# How come small engines are so weak?



## elemein (Apr 25, 2012)

I don't mean this as a put-down to small engines; sorry if it comes out that way with the word "weak", but I moreso have a question about why small engines make such small power with such high displacement.

For example, I have a 196 cc 6.5 HP small engine on my snow blower. Though, the Yamaha XT225 can put out around 17 HP with just ~25cc more. 

I'm thinking that it may be because of BSFC, as, most small engines are limited to 3600 RPM.

Though doing the math: 

9411*BSFC*6.5/11.96*3600 = 0.90 (90% VE, a bit optimistic but possible)
BSFC = 0.90(43,056)/(61,171.5)
= 0.63 BSFC

Which is a low BSFC...

Now, I know of course that small engines could possibly have a more staggered approach to airflow, and value force over the entire rev range instead of just a singular place, making 90% VE not even possible. That would defniitely raise the BSFC (for example, if 75% VE was the VE at 3600 RPM and that would make power more consistent over the entire rev range.)

Now, on the other side of things; I placed the Yamaha XT225 as a competitor; mostly because it has a high power rating for a similar displacement. 

Now, I am trying to work out exactly as to why small engines have such a low power rating against a similarily displaced motorcycle engine.

Is it because airflow for smaller engines were engineered to give a more steady powerband instead of a more concentrated powerband? It it because of the rev limiter? Or are small engines truly just less efficient?

Thanks to all answers!


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## chuck_thehammer (Oct 24, 2007)

it is really simple.

put a larger carb and rejet it to match the engine.

but also it is RPM's as well.

I have a 23cc two stroke that makes 2.5 hp. and I run it in a 1/4 r/c car.its 30 pounds and go's 50 mph. and the engine turns 13,000 rpms and its stock.

but I would not put is in anything for removing snow or mow grass.

I know that some 8 hp and 10 hp engines are the same. only difference is the carb. for ride-ing mowers.


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## justin3 (Apr 10, 2007)

This is a good question!

The way I look at it is, when you buy a lawn mower, you get an engine that is designed for its purpose, 3200~3400rpm..cut the grass. Sure you can squeeze a lot more power out of any small engine, change 

the parameters of the cam, advance or retard timing, bigger carb, port and polish...Billet aluminum this and that...But in the end your still spinning at the same RPM, just using more gas and producing more 

emissions. More power doesn't mean your snow blower won't get clogged and your lawn mower won't cut any better. people don't want to spend the money to get their lawnmower engine blue printed, bored 

and stroked. Its like buying a car, every car gets you from A to B, but do you want to spend $14,000 on a Volkswagen or $50-60,000 on a Dodge Charger. Same system applies here, if you want to buy the 

$10,000 Commercial rider with a Kohler Fuel Injected engine.. You can! but for most home owners... they don't need that much power.


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## DRslots (Oct 19, 2012)

*i get plenty of HP out of em*

:thumbsup:After machining, balancing, bluing, and port and polish. im getting 15 Hp out of the old B&s rotatiller engines with single or dual bearings, carb bore is factor also. on the dyno 15 hp under a 750 lb load at 5700 rpm.. so whats on your paper , compared to proper application, can be exceeeded.


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## ratosupport (Nov 12, 2012)

justin3 said:


> This is a good question!
> 
> The way I look at it is, when you buy a lawn mower, you get an engine that is designed for its purpose, 3200~3400rpm..cut the grass. Sure you can squeeze a lot more power out of any small engine, change
> 
> ...


In many cases, the recommended RPM is 2850-2950 due to emissions and noise restrictions. Just FYI.


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