Torque, or RPM? (Moped exhaust on 66cc)

Vikingimike01

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Joined
Jul 1, 2018
Messages
222
Hey forum!

I was wondering about this for some time now. Let's say, there's a 2 stroke that revs to 6000 RPM, and has 2ft lbs of torque. It's max speed is 30MPH, with a 44 tooth sprocket. (I'm not saying that those are my specs, I just made that up.)

What would be better for reaching more top speed?

Porting the engine, to reach a higher RPM, let's say 9000, but could be any number, at the expense of losing 0.2-0.4 ft lbs of torque
OR
Upgrading/modding the engine, so it has the same max RPM, but let's say 0.5-1 ft lbs of torque, and using a smaller rear sprocket? What's more efficient?

I'm not planning on doing any of theese, I don't feel too safe riding at more than 30MPH. I'm just asking, and someone may want to do that.
 


FNTPuck

Active Member
Joined
Jul 2, 2018
Messages
288
Hey forum!

I was wondering about this for some time now. Let's say, there's a 2 stroke that revs to 6000 RPM, and has 2ft lbs of torque. It's max speed is 30MPH, with a 44 tooth sprocket. (I'm not saying that those are my specs, I just made that up.)

What would be better for reaching more top speed?

Porting the engine, to reach a higher RPM, let's say 9000, but could be any number, at the expense of losing 0.2-0.4 ft lbs of torque
OR
Upgrading/modding the engine, so it has the same max RPM, but let's say 0.5-1 ft lbs of torque, and using a smaller rear sprocket? What's more efficient?

I'm not planning on doing any of theese, I don't feel too safe riding at more than 30MPH. I'm just asking, and someone may want to do that.
Age old question! If top speed is your concern, then RPM is usually the way to get there.

I prefer having more TQ and using gearing in general, since lower RPM is normally less stressful on parts and it gives you the option to play with your gearing more without sacrificing as much drivability as a high strung race motor...and is more fun to ride.

On the other hand, with 2strokes RPM really is king and they just don't have the displacement to make a ton of TQ to pull small sprockets so you need that RPM for high speeds. Those 70+ mph top speed bikes are cool to watch on the highway, but keep in mind most of them have very anemic gearing with terrible low end and would make horrible commuters. The bikes that win races don't gear for top speed - they have USABLE power to get off the turns quickly and accelerate faster.
 

JerboaJohn

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Jul 29, 2018
Messages
533
I like the torque too bcs you don't want to waste time getting to your preferred cruising speed w/e it is,, torque is acceleration and gets you up hills, so I'm a fan of low to mids
 

Vikingimike01

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Jul 1, 2018
Messages
222
I was thinking torque too. I'm also going for torque now, but I'm keeping the stock 44 tooth.

I own a banana pipe, What if I open it up, take the extra pipes and stuff out, and weld it back on, so it would be an expansion chamber kinda? I know its not an actual expansion shape, but it would be closer to it than just random pipes on the inside.
 

Street Ryderz

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Oct 14, 2016
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2,837
Torque is what makes the hp! if you take your torque say it's 2ft lbs and x that by the rpm we'll say 6k then divide by 5252 = 2.28 hp but torque will peak before the hp or the rpm limit so higher rpm is harder to attain as the torque doesn't climb as high with the rpm.Shifting the torque peak up in rpm is where true power is made along with more of it,If you can get your torque peak at or just below where the pipe stages it will carry flater or with less drop longer to suport the rpm climb past it to the hp peak.Making torque is a matter of flow and cylinder fill efficaintcy getting the most power density from each stroke and being able to burn it efficaintly with proper timming and air/fuel ratio's.
 

Vikingimike01

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Jul 1, 2018
Messages
222
So that would mean, if I do get the pipes out, it would make more torque? Thats what I understand from what you said.

I'd also loose some top end, right?
 

LewieBike

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Joined
May 21, 2014
Messages
431
Torque is also a function of cylinder diameter ratio to stroke length.

All other efficiencies being equal, like two engines with vastly different bore and stroke; say a stroke of 47mm and a bore of 36mm nearly 48 cc would be largely undersquare and would pull a lot of power from lower rpms than say similar engine with a 31.5 mm stroke and a 44mm bore ( oversquare), which would probably run with less vibration and have more mid and top rpm range. Being both engines are the same in transfer scavenging, intake induction, carb flow and the intake and exhaust systems are equally and optimally tuned.

I know I'd like to have the torque of the undersquare vs the oversquare engine but the oversquare would pull higher rpms and probably have less harsh vibration. It might also make more H.P. overall, but be a little peaky as it's power band would be less forgiving for trying to accelerate well from 12 mph. This could be fixed by adding a case reed system or rotary valve, but we're talking about a complete engine redesign.

CG engines are piston ported because it's easy and cheap to manufacture, especially with a dated engine design.
 

Vikingimike01

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Jul 1, 2018
Messages
222
Alright, but I still didn't get my answer. Other thing I'm thinking about, is buying a nice 50cc scooter exhaust, and possibly cutting off the muffler part.
 

LewieBike

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May 21, 2014
Messages
431
If you're still riding that deathtrap MTB, it doesn't really matter anyways as you'd just crash harder. More power and torque is going to pretzel that bike sooner.

Scooter exhausts weigh a lot probably more than your bike's frame does And would also require a lot of cutting and rewelding. Most of them are really large diameter made of thickwall tubing and some of their cylinders are laid out horizontal to keep the overall height of the engine low. They also as a rule come out of the engine from the right and will interfere with your pedals.

 
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Vikingimike01

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Jul 1, 2018
Messages
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Can you stop worrying about how/what my MTB does, and me crashing or whatever??

I wasn't asking about that, and you always come with that bs.

EDIT:

I see that you have added the information I needed, thank you. Yes, I was prepared for a lot of cutting and welding. I was looking for a smaller exhaust than what you have on that picture, and maybe cutting off the end (The blue muffler part) like this one:
1548538391245.png

My frame is actually quite heavy, it's an older, iron frame. It's a 26" bike, rated for 160kgs (Just a bit over 350 pounds)
 
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