Low RPM Torque Monster vice High RPM Screamer

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RobK

New Member
#1
#1
Has anybody really thought out the pros and cons of low rpm torque monster vice a high rpm screamer? How you set up any engine is a function of torque and horsepower curves, and the ultimate use of that engine. A big block Chevy truck may produce power in the low end of the curve and tow 10K pounds, but is no dragster. A Ferrari may have more total horspower, but it's available higher in the curve, and you wouldn't tow a camper with it.

I can understand if I was into drag or kart racing, I would want to squeeze every last rpm out of the engine. Seems to me if I want to focus on riding in the dirt, I'm more concerned about low end grunt; getting the torque and HP peaks in the low to mid range. I'm thinking the following would work well.

Arc rod
Flat top piston
Mildly ported 14cc or shaved 18cc head
low-midrange cam (recommendations?)
18lb springs
Bored carb or different carb
Header
High flow intake
advance timing

With this setup I could leave the governor and stock flywheel in place and add a TAV, and maybe tweak the final gear ratio. I think this would give me great acceleration and a top end around 45-50 or so depending on the gearing. Right now my nearly stock Serengeti Puma with a TAV will do around 37mph. Thoughts? Comments? Suggestions? Am I missing something? Do I have my head up my butt?
 
Motra

Motra

Active Member
#4
#4
Most of the mods you mentioned will lead to a high RPM engine. If you're leaving the governor operational, you might as well save your money. Advanced timing = high RPM. Large carburetor = high RPM. Billet rod and 18lbs. springs are only for high RPM, otherwise there's no benefit from using them.
 
MB165

MB165

Active Member
#5
#5
Im building something very close to your idea for my minibaja. I should be done in a week or so. One thing I kept in mind is, it is only a 200cc engine, so that limits us. There is no replacement for displacement.
 
R

RobK

New Member
#6
#6
Understood that the 18 lb springs and larger carb are really for higher rpm. However, the right spark advance and cam, combined with the increased compression ratio, should go a long way to improve torque. The rod is just relatively inexpensive insurance since I'd be replacing the piston any way. It also makes it easier to remove the governor if I choose to do so in the future.

MB165 - I look forward to hearing what your results are.
 
minidragbike

minidragbike

Supporting Speed Nut!
#9
#9
The more efficient you make the motor, the more TQ and HP it will make. So that means more TQ and RPM. But you loose low end TQ if you are talking about 1000 RPM TQ.

With a all out race motor the TQ band can start as high as 5,500 RPM. Max TQ might be at 5,700 RPM. So you then buy a race clutch, and set it up to engage at 5,500 RPM. At 5,500 RPM the motor may be making 17 pounds of TQ, and a max of 18 pounds at 5,700.

Anyone with a low RPM TQ motor will get blown away with a engine of the same size. Because a stock engine is only making 10 pounds at 2,000 RPM. And as soon as you start to modify the motor, the maximum TQ starts to rise up. Unless your only making the engine larger, without allowing it to breath better.
 
Modding_out

Modding_out

#10
#10
minidragbike said:
The more efficient you make the motor, the more TQ and HP it will make. So that means more TQ and RPM. But you loose low end TQ if you are talking about 1000 RPM TQ.

With a all out race motor the TQ band can start as high as 5,500 RPM. Max TQ might be at 5,700 RPM. So you then buy a race clutch, and set it up to engage at 5,500 RPM. At 5,500 RPM the motor may be making 17 pounds of TQ, and a max of 18 pounds at 5,700.

Anyone with a low RPM TQ motor will get blown away with a engine of the same size. Because a stock engine is only making 10 pounds at 2,000 RPM. And as soon as you start to modify the motor, the maximum TQ starts to rise up. Unless your only making the engine larger, without allowing it to breath better.
Click to expand...

Your the go-to guy....How would you make a stump puller?
 
Modding_out

Modding_out

#12
#12
Why a small carb? How would that help any thing besides throttle response? Wait...I think i get it, the engine isn't going to be spinning really fast so you want the small carb to provide as much gas a possible considering the air speed restrictions. Is that right?
 
C

C9H13NO3

Active Member
#13
#13
Modding_out said:
Why a small carb? How would that help any thing besides throttle response? Wait...I think i get it, the engine isn't going to be spinning really fast so you want the small carb to provide as much gas a possible considering the air speed restrictions. Is that right?
Click to expand...
Smaller ports allow for better velocity.

Velocity=torque (I think).
 
Neck

Neck

Growing up is optional
#18
#18
Torque......There's no replacement for displacement. Mini bike Paul bought a Bonanza with the big Harbor Freight clone, 13 HP? Anyway, he let me ride it with the back tire a little low on air. The engine had so much grunt, that it spun the wheel inside the tire and tore the valve stem off the tube. Nuff said?
 
Modding_out

Modding_out

#19
#19
Neck said:
Torque......There's no replacement for displacement. Mini bike Paul bought a Bonanza with the big Harbor Freight clone, 13 HP? Anyway, he let me ride it with the back tire a little low on air. The engine had so much grunt, that it spun the wheel inside the tire and tore the valve stem off the tube. Nuff said?
Click to expand...
One of these days I'm gonna get me one of those......:rolleyes:
 
R

RobK

New Member
#20
#20
I've thought about one of the 11 or 13 hp engines. I'm sure it would fit, but I'm concerned about lining up the clutch and sprocket. Also those engines would add about 40 pounds which is a lot. Since it's apparently pretty easy to bump a 6.5 up to the 10-12hp range, I think the lighter weight would be better.
 
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