how do you increase the horsepower of a four stroke engine cheaply?

sell whatever it is and buy a larger one. Eg: a 500cc engine will generally have more horsepower than a 125cc engine.

Do you care about reliability?

what sort of four stroke engine? whats it in? how many cylinders? fuel injection? carbs? air cooled? water cooled?

the obvious thing is that an engine sucks squeezes bangs and blows. If you burn more fuel then all things being equal, you will make more power. How do you burn more fuel? get more air and fuel into the combustion chamber, then, get the products of combustion out of the chamber efficiently so you can get more air and fuel in for the next cycle. How do you do that?

derestrict the input side: larger carburettor, derestrict the airbox, add more fuel and make the valves bigger. If you do that you need to derestrict the output side: larger exhaust valves, more efficient porting, larger diameter header pipes, larger capacity or less restrictive muffler. Turn the motor quicker: more revs equals more power cycles equals more power. Or add an oxidiser so you can dump more fuel in in a given cycle: Nitrous oxide injection is the obvious example.

Bear in mind that the stress is directly proportional to the power: will your piston/conrod/main bearings/clutch gearbox/oiling/cooling system stand up to an increase in power?. How much reliabiltiy are you prepared to sacrifice to get more power? Where do you want more power? up the top end at the expense of streetability? How much hotter can you make this engine without it turning into a hand grenade? Is it money well spent?

are you prepared to spend the money to get the results verified? i.e. dyno testing? because interweb claims or your mates saying "I got 400 hp by putting these computer fans in my airbox" you may as well save your money.

My opinion is that if you want to go faster or get better, you need to spend money on tyres first, suspension second, and power third. Thats after you spend money on yourself: training and protective kit.

but what the hell do I know?

Cheaply and more more horsepower aren't often used in the same sentence.
Some engines are cheaper than others. What kind of engine is it? What are you are trying to achieve, better acceleration or higher speed?
Some would say cold air intake and exhaust (ALL of it not just a baked bean can for muffler), but this can have limited effect unless you change the camshaft profile to suit. And changing the camshaft often isn't cheap.
Extra carburetors on a new manifold or engine management chip if EFI, but that ain't cheap, and will still require a new camshaft.
The most effective way and often the cheapest way is to replace the existing motor with a bigger one, but this will most likely require low volume certification, and probably brake upgrades. So the cost easily adds up.
A nitrous system is a cheapish and dirty way of boosting power, but without increased fuel supply and prolonged use you'll destroy your engine.
You could try using higher octane fuel i.e. 98, but if you can tell the difference with this, you've been tricked by clever marketing.
You'll probably find the cheapest way to make more power is to buy a more powerful car/bike.

ratrod wrote: Cheaply and more more horsepower aren't often used in the same sentence.
Some engines are cheaper than others. What kind of engine is it? What are you are trying to achieve, better acceleration or higher speed?
Some would say cold air intake and exhaust (ALL of it not just a baked bean can for muffler), but this can have limited effect unless you change the camshaft profile to suit. And changing the camshaft often isn't cheap.
Extra carburetors on a new manifold or engine management chip if EFI, but that ain't cheap, and will still require a new camshaft.
The most effective way and often the cheapest way is to replace the existing motor with a bigger one, but this will most likely require low volume certification, and probably brake upgrades. So the cost easily adds up.
A nitrous system is a cheapish and dirty way of boosting power, but without increased fuel supply and prolonged use you'll destroy your engine.
You could try using higher octane fuel i.e. 98, but if you can tell the difference with this, you've been tricked by clever marketing.
You'll probably find the cheapest way to make more power is to but a more powerful car/bike.

What you said.

ummm, its a honda xl100, i prefure accelteaion over top speed, i need simple methods of kicking up the power

its single cylinder carbed and its air cooled

El33T wrote: ummm, its a honda xl100, i prefure accelteaion over top speed, i need simple methods of kicking up the power

Didn't honda do a 100,125,185 and 500cc version of that bike?

Bonus if the 500 will fit, but even the 185 will be a massive improvement.

yep they did, i have the 100

ratrod wrote: You'll probably find the cheapest way to make more power is to buy a more powerful car/bike.

This is the answer

El33T wrote: ummm, its a honda xl100, i prefure accelteaion over top speed, i need simple methods of kicking up the power

aaaaa.....put some pedals on it............
There are less after market bits to put on bikes than cars especially little ones, so you can count that out.
If you can track down some different gears/sprockets that will help. Smaller on the front (if you can), bigger on the rear (most likely), it will sacrifice top speed though, kind of like on a pushbike really.
Do a google search to see if there are other engines sizes in that family that have a higher compression head, and fit that. It looks like an overhead valve as opposed to overhead cam so it will probably require different pushrods.
Is the engine in good nick? A simple ignition fresh up, plugs etc can make a supprising difference if their old.
I'd try the carb and exhaust method. Bigger carb of a wrecked bigger bike, remove the muffler and if you can bend pipe make up bigger diameter exhaust.
Bend the pipe up in the right shape, spray a line or normal spray paint down the length of it and where it stops burning, cut it of there.
It'll be loud as and probably piss you neighbours off, but I'm guessing that you live on a farm, so it'll probably only make the chickens stop laying.

I suspect that the 500 engine case mount points are common to stuff like GB's XT. XR and FT (I think, you would have to check carefully stuff like bolt holes, head heights and countershaft sprocket location. I do know that with minimal work an XT500 or XL500 motor will bolt up to a CB250RS frame....


I bet if you put a 500cc motor into this frame, assuming it would fit, the first time you blipped the throttle it would wind the thing in half then drop the motor onto the ground.....


Dude, see if you can source a bigger motor that will bolt up. something like an XR185/200 might work. But honestly for the time and fucking about with it why not sell the whole bike and get a bigger one: it will have better brakes and frame and suspension and all that good stuff. You run the risk of faffing about with your pride and joy, fucking it up, and then having no bike to ride.

Dont ask me how I know this, and dont you dare mention the words "TS125 " or "first bike" to me, mkay?

It is usualy quite easy to get say 10% more power cheaply, however 10% more of what a 100cc can manage is not much, so the result will not be much faster. All that work for little gain.

Going to a 125 or 150 would be a massive gain, something that would be impossible to get by warming up a 100...

robbieracer wrote: It is usualy quite easy to get say 10% more power cheaply, however 10% more of what a 100cc can manage is not much, so the result will not be much faster. All that work for little gain.

Going to a 125 or 150 would be a massive gain, something that would be impossible to get by warming up a 100...

True, but what a sense of achievement in tweaking 100cc's of pure power

I have done that tweaking 600cc to beat 1000, and 1000 to beat 1300cc etc, but it takes a lot of care, research and tinkering.

Put a sick as big bore exhaust on it au, itd go hard.