Chamber Shaping / CR
Turbo Mini Forums

Hello All !

In keeping your mini alive vol 1, you might remember the artical on Turbo charging.

It mentions the "beak" on the combustion chamber, and how it glows and causes detonation.

So....

If you were to remove the beak, as you normaly do to drop the CR.

Then skim the head to get same CR as when it had a beak, could you then run more boost before detonation occured, but still reduce lag due to the relitivly high CR?

Anyone tried it or have any thoughts?

I've never tried it, but it does sound like it could work

Give it a go, heads are fairly cheap, and if it works, it'll kick arse

Utter bollox i'm afraid.

Look at it this way. The temperature required to make cast iron 'glow' is lower than the temperature required to make alloy unstable. And what is Alloy? Only the pistons...

Removing the beak compromises the excellent standard chamber shape of the A-Series head. Flow induced swirl will all but dissapear, dropping off boost performance.

By all means partially remove as a means of lowering the CR as a comprominse, but don't skim it back!!!

D

I did wonder about the glow red comment. That's why I wrote this post!

I've got a unleaded MG head, but it's been skimmed to f**k and back!

I've removed the beak and taken a lot of material from the chamber but the chamber volume is only around 23cc's.

I might run it this summer, and look for a good untouched, unleaded casting to mod during the winter.

Thanks for you input!

Dave

Best unleaded casting is the one from the early mini coopers. These had around 23cc as standard, and will open out a lot more!

The last 1.3 metros also were quite large, around 22.5cc. Easy way to see if it's a good thick casting is to look at the little water holes at the front of the head. If these are 5mm+, then there is loads of meat available for moving from the chambers - avoid anything with less than 4mm thickness - chances are you'll go through when opening out the chambers...

D

I agree with Dave. I have cut two of my own heads and one for the mate and all these retain the beak. It is cut back quite a bit but the general shape resembles that of the std head. The biggest chamber so far was 35 cc and this was accomplished but cutting the roof of the chamber flat. Std chamber depth is around 8 mm (depending which casting is used) and I have cut mine to about 9,5 - 10,5 mm from head to head, depending how deep the valves were sunken in. All chambers are of the same depth. Of course this means that spring pondage and valve train geometry needs attention.

My gut feeling is that if you remove the beak you also destroy the swirl in the chamber that promotes fast burn => lack of punch at lower revs. Boost will take care of the power at higher revs. That glowing red thing is utter BS. Think about the chamber itself: there are always almost sharp corners that overhang the cylinder bore. If it were to glow it would start at the sharp corners instead of the round beak.

I did 21000 miles with the first head and sure as hell there were NO signs of melting nor glowing. The chamber colour was normal and uniform.

As for reducing CR, I cut a head to another mate and measured it before and after the beak cutting. (On this particular engine the piston/bore combo was such that I had to take the chamber to max because the mate wanted to run high boost, still I could not get the CR below 8.5:1) Removing the beak does not give that much extra cc even if it looks quite big. The gain was about 3-3,5 cc.

Jukka

all correct on the beak removal, i made my own head years ago when all the tech nowhow was get the C/R any way you can , so i did ,
ended up with bowl in my chambers, seems to work fine , low down power is crap , but when the power comes on , everything seems ok,
thats my 2p worth


and thats all its worth , before you say it jim.

Everyone knows that instructions only have to be read if the thing doesn't work....

no comment !

Team Racing

I'm about to cut a head for a friend of mine. It is unleaded which means I've got problems raising the roof of the chamber as the valves are NOT sunken in.
Where is the easiest place to remove metal then. Have you got any pictures Jukka? What about you Dave?

Basically retain the shape (also the beak) but cut back. How much depeneds to the core you are cutting. I did cut one head through to the water jacket but there was a slight core shift that could not be seen.

I?ll e-mail you some pics to give a general idea.

Jukka

I started (but never finished) a head that was 23cc unleaded to begin with. I dropped the roof by around 2mm. I opened out the rest of the chamber on the same milling machine. Admittedly the inserts are pretty thin now, but the chamber volume is 34cc, without the final grinding taking into acount... I'm sure it'll make 35cc when finished. As regards the core shift and breaking through, I selected this head from a few samples... I used a few short lengths of weldingf rod, bent to an L shape. Insering these through the holes in the head where the water circulates, lets you 'feel' for areas of concern, ie where there isn't much material...
Also, the thickness of the 'deck' of the head can be seen through these holes. Look at a selection of heads - there is quite a variation!

D

Excellent guys.
I received you e-mail Jukka. Very helpful.

I read that you have considerable guide boss remaining on the exhaust Dave. How much is considerable. Do you feel that it makes a difference?

Well, it isn't hogged out as much around the guide as it is on a normal stg 3 head - ie is a little restrictive... but i'm happy with that!

Also - my ports are almost the standard size - they are just cleaned up and the 'outer' side of the outer exhaust ports are curved to promote a better flow into the manifold. I kept the standard port size to keep air/gas speed high for bringing the turbocharger on song a little earlier. The theory is good, but it probably is a little restrictive at high RPM in reality

Edited by turbodave16v on 15th Nov, 2003.

Same here, I tend to be a little conservative with port sizes. I cut the valve throats as big as the seats allow and clean the throats after the seats are cut (a bit adventurous). I have removed most of the guide bosses. Intake ports are widened so that there is about 1 mm material left adjacent to the pushrod holes. The bottom of the intake port is simply cleaned. The roof is raised some. I use a tungsten carbide cutter to remove the bulk of material and a long 10 mm diameter cylindrical stone to create the final shape of the ports.

I usually open the ex ports quite a bit, but note that I have old pre A+ heads where the ex ports are usually smaller than A+ heads. Even here the shaping concentrates on the upper half of the port on centre manifold as seen from the manifold flange and to upper half/outer corners on outer ports.

All my heads feature ports size of an ar**hole compared to heads available from ANY reputable head shop in UK.

Jukka

I do also cut the ports slightly after the seats have been cut. Especiall when a bigger valve than before is being used. It's easier to do it properly after than to remove a bit too much before they are cut.
Great info on ports and bosses though. I'll get some pictures for you guys when I'm done.

One more thing.. At the manifold end. Some UK specialist opens this to fit the big-bore manifold gasket. This is way to big for a standard metty manifold isn't it?? I do not have one around me you see... Only the Mirage :)

I only use reinforced metro turbo gaskets and match the port to the gasket. But also here only on upper part of the port. I don?t see much point in cutting the port bigger where the minority of the flow is. It only serves to make the port bigger but not necessarily better. I guess it look better with HUGE ports but there is no real point. The manifold face is not the restriction...

For those who run Metro manifolds, check the manifold ports for mismatch. The manifold port on mine was 4 mm mismatched in #4 cylinder, manifold being narrowed than the head.

Jukka

i would love to see all the pics you guys have been sharing if poss.

minimark@blueyonder.co.uk

Everyone knows that instructions only have to be read if the thing doesn't work....

HI Guys.

Is there a chance i could get a look at some of the chambers your talking about in this thread as i'm currently building a 71 Turbo 1275GT and i'd like to grind my own head as i've got all the facilitis at my place of work..

Thanks
Karl.

P.s. I've had a turbo head of VMax back in 1993 and the quality wasn't upto much but i'm sure things have changed since then as with the years brings knowledge and experience.
In the end i went with a Brian Slark 36/32 @ 35cc very nice head...

Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.

Here is a pic of my old MED Head. this was when it was new, notice how badly pitted the exhaust valves are, and these where meant to be new!! The head also had a hole in it, anyway, rant over.
this was 28cc

And here is a pic of my current head, grinded by H.U.W. (Heads you win?) who ever that is, anyone know?? Chamber shape is not ideal, but it produces the power I need.

Team Racing

Hey Thanks Jimster.

One thing you don't mention is the Valve Sizes of both and the CC of the second picture which looks like a nice job but like you say Who the hecks (Heads you win).

Karl.

Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.

I haven't a clue who head you win is either sorry??
The valve size 29 / 35mm on both heads, the second head is 30cc's

Team Racing

I have a NA Heads you win... hes a guy from Newcastle who does a good job for the money IMO...

I brough mine at Stoneleigh one year... well about 4-5 years ago!

alex

AlexF

jim i think you know who HUW is !!!!!!!!!!!!!!!!!!!!

Everyone knows that instructions only have to be read if the thing doesn't work....

Jim that 30cc head looks huge!:)

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