Decompression plates

[ArunW]

Just out of interest could you use a decompression plate with a 12G940 head on a on a 1098cc engine to negate the valve clearance issue? And if so would changing the compression ratio slightly (probably increasing the displacement by around 6cc)negatively effect performance noticeably?

[oliver90owner]

How does this change the displacement?

[philthehill]

There is very little point in fitting a compression plate to negate the valves hitting the top of the block.
Better to sink the exhaust valves or face the valves to overcome any contact between the valve and top of the block. Lots of info on here by using the search facility.

I think that you mean that the combustion chamber volume would be increased by 6cc not the cylinder displacement.

The performance will be effected as you are effectively reducing the compression ratio but you will still retain some performance benefit as the 940 head has larger valves and better gas flow.

Fitting a compression plate will take the compression ratio back to somewhere near the original 1098cc as the 12G940 head (as fitted to 1275cc engines) has a smaller 21.4cc combustion chamber to that of the 12G202 (as fitted to 1098cc engines) which has a 26.1cc combustion chamber.

[ArunW]

You are correct that is what I meant as obviously this wouldn't change the displacement as all it would be doing is increasing the height from the top of the block to the head - thanks for the correction

On a slightly different note then, would you be able to to use a 12G940 head with a decompression plate without having to sink the valves if you needed the decompression plate anyway for supercharging? Essentially does anyone know weather the plate would create enough space between the head and the cylinder to negate the valves hitting or would you have to grind the seats and use the decompression plate?

[philthehill]

If you are supercharging I consider that you would be better off fitting the 12G295 head as it has larger valves than the 12G202 and most importantly the valves do not foul the top of the block. The combustion chamber of the 12G295 head is larger at 28.3cc so the head can be skimmed if required to suit the required compression ratio so alleviating the need for a compression plate.
Pushing the mixture into the cylinder does not require large inlet and exhaust tracts. You could get away with fitting larger valves to a 12G202 head and achieve the near same result as with a 12G940 head fitted.

cyl head details.jpg (870.85 KiB) Viewed 1583 times

[ArunW]

Thank you Phill, that is very insightful! I never though of it that way I've been thinking of forced induction for a while now and I was wondering what would be the best head for a 1098...

[RobThomas]

If you fit a Pierce head then you get massive valves, no clearance problem for the valves, more heat dissipation and 26cc (or is it 28?) chambers. Ideal for Supercharging...not cheap, though.

[philthehill]

Rob
Are you trying to give the man a heart attack.

Just looking at the heads - as you say not cheap.

If you are just relying on atmospheric pressure to fill the cylinders you do need large valves and good inlet/exhaust tracts but with a supercharger pushing the mixture into the cylinder at several times atmospheric pressure large valves and good inlet/exhaust tracts are not essential. The 12G940 and 12G295 heads are more than suitable for use when supercharging a 1098cc engine.

If you want big inlets how about this for a 12G940 head.

[ArunW]

Wow! And I think I'll give the Pierce heads a miss

[RobThomas]

Eeeek. I've got 3 of them boxed in the shed. Might be worth locking them away!

[RobThomas]

...or...
https://www.youtube.com/watch?v=jfSTP2P9WCQ

[philthehill]

Rob
Thank you for the you tube clip.
The problem with the Minor is getting all that power down onto the ground especially as the engine is in ahead of the front axle.
To counter the weight of the engine some have fixed lengths of railway track in the boot.
Five link suspension does help but does not overcome the lack of weight at the rear of the Minor.
Moving the engine/gearbox rearwards helps with the weight distribution on all the four wheels but moving the engine/gearbox is not always possible.
Whilst it is nice to have oodles of power if you cannot get it down onto the ground there is no point in having it - and as importantly to most of us spending all that money getting there.
Phil