commando compression (2012)

Hobot · Aug 11, 2012

Snatched off British Iron list in '05 - an article in an old Classic Bike magazine on
testing compression. Here more interesting Rule of thumb stuff.
Increasing CR has diminishing power returns (inverse gain) Example;
Increasing from 5 to 6 to 1 gives about 3.5% power increase.
>From 6-7 = 2.5%
>From 7-8 gives 1.4%
>From 8-9 gives 0.7 %
>From 9-10 gives 0.6%
>From 10-11 gives 0.5%

Not that much unless bumping up the bump stick that can use more CR.

iceteanolemon · Aug 11, 2012

hobot said:
Extra good each side is so close too as differences in combustion spikes can give some horizontal imbalance that might be felt. I've had that happen when carb bowl or gasket fell away, but was pretty subtle if happens on the fly. So what cam and compression ratio, gasket, base plate, to judge the sealing goodness? I've heard Combat can be kicked into the 180's. Yours seems rather good by below.

expect the following pressure from different Compression Ratios:
6:1 88-118;
7:1 103-147;
8:1 125-162;
9:1 140-176;
10:1 162-191;
11:0 176-206.

Yeha I was looking at that graph, interesting stuff to see the ration drastically diminish as the CR rises. getxs to be like squeezing water from a rock at a point.

I didnt know what the heck to expect on my bike but all numbers aside I am really happy the numbers are close for each side.... thanks for the contribution for sure!!!

Bernhard · Aug 17, 2012

Rohan said:
Bernhard said:

They are both within acceptable parameters.
nothing to worry your little head about really

Click to expand...

Within "acceptable parameters". What does this statement mean ?

Parameters;

A measurable quantity, e.g. temperature, that determines the result of a scientific experiment and can be altered to vary the result.

Hobot · Aug 17, 2012

It means same thing as medical phrase, "within normal limits", WNL, which means not obviously a problem in what was examined to some reasonable level. A number of terms can have exactly opposite meanings depending on the rest of the sentence. To Cleave or cleavage for instance, means to come together or split apart, often just depending on ones point of view.

mikegray660 · Aug 17, 2012

jsouthard said:
I just bought one of those fancy compression testers from harborb freight. I tested with the bikes cold but found 70 psi on the left cylinder and 50 psi on the right. Throttle wide open.

Why am I seeing such a difference from the 120-150 range others have post?

maybe because its a fancy compression tester from harbor frieght :roll:

drones76 · Aug 17, 2012

hobot said:
It means same thing as medical phrase, "within normal limits", WNL, which means not obviously a problem in what was examined to some reasonable level. A number of terms can have exactly opposite meanings depending on the rest of the sentence. To Cleave or cleavage for instance, means to come together or split apart, often just depending on ones point of view.

if this a new compression tester, I gotta get me one, or two....or four.

kraakevik · Aug 17, 2012

I'd be very skeptical of those readings if the bike runs at all. I had a trusted Sears compression tester that fooled me into removing a perfectly sound head with 30 psi readings on each side--I then tested my Combat engine that was returning 50 mpg and went like a scalded cat--the readings were even lower

Tim Kraakevik
kraakevik@voyager.net

Norton-Villier · Aug 17, 2012

Sorry debby and sierrascafe530, Boys will be boys

Phil

norvillnh · Mar 8, 2020

75 Commando with just under 87K on the top end. FWIW, 185 pounds both sides, throttle wide open. Since the top end rebuild, at 24K I have been useing 20-50 full synthetic after break in.

MexicoMike · Mar 9, 2020

Yeah - cams make a huge difference re compression PSI! We built 12.5:1 race motors that had lower compression gauge numbers than a stock 9:1 compression motor, all due to cam profile.

Also, as noted, a kickstarter is a poor method to spin a motor for a compression check since there can be variation every time you kick - weight/kick speed/position of the piston when the kick is started, etc. As mentioned, a leak down is a much better test though it takes more time/trouble/equipment.

commando compression (2012)

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