Topic: CB750: Which chamber volume is used when specify piston CR?

[PeWe]

There are several setups of pistons to CB750 where the CR is specified as 10.25:1/ 10.5:1 / 12.5:1 ....
- How are these numbers calculated? It must be with a certain chamber volume that can differ depending on head and how ported it is. Certain gasket thickness too.

I found a calculator that can be used to make relative changes and see how much it will affect the CR.
top setting is 2 for mm
http://www.csgnetwork.com/compcalc.html

I used std stroke 63 mm, chamber volume of 22.5, gasket height of 1 and fiddled with various dome cc and piston heights above or below deck (started with 0 on both)
When I got a CR around 10.25 I changed the chamber volume to my heads volume, 24.4 to see how the CR decreased. Then checked with thinner gasket as the 0.762 (0.030" MLS) to see how much this gasket could do.

Similar tests by finding closer to 12.5:1 by using same values except for the dome volume. When there I tested thicker gasket to see how much lower I got with my chambers and thicker gasket. Piston deck clearance might be adjusted by a thicker base gasket.

This to make it possible to find a CR around 10.5-11:1 despite larger chambers. Which pistons to use and gasket to adjust the CR with.
My hotter cams with longer duration need more CR to keep up the pressure.

- It is very interesting to know how the std CR is calculated for the aftermarket pistons to a CB750 K head.

[BPellerine]

pewe,dont know a lot about this ,but when they convert 883 sportsters to 1200 with stock heads they use a dished piston,which makes me wonder when you take a 750 out to 900 with close to stock head how much you raise compression.bill

[bwaller]

Who knows how accurate the advertised CR is with aftermarket pistons, especially old stuff. You're right at least you have different thickness gaskets to adjust, plus surface decking and piston dome machining I suppose. Measuring accurate chamber volume is easy enough, it's the piston net dome volume that is difficult to get accurately.

I had new pistons made and with that had an exact dome volume, the previous set I measured many times and never got the same result twice! If you're off by a cc or two it's useless.

Pick middle of the road...then you have options to take CR up or down.

[PeWe]

I think that the old big bore pistons for CB750 had often wider domes,  head chambers had to be adapted for it.
My head is adapted for forged RC 836 pistons it got 1983, chambers widened to 65mm. I found NOS cast RC 65mm pistons that matched and have OK CR. (Really old ones with single oil ring I replaced with the 3-rings from the other RC pistons I still had in a box, 2 of them with additional hole, dimple)

The big bore pistons of today are done for std chambers. I have (after I started this thread) got information about 22.5cc as base for  CR calculation.
CR 10.25 pistons reduced to around 9.7 with .040" gasket with my head. Thinner 0.030" gasket will give CR 10.1:1. Maybe not enough when a hotter cam with longer duration is used. OK touring engine when the risk of pinging will be reduced with lower CR.

I understand those guys that make a mold of the chambers and get pistons that match.

Pistons with CR 11:1 would be an alternative with thinner or thicker gasket.
CR 12.5:1 (calc w 22.5cc chambers) on std tap fuel (Shell V-power) might be too much? CR 11.5.1 with 24.4 chambers