are kenig hi comp.pistons any good ??

[brewsky]

My late (66) CA measures 150 psi, stock engine as far as I can tell.

Silver's Motor book says "8.3:1 (early) CA, 8.5:1 (late) CA, using CB pistons"...pg 15

Also under C/CA modifications section: "CB/CL pistons will raise the compression from 8.2:1 to 8.5:1"...pg 54

Also says "normal measured compression...approx. 150-175psi" (not specefied per model).

What it doesn't say is what is late CA with stock CA pistons?

It also doesn't say which CB/CL pistons will raise compression.... early, medium, late etc.?

[teazer]

There is still a lot of fog about which pistons are which. When I started to get involved with CBs many years ago, I was told that the very early pistons were 10:1 and those were used in the race kits. later on I was told different stories and I'm not as sure as LM as to what is what.

Somewhere I have a race kit 250 piston, but without a box or label, it doesn't add much to the discussion. Mt race bikes use specially modified 64mm pistons, so I have not learned much more until LM posted those details recently.

Those pressure numbers you quoted sound very high, even for a mild cam like the Dream. 150psi is about 10:1 CR and 175 is a number I have rarely seen on a 4 stroke.

Atmospheric pressure is around 15 psi, so compress that at 10:1 and you get 150psi. If the valves are open longer, the number tends to be lower at the low revs when it's kicked over but may be higher than that at running speeds with ram effects etc.

120-140 would sound about right for a CB and 100-120 for a dream, but it has been a really long time since I checked either. Anyone else checked cranking pressure recently?

[LOUD MOUSE]

I just discussed this with Bill.
This info was/is from 1997 (last revision) and data may be for using the early CB77 piston used in the CB77 but NEVER the CL77.
Bill was as surprised as I when I told him/showed him the difference when he visited a while back.
It's a simple thing to measure without assembling and running an engine.
Install a C/CA piston on one end of a wrist pin and a CB/CL on the other and ya can measure the heights.
Then with rings installed deck the pistons at top and cc the cylinders. (I used water as I was not going to have the parts wet for long)
I then assembled a CB/CL and C/CA head and cc'ed the same way.
Do this and ya will find the C/CA77 piston actually does cause a higher compression than the CB/CL.
I don't have a clue how much higher but for the $$$$ ya do gain a bit more power. ..........lm

My late (66) CA measures 150 psi, stock engine as far as I can tell.

Silver's Motor book says "8.3:1 (early) CA, 8.5:1 (late) CA, using CB pistons"...pg 15

Also under C/CA modifications section: "CB/CL pistons will raise the compression from 8.2:1 to 8.5:1"...pg 54

Also says "normal measured compression...approx. 150-175psi" (not specefied per model).

What it doesn't say is what is late CA with stock CA pistons?

It also doesn't say which CB/CL pistons will raise compression.... early, medium, late etc.?

[Spargett]

I recently tested my Superhawk compression. Just shy of 155psi for each. Left was a hair lower. CB pistons .25 over.

There is still a lot of fog about which pistons are which. When I started to get involved with CBs many years ago, I was told that the very early pistons were 10:1 and those were used in the race kits. later on I was told different stories and I'm not as sure as LM as to what is what.

Somewhere I have a race kit 250 piston, but without a box or label, it doesn't add much to the discussion. Mt race bikes use specially modified 64mm pistons, so I have not learned much more until LM posted those details recently.

Those pressure numbers you quoted sound very high, even for a mild cam like the Dream. 150psi is about 10:1 CR and 175 is a number I have rarely seen on a 4 stroke.

Atmospheric pressure is around 15 psi, so compress that at 10:1 and you get 150psi. If the valves are open longer, the number tends to be lower at the low revs when it's kicked over but may be higher than that at running speeds with ram effects etc.

120-140 would sound about right for a CB and 100-120 for a dream, but it has been a really long time since I checked either. Anyone else checked cranking pressure recently?

[busaken]

There is still a lot of fog about which pistons are which. When I started to get involved with CBs many years ago, I was told that the very early pistons were 10:1 and those were used in the race kits. later on I was told different stories and I'm not as sure as LM as to what is what.

Somewhere I have a race kit 250 piston, but without a box or label, it doesn't add much to the discussion. Mt race bikes use specially modified 64mm pistons, so I have not learned much more until LM posted those details recently.

Those pressure numbers you quoted sound very high, even for a mild cam like the Dream. 150psi is about 10:1 CR and 175 is a number I have rarely seen on a 4 stroke.

Atmospheric pressure is around 15 psi, so compress that at 10:1 and you get 150psi. If the valves are open longer, the number tends to be lower at the low revs when it's kicked over but may be higher than that at running speeds with ram effects etc.

120-140 would sound about right for a CB and 100-120 for a dream, but it has been a really long time since I checked either.

Anyone else checked cranking pressure recently?

after reading this post i decided to do a compression test on my 63 dream. since i rebuilt the engine in the winter, never did a compression test till now. readings is @150 psi on both cylinders mileage is @ 210 miles since rebuild. did this twice. engine cold. havent tried compression test with engine warm. mac tools compression tester. not bad for compression. seems to run great!

65 ca95 readings @ 145 psi both cylinders rebuilt engine winter of 09.

[Wilf]

Unfortunately there is no simple method to derive cylinder pressure (what you would expect to measure on a compression gauge) from the given compression ratio of an engine.

The suggestion that a 10:1 compression ratio with an atmospheric pressure of 15 psi should result in 150 psi cylinder pressure just doesn't work, and I think that is why the term 'normal measured compression' is used in owner manuals as a guide to estimating the state of engine wear.

Wilf

[teazer]

Actually Wilf it does. :-)

it's really simple - at a simple level. Compress a gas at STP into a space 1/10 the size and the pressure will be 10 time higher. So 14.7 PSI (1 bar) compressed at 10:1 will raise the pressure to 147psi all things being equal.

So what else comes into play?

Cylinder filling and that depends on ram effects (resonance) and cam timing. let's take teh latter first. If the cams close the intake valve after BDC, then in theory there is less time to compress teh gas and the effective compression ratio will be lower. that's how 2 strokes are ofter erroneously measured - from port closing.

So at low speed we would expect to see cylinder pressures lower than the theoretical Static compression.

So that brings us to the other major variable ( an there are others) and that's effective filling from resonance and inertia.

When the gas starts to flow into the cylinder, it has velocity and mass (albeit low) and that means it has interia. At all engine speeds cylinder filling is either less than or greater than 100% so teh effective compression is never quite what you expect it to be.

In the case on these Honda motors, the implication is that when turned over at around 500-1000 rpm, they manage better than 100% VE to fill the cylinder more than expected.

So yes, it's not as simple as 1 bar x CR=Compression pressure, but it's a reasonable place to start from and that was my previous point.