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MG Midget and Sprite Technical - Flat top pistons? - 1275 Mk3

Morning all, thought I had top and tailed this last week, when I decided to leave pistons as they are, as to get 10.5:1 CR the top of the block would have needed to be skimmed, as well as fitment of 6cc pistons, so engine out etc etc all rather expensive.

Anyway, after a very quick flying visit to the garage yesterday, it was suggested that another way to achieve a higher CR without removing the block or taking anymore from the head, was to fit flat top pistons. Richard wasn't sure he could still get them in the required size, but the thought had just recently crossed his mind, and whilst I'd already decided which route I was taking, he felt I ought to be aware of another (possible) option.

He did say that technically you can lose some torque compared to a dished piston, but in reality it wouldn't be that noticeable with my setup, on the road. Based on a back of fag pkt calc, he reckoned it would raise CR to between 10.5-11:1. Now, I reckon I would actually 'feel' that!

So... thoughts on these? Any other +/-'s that I should be aware of? What about valve clearance? Thanks.
M Weller

Flat top pistons are readily available from minispares so that should not be a problem.
And increasing the CR should increase volumetric efficiency and thus increase torque.

But a CR of 11-1 is a bit on the high side on RON95 so prepare to go for the less available higher octane fuels.

Again the question is:
Did you think you had enough power before you knew of this less than optimal setup?
If yes then slap it back together and enjoy the car!
If no and you can afford the work go for it
O K

Flat tops are brutally expensive

id becareful adding that much CR .... esp for the street

the problem is fuel quality ...its very poor and getting worse

If your going thwt high for CR... id schedual a trip to peter burguss for a new ignition system and have it recurved to match your engine...otherwise it could go POP goes the weasle

ive reduced my CR to 9.3 and local machine shops are advising agianst any thing beyound 10 for the street

with fuel quality burning so fast and hot its made street cars very difficult to tune

id definatly look at comtic head gaskets to dial in your CR as they come in many differant thickiness and are reusable

.
Prop and the Blackhole Midget

In the UK we appear to have higher quality fuel with less sulphur in it than in the states.

The standard UK unleaded fuel is 95 Octane.

Various companies supply super fuel with an octane number between 97 and 99 Octane but these are not available at all petrol stations.

They tend to be available in builtup areas rather than in Rural areas.
Eddie Cairns

As per my posts on the other thread, you need to consider the height of the piston as well as the dish volume. The extra 1.5mm of the taller piston accounts for 6cc, hence these pistons have the samle total volume (incl the unswept bit of the bore) as flat top pistons with the original compression height.

Of the two, I would go for the taller piston+dish any time, as the extra height increases squish and so make the engine more tolerant of higher CR. I suspect this is why that version of the pistons was produced for use in the Metro with its 10.5:1 CR. This ran happily on 95 octane BTW (all we had here for quite a while), but higher octanes are preferable.

Cam timing also has an effect on what fuel can be run, the later the inlet valve closes the lower the dynamic CR.

Another example of reduced deck height improving combustion outcomes is in my MGB, running 11.5:1 on pump fuel thanks in part to the pistons being flush with the top of the block rather than nearly 1mm down at TDC.
Paul Walbran

OK guys thanks for all your responses on this matter. I'd already made my mind up last week, but then this third option materialised late in the day, and got me thinking...

But based on the above, I'm going to stick with my original decision. I'll let you know how it all turns out!
M Weller

This thread was discussed on 31/10/2016

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