Topic: What is the combustion chamber volume difference on 550 / 650?

[Tretnine]

It's finally happened, I've become a huge bike nerd and this is what I'm doing with my morning. Attempts at compression numbers.

The 650 head volume is much larger than the 550, but I'm curious by how much.

Here's the real question... The 650 stroke is much longer and that makes up for the larger combustion chamber. With high CR 550 overbore pistons (61mm) would it be possible to put 650 head on bored 550 jugs without using a 650 crank? I'm doubting it, but you've got to explore all possibilities and all that.

[bwaller]

This is only half of what you asked for, but the 550 combustion chamber volume is 13cc's.

The CR will be too low with a 650 head.

Give us some measurements and maybe we can help.
1) piston compression height (distance from wristpin center to shoulder of piston crown)

2) total dome height (comp. height to top of dome)

Usually stock pistons are below deck approx .4mm so if you don't have to machine gasket surfaces and use stock thickness gaskets (head & base) you might be ok. If the CR is too high you can probably machine the piston crowns some.   

[mlinder]

No, it wouldn't.

I mean, yes you could do it, but it wouldn't work very well.
I asked Paulages once about using stock 650 pistons and a 650 head in the bored 550 jugs and bottom end.

There was some small difficulty with that, can't remember what it was, but that's probably the direction I'd head.

The longer stroke isn't waht makes up for "larger combustion chamber". It's the shape of the piston dome. It isn't like you can shove the straight edges of a piston into the semi-spherical chamber by giving it a longer con rod. The shoulder of both the cb550 and cb650 is probably a milimeter or so below deck height. Any higher and the pistons would be kissing stuff they shouldn't be.

Sorry, went off on a tangent there. Long story short: No, your compression will be too low.

[Tretnine]

You can make some rough guesses with these pictures (scroll down quite a bit), but the pistons just showed up about 45 minutes ago (Thanks USPS) and I'm leaving a bit early for work to buy a torch to try to get my cyl stud bolts out... I'll try to do some measuring tonight / this weekend.

Brady

[mlinder]

What pictures?

[Tretnine]

oh right, add link. http://forums.sohc4.net/index.php?topic=70052.0

[Elkie]

You'd likely have to get the valve pockets in those pistons enlarged as the 650 head uses larger diameter valves.

[mlinder]

Please see the pic for cylinder differences in Pauls thread at

http://forums.sohc4.net/index.php?topic=40414.0

Those 590 pistons are pretty much flat.

[Elkie]

Paulages response to just using 650 pistons and head in a 550. His thread is what I generally go to whenever I'm thinking about how I need to do something in my own build.

"yes you could, but most of the extra displacement found in the 650 was due to a stroked crank. the piston diameter increase is minimal at 1.3mm, while the stroke was increased 5.2mm. i imagine it would still improve performance a little, but most of it would be in the cam which will drop into the 550 head anyway."

Your not going to be able to use the 650 head without in some way modifying other parts. kits designed for the 550 are not going to give you enough compression with a 650 head. Using 650 pistons would be your best bet, with minimal changes to use the 650 head.

[bwaller]

It doesn't take much dome to bump compression with the small 500/550 chamber. I bet those pistons have an extra 1.25-1.5mm? If you use a street cam with those deep valve reliefs that'll reduce the CR some too.

You can mount both pistons side by side on a single wristpin to check shoulder height before you measure anything else.

[Hannibal Smith]

It doesn't take much dome to bump compression with the small 500/550 chamber.

Exactly, minor machining on the piston can +- compression by a mile with a CC that small.

[Tretnine]

Paulages response to just using 650 pistons and head in a 550. His thread is what I generally go to whenever I'm thinking about how I need to do something in my own build.

"yes you could, but most of the extra displacement found in the 650 was due to a stroked crank. the piston diameter increase is minimal at 1.3mm, while the stroke was increased 5.2mm. i imagine it would still improve performance a little, but most of it would be in the cam which will drop into the 550 head anyway."

Your not going to be able to use the 650 head without in some way modifying other parts. kits designed for the 550 are not going to give you enough compression with a 650 head. Using 650 pistons would be your best bet, with minimal changes to use the 650 head.

This is what I thought, that pretty much means using the 650 crank and opening a can of worms, which for the moment is not my plan for this bike.

[Elkie]

In regards to your original question, I would pm soos. He might know the value of the 650 combustion chamber, he has a lot of knowledge on the 650's.

[fastbroshi]

And please let us know what the compression height is. 

[paulages]

CB650 combustion chamber volume with the piston at TDC is 17.4cc.

[Tretnine]

Paul, to quote Heart, you're a 'magic man.' I will take some deck height measurements this morning when I manage to get off my duff.

[paulages]

Paul, to quote Heart, you're a 'magic man.' I will take some deck height measurements this morning when I manage to get off my duff.

ha! thanks, tretnine. for what it's worth, the 650 piston dome takes up about 12cc in the head, and the total head volume with gasket is around 30cc.

[Tretnine]

I linked some info into that thread again. I can relink later, but don't feel like it just yet. Got a friend in from out of town and I need to get some lunch. http://forums.sohc4.net/index.php?topic=70052.0