Some here know I put my 70’s in storage. I did it because after I improved The range into triple digits I was going on roads I probably shouldn’t have been on. I upgraded to a 500cc Honda. It can handle the roads I ride and lets me uncomfortably steal an exit or 2 on the interstates (70 mph here). What I am coming around to thinking is my 70 has as much or more grunt at 0 to 40mph as my 500 then falls flat. I know I’m geared just right so changing gears isn’t what I’m thinking. I’m wondering if it’s possible to retune it to give me better top speed torque. Not sure I asked that correctly. Can I give up some first gear grunt and get it back in 4th? Would a properly selected cam do that? I ask because my Grom was anemic in 1st but recovered once in 2nd and pulled nicely to redline in 4th and was a 125. It could handle the roads around here. I learned that the same motor is in the CBR500 and CMX500 but different tunes and they perform differently. My 70 (108 cc’s) doesn’t have maps to try so just wondering about cams.
A misunderstanding not a gearing question
- Thread starter Deoodles
- Start date
Actually, you've done a very good job describing the largely subjective side of engine performance. IOW, I get it. The answer is probably not going to be what you want to hear. In a nutshell, it's all about the shape of the power curve and the amount of graph area below it...usable power. Horsepower and mph (along with price tags) get virtually all of the attention because...drum roll please...they're easily stated. The quantitative side always is. Most will, quite naturally, do a little mental math, something along the lines of "X displacement will return Xmph for X dollars" and make a decision based on those criteria alone. Once in the saddle, however, the qualitative side (which I define as pretty much everything else) starts coming into view. That's where things get complicated...
The more miles you accumulate, the clearer the picture. IMHO, you're a quick study.
Until a little over 15 years ago, an honest 60mph bike was quite an achievement. 124cc tunes were the limit and very expensive, done right. Naturally, that was a driving force and, limiting speeds to ~60mph (and for short bursts at that), a lot of things sort of fell into place...including riding expectations. When the Wave 100 and Nice 110s reached the west, circa 2001, with tunes as big as ~140cc possible, a year later and 160ish displacements a year after that, the whole scene was turned upside down. Now, 80mph+ was possible for about half the cost of a high end Takegawa, or Kitaco, tune...better everything and way more power to boot (at least until the US economy was tanked and the Dollar lost half it's value against the Euro and the Yen, but that's another topic) what's not to like? The answer is "imbalance". There is nothing on a bike this size, or smaller...which includes the Z50, CT70, MSX125 (a.k.a. Grom) and the new-gen Monkey...that is truly suitable for high freeway speeds. The machines are too lightweight short wheelbased, the tire diameters take a beating at speed, due to basic laws of physics and the engines have to be taken to power levels that render than less than bulletproof. Look at the current offerings from Honda & Kawasaki; clearly, those manufacturers know what they're doing.
First off, comes the goal. Clearly defined, for a road bike, that means a realistic speed range...which, somewhat ironically, constrains peak power & displacement to sustainable (in practical terms) limits. Next up comes the rolling chassis; matching brake & suspension upgrades can easily surpass the cost of an engine, or engine mods. IMO, 60mph represents the high end of sustained cruising speed on any of these bikes. Doubtless, opinions will vary on this subject. That said, I've come to really appreciate the OEM approach to balance and practicality. It's nowhere near as easy as most believe and it surely doesn't come cheap. This is where the field gets thinned in a hurry. It takes just the right combination of dedication, expectation and budget to realize the kind of machine you want. Again, judging by your stated speed range, you're well on your way toward adjusting the balance.
FWIW, I started off the same way...big power as the top priority. My main rider was supposed to be the purple bike, which can easily sustain 65mph. It only took a few banzai blasts to realize that 80mph is best for a few "war stories". It didn't stay boast-worthy for very long (if it ever was). Circa 2018, a 190 Daytona motor can easily be tuned to exceed that. My red bike was supposed to be the "beater". But, even after a year of riding the much faster bike exclusively, the red bike has gotten virtually all of the miles...closing in on 25K as of the 15th of this month. The reason is that it met the power goals you mentioned in the OP. (Yes, that was enough to make me focus on the rest of the bike ever since.) The fact is, nothing feels "fast" for very long...even if it is dangerously overpowered. In an extremely long-winded way, I'm saying that it's not about the destination...it's about the journey and everything required to make that as enjoyable as possible. That, IMHO, is where drive and sustainability are to be found.
To answer your engine tuning questions directly, in context, you are looking to trade peak horsepower for a flatter torque curve...with the tradeoff being a big gain in under-the-curve (a.k.a. average/usable) power. Honda began doing this in the late `90s. First, they went to an under-square which typically results in a flat torque curve. Then, they really put a proverbial thumb-on-the-scale, going to a desaxe configuration. The Nice motor is a stomper, not a screamer; the power curve is (by traditional old-school standards) kinda weird...torque peak around 5K, hp peak at 8K, and all done by the time the tach shows low 9K rpm. On the SOTP dyno, it's an engine that pulls like freight train from just above idle into the mid-50s, then suddenly just runs out of breath, with no discernible power peak. The new 125 motor, as used in the MSX125/Grom, is just a slightly enlarged version of that configuration...~2mm on both bore & stroke...making virtually the same peak power but at slightly lower revs. That's why first gear felt "anemic" (it isn't really) but it pulled nicely to redline in top gear.
It may be possible to shift the power curve with a milder (less hp-biased) cam and re-gear the bike to match. Best advice I can offer...and it may come across as harsh...is to go for a 54mm crank. Nothing will give you the kind of torque increase of a longer stroke. That still comes up a little short of the 55.5mm arm of the Nice as well as the 57mm MSX crank. If you can accept boring your cases to accept a 54mm bore, the extra ~15cc should compensate for at least some of the torque curve difference(s). You'll be able to really "see" the difference once you've dialed-in the new, matching, sprocket combo. The other parameter...stay off the freeway, it's no place for anything under ~250cc & ~500lbs (combined bike + rider) mass, with ~17" wheels; those are minimums.
The more miles you accumulate, the clearer the picture. IMHO, you're a quick study.
Until a little over 15 years ago, an honest 60mph bike was quite an achievement. 124cc tunes were the limit and very expensive, done right. Naturally, that was a driving force and, limiting speeds to ~60mph (and for short bursts at that), a lot of things sort of fell into place...including riding expectations. When the Wave 100 and Nice 110s reached the west, circa 2001, with tunes as big as ~140cc possible, a year later and 160ish displacements a year after that, the whole scene was turned upside down. Now, 80mph+ was possible for about half the cost of a high end Takegawa, or Kitaco, tune...better everything and way more power to boot (at least until the US economy was tanked and the Dollar lost half it's value against the Euro and the Yen, but that's another topic) what's not to like? The answer is "imbalance". There is nothing on a bike this size, or smaller...which includes the Z50, CT70, MSX125 (a.k.a. Grom) and the new-gen Monkey...that is truly suitable for high freeway speeds. The machines are too lightweight short wheelbased, the tire diameters take a beating at speed, due to basic laws of physics and the engines have to be taken to power levels that render than less than bulletproof. Look at the current offerings from Honda & Kawasaki; clearly, those manufacturers know what they're doing.First off, comes the goal. Clearly defined, for a road bike, that means a realistic speed range...which, somewhat ironically, constrains peak power & displacement to sustainable (in practical terms) limits. Next up comes the rolling chassis; matching brake & suspension upgrades can easily surpass the cost of an engine, or engine mods. IMO, 60mph represents the high end of sustained cruising speed on any of these bikes. Doubtless, opinions will vary on this subject. That said, I've come to really appreciate the OEM approach to balance and practicality. It's nowhere near as easy as most believe and it surely doesn't come cheap. This is where the field gets thinned in a hurry. It takes just the right combination of dedication, expectation and budget to realize the kind of machine you want. Again, judging by your stated speed range, you're well on your way toward adjusting the balance.
FWIW, I started off the same way...big power as the top priority. My main rider was supposed to be the purple bike, which can easily sustain 65mph. It only took a few banzai blasts to realize that 80mph is best for a few "war stories". It didn't stay boast-worthy for very long (if it ever was). Circa 2018, a 190 Daytona motor can easily be tuned to exceed that. My red bike was supposed to be the "beater". But, even after a year of riding the much faster bike exclusively, the red bike has gotten virtually all of the miles...closing in on 25K as of the 15th of this month. The reason is that it met the power goals you mentioned in the OP. (Yes, that was enough to make me focus on the rest of the bike ever since.) The fact is, nothing feels "fast" for very long...even if it is dangerously overpowered. In an extremely long-winded way, I'm saying that it's not about the destination...it's about the journey and everything required to make that as enjoyable as possible. That, IMHO, is where drive and sustainability are to be found.
To answer your engine tuning questions directly, in context, you are looking to trade peak horsepower for a flatter torque curve...with the tradeoff being a big gain in under-the-curve (a.k.a. average/usable) power. Honda began doing this in the late `90s. First, they went to an under-square which typically results in a flat torque curve. Then, they really put a proverbial thumb-on-the-scale, going to a desaxe configuration. The Nice motor is a stomper, not a screamer; the power curve is (by traditional old-school standards) kinda weird...torque peak around 5K, hp peak at 8K, and all done by the time the tach shows low 9K rpm. On the SOTP dyno, it's an engine that pulls like freight train from just above idle into the mid-50s, then suddenly just runs out of breath, with no discernible power peak. The new 125 motor, as used in the MSX125/Grom, is just a slightly enlarged version of that configuration...~2mm on both bore & stroke...making virtually the same peak power but at slightly lower revs. That's why first gear felt "anemic" (it isn't really) but it pulled nicely to redline in top gear.
It may be possible to shift the power curve with a milder (less hp-biased) cam and re-gear the bike to match. Best advice I can offer...and it may come across as harsh...is to go for a 54mm crank. Nothing will give you the kind of torque increase of a longer stroke. That still comes up a little short of the 55.5mm arm of the Nice as well as the 57mm MSX crank. If you can accept boring your cases to accept a 54mm bore, the extra ~15cc should compensate for at least some of the torque curve difference(s). You'll be able to really "see" the difference once you've dialed-in the new, matching, sprocket combo. The other parameter...stay off the freeway, it's no place for anything under ~250cc & ~500lbs (combined bike + rider) mass, with ~17" wheels; those are minimums.
Thanks Bob, to be clear I’ve never taken my CT on the highway. I’m stealing an exit or 2 on a 500cc bike. Your answer is slightly harsh. If I read correctly I need to go bigger and more square with a longer crank; not exactly what I was hoping for. 
Going to have to put some real thought into your post.
Going to have to put some real thought into your post.
OLD CT
Well-Known Member
The stroker 124cc has that pull you are looking for. It gathers up MPH/speed at a lower rpm in every gear, than the 108. More low end grunt. It even has a better cruise ability and more throttle available at 55 to 60. It isn't as buzzy. Still would be scary on I 80, though! 1 or 2 short exits maybe.
It's reality that's harsh. I'm being brutally honest here because, despite the discomfort of confronting something we'd rather avoid, in the long term it's actually the quickest and least painful route to a desired outcome. IOW, there's no arguing, or negotiating, with machinery and physical laws.
Strictly speaking, given the choice of only one engine modification, it'd be the 54mm crank. That can be paired with a 52mm bore, saving you the added step of having the case bored. However, there's no getting around the fact that you'll need a taller (69mm) cylinder and a 54mm jug is going to cost virtually the same amount as a 52mm version and take the same amount of work to install. So, I figure "why not go for the extra displacement, all else the same?" The larger bore will tend to lower static CR a bit (not a bad idea for a road machine) and unshroud the valves, so it's hard to envision a downside of a 54x54 configuration, other than the case machining. Undersquares tend to have high compression, which is good for torque and overall efficiency, they just don't breath as freely as squares and oversquares.
In real-world experience, long stroke engines tend to "punch above their weight". It's stroke, above all else, that determines optimal gearing and pulling power. Consider the Honda Nice motor 50x55.5mm, Honda used a dished piston to lower static CR to 9.5:1 (very good for efficient flame propagation & breathing), the result is a solid 60mph+, under average conditions, with (what I call) an "imaginary top speed" (i.e. under ideal conditions) around 65mph and ideal gearing of ~7.4mph/1000rpm. On paper, it doesn't quite make sense...until you compare optimal gearing for tunes based on a 51mm stroke, such as yours. Even then, the new-gen motor still "punches above its weight" somewhat but, the effect of stroke increase explains most of it.
Strictly speaking, given the choice of only one engine modification, it'd be the 54mm crank. That can be paired with a 52mm bore, saving you the added step of having the case bored. However, there's no getting around the fact that you'll need a taller (69mm) cylinder and a 54mm jug is going to cost virtually the same amount as a 52mm version and take the same amount of work to install. So, I figure "why not go for the extra displacement, all else the same?" The larger bore will tend to lower static CR a bit (not a bad idea for a road machine) and unshroud the valves, so it's hard to envision a downside of a 54x54 configuration, other than the case machining. Undersquares tend to have high compression, which is good for torque and overall efficiency, they just don't breath as freely as squares and oversquares.
In real-world experience, long stroke engines tend to "punch above their weight". It's stroke, above all else, that determines optimal gearing and pulling power. Consider the Honda Nice motor 50x55.5mm, Honda used a dished piston to lower static CR to 9.5:1 (very good for efficient flame propagation & breathing), the result is a solid 60mph+, under average conditions, with (what I call) an "imaginary top speed" (i.e. under ideal conditions) around 65mph and ideal gearing of ~7.4mph/1000rpm. On paper, it doesn't quite make sense...until you compare optimal gearing for tunes based on a 51mm stroke, such as yours. Even then, the new-gen motor still "punches above its weight" somewhat but, the effect of stroke increase explains most of it.
I don't recall what carb, intake and exhaust you're running. That said, you're after more torque, not horsepower (AFAIK) and any given carb and exhaust will flow freely enough to support a given hp level, regardless of the displacement. In case you're wondering, the longer stroke + larger displacement will reduce rpm of the powerband...with a big increase in torque...just what you said you wanted.
That's enough carb to support 65-70mph hp levels and enough exhaust flow for ~80mph.
Don't feel bad, compared to me (for example), you've taken a very direct path. That's how one learns and it's part of the enjoyment, the adventure...and ultimately the satisfaction of a hard-won success.
Don't feel bad, compared to me (for example), you've taken a very direct path. That's how one learns and it's part of the enjoyment, the adventure...and ultimately the satisfaction of a hard-won success.
So let’s talk about what this motor would look like. I think boring the cases makes sense. Why go to the trouble of a build and let the cases be the limiting factor. It should have as flat a curve as possible and have a large portion of the graph below. How many cc is it. Is this a prepackaged kit or pieced. I haven’t seen a 52mm stroker crank for sale in quite some time.
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54mm x 54mm = 123.6CC
Here's most of what is needed.
54mm alloy cylinder
Matched piston
Your existing cam will "act smaller" with the larger displacement. Transration: the entire powerband will be shifted to lower rpm, which means a fatter torque curve and less of a power peak. Best guess, optimal gearing will end up ~10% taller (give or take) than what you're running at present.
Here's most of what is needed.
54mm alloy cylinder
Matched piston
Your existing cam will "act smaller" with the larger displacement. Transration: the entire powerband will be shifted to lower rpm, which means a fatter torque curve and less of a power peak. Best guess, optimal gearing will end up ~10% taller (give or take) than what you're running at present.
Tough call on the cases. I'm not a fan of unknown metallurgy and that's what you get with Chinese case castings. That may be needlessly prejudiced on my part. Haven't seen a case failure on anything smaller than 140/150cc in many years now. Given my druthers, I'd use Honda GB4 cases..."12v" style, with needle bearings, instead of integrally-cast bronze bearings, on the non-thrust sides of the tranny; then again, when's the last time anyone ran across a worn bronze tranny bearing...if ever? Are the Chinese cases GB4 knockoffs? This gets to be a slippery slope; the more 12V-era improvements you get into the more it could make sense to build a 12V-based motor. I still prefer CDI and 12v electrics over breaker point igntion and 6V. Then there's the question of bored-out OEM 4-speed cases. Feel the migraine coming on, yet? 
decisions...decisions...
I really don't know how the combustion chamber of your existing head compares to a 1970s era CT70 head and that's the "$64 question". If it's based on the 12v head, with the smaller combustion chamber, you'd be better off with a smaller-domed piston...assuming that the one in the dratv link will clear the head in the first place. If it's comparable, then I'd just go with the recommended piston.
FYI, even if we had the option of EFI, electronic engine management only goes so far in shaping the power curve...unless variable valve timing and 4 valves per cylinder are part of the equation. Short of that, the revband is determined by the cam profile and...to a lesser extent...by the intake runner length. For what you're trying to accomplish, more displacement and (effectively) less cam is the (only) way to go. Fortunately, it's easy & straightforward. EFI can be tuned with great precision, to dial-in just about any combo possible. The potential tuning parameter control, compared to pre-electronic engine control era tech is amazing. Ever map an ECU? It's not for the inexperienced, the faint of heart (or wallet), or anyone lacking deep knowledge of engine operating parameters; ask me how I know this.
decisions...decisions...I really don't know how the combustion chamber of your existing head compares to a 1970s era CT70 head and that's the "$64 question". If it's based on the 12v head, with the smaller combustion chamber, you'd be better off with a smaller-domed piston...assuming that the one in the dratv link will clear the head in the first place. If it's comparable, then I'd just go with the recommended piston.
FYI, even if we had the option of EFI, electronic engine management only goes so far in shaping the power curve...unless variable valve timing and 4 valves per cylinder are part of the equation. Short of that, the revband is determined by the cam profile and...to a lesser extent...by the intake runner length. For what you're trying to accomplish, more displacement and (effectively) less cam is the (only) way to go. Fortunately, it's easy & straightforward. EFI can be tuned with great precision, to dial-in just about any combo possible. The potential tuning parameter control, compared to pre-electronic engine control era tech is amazing. Ever map an ECU? It's not for the inexperienced, the faint of heart (or wallet), or anyone lacking deep knowledge of engine operating parameters; ask me how I know this.
kirrbby
Well-Known Member
I have lots if thoughts on this topic, more than I have time to type. But, fatcaaat has the tooling to bore cases now. Or, if you're handy with a router, I have a good video for you to watch.
Also, TRX70 engine cases are gb4 cases and typically sell pretty cheap.
The hardest pill to swallow is the price of the 54mm crank. But once it's in, the top end can be tried and swapped and modified and changed...until you hit it juuust right.
You have a nice 108cc engine running well. Put it on the shelf, build from scratch, build what you want the first time around if possible...(2nd)
Also, TRX70 engine cases are gb4 cases and typically sell pretty cheap.
The hardest pill to swallow is the price of the 54mm crank. But once it's in, the top end can be tried and swapped and modified and changed...until you hit it juuust right.
You have a nice 108cc engine running well. Put it on the shelf, build from scratch, build what you want the first time around if possible...(2nd)
OLD CT
Well-Known Member
Same here, lots of thoughts. If you now have points and you buy the new cases, you will have to buy a stator and a newer style oil pump. This project is going to apex 1000 dollars. 900 more will get you a 20 mph faster Daytona190cc with no messing around. It all depends on if you like building engines and have the time.
You guys are killing me. So many options and even then; will it meet expectations. I do need something to put on the bench this winter. My honest and first thought was why can’t I get a Grom motor in there and carburate it. If I could get that kind of power band into my 70 I would have it back out on the road. Not sure where I take this next.
kirrbby
Well-Known Member
Someone over on PM recently posted about his ZB50 with a Daytona 190. I think he said "it'll walk all over Groms", and the Groms had some minor mods. I could probably find the thread if you're interested. The Daytona is at tbolt, for $1200 I think.
My 124cc was a TRX90 engine, with a 4valve head, ATC 185 carb and intake, and a 5disc clutch.
From what I've read about Groms, it would definitely outperform them too.
It would hit 70, reliably, without any drama at all. Just zing up there any time.
What bike are you putting this engine into? Must be your H? 6v lighting?
I think it was Tweakin who built a 124 recently, with a tbparts V2 head. If you care to look up his final posts about it.
https://tboltusa.com/store/daytona-anima-190cc-flx-engine-p-908.html
My 124cc was a TRX90 engine, with a 4valve head, ATC 185 carb and intake, and a 5disc clutch.
From what I've read about Groms, it would definitely outperform them too.
It would hit 70, reliably, without any drama at all. Just zing up there any time.
What bike are you putting this engine into? Must be your H? 6v lighting?
I think it was Tweakin who built a 124 recently, with a tbparts V2 head. If you care to look up his final posts about it.
https://tboltusa.com/store/daytona-anima-190cc-flx-engine-p-908.html
Interesting stuff here. I would put this in my 12v 93 build. IDK about aftermarket motors. I’ve always thought they were inferior quality. I had a piranha 125 that could only get to 50 mph and the clutch was horrible after it warmed up. I ended up chunking it into the scrap metal bin at the dump. It had less than 2k miles on it. I can easily do that many miles in 1 season. I did see the electric start 190 750.00. If I end up spending 1k it better meet my expectations and not be crap. I need to be cautious before I commit $$ to the bike. I know it’s money lost in resale so I need the smile factor to be worth it.