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12G295 Chamber Volume


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#16 Swift_General

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Posted 09 November 2017 - 08:47 PM


Got to be said I can't get my head round what Swift_General has said, or is it just me?
I'm not having a go, I just don't understand it...
 
The change in chamber volume does depend on the cross sectional area and as said the skim,  but also how that CSA changes too, as it does when skimming. As the depth gets shallower the CSA gets smaller too so its the change in the area on a head chamber plus the skim. I can't see how this is worked out to 0.01cc using graph paper or am I missing something?
 
Cross sectional area has nothing to do with the volume of the chamber unless its a perfect cylinder or am I wrong? And a CSA should not be measured in CC, as CC is a cubic volume measurement not an area, surely an area is measured in cm squared not cubed? and unless its a perfect cylinder the CC cannot be calculated from an area, or am I wrong?
 
As the chamber in a head is an odd shape CSA has no place in calculating a 3D volume and is uneeded? or am I wrong? I fail to see what use graph paper is or am I missing something? Its a bit confusing.
 
A piece of perspex with a hole, a syringe and some ATF is all I have ever used, perfectly accurate and simple.
 
   


Yes the CSA of the chamber does change with depth, but if you are only taking a small amount of material off the head as you would be with a skim, that the area reduces by a fraction of a square centimetre will make such a small amount of difference as can be ignored.

But as you pointed out area is measured in square units, not cubic - I must have been im a rush, sorry!

I was trying to give a rough figure as to what the reduction of cc would be with a skim of a known figure as per OP's post but making some assumptions which I don't think will be far off - but ultimately in terms of calculating a CR (as assumption of original figures may be wrong) the only sure way is by direct measurement as has been suggested of all the individual volumes that make up the combustion chamber (with the exception of ring lands that I estimate).

#17 carbon

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Posted 09 November 2017 - 09:36 PM

Incidently, Im building the 1100 for my 60 mk1. Im retaining the std HS2, manifold and pea shooter single box exhaust.

I have an MG1100 2 ring cam on the shelf which Ive thought about installing but am tempted by a SW5 as Ive never used one before (Im a 266 fan!).

Obviously the carb and exhaust is going to restrict performance quite a bit, I know that. But its a very original early 60 so I have my reasons. The existing none original engine is a 69 factory replacement gold seal 848 which really struggles on the hills around here on Exmoor where I live. So the 1100 + 40 looky-likey is going in with a 3.44FD. It should prove a little more useful.

I wonder if its actually worth doing much work on the head as it will be effectively wasted due to my exhaust restriction.

Or will it?

Is it worth my while installing the SW5 and decent head?

Your opinions are welcome.

From memory the ADO16 1100 also used a pretty small bore exhaust, maybe 1.25 inch, and the rated power output on a single HS2 was about 48bhp and 60lbft for the 'high compression' version using standard cam.

 

I like your approach of keeping the external appearance standard, there are a lot of improvements which can be made which together should result in a really strong motor that pulls well up the Exmoor hills.

 

Some of the carb and intake mods which might be worth considering include:

- plain butterfly with the small bleed hole (this is probably what you already have fitted)

- throttle spindle waisted down, remove legs from spindle screws and fix with Loctite

- blend in carb body sharp internal edges just after piston (see Vizard for details)

- fit a decent filter such as K&N inside the original filter casing if possible, check the carb inlet has smooth radius or if space fit a stub stack

- port the internals of the cast manifold to open up intake slightly and remove any sharp bends which might restrict flow

 

Cam choice is a bit trickier. I'm also a fan of the MD266, the lift at lobe is only 260 thou but this should be good for steady idle and strong low down torque in an 1100. The SW5 should also be good for torque, and has the advantage of more lift at lobe. Pretty sure either MD266 or SW5 will work better than the 2-ring 1100 cam which has only 230 inlet duration.

 

But to answer your question about the 295 head, probably not much to be gained by modifying as the inlet ports are already generous. If you are using a 1.25 inch exhaust then not much point in opening up the exhaust ports. Best gains are probably to be had from detail to the valve seats.



#18 Cooperman

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Posted 09 November 2017 - 10:40 PM

A tip for measuring the chamber volume when the new valves have not yet been lapped in or even when the new valve guides have not been fitted, is to take the new valves and effectively 'seal' them in place using a run of RTV sealant around the as-yet not cut valve seats. Once the RTV has set, you can measure the chamber capacities accurately enough for the final calculations.

That way, you can deliver the head to the machine shop where the final seat cutting and face skimming can take place in just one visit.

Even if the seats have been lightly 'touched' by the grinding process, the RTV will fill the marks. The fact that the valves may sit a few thou deeper into the head is really not that critical. 



#19 Retroman

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Posted 10 November 2017 - 07:31 PM

 

Got to be said I can't get my head round what Swift_General has said, or is it just me?
I'm not having a go, I just don't understand it...
 
The change in chamber volume does depend on the cross sectional area and as said the skim,  but also how that CSA changes too, as it does when skimming. As the depth gets shallower the CSA gets smaller too so its the change in the area on a head chamber plus the skim. I can't see how this is worked out to 0.01cc using graph paper or am I missing something?
 
Cross sectional area has nothing to do with the volume of the chamber unless its a perfect cylinder or am I wrong? And a CSA should not be measured in CC, as CC is a cubic volume measurement not an area, surely an area is measured in cm squared not cubed? and unless its a perfect cylinder the CC cannot be calculated from an area, or am I wrong?
 
As the chamber in a head is an odd shape CSA has no place in calculating a 3D volume and is uneeded? or am I wrong? I fail to see what use graph paper is or am I missing something? Its a bit confusing.
 
A piece of perspex with a hole, a syringe and some ATF is all I have ever used, perfectly accurate and simple.
 
   


Yes the CSA of the chamber does change with depth, but if you are only taking a small amount of material off the head as you would be with a skim, that the area reduces by a fraction of a square centimetre will make such a small amount of difference as can be ignored.

But as you pointed out area is measured in square units, not cubic - I must have been im a rush, sorry!

I was trying to give a rough figure as to what the reduction of cc would be with a skim of a known figure as per OP's post but making some assumptions which I don't think will be far off - but ultimately in terms of calculating a CR (as assumption of original figures may be wrong) the only sure way is by direct measurement as has been suggested of all the individual volumes that make up the combustion chamber (with the exception of ring lands that I estimate).

 

Swift_General ; Thanks for explaining now I understand, it looks like we are all singing from the same hymn sheet, if not graph paper !! Hahah...it just looked like a complicated way to go about it.

 

As we all know cc's etc have to be accurately measured and the more highly tuned the more essential it is to be accurate...I always use the same method a Cooperman

 

MINIGMAN ;  A good head and cam is always worthwhile, its all about efficiency and they both help and I think like CARBON too; that modifying any SU to Vizard spec is a good mod and especially as the idea is to look standard.

I think the exhaust may be a bit on the small side, but if you are keeping a standard cast manifold you have no choice. But if you do go the standard cast route measure a few inside and pick the biggest they do vary a small amount, also clean the casting flash out and run a separate inlet and exhaust manifold....means sacrificing one but gets rid of the power robbing hot spot where they join.

 

I would go for a 3 branch cooper type and an RC40 maybe with a peashooter grafted in the last few inches just for effect and not restrict...not sure exactly how...

 

Showing my age I have had a few road going small bores that could kill XR2's and 205's...smile factor






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