Curious Question about Engine Displacement

[Nitroaddict]

A friend and I were having a conversation about nitro engines the other 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. I was under the impression that 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 my 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? Does anyone else have any factual data as to the torque increase possibility on larger BB engines?

 

[El Pirata]

Who was that friend??? I wonder...

 

[Nitroaddict]

Hahaa. El P - check your pm's.

 

[El Pirata]

I did. I think I said my peace on this subject the other night. I'm tired of the neverending debate so I'll just read for now...

 

[Bill Hoskinson]

Actually, you are both correct.... to a point.

This may get confusing, so bare with me here.....

It all comes down to the amount degrees of intake duration a given stoke will allow.

In a 2 stroke engine like our little glow fuel engines, the amount of intake timing the engine is capeable of using, is decided by the stroke of the crankshaft, reguardless of how large the bore size is in the sleeve.

Lets build a hypothetical engine here.....

Lets just say, we have a .21 ci big block, that has a .250" stroke. The stoke of the crank shaft will only allow X number of degrees of intake timing in the crank. (You cannot have the intake window below the carb open while the piston is traveling back down the bore) The number of degrees in intake duration available with our .250" stroke is capeable of withstanding, will allow our "hypothetical" .21ci size engine to rev up to say.....38,000 rpm.

Now, take this same engine, and bore it out to a .26 or larger ci, using the same .250"stroke.

What you run into, is a problem where the larger bore needs more intake duration (duration and timing are not quite the same thing) to achieve the same RPM that the smaller ci engine would. Basically, the larger engine "runs out of breath" at a lower rpm, even though it has a larger bore.

Even though a .21 and a .26 may have the same stroke, the .26 will produce more low end torque due to the larger CI, and run out of breath at a lower rpm than the smaller .21 engine.

Case in point- My MGT.

With my WRX Picco .26, geared 18/46, my truck has run a best top speed of 40mph. While with a smaller, 4 Port (modified) TT.21R (original engine) it has run a best of 41MPH geared 15/46.

The .21 will outrev the .26 hands down... But the .26 gets up to 40mph much quicker than the .21.

4 strokes and 2 strokes are two totally different animals fellas.:stupid:

 

[Nitroaddict]

Bill - so your saying that the .26 in theory would have more torque in the upper RPMs but less in the lower RPMs?

 

[El Pirata]

Glad to see "Professor Wizard" enlightened us with his wisdom. So according to your theory, which one will have higher RPM, torque and HP? Why?

 

[Error401]

Good post Bill.

Bigger bore, same stroke, more low end torque less top rpms.

Long strokes also have lower rpms, but are more effecient in F/A usaege.

Look at NASCAR vs Indy. Same thing with the bore, RPMs and HP (granted that they are 4 strokes, but analogy is the same).

 

[Bill Hoskinson]

4 strokes are totally different....

The camshaft determines how much intake and exhaust duration, and valve timing the engine has, reguardless of stroke.

The only thing that keeps long stoke engines (like a big block Chevy) from achieving the same RPM as a smaller CI (small block Chevy) is the amount of air and fuel that can be moved through the Cyl heads, and the strength of the crank, rods, block, wristpins ect.

The longer the stroke, the higher the piston speed. The higher the piston speed, the greater the forces are on the rotating asselbly.

Pro Stock 500ci engines regularly turn more RPM than most any sportsman racing engine of the same, or less CI.

Its all about $$$$ when it comes to 4 strokes. Titainium valves, valve springs, retainers, aluminum rods, ect ect ect....

 

[Error401]

yeah, I know. my point was that the F1 car engines turn higher rpm than the nascar engines of larger displcement. They develope their HP at different rpms (different powerbands) and have different top rpms, just like in your first post. 2 strokes are the same way. I bet if you scaled up a .21 to say, .26, it would probably scale the top rpms too. But then it wouldn't be a drop in replacement for a .21. there would be a point of diminshing returns to this, some point where the size of the piston and the rotating mass would in effect make the system calm itself down, but that's beyond my scope of understanding as to where that would begin.

 

[Bill Hoskinson]

Yes, true.... smaller displacement engines are "easier" to turn higher rpms.

Large engines, (lets say in excess of 500ci.) are capeable of turning up to 9,500 rpms, for a short amount of time. (6 seconds in the case of an NHRA Pro Stocker)

However, a smaller engines are able to turn higher than 10,000 rpms due to the fact that they are able to breathe at 10,000+ rpm through SMALLER VALVES. The smaller the valve, the lighter the valve is. The lighter the valve, the easier it is to control at higher rpms, and for a longer period of time.

For instance, Nascar allows up to (I think) 357ci. Most of the Cup cars use 3.00"-3.25" stoke cranks, with larger than production bore sizes of more than 4.125", with HONDA rod and main journals. (smaller & lighter) than a stock 350ci. (5.7L) 3.48" stroke and 4" bore

Smaller 4 stroke engine combos will always make less TQ and HP at lower RPMS than a larger engine with a longer stroke....:radio:

 

[Nitroaddict]

Bill let us assume - everything being equal, which of course woudnt be so....

Two nitro RC engines. Both have the exact same stroke and porting. One has a larger bore than the other. What would be the differences between the two regarding hp, rpm, and torque.

 

[Error401]

Originally posted by Bill Hoskinson
Actually, you are both correct.... to a point.

This may get confusing, so bare with me here.....

It all comes down to the amount degrees of intake duration a given stoke will allow.

In a 2 stroke engine like our little glow fuel engines, the amount of intake timing the engine is capeable of using, is decided by the stroke of the crankshaft, reguardless of how large the bore size is in the sleeve.

Lets build a hypothetical engine here.....

Lets just say, we have a .21 ci big block, that has a .250" stroke. The stoke of the crank shaft will only allow X number of degrees of intake timing in the crank. (You cannot have the intake window below the carb open while the piston is traveling back down the bore) The number of degrees in intake duration available with our .250" stroke is capeable of withstanding, will allow our "hypothetical" .21ci size engine to rev up to say.....38,000 rpm.

Now, take this same engine, and bore it out to a .26 or larger ci, using the same .250"stroke.

What you run into, is a problem where the larger bore needs more intake duration (duration and timing are not quite the same thing) to achieve the same RPM that the smaller ci engine would. Basically, the larger engine "runs out of breath" at a lower rpm, even though it has a larger bore.

Even though a .21 and a .26 may have the same stroke, the .26 will produce more low end torque due to the larger CI, and run out of breath at a lower rpm than the smaller .21 engine.

Case in point- My MGT.

With my WRX Picco .26, geared 18/46, my truck has run a best top speed of 40mph. While with a smaller, 4 Port (modified) TT.21R (original engine) it has run a best of 41MPH geared 15/46.

The .21 will outrev the .26 hands down... But the .26 gets up to 40mph much quicker than the .21.

4 strokes and 2 strokes are two totally different animals fellas.:stupid:

I think it was explained in this post.

 

[Nitroaddict]

Even though a .21 and a .26 may have the same stroke, the .26 will produce more low end torque due to the larger CI, and run out of breath at a lower rpm than the smaller .21 engine

I missed that statement. Wow. Exact opposite of what I thought would happen.

 

[HumboldtBlazer]

I like ponies Berry nitro doesn't smell good.


Great thread guys I think I just learned something.

 

[scottm]

OK heres my .01:

Torque is a product of the distance or radius of the crank pin from the crank centerline (stroke/2) and the force in the rod from combustion pressure on the piston (pin radius in feet x lb force in rod = ft-lb torque). A longer stroke produces more torque (all else being equal) because of greater mechanical leverage from the pin being further from the crank. But an engine with a bigger bore also produces more torque because the area of the piston (square inches) is bigger, and therefore the same combustion pressure (pounds per square inch) generates more force in the rod (pounds).

Here is an example. A healthy car engine can produce 1000 psi combustion pressure. A chevy with a 3.5" stroke and 4" bore has 12.56 sq in on the piston top (pi x r2), generating 12,560 pounds force in the connecting rod, pushing on the crank pin. The crank pin radius is 1.75" or .146 feet, generating a torque in the crank of 12,560 x .146 = 1,833 ft-lb torque. The torque measured in the crank on a dyno is much less than that, because the flywheel and all the rotating parts absorb the peak impulses and spread it out over time.

Now if you bored the Chevy out to say 4.5" bore, you will have more torque up to the point where the engine runs out of breath as Bill said. In other words, the point when flow restrictions in the heads and carb prevent the cylinders from being filled completely with the air/fuel mix. All else being equal the 4.5" chevy will have more torque at a lower rpm than the 4" chevy, BUT horsepower will be about the same. Why? Because power is a product of torque x rpm, but also from burning fuel and liberating heat. If both engines are the same except for bore, they will both breath the same amount of air through the carb and heads. Therefore the same amount of fuel will be burned in both, and the same amount of power will be generated. The difference is the small bore engine will peak at a higher rpm.

You can scale up the carbs and ports on 2 or 4 stroke engines so that air flow stays proportional to displacement, then power will grow proportionally to displacement, rpm being equal.

The ratio of bore to stroke as related to peak power is a separate but related issue. To make more power, more fuel must be burned. You can either go with more displacement, or process more air though a smaller displacement. If you are going with smaller displacement, you have a choice to force more air in with a turbo- or super-charger, or turn more rpm. If you are going to turn high rpm, you want a relatively short stroke as Bill said, to reduce piston speed and acceleration forces on the parts.

Another aspect is that high rpm engines require large airflow passages and long timing intervals to maintain flow volume as rpm goes up (ports on a 2s and cams on a 4s). This kills torque and power at low rpm, due in part to lost compresson from long valve overlap 4s or huge ports 2s.

Torque at the wheels is what moves any car. You can get it either with a small engine at high rpm and low gears, or use a bigger engine with taller gears. The bigger engine at lower rpm will last longer and sound better, so that is why it is better to use a bigger engine on a heavier car, model or 1:1.

For NA's specific question, notice that the Picco 26 and a good 21 make about the same power, 2.8 hp or so. The 26 does it at a lower rpm. Now look at the carbs. They are the same! Coincidence? No! Whew, i'm tired!

 

[Robie6]

unless I'm misunderstanding Bill, which he said dont let the induction timing go past TDC?? all 3 of my engines induction ports have a closing window at least 50 degrees past TDC. The 26 I just finished porting, rebalancing to 55% has the induction timing of} 44 degrees ABDC & a closing side of 65 degrees ATDC for a total duration of 199 degrees. Most .21 & larger engines can have a total duration of up to 204 degrees without losing but gaining torq but remember to go to 204 degrees other things HAVE to be done like opening & flowing the crank, maybe a scalop or two in well placed areas on the crank counter balance. A 3 needle carb works really well in this application cause you have the option of setting the low speed needle SEAT just about anywhere in the venturi which translates where the slide is completley past (not the low speed needle itself but the barrel) the seat at low or mid range. Porting is a must when you go this far, but the port job is only to enhance & handle the engine work just mentioned. :thumbsup:

 

[Bill Hoskinson]

No, all I said was that you cannot have the window of the crank open while the piston is traveling back down the bore of the sleeve...... I was merely trying to explain how the length of stoke limits the amount of duration on the intake timing of these engines.

Yes, most if not all engines to have some "overlap" in the intake timing. I didn't specify any specific degrees of timing in my reply, and never mentioned anything about TDC.

 

[Robie6]

well ya dont have to get your feathers up in a ruffle over it

 

[Bill Hoskinson]

:stickup: lolololol.....