A friend and I were having a conversation about nitro engines last night, and an interesting question came up regarding .21 engines vs .26, .27, .28 engines.

Now - The general concensus is that the larger engines will produce more torque. This doesn't make sense to me. To generate more torque, you need to increase the stroke of the engine. If one wants to increase hp and rpm, then you increase the bore of the engine. Now, the .26 ect engines are the same as a .21, just bored out to .26 or larger. Technically, since the stroke is the same, and the engine is now over-bored (bore is greater than stroke), the engine should become more higher revving, have more hp, but less torque. This is the theory, but I have had many people tell me that the .26 engines have more torque than the .21's

What does everyone else think?

I've heard the same: picco .26 has more torque, but less RPMs.

I honestly have no clue, but if you're dead curious I can ask our engine guru. He works on that .26 ALL THE TIME! It may be a while until I see him again though. *shrug*

Yes, Rob - give him a jingle. Please though, get some detailed explaination as to why for me. I'm a technical guy.

It sorta makes sense to me. If you stayed at the cylinder diameter of a .21 motor, you would have to increase the stroke to increase torque. But, when you increase the diameter of the cylinder to the size of .26 motor, you have more force pushing on the piston, creating more torque.

I've never heard that you bore out a motor to increase the RPM. IMHO, you would decrease the stroke to increase RPM.

Doesn't HP = RPM * Torque.

I am NOT an engine guru by any means, this is just my thoughts on this.

Matt

A short stroke engine is a higher revving engine. The longer the stroke, the more Oomph it has through rotating mass, thus creating more torque. I can be wrong on this, if someone has some facts otherwise, please share. This is something I am not 100% clear on, and could use clarification.

Either increasing bore or stroke will give more tork and more power (everything else being equal)
Also even at the same displacement a long stroke will have more tork than a short stroke, but theoretically the exact same horsepower.

ps tork = horsepower at 5250 rpm

wow, this is the first question ive saw you ask nitro. you would think that more displacement would equal more torque and less rpms. but if you think about it a short stroke engine would crank out more rpms because the stroke of the piston is shorter, so that the piston on a shorter stroke engine wouldnt have to move as far for each stroke(gaining rpms but loosing torque). i think it has more to do with the overall displacement though.

guver - If using your explaination, I input my same question. I get the answer I came up with. You say all things being equal, the longer the stroke the more torque. You also state that increasing displacement will increase both torque and HP. That being said, one would assume that when increasing displacement, you are only increasing bore and leaving the stroke the same (as in the .26) - then the engine would in essence create the same torque as before, as you technically decreased the stroke of the engine in relation to its size. I'm just guessing here.

Alright...This is what I think.

If the engine is bored out more then its stock form, the circumference of the piston sleeve changes. Since there is a larger area, the more fuel/air mixture is needed to create a even combustion cycle. If it uses the same amount of fuel/air mixture as the original block size, then it would actually create less power since the ignited fuel/air mixture would take up less cubic centimeters so there for, there wouldn't be a sufficient amount of power. Now when you add more fuel/air to take the place of the extra cc's of a bored out engine, the ignited mixture would take up the extra cc's causing more torque.

I just think the above and cannot fully back up my theory but I hope this sparks some ideas.

i think with the same carb and needle settings it would be too lean because the increased displacement of the chamber would fill with air. they make bigger carbs for bigger engines, but my brothers .18 fantom has the same size carb as my .15.

I think I outta kick your arse for making me think too hard on a Friday. j/k ya know

Increasing bore size wouldn’t increase engine rpms. The rpm “limit� is mostly dictated by the weight of the rotating assembly and its balance (provided there is enough fuel and air to carry it to destruction). By increasing the bore size, the larger piston is naturally heavier and therefore needs a heavier counter weight for proper balance. It will not turn the rpms “quicker� either. In fact (although we could be talking nanoseconds here), it would actually slow the rpm gain. Although there is more volume with more air and more fuel being burned, the rotating mass is heavier thus making it slower to gain. With keeping in mind we are talking about rpm climb losses of nanoseconds and weight increases of milligrams, this is all pretty insignificant but I feel quite confident that it’s true.

Bore and Torque and Horsepower:

I’ll tell you what I’ve done (because curiosity got the best of me) to find out what I’m now reporting. This gets kind of weird and since I could only “test� four strokes, I’m not sure how to apply it to our tiny 2 strokes.

I have DesktopDyno software. Its “virtual� I know, so one can’t really take the figures to heart but it is a great tool to see where changes of this or that changes this or that. Got me? Ok. I started off by putting together your basic single cylinder lawn mower engine. These figures are from increasing the bore only.

3.000 in. bore @3500 rpms—12 hp—18ft lbs torque. @4000 rpms—13 hp—17ft lbs torque (those were the peaks (13 hp and 18 torque) with 500 rpms difference).

3.030 in. bore @3000 rpms—12 hp—18 ft lbs torque. @3500 rpms—13 hp—18 ft lbs torque.

As I increased the bore size in increments of 0.010 each time, the number changes were bare minimal (mostly stayed the same) BUT, they moved lower and lower down the rpm scale. By the time I had the engine at 3.080 bore, and although it still didn’t ever create more than 12 hp and 18 torque, it was doing its peak much earlier @2000 rpms.

This is where it gets kinda weird.

I built your basic two cylinder engine doing the same as above. The hp and torque increases were still minimal but there was more of an increase of each with each increase of bore size. And too, it more or less fallowed a similar path of making the hp and torque at lower rpms.

I jumped up and built a 4 banger. Then jumped over a 6 and went straight to an 8 cylinder and just for the helluvit, built a V12.

During all the “testing�, the more cylinders there were, merely increasing the bore size made more and more increases in both hp and torque. Also, the more the cylinders the hp and torque increased steadily within certain rpms as oppose to the single which had little to no increase but merely moved their place in the powerband.

It makes sense but is kinda weird all the same.

I feel that the folks that are saying the .26 has more torque than the .21 are mistaken. I think that because the .26 has its peak powers at a lower rpm than the .21, the .26 pulls harder “off the line� (so to speak) and fools the eye. It seems the .21 will pull down the same numbers just later in its rpm range.

Disclaimer—The above is not based on fact but merely my findings and my speculation and theories. The debate is now open.

im guessing your talking about a riding mower right? i started thinking of a push mower and saw 12hp, i was like dang. btw, what size stroke was this simulated model?

2.500 stroke on both the single cylinder and the twin. Any thing above that (the 8 and 12) I just built to factory specs for the most part.

how many cc's or cubic inches is that??

I think a simple way to state is that any increase in displacement will increase tork and horsepower at the same time whether you increase bore or stroke. All things being equal of course.

tork and horsepower are mathematically related to each other and when tork is doubled at a specified rpm you cannot help but double your hp at the same time.

3in bore with 2.5 inch stroke would be about 23.55 cubic inches or 59.817cubic centameters i think. or would it be 17.6625ci and 43.273125cc? wait i think its the second one, but who knows im tired.

How about this? Tork is meaningless unless an rpm is specified cuz tork doesn't do any work. Horsepower does work.

Hehe, I couldn't do the math without looking up the formula myself Eric.

Hey! It’s built in the software!!!

3.000 bore—2.500 stroke = 289.58cc and/or 17.7 ci.

i was thinking it would be (bore/2)squared x 3.14 x stroke
is this right? i know 3.14 is only pi to 2 decimals but i dont remember much more than that.

oh and billyman, wouldnt it be 44.958 cc's? that would be 17.7 x 2.54

Wow. Whatever did I start, lol.

you started the destruction of a calculator, man i was using this one that for some reason was sitting by my computer, it sucks. wish it was mine so i could break it. btw i think your right billy man i just converted to centameters at the wrong time.

I used a separate online displacement/cc calculator and got the same thing (kinda, roughly)

17.671499999999998 cid. (rounded off to 17.7 by DesktopDyno)

289.6967213114754 cc (Desktop only got 289.55).

http://www.straightliners.co.uk/calcs/eng-disp-calc.htm

What I think is..

When you increase the Bore of the engine.. the same amount of pressure (expansion) is applied at the same rate per square inch over a bigger area.
Result? More "bang".

I also think that while stroke does change max RPM, its ultimately related to other factors. (Ferrari V12 that has the throttle response of an S2000 4 banger!)

Its important that any change we talk about DOES NOT AFFECT other factors in a LINEAR fashion. Every single calulation has an "optimum" point to achieve what we want.
Balancing all these points is what building a great engine is all about.

Another thing... why does a diesel engine produce more torque than a gas burner? The theory I heard is that diesel explosions are "slower" than the "flag bang" type explosion gas engines produce. So in effect, the power stroke "lasts" a longer time. This also explain why the 1.8 liter VW TDI tops out at a tops of 4k RPM. A far cry from the 8k-9k that gas burners can go up to.
And since nitromethane has a higher calorific value than gas, its "gang" is more intense and shorter than one produced by gasoline. Hence higher RPMS can be achieved (taking displacement into account)

I wish I had the energy to whip out my physics book.. look up the formulas and use the ideal gas law to come up with a mathematical model for this! (ahem.. anyone else up to it? or know software that does this mathematically?)
Billyman.. where did you get that software? Sounds like that thing can keep me busy for days on end!! lol

ram

A gas engine puts out more hp than a dsl. All things being equal of course.