Bigger Valves,head porting,exhaust mods,superchargers
- Thread starter Tim S
- Start date
Yea, you can have the intake and exhaust ported to a larger size with larger valves to accomodate, or just have them cleaned up of any casting marks for better flow. You can get a bit more power by port matching and such.
If you are looking fo a bit more power without spending to much, you can purchase a more aggressive cam, and a freer flowing filter, maybe a K&N style filter, or a Uni for dirt/trail riding.
If you are looking fo a bit more power without spending to much, you can purchase a more aggressive cam, and a freer flowing filter, maybe a K&N style filter, or a Uni for dirt/trail riding.
The biggest power gains will come from port work. It is possible to fit larger valves, but this is a can of worms as there are variations in head design over the years. A competent performance engine builder should be able to figure it out for you, though the costs may add-up. As I recall, the largest intake valves Honda ever used with the 72cc engine was 23mm. It may be possible to go as large as 25mm, depending upon the specific head on your motor.
Even with the stock size intake valve, opening up the ports can really wake up the motor. You want to begin by removing all casting flash, then open the short-side raidus of the port just beneath the valve, then enlarging the manifold end and blending the two ends together in a smooth transition. In effect "straightening out" the port. This can be done on the exhaust side as well. The intake port should be left with a relatively rough finish for fuel atomization; the exhaust port can be polished smooth. Do not grind away the exposed portions of the valve guides.
Once the ports have been improved, you'll need a hairier cam to take advantage of the increased breathing capability. 72cc is still small displacement, so you'll need to rev the motor fairly high to make power. Cap it off with an 18mm carb, K&N filter and a decent exhaust and you should see 50mph on the flat.
Adding spark advance, either the old mechanical type as used with the "H" motors or CDI type will make the engine rev better and have crisper throttle response. Increasing compression will also help, especially with top end hp. Again, you have to do your homework to get the correct hi-comp piston.
Adding an 85/88cc kit is almost a no-brainer as the cost is nearly the same as boring/honing & a new piston assembly for your existing cylinder. Plus it's another 5mph+.
Forget the forced induction and nitrous. The setups are complicated, fiendishly tricky to setup and expensive. I've yet to actually see a supercharger or turbo that actually worked properly on one of these bikes. Nitrous is good for about 2-4hp, if you really know what you're doing. Expect a toasted piston, or three, along the way. Go too far and expect the engine to go "bang" in a big way
Even with the stock size intake valve, opening up the ports can really wake up the motor. You want to begin by removing all casting flash, then open the short-side raidus of the port just beneath the valve, then enlarging the manifold end and blending the two ends together in a smooth transition. In effect "straightening out" the port. This can be done on the exhaust side as well. The intake port should be left with a relatively rough finish for fuel atomization; the exhaust port can be polished smooth. Do not grind away the exposed portions of the valve guides.
Once the ports have been improved, you'll need a hairier cam to take advantage of the increased breathing capability. 72cc is still small displacement, so you'll need to rev the motor fairly high to make power. Cap it off with an 18mm carb, K&N filter and a decent exhaust and you should see 50mph on the flat.
Adding spark advance, either the old mechanical type as used with the "H" motors or CDI type will make the engine rev better and have crisper throttle response. Increasing compression will also help, especially with top end hp. Again, you have to do your homework to get the correct hi-comp piston.
Adding an 85/88cc kit is almost a no-brainer as the cost is nearly the same as boring/honing & a new piston assembly for your existing cylinder. Plus it's another 5mph+.
Forget the forced induction and nitrous. The setups are complicated, fiendishly tricky to setup and expensive. I've yet to actually see a supercharger or turbo that actually worked properly on one of these bikes. Nitrous is good for about 2-4hp, if you really know what you're doing. Expect a toasted piston, or three, along the way. Go too far and expect the engine to go "bang" in a big way
oilspot
Member
flycutting the head?
You mention raising the compression. Any reason not to take a calculated flycut of the head to raise the compression?
I was reading this thread and this post about doing the light head porting.racerx said:The biggest power gains will come from port work. It is possible to fit larger valves, but this is a can of worms as there are variations in head design over the years. A competent performance engine builder should be able to figure it out for you, though the costs may add-up. As I recall, the largest intake valves Honda ever used with the 72cc engine was 23mm. It may be possible to go as large as 25mm, depending upon the specific head on your motor.
Even with the stock size intake valve, opening up the ports can really wake up the motor. You want to begin by removing all casting flash, then open the short-side raidus of the port just beneath the valve, then enlarging the manifold end and blending the two ends together in a smooth transition. In effect "straightening out" the port. This can be done on the exhaust side as well. The intake port should be left with a relatively rough finish for fuel atomization; the exhaust port can be polished smooth. Do not grind away the exposed portions of the valve guides.
Once the ports have been improved, you'll need a hairier cam to take advantage of the increased breathing capability. 72cc is still small displacement, so you'll need to rev the motor fairly high to make power. Cap it off with an 18mm carb, K&N filter and a decent exhaust and you should see 50mph on the flat.
Adding spark advance, either the old mechanical type as used with the "H" motors or CDI type will make the engine rev better and have crisper throttle response. Increasing compression will also help, especially with top end hp. Again, you have to do your homework to get the correct hi-comp piston.
Adding an 85/88cc kit is almost a no-brainer as the cost is nearly the same as boring/honing & a new piston assembly for your existing cylinder. Plus it's another 5mph+.
Forget the forced induction and nitrous. The setups are complicated, fiendishly tricky to setup and expensive. I've yet to actually see a supercharger or turbo that actually worked properly on one of these bikes. Nitrous is good for about 2-4hp, if you really know what you're doing. Expect a toasted piston, or three, along the way. Go too far and expect the engine to go "bang" in a big way
You mention raising the compression. Any reason not to take a calculated flycut of the head to raise the compression?
I can only take an educated guess on this one. The head is a bi-metal casting, at least on the vintage motors I usually deal with. Ever take a good look at the combustion chamber? It's hemispherical, which allows larger valves than would otherwise be possible. Cutting the head would created valve clearance problems, then you'd have to cut reliefs into the piston crown. This would also alter the combustion chamber...overall volume and configuration. Squish and quench areas would be affected. Lastly, there could conceivably be problems with the timing chain and getting the cam indexed correctly.
There are any number of piston designs available. Unless you're an engine development engineer, it's better to eliminate variables from the equation, not introduce them. If piston mods would be necessitated by head milling, it just makes more sense to simply change-out the piston for a proven design and be done with it. In the end, it will cost you less anyway.
There are any number of piston designs available. Unless you're an engine development engineer, it's better to eliminate variables from the equation, not introduce them. If piston mods would be necessitated by head milling, it just makes more sense to simply change-out the piston for a proven design and be done with it. In the end, it will cost you less anyway.
oilspot
Member
How much do you usually increase the CR?
I respect your knowledge and know-how on these bikes, and appreciate all your feedback. I just don't want you to take my questioning wrong.
I'm just wondering if something as small as a .005 cut on the head could boost compression to a desireable setting. If so the changes to timing, and concerns with piston/ valve clearance may be minimal. The head being hemispherical may actually make it possible to reduce the cc's of the head with a very small cut.
All of this would of course take some sitting down with a calculator etc.
Like I said, I'm not asking in disrespect!!! I just like asking too many questions.
I respect your knowledge and know-how on these bikes, and appreciate all your feedback. I just don't want you to take my questioning wrong.
I'm just wondering if something as small as a .005 cut on the head could boost compression to a desireable setting. If so the changes to timing, and concerns with piston/ valve clearance may be minimal. The head being hemispherical may actually make it possible to reduce the cc's of the head with a very small cut.
All of this would of course take some sitting down with a calculator etc.
Like I said, I'm not asking in disrespect!!! I just like asking too many questions.
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I have no experience at all on these motors but have built motors for numerous NHRA super stock cars in the past. With this small a motor you will have to be real careful with porting and valve size so that you get the max flow without killing the air velocity. It might make power on a dyno but it wont accellerate in the bike if the air velocity is too slow. A flow bench wont tell you that. Lots of stuff that looks good on a dyno is a turd in the car. Just something to think about, bigger isnt always better. Im sure racerx is a much better authority then me on these motors so listen to him.
Most engines are limited to about 10.5:1 on pump gas. That's just a rule of thumb, there's leeway. There's also such a thing as actual compression ratio vs nominal. In other words, what a given combo works out to mathematically compared to actual measurement.
Of course milling the head will increase compression. Figuring out valve clearance and actual compression ratio would require CC'ing, checking valve clearance and testing the engine's output. The first two are relatively easy. What's harder to determine is what effect, if any, the revised shape of the combustion chamber has on flame propagation. Also, if the steel "skull" cast into the aluminum head has locking tabs on the lower edges, cutting into the mating surface of the head could conceivably cause problems with sealing, cracking. I have no idea what the odds are on that one.
The hemispherical design is actually a mediocre configuration for a combustion chamber. The most efficient designs are heart-shaped (aka "cardioid") with a flat section on one side. This, along with efficient port design causes the air/fuel mixture to swirl into the combustion chamber, where it gets "squished" from the flat section into the open part of the chamber created by the head. Properly designed, and using a flat top piston, this creates lots of turbulence inside the combustion chamber which promotes even burning and resistance to detonation. That's why it's possibe for contemporary engines to run in excess of 11:1 on pump gas and meet emission standards. CT engines are old tech and the hemispheric combustion chamber requires a pop-up piston to get decent compression. The reason for it's use is the ability to fit larger valves than would otherwise be possible.
Now, take a good look at a CT70 head. The steel insets are different from one head to the next, something which you won't be able to tell by looking at a single example. There's also a chamfer at the outer edge. Thinning this down might create a hot spot. Then again, 5 thou, isn't much material. But it might not be enough to make a power difference either.
The point I'm going for is that while it is possible to experiment with virtually any engine parameter, such a "roll your own" classic hotrodder approach involves a lot of trial and error. That takes time, money and specialized talent. You can expect failures along the way and it takes specialized equipment to measure what are usually small changes. Unless your situation is unique, milling the head for a CR increase probably isn't the way to go. I've seen these engines built to produce 15-17hp, and new gen engines like the Honda Nice tuned to 23hp. None of them had the stock head milled. That kinda says it all.
If you just want the adventure of experimenting with this, then go for it But, if you're looking for the most bang for the buck, there are proven ways to go about that. A $135 88cc kit will give you more power than raising compression ratio by three points and running race gas and you won't have to worry about the results, someone else already covered that. The one area where you can improve power is in the ports. With a little bit of know-how, it is possible to get some decent gains by slightly enlarging and recontouring the ports. Lots of guys have done this successfully. Just do your homework before breaking out the Dremel or die grinder. It's easy to get carried away and make a mess.
Of course milling the head will increase compression. Figuring out valve clearance and actual compression ratio would require CC'ing, checking valve clearance and testing the engine's output. The first two are relatively easy. What's harder to determine is what effect, if any, the revised shape of the combustion chamber has on flame propagation. Also, if the steel "skull" cast into the aluminum head has locking tabs on the lower edges, cutting into the mating surface of the head could conceivably cause problems with sealing, cracking. I have no idea what the odds are on that one.
The hemispherical design is actually a mediocre configuration for a combustion chamber. The most efficient designs are heart-shaped (aka "cardioid") with a flat section on one side. This, along with efficient port design causes the air/fuel mixture to swirl into the combustion chamber, where it gets "squished" from the flat section into the open part of the chamber created by the head. Properly designed, and using a flat top piston, this creates lots of turbulence inside the combustion chamber which promotes even burning and resistance to detonation. That's why it's possibe for contemporary engines to run in excess of 11:1 on pump gas and meet emission standards. CT engines are old tech and the hemispheric combustion chamber requires a pop-up piston to get decent compression. The reason for it's use is the ability to fit larger valves than would otherwise be possible.
Now, take a good look at a CT70 head. The steel insets are different from one head to the next, something which you won't be able to tell by looking at a single example. There's also a chamfer at the outer edge. Thinning this down might create a hot spot. Then again, 5 thou, isn't much material. But it might not be enough to make a power difference either.
The point I'm going for is that while it is possible to experiment with virtually any engine parameter, such a "roll your own" classic hotrodder approach involves a lot of trial and error. That takes time, money and specialized talent. You can expect failures along the way and it takes specialized equipment to measure what are usually small changes. Unless your situation is unique, milling the head for a CR increase probably isn't the way to go. I've seen these engines built to produce 15-17hp, and new gen engines like the Honda Nice tuned to 23hp. None of them had the stock head milled. That kinda says it all.
If you just want the adventure of experimenting with this, then go for it
But, if you're looking for the most bang for the buck, there are proven ways to go about that. A $135 88cc kit will give you more power than raising compression ratio by three points and running race gas and you won't have to worry about the results, someone else already covered that. The one area where you can improve power is in the ports. With a little bit of know-how, it is possible to get some decent gains by slightly enlarging and recontouring the ports. Lots of guys have done this successfully. Just do your homework before breaking out the Dremel or die grinder. It's easy to get carried away and make a mess.
oilspot
Member
You've made a lot of good points.
I may big picking up a large "lot" of engine parts some I may do some experimenting. I've got limited access to a mill so I may decide to give it a shot, and might decide aginst it.
This is the type of crap I throw around in my head, I know it's strange but I enjoy modification q&a's, and not just with honda, with damn near anything mechanical.
I guess I never grew out of the "why" questions when I was a kid.
I really appreciate your feedback!!!
I may big picking up a large "lot" of engine parts some I may do some experimenting. I've got limited access to a mill so I may decide to give it a shot, and might decide aginst it.
This is the type of crap I throw around in my head, I know it's strange but I enjoy modification q&a's, and not just with honda, with damn near anything mechanical.
I guess I never grew out of the "why" questions when I was a kid.
I really appreciate your feedback!!!
Nothing asked, nothing learned. There's a certain satisfaction in experiments that go right.
Back in the day, we used to "mill" heads using 220 wet/dry sandpaper over a slab of thick plate glass. It took what seemed like forever. I wouldn't attempt that now but, with 600 grit and machinists dye, it's still a fair method to check for warpage.
You never know what you might learn.
Back in the day, we used to "mill" heads using 220 wet/dry sandpaper over a slab of thick plate glass. It took what seemed like forever. I wouldn't attempt that now but, with 600 grit and machinists dye, it's still a fair method to check for warpage.
You never know what you might learn.
You can get them there, Eastwood, Summit, Jegs etc. Stay away from the cheap import stuff, the stones just wear out faster and you dont save anything. I guess it depends how serious you want to get as to what kit you get. Even good hardware stores carry stones and grinding bits if you just want to play around some. On those heads you might get away without flex shaft stuff. Then you can build or buy a flow bench, mo money, mo money. Id think cleaning up the seat pockets and short turn radius and smoothing things out leaving a semi rough finish {not shiny}will get you 90% of whatevers there to get. Dont forget the intake manifold too. Power is tough to get on a small motor, just dont go for max size and kill velocity. In order to get the benefits of porting the motor has to be turned higher RPM. It really isnt an ideal combustion chamber to start with as was mentioned but its all you have.
I'd start out with small carbide burrs, also available from Eastwood, followed by the smallest cartridge rolls you can find, with a bit a grinder's grease. The intake port should be left with a scuffed finish, the exhaust port can be polised to a mirror shine if you like (waste of time).
The intake port will benefit from raising the roof, in effect, straightening the port and blending the transition to the pocket. You also want to remove casting flash and taper the metal around the valve guide into more or less of a point, like the bow of a canoe. That will eliminate unwanted turbulence.
The exhuast port will benefit more from overall enlargement and work on the short turn radius. Unfortunately (or fortunately) your limited by the amount of material in this area. You still need a place for a headpipe to seal against.
Mtkawboy is correct, you need to spin the motor faster to get the most out of this kind of mod. That means a more aggressive cam, bigger carb/exhaust and less low-end torque. It is possible to get 50mph+ from 70cc, but it'll be pure screamer power, no cajones below 5K
The intake port will benefit from raising the roof, in effect, straightening the port and blending the transition to the pocket. You also want to remove casting flash and taper the metal around the valve guide into more or less of a point, like the bow of a canoe. That will eliminate unwanted turbulence.
The exhuast port will benefit more from overall enlargement and work on the short turn radius. Unfortunately (or fortunately) your limited by the amount of material in this area. You still need a place for a headpipe to seal against.
Mtkawboy is correct, you need to spin the motor faster to get the most out of this kind of mod. That means a more aggressive cam, bigger carb/exhaust and less low-end torque. It is possible to get 50mph+ from 70cc, but it'll be pure screamer power, no cajones below 5K
yeah, i am debating what exactly i want to do. I currently have a 72 ct70H with the 85cc kit, 22mm intake, 20mm carb w/kn filter, and a v-8 speedsource pipe. I will probably use it mostly like a moped in my town so i wont need the low end torque. I really don't want to get much lower in that area though. I do have an extra head so i might as well try it and see if i like it. Thanks for all the help everyone.
I would be fun to play around with but at normal riding speeds I doubt you would even feel the difference, mostly you would succeed in killing low end torque. It might pick up some at the very top of the RPM scale but theres always a price to pay at the bottom. Making it bigger & higher compression are the only things that gives you both ends of the power range in my opinion. Porting, cams & big carbs all have a price at the bottom. Moderate carbs may work too if you dont get too big. This is just motor in general specific, no matter what size the motor is. As in other facets if life bigger is really better, but we have to work with the tools we have, at least I did.