commando compression (2012)

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How much compression is normal on these things? I never looked and notice lots of different values around.

I want to do a compression test and see if I am somewhere normal for compression. My bike doesnt smoke or anything but I am just curious...

71 750

Cheers,

Ben
 
iceteanolemon said:
How much compression is normal on these things? I never looked and notice lots of different values around.

I want to do a compression test and see if I am somewhere normal for compression. My bike doesnt smoke or anything but I am just curious...

71 750

Cheers,

Ben

The reading you get on a compression gauge can vary quite a bit both from gauge to gauge and engine to engine. The cam timing will change it and I seldom see two barrels that are the same height.
That said- 150 psi would be in the area- give or take 10 psi. AT WIDE OPEN THROTTLE.
A leakdown test is better unless you are checking cam timing. 10% leakdown is good. Jim
 
I did a comp test on mine a few years ago and from memory the numbers were disappointing, below 100 with the kickstart then went up to around 145 or so with the E start. That was with the thick copper headgasket, should be a little higher now with the thinner composite gasket.

Glen
 
Such a sliding scale on engine configuration, mood or state of charge in the starter, air and humidity and phase of the moon at sea level or hurrance eyeball.
Here's a down and dirty rule of thumb to apply to Norton twins in general.

results are typically between 15 and 20 × static compression ratio (9:1 = 135 to 180 psi) 5% difference between cylinders is OK. 100 lbs on a fist kick and a cold motor means compression sealing not your problem.



I found an article in an old Classic Bike magazine on
testing compression which may be of some general assistance.
Here's the gist of it:~
In the absence of manufacturers quoted figures 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.

Dave & Marlene wrote:

On your compression ratio question, 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%

Maximum usable CR depends among other things, compactness of
combustion chamber, speed of burn, more compact, faster burn, can
use more CR.

How much turbulence, swirl in combustion chamber, more swirl or
more compact can use more CR.

Cam timing, the later in the cycle that the intake valve closes
the less cylinder pressure & the more CR that can be used.

A good design rule for small bore engines, around 80mm or less, is
that the CR can be one tenth of the fuel octane number plus 0.5
ratio if everything is optimum. Example, 92 octane fuel = 9.2 CR
plus 0.5 CR = 9.7 to 1 CR.

I am referencing the book "The Design of Competition Engines" by
Philip H. Smith, a well known racing engine designer.

Regards,

Dave Russell
 
"Cam timing, the later in the cycle that the intake valve closes
the less cylinder pressure & the more CR that can be used."

Late intake valve closing will produce less cranking compression however it may not produce lower running pressure. The idea of closing the intake valve later is to take advantage of the inertia of the air that will continue filling the cylinder after BDC. If that is working you may end up with higher cylinder pressure. [just like a supercharger]
 
For hope of kick starting on 87 octane I hurt my brain on cam selection to get 7.5 dynamic CR in 10.5 static CR Peel which could see up to 17:1 effective CR.

I really truly await a saga report on attempts to perform useful leak down test on an intact Commando.

In practice street machine low compression d/t leaks are a non issue until smoke out the pipes or the case breather till bad enough not to start or run.
 
My rebuilt stock 850 Mk1 Commando consistently gave 170 psi on both cylinders.
Adiabatic heating is good for quite a few psi....

It is worth pointing out that compression testing should always be done
with the ENGINE HOT, and the THROTTLE WIDE OPEN.

Cold testing will consistently give inconsistent numbers...
 
lots of good tips here! If I get a chance i nthe near future I am gonna do a compression test and see what turns up. Good or bad Ill post up my numbers :)
 
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?
 
Dry and cold and maybe timid kicker - not that bad, now warm up then put a bit of oil in jugs, giver WOT and romp on her a few quick kicks and then report back.
 
hobot said:
Dry and cold and maybe timid kicker - not that bad, now warm up then put a bit of oil in jugs, giver WOT and romp on her a few quick kicks and then report back.


Oh crap I forgot about this post!! I need to remember this stuff so I can get a post on here!

Im intentionally set the compression tester on top of my tool box to remond me but I guess I have selective memory!
 
hobot said:
Dry and cold and maybe timid kicker - not that bad, now warm up then put a bit of oil in jugs, giver WOT and romp on her a few quick kicks and then report back.

Warm it up and wide open yes. If you put oil in the chamber you will get a false high reading. Jim
 
Ugh sorry, then put the oil in before warm up: )
Jim have you ever heard of Commando success with a leak down test set up. I've got a gauge set I learned I'll never need again.
 
Does it go without saying that the opposite plug is removed while doing comp testing?
 
hehehe, not anymore.
I'm of opinion its about worthless to bother testing as will rarely nail down the problem to shop for ahead of time, so basically if not running right must open up for a look see then renew everything or just fix the obvious and put back and fix the more hidden issues only discovered after the new run in. Please correct me if my logic is off on this.
 
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?

Sounds like your top end is flopped out. Now just figure out whether it's rings or valves.
 
Sounds like your top end is flopped out. Now just figure out whether it's rings or valves.

Naw naw naw naw Nah, how's my Commando obnoxious logic looking now....
 
hobot said:
Ugh sorry, then put the oil in before warm up: )
Jim have you ever heard of Commando success with a leak down test set up. I've got a gauge set I learned I'll never need again.

Yes- I use a leakdown tester regularly. It tells you a whole lot more than a compression tester can. When my motor was new it tested at around 8% leakage with conventional rings. Last time I tested it it was over 15% but still runs strong. It is time for a refresh.

A fresh motor with Total seal rings will show around 5% or a little less.

I don't even bother with a compression gauge. Too many variables that affect the reading like cam timing and cranking speed or valve guides leaking oil into the cylinder or a little extra fuel from the carb. Jim

Hobot, what issues have you had with a leakdown test?
 
concours said:
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?

Sounds like your top end is flopped out. Now just figure out whether it's rings or valves.


Harbor Freight is not known for great stuff. How fancy can it be? I'd try a decent gauge before blaming the bike. The last two times I had to return pure junk to Harbor Freight.
 
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