Ok i bought me a 1293 from Jordie today :grin: The bores look fine and there is no play in the pistons and it turns over nicely ! But im curious to know why or what is the difference between flat top pistons and dished ?????
I sound a little stupid here been a guy who can paint a car and change an engine but ive never really got my self involed with the more technical stuff, and just wondered whats the crack does it give a better compression ? or what ?
Help me out !!!!!
Started by
Guest_Beno_*
, Aug 22 2005 08:05 PM
5 replies to this topic
#2
Jammy
-
- Members
-
- 25,397 posts
Moved Into The Garage
Posted 22 August 2005 - 08:35 PM
Dished give a lower compression ratio, needed on turbos. The higher the compression ratio the more power is acheived.
#3
Guest_Beno_*
Guest_Beno_*
-
- Guests
Posted 22 August 2005 - 08:38 PM
so does this mean the engine will be slower ?
#4
dklawson
-
- TMF+ Member
-
- 10,923 posts
Moved Into The Garage
- Name: Doug
- Location: Durham, NC - USA
- Local Club: none
Posted 22 August 2005 - 08:39 PM
The dished top pistons actually have a hemispherical depression on their tops and are lower compression. My -S had flat-top pistons in it when I tore it down for rebuild. I chose Hepolite dished pistons for the rebuild because I had to bore the block out for new pistons (increased displacement) and fuel is lower in octane than it was years ago. I think you can get quality pistons in both types and which you select depends on how you're going to drive your car and your wallet. For maximum compression and power the flat-tops are good, but you need higher octane fuel or to retard the ignition timing quite a bit.
EDIT:
The engine won't be slower, just develop a little less power unless you advance the ignition timing a bit. It's a tradeoff.
EDIT:
The engine won't be slower, just develop a little less power unless you advance the ignition timing a bit. It's a tradeoff.
Edited by dklawson, 22 August 2005 - 08:40 PM.
#5
Bungle
-
- Members
-
- 28,971 posts
Original Spamster
- Location: Cornwall
- Local Club: cornish mini club
Posted 22 August 2005 - 08:41 PM
not if the cc on the head is small :smartass:
#6
Sprocket
-
- Members
-
- 7,266 posts
Great on Injection faults
- Location: Warrington
- Local Club: Manchester Minis
Posted 22 August 2005 - 08:45 PM
It raises the compression ratio. All depends on the maths of the engine and the final CR to be achieved. Normaly the oversize pistons retain the standard dish of 6cc and reduce the compression hieght of the piston( the distance from the crown to the center of the gudgeon pin) to keep the compression ratio constant. An oversize piston with flat top pistons will have a fairly high compression ratio depending on the compression hight, not considering if the head has been skimmed either. At a rough guess the CR will be around 11:1, so be carefull when setting up fueling and igniton. Adviseble to use super unleaded 
0 user(s) are reading this topic
0 members, 0 guests, 0 anonymous users
