Bore x Stroke vs Harshness on oil

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Seems better engine building techniques, lighter parts and better metallurgy are making long strokers a non-issue.

Honda K24 engine has a 87x99mm stroke, an 'abysmal' 1.5:1 rod ratio.
The Mazda 2.5L MZR and Nissan QR25 have a 89x100mm bore/stroke, although the Nissan engine has been problematic.
Toyota's new AR engine has 2 varients:
2.5L I4 @ 90x98mm
2.7L I4 @ 90x105mm
It seems some manufacturers are going for long stroke undersquare configs these days. This could have more to do with emissions and burn characteristics, or compensating for torque lost by new emission control techniques enabled by better machining and metallurgy.

But hey, one of the most undersquare engines ever made was a Willys Jeep four cylinder and those took us through wars and back, albeit very slowly
 
Originally Posted By: ItsuMitsubishi
Seems better engine building techniques, lighter parts and better metallurgy are making long strokers a non-issue.


I don't know that it ever WAS an issue. Short strokes became the popular way to make big power in the 50s and 60s, but prior to that long strokes ruled the roost.

As for reliability, I can think of at least two vintage engines with "abysmal" rod ratios that are widely considered two of the most reliable powerplants ever built: the Chrysler 225 slant-6 and the Chevy 350. The slant-6 also had a very long stroke (4.125"!!) and was undersquare (3.4" bore) on top of that. The 350 just had a "bad" rod ratio due to the low block deck needing short rods when the stroke was increased form the 265/383 crank of 3" stroke, but was oversquare with a still reasonably short stroke (3.48").
 
Originally Posted By: mechtech2
Short rods are not all that bad.
The pistons 'dwell' less at /near TDC, and less cam is needed .


The downsides being that a little more power is lost to higher friction due to higher side-thrust against the cylinder walls, the cylinders wear egg-shaped faster for the same reason, and the non-sinusoid motion of the piston puts higher stresses on the connecting rod bolts. But even at traditionally "bad" rod ratios, those things seem to be a non-issue most of the time.
 
Originally Posted By: 440Magnum
Originally Posted By: ItsuMitsubishi
Seems better engine building techniques, lighter parts and better metallurgy are making long strokers a non-issue.


I don't know that it ever WAS an issue. Short strokes became the popular way to make big power in the 50s and 60s, but prior to that long strokes ruled the roost.

As for reliability, I can think of at least two vintage engines with "abysmal" rod ratios that are widely considered two of the most reliable powerplants ever built: the Chrysler 225 slant-6 and the Chevy 350. The slant-6 also had a very long stroke (4.125"!!) and was undersquare (3.4" bore) on top of that. The 350 just had a "bad" rod ratio due to the low block deck needing short rods when the stroke was increased form the 265/383 crank of 3" stroke, but was oversquare with a still reasonably short stroke (3.48").





Hey isn't the 350 about a 1.62 ratio? I'm going off memory, please correct me if I'm wrong. 1.62 ain't bad at all! I'm gonna have to look up the 225 slant 6. 1.5 and 1.48 are really bad, but who could tell you a Honda K24 had a 1.5 ratio from just driving it? We've come a long way!

Originally Posted By: mechtech2
Short rods are not all that bad.
The pistons 'dwell' less at /near TDC, and less cam is needed .


Yessir, this has been Honda's 'secret' to intake port excitement for a while. Most Honda engines have low rod ratios this is why the old ones piston slap so badly. But the method is to exploit the sharp acceleration away from TDC, which excites the intake port more and mitigates the evacuation of intake charge and raw fuel allowing large overlaps due to the piston pulling the intake charge down, away from the exhaust valve sooner, allowing less time for unproductive evacuation to occur. The benefits of overlap is still viable at high RPMs. Combined with high static compression ratios for the top of the RPM band, and retarded cam timing lowering the low RPM dynamic compression ratio for driveability, and long duration on the high cam, the Honda 'secret' to making high RPM, high HP from small displacement is fully disclosed. I still prefer longer rod engines, they're just straight up smoother at high RPM and maintain a high peak cylinder pressure for longer, thus using the power stroke more efficiently. If inertial and dimensional limitations weren't an issue, I'd love to see an undersquare, long stroke motor with a high rod ratio greater than 1.9. It would be an efficiency queen!
 
Originally Posted By: ItsuMitsubishi
Originally Posted By: 440Magnum
Originally Posted By: ItsuMitsubishi
Seems better engine building techniques, lighter parts and better metallurgy are making long strokers a non-issue.


I don't know that it ever WAS an issue. Short strokes became the popular way to make big power in the 50s and 60s, but prior to that long strokes ruled the roost.

As for reliability, I can think of at least two vintage engines with "abysmal" rod ratios that are widely considered two of the most reliable powerplants ever built: the Chrysler 225 slant-6 and the Chevy 350. The slant-6 also had a very long stroke (4.125"!!) and was undersquare (3.4" bore) on top of that. The 350 just had a "bad" rod ratio due to the low block deck needing short rods when the stroke was increased form the 265/383 crank of 3" stroke, but was oversquare with a still reasonably short stroke (3.48").





Hey isn't the 350 about a 1.62 ratio? I'm going off memory, please correct me if I'm wrong. 1.62 ain't bad at all! I'm gonna have to look up the 225 slant 6. 1.5 and 1.48 are really bad, but who could tell you a Honda K24 had a 1.5 ratio from just driving it? We've come a long way!

Originally Posted By: mechtech2
Short rods are not all that bad.
The pistons 'dwell' less at /near TDC, and less cam is needed .


Yessir, this has been Honda's 'secret' to intake port excitement for a while. Most Honda engines have low rod ratios this is why the old ones piston slap so badly. But the method is to exploit the sharp acceleration away from TDC, which excites the intake port more and mitigates the evacuation of intake charge and raw fuel allowing large overlaps due to the piston pulling the intake charge down, away from the exhaust valve sooner, allowing less time for unproductive evacuation to occur. The benefits of overlap is still viable at high RPMs. Combined with high static compression ratios for the top of the RPM band, and retarded cam timing lowering the low RPM dynamic compression ratio for driveability, and long duration on the high cam, the Honda 'secret' to making high RPM, high HP from small displacement is fully disclosed. I still prefer longer rod engines, they're just straight up smoother at high RPM and maintain a high peak cylinder pressure for longer, thus using the power stroke more efficiently. If inertial and dimensional limitations weren't an issue, I'd love to see an undersquare, long stroke motor with a high rod ratio greater than 1.9. It would be an efficiency queen!


I do believe the "endurance" class engines aim for an RS of 1.9, if not higher.

Interesting dissertation on Honda's use of low rod ratios to help define the characteristics of their engine's operating dynamics. I'll be filing that one in the back of my mind for sure!
thumbsup2.gif
Well written.
 
Originally Posted By: 440Magnum
Chevy 302 (the original LT-1) .


Just thought I'd point out that the original LT-1 was developed for use in the 70' vette/z28 and was a 350.

The 302 was just known as the DZ302.
 
Thanks for that clarification.

But the DZ had zero low end, and was really 'cammy' stock.

Lots of fun, but not really a great daily engine.

I've had 3 LT-1's, and they were fantastic. My 370 horse 350 in a 70 Vette was smooth and tractable in any gear, and could still wail on the top!
 
Last edited:
Originally Posted By: ItsuMitsubishi
Originally Posted By: 440Magnum
Originally Posted By: ItsuMitsubishi
Seems better engine building techniques, lighter parts and better metallurgy are making long strokers a non-issue.


I don't know that it ever WAS an issue. Short strokes became the popular way to make big power in the 50s and 60s, but prior to that long strokes ruled the roost.

As for reliability, I can think of at least two vintage engines with "abysmal" rod ratios that are widely considered two of the most reliable powerplants ever built: the Chrysler 225 slant-6 and the Chevy 350. The slant-6 also had a very long stroke (4.125"!!) and was undersquare (3.4" bore) on top of that. The 350 just had a "bad" rod ratio due to the low block deck needing short rods when the stroke was increased form the 265/383 crank of 3" stroke, but was oversquare with a still reasonably short stroke (3.48").





Hey isn't the 350 about a 1.62 ratio? I'm going off memory, please correct me if I'm wrong. 1.62 ain't bad at all! I'm gonna have to look up the 225 slant 6. 1.5 and 1.48 are really bad, but who could tell you a Honda K24 had a 1.5 ratio from just driving it? We've come a long way!

Originally Posted By: mechtech2
Short rods are not all that bad.
The pistons 'dwell' less at /near TDC, and less cam is needed .


Yessir, this has been Honda's 'secret' to intake port excitement for a while. Most Honda engines have low rod ratios this is why the old ones piston slap so badly. But the method is to exploit the sharp acceleration away from TDC, which excites the intake port more and mitigates the evacuation of intake charge and raw fuel allowing large overlaps due to the piston pulling the intake charge down, away from the exhaust valve sooner, allowing less time for unproductive evacuation to occur. The benefits of overlap is still viable at high RPMs. Combined with high static compression ratios for the top of the RPM band, and retarded cam timing lowering the low RPM dynamic compression ratio for driveability, and long duration on the high cam, the Honda 'secret' to making high RPM, high HP from small displacement is fully disclosed. I still prefer longer rod engines, they're just straight up smoother at high RPM and maintain a high peak cylinder pressure for longer, thus using the power stroke more efficiently. If inertial and dimensional limitations weren't an issue, I'd love to see an undersquare, long stroke motor with a high rod ratio greater than 1.9. It would be an efficiency queen!
Geometrically, Long rods "always" provide for a less angular(severe) vector between the bore CL and rod. The increased length can become a reliability/failure issue at high rpm due to mass driven high lateral forces and moments, and, tensile elongation at TDC. The TDC/BTC Dwell is more affected by crank throw. Overlap Reversion/Evacuation more by increased Bore(with cam variable held constant). I think Hondas power "secret" has more to do with VTEC cam profile, tumble intake charge and the High static compression. Its not much of a secret. Minor rod ratio compromise is not much an issue in gasoline engine "street" operating ranges of 2000-4000 RPM given proper piston skirt design.
 
Last edited:
Originally Posted By: JT1
Did those old solid lifter engines really need the valve lash adjusted often?


They did to run sharp. But I've driven little bitty 4 bangers with solid lifters that would go many thousands between adjustments.

I guess it's related to the duty cycle.
 
Originally Posted By: JT1
Did those old solid lifter engines really need the valve lash adjusted often?


Define "need." The slant-6, for example, could run 200k= miles without a valve adjustment... easily. But it would run *better* and quieter and more smoothly with periodic adjustments for sure. The really high-performance v8s with solid lifters (think Mopar 273 Commando, 426 Hemi, etc.) were more sensitive since their performance depended on accurate valve timing much more than a low-RPM torque engine.

Air-cooled engines like VWs were another matter. While the Mopars I mentioned would tend to get loose and clattery valve adjustment over time due to wear on parts, VWs could often get *tight* valves due to the valve stems stretching as they were heat cycled. That would lead to a burned valve without regular adjustments.
 
The 134 cubic inch 4 cylinder engine in my 51 Willys 4x4 truck has a 3-1/8 bore and a 4-3/8 stroke.The connecting rods are 9.18 inches center to center.With 5.38 gears it's always spinning fast.Think this engine is hard on oil?
 
Originally Posted By: Truckedup
The 134 cubic inch 4 cylinder engine in my 51 Willys 4x4 truck has a 3-1/8 bore and a 4-3/8 stroke.The connecting rods are 9.18 inches center to center.With 5.38 gears it's always spinning fast.Think this engine is hard on oil?


Not for any of those reasons.... but if its like my '49 Plymouth and doesn't have a 100% full-flow oil filter, it could be hard on oil for that reason. Or if it is prone to fuel dilution, etc. There are many other things come into play.
 
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