josh_did_what

So for my engineering class, our assignment is to make a rc drag racer with 2 stroke chainsaw engines. Basically no one in my group knows anything about engines, so i was instructed by a smaller RC forum to come here. What i want is some ideas regarding increasing the power. Right now the only things we have are maybe taking off/moddifying the rev limiter and seeing if we can get a polish/porting job from a local motorcycle shop. Anyone have any ideas?

PS- the engine is off a Skilsaw 1631 Type 3. AKA a 40 year old engine, so i dont have specs.

Thanks!

traxxaspede

you might look into getting upgraded exaust? i dont know much about any kind of engine but i know the exaust pipe has a big impact on 2 strokes. also check out the large scale or drag racing section of this forum. make sure everything is clean and lubricated and run good fuel. thats all i can say, i hope it helps.

yahman1254

My Feedback: (1)

look at the gas car section. and large scale.

josh_did_what

Ok, ill move to that forum, I didnt see it.

SAVAGEJIM

Here us the large scale forum, making your own gas powerd RC can be a rewarding experience, and I really hope you do well especially for your class.

What are the objectives of your project? I.e. do you have to achieve a certain top speed? Dont try for anything crazy like 80MPH, several have done that with production RCs, but they have spent easily hundreds in upgrading and modding their engines, exhaust pipes, etc. to reach those speeds.

http://www.rcuniverse.com/forum/forumid_234/tt.htm

Here is another link of porting (look further down, my initial measurements of the engine timing profile is seriously off in the first few posts)
http://www.rcuniverse.com/forum/m_8235025/tm.htm

SAVAGEJIM

I forgot to mention, here are some scratch built projects on making a gas powered RC, these should give you some good ideas to help you out:

http://www.rcuniverse.com/forum/m_5767129/tm.htm
http://www.rcuniverse.com/forum/m_8418631/tm.htm
http://www.rcuniverse.com/forum/m_7677148/tm.htm
http://www.rcuniverse.com/forum/m_8364687/tm.htm

hands without shadows

You could stroke it, bore it, port it, lighten the crankshaft, and bore out the carb.

Realistically a port job from a shop and a nice exhaust would be most effective and fairly easy to do on your part. I would imagine the cranks on chainsaws are pretty heavy, taking it to a machine shop to have it lightened and balance would add a lot of snap. While your in the engine I would freshen up all the bearing, gaskets and inspect wear parts for any stress crack or other obvious damage. A crank bearing or connecting rod failure isnt pretty!

If you have access to a newer engine I would use it. Stihl gets up to the 120cc/8 horsepower range. According to a snippet of google info yours was rated at 3.5 when it was new.

txtkiller

It's all about Power to Weight ratio = speed.

SAVAGEJIM

Actually, torque to weight ratio, power is absolutely useless because you have to take the extra step of factoring out RPM and the HP-torque formula constant (5252).

So, to simplyfy, it would be best to take the HP given to you and factgor out those two factors, and then you can do some more direct calculations on just what kind of acceleration you can get at a given RPM.

Also, knowing the ttorque of the engine at the flywheel, you can calculate the optimum gear ratio to be your correct torque multiplier to give you the bets torque at the wheels. And knowing the torque curve of the engine in question, if you go multi-speed, you can set up your gear ratios and shift points such that the engine is running at the RPM bandwidth such that it remains within the meat of the torque curve.

hands without shadows

SAVAGEJIM, I understand what you are thinking but its not quite true. I think you are basing your statement on something like "a car with 5 ft-lbs of torque and a top speed of 60 mph will accelerate faster than a car of same weight with 2.5 ft-lbs of torque and a top speed of 60 mph." Thats true but torque isnt the deciding factor. The faster car has twice the horsepower. If the cars had the same horsepower than the car with 5ft-lbs of torque would have a top speed of 30mph. Torque is nothing without speed, and torque with speed is called power.

txtkiller

Well said!!! RPM is useless Rpm is generaly the same for these small engines. The only thing that changes is HP. MORE HP the faster you can go. But more weight the slower you go. Correct gearing will help the weight problem.

olkleaf

[quote]ORIGINAL: txtkiller


rpm is not use less let me but it this way which engine would you rather have a 5hp@20,000 rpm engine or a 5hp@32,000 rpm engine?
if rpm was useless why would formula 1 have their engines revving at 20,000 rpm?
usually the more rpm an engine can make the more power it can make ...

a perfect example of this is [link=http://www.youtube.com/watch?v=fhh0F9bSmh4&feature=channel_page]this guys ntc3[/link] it is beating peoples big block engines

for nitro engines rpm can be very different ... a .18 32,000rpm and a .12 at 42,000 rpm... lets keep in mind that your car (actual real car) can only do about 8,000 rpm .. if a .12 makes about 1.5hp and a .18 makes 1.8 hp (both stock) and the .12 revs at 42,000 and the .18 revs at 32,000 i rather have the .12 they are making about that same power and the .12 is making 10,000 rpm more then the .18 . plus all you have to to is gear it little to make up for the .2hp difference plus smaller engine less weight less power is need

Access

People need to learn to look past the numbers, the value of an engine is how it drives, not how it rates on some numeric scale. I can have two engines, engine A might have a highly rated horsepower, torque, rpm, and all that, engine B might have lower ratings in all those things but still drive and perform better. There are many things that these numbers don't describe, yet they still matter. If people overbuy on the numbers, manufacturers design to maximize some number while making tradeoffs that hurt the overall driving and performance. Look at digital cameras (megapixels), audio amps (THD), computers (clock speed), etc.

SAVAGEJIM

You hit on something that everyone falls into at one time or another, and yes, I admit, I was one of those people. And this is the fallacy of peak HP and peak torque numbers.

The most important factor of an engine if it will perform is the torque curve. You can even skip the HP curve and strictly look at the torque curve to see where the meat of the most accleration will occur. So, given your RPM as your independent variable, you can look to see which RPM bandwidths the engine will accelerate the RC best at.

If you want to look strictly at the HP curve, looking at it to see where the RC will accelerate best at is a bit trickier. But you can scale that curve by scaling out 5252 and doing algebra on the HP formula to calculate torque vs RPM and then plot your torque curve.

RPM is not useless in our combustion engines since combustion engines must be moving at some RPM to be making any amount of torque. Motors are a different story since motors can produce torque at zero RPM at the instant electricity is applied.

txtkiller

It's all about HP and weight ratio. It's a proven fact.

K4neX

Good advices given here, but also remember to check the condition / behaviour of the engine you have chosen by looking at the P-T graph, it tells you lots of thing about the engine.