Compression ratio.

I intend to check the compression ratio on my Commando sometime this Winter, as I believe it may be less than 8.5:1, and would like to put it at 9.5:1 to 10:1. (it's a MK3 with an RH4 head.

Last time I did it (20 or so years ago on my former Norton), I did it as per Paul Dunstalls "Norton tuning," with the engine built and with with the spark plug hole set in a vertical position with the engine at TDC, and adding fluid through a burette until it was half way up the plug hole. Then, I used water as the fluid medium and rubbed some grease in between the piston and bore to stop the loss of the fluid down the bore, although I note that Paul Dunstall advocates a "light oil."

Reason for the post is to ask for any advice as to if this is still a suitable method or are there any better ways of doing it?

why do you want to increase the CR? unless you have access to high octane gas, and intend on driving it infrequently i'd give a lot of thought to it. While it certainly can be done - the higher the CR the more temperamental it will become. there's a reason why all those 7.5 CR BSA'a and triumphs (and 8.5 nortons too) kept chugging along....

mikegray660 wrote

why do you want to increase the CR?

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Answer, because I want more power :twisted: :twisted:

As I understand it, if the C/R is kept below 10:1 there shouldn't be much of a problem with pre ignition.

mikegray660 wrote;

the higher the CR the more temperamental it will become.

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I like lots and lots of pain. I run two two Brit bikes :lol: and just have to keep "fine tuning."

I run a 750 up to that 10-1 and have no problem with fuel. But taking the head off and putting on a sheet of glass with some grease would be your best way to CC the head. If your pistons are above or below at TDC that could just be done with math. There's no way to seal the rings without making a mess of the job. The last thing you want is gob of oil on top of your piston because running the bike like that would have it burning into the ring lands. Be advised that this can be a good way to power but at the cost of vibes so the bike will feel much more powerful but might not go that much quicker. Best to keep the modifications reversible and do some quarter mile times before and after, the half second or second you gain may have you uncomfortable on a long run.

I asked same question a while back for similar reasons. See quote below but Be Aware that if cam profile is not aggressive enough with enough over lapped valve opening and late intake closing - then proneness to detonation goes way up as many found out the hard way thinking they'd detune a Combat by merely installing a standard profile cam > bing bang boom

I suspect going up to 10 in MK3 would give a bit nicer crisper response to throttle and add about 500 more rpm to the sense of increased pull then just sorta settle down as usual making more noise than 'gathering' speed. I suspect sensitive pilots require at least a 5% difference to detect in seat of pants meter, most need 10% to notice.

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

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norbsa48503 wrote:

But taking the head off and putting on a sheet of glass with some grease would be your best way to CC the head. If your pistons are above or below at TDC that could just be done with math.

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I am aware of this method. If the pistons are flush or just below the top of the barrel it will be quite straightforward (I can't remember for sure), otherwise I suppose I'll have to get the calculator out! My pistons do have cutouts for in and ex valves so I will have to use the grease and glass method for the cutouts if the pistons are above the barrels.

norbsa48503 wrote;

but at the cost of vibes so the bike will feel much more powerful but might not go that much quicker

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I was hoping that the isolastics would continue to work their magic regards the vibes, but a good point to bear in mind. I had my T160 bored and stroked to 930cc with high comp pistons. Previously it was a 750 with low comp pistons, and it went from absolutely smooth with no vibes (mine also has rubber mounted footrests) to noticably vibrating at certain points in the rev range, but it has much more torque and power now.

norbsa48503 wrote;

Best to keep the modifications reversible

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I noticed that Jim Schmidt offers 0.020" copper head gaskets which give approxamately 0.5:1 increase in CR.

hobot wrote;

Be Aware that if cam profile is not aggressive enough with enough over lapped valve opening and late intake closing - then proneness to detonation goes way up

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I should be OK as I am running a PW3 Thanks for the additional info.

Reggie said:

mikegray660 wrote

why do you want to increase the CR?

Click to expand...

Answer, because I want more power :twisted: :twisted:

As I understand it, if the C/R is kept below 10:1 there shouldn't be much of a problem with pre ignition.

mikegray660 wrote;

the higher the CR the more temperamental it will become.

Click to expand...

I like lots and lots of pain. I run two two Brit bikes :lol: and just have to keep "fine tuning."

Click to expand...

Reggie
just 2 B an a-hole - the best way to make your bike quicker and remain reliable is to lose weight - the bike and more importantly the rider - i don't have the exact figure (i'm sure someone will post it) but i believe its something like every 7-8 pound shed is like gaining one hp - how much hp can you can ? i lost 30 lbs, felt better, can ride longer - and the bike "seems" faster - of course puching it though corners with a smaller ass helps too

Reggie, no wonder you feel sluggish, a PW3 needs some hi-ish CR to work as designed. It should be fine on pump gas too just not snap throttle high gear lugging but with smooth throttle about as fast as could take it anyway, have happy at it. May even find more zip going with lower octane in your mild combo as the faster burn gives more torque pressure where it matters 11' ATDC most.
Exhaust sounds will be nicer more distinct too, can't ignore the Norton purring and barking pleasure.

Rule of thumb is CR 1/10'th the octane rating plus .50 CR - if Al head and mixture and gearing proper along with cam valve events. Yours sounds ripe for it.

4.5 lb mass lost is said to be equivalent to 1 hp power gain for acceleration.
After the stands gone and fenders and side covers and battery, costs more to lose mass than some simple engine mods give back in hp equivalent of riding j-bird naked. Sans boots, leathers gloves helmet and full bladder can be felt in pull.

There are online calculators for hemi's but almost none that can directly account for the .050" stick em up 10:1 Combat pistons. i'd use plain engine oil filled by syringe measure fast as practical, then suck out most with a tube on syringe, then flush with diesel chased by a light oil spray before sump drain and start up.

W/o big bux efforts just to get a bit more crispness and bit longer pull in a gear is a good success and worth pursuing in the long run.

mikegray660 said:

just 2 B an a-hole - the best way to make your bike quicker and remain reliable is to lose weight - the bike and more importantly the rider - i don't have the exact figure (i'm sure someone will post it) but i believe its something like every 7-8 pound shed is like gaining one hp - how much hp can you can ? i lost 30 lbs, felt better, can ride longer - and the bike "seems" faster - of course puching it though corners with a smaller ass helps too

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Body weight helps in starting though :lol:

Jean

Wow, i have been sick and lost over 20 pounds recently.

So my bike has gained four horsepower?

Sure does NOT feel like it when I ride it.

There is a gray area of understanding between compression ratio and compression. Compression ratio is determined by comparing the volume of the cylinder above the piston at bottom dead center and top dead center. If the volume at the top is 1/10 of the volume at the bottom, you have a 10:1 ratio. Then the volumetric efficiency (breathing) of the motor comes into play. Many factors can change actual compression within the same ratio; valve timing and overlap, rod length, port size, intake length, exhaust length and volume, ignition timing, cam duration and lift and rpm. A more descriptive figure than compression is Brake Mean Effective Pressure, which is the downward force on the top of the piston when compression is at its highest. To use an extreme example, race motors with 15:1 ratios aren't the same in every other way as street motors running 10:1. Almost every other factor mentioned is different, also. Ideally, an efficient 15:1 motor should be able to produce a BMEP of 225 psi (a 15:1 compression of atmospheric air pressure which is 15 psi more or less at sea level) but 200 psi is about the highest achievable BMEP for a normally aspirated motor. Super- and turbocharging can increase this by as much as the motor can mechanically withstand, but without force-feeding, 200psi is about it. So you see, increasing horsepower is a little more complicated than shaving the head, slapping in a thinner head gasket and reducing the TDC combustion chamber volume to as near zero as possible.

Book em Danno!
I only learned to talk this level gear head since getting a Combat about a decade ago.
Now I'm shooting for 17:1 effective CR on 91 octane farm fuel + 116 octane creek water in a special. To test if I've understood correctly, hope to have heck of a trials bike using a drag race only cam in 920 cc.

Coke build up also lowers detonation threshold, water sprayed in carb can help clean that out, similar to boiling pistons in water, turns to grey dust that blows out.
Ethanol ups octane but leans mixture over plane gasoline.