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Need to increase combustion chamber size...best shape

Posted by pralahda 
pralahda Avatar
Wayne Barrett
NSW, Australia   aus
1955 BMW r25
1965 MG MGB MkI "The Red Rat"

As discussed last week I have stripped down the B once again and found a couple of things..

1. Combustion chambers (by burette) are between 33cc and 35.3cc., also 9.8cc - 10.2cc above piston, add 7cc for head gasket....

I worked this out to be approx 10.5 to 11.2 : 1 Comp ratio across the cylinders.

I want to lower the comp ratio and develop a better combustion chamber shape.

I have been following Perter Burgess book and the Huffacar Book from this site....but both only talk about shape to achieve 36cc across the chambers....

As I have much metal to play with to reduce comp ratio to 9.8 :1 ....what is the best shape I can aim for....Should I remove lip from upper side of chamber to improve swirl or from the bottom...

What is the maximum I should allow the valves to sit proud of their seats.

Note I am going for 4 angles on valve seats....


Answers ..with pictures if possible from those in the know would be of great assistance....


Cheers and thankyou in advance....wayne



The Red rat
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Glen Avatar
Glen Rib
Red Oak,NC, USA   usa
1968 MG MGB "POS"

What cc are you looking for? I would say equalise the chamber cc's then mill the head to whatever cc you need. Not sure if you can change the chamber shape to the point it makes it much more efficient. Check shawns site and he has pics of what he does to the chambers. The shape pretty much stays the same but gets smoothed out. I would like to know the hp differences between shawns port job only compared to his chamber work only to see what each mod gains in horsepower.
pralahda Avatar
Wayne Barrett
NSW, Australia   aus
1955 BMW r25
1965 MG MGB MkI "The Red Rat"

Glen

I have too much compression at this stage....I actually want to increase comb chamb size...The head has been milled too much in the past and I did a chamber clean up and port (flow) job 2 years ago.

What I have is too much metal in chamber...I want to take some more out and want to use the oportunity to improve swirl ....

ie...I am running too much comp. approx 11:1 and want to bring it back to 9.8...to do this I must increase the combustion area by taking out metal or putting an autin 1800 head gasket on. they compress to 58 thou and the MG gasket compresses to 21 thou...

Thanks any-way......wayne
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Glen Avatar
Glen Rib
Red Oak,NC, USA   usa
1968 MG MGB "POS"

You can change the pistons or you might be able to mill a little off the top depending on the pistons. On the head itself remember that the chamber itself doesn't cover the whole bore so you should be able to just increase chamber diameter without having to go deeper. I would take material off the opposite side of the plug.Keep the heart shape just "open it up" some.Hopefully someone will chime in and mention if any water jackets are in the way. ; )
Jerry Avatar
Jerry Harrington
north east, USA   usa

pistons can be dished more...first and least dangerous....if a head mistake is made can not go back...
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Steve64B Avatar
Steve Opitz
Phoenix, AZ, USA   usa
1966 MG MGB

Wayne,

Removing more metal from the combustion champer might be a problem. Here's another option. This link will take you to a site that sells copper head gaskets that can be ordered in thicknesses greater than stock. This will allow you to get back the chamber space you need to lower the compression ratio.

http://www.headgasket.com/gaskets.html

Steve
underdog Avatar
Jim Underwood
Pittsburgh, USA   usa
1972 MG MGB
1980 Triumph TR8 "Fabulous Trashwagon"
1999 Chevrolet Corvette "Darth Vader"
1999 Chevrolet S10 "Spare Change"
2003 Jaguar S-Type "Eleanor"

What I did on mine was remove material around the valves from the sides of the chamber. This helps to unshroud them promoting better flow. You need to use the head gasket as a guide to how far you can go. I believe I also worked the area around the plug and smoothed the heart protrusion. I had a book that showed it but I think I lent it out & it never returned. Just to qualify my remarks, I did this over 10yrs ago so I'm going from memory.

I can tell you it is very labor intensive and difficult to get all the chambers the same. Grind a little, check the ccs, grind some more ect.

Weather or not this is going to get were you need to be is another question. Problem with thick gaskets or milling the piston tops is that increases the squish area and agravates detonantion.

I know you have port work done already but I'd be inclined to get another 18v core and start over.
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mac townsend Avatar
Fairfield, CA, USA   usa

your best bet would be to even out what you have then find pistons with a deeper dish, to reach the desired.



1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed
MGB Tips and Tricks: www.mgrescue.com
MGB65B Avatar
Thomas Aczel
Sydney, Kurrajong Hts, Australia   aus
1962 MG MGA MkII
1965 MG MGB "Dad's Little White Car"

Wayne give Rams Heads a call. They have HUGE experience and a great reputation for cylinder heads. They do sub-contract port flow studies for some pretty big concerns. They work on everything from ex-works Ferraris to racing carts. They did an excellent job on my 18V 'big valve" head (real seat of the pants difference) and a STUNNING job on my 850 Commando head.

http://www.ramsheadservice.com.au/


PS Give Bernie and Foley my regards; they're great guys
And you can call in for a "cuppa" with me when you're in Windsor



Edited 2 time(s). Last edit at 06/07/2008 05:37PM by MGB65B.
pralahda Avatar
Wayne Barrett
NSW, Australia   aus
1955 BMW r25
1965 MG MGB MkI "The Red Rat"

Thanks guy's

Hey Tom, I will give the Windsow mob a call after the long weekend...

By the way I went to 41 inlets last pulldown....

I am also at my last oversize, but I like it at 1868cc so I am going for a re-sleave....

Oh ! why do we muck about so much with these bloody things.....

....My wife thinks I am off my head.....it was going perfectly well before.....why do we now have a car totally dismantled in the shed...

I informed her that I wanted to put in a 5 speed...and as always it went from there...

Should be finished in next month or so.....should be able to start posting photo's soon...

It is Sunday on long weekend here and my dial indicator has just stuffed up....you would think that an auto shop would remain open so I can purchase a new one at my conveinience......

Well I without the indicator I am stuffed so it looks like a day inside helping the wife tidy the house........but then again I could go and rearrange the shed again.....

decisions decisions....

Wayne...........


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