Overbore effect on compression ratio ?

[old bones]

At what point can we get into the danger zone with a 5.5 or 6.0 head??. I have good babbitt. How much will it move at .030 .060 ect. through .125 ?

[Kurt in NJ]

Bore changes don't change compression ratios as much as stroke changes----you will hit water before you get compression high enough to hurt good babbitt ----

[Tom Wesenberg]

You just have to do the math. If the diameter is increased by 3%, then you square that and compare it to the old compression ratio. So, lets say the old ratio was stock 4.22 to 1, then you multiply 1.03 x 1.03 = 1.0609 x the old compression ratio. 4.22 x 1.0609 = 4.477.

[tbirdtbird]

babbitt is good up to and including CR 7.0

[Pete]

Originally Posted by tbirdtbird

babbitt is good up to and including CR 7.0

That's a conservative number.
Ed Winfield who I'm sure everyone into model A's has heard of, ran 11 to 1 in his race engines in the 30's. He ran babbitt and the engines easily went 100 miles in dirt races which are a lot harder on an engine than pavement racing. He turned the engines 5000 rpm twice every lap. He was almost unbeatable.
Pretty good testimony to babbitt.

[johnneilson]

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Mr. Brierley has run his Cook OHV at Bonneville on fuel with a turbo charger many times. Yes, still on Babbitt. 166 mph is not too shabby.

J

[caboose]

The compression ratio is the ratio of the volume of the combustion chamber (a) plus the volume of the cylinder (b) divided by the volume of the combustion chamber (a).
Or, (a + b)/a. This reduces to 1 + b/a.

The new combustion ratio would be (a + (1.0609)b)/a. Or, 1 + (1.0609)b/a.

If the original combustion ratio is 4.22, then 1 + b/a = 4.22, so b/a = 3.22.

The new combustion ratio would be 1 + (1.0609)(3.22) = 1 + 3.416 = 4.416.

Slow: one finger typing on an iPad.

[Jim Brierley]

167.5 John! Babbitt mains but switched to insert rods because of a piston change. Ran babbitt rods with 12:1 compression for years.

[pooch]

Originally Posted by old bones

At what point can we get into the danger zone with a 5.5 or 6.0 head??. I have good babbitt. How much will it move at .030 .060 ect. through .125 ?

30 thou overbore will be 4.288 to 1 ratio .

[tbirdtbird]

Jim, very impressive indeed!!!

[Kohnke Rebabbitting]

The spec's on the Rajo, and Gulavan "SP"?, duel over head cam engines, with the Hemi shaped combustion chamber was 14 to 1 on babbitt.

Herm.

[johnneilson]

Originally Posted by Jim Brierley

167.5 John! Babbitt mains but switched to insert rods because of a piston change. Ran babbitt rods with 12:1 compression for years.

Just checking to make sure you are still on line...........

Good quality Babbit is great, I am just too lazy.

John

[edmondclinton]

Originally Posted by caboose

The compression ratio is the ratio of the volume of the combustion chamber (a) plus the volume of the cylinder (b) divided by the volume of the combustion chamber (a).
Or, (a + b)/a. This reduces to 1 + b/a.

The new combustion ratio would be (a + (1.0609)b)/a. Or, 1 + (1.0609)b/a.

If the original combustion ratio is 4.22, then 1 + b/a = 4.22, so b/a = 3.22.

The new combustion ratio would be 1 + (1.0609)(3.22) = 1 + 3.416 = 4.416.

Slow: one finger typing on an iPad.

Well don't forget to subtract the volume occupied by the piston above the deck. Plus add the volume of the gasket.

[hardtimes]

Originally Posted by Kohnke Rebabbitting

The spec's on the Rajo, and Gulavan "SP"?, duel over head cam engines, with the Hemi shaped combustion chamber was 14 to 1 on babbitt.

Herm.

Hey Herm,
Is this pictured engine a product of your work ?

[glenn in camino]

My 31 sport coupe is .125 overbore and has a B, police head Although it ran hot at first, It's ok now and is my best running Model A.