Raising CR on MK2a 850

Not sure on that. Here is an early 750 Atlas head from the Atlantic Green site. The squish looks very similar to the later Commando heads. There is also a photo of the 650SS head, smaller squish on that one.

In fact, the early Atlas, 650SS and 88SS have room for the cylinder spiggotts, no squish band at all.
(As do all the early 500 and 600 dommies).

Dyno Daves pic of early 650SS with accomodation for cylinder spiggotts.


Once the valves are re-angled and enlarged, the later heads loose those half-sided squish bands too ??
Unless they are deliberately added back in.
The question then is - which is better ??

Heres the link to some of the factory ' hot up ' bulliten : http://commando.yolasite.com/performance-mods.php
as mentioned there , the pushrod length and head hight are interrelated .

They say the Big Chief wanted a 850 SS Commando , but noise reglations prevented its production . 1 3/4 bore pipes would fix that . :lol: 8)

There is almost 4 pages of text to go with those diagrams Matt, for each of the Tuning Sheets, so without that the diagrams are only part of the story...

The text mentions that big bore pipes, mufflers AND carbs and throats were all part of the package.
It certainly wouldn't pass the noise requirements, thats why the bikes could not be sold new like that (?).
Note that there is no squish band shown in the head modifications, how does that work then ?

As to which is better, in the Vincent world we have stock heads, which are hemisperical with no squish bands, and Terry Prince racing heads, which also work well on street bikes. The squish is said to make a little more power at top and a whole lot more in the middle. Terry's 600 cc top end with squish head will make a Comet go like a standard 1000 twin.

Glen

Is it the squish band that gives it the go though, or a whole package of improvements ?

The early 650SS Nortons were as fast, without squish band, as anything that came later ?

Getting the squish band right is one of the tricks to tuning BMW airheads.. A good working squish helps run a high CR without pinging.

Thanks for the comments so far gents, well, those that are relevant at least... Perhaps someone would like to start a separate squish thread!?!
To update y'all:
It seems that my bike has not had anything skimmed thus far, perhaps it's had valve seats cut, I don't yet know.
But I do know that the pistons come flush to the top of the barrel, and that if I measure the combustion chamber volume, and add the thickness of the head and base gaskets fitted... the actual CR is only 8.1!
To answer the gent who asked, it also has a standard cam. And is on a standard bore.
Even if I left out the head gasket and base gasket entirely, it will only be 9.1 (according to my calculations)! It seems to me that if I built it without a base gasket, and used JS 0.003 copper ring gaskets, it would almost measure the same as no gaskets at all, and i don't screw up a good head etc by skimming. So this may be my choice.
Next job (when I've finished my cup of tea, will be to do a dry assembly of the top end, sans gaskets, just to see what kind of valve to piston clearance there is (or isn't).
Unless of course, someone can tell me the clearances on a non skimmed box stock 1974 850 MK2A?
Lastly, has anyone used JS 0.003 gaskets? I know Jim is a top bloke and they should therefore be fine, but I've never used anything like that at all, so any feedback from users would be greatfully received.
Thanks,
FE.

Fast Eddie said:

Thanks for the comments so far gents, well, those that are relevant at least... Perhaps someone would like to start a separate squish thread!?!

FE.

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Eddie, although the squish band may not be what you want discussed here, you have asked a question on an open forum and the squish band discussion is related, as is ignition timing. When you bring the head closer to the piston, you change the squish band distance. From the reading I have done, I believe makes it more effective, to a point.

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.


Glen

worntorn said:

Fast Eddie said:

Thanks for the comments so far gents, well, those that are relevant at least... Perhaps someone would like to start a separate squish thread!?!

FE.

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Eddie, although the squish band may not be what you want discussed here, you have asked a question on an open forum and the squish band discussion is related, as is ignition timing. When you bring the head closer to the piston, you change the squish band distance. From the reading I have done, I believe makes it more effective, to a point.

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.


Glen

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Fair point Glen, but on that topic, there's no way the squish band makes any difference on an un modified 850. The piston is no where near close enough to make it work. 750s may be completely different!, I don't know.
Rgds,
FE.

Fast Eddie said:

Lastly, has anyone used JS 0.003 gaskets? I know Jim is a top bloke and they should therefore be fine, but I've never used anything like that at all, so any feedback from users would be greatfully received.
Thanks,
FE.

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I talked to Jim @ J&S & he said the .003" copper gasket is the most trouble free of all the solid copper gaskets. Note in the picture below, the copper wire around the pushrod tunnel & oil passage. They are secured to the head W/weather strip adhesive prior to assembly.

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, so why would you change a Norton head design to copy the Triumph ? A better way to go might be to use Jim Scmidt's rods, and buy pistons which have raised crowns, even it the plugs are a bit close to the crowns, and the piston weight is greater.

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, so why would you change a Norton head design to copy the Triumph ?

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Not quite sure who you are addressing there matey, who has said they want to copy the Triumph head design??
FE.

It seems fairly common practice amongst the faster racing guys in the US, to hemisphere the heads removing the squish band, and re-angling the valves while porting to the max. When that is done, and bigger overlap cams are fitted the usable rev range must rise, however the longer stroke then gives a problem with cranks, crank cases and bearings. It seems to me that the guys are looking at single changes in isolation. The strength of the commando engine lies in it's pulling power and raising the comp. can add a bit right across the rev range, however if you are using petrol as fuel you can get pinging, and reducing the squish band clearance is asking for it to happen. I would try a well designed two into one exhaust before raising the compression on a street bike. I noticed the mention of using larger diameter pipes, I wouldn't go that way.

Been using Omega 10.25 to 1 pistons since the early eighties.
No problem, just goes very nicely.
95 octane petrol.

Is it possible to get reasonably light 12 to 1 pistons for an 850,which are compatible with Jim Scmidt's rods ?

Flo said:

Been using Omega 10.25 to 1 pistons since the early eighties.
No problem, just goes very nicely.
95 octane petrol.

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That's interesting Flo.
Have you measured the combustion chamber volume at all, to check the actual compression ratio?
And may I ask what cam you are running?
My bores are standard, and by chance, a friend has some new Omega 10.25:1 in a standard size that he doesn't want. I think they'll be too much for what I want (running a standard cam), but then again, I could machine them down a little I suppose...!
FE.

acotrel said:

It seems fairly common practice amongst the faster racing guys in the US, to hemisphere the heads removing the squish band, and re-angling the valves while porting to the max. When that is done, and bigger overlap cams are fitted the usable rev range must rise, however the longer stroke then gives a problem with cranks, crank cases and bearings. It seems to me that the guys are looking at single changes in isolation. The strength of the commando engine lies in it's pulling power and raising the comp. can add a bit right across the rev range, however if you are using petrol as fuel you can get pinging, and reducing the squish band clearance is asking for it to happen. I would try a well designed two into one exhaust before raising the compression on a street bike. I noticed the mention of using larger diameter pipes, I wouldn't go that way.

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Ah, I see what you were getting at. And I totally agree that the squish should be kept / used if possible. I used to weld up Triumph combustion chambers and re machine to a bathtub shape and machine the pistons to suit in order to get a very effective squish. Bloody lot of work though! Actually T140s have a nice big chamfer in the head and a matching chamfer on the piston, as standard the gap is too big to work, but some carefull skiming of the barrel can yeild a very easy, cheap and fairly effective squish band.
FE.

Fast Eddie said:

worntorn said:

Fast Eddie said:

Fair point Glen, but on that topic, there's no way the squish band makes any difference on an un modified 850. The piston is no where near close enough to make it work. 750s may be completely different!, I don't know.
Rgds,
FE.

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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.

acotrel said:

however if you are using petrol as fuel you can get pinging, and reducing the squish band clearance is asking for it to happen.

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That is not correct. 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.