Compression Ratio

If it's broken or just needs tweaked

Moderators: Volker_P, tidd650

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Chris
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Postby Chris » Wed May 09, 2007 10:49 am

Hey, Pinhead. I found this little bit today and thought of this thread:

'Below is Action Fours overbore kit. Ups the compression from 9:1 to 10.5:1, and increases displacement from 627cc to 675cc. Unfortunately, no longer made and hard to come by. But check out the dome and the huge valve notches compared to stock! I just bought this set on Ebay, and it will be going into my project motor. I am looking into having them blueprinted and another set fabricated for a different motor. '

It was here: http://www.myhonda650.com/cb650mods.htm almost half way down the page. It was his mention of blueprinting this kit that I thought might interest you. Also, you might want to try a google search for the 'Action Fours' brand name and maybe get lucky?
1980 CB650c

Pinhead
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Postby Pinhead » Wed May 09, 2007 12:50 pm

Volker_P wrote:Sounds like you have thought and found out considerably about the topic. I am sorry I do not know much on details here.

But now I wonder whether it could make sense to use smaller diameter ports to enhance air velocity and backpressure in case reversal starts (of course on the cost of high rev performance)?


If I get lucky enough (and can cut large enough valve reliefs), I may get a set of Omni Valves. The limiting factor, however, is piston-valve interference. If I can get deep enough notches, they may work. They act like a "check valve" in the intake, thus eliminating reversion.

HOWEVER, if there isn't enough material in the piston to cut deep enough valve reliefs, getting higher compression pistons or milling the head will be my next option. I've read about the Action Fours piston kit and really wish I could find a set. I've searched all over the web, however, and haven't had any luck. The only other option I can think of is having custom pistons made. That'd be an expensive endeavor, however, and I doubt I would go that far.

The ports could probably use a little work, but I've never personally ported anything, and wouldn't even know where to start. From seeing pictures and reading on the SOHC4 forums, I know that the 650's ports are a LOT better than the 550's and 750's. I do know some people that know how to port for velocity while generally keeping the close to the same top-end flow, but they're pretty expensive (and I'd have to ship my cylinder head out).

I guess I just need more information before I start deciding which direction I want to go. So how do I find out how thick the pistons are without cutting a hole in them??

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Volker_P
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Postby Volker_P » Thu May 10, 2007 12:50 am

Pinhead wrote: I guess I just need more information before I start deciding which direction I want to go. So how do I find out how thick the pistons are without cutting a hole in them??

I think I have pistons for which drilling a hole or cutting with a saw would not mean too much decisive additional damage. :lol:
But as I just reassembled the engine after having a look to get it complete and closed, this would mean much work. But I think one could measure the profile from inside and from outside and then relate both profiles with two or three accurate relative measurements. But first of all, I would clarify if piston top cutting is required at all for the compression ratio you like to have.
Last edited by Volker_P on Fri May 11, 2007 3:12 am, edited 2 times in total.

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Postby Pinhead » Thu May 10, 2007 12:48 pm

Volker_P wrote:
Pinhead wrote: I guess I just need more information before I start deciding which direction I want to go. So how do I find out how thick the pistons are without cutting a hole in them??

I think I have pistons for which drilling a hole or cutting with a saw would not mean too much decisive additional damage. :lol:
But as I just reassembled the engine after having a look to get it complete and closed, this would mean much work. But I think one could measure the profile from inside and from outside and then relate both profiles with two or three accurate relative measurements. But first of all, I would clarify if piston top cutting is required at all for the compression ratio you like to have.


I agree. If I simply get a thinner gasket set or only plane the head, I probably won't have to go too deep for the valves. However, if I were to get a set of these I would definitely have to cut deep valve reliefs in the pistons. I'd really like to use them, but it may not be possible due to valve clearance issues.

I made a post outlining some of the mods that I'd like to do here (and explained the above valves). The rest of them are at MPG Research.

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Chris
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Postby Chris » Fri May 11, 2007 12:04 am

I just spent over an hour reading those threads, Pinhead. I'm supposed to be writing a report for work! Interesting stuff, however. Thanks.
1980 CB650c

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Volker_P
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Postby Volker_P » Fri May 11, 2007 3:09 am

These two parts valves are an interesting idea but no time to read all this. But my concern would be inertia forces. The flap seems free on the valve, so it is not opened mechanically but by the pressure of the incoming gas (or closed again in case of inversion). So this mechanism will get certainly quite restricting at high rpm which will ask for compensation by using a higher valve diameter or/and lift. Any dirt between flap and valve stem might end up in blocking the free movement so the life-expectation seems to be away from all-day use requirements. Maybe not a point when running the engine with methanol.
In summary for me this looks like a trick for acceleration competitions especially in combination with some air-loader but not for a reasonable tuning option for a more or less conventional high rpm engine like the CB.

Looking at the area of the rim and having in mind inertia forces I would even not be surprised if this is just a joke and/or the observed increase is just because increased compression ratio and the modified chamber flow geometry. Could be interesting to verify the difference between a movable and a fixed flap valve of the same shape or to make a simulation on a computer to get some insight.

But besides having some idea of physics, I am no engine expert at all and so my concerns do not base on experience. Anyway, in my opinion the CB650 has the highest need for improvement in the chassis.
Nevertheless I agree that playing around with options to improve the engine can be funny and instructive. But one must be aware that most of the money and time is spent on exactly this. :D

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Buber
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Postby Buber » Fri May 11, 2007 7:00 am

Chris wrote:You mentioned gas prices. Are you sure that working this engine will improve your fuel mileage?


I'm terribly sorry for spamming, but I cannot help myself :D - Guys, what do you know about the petrol prices, eh?
When you will reach our comfy european 1,1-1,2 euro per liter (which will compare to 5,5 u$ a gallon) then you will cry for the good old times to come back.

How that was? "better is the enemy of the good" My CB amazes me every time on a gas station. Although i use it in a mild way (I rarely go over 130km/h) it does repeatedly 4,4l/100km in teh city traffic! No kidding! And I'm no slouch on the green, I promise!

Nevertheless, there is an inkling feeling to get this 63 hp camshaft..... :wink: someday...

Regards!

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Postby Pinhead » Fri May 11, 2007 10:40 am

Buber wrote:
Chris wrote:You mentioned gas prices. Are you sure that working this engine will improve your fuel mileage?


I'm terribly sorry for spamming, but I cannot help myself :D - Guys, what do you know about the petrol prices, eh?
When you will reach our comfy european 1,1-1,2 euro per liter (which will compare to 5,5 u$ a gallon) then you will cry for the good old times to come back.

How that was? "better is the enemy of the good" My CB amazes me every time on a gas station. Although i use it in a mild way (I rarely go over 130km/h) it does repeatedly 4,4l/100km in teh city traffic! No kidding! And I'm no slouch on the green, I promise!

Nevertheless, there is an inkling feeling to get this 63 hp camshaft..... :wink: someday...

Regards!


I filled my bike up yesterday and was surprised at what I saw. With my exhaust opened up, the air filter cover removed, side-gapped spark plugs, and 102.5 main jets, I'm getting 65mpg in town. This is stop-and-go traffic light driving. I think my idle jet circuit is somewhat clogged; it's still got a slight hesitation below 2000 rpm with any kind of load. Once it gets above 2k it runs like a raped ape.

According to this, you're about 53.4mpg. With the above mods, I get 65mpg or 3,6l/100km in town. At a steady pretty 70-75 mph (before the opened exhaust and side-gapped plugs), I was getting 47mpg or 5l/100km. I don't drive on the highway that much, though, so it'll be a while before I know what my new highway mileage is. I DO know, however, that it's got quite a bit more power at 65-75mph with the opened exhaust and new plugs than it did before I did the changes, though.

I have a somewhat annoying condition in which I can't leave anything alone. If I think there's anything that can be done to get a slight improvement in fuel efficiency, I do it. I do a lot of research before I perform each mod, but if they look worthwhile they'll get put in. One example is ozone and water injection (raised my car's mileage from 16 to 23 mpg). Another is PowreLynz (I haven't done it yet because I haven't torn my engine down). Even though I'm getting really good mileage, there is still a lot of room for improvement. I'm wholly confident that with all of the mods that I plan on doing, I could hit that magical 100mpg mark with at least the same power (probably more).

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Chris
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Postby Chris » Fri May 11, 2007 11:13 am

Yeah, Buber, I hear you. I understand that fuel has always been more expensive over there... for longer than I've been alive anyway. We're in a bad situation everywhere when it comes to gas prices. As long as people keep buying it, the prices will continue to increase. Even if people don't buy it, refineries will cut production to keep the price up. It's a catch 22. So, my feeble attempts to lessen the burden on my own wallet will have no effect on the economy as a whole, but it sure helps my personal budget :wink: The same will happen with any alternative fuel that's introduced, it may be 'greener', but you can bet it won't be any cheaper. Not so much because of the production costs, but because the fuel companies realize that if people can afford to pay $4/gal. for gasoline, then they can afford to pay the same for an alternative. Round and around we go.

I'm more than happy every time I fill up my 650 at the pump. That's the main reason why I decided to to finally get a motorcycle. The next step for me will be foot power, if it comes to that. God, I hope it never comes to that :lol:

I love internal combustion engines and I love my CB650 :)

On a side note, I have a link here somewhere with instructions and blueprints for building your very own ethanol still. It even has a Q&A with instructions for obtaining a federal license to operate said contraption.

Sorry for the thread hijack.

Pinhead, I trust that you've already fiddled with chain sprocket sizes?
1980 CB650c

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Postby Pinhead » Fri May 11, 2007 6:21 pm

Yeah, I got a 1-tooth bigger front sprocket. I'm contemplating dropping another 2 on the rear. I'd have to get a stronger clutch, though, to make it work well. It's got enough torque to take off in 2nd gear right now... BUT as it is, if I snap the throttle wide open around 6k rpm (in any gear) the clutch lets loose. I've adjusted my clutch cable to no avail. I'm thinking stronger springs would come in handy. That and a cheap set of clutch plates.

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Chris
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Postby Chris » Fri May 11, 2007 11:25 pm

I have clutch slippage like you describe even with the stock sprockets. I was thinking maybe it could be the type of oil I'm using? I know today's engine oils are formulated alot different than they were 30 years ago. Hell, maybe it is just a worn clutch. It doesn't do it all the time, only when I really get rough with it. Will look into a clutch rebuild someday. Because it doesn't happen all the time, it's not one of my highest priorities... yet.
1980 CB650c

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Buber
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Postby Buber » Sat May 12, 2007 7:16 am

Pinhead wrote:I filled my bike up yesterday and was surprised at what I saw. With my exhaust opened up, the air filter cover removed, side-gapped spark plugs, and 102.5 main jets, I'm getting 65mpg in town. This is stop-and-go traffic light driving. I think my idle jet circuit is somewhat clogged; it's still got a slight hesitation below 2000 rpm with any kind of load. Once it gets above 2k it runs like a raped ape.


Hey, hey, hold it there! This is VERY interesting! Actually I tried the air filter cover removal, but it made the mixture so lean, that I could hardly accelerate. But i did few extra holes in the end of muffler, and it leaned it just a bit. Still, would you be so kind as to say something more about "side-gapping"? is that simply making the gap bigger, up to 1mm? And the jets. I heard so much about it, but there's this hesitation that they will actually draw MORE fuel. I'm not worried about performance. I know it will be better :)
over 60mpg..... I must get it! And the sprockets. ! tooth bigger on teh fron, or 1 tooh less on teh rear? Or which one comes first to replacement, eh :) ?

Still, I would appreciate some more insight into "home-performable" mods like You described above.

Thanks!

P.S. Ok! I found what side-gapping is. 1st thing I will do when coming back home!

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Postby Pinhead » Sun May 13, 2007 11:18 pm

Buber wrote:
Pinhead wrote:I filled my bike up yesterday and was surprised at what I saw. With my exhaust opened up, the air filter cover removed, side-gapped spark plugs, and 102.5 main jets, I'm getting 65mpg in town. This is stop-and-go traffic light driving. I think my idle jet circuit is somewhat clogged; it's still got a slight hesitation below 2000 rpm with any kind of load. Once it gets above 2k it runs like a raped ape.


Hey, hey, hold it there! This is VERY interesting! Actually I tried the air filter cover removal, but it made the mixture so lean, that I could hardly accelerate. But i did few extra holes in the end of muffler, and it leaned it just a bit. Still, would you be so kind as to say something more about "side-gapping"? is that simply making the gap bigger, up to 1mm? And the jets. I heard so much about it, but there's this hesitation that they will actually draw MORE fuel. I'm not worried about performance. I know it will be better :)
over 60mpg..... I must get it! And the sprockets. ! tooth bigger on teh fron, or 1 tooh less on teh rear? Or which one comes first to replacement, eh :) ?

Still, I would appreciate some more insight into "home-performable" mods like You described above.

Thanks!

P.S. Ok! I found what side-gapping is. 1st thing I will do when coming back home!


When I got my bike my jets were all clogged so I changed them out. I heard that from the factory they came lean, so I went up to 102.5 size main jets (stockers are 90's). With the air filter cover on, it ran stinkingly rich so I took the cover off, which lessened the problem considerably. I think I still have the stock paper filter (no K&N high-flow or anything like that). Then I opened up my exhaust and that made the mixture just about right. I always run 87 octane regular.

The stock sprockets on my '79 is 16/40 front/back. I went up to 17/40 and it dropped my highway engine speed by about 500 rpm or so, maybe a little more. I don't think you can go any bigger on the front because it'd have trouble rubbing the on crankcase. My next sprocket change would be a 38 tooth rear sprocket, though I think I'll do some more work on the engine and clutch first.

It's still not running completely right, but I just found out that both of my coils are cracked. That could be causing my hesitation down low. They'll probably be the next thing I replace.

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Volker_P
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Postby Volker_P » Mon May 14, 2007 2:00 am

I also have the 17 front sprocket and had to modify the guard to get it in place. Maybe 18 is possible without the guard but I do not want to risk to have stones kill my oil pump. But as I have the Custom with the 16" rear wheel, this basically comes out to the Z-model stock transmission (100km/h at 5000rpm).

I can achieve 65mpg in the mountains when I let it roll down but I agree this is hard to beat. But beware of the illusion to reduce rpm too much as the 4th and 5th gear will considerably suffer. The oil pressure will also go down which does not matter much at idle but is bad at load. You can do things like that with ball and roller-bearing engines but not with the CB650 engine. It is completely designed for high rpm and can stand them well and without execessive wear as long as your oil level and pressure is allright. I once thought like you and I think it partly was the origin of the crash of my 4th gear wheels some years ago. 5th were close to go off, too. I guess I was very lucky that these hardened parts did not kill anything else.
So I like to write it down a bit more drastically: If you like your engine, don't even think about your below 2000rpm performance. :!:

I do not think that it comes out to a positive calculation to put much money into fuel saving measures that include major engine modifications. Any reducement of the rpm vs speed ratio will end up in higher wear unless you tune up the oil pump also. You may put a manometer at the oil pressure switch to get an idea about the pressure curve (when it is warm). Too light "easy" oil is not a good idea for that reason when you prefer to ride below 3000rpm in town. Anyway, the problem with the transmision will stay. Sit down when asking for the price of new Honda transmission sprockets. :shock: You can blow much fuel for that.

Oil certainly is something that is also decisive for the clutch. If it is written "easy" on the label, it is probable to cause trouble. The engine is specified for 1980's oil qualities, so nowadays cheapest is sufficient. A washer below the springs may help to avoid looking for slightly stronger spares.

BTW, I side-gapped my dirtbikes' spark plug this weekend and will try it out the next ride. I am curious what will happen. :)

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Buber
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Postby Buber » Mon May 14, 2007 7:13 am

Hmm, looks like i will leave the sprockets alone.
Then on the mixture - I'm just wondering where is the difference in performance and fuel consumption: smaller jets and normal airbox, or bigger jest and open airbox. it seems that at the end you will be getting simply more or less the same mixture. Where's the gain? (or my knowledge is not good enough :wink: )

Thanks guys for info so far. there's always something to learn.....


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