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Herald 13/60 engine to Spit Mk 3 spec?


timraven

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I'm currently rebuilding a GE prefix engine to put in my Spitfire, I do however have a few questions :)

I'm looking to get the engine to Spit Mk3 FD prefix spec, am I right in thinking that only the cam and compression ratio is different? I've found the GE engine has a CPR of 8.5:1 , and the FD a CPR of 9.0:1 . Bearing this in mind, skimming the head should sort that? Any light on how to calculate the amount I need off, I don't know if it's been skimmed before so are there any standard measurements on the head I should do to find out?

I'm going to get unleaded exhaust inserts put in, and probably new valve guides too. Also tempted to get it acid dipped to clean out the waterways. Any other head jobs that an engine place will need to do? I'll do the flow work myself to save on costs.

Are pinto valve seals worth the money?

I'm very new to this, so bear with me :P

Cheers
Tim

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Hi Tim-

To go from 8.5:1 to 9.0:1 on a 'virgin' 1296 head on a standard bore and deck block, removing 0.030" will do it.  

I put together a head skim calculator for my own use that you might find handy:

http://www.auskellian.com/paul/links_files/CR%20head%20skim%20calculator_v3.xls

Go to the 1296 tab and input your starting and target values in the blue cells and see the results.    

It's a very very good idea to measure the head volumes before skimming in case the head has already been skimmed.  A virgin 1296 head that started life as 8.5:1 should have a volume of about 37.4cc per head recess.  



One other useful thing about my calculator is you can infer compression ratio from measured head volume (knowing the other contributions to head volume, which are primarily the gasket and deck height).  It's all there in the calculator.

Dipping, die pen and pressure testing are good ideas to make sure the head is sound before investing more in shaving and seats.  I can't comment on Pinto seals-

-Paul

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Starman,
Congrats on your calculator, but.......... (there's always a but!)

You appear to caculate the volume of the old compression chamber by using the original  CR. Cell B25 = Bore displacement/(Cell B23-1)   Cell B23 is an absolute statement of the CR.
This is unwise, for the reasons you mention, that the DPO may have already fiddled with chamber volume or shape.
I also congratulate you on insisting on measuring the chamber volume.  IMHO this is the essential first step.

And then, you calculate the necessary skim in Cell  B39  ( CCs to remove/ B38 ), by taking the difference between the calculated old and new chamber volumes and using a constant that appears in Cell B38, when B38 is hidden on the spreadsheet.  I presume that B38 contains an estimate of the surface area of the chamber on the face of the head.
Thus your value for the skim is one which we cannot rely on, because you don't give an essentail factor of the calculation.

Please make clear how you estimate the chamber area on the face.  This is not simple due to the irregular outline of the chamber edge!   I place graph paper on the face and with my dirty thumb or a soft pencil outline the edge by rubbing it.    Counting the enclosed squares on the graph paper, plus the proportion of partly enclosed squares can give a estimate with millimeter paper of +/- 1mm, which may transfer to +/- 0.1cc, an acceptable error.

I hope that you will take these criticisms in the constructive spirit in which I offer them.  Spreadsheet calculations can work like magic, but if they are not robust - they are magic!

May I recommend to Tim that for background information and a more detailed method of calculation that requires pencil and paper and a calculator, he reads my article on compression ratio, on the old Totally Triumph Net?
See: http://www.totallytriumph.net/spitfire/skimming_your_head.shtml

John

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Back to Tim's original question - yes, to convert a Herald 13/60 motor to Spitfire Mk3 tune, change the camshaft and compression ratio. Factory Spitfire Mk3 engines also had camshaft bearings, which can be fitted to a Herald block after boring out the camshaft galleries.

You can push the compression ratio above 9.5:1. I've run a Herald with aftermarket camshaft and CR of 10:1 for years, and with a properly curved distributor and 96 octane petrol, it never pinks.

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Thanks for the replies guys, they've been really helpful. I've just measured my head volumes, came to about 37cc as well, so looks like it hasn't been skimmed before :)

Thanks for that link John, I'll work my way through them as well as bearing in mind the spreadsheet Paul put up.

I'm looking at using one of Canley Classic's Mk3 profile cams that run on journals, and so can fit straight in without needing cam bearings.

Cheers
Tim

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JohnD wrote:
Starman,
Congrats on your calculator, but.......... (there's always a but!)

You appear to caculate the volume of the old compression chamber by using the original  CR. Cell B25 = Bore displacement/(Cell B23-1)   Cell B23 is an absolute statement of the CR.
This is unwise, for the reasons you mention, that the DPO may have already fiddled with chamber volume or shape.
I also congratulate you on insisting on measuring the chamber volume.  IMHO this is the essential first step.

And then, you calculate the necessary skim in Cell  B39  ( CCs to remove/ B38 ), by taking the difference between the calculated old and new chamber volumes and using a constant that appears in Cell B38, when B38 is hidden on the spreadsheet.   I presume that B38 contains an estimate of the surface area of the chamber on the face of the head.
Thus your value for the skim is one which we cannot rely on, because you don't give an essentail factor of the calculation.

Please make clear how you estimate the chamber area on the face.  This is not simple due to the irregular outline of the chamber edge!   I place graph paper on the face and with my dirty thumb or a soft pencil outline the edge by rubbing it.    Counting the enclosed squares on the graph paper, plus the proportion of partly enclosed squares can give a estimate with millimeter paper of +/- 1mm, which may transfer to +/- 0.1cc, an acceptable error.

I hope that you will take these criticisms in the constructive spirit in which I offer them.  Spreadsheet calculations can work like magic, but if they are not robust - they are magic!

May I recommend to Tim that for background information and a more detailed method of calculation that requires pencil and paper and a calculator, he reads my article on compression ratio, on the old Totally Triumph Net?
See: http://www.totallytriumph.net/spitfire/skimming_your_head.shtml

John


Thanks much John.  I'm honored that you looked this over and took the time to comment thoughtfully as I really respect your knowledge and experience.  

Yes, as an initial value I do calculate the starting volume (cell B25) based on CR and other parameters, but this is just a starting or default value if you will to seed the spreadsheet in case somebody "believes" but doesn't know their actual starting volume.  That's why cell D26 should be filled-in with the actual measured value, in which case the if-statement in B30 uses D26 and not B25 (the note for D26 says "input measured value of head volume here, if available--otherwise, leave blank and nominal value used", where the 'nominal value as I call it is the calc'd factory value in B25).

Indeed, the skim in cell B39 is based on the value hidden in cell B38 (row 38 is hidden; I protected the spreadsheet to limit changes by others to only input cells).  This value is 3.5cc per mm, which is a very good approximation for the first few mm of skimming.  Here's why (the following words in italics are in my comment note for cell B38): area of Vhead w/out milling = 35 sq cm (measured); 1st mm of milling will remove ~3.5cc from Vhead, and each successive mm of miling will remove ~0.05cc less than the previous one (e.g., 2nd mm milled will remove 3.45cc, 3rd mm will remove 3.40cc, and so on).  I came up with a variable equation, solving for the solid integral, but the beveled step in the head that causes the incremental volume to change as it is skimmed actually results in such a relatively small increment of 0.05cc per mm and given that shaving more than a total of a few mm off a head risks miling into the water jacket, I approximated this as a constant to keep it simple.  Besides, I suck at macros  :X  I actually got this figure of 35 sq cm by pencil-rubbing the outline of a head recess edge at the mating surface, laying it over graph paper and manually counting the area.  I've shaved several heads (up to 2.5mm) using this tool as a guide, and the measured results have all been within a total error bar of 0.2cc of predictions, which is pretty reasonable I think given the potential stack-up of measurement and metrology errors, so I like to think it's validated by actual test.

Thanks again for looking it over and I hope this addresses your comments.  I left this as a mostly transparent spreadsheet rather than some 'plug and chug' thing that the 'net is littered with so that one can follow all the calcs and logic, as you have.  I believe in peer review!  As a result of my job and hobbies, I'm a firm believer in understanding underlying principles, and that engineered things can be understood, modeled and predicted (but it gets pretty scary with complex systems, and I'm an I&T guy, not a model jockey).  But there is no substitute for empirical, observational measurements and data and that hubris is followed almost immediately and invariably by a sharp and humbling bite in the arse  ;D

-Paul

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  • 1 month later...

Hiya again chaps,
It's been suggested to me that it may be worth getting the big valve head to go on, ie the late MkIV Spit or Toledo one.
I'm guessing the only difference between these will be the combustion chamber volume, and hence if I get a Tolly one I can skim it to get a 9:1 CPR?

I'm also looking at running it on HS4's (is this a good idea?), as they are what I have at the moment, with a standard 1500 cast exhaust. With a light porting (just removing rough bits, and matching the ports etc), any idea what needles I'd need?

Overall the engine won't be highly tuned, just a light porting, skim to 9:1 (standard Mk3), big valve head (maybe), mk3 grind cam, HS4's and 1500 exhaust. Any advice on anything else? I don't want it undriveable, just a nice amount of power.

Cheers
Tim

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Do your self a favor and buy one of the 4-2-1 sports exhaust manifolds. The hs4's will be fine with the sports exhaust manfold and don't port it your self. Send the head you currently have to a head guy ask for a fast road head. This will not be very expensive but will mean you get a nice increase in power without any loss else where.

Peter Burgess will do what you want for a low price.
http://www.mgcars.org.uk/peterburgess/page17.html

and the price list. Look for spitfire 1500
http://www.mgcars.org.uk/peterburgess/page12.html

He will also fit unleaded valve seats skim the compression ratio up and you will have a head ready to fit.

Chris.

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Nick_Moore wrote:
Factory Spitfire Mk3 engines also had camshaft bearings, which can be fitted to a Herald block after boring out the camshaft galleries.


No line boring required. There are two different cam journal diameters (with & without bearings).

Not many around now, but the head to get was from a 1300TC FWD. This was the 'big valve' jobbie, although I'm sure a 1300 Spitfire NkIV head will be fine.

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Richard_B wrote:


No line boring required. There are two different cam journal diameters (with & without bearings).

Not many around now, but the head to get was from a 1300TC FWD. This was the 'big valve' jobbie, although I'm sure a 1300 Spitfire NkIV head will be fine.


The big valve heads are on 1500FWD engines I am sure.

1500TC has smaller than standard ones.

1300TC is standard Mk3 spitfire head.

Cheers

Colin

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2402 wrote:
Recomend you dont touch the head then. just raise the comp ratio. As for the exhaust manifold this is just a replacement part and you dont need to tell your insurance about. Its the rear silencers that you need to tell them about.

Chris.


You need to tell them about Everything, particularly the manifold as it is a performance enhancing product.

You will need to tell them it is a Herald engine as well, and provide proof that it is mechanically identical to a Mk3 spitfire engine, as if you have an "incident" and the inspector knows how to read Triumph engine numbers, you will have failed to disclose a notifiable modification, and be uninsured.

You could just get the engine stamped with a Mk3 spitfire engine number of course.


Cheers

Colin

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