Topic: Cylinder Height Question

[Rosinante]

A friend obtained a good used cylinder set for what appears to be an earlier (non-77/78) CB750.  Not sure what this part is called....it is four cylinders, with fins, etc.  He reused the old head and pistons.  He says the bike runs fine, but all cylinders now measure 100 psi of compression.  Here are my questions:

Are all SOHC4 cylinder sets the same height or, alternatively, are some cylinder sets just a few thousandths taller?  If this cylinder set is too tall, then that would enlarge the combustion chamber and explain the oddly low compression numbers.

Any other ideas for how this rebuilt engine makes only 100 psi of compression?  The valves are sealed perfectly and a new head gasket was used (of course).

[Rosinante]

We believe this bike is a 1972 CB750.

[Don R]

 Some cylinders may be taller than others, I've read that Honda allowed the compression to lower at times without officially announcing a change. I had one that I was sure hadn't been apart before that was lower than the one before it.
 That said, a compression test on a small cylinder may read low results with a large automotive gauge. Modern thicker gaskets also affect the deck height and compression.

[Rosinante]

Yes, a thicker cylinder base gasket would lower compression, but not by 1/3, as here.  I think.  I suspect this cylinder set is from a later bike, such as 77/78.

[pjlogue]

If the engine was just rebuilt it takes a while for rings to seat in the honed cylinders.  Also, use a compression tester that has the Schrader valve close to the cylinder, not close to the Gauge.

-P.

[calj737]

There are two gaskets in play here; the base gasket and the head gasket. Using thick options of each will increase the cylinder volume and lower the compression. It’s also possible that the used cylinder block was worn and needed a fresh hone at least and possibly new rings/pistons/overbore. Does the motor smoke at all while idling or running? These are indications of cylinder/valve wear and can contribute to your observed issue.

The point about a motorcycle compression tester is very important. Long nosed models typically used for cars register lower compression. I’m not sure I concur about seating the rings being an issue though. Compression should be very close at the start-up as it will be after several hundred miles unless a great deal of machining was done.

[Don R]

  Some guys mill the head a bit to square it up, allow for the fat gasket and help the "now too thin" O rings to seat also.
  If it runs good, I'd just ride it.

[HondaMan]

Yes, there are "taller" cylinders in the late 1974 and some of the 1975 (K5) series of CB750s, most notably those that Honda advertised at the time as getting 50 MPG gas mileage. They were about 1mm taller overall, but few have measured them. Normally these can be milled back if the oil ports in the back of the cylinders are correspondingly milled down again. The modern head gaskets are 0.2mm thicker than Honda's OEM gaskets were: that drops compression a little bit. The cylinder base gaskets in some of the gasket sets are also extra thick in some brands: that lowers it, too. If you add all of them at once you can see the compression (after rings are seated) on the order of 105 PSI if all the 'wrong' parts are added at once.

The cams, too, make a difference. If you use a K4 or K5 cam in the K2's place, it loses some intake duration and thus compression with it.

Try this: squirt a bit of oil into each cylinder (sans sparkplugs) and wait about 5 minutes, then test compression with the electric starter and all sparkplugs out. Open the throttle to at least 1/8 to 1/4 when cranking with the electric start. This will tell you what the collection of parts you have can generate as PSI.

[grcamna2]

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[Rosinante]

Waiting for feedback from the guys working on this bike, about five hours away.  I am always pleased and flattered when Mark weighs in on my question and i want to consolidate my questions to be respectful of his time if he is still watching.  In the meantime.....

I wonder whether we can calculate the compression ratio, or at least estimate it, from data we have.  These cylinders make roughly 100 psi.  Displacement is 736 cc, right?  Atmospheric pressure is roughly 14.7 psi (the bike is in Eugene OR at 430 feet above sea level).  Bear in mind that compression testers register 0 even when atmospheric pressure is 14.7 psi.  Those gauges measure only pressure in excess of 14.7 psi.  Not sure exactly how that fits into the equation but I think it does.  For example, at 9:1 the piston is reducing 736 cc down to 81.77 cc [736/9].  But we can't just figure compression will be 14.7 * 9 since that would be only 132.3 psi.  It only gets correct when we add the missing 14.7, making 147 psi.

Okay.  We have a motor that should make 147 psi and is only making 100.  Now....100 / 14.7 = 6.8 but this is going to overstate the compression ratio because we have not accounted for the 14.7 lbs of atmospheric pressure our instrument is not noticing.  Any math experts here?  I think we should be able to calculate the area inside the cylinder at TDC, and also the actual compression ratio, using numbers we already have.  Ironically, my business card used to say "Statistician" on it.  Long story.

But at the end of the day I think the actual CR for this bike is close to 6:1.  If this were my bike, and we find out the cylinder head deck height is 1.5mm too high, and compression ratio is around 6:1, I'd tear the thing down again and take the cylinders to a machine shop.  there is a huge performance difference between a 6:1 engine and a 9:1 engine.

Discuss.

[Scootch]

I'm not a math wizz so I would use a compression ratio calculator. Here's one...
https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator

[MauiK3]

Wouldn’t compression be just the change from zero at whatever altitude to the compressed state?
It’s all relative.
Sounds like you have a set of parts stacking up to a low compression engine.
What about your pistons?
Right height, distance from top of piston to wrist pin?
Rod length?

[ekpent]

All that math etc. is fine if the gauge/tester is actually getting a correct reading. If it runs good and doesn't smoke and leak oil ride it.

[HondaMan]

Some things to remember about the engineered "compression ratio" or the numbers stated in the Honda manuals are:
The 'compression ratio' is determined in large part by the cam in use, and the size of the combustion chamber, minus the height of the piston dome. In the K0-K6 engine, at least this last item is a 0.0. The ratio is then:
{(swept volume of the bore)+(chamber volume at TDC)} / (chamber volume at TDC) X (intake valve open duration of intake stroke as a decimal)
The K0-K5 combustion chamber is nominally 22.6cc...(and I won't enter the F0/1/2/3 head's fray for the moment)...
And, at cranking speeds, since the intake air has no kinetic energy to increase the intake charge, I'll use 1.0 for the (intake open duration) - this number gets a little bigger as the engine speed increases and the intake charge flow's speed rises because the intake valve opens before TDC and close ABDC.
This then works out as:
[(184.0cc)+21.0cc)} / (22.6.0cc) X 1.0 = 9.07:1, or with normal mechanical losses, about 9:1. Those with the temerity to calc up the increased intake charge with intake speeds will find it to be around 9.2:1 at 7500 RPM.

The "compression tester" gages are usually built to just measure PSI, so multiply your [sea level?] air pressure by the 9.07 (if you wish to be precise) and that makes the PSI gage read 133.3 PSI for a perfect engine. That's a pretty high reading for these engines, in my experience: they are usually more like 110-120 PSI in real life (or 105-110 up here in Denver).

And, cams that open the intake valve sooner than the 5 degrees these (are stated to) do and close later than the 5 degrees (more than that when the cam chain wears) will increase the effective compression more, but will usually read less with a compression tester because of the slow static-air speeds.

[Rosinante]

Oh Geez.  It's gonna take me a good long time [ when I am sober ] to process all that, but it already makes sense. 

As an aside, I have an air cooled performance motor that benefits from some of these principles.  9.3:1 pistons and long narrow intake runners with a long-duration cam.  This means that intake air velocity is high, and the cylinders have the opportunity to get PACKED with intake air before the intake valve snaps shut.  Gobs of torque.  For this motor, peak torque and peak horsepower are the same.  Meaning that those curves cross at around 5250.  Other similar motors for this normally race-application care use fatter intake runners and cams with more lift than duration, resulting in high-rpm power.  So....cylinder "packing" comparisons between these motor configurations would unfair to one or the other of the motors.  For mine, the sweet spot is 5-6K.  For the others....more like 7-8K.

[PeWe]

85.0mm is std height for CB750 cylinders?
At least those I have measured. K1, K6.

[dave500]

you did check the compression on a hot motor with the throttle held wide open?the static compression ratio is calculated by swept bore and combustion chamber volume,dynamic compression is calculated once both valves are closed on the compression stroke,hotter cams give less,thats why blocks or heads get milled or domed pistons used to get it back up after that loss,long hose testers are fine so long as a schrader valve is at the lower end,might just take a couple more pumps to fill it.

[HondaMan]

85.0mm is std height for CB750 cylinders?
At least those I have measured. K1, K6.

That sounds right, Per. If you find some late K4 and/or early K5, see if you can locate one of the 86mm tall ones over there? I have no idea if Honda also marketed that change in Europe, as the gas shortage was purely American, due to politics here in those days, so Honda was trying to put on their "good guy image" over here.
I had some (2) in engines about 4 & 5 years ago that came to me for rebuild. The difference is down at the bottom, which makes the "736cc" emblem seem more pronounced when you look at it. It isn't, but just sorta looks that way.

[HondaMan]

Oh Geez.  It's gonna take me a good long time [ when I am sober ] to process all that, but it already makes sense. 

As an aside, I have an air cooled performance motor that benefits from some of these principles.  9.3:1 pistons and long narrow intake runners with a long-duration cam.  This means that intake air velocity is high, and the cylinders have the opportunity to get PACKED with intake air before the intake valve snaps shut.  Gobs of torque.  For this motor, peak torque and peak horsepower are the same.  Meaning that those curves cross at around 5250.  Other similar motors for this normally race-application care use fatter intake runners and cams with more lift than duration, resulting in high-rpm power.  So....cylinder "packing" comparisons between these motor configurations would unfair to one or the other of the motors.  For mine, the sweet spot is 5-6K.  For the others....more like 7-8K.

Yep, that seemed to be Honda's intent also, on the CB500/550 design. They wanted a wide, flat powerband to make a smoother-clutching roadster with a lower seat height (and weight) that could tour with the 750, in the hopes of selling them to those with inseams shorter than 30" (For the K0, in the Chicago area, shops were required to measure customer's pants inseam and Honda would not honor the Warranty unless this was 30"+, or 29"+ when I was in Peoria). The first time I was blessed to throw my leg over a freshly-assembled (Green) CB500 and buzz it around the block for the test ride, I called my brother (who hated me for my 750, but has a 28 inch inseam) to tell him about it, and he had me buy the other CB500 that was in the other crate, not yet assembled (it was Candy Gold). He rode that bike over 50k miles, 1/4 of that next to my 750, many times at 3-digit speeds, before giving it to our littlest brother. That bike has the most linear powerband I've ever ridden. This is due largely to the long intake runners and 6500 RPM torque peak that feels linear to the 8000 RPM HP peak. The 550 comes 'on' a little lower in RPM.

[newday777]

Oh Geez.  It's gonna take me a good long time [ when I am sober ] to process all that, but it already makes sense. 

As an aside, I have an air cooled performance motor that benefits from some of these principles.  9.3:1 pistons and long narrow intake runners with a long-duration cam.  This means that intake air velocity is high, and the cylinders have the opportunity to get PACKED with intake air before the intake valve snaps shut.  Gobs of torque.  For this motor, peak torque and peak horsepower are the same.  Meaning that those curves cross at around 5250.  Other similar motors for this normally race-application care use fatter intake runners and cams with more lift than duration, resulting in high-rpm power.  So....cylinder "packing" comparisons between these motor configurations would unfair to one or the other of the motors.  For mine, the sweet spot is 5-6K.  For the others....more like 7-8K.

Yep, that seemed to be Honda's intent also, on the CB500/550 design. They wanted a wide, flat powerband to make a smoother-clutching roadster with a lower seat height (and weight) that could tour with the 750, in the hopes of selling them to those with inseams shorter than 30" (For the K0, in the Chicago area, shops were required to measure customer's pants inseam and Honda would not honor the Warranty unless this was 30"+, or 29"+ when I was in Peoria). The first time I was blessed to throw my leg over a freshly-assembled (Green) CB500 and buzz it around the block for the test ride, I called my brother (who hated me for my 750, but has a 28 inch inseam) to tell him about it, and he had me buy the other CB500 that was in the other crate, not yet assembled (it was Candy Gold). He rode that bike over 50k miles, 1/4 of that next to my 750, many times at 3-digit speeds, before giving it to our littlest brother. That bike has the most linear powerband I've ever ridden. This is due largely to the long intake runners and 6500 RPM torque peak that feels linear to the 8000 RPM HP peak. The 550 comes 'on' a little lower in RPM.

My friend Steve that has Honda Restoration (NOS parts seller) has this K0 available he is selling. I derusted the tank for him last fall, it needs the carbs cleaned still as it's been setting in his museum since 2014. His riding days are over.

https://www.hondarestoration.com/index.asp

[HondaMan]

My friend Steve that has Honda Restoration (NOS parts seller) has this K0 available he is selling. I derusted the tank for him last fall, it needs the carbs cleaned still as it's been setting in his museum since 2014. His riding days are over.

https://www.hondarestoration.com/index.asp

Hey, I've bought parts from there before!
That 500 looks just like the one I had. When I had my shop a customer with it came in one day to tell us his wife just had their baby, and he had to sell his then-2-year-old 500 to pay the hospital. I bought it from him and rode it while I repaired the crash damage to my 750 that winter, then sold it the next Spring. Great bike!

[newday777]

This one has under 10,000 original miles.