it's the original tacho; it certainly looks original from the yellow plastic case, bezel shape, face, shitty lighting, etc. .

Reason I was asking, there are several tachs that were used on other Honda "F" series that look exactly the same, but do have a different drive ratio than the CBX .

bdento59 · Post by **bdento59** » Tue Sep 09, 2008 9:28 am

EMS wrote:there are several tachs that were used on other Honda "F" series that look exactly the same, but do have a different drive ratio than the CBX .

Well, in that case, I have no effen idea unless you can tell me how to det'n this w/o opening it up "again". Maybe someone can loan me a verified CBX tacho and I can plug it in and see if there are any diffs. Seems easier than buying a $185 tach from mcmaster-carr. Thanks.

alimey4u2 · Post by **alimey4u2** » Tue Sep 09, 2008 9:57 am

Osilloscope available ? Plug it into a coil excitation pulse, measure the time base. Digital tachometer available triggered by a coil pulse ??

dan1995 · Post by **dan1995** » Tue Sep 09, 2008 10:22 am

It's been raining a lot over there hasn't it larry

Don #6141 · Post by **Don #6141** » Tue Sep 09, 2008 10:30 am

bdento59 wrote: I think I'm going to go down a few teeth in the rear when it comes time for new sprockets, as the bike just seems a bit frenetic on the highway in high gear.

I felt exactly the same way about my '79 . . . . and was contemplating a sprocket change but was talked out of it by some of the old time CBX'ers with cooler heads ;)

I think it all depends on what you do with your bike and how you like to ride it. If it's a freeway cruiser, then you might like it better with some different gearing, but if it's a sportbike that you ride for pleasure and you enjoy accelerating out of the corners like I do, then any ratio change is going to affect the way you like to ride your bike. I've pretty much made up my mind to stich with the stock gearing because I'd lose more than I gained by going to a smaller rear sprocket

Don

EMS · Post by **EMS** » Tue Sep 09, 2008 11:15 am

Ideally, a vehicle is geared such, that the available gears will provide nice , seamless performance over the speed range.
In 1979, the CBX had a top speed of 135 mph give or take a few miles.
135 mph spread over five gears should give you a decent performance in almost any situation.
Now, how fast are we going here in the U.S.

I know, some of you who are 6 ft 4 are getting your CBX up to 165 mph on a regular basis

, but the rest of us slow-pokes ??? 100 mph?? Maybe occasionally.
80-85mph for somewhat extended periods of time. So, if the CBX would do about 12 miles per 1000 rpm in top gear, you would be around 7,000 rpm at 85mph and around 5000 rpm at 60 mph. considering the power band of the bike. That wouldn't be too bad, I think. That would be a 16% change in final drive ratio. Or from 42 to 48 teeth

or from 18 to 15 in the front. The latter being a popular change.

alimey4u2 · Post by **alimey4u2** » Tue Sep 09, 2008 1:09 pm

dan1995 wrote:It's been raining a lot over there hasn't it larry

How did you guess Dan...

dan1995 · Post by **dan1995** » Tue Sep 09, 2008 1:17 pm

Your acting more like Goerge everyday....Too much time on your hands