Bore/Stroke Ratios

Posted by 58L8134 On 2014/2/24 13:26:33

Hi

Rereading a January 2011 Hemmings Classic Car article detailing the 1933-54 Pontiac Straight Eights engines, as well as recalling anedotes of an old-time mechanic familiar with those cars, set this line of thought in motion. The article details multiple engineering features designed into those Pontiac eights that contributed to their longevity. Although Pontiac and Packard were in different price segments for the most part, their long histories with straight eights makes for interesting comparisons.

First was a relatively low bore-to-stroke ratio in an era of long-stroke L-heads. For comparison, here's some numbers:

Year/Make......Cu.In......Bore & Stroke...........S/B Ratio.........Torque @ RPM
'33 Pontiac Eight....223.4........3.187 X 3.500.........1.09:1...153@1,600
'35 Packard 120.....257.2........3.250 X 3.875.........1.19:1...203@ 2,000
'35 Packard Eight...320.0........3.187 X 5.000.........1.57:1...260@ 1,600
'35 Super Eight......384.4.......3.500 X 5.000............1.43:1...300@ 1,600

One point was the Pontiac shorter stroke allowed main and con-rod bearing journals a degree of overlap which contributed to rigidity, in five main bearings. Intially that was the case for the 120, also a five main unit. For the Senior engines, nine main bearing providing the support and stiffness. By 1940, we find:

Year/Make............Cu.In.......Bore & Stroke...........S/B Ratio.........Torque @ RPM
'40 Pontiac Eight.....248.9.........3.25 X 3.750......1.15:1......175@1,600
'40 Packard 120......282.0.........3.25 X 4.25.........1.31:1.........225@ 1,700
'40 Super Eight 160..356.0.........3.50 X 4.625......1.32:1...292@ 1,800
'40 110 Six.............245.3.........3.50 X 4.25......1.21:1............192@ 2,000

While the 120 ratio increased with displacement, the 356 c.i. was significantly reduced from the old Senior engines. The Six is added to comparison.

The 1948 comparison:

Year/Make...........Cu.In......Bore & Stroke...........S/B Ratio.........Torque @ RPM
'48 Pontiac Eight....248.9........3.25 X 3.75......1.15:1.........190@2,200
'48 Packard Eight...288.0........3.50 X 3.75......1.07:1............226@ 2,000
'48 Super Eight ......327.0........3.50 X 4.25......1.21:1......266@ 2,000
'47 Packard Six......245.3........3.50 X 4.25......1.21:1......192@ 2,000

The new 1948 generation engines reduced ratios further, the 288 c.i. lower than the smaller displacement '35 120 unit. This also highlights the missed opportunity a decade prior when the Six went to 245 ci to utilitize the tooling to create a 288 ci to replace the 282 ci and further the 327 ci. Just on the greater tooling untilization and reduced manufacturing costs, one would think this would have been obvious at the time.

The final Straight Eights, 1954:

Year/Make...........Cu.In......Bore & Stroke...........S/B Ratio.........Torque @ RPM
'54 Pontiac Eight....268.4........3.375 X 3.75......1.11:1......226@2,200
'54 Clipper Eight....288.0........3.50 X 3.75.........1.07:1......260@ 2,000
'54 Cavalier... .......327.0........3.50 X 4.25............1.21:1.........310@ 2,200

Most striking is the low bore/stroke ratio of the 288 ci, that by '54 it had torque nearly equal to the '48 327 cu.in.. The affect of increased compression ratios for both makes.

My understanding is the pre-war years with mid-60's fuel octane and compression ratios in the 5.5 to 6.5 range, the higher B/S ratios produced more torque at lower rpm's. That was their strength, conversely higher sustained rpms reduced engine longevity. As postwar fuel quality and octane improved, higher compression was possible, above the 8.1 range limit for L-heads. One wonders what the 1948 288 and 327 could have been as higher compression OHV designs

Of the anedotes related by an old-time Pontiac mechanic, he maintained those straight eights really were good for the 100,000 miles they advertised. He claimed they rarely did complete overhauls on those engines until after that mileage if maintained with regular oil changes. I didn't dispute him, though I think the last point is the reason.

So, would anyone care to enlarge on the relative merits and deficits of various bore/stroke ratios?

Steve