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Old 04-03-2011, 10:58 AM
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Big Bore Vs Small Bore

I Know This Has Been Beat To Death But I Got To Ask If You Was Building A 540 Bbc One Has A Big Bore Short Stroke An The Other Has A Small Bore Long Stroke What Is The Pros N Cons An Also If Yall Can Pick Out A Cam For Both Motors Would Like To See The Difference In The Timing Numbers Thanks Johnny

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Old 04-03-2011, 12:08 PM
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I would like to hear something about this too. Have often thought about it be never really checked into it.
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Old 04-03-2011, 01:01 PM
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Assuming the parts cost/availability is all the same, and they'll both fit in the same engine compartment, always choose a large bore/short stroke combination over the small bore/long stroke engine of the same displacement. There just isn't any downside, and lots of upside.
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Old 04-03-2011, 01:02 PM
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It can be bet to death and probably will again.

If you just take basic physics and apply that to each.

The force exerted on the piston is Pressure x Area. Increase either one of those and more force is created. Increased area (bigger bore) with the same pressure or increased pressure (boost or compression) with the same area. either one will create more force or power.

A longer stroke creates more leverage as the crank arm is longer. The rod connects farther from the centerline of the crank.

Which one of those options can you utilize more effectively? That will be where the debate begins. There is a limit to how big of a bore you can get. Cylinder wall thickness and cylinder spacing. There is a limit to the stroke due to the block. Can it clear the pan rails and the bottoms of the cylinders. And how tall the deck is limits this.

In my opinion. The bigger bore scenarion is best. You can have more force exerted on the piston and the force can be exerted longer to the crank due to rod lengths remaining loonger. More stroke is good but most of the force is exerted to the crank long before it rotates 90* and the point of maximum leverage is present.

Each have their effects on airflow as well but maybe someone else can type all of that up.
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Old 04-03-2011, 01:19 PM
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Bore vs Stroke 540

Quote:
Originally Posted by Infomaniac
It can be bet to death and probably will again.

If you just take basic physics and apply that to each.

The force exerted on the piston is Pressure x Area. Increase either one of those and more force is created. Increased area (bigger bore) with the same pressure or increased pressure (boost or compression) with the same area. either one will create more force or power.

A longer stroke creates more leverage as the crank arm is longer. The rod connects farther from the centerline of the crank.

Which one of those options can you utilize more effectively? That will be where the debate begins. There is a limit to how big of a bore you can get. Cylinder wall thickness and cylinder spacing. There is a limit to the stroke due to the block. Can it clear the pan rails and the bottoms of the cylinders. And how tall the deck is limits this.

In my opinion. The bigger bore scenarion is best. You can have more force exerted on the piston and the force can be exerted longer to the crank due to rod lengths remaining loonger. More stroke is good but most of the force is exerted to the crank long before it rotates 90* and the point of maximum leverage is present.

Each have their effects on airflow as well but maybe someone else can type all of that up.
(My 2 Cents)--Following the lead of the quote above, you will "feel" more low end torque with the small bore long stroke model, like the favorite "383" Small block, great for the street or 1/4 mile passes. However if you want to rev the engine for more than a few minutes, short stroke and small bore works best. Example: A cheap "short track" race engine is a 400 SB with a 327 large journal crank, same with my '67 427, It winds like a 302 trans am engine and does not come appart in a mile or two. I have ran this confiuration for three summers in my runner bottom v-drive, while most everyone else is rebuilding their "Stroker big blocks" not saying one is more robust however look at how they are used in racing.

Last edited by Hondo78; 04-03-2011 at 01:21 PM. Reason: Misspelling
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Old 04-03-2011, 01:59 PM
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A larger bore means less shrouding of the valves, so that gets my vote.
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Old 04-03-2011, 03:23 PM
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some good info --i was thinking the same about the valves now for the cam would you have two cams made one to fit each one or just one for each motor

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Old 04-03-2011, 11:42 PM
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The old wives tale of long stroke making more torque has been disproven a few million times. Yes, the long stroke has more "leverage" on the crank, but it also has less peak force from the piston.

If you take two identically spec'd mild engines (same heads, cam, etc) and play with bore and stroke only, they will make identical numbers on the dyno. The real difference comes if you want to up the ante a bit. The large bore will win on several counts; valve shrouding/breathing, rotating mass, lower piston speed, fewer harmonics... it just makes sense that you can't spin a long stroke, small bore engine too fast - the small valve area prevents the breathing that it would need, and the heavy rotating mass will start flying apart long before the short stroke would.

But its a pet peeve of mine when someone says, "get a 383, that long stroke makes a lot of torque." No, the additional cubes make the torque. It doesn't matter what bore/stroke combo you use to get it.

If you never plan on making big power, either combo works fine.

When I'm building an engine, I select a target HP, then I select a range of displacement that I want, (bigger for streetable torque, smaller if I need to save weight and the combo is suited to higher RPMs) then choose an engine with the bore/stroke I want that suits the purpose... but I almost always choose the large bore/short stroke in case I want to put it in a different vehicle and stab in a big cam and heads.

Small bore, long stroke engines are making a bit of a comeback for emissions reasons. Smaller bores and longer strokes (depending on rod/stroke ratio) tend to make faster pistons speeds with greater piston dwell. With that combination you can decrease the amount of ignition advance and lower NOx and sometimes the additional turbulence can increase MPG. Whether or not its a viable tradeoff for performance reasons depends on the build.
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Old 04-04-2011, 04:02 AM
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Thanks Curtis73. Well put. I'm not an engine builder but if given a choice I would always choose as much of an over-square engine as I could. Bigger bore and smaller stroke. The examples I have observed, the big bore engines have more snort. A good example is Oldsmobile 400. The earlier versions are big bore short stroke and then in 68 went with more stroke and less bore to make the same 400. The earlier 400's way out performed the 68 and later versions.
The 427 Ford is a legendary short stroke engine. Not quite as legendary but a real sleeper of an over square engine is the 455 Buick. The 1970 Stage 3 engine might have been the baddest factory muscle car on the street by some accounts.

Last edited by willowbilly3; 04-04-2011 at 04:14 AM.
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Old 04-04-2011, 02:07 PM
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Quote:
Originally Posted by crowerjunky
I Know This Has Been Beat To Death But I Got To Ask If You Was Building A 540 Bbc One Has A Big Bore Short Stroke An The Other Has A Small Bore Long Stroke What Is The Pros N Cons An Also If Yall Can Pick Out A Cam For Both Motors Would Like To See The Difference In The Timing Numbers Thanks Johnny
In terms of total power this doesn't have a big effect all other things being equal between two engines. The basic formulas only show that mean effective pressure and displacement to be the big drivers.

But when you get into specific engineering you'll find that where the torque and horsepower peaks occur will move around and power area under the curve will also. Generally the longer stroke engine will show a slightly more robust curve under the torque peak and a little less from there to the power peak compared to a bigger bore with shorter stroke for the same displacement. What this means is a harder pull out of the hole or out of corners for the longer stroke engine, but the shorter stroke, larger bore tends to pull harder on the top end. But like I said earlier, the difference at the point of favor for either is not so large that other factors like gearing can't overcome or hide.

The usual reason for using a shorter stroke is to reduce piston velocity. This plays into crank balance and piston, ring and cylinder wall wear issues more than power output. A slower moving piston is easier on crank balance and reduces component wear of the piston, rings and walls. the piston does not move anything like the crank and the big end of the rod. It moves from stopped at either Top or Bottom Dead Center, accelerating through a co-sine function to stop and reverse direction at the Dead Center extremes which puts unbalance-able loads into the crankshaft. So slowing the piston's velocity lowers the amount of these loads and the engine responds by running smoother. though this is something that can be hidden to a large extent in the selection and design of the engine mounting system.

Back in my Ford FE days we ran two different 396ish engines in 400 inch limited classes. One a 427 with a 352's 3.5 inch stroke, the other a thirty over 390 with the 3.78 inch stroke common to the 390, 406 and 427. They ran so close to the same, I know the driver could not tell which was in the car.

Bogie
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Old 04-04-2011, 02:21 PM
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so a short stroke puts less stress on the crank so the tq curve would be like a 106 compared to a 114 is that what ya saying
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