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Discussion Starter #1
Has anyone ever done any testing to see what the drop in efficiency is as engine cc's goes up?

For example, if a motor is at a certain cc and makes "x" amount of hp per cc (or CI), and you increase the size of the engine, going to a larger displacement, what would you expect the hp output to be if the engines are in virtually the same state of tune?

Reason I am asking, I've got my little Honda 250R that currently is at 16.35 cubic inches (268cc's) and putting out 56.4 hp. That makes the HP per Cubic Inch right at 3.45 hp/ci. What could I expect that number to be if I increased my displacement to say a 310cc engine, and then say a 330cc engine (and so on.... 350cc, 370cc, 431cc etc.)

I have been contemplating this for a bit and am rather curious about it.

As engine size goes up, is it to be expected that efficiency would drop?

Thanks.
 

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That is a great question. There are a host of reasons, and even more theories lol, as to why the hp/cc goes down as the displacement increases. And the difference is even seen in 4strokes.
We too have found that the smallest motors we build also have the highest hp/cc. All opinion aside, the general rule must follow the real world.
Consider this: 125cc gp motors have made close to 50hp on 100octane gasoline, and a 500cc single that makes 100hp on gas is eqaully impressive.... There are some snomo motors that are in the low 90s/333cc with quite narrow powerbands. And there might well be some of the most advanced "large" 2-stroke cylinders as an industry.
 

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You guys should look at those large super tanker power plants. As far as efficiency goes they are pretty decent for the amount of power they make.

There probably is some correlation between the size of the piston and the amount of surface area causing friction. The super tanker pistons look to be fairly small in diameter for the amount of crankshaft stroke.
 

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the relationship to the bore and stroke is more important than the final displacment number, some bore and stroke combos work better as a smaller cc setup and overcome their size by the ability to hit high rpms and do so faster therfore moving just as much air and fuel .as an example :stroking engine "X" could produce 10% more power but just increasing the bore size on "X"to equate the same given cc will only add say 5%,thats espeically true if yo udont have the needed crankcase volume for the bigger bore ..the more fuel and air the engine can effecinelty pump through it, the more output its going to make ,the displacment stamping on the side of the jug is not the whole story .thats how a good 250cc can often spank a 500cc
 

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Discussion Starter #7
So what you all are saying is that there are really too many factors to really put a good number to how much the efficiency of a motor goes down as displacement goes up.

Perhaps a better topic might be is "what is the most efficient engine configuration?" (ie - square, over-square, under-square, etc....)

My take on it is, in fact, that efficiency does go down as displacment goes up. It's just hard to say by how much.

Am I correct in my statement?

Thanks.
 

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The facts are that the most powerful motors are where the money is spent to develope them. And that is why i mentioned the 1000cc snomobiles, and the 125cc GP motors. There is currently lots of $$ spent developing them...

As to torque gains, you must understand that torque is not an energy measurement. It is force. The torque of a motor is directly related to the rpm that the power is made at. Remember that HP=torque X rpm/5252 .

Power is what we are after.
 

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I truely believe you hit a real world plateau.



when i use to build v8's it was the same thing, you can make a OUTSTANDING amount of power with a 350sbc, or a 383... but its making the slight percentage more in power that took 2x's the amount to achieve...



its the same with 2 stroke and 4 stroke small cc motors. I mean 4-5 years ago getting a 70RWHP 450 was unattainable. now we have some that are 75+ you then get these monster 520's 540 based 450's that are in all reality not making that much more power when comparing them... but then you have to realize your hitting the max that platform is going to make hp is (rpmXtorque)/5252. there is only so much RPM your going to get that motor to pull, now you put a huge slug of a piston behind it, and increase your torque number but lose RPM.... its a never ending fight.



thats why on flat land you can build a 250cc 250R motor that will straight whip a 310,330,350, even 500... but you have to look at the whole solution, you have a high revving motor that makes GREAT power... but little to no torque... you put that thing in front of a hill the situation reverses...



i guess without babling anymore, I believe that smaller motor almost always has more advantage when it comes to intake and exhaust flow. especially the 450's, you have a engine that as it grows becomes increasingly held back by its head. hell look at the 04 vs 06 450R, look how the 450R's do compared to the YFZ... I do not believe we will see the day where an aftermarket head will be avaliable. but we are limited to what we can get that head to swallow and then fart out the backend... The banshee's and 250Rs the same thing, we where plataued for along time before Calvin Christ came and showed us the way to 2 stroke enlightenment. Impressive numbers where made off stock cylinder stock borexstroke bikes. but thats because we could make the power with the transferports, exhausts and other things.. I remember when trinity and others came out with "420" big bores and they where not making anymore power then stock strokexbore bikes.. they had more torque... but nothing to write home about..



holy (censored) did I start babling again....
 

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volumetric effiency, if all factors (case volume, transfer area, exhaust tunnel diameter, pipe, etc...) are tuned the same for bigger cc's as a basic rule of thumb we go by is 10hp for 100cc. not always the case but usually fairly close. jmo one of the biggest factors is ignition timing that holds back bigger bore longer stroke engines, sure its easy to acheive finish timing around 14-16 degrees btdc with any stock ignition but the curve up thru can almost always be improved for differnt engine combos. there are a few stand alone ignition systems out there that can be adapted to twin or triple cylinder two strokes (have installed many with excellent results). ignition / compression and pipe will make or break a big bore. again just my opinion
 

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I think allot of factors come into play and with big bore kits that are adapted to smaller designed motors and they don't live up to expectations

Many times the balance is thrown off maybe because of to small of a crankcase or to short of a stroke.

Proper ignition timing curves is a excellent point on these big bore kits that utilize a factory curve designed for a smaller bore, like Moser stated total ignition advance can be adjusted to equal max hp on 2 engines but a programmable cdi can find horsepower before or after that point by tweaking the curve to the likes of the motor.

But I will also say that a larger bore may have a harder time with residual gases hanging around hurting the combustion process thus killing hp.

Seams also that most extreme cases of cc to hp output are the small GP style motors and they tend to be more square in the stroke to bore ratio and trying to have a square large cc motor kills the rpm potential because of piston speeds.
 

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I would have to agree ignition is EVERYTHING. Some other factors are most of the cylinders, pipes and port timings tend to be built around a snowmobile style. What I mean is all this stuff is really designed around a targeted rpm not around a usable power curve for a transmission based engine. Another issue is these engine are built specific and are only built by a few people so the R&D is minimal not like a cub cylinder.

Lastly ALL THE OLD 2 stroke" TECH" (transmission based) is based on small displacement engines. I'm not saying its wrong but they are just guidelines and starting point rules of thumb none of it was designed for these large cc engines. This is why everyone looks to the snowmobile world because they have been dealing with these large cc for along time and they making great power already.
 
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