13 replies to this topic

[minimagic1380]

hi guys ive just broke a ring in my 998 so i want to have it bored +60 but what pistons are better flat top or domed? and is there any other advise you can offer to get the best out of it.
thanx scott

[Stevee]

Flat top type pistons will give you higher compression

as

Domed (dished) pistons will give you a slightly lower compression

[liirge]

Wooaahh, depends

Domed and Dished are 2 different things!

Dished pistons are as they sound, they have a dish in them to reduce the compression ratio, or to help combustion characteristics in some cases. Domed pistons are dome shaped, and they will RAISE your compression ratio over flat-top pistons.

Domed piston pros:

-Piston crown is in compression under load
-Dome pushes incoming mixture to top of cylinder and reduces "short circuiting"
-Scavenging is better than with a flat top, so engine runs less erratic under light loads.

Cons:

-Heavier than a flat top.
-Combustion pressures try to push the center of the dome down, causing some distortion of the top ring groove.
-Slightly "fuzzy" port timing due to the curve of the piston crown.
-Greater surface area increases flame quench at the periphery of the combustion chamber.
-Combustion chamber shape is not as efficient

[minimagic1380]

ok thanks guys so what about dished over plats like these
http://www.minispare...ty=pb&pid=34844

Edited by minimagic1380, 18 March 2011 - 06:51 PM.

[Stevee]

whoops

sorry mix up then sorry

[liirge]

Well you will lower the compression ratio so you will have to skim the cylinder head to make up for the lost compression, but if you rectify this than dished are better

[minimagic1380]

and how much should i have off to make the cr back up to what it should be? and can i go over for more compression?

[liirge]

yes you will have to work out your current compression ratio then decide where you want to go from that, but if you want to keep it the same as it is then you could tell a a friendly machine shop to take out how ever many cc's are in the dish of the piston, off the cylinder head, however I fear Most will want an amount to shave off in Thou,which case you will have to work it out, or they may do it for an extra charge. If you look on google there are several good websites that show in depth how to work this stuff out, or try a search on here!

[liirge]

10.5 to one and above will mean you have to run premium fuel,

[minimagic1380]

ive found some pistons to buy at the right price but they say lower piston to bore clearances are needed what does this mean?
thanx scott

[liirge]

it is what it sounds like, the amount of clearance between the piston and bore going all the way round, I would get hold of the manufacturer to find the recommended bore clearance. If you are getting your engine re-bored the machinist will ask you what clearance you want, and you should state what ever the manufacturer recommends, larger clearances are generally for higher revving motors.

[minimagic1380]

also ive found some pistons at +120 i didnt think tou could go that big! and what does that take your cc to?
thanx scott

http://www.minisport...category5_1.htm

Edited by minimagic1380, 18 March 2011 - 09:06 PM.

[liirge]

Look in my signature! that's part of my engine build!

[liirge]

its a 1096 by the way!
Also those 120 pistons use the old style Circlip type conrods and gudgeon pin