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Bore vs Stroke #5317372 01/09/20 05:14 AM
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ChemLabNL Offline OP
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Re: Bore vs Stroke [Re: ChemLabNL] #5317388 01/09/20 06:04 AM
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BMWTurboDzl Offline
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Ya. I watched that last night. Learned something new for sure.


“It took untold generations to get you where you are. A little gratitude might be in order. If you’re going to insist on bending the world to your way, you better have your reasons.”

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Re: Bore vs Stroke [Re: ChemLabNL] #5317425 01/09/20 07:07 AM
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Yeah, the best flowing cylinder head is ideally one that has valves a little larger than the bore

Re: Bore vs Stroke [Re: ChemLabNL] #5317514 01/09/20 08:48 AM
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I like his last comment where he mentioned that just because an engine has a large bore doesn't mean it can't be efficient, and just because an engine has a long stroke doesn't mean it can't make a lot of power.

For example, let's take an example I'm very familiar with. A simple 350ci Chevy small block with a 4.00" bore and 3.48" stroke. Let's take 2 of them, with the same heads, but one to be focused on a better horsepower curve and the other focused on a better torque curve. The prep of the cylinder heads, not just the flow, is important to efficiency and power. For maximum power, I'd want to recess the valve more in the seat, put it on a steeper 50-55* seat angle, and have more gradually steeper angles leading up to a rather large (>90% valve dia.) throat. This allows for less turbulent port flow at high lift and high rpm so you can maximize the peak power curve. On the other engine, I'd want to sink the valve as little as possible on a less steep 42-45* seat angle and less angles going to a narrower throat and preferably a more prominent short turn radius in the port itself. This forces the air to move around more at lower lift, creates more turbulence, and thus helps efficiency at low rpm.

The piston shape takes a factor as well. A piston with a flat top or D-shaped dish with a flat quench pad will create more turbulence and be more efficient than a piston with an open dish or dome top. The same applies to the chamber shape in that a shallower, more closed chamber will be more turbulent and more efficient than a big open chamber. Spark plug placement closer to center of the chamber means shorter distance of flame travel and thus more efficient. Pistons with a shorter top ring land (top ring is closer to the top of the piston) can be more efficient as it is exposed to more heat and can help with sealing at the expense of high rpm stability. You also wouldn't want a high top ring on a turbocharged/supercharged or nitrous engine.

I'm getting way off on a tangent. I like this topic a lot.


"He who is without oil, shall throw the first rod." - Compressions 9:1
Re: Bore vs Stroke [Re: RDY4WAR] #5317625 01/09/20 11:35 AM
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Originally Posted by RDY4WAR
I like his last comment where he mentioned that just because an engine has a large bore doesn't mean it can't be efficient, and just because an engine has a long stroke doesn't mean it can't make a lot of power.

For example, let's take an example I'm very familiar with. A simple 350ci Chevy small block with a 4.00" bore and 3.48" stroke. Let's take 2 of them, with the same heads, but one to be focused on a better horsepower curve and the other focused on a better torque curve. The prep of the cylinder heads, not just the flow, is important to efficiency and power. For maximum power, I'd want to recess the valve more in the seat, put it on a steeper 50-55* seat angle, and have more gradually steeper angles leading up to a rather large (>90% valve dia.) throat. This allows for less turbulent port flow at high lift and high rpm so you can maximize the peak power curve. On the other engine, I'd want to sink the valve as little as possible on a less steep 42-45* seat angle and less angles going to a narrower throat and preferably a more prominent short turn radius in the port itself. This forces the air to move around more at lower lift, creates more turbulence, and thus helps efficiency at low rpm.

The piston shape takes a factor as well. A piston with a flat top or D-shaped dish with a flat quench pad will create more turbulence and be more efficient than a piston with an open dish or dome top. The same applies to the chamber shape in that a shallower, more closed chamber will be more turbulent and more efficient than a big open chamber. Spark plug placement closer to center of the chamber means shorter distance of flame travel and thus more efficient. Pistons with a shorter top ring land (top ring is closer to the top of the piston) can be more efficient as it is exposed to more heat and can help with sealing at the expense of high rpm stability. You also wouldn't want a high top ring on a turbocharged/supercharged or nitrous engine.

I'm getting way off on a tangent. I like this topic a lot.



Great info - thanks for sharing. Thanks to the OP, as well!


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Re: Bore vs Stroke [Re: RDY4WAR] #5317668 01/09/20 12:30 PM
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OVERKILL Offline
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Originally Posted by RDY4WAR
I like his last comment where he mentioned that just because an engine has a large bore doesn't mean it can't be efficient, and just because an engine has a long stroke doesn't mean it can't make a lot of power.

For example, let's take an example I'm very familiar with. A simple 350ci Chevy small block with a 4.00" bore and 3.48" stroke. Let's take 2 of them, with the same heads, but one to be focused on a better horsepower curve and the other focused on a better torque curve. The prep of the cylinder heads, not just the flow, is important to efficiency and power. For maximum power, I'd want to recess the valve more in the seat, put it on a steeper 50-55* seat angle, and have more gradually steeper angles leading up to a rather large (>90% valve dia.) throat. This allows for less turbulent port flow at high lift and high rpm so you can maximize the peak power curve. On the other engine, I'd want to sink the valve as little as possible on a less steep 42-45* seat angle and less angles going to a narrower throat and preferably a more prominent short turn radius in the port itself. This forces the air to move around more at lower lift, creates more turbulence, and thus helps efficiency at low rpm.

The piston shape takes a factor as well. A piston with a flat top or D-shaped dish with a flat quench pad will create more turbulence and be more efficient than a piston with an open dish or dome top. The same applies to the chamber shape in that a shallower, more closed chamber will be more turbulent and more efficient than a big open chamber. Spark plug placement closer to center of the chamber means shorter distance of flame travel and thus more efficient. Pistons with a shorter top ring land (top ring is closer to the top of the piston) can be more efficient as it is exposed to more heat and can help with sealing at the expense of high rpm stability. You also wouldn't want a high top ring on a turbocharged/supercharged or nitrous engine.

I'm getting way off on a tangent. I like this topic a lot.


Great post! Really susses out a lot of the detail with respect to the topic.

Another issue is of course valve size and shrouding, which, within a given architecture (key) can put a significant cap on output potential. Comparing two engines of similar displacement and architectures:

1. The venerable SBC in 305 form
2. The SBF in 302 form

both engines are of very similar displacement, but the 302, having a 4" bore when compared to the 305, which had a 3.736" bore meant that the 302 benefited from a much more flexible selection of cylinder head designs and configurations, similar to the 327 and 350, both of which shared the same diameter bore. With the 305 there was a limit with respect to both valve diameter as well as the flow restriction as a result of valve shrouding from the smaller bore when working within the constraints afforded by it. This is a key reason as to why the 305 was never a popular performance build and was typically swapped out for a 350 so that heads with larger diameter valves could be employed.

Of course with multi-valve heads on an engine that doesn't use pushrods like a Modular, Coyote, Honda...etc, having to deal with side-by valves in a pairing isn't an issue, so bore size becomes far less of constraint with respect to potential power output.


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Re: Bore vs Stroke [Re: ChemLabNL] #5319489 01/11/20 09:50 AM
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Jason correctly shows how a long stroke engine has an efficiency advantage in thermal loss of combustion heat. Aircraft piston engines overcome the large bore flame front disadvantage by having 2 opposing spark plugs and 2 flame fronts. They also use air cooling and operate at a 450 deg f internal cylinder head and upper cylinder temperature. This transfers less combustion heat. Additionally, using fewer large displacement cylinders offers a surface area advantage for efficiency. The result is that aircraft engine designers in the 1930's knew all about what Jason presents here. So much so, that some engines even used opposing pistons to eliminate the cylinder head entirely. In much the same way as the current Achates opposed piston engine.

Only recently have we been able to meet or exceed the efficiency of yesterday's best engines. "Magic" numbers worth keeping in mind are about: 210 grams of fuel per KWH or 0.34 pounds of fuel per HP hour which also equates to 41% thermal efficiency. (the best today's Prius can do)


People who count on their fingers should maintain a discreet silence.
Re: Bore vs Stroke [Re: ChemLabNL] #5321261 01/13/20 06:11 AM
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Excellent vid; stuff I learned many years ago in school.

Also, not discussed but certainly a tie-in topic is that of thermodynamic laws, etc. Generally the efficiency of a typical ICE is directly tied to the CR; that is the key factor in efficiency, all other things being held constant.

Modern engineering models and tools make for wonderful engines. Things like multi-valves, VVT, fuel injection, etc all make decently efficient engines that run reasonably clean. Especially contrasted to those of yester-year.

We still waste a lot of energy though (I'm talking about thermal waste, not referring to large consumption which will trigger a verboten discussion on "green" vs fossil ... taboo, you know). Even some of the best traditional designs are only 30%+ efficient; most energy escapes in lost heat and a bit in sound energy.


The act of preventative maintenance, in and of itself, is FAR MORE important than brand/grade/base choices among lubes and filters.
- under maintaining something is akin to abuse/neglect; that can kill equipment by shortening the lifespan
- over maintaining something has never been proven to be anything but a waste of time and money
Re: Bore vs Stroke [Re: dnewton3] #5321325 01/13/20 08:01 AM
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Originally Posted by dnewton3
Excellent vid; stuff I learned many years ago in school.

Also, not discussed but certainly a tie-in topic is that of thermodynamic laws, etc. Generally the efficiency of a typical ICE is directly tied to the CR; that is the key factor in efficiency, all other things being held constant.

Modern engineering models and tools make for wonderful engines. Things like multi-valves, VVT, fuel injection, etc all make decently efficient engines that run reasonably clean. Especially contrasted to those of yester-year.

We still waste a lot of energy though (I'm talking about thermal waste, not referring to large consumption which will trigger a verboten discussion on "green" vs fossil ... taboo, you know). Even some of the best traditional designs are only 30%+ efficient; most energy escapes in lost heat and a bit in sound energy.


I deal with this a bit with people tuning their older cars, and even some newer ones. They want to keep their spark advance pulled back on a set mechanical curve with the distributor. As a result, they only have 20-30 degrees of spark advance at part throttle and cruise. Then they wonder why the engine is unresponsive and gets bad gas mileage. Normally their exhaust gas temperatures are exceeding 1,000*F just cruising which is near double what you'd like to see. I mentioned increasing part throttle timing to 40+ degrees and they look at me like I'm stupid. I've done it on several where I add a vacuum advance canister, sourcing from manifold vacuum, and get that timing up to 40+ degrees at part throttle and cruise. The EGTs come down, the car is actually responsive, cruises much smoother, and picks up as much as 40% better fuel economy.

The thing I've noticed about BSFC (brake specific fuel consumption) is that the best results I've ever achieved with that was with no emissions equipment installed at all. No cats, no EGR, nothing. Just free-flowing and high velocity exhaust design with smooth merges and transitions. The engine has always picked up power and become more efficient. It makes me wonder if we couldn't have far more efficient engines if they weren't choked down so heavily having to breathe in their own farts.


"He who is without oil, shall throw the first rod." - Compressions 9:1
Re: Bore vs Stroke [Re: RDY4WAR] #5321463 01/13/20 10:57 AM
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Originally Posted by RDY4WAR
... The thing I've noticed about BSFC (brake specific fuel consumption) is that the best results I've ever achieved with that was with no emissions equipment installed at all. ...
Maybe at high loads, but can you beat the part-throttle BSFC some modern production SI engines manage using cooled EGR?

Last edited by dnewton3; 01/14/20 07:36 AM. Reason: fix quotation reference

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Re: Bore vs Stroke [Re: ChemLabNL] #5321883 01/13/20 06:31 PM
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It is great theory but we get what the manufacture thinks in the best compromise for the results they want to achieve. with race engines again the best has been worked out by the engineers and pro racer teams.


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Re: Bore vs Stroke [Re: ChemLabNL] #5325327 01/17/20 01:08 PM
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ChemLabNL Offline OP
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"Aircraft piston engines overcome the large bore flame front disadvantage by having 2 opposing spark plugs and 2 flame fronts"

Cool!

Didn't BMW or Mercedes try a dual spark plug?

Re: Bore vs Stroke [Re: RDY4WAR] #5325461 01/17/20 03:29 PM
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Originally Posted by RDY4WAR

It makes me wonder if we couldn't have far more efficient engines if they weren't choked down so heavily having to breathe in their own farts.


The Ford 5.4L 3 valve V8 is a notorious fuel hog in practice. But of particular interest is the fact that it's a long stroke engine with superb internal dimensions for both thermal efficiency and lean-burn combustion. Soooo, some enterprising team decided to re-configure the ECU to deliver a larger throttle opening at partial load. Along with a very lean air/fuel mixture (as lean as 20 to 1) and appropriate spark advance (more advance than OEM programming) .

The result was a stunning improvement in BSFC.



Quoting myself from the past:


Lean operation results in lower combustion temperatures, and longer combustion burn times. At modest RPM's lean burn results in lower cylinder head temperatures, as even though the combustion is slower, it completes well before bottom dead center.

Also, lean operation is, absolutely, without a doubt, a more efficient way to make power. There is no "false economy" with it. BSFC improves as mixture is leaned beyond stoic, and in a properly designed combustion chamber, will peak at between 17/1 and 20/1 air/fuel (depending on the engine of course) .

As noted, lean operation results in a power loss. This requires more throttle opening to regain the lost power. This has a positive result, in that pumping losses are reduced, further improving BSFC numbers.

Today, there is a way to achieve a lean burn engine, with low NOx. The engine cycles between stoic and as lean as 20/1. With short periods at stoic. The exhaust contains a Lean-NOx trap that converts the stored NOx during the short period of "rich" operation.

In a Ford 5.4L BSFC numbers are as follows:

1) Stoic 272 g/kwh
2) Lean burn 20/1 near unstable, 222 g/kwh
3) cyclic mode 255


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Re: Bore vs Stroke [Re: ChemLabNL] #5325626 01/17/20 07:23 PM
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Originally Posted by ChemLabNL
"Aircraft piston engines overcome the large bore flame front disadvantage by having 2 opposing spark plugs and 2 flame fronts"

Cool!

Didn't BMW or Mercedes try a dual spark plug?


Dual plugs exists on the current HEMI, the Mazda 4-popper that was fitted to the Ranger/B2000 and many other engines over the years. My grandfather's best friend had a massive 4 cylinder with dual ignition in his 1920's vintage antique boat. Had two distributors, one slightly out of phase with the other.


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Re: Bore vs Stroke [Re: ChemLabNL] #5330022 01/23/20 05:23 AM
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The 6.2L Ford SOHC truck engines have 2 plugs per cylinder also. A fairly efficient engine given the displacement and loads it pulls. People often complain about the fuel economy of large work trucks, but I think they are missing the overall viewpoint.


The act of preventative maintenance, in and of itself, is FAR MORE important than brand/grade/base choices among lubes and filters.
- under maintaining something is akin to abuse/neglect; that can kill equipment by shortening the lifespan
- over maintaining something has never been proven to be anything but a waste of time and money
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