Compression Ratio

If it's broken or just needs tweaked

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Pinhead
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Compression Ratio

Postby Pinhead » Wed May 02, 2007 12:15 am

What are the exact engine specs on the '79 CB650? I need to know the bore, stroke, head gasket thickness, and combustion chamber cc's. Is there enough room to shave the head/cylinders for higher compression? I want ~10:1 but if that's not possible with a simple head/cylinder planing, how high do you guys suppose I could go? 9.5:1 wouldn't be too bad. I've been looking around everywhere on the 'net for pistons with no luck.

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Volker_P
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Re: compression ratio

Postby Volker_P » Wed May 02, 2007 2:32 am

You can find some of the data you look for at http://cosky0.tripod.com/.

Pinhead
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Postby Pinhead » Wed May 02, 2007 9:41 am

I guess all I need is the compressed gasket thickness and the combustion chamber volume. ??

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Chris
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Postby Chris » Thu May 03, 2007 12:59 am

Just some random thoughts that kinda pertain:

If you end up boring your cylinders and adding oversized pistons, your compression will go up automatically (larger bore with the same chamber volume).

Also, I was on the Honda website earlier today looking at this years twins and noticed they only have a 8:1 compression ratio. I thought to myself, "Jeez, they oughtta be able to go alot higher than that with aluminum and water cooling. People with aluminum car engines and electronic(computerized) fuel-injection are pushing 11:1 on premium pump gas." I mentioned this to one of the fellas at work while we were on break and he says that low compression helps develop bottom end torque. This was kinda confusing to me, but according to him (his game is offroad trucks) an engine uses some of the energy from the firing cylinder to compress the fuel in the compressing cylinder. Does that make sense? It did to me after I thought about it for a while. Compression would make a huge difference when you only have 2 cylinders to work with. I would want to waste as little of the combustion energy from the firing cylinder as possible in compressing the mixture in the next cylinder to fire. The reason I mention aluminum and water cooling is that higher compression translates to higher combustion temperatures... better keep it cool.

We have a huge advantage over car engines in RPM as well. Most V8's over here turn about 6,000 RPM. Our bikes can turn twice that without even feeling it. Horsepower= (torqueXrpm)divided by 5,252 . Don't ask me why 5252. I asked that same question when I was in school and I can't remember the answer. It's just one of those magic numbers. Look at any dyno graph and the horsepower and torque curves will always intersect at 5252rpm. That's because a dyno doesn't really measure horsepower. A dyno measures torque throughout the rpm range and then this formula is used to figure horsepower. Point is, higher rev's equals more potential for horsepower.

You can find CB650 engine specs here: http://sohc4.us/node/index.php?option=com_content&task=view&id=54&Itemid=52

The chart says that SOHC 650's have a 9.0:1 compression ratio. On older engines, carbon build-up in the chambers and on the top of the pistons will raise that a tad.

Personally, I'd leave the internals alone and just concentrate on letting it breathe easier. Or get a bigger engine.
1980 CB650c

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Volker_P
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Postby Volker_P » Thu May 03, 2007 2:28 am

The remark on bottom end torque sounds like it makes sense. On the other hand, Diesel engines which are known for bottom end torque have much higher compression, so I guess it cannot be a very dominant effect.
I may have an old, compressed head gasket to measure it, but I guess it will take some time to find it. With regard to the oil lines leakage, I'd rather try planing at the cylinder bottom (new bore polish recommended).
The 650 engine is not that bad and carbs and exhaust are quite optimal adapted to it. I guess you really need to know what you do if you want to recieve a satisfactory improvement. Otherwise it probably ends up in a slightly enhanced top power at the cost of much higher fuel consumption and bad engine characteristics in mid-rpm range.
I would prefer to go riding with the time and the money. :wink:

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Chris
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Postby Chris » Thu May 03, 2007 11:33 am

Good point on the diesels. That's practically all they run on is compression. That's why I was kinda confused. I've always thought that more compression leads to more power as long as things don't burn up.

I bought a cheap cylinder head cc-ing kit by mail order years ago. It consists of a thick plate of clear plexiglass large enough to cover the combustion chamber and a graduated cylinder (cc's is best). There's a small hole in the plate. All you do is seal the plate over the chamber with grease or petroleum jelly, fill the graduated cylinder with rubbing alcohol (add food coloring to make it more visible), carefully pour the alcohol through the hole in the plate (don't spill any), observe the amount of alcohol remaining in the graduated cylinder and do the math to find chamber volume :wink: Some of the more expensive kits use a fancy glass buret on a stand with a little petcock at the bottom of it, in case any of you have one of those in your chemistry sets at home.

In addition to the combustion chamber volume, compressed gasket thickness, and bore X stroke, you will need to measure the deck height. The deck height is the volume in the cylinder above the piston when the piston is at top dead center. Some times this can be quite a bit and sometimes it can be '0'. If you can get a good seal around the piston, you can check this volume the same way I described the head cc-ing.

Compression ratio = (PV + DV + GV + CV) divided by (DV + GV +CV)

PV = Piston Volume
DV = Deck Clearance
GV = head Gasket Volume(= Bore X Bore X 0.7854 X Thickness of gasket)
CV = combustion Chamber Volume

The book I got this formula from uses measurements in inches. I'm not sure if that makes a difference or not. Just in case, 1cu. in. = 16.4cc
1Liter = 61 cu. in.
1980 CB650c

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Postby Pinhead » Thu May 03, 2007 2:42 pm

I would think those low-rpm optimized engines are lower compression because at those low compression ratios you have absolutely no worry about detonation. The small camshafts increase the dynamic compression and therefore less of it will "bleed" into the intake (reversion). An engine like ours (camshaft profile) lacks a lot of torque for a couple of reasons... Obviously size is a factor, and the "bigger" camshaft (high-RPM tuned) doesn't completely fill the cylinder at low rev's. If you put a smaller camshaft in (increases low-RPM torque) the peak torque moves lower in the rev range, and therefore the "full" compression moves lower.

I have no worry of detonation... I have some tricks that I'll use to help eliminate that. I won't reveal them until I can prove they work.

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Postby cb650 » Thu May 03, 2007 5:52 pm

Remember on most auto engines they have a intake manifold so cyls interact more.


Terry

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Chris
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Postby Chris » Thu May 03, 2007 10:58 pm

I'll be looking forward to seeing how things work out for you. I hope the formulas were some help. Have you considered some type of forced induction? It seems to be popular with some of the newer bikes.
1980 CB650c

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Postby Pinhead » Fri May 04, 2007 12:59 am

Chris wrote:I'll be looking forward to seeing how things work out for you. I hope the formulas were some help. Have you considered some type of forced induction? It seems to be popular with some of the newer bikes.


Thanks. Hopefully I'll get this stuff done before gas prices jump up to $4 a gallon...

I've never worked with turbo/super charging and don't really like the complexity. I know it's not that more complicated, but there's not really much room on the bike as it is, and adding more equipment would make it even more cramped. I don't like the idea of forced induction on a non computer-controlled engine; there's just not enough safety measures on these engines.

Does anybody know how thick the crown of the stock pistons are? How deep can I cut valve reliefs?

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Volker_P
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Postby Volker_P » Fri May 04, 2007 1:04 am

Well, there is a 50hp and a 63hp camshaft for the CB650. With the 63hp camshaft you do not get only more top end power, also the torque in 3000-4000rpm increases somewhat. I tried it out last year. The 50hp camshaft lobes are about 3mm lower and less steep. So it depends what you would like to call a "smaller" camshaft.
I remember that I read an article (probably in sohc.us?) about intake channel modification for the CB750 K-engine. I guess that could be the key for the 650 engine, too. However they did very much effort with velocity profile mesurements etc.

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Postby Pinhead » Fri May 04, 2007 1:51 am

The stock ports on the 650s are much better than the SOHC 750's. Much.

The US '79 model had the 63hp and the Euro model had the 50hp cam, I believe.

I guess "smaller" isn't really a good description. I mean low-RPM tuned. Low lift, short duration, little overlap.

For instance, the peak torque on one of our engines is in the 5000rpm range. A "smaller" cam would put the peak torque lower in the RPM band, say to 4000 or so.

Peak Torque = Peak Volumetric Efficiency.

Our engines aren't designed for low-rpm torque, though (even with a low-RPM tuned cam). The short intake tracts don't help low-rpm torque much. Because of this, reversion runs rampant at low revs. The air simply doesn't have enough inertia to continue to fill the cylinder, and therefore it reverses. That's why the bigger camshaft would also increase low-mid to mid rev's. The head itself doesn't lend itself to low-rpm efficiency.

If there was a way to eliminate reversion... :wink:
Last edited by Pinhead on Fri May 04, 2007 4:31 pm, edited 1 time in total.

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Volker_P
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Postby Volker_P » Fri May 04, 2007 2:53 am

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)?
BTW, 50hp was a German assurance class limit, and insurance fees were that horrible in early eighties here that many manufacturer offered restricted versions. But you could also buy the unrestricted version here in case you could afford.

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Chris
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Postby Chris » Fri May 04, 2007 12:07 pm

So that must be why the '79 was the fastest in quarter-mile testing. I read on one of those links I gave you that it got into the 12's while later models were a little slower. I figured it was just the rider, but now it makes sense.

You mentioned gas prices. Are you sure that working this engine will improve your fuel mileage?
1980 CB650c

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Ibsen
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Postby Ibsen » Fri May 04, 2007 4:08 pm

The 79 European Z model had a 63hp camshaft. At least in England and Sweden. As Volker said, I think the 50hp cam was used for the German market.

The compression ratio is the ratio between the volume above the piston when it’s at its BDC (at the bottom of the cylinder) and the volume above the piston (the combustion room) when it’s at its TDC (at the top). With a compression ratio of 9:1, the combustion room volume is 1/9 of the total volume when the piston is at its BDC, the volume of the compression room included.


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