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# Thread: Bore and Stroke increase limits on motorcycle engines.

1. ## Bore and Stroke increase limits on motorcycle engines.

Aoa,

Sorry in advanced if this topic has been touched upon before, but I'm curious to know if anyone can tell me what they know from experience on this topic.

Amongst modders and performance enchancers we know that by increasing the bore and stroke of an engine or their ratio we can increase the displacement (CC)/horsepower, but there must be limits in terms of % or mm x mm for 70, 100, 125 cc engines as the pistons and new other components need to fit right.

Engine Displacement = Pi/4 x Bore^2 x nCylinders

2. Maximum Bore sizes

CD 70 = 62mm
CD-100 = 62mm (Both 70 and 100 have same chamber blocks)
CG-125 = 70mm

3. Thanks for the quick reply. I read that it's mostly the bore you want to increase in order to increase rpm/bhorsepower. Increasing stroke is more for balancing with bore -

What's your comment on this, any benefits to increased stroke as well, the limits? Or just focus on bore increase?

4. So I did some simple calculations using the displacement formula, and this is what you get for increasing the bore on CD-70 and CG-125

(CD-70) - (increased bore only to 61mm instead of 62mm to be safe) --> (Pi/4) x (61)^2 x 41,4 ~ 121 cm^3
(CG-125) - (increased to 69 mm instead of 70mm to be safe) --> (Pi/4) x (69)^2 x 49.5 ~ 185 cm^3

CD-70 - old power to weight ratio (72cm^3/82kg) = 0.87 , new (121/82) = 1.48
CG-125 - old power to weight ratio (124cm^3/99) = 1.25 , new (185cm^3/99) = 1.87 , (yd-125 which weighs 1kg less gives 1.89)

So basically you can just get get a CG-125, increase the bore to 69mm and your new 185cc engine should let you beat a 6.6lakh CBR150?
And if you add a larger frontsprocket you could hit 200km/h? Maybe that would work on the Deluxe with 5gears.

Maybe one needs larger tires for better grip at higher rpms/speeds..

-Stronger brakes to deal with higher speeds.
-Carburator that gives enough air for 200cc.
more??

Maybe it's not possible or realistic to bore a 125 to 185cc because of it requires too much customisation of the engine parts. Maybe 160-165 cc is more plausible?

It all sounds fun while doing the maths here but I suppose there are many other factors one needs to consider, interms of enginehealth, other components etc. The manufacturers know which optimal settings the bike should have in order to keep both the driver safe and the bike maintained for a long time.

I don't have any engineering background, that's why I'm asking here to see if someone with the knowhow and experience can tell me better.

5. Originally Posted by mateen001
So I did some simple calculations using the displacement formula, and this is what you get for increasing the bore on CD-70 and CG-125

(CD-70) - (increased bore only to 61mm instead of 62mm to be safe) --> (Pi/4) x (61)^2 x 41,4 ~ 121 cm^3
(CG-125) - (increased to 69 mm instead of 70mm to be safe) --> (Pi/4) x (69)^2 x 49.5 ~ 185 cm^3

CD-70 - old power to weight ratio (72cm^3/82kg) = 0.87 , new (121/82) = 1.48
CG-125 - old power to weight ratio (124cm^3/99) = 1.25 , new (185cm^3/99) = 1.87 , (yd-125 which weighs 1kg less gives 1.89)

So basically you can just get get a CG-125, increase the bore to 69mm and your new 185cc engine should let you beat a 6.6lakh CBR150?
And if you add a larger frontsprocket you could hit 200km/h? Maybe that would work on the Deluxe with 5gears.

Maybe one needs larger tires for better grip at higher rpms/speeds..

-Stronger brakes to deal with higher speeds.
-Carburator that gives enough air for 200cc.
more??

Maybe it's not possible or realistic to bore a 125 to 185cc because of it requires too much customisation of the engine parts. Maybe 160-165 cc is more plausible?

It all sounds fun while doing the maths here but I suppose there are many other factors one needs to consider, interms of enginehealth, other components etc. The manufacturers know which optimal settings the bike should have in order to keep both the driver safe and the bike maintained for a long time.

I don't have any engineering background, that's why I'm asking here to see if someone with the knowhow and experience can tell me better.
For cylinder size you are somewhat right but as you said there are other consideration as well. The most important is Gear Ratio. You can increase the head size but it will help upto some extent. Your bike gears will be bottle neck for that. Thats the reason in bigger cc bike Clutch Plats and Gears are also bigger to support and transform the power to Sprocket.

6. Originally Posted by angel82
For cylinder size you are somewhat right but as you said there are other consideration as well. The most important is Gear Ratio. You can increase the head size but it will help upto some extent. Your bike gears will be bottle neck for that. Thats the reason in bigger cc bike Clutch Plats and Gears are also bigger to support and transform the power to Sprocket.
Could you please elaborate? If we look at heavy bikes they have atleast 5 gears, some have 6, just like cars. Mountain bikes have more gears for higherspeeds so I guess if the above work is carried out on say - Honda Deluxe, 125RP or yd-125 they would make use of this power better at the 5th gear.

As for clutchplates, could you just gear larger clutchplates the ones required would not fit with a 125 default engine?

In the West I think this work is done more professionally or accurately, as you can actually buy big borekits custommade for your type of machine - 125cc to 175cc package or more. In Pak the mechanic will send you to the "khraadiya wala" and you tell him to bore it so that a 50, 100 or 150* piston will fit? - I mean, if the max bore for a CD-70 is 70 mm, how do you know if the Khraadiya guy only increases the bore from (47mm default) to 52, instead of 70'' - makes a huge difference in cc.

Actually the reason why I started this thread is to get information around to customize an old CD-70 and I want to have good job done, but I need to gather info first.

7. Originally Posted by mateen001
Could you please elaborate? If we look at heavy bikes they have atleast 5 gears, some have 6, just like cars. Mountain bikes have more gears for higherspeeds so I guess if the above work is carried out on say - Honda Deluxe, 125RP or yd-125 they would make use of this power better at the 5th gear.

As for clutchplates, could you just gear larger clutchplates the ones required would not fit with a 125 default engine?

In the West I think this work is done more professionally or accurately, as you can actually buy big borekits custommade for your type of machine - 125cc to 175cc package or more. In Pak the mechanic will send you to the "khraadiya wala" and you tell him to bore it so that a 50, 100 or 150* piston will fit? - I mean, if the max bore for a CD-70 is 70 mm, how do you know if the Khraadiya guy only increases the bore from (47mm default) to 52, instead of 70'' - makes a huge difference in cc.

Actually the reason why I started this thread is to get information around to customize an old CD-70 and I want to have good job done, but I need to gather info first.
@vickytulla can shed some light properly because he has altered his 70cc big time

8. on papers its looks like it can reach 200kmh or beat a CBR .... in reality if you put all these calculations into a CG it hardly reaches to 130~140 ....

I increased the Bore of a CD70 to 124cc by using a quad Bikes Head Cylender crank weight piston .... in simple a complete swap of power plant...

according to math which we did at that time with such a 70cc light weight frame slim tyres it should reach above 150+kmh but in reality it only increased its accelration more the CG and top speed chaged from 80kmh to 100+kmh ....

reason was it needed ignition advancer and better intake better exhaust larger ports... its a complete world one has to understand first...

the gear ratios are all revised, final reduction ratio is to be choosed wisely and lastly it needs a better clutch box..

Originally Posted by mateen001

So basically you can just get get a CG-125, increase the bore to 69mm and your new 185cc engine should let you beat a 6.6lakh CBR150?
And if you add a larger frontsprocket you could hit 200km/h? Maybe that would work on the Deluxe with 5gears.

9. So mechanically speaking, it is very possible to make the CD-70 hit 150km/h+ , provided you make up for all that the engine requires to give the true performance of 124cc.

Did you look around to see if you could get these missing/required upgrades? (ignition advancer, better intake/exhaust, larger ports) ?

A few months back I saw that my uncle had an old and rusty kinghero - 70 (model 2006 I think) just staying idle in a dark room. I'm new to motorbikes so I decided that perhaps one could get it working. Then I have this distant cousin, who used to be a racer in Lahore and also ran his auto-shop specifically dedicated to modding and tweaking motorcycles.

He not only got this motorcycle working, he inspired me to turn it into a Jinn**

We had a complete overhaul and enginework done (though I have no idea about how much the bore x stroke was increased, we added a 100* type piston), new clutchplates and a larger frontsprocket:smaller backsprocket. It's funny, after doing this the bike would hit 40km/h+ on the first gear and 70km/h on the 2nd gear

I was really hoping to hit 120km/h+ and "break the needle", but I had to leave Pak at the time and didn't have enough time to complete the "running" for 500-600km..

Anyway it's not a rumour - alot of modded 70 machines (old Honda's especially) have been tweaked to break the needle on the speedometer, going past 120km/h. I once found a tweaked 70 from 1986 on olx.com.pk, where the seller said it had a topspeed of
135km/h.

So if a guy in Karachi can make an old cd-70 hit 135km/h (no weight reductions) then I'm sure a cg-125 can be made to hit 160-170km/h.

Is there any issue with engineoverheating? The CBR150 supposedly has liquidcooling.

10. Originally Posted by mateen001
.

Did you look around to see if you could get these missing/required upgrades? (ignition advancer, better intake/exhaust, larger ports) ?
yes all these spare parts are avaiable but I lost interest and put it to use on every day ..

yes a cd bored out to 160cc can perform good but still it will not be able to catch up with a CBR150 as its a four valve High compressor sports engine..

11. @poster
increasing only bore size will not give u result

Stroke= down or high
Gear Ratio= 3rd and 4th gear
Head= Bigger Valve+CAM+Bigger Squish Band

Then Current which makes quick RPM

12. what is tha standard piston size of honda cg 125 ???

13. Would there happen to be a comprehensive techguide on all the things one should know when bore'/stroke'ing and upgrading your engine for boosted cc/performance?

Last time I had a bikeengine tweaked it was thanks to mechanic friends and relatives, but it helps to know what you're doing, what needs to be done.

14. interesting , i have a 100cc layin there in the dark ever since i got my cruiser. could perform something like this on it just for fun , any recommendations of such a mod shop in karachi that deals with 125/150/70s ?

15. An interesting and low price remedy for speed will be... yama4 bike(its not junoon)
Bore x stroke= 49 x 54
Intresting part is it has stroke i:e 54mm
u will get a good 2005 model for 20-25k... 2005 ones have Japanese parts...i bought mine for 20k but had rod knock issue..which was fixed..
its engine combustion side is similar to Gy6 in america or yamaha Mio in malaysia.. they have insane big bore kits like 70mm bore but their cylinder is a little different in shape thus allowing that 70mm insane bore. that will cost 16k or more.or u can bore existing cylinder however 70 mm is not possible in yama4 beacuse its piston area is near to timing chain section ..but a very safe 62mm z possible..but taking this project will require u some good mechanical skills.. and availability of some good tools... but the problem z where to find this 62mm piston in Pakistan? will the 3 wheeler (auto rikshaw) be 62 mm more or less and what would be its piston pin diameter and also its length from piston pin to piston top...suzukie gs 150 has 57 bore but thats low..
u will require to cut ur crankcase to fit the new big glass from cylinder...if any of u guys have idea about big bore pistons available in Peshawar then educate me :-)

16. Originally Posted by junaid58
An interesting and low price remedy for speed will be... yama4 bike(its not junoon)
Bore x stroke= 49 x 54
Intresting part is it has stroke i:e 54mm
u will get a good 2005 model for 20-25k... 2005 ones have Japanese parts...i bought mine for 20k but had rod knock issue..which was fixed..
its engine combustion side is similar to Gy6 in america or yamaha Mio in malaysia.. they have insane big bore kits like 70mm bore but their cylinder is a little different in shape thus allowing that 70mm insane bore. that will cost 16k or more.or u can bore existing cylinder however 70 mm is not possible in yama4 beacuse its piston area is near to timing chain section ..but a very safe 62mm z possible..but taking this project will require u some good mechanical skills.. and availability of some good tools... but the problem z where to find this 62mm piston in Pakistan? will the 3 wheeler (auto rikshaw) be 62 mm more or less and what would be its piston pin diameter and also its length from piston pin to piston top...suzukie gs 150 has 57 bore but thats low..
u will require to cut ur crankcase to fit the new big glass from cylinder...if any of u guys have idea about big bore pistons available in Peshawar then educate me
First instead of finding the biggest piston you can find I would suggest you to find such piston which are design for higher compression incase bore with low compression than stock would result in low power and low fuel efficiency and higher compression would result in more power and high fuel efficiency I would recommand you piaggio piston and copper gas kit with double thickness the stock

17. is it possible to alter 1986 cd70 to 86 bore?? bike`s engine is going to be overhauled so decided to change head to 86. how much it cost me and what abt fuel consumption and power . ?

18. Originally Posted by mateen001
Would there happen to be a comprehensive techguide on all the things one should know when bore'/stroke'ing and upgrading your engine for boosted cc/performance?

Last time I had a bikeengine tweaked it was thanks to mechanic friends and relatives, but it helps to know what you're doing, what needs to be done.
Adding more power to the bike will only increase the acceleration significantly....topspeed increases will be minimum....this is due to air resistance and the way it increases with velocity by a factor of square....

If you want to go faster on a bike, your main priority should be to make it more aerodynamic....Bikes already have excellent power:weight ratio....

Lets take an example of CBR600RR , a decent sport bike....It's got ~120hp of power and weighs just less than 200kg. That's a power to weight ratio of about 600hp/ton. That is Buggatti Veyron territory ! But does it do 400km/hr ? Nope, it get's stuck at ~260km/hr....

That is because bikes are not nearly as aerodynamic as cars are...That is why they are also quite inefficient...

19. Originally Posted by ivan_zamataev
Adding more power to the bike will only increase the acceleration significantly....topspeed increases will be minimum....this is due to air resistance and the way it increases with velocity by a factor of square....

If you want to go faster on a bike, your main priority should be to make it more aerodynamic....Bikes already have excellent power:weight ratio....

Lets take an example of CBR600RR , a decent sport bike....It's got ~120hp of power and weighs just less than 200kg. That's a power to weight ratio of about 600hp/ton. That is Buggatti Veyron territory ! But does it do 400km/hr ? Nope, it get's stuck at ~260km/hr....

That is because bikes are not nearly as aerodynamic as cars are...That is why they are also quite inefficient...
I agree with your point that bike even after having same power to weight ratio are slower than car but the reason is not the aerodynic bike are inefficient and less powerful due to the gear ratio difference bikes manufacturer have to make small gear box to make the bike as much as compact as possible

20. Originally Posted by saboor73
I agree with your point that bike even after having same power to weight ratio are slower than car but the reason is not the aerodynic bike are inefficient and less powerful due to the gear ratio difference bikes manufacturer have to make small gear box to make the bike as much as compact as possible
buddy this is not religion we're arguing about...it's scientific facts...

make a fiber glass tube around the bike, then see it go above 200km/hr....

Also, when you do such stupid things, do check tyre max speed ratings...

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