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Jim Bettison (
Posted on Thursday, 08 November, 2001 - 22:37:   Edit PostDelete PostView Post/Check IP

Can anyone tell me the combustion chamber volume for the 4 1/4 litre MkVI engine? For reference: I define the combustion chamber volume as the space in a cylinder above the piston when the piston is at TDC. It includes the effect of the gasket; and the valves are (notionally) closed. It doesn't include the swept volume of the piston.
Jim Bettison.
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Richard Treacy (
Posted on Monday, 12 November, 2001 - 21:28:   Edit PostDelete PostView Post/Check IP

Nomonal combustion volume allowing for minimal soot is about 110ml. With 6.4:1 cylinder head standard compression ratio: 110.86 x 6.4 (comp) x 6 (cylinders) = 4257 cc (The engine capacity of a standard bore 4 1/4 Mk VI is 4257 ml).
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Norman Geeson (
Posted on Wednesday, 28 November, 2001 - 06:47:   Edit PostDelete PostView Post/Check IP


The compression ratio as you are probably aware is calculated from: The swept volume + The volume of the combustion chamber divided by The volume of the combustion chamber.Without knowing the combustion chamber volume one has to accept the published compression ratios to work the formula backwards. It is very doubtful if the C/R's are actual exactly 6.4:1 and 6.75:1 respectfully in the same way that the engine capacities are not exactly 4257cc or 4566cc. However for practical purposes they should suffice.

On the various early post war cars this produces the following combustion chamber volume per cylinder.

4.25 Litre 6.4:1 C/R = 131.402cc
4.50 Litre 6.4:1 C/R = 140.955cc
4.50 Litre 6.75:1 C/R= 132.375cc

Swept Volumes per cylinder are;

4.25 Litre 709.571 cc
4.50 Litre 761.159 cc

R-R listed the compressed head gasket thickness on the 4.5 Ltr at 0.055 inch in the case of a copper asbestos head gasket, although you should find it is actually slightly less.In addition the original type steel gasket for the larger engine was given at 0.25 inch compressed thickness. I have no reason to believe the compressed thickness of the copper asbestos on the 4.25 Ltr is any different than the 4.5 Ltr.Should you need detail on the exact practical compressed thickness please contact me as I have original samples of each in the compressed state but they have been loaned temporarily to an agent.

In connect with trying to obtain the combustion chamber volume by filling the chamber with oil through the plug hole, I am afraid you will have no chance as the leakage by the rings will not produce the required result.It is also not quite so easy on this engine to carry out the same type of check with the cylinder head inverted unless extra provision is made to added around 0.012 inch to the compressed head gasket thickness.This is due to the engine design as the pistons do not come to the top of the cylinders by 0.015 to 0.017 inch when the engine is cold.

I do not really understand the background to the question, however it may be worthwhile relating that the combustion chamber is a different shape on the larger engine.Should the compression ratio be increased on the 4.25 Ltr the gross BHP output will not be raised anywhere near what would be expected unless many other things are done. R-R found that out when trying to alter the compression ratio on the 4.25 Ltr and this fact is documented and was the reason they never provided a high compression head gasket for the 4.25 Ltr.
I trust that the data you required is somewhere in the above.

Bests Regards
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J Bettison (
Posted on Thursday, 29 November, 2001 - 22:34:   Edit PostDelete PostView Post/Check IP

Thanks again for your invaluable supply of info. It will go into the general file, and may be of considerable use in adducing a solution to the present problem.
Originally, we asked the question about comb chamber volume because we thought that we might need to compute the original ex-works head height (the distance between block/head interface and, say, the rocker cover flange) of our engine by ascertaining the lost volume and thereby adducing head height reduction. We thought that there might be quick answers to our question, in the context that several "modern" engines used in Oz publish the comb chamber volume in their manuals, as well as the limit to skimming/planing for the head. We now infer that RR-B didn't - at least not for that engine.
We should, of course, have asked the question about head height at the same time. A few days ago, in the dialogue I've also got going on the RREC web page, Alan Padgett came up with what seems a likely figure of 3.75".
As you may have guessed, this activity is (still) all about "taming" B10J, fitted in B20JO, about which we corresponded in the last few months. You might be interested to know that we should receive a set of "new" carbs (H6, 1.75") plus manifolds, in a couple of days - remembering what you told us about the limitations of H4 of 1.5" choke.
For interest, as much as anything else, we will do rear wheel dynamometer runs before and after the H6 fitment. I expect that we will also make up a new exhaust system, running a set of individual matched branches to a header down near undershield level. (We will do this with minimum disturbance, so that the original can be reinstated, if desired.)
Jim B.
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Norman Geeson (
Posted on Monday, 24 December, 2001 - 08:44:   Edit PostDelete PostView Post/Check IP


I have tried to post a reply before, but it seems to have been lost in space.
In short I believe you have misplaced the digits and decimal place when computing the cylinder head height at 3.75 inches. I have recently measured a 4.25 Ltr head that I know has not been removed for the last 32 years, so it is probable that it has not been skimmed.The height from the rocker cover facing to the cylinder head facing, adjacent to the rearmost head stud on the exhaust side is 4.375 inches.If this data is not sufficiently accurate please advise me and I will endeavour to look up the original drawing.

I would be most interested to know what extra power you achieve from fitting a branched exhaust.

The first and most vital obstruction on the 4.25 Ltr exhaust is the throat area between the edge of the cylinder liner and the roof of the combustion chamber.This is not so much of a problem on the 4.5 Ltr and 4.9 Ltr because increasing the bore size also increased the throat or put another way decreased the obstruction.It is the main reason why the 4.25 Ltr does not take kindly to increased compression ratios, unless the cylinder size is enlarged. Be very careful of enlarging the bore size to attain any extra performance on the 4.25Ltr.You should be able to run on parent bores with 4.5 Ltr oversize pistons although I have not tried this personally.Certainly, as you are aware, the short liners only spell trouble and if linering full length types are the only way to proceed, that is providing you are staying with standard 4.25 Ltr bore sizes.The cylinder block in your J series should be an early type 4.5ltr block, they were changed over well before the introduction of the 4.5 Ltr around late 1949.Unfortunately most of these early 4.5 Ltr blocks suffered from poor casting core positioning and even R-R ran through the side of some bores during machining, a fault repeated on the early 4.9 Ltr engines, so I would not recommend boring AND lining to obtain 4.5Ltr on an unknown block.

You would also be advised when operating in OZ to ensure the engine is fitted with the high speed water pump and fan, a subject I am presently writing up for the RREC bulletin.

Keep us all advised on your progress.

Bests Regards
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Jim Bettison (
Posted on Wednesday, 26 December, 2001 - 21:22:   Edit PostDelete PostView Post/Check IP


Hope you had a good Christmas. We did; but we in Adelaide complained because the temperature was around 19degC max (although the East Coast around Sydney had/is having a very bad bout of bushfires). I imagine that our weather would have been a heatwave for you, from the international weather reports!

Thanks for your last e-mail, and some of the insights it contained. Yes, I did b-up the head height dimension, which should have been 4.375" - which is the dimension we will start at. It is also an apparently logical figure (if such exists in engine design, which this layman sometimes doubts ...)

Unfortunately, I haven't been able to do a "before" dynamometer run, as the instrument has suffered a critical failure, and its owner is awaiting a spare part (the wrong item having already been sent once); and we couldn't wait ... I'll copy you in on what we achieve with the new branches, etc, and also struggle with my PC to get you a picture of what we fabricate. The exercise is hastened because the existing rear manifold is nearly on its last legs - if we change it now, it should be repairable, and join the box labelled "Items To Be Used For Restoration To Original". The new branches are in fabrication. Flanges have been milled from (I think) 6mm steel, with porthole sized to accept a suitable size and grade of steel. We will be using 1.875" steel tube, which will swage neatly to the head port size. We would probably have used stainless if it was available, but only easily available stainless tubing is in either 1.75" or 2", neither of which would make for a smooth or neat job. We will go 3 > 2, then 2 > 1 as branches, with swaged headers from 2" tube stock. The final pipe will be flanged and go to the existing front exhaust box. The existing pipe will be kept ("ITBUFRTO" again ...). Advice has been offered that we should carry right through with two pipes, a la R-Type; but I think we'll save that for now.

Your comment about the obstruction at the throat formed at the liner/CC roof is most interesting. Accepting what you say, it seems to me that we might adduce that heavy skimming of the head (which I suspect is the case with B10J) would exacerbate the problem, and that overboring to the 4.5L dimension would alleviate it. Comment, svp? I think that we might have a second line of approach; viz., the pistons. We will almost certainly use JP Pistons, who are 10 minutes down the road, to provide a fitted set of pistons to the new dimension. One possibly that we had intended to explore (before the info in you last e-mail) was to restore lost head volume (and higher than comfortable compression) by having JP develop pistons with a special crown (which isn't difficult or too expensive from them). Your comments again appreciated. Certainly I am sure that we will heed the advice from several quarters not to overbore and resleeve to 4.5L dimensions.

At present we are "settling-in" the "new" H6s, which arrived last week, and have cleaned-up quite well. One thing we have already found; the H6s can be adjusted down to a fuel/air ratio of 17.5 (which is far too lean for practical purposes) whereas the H4s "bottomed out" at 14.5. In both cases we're using SG needles (I think the H4s could only achieve around 11.5 or 12 with SF needles - can check the work book if you're interested). The car is, of course, in best "colonial" configuration, fitted with an oil bath air cleaner. Another interesting observation is that NGK plugs seem to work noticeably better than Champions, both in terms of engine running and plug colours. (We are presently using the local Premium Unleaded (PULP) which is a 98 RON witches brew ... will at a later stage change the valve seats.)

The car is fitted with HS water pump and 18" fan ("Colonial" again ...). Cooling and cooling systems are systems whose operation is a sine qua non in our climate. Even with the 9-slat/side Bentley radiator shell, air flow and cooling is, in some locations (particularly hot, dry, high), marginal in my experience (and not only of the Bentley, it must be said). In these circumstances three factors are important; the condition of the thermostat (particularly its ability to open fully - some non-factory units have limited opening); the clean-ness of the water passages; and the heat-carrying and heat transfer capacity of the coolant. When we had B10J in pieces, we spent a long time in the block with an old speedo cable in a drill, with all access plates off, mining a fairly high grade iron or aluminium ore ... the result is showing up in B20JO's perormance in my empirical test. My way home from Adelaide to the Adelaide Hills has a stretch of about 8km with an average grade of 7%; I reckon a system is OK if we can do the stretch in not more than around 6 minutes (preferably 5 minutes) in an external ambient of around 30degC. B20JO is OK.

Mind you, we have the hardest mains water in Adelaide of anywhere in Australia, and Australia has some of the world's hardest water ... If you want to get me started on the virtues of ethylene glycol - based coolants for anti-corrosion and heat transfer, just say so ... but perhaps this is enough for one posting.

With very best regards,

Jim B.
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Jim Bettison (
Posted on Thursday, 27 December, 2001 - 21:39:   Edit PostDelete PostView Post/Check IP

Norman (and others):

One correction and one elucidation:

The exhaust flanges are machined from 10mm steel, not 6mm;
The tubing sizes given are all OD. Hence a 1.875"OD tube is 1.75" ID.


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