Topic: What size would a .30 over cb750 be?

[75 super sport]

Had a guy offer to put a big bore kit into my 1975 cb750. He said it would be .30 over. what size would that make it?

[lucky]

For the 736cc engine, @8.6:1 compression with .030 over is 754.523cc

Bore X stroke+.030 X ratio = cu. centimeters x4=

[Don R]

I've never seen it done like that before. what does the CR ratio have to do with CC's?

[raymond10078]

Let's try again.

(Cylinder Diameter divided by 2) x (Cylinder Diameter divided by 2) x pi x stroke x 4 cylinders = displacement.  Has nothing to do with compression ratio.

Original Bore and Stroke is 61 x 63 mm, or 6.1 x 6.3 centimeters.

0.030" is 0.762 millimeters, or 0.0762 centimeters.

New bore is 6.1762 centimeters.

New displacement is then ~ 755 cubic centimeters.

[lucky]

Let's try again.

(Cylinder Diameter divided by 2) x (Cylinder Diameter divided by 2) x pi x stroke x 4 cylinders = displacement.  Has nothing to do with compression ratio.

Original Bore and Stroke is 61 x 63 mm, or 6.1 x 6.3 centimeters.

0.030" is 0.762 millimeters, or 0.0762 centimeters.

New bore is 6.1762 centimeters.

New displacement is then ~ 755 cubic centimeters.

Did you round that final number off?

[lucky]

I've never seen it done like that before. what does the CR ratio have to do with CC's?

The compression ratio is important because the cc's is the measure of how much space is in the combustion chamber. With different piston shapes the amount could be different.
For example valve reliefs cut into the piston top  would have more volume .

A flat top piston would have less.


But why quibble over 1/2 cc total on the 4 cylinders?

[lucky]

Let's try again.

(Cylinder Diameter divided by 2) x (Cylinder Diameter divided by 2) x pi x stroke x 4 cylinders = displacement.  Has nothing to do with compression ratio.

Original Bore and Stroke is 61 x 63 mm, or 6.1 x 6.3 centimeters.

0.030" is 0.762 millimeters, or 0.0762 centimeters.

New bore is 6.1762 centimeters.

New displacement is then ~ 755 cubic centimeters.

"Let's try again."

So condescending. 

[HonderCB]

Let's try again.

(Cylinder Diameter divided by 2) x (Cylinder Diameter divided by 2) x pi x stroke x 4 cylinders = displacement.  Has nothing to do with compression ratio.

Original Bore and Stroke is 61 x 63 mm, or 6.1 x 6.3 centimeters.

0.030" is 0.762 millimeters, or 0.0762 centimeters.

New bore is 6.1762 centimeters.

New displacement is then ~ 755 cubic centimeters.

"Let's try again."

So condescending.

particularly ammusing since he came up with the same number as you, lucky.   

[raymond10078]

Sorry - didn't mean to sound "mean".

However, displacement has nothing to do with compression ratio.  Swept volume is swept volume.  Domes, piston reliefs are not related to displacement.  Never has been, never will.

As yes, I did round the answer.  I recall the answer was about 754.975xxxx - anyway, it was very close to 755.  Extreme precision didn't seem necessary, or appropriate.