Topic: Anyone wanna check my math on this (compression ratio calcs)?

[JLeather]

Trying to figure the compression drop of using CB750 DOHC rods (1.5mm shorter).  If I started with a Wiseco 836 10.25:1 kit it'd be 209cc per cylinder (bore x stroke).  To calculate the head cc for 10.25:1 it would be (209+X)/X = 10.25 where X is the head cc.  Solve that it yields about 22.6cc.

Now, a 1.5mm reduction in rod height is like adding that volume to the head.  So 1.5mm x 65mm yields an extra 4.97 cc (lets just call it 5cc) upping the head volume to 27.6cc.  Now it ends up being (209+27.6)/27.6 which yields me 8.57:1 static CR.  That right?  Obviously I'll have to titrate the head for an accurate combustion chamber since it's been ported and worked over, but it can't be much larger than a stock head (maybe another cc?).

Did I completely screw anything up here?

[NitroHunter]

Here is a cool online calculator.
http://grapevine.abe.msstate.edu/~fto/tools/vol/index.html

I read a post from SteveD CB550 F where he said the stock head is 22cc.

Remember to account for the head gasket.

[NitroHunter]

John check your PM

[Artfrombama]

I get 3.06cc more volume

π(pi) x R² x H = volume or
3.1416 x 65 x 1.5 = 306.3 cubic millimeters

306.3 cubic millimeters = 3.06 cubic centimeters

[oldbiker]

Jleather,  you are just about right. 5cc is pretty close.
Artfrombama, in your calculation you multiplied Pi x diameter x 1.5 milimeters (should have been radius squared not diameter) and cc is cubic milimeters dived by 1000 not 100.

[KingCustomCycles.com]

I have a volumetric effeciency calculator here that might help with actual gains. They will always be less than calculated. 

http://www2.tech.purdue.edu/at/courses/aeml/#second

Look under Powerplant Materials, calculator

[bryanj]

Dont forget you add that 5cc to the cylinder capacity as well as the "cylinder" produced by the piston at BDC is longer so

(209+5+27.6)/27.6 which i make 8.754 to 1

[JLeather]

BryanJ, I don't believe that's how it works.  The stroke hasn't changed at all, so the volume displaced by the piston is still only 63mm by 65mm.  The only way to change the volume displaced by the piston is to alter either the bore or the stroke, and neither of those is altered by shorter rods.

Yes, the piston at BDC is lower in the cylinder, but it is also lower at TDC which is why the extra volume is only added to the combustion-chamber.  But this is why I posted it up here, to make sure my assumptions are correct.

[Tower]

Don't know if this was covered in PMs but here's my 2cents:

Compression Ratio = Total Volume / (Total Volume-Swept Volume)

Swept Volume = (3.1415926 / 4) X Bore x Bore x Stroke

Total Volume = Swept Volume + Head Volume + Gaskets Volume +/- Piston Gap Volume +/- Piston Dome Volume

Head Volume = commonly referred to as head cc, measured with spark plug and valves installed, using a graduated or other measured volume of liquid to fill the space inside the head (and a glass cover to ensure surface tension does not overstate volume).

Gasket Volume = (3.1415926 / 4) x Bore x Bore x crushed height of gasket(s)

Note: this includes head gasket and not the cylinder gasket

Piston Gap Volume = +/- (3.1415926 / 4) x Bore x Bore x distance between piston (less any dome) at TDC and top of cylinder.

Note: + when piston is below top of cylinder, - when piston is above top of cylinder


Piston Dome Volume = + amount of displacement, if any from material present above top of piston edge;
- amount of volume cutaway for additonal valve clearance

Note: Since stock 750 pistons have a flat top, the Piston Dome calculation nets to zero, but if any aftermarket pistons come with domes or cutaways then it applies)

[754]

Sharpen up the piston cutting tools...

[NitroHunter]

OK guys, I’ve given this WAY too much thought…

First, let’s start with a stock motor, advertised compression ratio of 9:1.
This is a flat top piston, so there will be no dome factored in.
Let’s also say it is zero decked, that is the top of the piston is flush with the top of the block.
I don’t know the exact head gasket thickness, so I’ll use .040”.
I’ve never cc’d a stock head, so trial and error produced the results shown.

Displacement (61 bore x 63 stroke) = 184.0666 cc
+ Head gasket (61 mm X .040”) = 2.9689 cc
+ Head cc = 20.0 cc
= Total volume = 207.0355 cc
Divide by volume of head + gasket = (22.9689cc)
CR = 9.01:1

Using the above data, lets solve for 836cc with flat top pistons:
Displacement (65 bore x 63 stroke) = 209.01935 cc
+ Head gasket (65mm X .040”) = 3.37134 cc
+ Head cc = 20.0 cc (from above calculations)
= Total volume = 232.39069 cc
Divide by volume of head + gasket = (23.37134cc)
CR = 9.94:1

Now for 836cc 10.25:1 pistons:
Dome volume of .78cc, includes valve pockets.
Remember the dome is taking up space compared to the flat top, so:
Flat top volume = 232.39069 cc (from above)
- Dome = 0.79 cc
Cylinder total volume = 231.60069cc
Divide cylinder total volume by volume of (head + gasket - dome) (22.58134cc)
CR = 10.25:1


Now to see what happens with the same motor, but using 1.5mm shorter rods. Since the piston will now be down in the bore at TDC, let’s add it in:
Short rod added volume (65mm X 1.5 mm) = 4.9773cc
+ cylinder total volume (from above) = 231.60069cc
= Short rod total volume = 236.5779
Divide short rod total volume by volume of (head + gasket – dome + SRAV) = (27.5586cc)
CR = 8.58:1

Thank you and good night. 

[Tower]

Yes, but do you know for sure that the piston wrist pin is in the same position relative to the top of the 836 piston as in the stock 736 configuration?  He He.

This will alter the piston deck gap volume.

[NitroHunter]

AAAAAAAAAAAAGGGGGGGGGGHHHHHHHHHH! 

[JLeather]

Sweet.  8.58:1, sounds just about right for a turbo 

Now, wonder how much I can stuff in it before the DOHC rods let go?

[NitroHunter]

To anybody that reads this:

   Calculation disclaimer!  

These figures are for a 'theoretical' engine that is based on our 750's and should be used FOR REFRENCE ONLY.
Let me say this again: I've never cc’d a stock head. Head gasket compressed height may be different, and God only knows how far the pistons really sit down in the bore.

But it WAS fun - in an anal-retentive sort of way...

[bryanj]

OK think of it this way:-

With piston at TDC fill with a fluid and measure

With piston at BDC fill with fluid and measure

If piston is farther down the bore you will get more fluid in
QED