Cam Differences for a C70

Discussion in 'Tech Area' started by whereshaldo, Mar 10, 2018.

  1. whereshaldo

    whereshaldo Member

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    I've been lurking around here for a bit but finally need to come out of the shadows. I've got a bone stock '81 C70 than that I've swapped out the 14t sprocket for a 15t. Bike runs great and cold has 190psi compression. My big complaints are that i don't have enough mid throttle oomph and that I max out at around 36mph (at least here in hilly Seattle in winter). Even on moderate grade hills I seem to loose speed over the duration of the climb and on the steeper hills I'm forced into 1st just to keep from bogging down. That is part of the reason I went with the 15t sprocket, the 14t is just too low geared in both 1st and 3rd and I just end up pegged at max RPM but wanting more speed.

    I'm looking at swapping out cams. There seem to be three:

    DrAtv Mini-Monster
    DrAtv Long Duration Power Cam
    Faito Stage 1 from MotoGaga

    Is there one of these that's better suited for giving me more mid-throttle torque, a few more overall MPH that doesn't sacrifice low end power entirely? I'd prefer to stick with the stock carb, just looking to slightly change the overall powerband.

    Thanks in advance,

    Hal
     
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  3. OLD CT

    OLD CT Well-Known Member

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    Hi Hal, 81 have low compression pistons to begin with. it's not really a good idea to put a hotter cam in a stock piston bike. Hotter cams rely on higher compression pistons to work. In my opinion it's a waste of time and money. I would look into a stroker crank, dratv 11:1 compression 88cc piston kit and a bigger carb. Then it would be worth it. Otherwise enjoy the 36 mph top end.;) I tried a hotter cam in a 82 ct70 and it made it slower...
     
    #2 OLD CT, Mar 10, 2018
    Last edited: Mar 10, 2018
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  4. whereshaldo

    whereshaldo Member

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    Which cam did you try? I agree about the necessity for compression and hotter cams, but if I'm at 190 cold, which is at the higher end of what Honda claim for these engines, I was just hoping for a bit more. Maybe just a diet?

    Along these lines, are there any good gear calculator charts that determine engine rpm and different combinations of front and rear sprockets?
    My assumption is that Honda had this bike maximized at 70cc but half the fun is in screwing around. I've put 1200 miles on the bike since the middle of December and have been making incremental adjustments almost weekly to just see how a single change affects performance.

    Hal
     
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  5. OLD CT

    OLD CT Well-Known Member

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    My 82 has a different style cam and head style, than the 81. I tried the 6mm lift race cam. You would use the non bearing style cam. If you want to try a cam go with the mini monster, I have used them with a slightly bigger carb on other bikes. Usually with a fresh 88cc kit. The m/m cam is the one to try.
     
  6. vintagehondatrailbikes

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    Would you have interest in 88cc? Tbparts.com (sponsor here) has an awesome complete race head kit. You can keep your stock exhaust and carb with a larger jet.
     
  7. racerx

    racerx Administrator
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    190psi is as good as it gets. I can't recall ever seeing a stocker that tested that high. As for gearing limitations, a 72cc motor is factory-geared for ~4.77mph/1000rpm, in top gear. They won't pull much more than this due to the torque curve. Stabbing-in a "hotter" cam may increase hp but, it does so by shifting the powerband to a higher rpm range. So, take a motor that's reaching peak hp at, say 8000rpm and running out-of-breath at ~9000rpm and you have a ~43mph bike...pretty typical for a stock CT70. Stab-in a hotter cam and you very well may gain 1-1.5hp, depending upon the rest of the setup and where you began but, any mph increase is going to be with the engine spinning more rpm. High 40s-to-50ish top speed require twisting 72cc of rip-snorting Honda power to 10,000rpm...which they can do, with relative ease.

    You don't really need a gearing chart, or the tranny + primary ratios. Start with the stock sprocket ratio, then calculate your changes in ~3% increments...choosing your sprockets from what's available. There's an even simpler method...upsize the C/S sprocket by one tooth. If the bike feels weak, upsize the wheel sprocket by one tooth; that'll probably be the absolute limit of engine torque.

    If you want more mph per rpm, add displacement, via a longer stroke crank. That's where changes register most on the seat-of-the-pants dyno.
     
  8. whereshaldo

    whereshaldo Member

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    Thanks racerx that's a whole lot to think about. I did a lot of prep to the cylinder walls, head and the mating surfaces before assembly, so I'm please with the compression level. I've been spending a lot of time just trying to get everything tightened up as much as I can and cut down on excess vibration as much as possible. That alone has made a huge difference at least in how the bike feels at high RPM. I'm running a 15T C/S sprocket right now and was actually thinking of trying a 1T larger rear sprocket. That should slow me down a bit, but give a bit more torque, which at this point I'd almost prefer. The headwinds and the hills in Seattle are a little too much for the bike right now.

    hal
     
  9. racerx

    racerx Administrator
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    IMO, going +/- one tooth, at the wheel, is the ideal way to dial-in gearing with precision...just not the easiest. IDK about your model but, on a CT70, that's working in ~3% increments and there is a little bit of "wiggle room" to accommodate rider preference.

    Ultimately, what you're going to find is that there is a limit to what the engine can pull, in top gear. Go 3% over-the-top and the motor will fall on its face; chances are, you'll also notice first gear working the engine harder, from a dead stop, too. I mainly work with 4-speeds, in high-powered (60mph+) applications. What I've found is that the same gearing that's optimal for the road is also best for most offroad applications, too. Why? the engineers knew what they were doing. Going too low makes first gear useless, top gear "too busy"; going too tall results in the bike struggling at both extremes. The only exception might be a specialized low-speed situation, like swap meet, where being able to basically dump the clutch, just revving above idle, is a plus...and top speed/cruising speed are irrelevant. I'm assuming that there are some places in your locale that are not downhill with a tailwind...the only other exception.

    Now, all of that having been said, it is possible to alter the speed ranges of each gear, to good effect. However, that's far easier with 4 gears. The 3-speeds have a huge 1-2 ratio gap, so second gear has to cover a wide range. In certain situations, going with a taller sprocket combo can increase top speed in 3rd gear, with 4th becoming more like an overdrive. Yes, I do realize that's not going to help you very much, unless you install a 4-speed tranny. I mention it in the interest of completeness. In practical/directly applicable terms you're going to have to experiment. I'd suggest starting with a 14t C/S sprocket...easy, quick & cheap. See what that delivers, then break out the calculator and decide which way to go with the wheel sprocket. I recommend upsizing the wheel sprocket, rather than downsizing the C/S sprocket, except for testing purposes...for increased chain clearance and chain life. C/S sprockets smaller than 15T increase chain wear.
     
  10. whereshaldo

    whereshaldo Member

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    Sadly, I have to agree with what you're saying. My general observation with the bike is that the engine as stock is about as maxed out as its going to get without increasing displacement or stroke. Its possible that I am under-carbeureted, the bike seems to top out below what it seems like peak RPM could be. I'm going to play with valve adjustment a bit more and make sure I'm completely in spec.

    The 4 speed gearbox, sounds intriguing. Is there an easy drop in 4 speed replacement? I'm not keen on having to completely break the engine down now that I've got it running so well, but if I could find a spare engine I'd be inclined to play with the gearbox.

    Looking at the wiki, the C100EX or the SuperCub 110 are the two that came with 4 speeds. If I could have one of those, I'd just take the whole bike at that point. Is there another option for a 4 speed gearbox?

    Hal
     
  11. dirtbkr188

    dirtbkr188 Active Member

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    The '82-'85 ATC70 has a four-speed trans, with '82-'84 being a 4-down shift pattern and the '85 is 4-up. The '86-'87 TRX70 is the same as the '85 ATC, 4-up.
    The CL70, SL70, and XL70 are also 4-speed, with a 1-down 3-up shift pattern.
    The CT70H is a 4-down shift pattern.
    All of the above should be a direct swap-in to the C70 cases, some require that you use the kickstarter assembly from the donor engine due to a different pinion gear.
     
  12. kirrbby

    kirrbby Well-Known Member

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    You do have to check the CL70's. The early CL's had a 4down drum, same as the H.
     
  13. racerx

    racerx Administrator
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    The speedometer is actually a very good "poor man's dyno". Horsepower equals miles per hour, simple physics. Adding a 4th gear ratio, to the trans, will not increase top speed; it will, however, improve the ratio spread, between gears and that is a nice improvement, to the discriminating rider. If you want more mph, you'll need more cam and a bigger carb, I'd go for an 18mm metered air & fuel leak ;) Figure just shy of 5mph for each 1000rpm the engine can pull, under load...and ideal conditions.

    If you want more impressive seat-of-the-pants dyno results, go for more displacement. Displacement equals torque. And, all else the same, nothing impacts torque and the shape of the torque curve, like a longer stroke.
     
  14. whereshaldo

    whereshaldo Member

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    Ok, I'm coming back to the gearing issue with these little bikes. I've got everything tuned as good as I've ever had it. Runs really, really well. Just looking for a little extra oomph.

    So currently I'm running a 15 x 36 for front and rear sprockets. This gives me a ratio of 2.4. Stock with a 14 x 36 is a ratio of 2.57.

    Ideally I'd love to run a 14 x 37, which would put me right in the middle at 2.47, but I just can't seem to find a 37 tooth rear sprocket for the C70.

    The next option is to run a 14 x 34 which gives me 2.43. Is this enough of a shift to be noticeable? Otherwise I've got to get into some really odd combos to get where I want.

    Hal
     
  15. kirrbby

    kirrbby Well-Known Member

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    14/37 is 2.62
    Oh, I see. 15/37 is 2.47
     
  16. whereshaldo

    whereshaldo Member

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    I thought I'd post a follow up on this ongoing project. I eventually got enough sprockets to try all the combinations I listed above and I finally settled on the 15x37 as the happy middle. There really isn't enough difference between the 15x36 and the 14x34 to be noticeable. 15x37 gives you a slightly longer 1st gear but still maintains the torque you need for Seattle hills.

    Right now I have the Long Duration Cam from Dr. ATV in the bike as well. It definitely gives a bit more pep on the upper end of the throttle but the trade off is that you look all of the oomph off the line which makes hills difficult to get going. I played with the carb jetting too and with that cam I needed to go to a 92 main jet to not bog out in mid throttle (it did have a 90 rather than 88, which is odd).

    I think I may go back to the stock cam which is what everyone said to stick with. If I go that route I may try to take the carb back to the stock 88 jet as I always felt like it ran too rich.

    hal
     
  17. racerx

    racerx Administrator
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    On the gearing front, you're now ~4% taller than stock which is the ragged edge with this amount of torque...as expected. I really dislike the idea of running 14t on the C/S, 15t is minimum for chain life.

    As for the power curve, this is a little more subjective. I don't know the specs of the stock bumpstick. That said, there really is quite a spread - horsepower-wise - between the mildest and wildest stock grinds. The 3-speed CT70 of the 6v era was rated between 4.2-4.7hp, depending upon the source one consults. At the other end, the CL70, was rated at 6.5hp...at 10,000rpm. That's 2K higher than the CT70 and is the point. By optimizing the cam to raise the power peak by 2000rpm, there's a corresponding increase in horsepower. FYI, only torque is actually measured on a dyno; hp is a mathematically-derived value based on measured torque at a given rpm. Torque is what gets you moving, i.e. acceleration. But, it's hp that equals mph. Thus, there's a good 4-5mph difference in top speed potential. The trade-off is the shape of the torque curve. I'm not so sure that I wouldn't go for the peak power, with the stock stroke crank in place. That's the only way you're going to increase usable road speed.

    It's not clear as to which cam you have. The SL/XL70 grind, often referred to as the "fast road" cam is really a painless upgrade. OTOH, if you've gone for the hotter 6V "race" cam...the one that requires different valve springs, then the next logical step is a larger (18mm) carb to let the thing breath more freely. With this tall-tired bike and a rev-happy, horsepower-biased, tune, I'd expect a top speed around 50mph. That's enough to give you 40-45mph usable power and on the cheap, too.

    If you're really looking for more torque, stab-in a stroker crank. That'll make all the difference...as in "night & day". Of course, then, you'll have to re-gear and the 18-20mm carb will be mandatory. That might be more than you care to deal with. Still...food for thought...a 50mph cruiser can go just about anywhere but a freeway.
     
  18. whereshaldo

    whereshaldo Member

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    I'm not going to repost your entire thread just for brevity, but as usual, your commentary is very helpful.

    the cam I have is this one: http://dratv.com/lodupocam.html which is what the Dr. recommended. I think its the SL/XL70 grind but he's never that clear on details. I'm also running the big valve head as that was the only thing that would work (this one: http://dratv.com/hewivain.html)

    I'm not trying to take the bike on the highway, just build a reliable city bike for getting to work. Being in Seattle we have a lot of hills, so I'm willing to sacrifice a little speed to get more power. If I could find the happy medium of a solid 40mph on the flats (and a headwind), hold 30mph on the gradual hills and 20+ on the steeper hills, I'd be happy.

    At the end of the day spending $350 or so for a Lifan 125 may be the easiest option for not really that much more money than continually upgrading cams and carbs and gears. I'd also love to keep the stock airbox and have seen some posts about how to do that, but only because that seems like the safest way to ensure reliable running in really wet weather. I know you're a fair-weather rider but the parking at my work is so terrible that the Honda has really changed my life.

    If I don't go the lifan route, I'll probably just get the new Super Cub instead, but then I need to worry about locking it up, whereas my ugly bike does not attract any attention at all.

    H
     
  19. racerx

    racerx Administrator
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    That cam really should do the job, without killing the low end torque. It's just not that much more aggressive than the stock cam. With a typical CT70 K0, this cam + 52mm (88cc) bore-up kit will deliver high-40s, wearing the stock induction assembly & exhaust. I did see that the aftermarket head has a larger chamber, which reduces compression...we just don't know how much. The valves are larger than stock, too. I could see this combo making the low-end power curve kinda soggy.


    As for "continual upgrades"...totally unnecessary, once you get the combo dialed-in. Frankly, you should be close, as-is. The head appears to be the variable, in this equation.

    Why did you replace the original head?

    A stroker crank would likely increase compression, bringing things back into balance. That's the key to an engine that performs as expected...approaching it as a complete, coordinated/balanced/matched system. I reckon that'd give you more like 60-65mph top speed potential, which should translate into a solid 50-55mph cruiser. Obviously, you don't have to ride that fast. The L125 motor should give about the same power, with a bit more of a torque (as opposed to horsepower) bias.

    FYI, I'm not even suggesting that you'd ever take a bike this size onto a limited access freeway. I think more like an engineer and that reference makes a point quickly. A lot of guys become transfixed by top speed numbers and cheap horsepower, overlooking the rolling chassis improvements, i.e. suspension & brakes, to balance-out that increased power. Mathematically, increasing speed from 40mph to 60mph roughly doubles the operating stresses and hp requirement; taking that up to 80mph has the same percentage increases, above 60mph. In that context, it's easy to see how quickly, dramatically, the balance is upset and it's all safety-related. Again, just because you have the power doesn't mean that you must use it...but most red-blooded gearheads just cannot resist. I designed an 80mph CT70 back in `02, when that was quite an achievement. Trust me, even with the requisite braking, suspension and steering upgrades, it's not as much fun as one might think. All of that having been said, here in the upper midwest, there's not a whole lotta grades that extend any real distance. I've ridden a few stretches that were 10% for maybe 3 miles, or thereabouts, had no problem maintaining 50mph...from 110cc. But, I believe that you face some very different terrain and that does make a difference. There's no substitute for power, when it comes to maintaining speed while ascending grades. Your bike has a couple of inherent mechanical advantages over a CT70 or Z50...larger-diameter tires & wheels, longer wheelbase. Those are huge. The biggest challenge may be braking. Drums don't dissipate heat all that well. OTOH, engine braking should do the job. I've ridden on grades in excess of 30% on my CT; first & second gear work very well keeping the speeds below 30mph, on descent...that brakes can take over from there, easily.

    From my perspective, of this moment, it looks like 110-125cc is the answer and the cost should be close either way...stroker crank, etc vs L125. The Lifan option is, of course, easier. Maybe you could make an offer on the L125 being offered by one of your fellow members, and do the engine swap on the cheap.
     
  20. whereshaldo

    whereshaldo Member

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    I put that head on because it was the one I could find that worked for my 81 when I was rebuilding. The old head was totally rotted out. At that point in time I'd only discovered the Yahoo group and Dr. ATV -- this forum, even though not focused on C70s in particular, is more responsive and knowledgeable about these engines. I fiddled around a bit more last night and the bike seems to be settling down nicely. The last two rides seemed good but I can't tell you how fast I'm going because the speedo cable broke again!. The two things I notice as not being totally correct are that I need to keep the idle a little higher than it should be because once its really warmed up it tends to die. The other is that at the top end of the throttle in 3rd, I seem to be getting a little clutch slipping -- I can hear the engine revving but without an increase in speed. Its really just the last 1/4 turn of the throttle but its definitely slipping.

    I may try one size larger main jet this weekend to see what that does and play with the air mixture to make sure I'm not running too lean at idle. But overall, I'm pretty happy -- there is still a lack of engine torque on big hills. I just did some measurements and I'm talking about an 8% - 10% grade-- a gain of about 50 feet over two blocks. I know a stroker would fix this, or I can just live with being slow.

    Hal
     
  21. whereshaldo

    whereshaldo Member

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    RacerX
    With regard to my comment about the clutch slipping, I thought about this last night and adjusting the clutch isn't going to solve the clutch slipping when at high revs in 3rd. Do I need heavier clutch springs or what would mitigate the issue of the clutch slipping at high revs.

    hal
     

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