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lendaddy's Avatar
 
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The stroke vs bore thread

What is considered the better route to displacement increases? I would assume you would want a short stroke motor for quicker revs and lower piston speeds but I could be wrong Also, would not a short stroke motor have inherently better balancing as well? I realize that torque is improved with long stroke motors but how much?

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Old 12-12-2003, 07:48 AM
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It is a misconception that long stroke motors produce more torque than short stroke motors. This misconception is based on the fact that long stroke motors are often tuned to produce increased torque as a route to horsepower because the alternative (higher RPMs) isn't possible due to unacceptably high piston speeds. In order to realize the potential for the theoretical increase of power possible with a short stroke engine, the potential for higher rev capacity must be realized. This means that the valve train must be capable of such RPMs, and that the induction track will have the capacity to move air into the engine while not forcing the air to move at unreasonable speeds. So, for a Porsche engine, the long and the sort of it is (ha ha), either approach is fine. Porsche has never really made a "long stroke" engine anyway, just short and shorter. It's relative. Other factors seem to be of more consequence (availability of pistons and cylinders, camshafts, etc). Me? I'm building a short stroke because I like the IDEA of it, and it was a good way to go given the parts that I started with.
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Old 12-12-2003, 08:27 AM
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A longer stroke motor will produce more torque absolutely with a few qualifications. The indicated mean effective pressure in the cylinder over the time span of one half cycle must average to the same value as the smaller stroke motor. If this is true the engine must produce more torque. The lever arm of the crankshaft is longer.

Just changing the stroke of the engine will not ensure this result. There are so many open variables in an engine that a major change like stroke must be made with many other changes. Possible intake/exhaust runner length, camshaft, and valve changes just to name a few. This would take a pretty hefty R&D budget.

Stroke vs. bore changes seem to be like turbo vs supercharge, a personal ideology. Don't even get into the long vs short rod debate.

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Old 12-12-2003, 10:34 AM
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Quote:
If this is true the engine must produce more torque. The lever arm of the crankshaft is longer.
True, by a whopping 4.4 mm in the case of the 2.4/2.7's 70.4 mm crank compared to the 2.0/2.2's 66 mm crank. But be sure to add in (subtract?) the energy involved to accellerate the reciprocating masses harder in each stroke. You also have increased friction due to larger rod angles resulting from the shorter rod length, basically a short stroke/long rod engine is better at converting the pressure on the piston into lateral motion, while a long stroke/short rod engine wastes more energy trying to push the rod sideways.

The big benefit of a "long stroke/small bore" engine is that it is easier to optimise the combustion chamber shape while maintaining a high CR. One of the biggest challenges for F1 designers working with VERY short stroke (40mm's) - BIG bore engines (94-100 mm's) is that the combustion chamber gets wider and wider and flatter and flatter because of the big bore and the big valves needed to pull all of the air needed at 19000 RPM. It gets real tough to have clearence for valve overlap at TDC without pockets in piston, which reduces the CR again. The 2.0S's had a similar problem.
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Old 12-12-2003, 11:38 AM
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Flame travel speed is also a problem especially with large bores at high rpm's(combustion chamber shape plays into this). But spark ignited engines have an upper limit usually stated at 6". No consumer car engine is even close, but it is good to note that there is a limit.

I also have several books that state that frictional losses associated with side load changes is negligible. Larger bores also have have more surface area for frictional loss. These are all part of the qualifications statement from the previous reply.

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Old 12-12-2003, 11:54 AM
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Matt, you are overlooking a key point.

It is true the long stoke has more leverage, but the short stroke has more piston area. Hence the short stroke piston is exerting more force on it's shorter lever arm (assuming displacement is kept constant).

The change is directly proportional, so all else being equal both engines will produce the same torque.
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Old 12-12-2003, 12:11 PM
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the question i have for this thread is how much is it to bore out and up the displacement on my 3.0--->3.2?
Old 12-12-2003, 12:26 PM
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Check with Bruce M. I remember him having a stroker turbo and being very pleased with the way it made some big HP/torque numbers.
Old 12-12-2003, 01:08 PM
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autoxracer31, generally boring isn't done with these motors, usually the method of increasing bore is to buy new P&C which I believe runs around $3000, maybe more maybe less, I think the range could be $2k-$4.5K with 3K-3.5K being the norm.
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Old 12-12-2003, 02:58 PM
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The Rod ratio also needs to be factored in. Steve
Old 12-12-2003, 03:20 PM
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Lightbulb SAE Paper on Short Stroke vs. Long Stoke Benefits

I came across this on another BBS and figured I'd share it here since the question of stroke versus HP and torque comes around every few months. This would seem to be pretty close to the final word. Unfortuantely, like many things, it isn't so much the final word as a whole lot of words.

The following excerpts are from SAE TECHNICAL PAPER 980126- The Influence of Stroke-to-Bore Ratio and Combustion Chamber Design on Formula One Engines Performance, available from the SAE.

"...within real life design constraints. The effects of the
stroke-to-bore ratio on both the volumetric efficiency and
the thermal conversion efficiency have been investigated.
Flame front area maps, wall areas wetted by burned
gases, mean flow field patterns and main turbulent
parameters have been compared for two different S/B
ratios. Since higher intake and exhaust valve areas per
unit displaced volume result in a higher volume of piston
bowls, a lower S/B ratio leads to a lower compression
ratio, which strongly limits the indicated mean effective
pressure. Therefore, in the second part of the paper, an
analysis of the influence of the piston shape on combustion
process has been performed in order to optimize the
imep of the lower S/B ratio engine.

The overall technical objective of racing engine design is
the achievement of the highest possible power levels. A
simple analysis can reveal the effects of the main engine
parameters on the brake power Pb delivered by a fourstroke
cycle reciprocating internal combustion engine

For a given total displaced volume, high engine speed, high compression
ratio, high volumetric efficiency and low stroke-to-bore
ratio are required in order to obtain high levels of power.
High rated engine speeds dictate a diminishing of the
stroke due to mechanical stress and friction considerations.
High volumetric efficiencies call for higher valve
areas per unit displaced volume, resulting in a bore that
increases with the rated engine speed.
On the other hand, the diminishing of the S/B ratio determines
a lower thermal conversion efficiency since it leads
to a combustion chamber design far from the optimum
one required for an efficient combustion process.

In fact,as the S/B decreases, combustion duration increases, as
well as cycle-by-cycle variability and chances of misfire
or partial burning.
Since performance improvements call for low stroke-to-bore
ratios, a detailed study of in-cylinder processes is
required in order to achieve a compromise between thermal
conversion efficiency and volumetric efficiency.

Since higher intake and exhaust valve areas per unit displaced
volume resulted in higher volume of the piston
bowls, the diminishing of the S/B ratio led to a lower compression
ratio. Although this is often sufficient for avoiding
knock, it strongly limits the imep. Therefore, in the second
part of the paper, an analysis of the piston shape influence
on combustion process has been performed in
order to optimize the compression ratio of the lower S/B
ratio engine.

The objective of a combustion chamber is to produce
fast, stable and repeatable combustion processes with a
high thermal conversion efficiency. The objective of racing
engines is to provide a high specific power by achieving
a high thermal conversion and high volumetric
efficiencies.
For a fixed total displaced volume, high power density
requires high rated engine speeds. Hence, to obtain high
volumetric efficiencies at high engine speeds and to limit
friction and component mechanical stress, low S/B ratios
must be adopted.

Due to the higher maximum speed and volumetric
efficiency, for constant fuel conversion efficiency, the
engine with higher bore to stroke ratio should have about
a 5% gain on brake specific power. The poorer engine
fuel conversion efficiency is expected to reduce this gain.

A CFD analysis was carried out in order to gain more
insight regarding the effects of S/B ratio changes on racing
engine performance. Two different S/B ratio engines
were compared: since the reduction of S/B ratio was performed
within actual design constraints, the low S/B ratio
engine presented a 3.2% lower compression ratio. The
analysis of the influence of S/B ratio on volumetric efficiency
and thermal conversion efficiency led to the following
conclusions:

Decreasing the S/B ratio results in the improvement
(up to 4%) of the volumetric efficiency in the range of
medium to high engine speed, while it determines a
reduction in the volumetric efficiency at low speeds.

The thermal conversion efficiency falls while S/B has
decreased as a consequence of the much lower
burning rate due to both the lower compression ratio
and the lower fuel mass effectiveness. The reduction
of imep is balanced by the increase in the maximum
rated power rotational speed. However, as a result,
the two different S/B ratio engines present the same
indicated power.

In order to achieve the highest possible gain from a
S/B reduction, an optimization of the combustion
chamber, with particular emphasis on compression
ratio, is required.

Calculations clearly show the important role played
by compression ratio on engine performance. An
increase of the compression ratio of 3.2% determines
a power gain of about 2.9%.

Finally, computations reveal that a proper optimization
of the piston shape could produce significant
improvement in the combustion process quality."
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Old 01-30-2004, 02:17 PM
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I'm still waiting for the technology of oddly shaped I/E valves that together make a circle and each have a spark plug(multiple) imbedded in the center. No head space wasted and multiple ignition points to configure flame propigation.

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Old 01-30-2004, 04:44 PM
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