Raising CR on MK2a 850

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I believe that the squish causes better mixing in the combustion chamber which improves power output, however in two stroke motors the squish area is where the signs of detonation are usually seen first, albeit on the exhaust side to the head. I've seen the boat shaped combustion chamber thing done to a Harley head, however I think it might shroud the valves. The best result I had with a 650 Triumph engine was by fitting standard flat top pistons from a 350 BSA by machining the sides of the crowns to closely fit the head, as in a two valve Jawa speedway motor. I think the 12 to 1 comp. Triumph pistons are an abomination. They are always coked up on the side of the crown away from the spark plug. You will note that when a piston gets burned by running the mixture too lean, in most motors the damage occurs under the spark plug. I think that might be unusual in a Commando engine.
A friend owns a 350 Manx which was raced in Europe by Jack Ahern. It has a bit of developmental stuff in it, and the head has a second squish band which has been welded up to remove it.
 
worntorn said:
Rohan, the 62 650SS ran the 1/4 mile in 14.5 seconds @92mph whereas the 1969 750 Commando ran it in 12.69 @103mph (Norman White later did 12.3?)
This is more difference than just 100cc s ought to give. Compression and cams were virtually the same.
The heads are very similar other than the addition of a squish band.
Click to expand...

Unfortunately, we are comparing apples and oranges here.
The Commando was geared down with a 19t gearbox sprocket, where the 650 was geared for normal riding.
If they are geared the same, the difference is nowhere near as apparent.

We diverge a little here - although gearing is just as applicable to an 850...
 
Dances with Shrapnel said:
Squish band is often times referred to as quench area. The "squish band" helps inhibit pinging. There's a point of greatly dimishing returns once you get into a few tens of thousandth clearance.
Click to expand...

The squish area can't have the piston too tight to the head ? - carbon formation could reduce the clearance to less than zero - and mention was made somewhere recently about the piston and conrod growing slightly, both with temperature and g-forces. Banging metal things together doesn't work for long....
 
Eddie, how much squish space are you measuring on the stock 850?
I find info claiming squish bands begin to become effective as wide as nearly 4mm, but do more at around 1.5 mm
This could account for the fairly noticeable gain felt by just increasing 850 compression half a point. According to Hobots chart, this only gives about .75 hp gain, yet I felt it was much greater than that, sespecially in midrange which is where the squish really helps.[/quote]
Even modern family hatchbacks run squish's of around 0.060. I don't know how big you can go for it still to work properly, but I do not believe a squish works properly at 4mm, which is what mine currently is.
I was taught that for a squish to work properly, combustion does not take place in the squish band. When you strip a motor with a good squish, the squish band is clean. HRC used to set up race bikes with ZERO cold clearance! I'm not suggesting we do that! I have run squish bands in racing Triumph twins (one running a Norton crank) between 0.040 and 0.060, which have run 'clean'. Below 0.040 I saw piston to head interference, above 0.060 I saw evidence of combustion.
I've just come from the shed, I'm thinking I could use 10.25:1 Omega pistons to get a squish of circa 0.050, then machine a dish into the crown to get the CR down to my target of 9.5:1 ish... That might be b*llocks though!!
Rgds,
FE.
 
Fast Eddie said:
HRC used to set up race bikes with ZERO cold clearance! I'm not suggesting we do that! I have run squish bands in racing Triumph twins (one running a Norton crank) between 0.040 and 0.060, which have run 'clean'. Below 0.040 I saw piston to head interference, above 0.060 I saw evidence of combustion.
Click to expand...

How tight you can set your squish clearance is a function of:

alloy versus cast iron cylinder barrels (thermal coefficients of expansion),
hot versus cold clearance,
whether you are using steel, aluminum or Ti rods and rod loading (ie rpm and subsequent stretch of rods and thermal coefficients of expansion) and
if you are using a flat squish area or squish around a piston dome (how much piston rock fore and aft and piston slide left and right)
 
Dances with Shrapnel said:
Fast Eddie said:
HRC used to set up race bikes with ZERO cold clearance! I'm not suggesting we do that! I have run squish bands in racing Triumph twins (one running a Norton crank) between 0.040 and 0.060, which have run 'clean'. Below 0.040 I saw piston to head interference, above 0.060 I saw evidence of combustion.
Click to expand...

How tight you can set your squish clearance is a function of:

alloy versus cast iron cylinder barrels (thermal coefficients of expansion),
hot versus cold clearance,
whether you are using steel, aluminum or Ti rods and rod loading (ie rpm and subsequent stretch of rods and thermal coefficients of expansion) and
if you are using a flat squish area or squish around a piston dome (how much piston rock fore and aft and piston slide left and right)
Click to expand...
I agree with that Mr Shrapnel, and would only add the element of crankshaft flex. I know from my own experience that I could run a tighter squish with a Nourish crank than a Norton crank.
Love the name by the way! I have a collection of very expensive engine shrapnel myself! None motorcycle racing family friends just don't get it at all!
Rgds,
FE.
 
Rohan said:
worntorn said:
Rohan, the 62 650SS ran the 1/4 mile in 14.5 seconds @92mph whereas the 1969 750 Commando ran it in 12.69 @103mph (Norman White later did 12.3?)
This is more difference than just 100cc s ought to give. Compression and cams were virtually the same.
The heads are very similar other than the addition of a squish band.
Click to expand...

Unfortunately, we are comparing apples and oranges here.
The Commando was geared down with a 19t gearbox sprocket, where the 650 was geared for normal riding.
If they are geared the same, the difference is nowhere near as apparent.

We diverge a little here - although gearing is just as applicable to an 850...
Click to expand...

I am away from home right now, but will look at the 650 test when I get back. I believe it was fitted with a 20 tooth.
My good friend who drag raced Commandos for British Mcs in Vancouver got the best times with a 21 tooth.
One less shift, just let it wind in third.
In any case, the 650SS was not as fast as anything that came after. Good bike tho, I have one and quite like it.

Glen
 
worntorn said:
In any case, the 650SS was not as fast as anything that came after. Good bike tho, I have one and quite like it.
Click to expand...

118 mph recorded in a roadtest....

19t Commando, stock out of the showroom, would have trouble catching that.
Redlined at 107 mph
 
That was with the wind pushing, not quite fair :wink:
If the 650SS is lacking anything, it is midrange punch. The commando combo of bigger bore and use of squish band seems to give a big midrange?
 
acotrel said:
There are three major differences between 750cc Triumph and Norton twins. Triumphs have separate inlet and exhaust cams, Nortons have better heads and longer strokes. Nortons are faster,

maybe , i had the Commando geared for 112 in third , and the Bonnie would run 110 in third , geared for 95 in third , with a ' few mods ' .

The FOURTH differace is a TRIUMPH running GEAR DRIVEN ' Twin Cams ' .

The gear drive giveing accurate cam timing , without the drawbacks of chain drive .

Then theres the lighter followers & pushrods . Turns out the S&W valve Spings were at the behest of JoMoCo , thus TRIUMPH . :D

A Norton had a more EFFICENT head , thus the Commandos 50 - 70 mpg with Mk II Amals , Vs the Bonne's 45 , with equally effeiant Monoblocks .

The Sqish Bands secret is the Sudden / Instantaeneous INCREASE in Comp Ratio , as the effect occurs in proximity to the piston .
This allows a higher C.R. , better ' turbulance '/ mixing , with the old 600 chamber spaceing on the 750 / 828 . :lol: 8)
Click to expand...
 
Dont look a gift Horse in the Mouth : Throw in the Omegas , as is . pushrods 'n all .

If the damn things uncooperative , then throw in a thick gasket , here n there .

Thunderbirds are Go . :? Er , COMMANDOS . :D
 
Raising CR on MK2a 850
 
I know that the crank whips, I've seen it, but do the con rods really stretch??

I'm not saying they don't, I'm just gobsmacked if they do!!

cheers
wakeup
 
Wiith the Bonnie , after being thrashed mercilously by the yokle after being sold , 9 Ex Japper , CV Carbs and run to flat in gears and top )
would only do 14.5 @ 90 odd . Tear Down showed say 25 slack in main bearings , wear everywhere else ( bar valve gear ) and one intact piston ring .

Two Ways the ' take up ' occurs . Assumeing red line , after all , the idea is PERFORMANCE , the Loadings are amplified . -
The Pistons want to keep going UP , tHERE is 12 1/2 slack ( 1/2 ) there , a few at the big ends , and another few at the gudgeons .

So thats around 15 thou closer , with a good few revs on ( only 7500 :( ) NOT Allowing for THERMAL displacement , and ELASTICITY .

The VIRTUE of the ALLOY Rods , is there Elasticity / shock absorbing capeability . They shuck aroud , Exceeding Elastic Limits gets PLASTICITY .
They dont go back , so stay strethed , compressed . or bent . :shock:

Thus in a Drag race nitro V* theyre a service replacement item . Second hand ones there art much use other than as ornaments .

Someone'll say youll get about 20 thou ' stretch ' all combined , with a healthy engine at 10.000 rpm's .
This is why Flat Top piston / head combinations , with chambers , require ' clearance , piston to head - static .
 
We'd be 'gobsmacked' if you've "seen" the crank whip....

There are a few TONS ? of force applied to the rod on its reversal of direction - anything up to 14,000 times per minute.
(2 reversals per stroke).
We'd be surprised if the rod didn't stretch/compress, ever so slightly, every time.
Thats why fatigue failure is called fatigue failure ??
Not that Norton rods are afflicted with this - in fact they have a good record.
D-rod discussion pending...

Not to mention that the rods warm up, so there is some slight expansion there.
Piston rock, etc etc...

There is also that hint that squish bands are ineffective if the clearances are too large.
??

All on the periphery of upping the COMPRESSION RATIO.
 
Matt Spencer said:
Dont look a gift Horse in the Mouth : Throw in the Omegas , as is . pushrods 'n all .

If the damn things uncooperative , then throw in a thick gasket , here n there .

Thunderbirds are Go . :? Er , COMMANDOS . :D
Click to expand...
Good point ref the Omega's I'll check them out properly, measure them up re likely CR and squish, and see. Will keep y'all posted!
FE.
 
Rohan said:
We'd be 'gobsmacked' if you've "seen" the crank whip....
There are a few TONS ? of force applied to the rod on its reversal of direction - anything up to 14,000 times per minute.
(2 reversals per stroke).
We'd be surprised if the rod didn't stretch/compress, ever so slightly, every time.
Thats why fatigue failure is called fatigue failure ??
Not that Norton rods are afflicted with this - in fact they have a good record.
D-rod discussion pending...
Not to mention that the rods warm up, so there is some slight expansion there.
Piston rock, etc etc...
There is also that hint that squish bands are ineffective if the clearances are too large.
??
All on the periphery of upping the COMPRESSION RATIO.
Click to expand...


Stand by to be gobsmacked. I worked at NV in 69/70, at the time there was concern about oil consumption exceeding petrol consumption. I think it was part of the oil consumption exercise, but in any event a Commando bottom end was set up so that it could be powered by a large electric motor. The barrel joint had a thick piece of perspex to stop oil spray. When being driven at up to near max rpm, when viewed with a strobe the crank could be seen to whip. Don't ask for numbers, because I can't remember, but it was very visible. There were also claims that on very high mileage motors, oil could centrifuge from the central oilway and escape into the crankcase. One of the team claimed that the escaping oil was microscopically eroding the flywheel.
One of the outcomes of this exercise was that the Superblend mains were incorporated, but some time down the track.

This is the first time I've seen fatigue failure in this thread, obviously that will apply, its extent will be governed by the weights of the con rod, gudgeon pin (wrist pin to you Yanks) piston, circlips etc) I've forgotten how to calculate this but I'll accept your guess. I'm sure that there would be someone here who can quickly knock out the various stresses on a conrod.

Thermal expansion I absolutely agree with and have no problems. Assuming that the inside of the crankcase doesn't get much over (say) 100 degrees C then the expansion of a typical (assuming 150mm or about 6" between centres, al.alloy) conrod would be about 0.35mm, or about 0.013"

As upping the CR will (at least should) increase revs, combustion chamber temp, oil temp, forces on all rotating/reciprocating bits I don't think its a peripheral debate.
cheers
wakeup
 
Update for y'all Gents,
I collected a pair of 10.25:1 Omega pistons today. They are .100 taller than stock.
According to my calculations and measuring, based on using a standard flaming ring head gasket and standard base gasket, these will give me a squish clearance of .060, which basically is ideal' and should give a working squish.
As to CR, even not allowing for the valve pockets machined into these as standard, or the fact I will remove some material from around the ex valve cut out to prevent a possible 'hot spot' the CR will be just under 9.8:1.
So, they seem pretty bloody ideal to me, and they will be going in!
I won't have it running until spring now, I'll update you then as to how to goes... Or not!
Thanks for all the advice and input.
Rgds,
FE.
 
Eddie, have you weighed your new pistons and compared them with the ones you have replaced ? The rods must stretch, otherwise they would break. If you over-rev a motor and the rods stretch so much that they exceed the elastic limit, plastic deformation occurs and the damage is irreversible. If they don't break immediately, they can do that at any time afterwards when fatigue completes the process. Keeping piston weights low is a good thing to do, and if the high-comp pistons are heavier, it is a backwards step fitting them.
 
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