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Old 01-11-2015, 02:00 PM   #1
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Cool Difference between ON and OFF road engines

Hello friends. Am new to the group but always followed the exchanges that were posted here.
I saw that many participants have a great knowledge in RC's and would like to share my knowledge few and humble too.
My question is: - What is the difference between ON and OFF road engines? There is differences in RPM, torque or something else? I saw that "normally" there is a difference in the carburetor throttle position (inverted).
I have a Ofna GT-V2 and want to put a .21 pro motor but the motor road on the carburetor is reversed.
Thank you very much!

P.S sorry my english ... I'm from Brazil lol
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Old 01-11-2015, 02:23 PM   #2
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Timings.....

Because offroad driving is more about driveabillity and torque it is better to have a low exhaust timing. That will keep the top RPM low and the bottom torque high and with that a wide torque band.

To get the best out of onroad RPM is needed, this means a high and wide exhaust port (M.E.S.) and also larger timings on the intake to get enough fuel inside which is needed.
Sadly with an onroad setup the bottom torque is less so a high engaging clutch is needed for a fast acceleration.
The high speeds of an onroad and the low placed bodies is giving a lot of airflow giving a lot of cooling, that is why onroad engines can work with smaller cooling heads.

But yes, an onroad engine can be used in an offroad and otherwise. The type of exhaust (mainly the length of the manifold) is also giving some changes to the engine, a longer exhaust is giving more torque and a shorter exhaust more RPM. In some ocasions is is also done, an offroad engine in an onroad car on a short track is a good choise.
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Old 01-12-2015, 03:24 AM   #3
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Timings.....

Because offroad driving is more about driveabillity and torque it is better to have a low exhaust timing. That will keep the top RPM low and the bottom torque high and with that a wide torque band.

To get the best out of onroad RPM is needed, this means a high and wide exhaust port (M.E.S.) and also larger timings on the intake to get enough fuel inside which is needed.
Sadly with an onroad setup the bottom torque is less so a high engaging clutch is needed for a fast acceleration.
The high speeds of an onroad and the low placed bodies is giving a lot of airflow giving a lot of cooling, that is why onroad engines can work with smaller cooling heads.

But yes, an onroad engine can be used in an offroad and otherwise. The type of exhaust (mainly the length of the manifold) is also giving some changes to the engine, a longer exhaust is giving more torque and a shorter exhaust more RPM. In some ocasions is is also done, an offroad engine in an onroad car on a short track is a good choise.
Thank you Roelof. It was a great lesson One interesting point that you said is about the exhaust and their lengths. I had forgotten these details that influence very well.
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Old 02-24-2015, 01:42 PM   #4
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Quote:
Originally Posted by JKit View Post
Thank you Roelof. It was a great lesson One interesting point that you said is about the exhaust and their lengths. I had forgotten these details that influence very well.

Also to add to Roelof's explaination the crank shaft timings can sometimes be adjusted to give a broader engine power by opening the crank shaft much closer to the time the transfer port closes rather than the much earlier opening time in on road engines, the exhaust port timing and the manifold/pipe combo extends the wave reflection time which in turn broadens the power band at the expense of peak power, this also reduces peak RPM thus fuel economy is better in off road than on road.

It's tayloring the power band to the application essentially, a usable broad power band is much more useful in off road, where as peak power and rpm is king in on road.
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Old 04-07-2015, 04:38 AM   #5
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then whats the difference with long stroke - short stroke, i thought it was timing too, but im not engine expert. =(
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Old 04-07-2015, 07:12 AM   #6
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Not with the timing. only the bore/stroke spec is different. Because the longer stroke (crankpin further away from center) it will create more torque. Also the longer stroke of the piston will create different pressure changes within the crankcase making a better pump to suck the mixtrure from the carburator and to pump the mixture into the transfer chanels.

I just wonder why super long strokes are better because RPM=horsepower.
A longstroke has the problem of a longer move of the piston so with mass and lubrication limits a longer stroke should run a lower top RPM. That is why F1 engines are running short strokes.

But maybe the 2-stroke concept has influence on the theory of needing more RPM for more HP.
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Old 04-07-2015, 07:34 AM   #7
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I think it's because unlike F1, we don't have 7 speed gearboxes. The normal rpm range of an F1 engine is a lot narrower than ours. With the current fuel flow and rpm limits (and 7 speeds) in F1, they only need to make good power between 10 and about 13k and even then they have hybrid power to boost the bottom. We need to make good power from 20k all the way to 50k. Long strokes seem to be able to still make decent 50k power but are much better in that 20-30k range.
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Old 04-07-2015, 07:43 AM   #8
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One other thing. F1 is four stroke, we are two stroke. So what? you might think. For whatever reason, long strokes are usually better in two strokes while four strokes like short. I suspect a lot of the reason for this has nothing to do with stroke physics and more to do with valve area. A four stroke with overhead valves is limited in how big the valves can be by the size of the cylinder. Go to a larger bore and shorter stroke for the same displacement and you have already improved breathing by reducing the shrouding effect on the valves. Make the valves a bit bigger as well (or increase the number of valves) and you have improved breathing even more. This isn't really an issue with two strokes since a smaller bore will mean slightly narrower ports but the longer stroke also makes them taller so it mostly evens out.
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Old 04-10-2015, 12:37 AM   #9
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Well I tested a short and long stroke engine recently identical timings otherwise from the same brand, and the long stroke pulled like a freight train and revved higher too, so theory seems to go out of the window here! Can't explain it but the results on track were very apparent.

Maybe the long stroke can produce power earlier so it can climb the Rev range earlier, or maybe it was just a case of gearing.
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Old 05-20-2015, 01:54 PM   #10
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Quote:
Originally Posted by banana rt View Post
then whats the difference with long stroke - short stroke, i thought it was timing too, but im not engine expert. =(
Here is an extract from a book on two stroke engine modification/design:

"While on the subject of bore/stroke dimensions, I would like to say that there is
much in favor of long stroke two-stroke cycle engines in many applications. They are not
superior (as many people seem to think) compared to the present day short-stroke designs
in terms of low-speed torque, as torque is entirely a function of displacement and bmep,
and wholely unrelated to bore/stroke ratios. With a long stroke, there is (at any given
displacement) a reduction in bore, and with it a loss of piston area against which gas
pressure can exert its force, that exactly balances the loss of leverage in a short-stroke
engine (which is, in turn, compensated by a gain in piston area). The only thing wrong
with the long-stroke engine is that its crankshaft speed is limited by inertia loadings, and
that in turn limits its absolute power potential as compared with the “modern” shortstroker.
On the other hand, it is compensated by having a much more compact
combustion chamber, which makes for more efficient burning, and by lower thermal
loadings on the piston as a result of the smaller crown area into which heat from the
combustion process may soak. Finally, there is an advantage in port area for the longstroke
design resulting from its relatively large cylinder wall area. This area increases in
the long-stroke engine because displacement rises only in direct proportion to stroke, but
is increased by a factor of 3.1416 (the constant, π) with enlargements in bore. These are
very real advantages, but they are not enough, usually, to prevail against the short-stroke
engine's sheer ability to rev. Crankshaft speed is the only thing subject to much juggling
in the horsepower equation- and is a far more potent factor in determining power output
than the relatively slight improvements in bmep obtainable with the marginally better
combustion chamber and porting in the long-stroke engine. A 10-percent improvement
in our Kawasaki F-5 engine's bmep (a large improvement indeed) would raise its output
to 52.3 bhp; leave the bmep unchanged, but shorten the stroke and spin it 11,000 rpm and
you would have 61.3 bhp. There is indeed no substitute for revs."
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Old 05-20-2015, 09:13 PM   #11
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The problem is that we are approaching the flame/burn rate limit of nitro at the rpm we run.
Combined with primitive carburetion the combustion is somewhat short of efficient.

The three main beneficial effects that longer stroke have are- the combustion chamber is more compact giving a shorter flame path, less advance is required and the piston spends more time around tdc to allow for more complete combustion and less fuel exiting the exhaust.

Hence more complete combustion, better power spread and better efficiency/runtime.

The piston also spends less time at bdc so taller exhaust ports are needed for the same time/area.

When will we see bores offset from crank centerline?
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Old 05-21-2015, 05:04 AM   #12
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Quote:
Originally Posted by Roelof View Post
Not with the timing. only the bore/stroke spec is different. Because the longer stroke (crankpin further away from center) it will create more torque. Also the longer stroke of the piston will create different pressure changes within the crankcase making a better pump to suck the mixtrure from the carburator and to pump the mixture into the transfer chanels.

I just wonder why super long strokes are better because RPM=horsepower.
A longstroke has the problem of a longer move of the piston so with mass and lubrication limits a longer stroke should run a lower top RPM. That is why F1 engines are running short strokes.

But maybe the 2-stroke concept has influence on the theory of needing more RPM for more HP.
The Novarossi 12 longstroke series was a change in the block and lowering the wrist pin. Crank pin stayed the same.
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Old 05-23-2015, 12:56 AM   #13
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Good explanation, thanks for that.


Quote:
Originally Posted by JTeamtech View Post
The problem is that we are approaching the flame/burn rate limit of nitro at the rpm we run.
Combined with primitive carburetion the combustion is somewhat short of efficient.

The three main beneficial effects that longer stroke have are- the combustion chamber is more compact giving a shorter flame path, less advance is required and the piston spends more time around tdc to allow for more complete combustion and less fuel exiting the exhaust.

Hence more complete combustion, better power spread and better efficiency/runtime.

The piston also spends less time at bdc so taller exhaust ports are needed for the same time/area.

When will we see bores offset from crank centerline?
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Old 05-25-2015, 02:21 AM   #14
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Quote:
Originally Posted by JTeamtech View Post
The problem is that we are approaching the flame/burn rate limit of nitro at the rpm we run.
Combined with primitive carburetion the combustion is somewhat short of efficient.

The three main beneficial effects that longer stroke have are- the combustion chamber is more compact giving a shorter flame path, less advance is required and the piston spends more time around tdc to allow for more complete combustion and less fuel exiting the exhaust.

Hence more complete combustion, better power spread and better efficiency/runtime.

The piston also spends less time at bdc so taller exhaust ports are needed for the same time/area.

When will we see bores offset from crank centerline?
You say:stay more time around tdc and less in btc.
How the hell it does it??
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Old 05-25-2015, 04:45 AM   #15
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The piston is more arround BDC than at TDC. It is due the angle change of the conrod.

https://forum.efilive.com/showthread...-Piston-motion
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