What is the difference between these two cylinder heads?

[Jaywhy]

Could someone possibly (BMC) tell me what is the difference between a 12g940 and 12g295 cylinder head? If fitted to a 1098 minor what would I achieve in performance between the two?

[philthehill]

Depends what you want to do to and with the 940 head.
The 12G295/12g206 head has large valves 1 7/32" inlet and 1" exhaust valves and 28.3cc combustion chambers (standard) and will fit directly onto the 1098cc engine. Would be better with a mild skim to increase compression.
The 940 head has larger valves 1 5/16"/1.4" inlet and 1 5/32" exhausts and 21.4cc combustion chambers (standard) and will give better performance but will need some work to fit it and extract the best performance from it i.e. bigger carb/carbs, free flow exhaust and better cam. A 940 head can have either 9 or 11 studs holding it to the cylinder block.
The standard 1098cc cylinder head 12G202 has 1 5/32" inlets and 1" exhausts and 26.1cc combustion chambers.
Plenty of info on this web site regards application; and fitting just use search facility.

[bmcecosse]

Pretty much as above - but a 295 head WILL need a good skimming (60 thou) to get a decent compression ratio - otherwise it is a straight bolt on. Main problem is sky high asking prices for these heads! 940 head has much better gas flow as standard - and the shallower chambers raise the compression ratio nicely - actually probably slightly too much! The exhaust valves may need 'sinking' by up to 40 thou - but possibly not - if the valves have already recessed slightly. It also needs a 1275 head gasket and probably a 2 minute rub with a file across the top flange of the water pump. The rockers also need to be Mini 'sinter' type -or re-arrange the existing rockers (with some filing + add washers) to align them nicely over the valve tips. Advantage also is these heads (~ £50 if patient) are a lot less expensive than 295 heads! I can say that I removed a quite well modified 295 head, and fitted a slightly modified (exhaust ports eased at the bend) 940 head - and there was a considerable improvement in performance - it actually surprised me just how much better the engine was. I already had a larger carb and alloy inlet manifold - these are going to be needed with either head on a 1098 to get any worthwhile improvement.

[daveyl]

I see 'minispares' sell a larger inlet valve for the small bore engine. '12G296' is listed. Quite a bit of work would be required if you were to do it yourself. As well as a Valve seat cutter, a carbide burr set and maybe a copy of the 'Vizard',
And also a head skim.

[alexmcguffie]

I've had a 12G295 fitted to one of my Minors for 20 years now and it does make it much more driveable. The head is standard other than being skimmed to raise the compression. I run it with twin 1 1/4's and an SS exhaust manifold. When I used to use it every day the other impressive thing was the increase in MPG. I seem to remember that on long motorway trips at 60MPH I was getting around 50MPG.

If you already have a 12G295 then it depends if you see it as an investment or something to be 'used'. Once you skim it by 60thou you de-value it to the 998 Mini drivers out there.

A 12G940 head will be a cheaper option with better results although won't look 'standard' under the bonnet. As bmc says though, watch out for the exhaust valves. Some engine builders will say sink the valves into the head, others will say pucket the block.

Alex

[bmcecosse]

Don't worry about a 60 thou skim on a 295 - it's still perfectly usable on a 998 Mini engine - even the later flat top piston versions.

[bmcecosse]

Good price so far for this head - http://www.ebay.co.uk/itm/classic-mini- ... 1e873fd8f3 And this one could be a good buy - if you can collect. http://www.ebay.co.uk/itm/Classic-Mini- ... 417ce0ba6e

[Jaywhy]

Thanks everyone for your input it is much appreciated
Message for Alex - did the 295 head you fitted to the 1098 engine give you anymore power, particularly when climbing hills ?

[alexmcguffie]

The head would give more power due to bigger valves and better flow and also combined with the carb and exhaust change although I don't have any bhp/torque figures. David Vizards book could give you an approx number based on the engine modifications.

It been a few years since I've driven it as its currently in storage at my parents house 250 miles from where I live. It was definetly torquier to drive and hence would be better on hills. As others have said though, a 12G940 would give the same or better effect.

[philthehill]

The attached photo is of the 998cc engine fitted with a 12G295 head (skimmed 60 thou), twin 1 1/2" HD4 SU carbs (they are good and I found them better than the standard H4), Multi branch exhaust manifold, 948 cam, lightened flywheel and 998cc rods/pistons etc. etc. previously fitted to my Minor and it flew up the hills.[frame][/frame]

[alexmcguffie]

Phil,

What CR were you getting with 60 thou skim on a 998? I'm guessing 11:1 or 12:1 but thats a very rough guess after a busy day at work!

Alex

[philthehill]

alex
The compression ratio was about 10.5/11.0 - 1, but I cannot be definite. I used to have to run it on 5 star to get the best out of it.
My current engine 1380cc with a 940 head is running 19.5cc combustion chambers which gives a compression ratio of whow. The block has been decked (skimmed) to reduce the cc above the piston so that also adds to the compression ratio.

[IslipMinor]

The standard 1098 has a combustion chamber of 26.1cc vs an unskimmed 12G295 of 28.3cc, so an unskimmed 12G295 head on a standard bore 1098 engine will give you around 8.1:1 CR (standard 1098 is 8.5:1).

Just skimming the head by 0.060" will increase the CR to 9.1:1.

No head skimming + flat top pistons gives 9.6:1, and skimmed 0.060" + flat-top pistons results in 11.2:1!

On a 998 with flat-top pistons and unskimmed 12G295 head, the CR will be around 8.8:1 (the Cooper 998 had a raised 'D' section on top of the flat-top pistons to increase the CR). Skimming the head by 0.060" will give a CR of 10.3 on a 998.

[MarkyB]

Doesn't the likelihood of pinking increase with C/R?
How high is OK on pump petrol?

[bmcecosse]

Within reason - and using the 'better' grade of pump fuel - it can be controlled with careful adjustment of the ignition timing curve. There are of course expensive 'octane boosters' out there - to add to the fuel. Not really practical if doing any decent mileage with the vehicle.

[alexmcguffie]

philthehill, only tea strainers on a carb inlet!

[IslipMinor]

How high is OK on pump petrol?

Yes it is about CR, but really more the combination of cam and CR. A high overlap cam will not allow the cylinder pressure to build up as much as a more mild one, so less chance of detonation.

Our 1380 runs on about 10.5:1 with a 276 cam and does not pink on super unleaded. On 95 need to be a bit careful on full throttle and lowish engine speeds, but occasionally have to use it, as the 6.5 gallon Minor tank doesn't always give a wide range of choices of where to fill up.

[philthehill]

Alex
Yes they were 'tea strainers' in those days.
Currently do not run any form of air cleaner on the 45 Webers now fitted to the car. Tried foam covers over the trumpets but did not seem to make much difference so they were ditched. The car is only run in a clean track environment so no real need for air cleaners as such
Richard
Thank you for doing the calculations, much as I thought. Since writing the above post I have had a vague recollection that the head was possibly skimmed by as much as 80 thou and not the 60 thou as first thought. I do know that I had to put packing pieces (Pt No: 2A515 (92 thou thick) which are normally fitted on top of the rocker pedestals) under the rocker pedestals to bring the valve gear geometry back to the right orientation with the valves and push rods so there must have been quite a bit taken off the face of the head.
I just used 100 octane (5 star) or higher which was available at the time of running with the 998cc engine, 35/40 years ago.
A few years back I used to be able to get fuel locally containing tetraethyl lead (one of the 5% retained sales outlet) and ran my 1380cc engine on it but now I have to run it on the highest octane I can get. So far I have not experienced any pinking.
Richards point regarding cam overlap is a valid one and as I run either a 649 or Megadyne 320 scatter cam it is most likely the reason why I do not experience pinking.

[bmcecosse]

I don't agree with the big cam not causing pinking! At low revs - when it is v inefficient that may be so - but at high revs the whole point of the big overlap is to ensure as much exhaust is scavenged, and air is crammed into the cylinder as possible - making the barometric (the one that matters) compression ratio as high as possible - and so pinking and indeed detonation are highly likely. But you probably can't hear it above all the other calamity going on. The ignition timing needs to be carefully controlled to prevent this. I do wonder - has anyone developed a knock sensor for use on the A series -with Megajolt or similar. ? It is the means for controlling timing on modern engines.
The tea strainers...we ran an 8 port 'works supported' engine with twin 40 DCOE. The owner fitted tea strainers (this was a Gp2 Rally Mini - so going in the forests..) and the power on the rolling road was disappointing. Removed the strainers and gained 10 bhp immediately - surprising just how much these things disrupt air flow.

[philthehill]

bmc
I do not disagree or agree with you!
The majority of pre-detonation (Pinking) occurs when the engine is under load/hard pulling and that is usually at low revs on a standard engine. A high lift, large overlap cam usually does not start to work (come on cam) (with my 1380cc engine/320 cam combo 5500rpm) until the higher revs are reached. The rates of air flow into and out of the cylinder are very high and the crankshaft degrees of actual detonation/power applied to the crank is low so the possibilities of pre-detonation are reduced and to put it simply there isn't enough time for the mixture to pre-detonate before the next cold charge enters the cylinder and of course the colder the mixture the denser the mixture which allows more mixture to be got into the cylinder.
It is all about getting the right combination of mixture, timing, compression, cam/valve gear, plugs and fuel etc. etc. to combat pre-ignition.