Rod/Stroke Ratio - B16A Vs. B16B

 
Old 10-12-2018, 11:49 AM
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Default Rod/Stroke Ratio - B16A Vs. B16B

Just for the sake of argument, I was wondering what the thoughts were on this. Almost no one builds B16B's anyway, and the difference here would likely be fairly minute, but I'm still curious as far as high, smooth revving engiens go:

B16A has a rod/stroke ratio of 1.73 - The rods are shorter than in the B16B
B16B has a rod/stroke ratio of 1.84 - The B16B is quite literally a de-stroked B18C5 engine, with the same taller deck height of the B18 in comparison to the B16a engines, but with the same height 77.4mm crankshaft as in the B16's.

Just looking for a small discussion here, or what people think - Would the longer rod ratio on the B16B make it more suited for higher revving? On one case I would think so, as it puts the rods at less of an angle at the bottom and center of the cylinder, however the longer rods would also inevitably weigh a little bit more. I also know that the B16B crankshaft actually weighs more than the B16A due to having added counterweights, but for sake of argument let's assume that both engines have the same lightweight crank, completely balanced.

/pointless rambling. I almost built a B16B awhile back but we ended up going a different direction. Something about them still interests me, but more displacement is always hard to argue against with the B18 or B20 builds.
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Old 10-12-2018, 03:19 PM
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Default Re: Rod/Stroke Ratio - B16A Vs. B16B

For majority of setups it's not going to make a difference. The difference between the two would likely only be a few hundred RPM in what was already extremely high anyway. I'm talking like 10,500 vs 10,700rpm if the bottom end was the only factor. There are a lot more factors than just rod/stroke and crank weight though. Piston weight and design, pin weight, friction losses, etc etc. For the sake of your discussion I would say the longer rod and higher rod/stroke ratio of the B16B would be more ideal for extreme RPM. Having a longer/slightly heavier rod isn't necessarily a bad thing. Yes it's a little heavier but that means there's more material. More material means more resistant to stress and fatigue from repeated high RPM operation. Rod stress increases exponentially with RPM so the higher you go the more they're going to want to come apart. Before you had an issue with the rod/stroke ratio, etc of either engine I would bet that there would be many other limiting factors like header design, intake design, head flow and so on.

1.73 rod/stroke is already very high and that's the lower of the two engines you have listed. For perspective: an F20B has a rod/stroke ratio of 1.65:1 and will happily do 9500+ with just valvetrain. I can't see many situations where it would be more efficient to go higher than that when you could simply make the engine larger. As you said more displacement is hard to argue against.
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Old 10-15-2018, 04:30 AM
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Default Re: Rod/Stroke Ratio - B16A Vs. B16B

Good input Aradin. Rod bolts also become a crucial factor when going north of 10K, every factor can make a difference that becomes important at those levels of revolution - any inch you can gain in terms of making the car rev smoother/more reliably is worth it.

Like we're both saying though, I'd take a B20 revving to 9-9.5K over a B16 revving to 10-10.5K any day. There's definitely something atmospherically and sensationally pleasing about a tiny engine revving to the moon though.
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