Hot Take: Why Bore and Stroke Don't Matter

Hot Take or Hot Tech? This will no doubt be a controversial topic, but before you get ready to throw the gloves off bear with me for a moment while I explain mathematically, with 100% certainty why bore vs. stoke won't make any difference for a given displacement.

Size matters, clearly, but for a given displacement is there any difference in how an engine performs if we swap the bore and the stroke to get to that displacement? The short answer is no, but for the long answer I have some math for you. The formula is [Average PSI x Piston Face area x ((Stroke length / 12)/2/2)] To illustrate this point we will start at the point in the cycle that the fuel is ignited, somewhere around 35 degrees Before Top Dead Center (BTDC). As the flame front expands burning freshly atomized fuel and air the pressure curve in the combustion chamber slowly rises, meanwhile the piston is still heading up towards Top Dead Center (TDC). This pressure curve reaches it's highest point around 25 degrees After Top Dead Center (ATDC). Peak pressure varies depending on flow characteristics but for a healthy street/strip engine this can be as high as about 1000 PSI (Pounds Per Square Inch). As the burning mixture forces the piston away from the combustion chamber the pressure steadily decreases to about 100 PSI by the time the exhaust valve opens then abruptly falling almost to Zero. This means we must average this PSI to about 500.

Now in our formula we take this average PSI and multiply it by the surface area of the piston. To get that we'll us the example of a 427BBC which has a 4.25" Bore. The formula to get the surface area would then be [4.25/2 = 2.125] Next [2.125²x3.14] = 14.18 Square Inches. So now we multiply our 500PSI by the 14.18 Square inches to get 7090 Pounds. This is the total average amount of pressure on our piston and thus on our rod journal, or crank pin. Then we need to multiply this by the amount of leverage our crankshaft has. If our crankshaft had a 24" stroke it would be easy because one foot-pound simply means we have "one pound of force-on a foot long leverage point". Obviously we would divide the 24" stroke by 2 to get a leverage point of 12" and be done... However, the stroke on a 427 or almost any engine for that matter isn't that long. So to get the correct number we must first divide our stroke by 2 to get the correct leverage point length between main journal and rod journal, for a 427BBC this would be [3.76/2=1.88] Then take that number and divide by 12" to get 0.157. Then remember how we had to average the pressure curve of our flame front earlier? We must also do that with our crankshaft because the leverage point begins at zero and ends at zero, technically it ends when the exhaust valve opens but let's not complicate this any further than we have to. So then we divide 0.157 by 2 to get an average effective leverage point of .078. So if we now plug all of these numbers into our formula it looks like this 500x14.18x0.078 = 553.02. Our little 427 makes 553ft-lb of torque, Not Bad! But here's where it get's interesting, what if we decide to make a 427 with a 4.125" Bore and a 4" Stroke instead? Of course for this test we'll leave everything else the same; same heads, same cam, same manifold, etc. Let's plug the numbers in and find out!

500PSI*13.36Sq.In.*0.083Leverage = 554.44. 554ft-lb of torque. This extra 1 ft-lb is just a rounding error from working with so many decimal points and on a dyno the same engine run back to back will have more variation than that. For all practical purposes, it is, THE SAME. It doesn't matter what bore and stroke combination you use this formula on, if the total displacement is the same they will produce the same amount of torque and since Horsepower is just torque*RPM/5252, the same HP. If we plug the numbers in for a 4"Bore*3.48" Stroke (350SBC) we get 455ft-lbs, if we use 4.25"Bore*3.0855"Stroke (Also 350CI) we get? You guessed it, 455ft-lbs. Now at this point I'm sure many of you are saying... "But since the stroke is different, peak torque and peak HP will be at different RPM!" Wrong again sir! And next I am obliged to show you the math on that as well.

To start this next segment, it's important to understand that the crank doesn't spin in a vacuum, it only does what it does because it is forced to by the combusting fuel forcing pressure upon it. Assuming that this fuel is perfectly atomized gasoline, the laminar burning velocity is 1.64 feet/second or 1 Inch every 19.6 Milliseconds, traveling from the ignition point outward in the shape of a sphere. So, if we take a simple steel box that is 3"wide*3"deep*4"Tall, then fill it full of perfectly atomized fuel and light it directly in the center, assuming we have a magical ignition source that doesn't impede the flame front, how long will it take for the flame front to reach every part of the surface inside of the box? About 352.8 Milliseconds. This would have to assume that there is no pressure wave which increases the speed of the ignition but again, let's not get too technical.

So now let's take this same box and lay it on it's side so that now it is 4"wide instead of 4"Tall, would that change how quickly the flame front reaches every surface of the box? No, of course not. This is all we are doing when we swap our bore and stroke but keep the same cubic inches, the flame fills the volume at the same rate either way. So if we have 100cc worth of combustion chamber at TDC and have 53.31 CI of displacement at BDC it will take the flame the same amount of time to move the crankshaft 180 degrees which translates to our torque peak being produced at the same RPM. I used 53.31 CI because that's the actual volume of each cylinder on a 427 by the way.

So now that we've determined that displacement is displacement and will have the same power at the same RPM regardless of stroke and bore combination let's talk about the ways that stroke length does affect an engine's characteristics. First and foremost is longevity, and even this has more to do with rod to stroke ratio than purely stroke itself but let's dive in shall we. Let's say we've got a factory 350SBC which will have a 3.48" stroke with a 5.7" connecting rod. This gives us a rod ratio of 1.64:1. The higher the ratio the straighter up and down the connecting rod remains throughout the stroke cycle. So if we wanted to retain this to prevent placing more side load friction and wear on our cylinder bore and piston but wanted a 3.75" stroke we would now need a connecting rod that is 6.15" long. The problem with that is that our deck height is only 9.025" which means we will have to use a shorter piston and possibly even place our wristpin up into the oil ring area. The shorter piston becomes less stable especially at high RPM's, it will tend to rock more at TDC which gives it more potential to touch the head if we have a tight quench area. Most people don't tend to report oil usage with a wristpin that is up in the oil ring but in theory it doesn't seem like the best design to prevent oil consumption. Meanwhile the longer stroke also increases piston velocity and since the piston has to stop at the top of the stroke and at the bottom this results in the piston being jerked back and forth much more violently. So yes, strokers do have drawbacks when it comes to high RPM's but not for the normal performance reason's that a lot of people first think about.

The second affect stroke has, again comes down to rod ratio and actually IS rooted in performance, but only when operating at speeds in excess of about 7000RPM. Remember that the higher the rod ratio the straighter up and down the rod is throughout the cycle. This means that the piston has more dwell time at the bottom and at the top the higher your rod ratio is. This can help fill the cylinder with more atomized fuel but again this benefit is normally only seen at extremely high RPM's.

So are there any pro's to a lower rod ratio? Yes, one. Because of the fact that a lower rod ratio forces a piston past TDC so fast they can be less prone to detonation. The highest moment of risk for detonation is at lower RPM up through your torque peak at high load. This is likely where strokers get the reputation of being a better "torque" motor. But again, that only applies if you increase your stroke without lengthening your rods. Is that worth it? That's for you to decide.