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Strange combustion chamber shape on 1275 head
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Author:  1380 K [ Fri Aug 24, 2012 8:06 pm ]
Post subject:  Strange combustion chamber shape on 1275 head
Hey there. I am currently replacing my cylinder head as the previous owner got the engine rebuilt but nothing was done to the head. However tonight when i took the head of i found this.

Image

Is this a bad modification or good?Anyone seen this before?
Author:  drmini in aust [ Fri Aug 24, 2012 8:46 pm ]
Post subject: 
Maybe it's had a blower or a turbo on it?

Mine is similar, but more so- it got 86.6HP ATW so I'm not complaining.
Only thing to watch (apart from chamber volume) is it will probably need lots more advance than a standard 1275 head shape. Mine runs 38° total advance @3500 up, and 20° static.
If I give it less, the HP and torque drops off.

[edit] Inlet valves are 1.401", exhausts are 1.219" (race valves)- not much room left!

Image
Author:  1380 K [ Fri Aug 24, 2012 8:55 pm ]
Post subject: 
Actually now that you mention it the previous owner was think bout going down that track. Do you know at all what benefit it would provide. He said the engine was set up with high compression(not sure how pressure is changed)This is the combustion chamber of the head im putting on. Just wondering if it might be a step backwards. The new head however has been port and polished. Thanks

Image
Author:  drmini in aust [ Fri Aug 24, 2012 9:08 pm ]
Post subject: 
Compression ratio is lowered by grinding out the chambers, or raised by shaving the head face.

Mine has had both done, the C/R is currently 11.0:1 and with the RE282 cam it still runs happily on BP Ultimate 98. 8)
Author:  1380 K [ Fri Aug 24, 2012 9:12 pm ]
Post subject: 
Found some invoices from the engine build and ignition timing with that head one was 8 degrees BTDC and compression ratio 10.2:1. I guess ill just put the new head on and see how she goes.
Author:  Convertible Mini [ Sat Aug 25, 2012 5:26 am ]
Post subject:  Compressioin ratio
Looking at the first head the chambers look bigger, so by fitting the done up head you have which has smaller chambers the compression is going to go up. I would suggest measuring the Volume of Both heads to compare and see what you will end up with if you do put that head straight on. You don't want to end up with too high a comp ratio.
Author:  1380 K [ Sat Aug 25, 2012 9:46 am ]
Post subject: 
How do I do that
Author:  drmini in aust [ Sat Aug 25, 2012 10:25 am ]
Post subject: 
1380 K wrote:
How do I do that

Here's how I do it-
1. clean the chambers out with a wire brush
2. Clamp the head in a vice and check it's level L-R and F-B
3. smear 2 valves lightly with grease on the face and drop them in
4. fit a spark plug
5. grease the head face lightly around the chamber and sit a CD over it
6. Use a burette, measuring cylinder, or a 30mL horse syringe (my choice) to fill the chamber to the bottom of the CD hole. I use water with 20% metho added (it lowers the surface tension). Measure off how much goes in. I can read accurate to 1/2 a mL this way.
Note 1mL = 1cc (from skool, remember??)

This will give you the chamber volume. Std for a 1275 head is 21.4cc.

Also measure head thickness with a vernier, top to bottom- std was ~2.750". This will give an idea how much has been taken off in the last 30 or 40 years.
Author:  1380 K [ Sat Aug 25, 2012 10:32 am ]
Post subject: 
Did a quick check. Old head approx 30cc new head approx 27 cc. Old head has been decked quite a bit
Author:  1380 K [ Sat Aug 25, 2012 10:33 am ]
Post subject: 
Did a compression test before taking the head off. Cylinder average was about 1200 kpa
Author:  Mike_Byron [ Sat Aug 25, 2012 10:35 am ]
Post subject: 
Its fairly easy really

Just lie the head on its back with the combustion chambers facing up with the valves and springs (also the spark plugs) in place. Make sure it is perfectly flat and prop it with whatever you have at hand to do so.

Make sure the combustion chambers are clean - no carbon deposits etc.

The get some clean engine oil and suck it up into a 5ml medical syringe (thirty cents each from a chemist shop) and slowly empty it into the chamber. Repeat until the chamber is totally full with the miniscus of the oil just above the surface of the head but not spilling over. Calculate accurately the amount of oil that went into the chamber. It should be somewhere in the range of 20-30 millilitres of oil. One millilitre of oil equals 1 cubic centimetre of combustion chamber capacity (or volume). Take into consideration that it may be 26.3 ml or some other decimal point and not necessarily a full ml.

Repeat for each cylinder and average the capacity over the four chambers.

Then there is formula for working out the compression ratio - I dont have that formula in front of me ATM. Perhaps Google or someone else can supply it.

Theminiscus is the bow of the oil when you look at it. The surface tension thing that holds it together and stops it flowing out of the chamber. Google that for a better explanation if I havn't epressed that clearly enough.

Mike
Author:  drmini in aust [ Sat Aug 25, 2012 10:38 am ]
Post subject: 
C/R = [displacement + CV]/CV
where CV = chamber vol + piston dish + deck height + compressed gasket volume.
Author:  Phat Kat [ Sat Aug 25, 2012 10:53 am ]
Post subject: 
drmini in aust wrote:
C/R = [displacement + CV]/CV
where CV = chamber vol + piston dish + deck height + compressed gasket volume.


Don't forget your top ring land to crown :)

Here's the formulae I use (the others work too, they all work... just a different way of expressing it)

Swept Volume per cylinder (Vs) = (bore squared) x 3.142 x stroke / 4

Then compressed volume (same as the doc) (Vc) = combustion chamber volume + piston dish volume + crown to deck height volume + compressed gasket volume (chech the minispares.uk website, they had their volumes listed with their gastkets) + first ring land to crown volume


Then its just Vs + Vc / Vc



Image

I like to use something like kero for measuring, just the thinest liquid I can get, you want the least surface tension possible so that you can get the best reading
Author:  drmini in aust [ Sat Aug 25, 2012 11:28 am ]
Post subject: 
PK, ring land vol is only around 1cc max, I keep that up my sleeve.. :wink:
Author:  1380 K [ Sat Aug 25, 2012 11:30 am ]
Post subject: 
What's a good compression reading ( Kpa) on a 1380 with performance cam etc at what pressure does it become too high?
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