According to Smokey Yunick (who knew much more about engines than I ever will) you want the longest connecting rod that you can stuff inside the block. Going in that direction is what I do. Torque is more than just bore and stroke. It has a bunch to do with camshaft selection and the heads on the engine. All the parts need to be dialed in with each other. In my experience, many people think they can just turn a bunch of rpm to go faster. Working on the chassis and getting the suspension to hook up coming off the corner will help more than turning 8,000 rpm.
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Well have you chosen a piston yet? You can dominate races with long or short connecting rods, its just a matter of piecing the engine together correctly and tuning. I would look at the heads and how they flow and connecting rod length to boot. When building an engine, you do not need to turn extreme rpms to make power and torque (just look at Engine masters), you late model guys get up to 7,500 - 8,500 at the highest revolution???? You want to adhere to your operation range closely when selecting camshafts/valvetrains, heads, ignition curves etc etc. You want the powerband to span the rpms you use the most.
If the heads lack some flow I would use a longer rod since the TDC speed is slower than a short rod at TDC. This allows a lower flow rate head to not chase the piston as much on the induction stroke. If the piston is too fast ATDC on the powerstroke as well, the flame front will not produce the maximum amount of pressure/power possible simply because it may just be filling a slight void right next to the piston crown as it moves down to TDC rather than actually forcing. We have to remember to look at the valve event timing as well. Look at burn patterns on the crown when the engine is torn down.
Getting the correct flow velocities, sufficient cfm, unshroud areas, add minor modifications (legal ones of course) along with optimum cam timing is a must.
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+1 True,,,
There is only one glitch to long rod engines and what you gain in piston dwell time at and ATDC is also wasted time at and ABDC.
Your most effective cranking (combustion) pressure is from 15 to 55 degrees ATDC,,, beyond that, there is little effective combustion pressure left to turn the crank, besides, another cylinder is already going through the same cycle the previous one did.
Cylinder heads are where "ALL" your power is produced,,, pistons are nothing more than the combustion chamber floor that heads south, like an elevator going down.
Concentrating on VE (volumetric efficiency) is the key to power production,,,
This is the type of chamber to use,,, if rules allow it.
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very little combustion chamber to speak of and compression is determined by the dish in the pistons,,, as it should be. This fast burn head, owes it's existence to Robert Yates, who in turn revived the design that Michael May worked on, many years ago.
The design came into existence at the end of World WarII, where it was used on RR Merlin engines running a crank driven centrifugal Super Charger with water/methanol injection.
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They, "FTLracing", did a comparison of a 5" (1.492) ratio rod vs 7" (2.0) ratio rod as an example.
This is a little, of what they had to say.
copy/paste
Effects of a longer Rod
* Less rod angularity reduces wear.
* Lower piston velocity and acceleration reduces tensile loading of the rods.
* Less ignition timing is required which resist detonation.
* Compression can be increased slightly before detonation is a problem.
* Less intake runner volume is required and high rpm breathing is improved.
* Reduces scavenging at low rpm (weaker low RPM power).
* Longer TDC dwell time. (high RPM efficency).
Effects of a shorter Rod
* Increased rod angularity increases wear.
* Increased piston velocity and acceleration increases tensile loading of the rods.
* Increases scavenging at low rpm (increased low RPM power).
* Reduced TDC dwell time. (Reduced high RPM efficiency).
What they forgot to mention about the long rod is that the positive gain at TDC, is partially offset by wasted dwell time at BDC. The other flaw IMHO, is the close proximity of the wrist pin hole and the bottom oil ring rail, on the piston.
They used a 3.5" stroke for both rods, which is very close to a 350's 3.48" stroke.
As the graph shows, even 2" longer rod does not perform miracles.
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Basically my question is this, why are so many people fixated on building short stroke engines when they can build a larger engine with the right valvetrain and spin it just as high and enjoy all of the benefits of the larger engine?
A big bore small stroke motor will almost always make more power than a small bore short stroke, simply due to a better burn in the combustion space. We have spent years testing this on the dyno with race motors. For us the debate is over.
Second, piston speed and part quality determine how long a stroke you can have versus designed RPM peak you want to achieve. The longer the stroke the higher the piston speed the harder it is to change direction and the more stress you'll have on the part. Part being piston, rod, crank. Too much stress and it breaks.
Your 352 example is fairly close to the concept we use on our race motors. They seriously out perform a standard bore/stroke SBC race motor.
Just because a motor design has a long stroke doesn't mean it's going to make a ton of torque. Because it's a long stroke it can't rev high, so it has to be designed around making a lot of torque, re: cam, induction, heads.
In your example I assume the identical cylinder heads are used with both examples. If that were the case the shorter stroke engine would have higher RPM ability.
Piston speed determines port velocity, higher RPM equals higher piston speed and port velocity. At some point the port velocity reaches its peak, called choke.
Increasing stroke increases piston speed, with the longer stroke port choke happens at a lower RPM. For the long stroke engine to RPM as high as the short stroke engine the entire induction system must be sized to meet the demands of the higher piston velocity.
A long stroke - or short rod engine allows the piston to spend more time (in crankshaft degrees around bdc. A short stroke/long rod hoss has the piston slower around tdc. Therefore a long stroke engine like a 400 sbc or a short rod engine - 302w will feed better off a short intake duration cam and work badly on slow burning fuels like alky or nitro methane.
A long rod/short stroke engine generally likes big camshaft overlap, slow burning fuels and wide lsa's.
The torque vs hp argument is more about piston speeds and less about crankshaft leverage. Check Isky's cam grinds - their long stroke short track grinds have VERY narrow lsa's. they seem to be the only company addressing this factor.
As a rule, anytime you increase cubes you increase torque (not necessarily horsepower as much). Older, workhorse engines remembered for their torque were probably engineered as long stroke motors probably because they were cheaper. You could use a cheaper valve train and getting a good flame front across a smaller slug is easier.
Like everyone else I have an opinion. My experience is racing and racing engines will not run on the street.
First a piston can not travel faster that 2700 feet per minute.(3" stroke 11000 tops)
Having said that bore has no effect on RPM.
If you plan to run an engine at a very high RPM you must have a minimum of .001 per inch clearance on everything (2" crank .002 - 4" piston .004 etc)
You must then use 50 weight oil and contrary to what you would think keep oil pressure around 60 lbs at high RPM. More pressure only creates heat and will break your oil down. Cubic inch x rpm you want to run will give CFM. Stay under for carburetion. Less gas gives more power but also more heat.
Let's compare the 455 Olds (4.126"B x 4.25"S) to the 455 Buick (4.313"B x 3.90"S)
The main reason shorter strokes are better for higher winding is because force increases by the square of speed. Increasing stroke increases piston speeds. Doubling pistons speed SQUARES its force inertially. What that means practically is that a modest increase in stroke can easily double or triple the stress seen at the piston and pin. This limits peak RPMs. The Buick's pistons at 6000 RPMs are moving much slower than the Olds' pistons at 6000 RPMs. Just doing some quick math in my head, the extra .350" stroke on the Olds would mean forces at the piston would be almost twice what they are in the buick at the same RPM
The second reason (comparing the buick and olds) is breathing. The undersquare olds can't fit big valves, but its supplying the same cubes. Therefore the intake velocity peaks earlier, meaning it makes torque at a lower RPM.
The third reason (which has been beaten to death over the years) is the length of the crank throw. Adding stroke means you are lengthening the lever, so the same force on the rod creates more torque at the output. In the real world this has less of an effect than you might think. One of the magazines did a test (using these two engines actually) with identical head flow numbers, cam numbers, etc, and they turned up nearly identical on the dyno. Evidently the shorter lever on the buick crank was made up for by the extra cylinder pressures it was able to generate
I think the truth is somewhere in the middle. You can make a long stroke rev high, but if you want it to last more than one stomp of the pedal it takes more money. I think when people say "long stroke = low end torque" they mean that given its handicaps to revving high, you might not want to open your wallet that far.
Since the larger bore setup has 9.3% more piston area, equal pressure in the cylinder would produce that much more pressure on the connecting rod, raising the torque output even without the leverage of the crank throw.
Maximum piston speed is about 9.5% more with the longer stroke motor. A Buick 455 is about 6370 fpm @ 6000 rpm while an Olds 455 is about 6975 fpm.
If you spin a long stroke motor too fast it's like whiplash, something gives/breaks.
You get around this in race applications with billet cranks, billet or Ti rods, strong light pistons. But even those have limits. Sooner or later they will experience whiplash if you rev them too high by exceeding designed piston speed velocities. Curtis is correct, the calculated, and engineered, G loading maximums are what govern the RPM capability of a bottom end on a motor.
You are going to have much higher g loads moving a piston up and down a 4" stroke 6000 times per minute versus a 3" stroke moving up and down 6000 times per minute. That is the KEY POINT on the bottom end design and it's mechanical RPM capability. Doesn't have anything to do with power.
A larger bore engine can utilize a cylinder head with a wider combustion chamber,which allows bigger valves and ports to be used which leads the way to more RPM.The wider chambers are more prone to detonation but the higher engine speeds reduce detonation so it all falls together.Short stroke engines built from production based engines and being run with unlimited induction systems{like a tunnel ram} suffer from having connecting rods that are a little too long which can adversly effect cylinder filling in some case but can be benificial in an enging with a restricted induction system.Most modern high RPM drag engines built today use special low deck blocks which allow both a short stroke and a short rod.These combinations tend to get the benifit of steeper rod angles earleir in the combustion cycle with has typically been the domain of long stroke short rod engines,and the result is more power in the beggining of the powerrange with no real loss at top end when compared to a taller height long rod engine.
Long stroke engines beifit from a smaller bore because it helps control detonation when you are building an engine to make peak cylinder pressures at relitivy low speeds.The downfall to this is that the smaller bores limit breathing potential,but since the engine inst intended for high rpm this is a benifit because the builder will choose a cylinder head with smaller ports which will build more velocity at low speeds and make the engine tolerate a little later intake valve closing event if everything is matched right.This is how low RPM engines get a little extra power at the top fo their range.
In the end, given the same cubic inches and cylinder pressure, more stroke does not make more torque by its self. One of the reasons that the shorter stroke motor makes more power at high RPM is that there is less internal friction because of the lower piston speed. Also, the heads are allowed to flow better in the larger bore. I have seen tests where the longer stroke motor made a little more torque at very low RPM's. My only guess to why this happens is that the higher piston speed creates more velocity, and since the motor is not at a very high RPM, the reduction of friction from the short stroke has not had a chance to make a big effect.
So basically a shorter stroke engine is most beneficial when used in extreme applications like racing but doesn't really matter as much as overall engine size does when it comes to the street/strip duty that probably 95% of put our engines through.
ive experianced . Some suckers are saying 74 % for a solid Norton , but individual
