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Flat top pistons


Harvey

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The Xflow has a heron design of combustion chamber that is based in the piston, the cylinder head is flat. Hi compression pistons have cut outs for the valves, and thier base of the top is flat - is that what you were refering to?

 

I am sure PC will be along with a better explination.

 

Greg, Q 86 NTM (Green 185BHP XF)

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Greg,

Apologies you are pushing my technical knowledge to the limit.

Not sure is the answer.

Looking at the Accralire pistons in the Burton catalogue there are 4 types with cutouts for the valves listed. There are also ones listed as "flat" on pre x flow 1.5 and on a 1699 x flow.

 

I am looking in to buying an engine with flat top pistons and wondered what the performance difference is.

 

Sorry if that does not make much sense.

 

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Vulcan used to machine a combustion chamber into the X- Flow head so that flat top pistons could be used in place of the standard heron design. You may want to get in touch with them.

 

Their website is here.

 

At a guess I suspect a flat top piston will be lighter than the OE parts. This may allow the engine revs to rise and fall quicker although I suspect any change would be minimal.

 

Another possible consideration would be that the edges of the heron pistons could increase in temp to a point that may cause pre-combustion. This would of course only apply to very highly tuned engines (poorly set up) and is unlikely to be of any concern to the majority of people. I havn't personally heard of anyone suffering such consequences.

 

I am clutching at straws for any possible differences mind 😬! Don't take any of what I've written as gospel as I could be completely wrong 😳. Hopefully Roger King will be along soon to offer more insightful prose *thumbup*!

 

Peardrop

 

Edited by - Peardrop on 19 Nov 2002 20:11:00

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I would use the flat head and Accralite bowl in crown pistons. They're very lightweight, available, and very reliable. The flat top pistons (People used to use a machined Twincam piston) are more suitable to a very high revving 1300, and allow the use of the longer 1100 rod in the 711M AA block.

 

The heron design Accralite is more than man for the task. Also, chambered heads leave a very sharp short turn radius in the inlet port, and restrict maximun valve size.

 

Chris.

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Well there you go, those with more knowledge than me have answered - thanks Gents I've learned something already today.

 

Harvey, I would assume that your head dictates what pistons (heron/cut outs or flattop) you need. One issue with the Herron design is that the pistons are slightly heavier as the crown is bigger - hence a lower theoretical rev limit.

 

Greg, Q 86 NTM (Green 185BHP XF)

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Thanks for the info folks.

 

It is a complete engine I have been offered with reasonable power claimed(160ish). I have never seen one offered for sale before and wondered why they seem to be so rare.

 

Anybody else with any thoughts?

 

Cheers

 

Harvey

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My old hillclimb 1700cc engine ran forged flat-top Lotus pistons with a flat head. Pistons and block were machined so that the pistons ran down the bore, effectively forming a combustion chamber. Valve pockets were machined into the crown of the piston. This set-up needs a lot of advance and has a limited lifespan, which in a competition engine is not a problem, but I wouldn't use it for a high mileage road engine. Incidentaly, it made 180bhp on Connaught's dyno.

 

Chris.

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Sometimes it pays to take the cynic's view with technical matters - namely, if flat pistons with a flat head were a good idea, would the manufacturers spend money machining specially shaped combustion chambers into their heads/pistons?

 

Bearing in mind that the accountants look to shave every last penny from production costs, the answer has to be NO!

 

A proper combustion chamber design (be it in head or piston) gives a shorter flame path, better squish, lower surface area to absorb heat, reduced chance of detonation, etc. A flat/flat set up has none of this and is not a good idea.

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