flame propagation v high comp

[Polska]

really flat top pistons v dome pistons
is it better to gain power from better flame propagation and more complete combustion as with modern production engines and flat top pistons or form high compresion ratio dome style piston as in race cars?

its all in turbulence right?? or am i all wrong about this?

[ed]

right about the turbelence, semi right about the compression thing....

...compression is not dependent on 'dome' type pistons. you can easily have a flat top piston running a high compression ratio. 'race' engines havent used dome tops for ages (well maybe some pushrod hemis still run them). either way, higher compression gives you higher peak cyl pressures, which generates more torque... you just need a fuel good enough to resist pre-ignition when you start pushing the numbers really high

the key to a good burn is a good mix, good mix comes from turbulence - the turbulence primarily comes from 2 areas, swirl and squish. swirl is a matter of port design, sqush is from you piston/combustion chamber interface. the flat top piston design with large squish area obviously generates the best turbulence in this regard

article on squish/comustion/detonation:

http://www.v-eight.com/tech_forum/viewtopic.php?t=39

articles on intake swirl:

http://www.theoldone.com/articles/Ci...rl_Power_1.jpg
http://www.theoldone.com/articles/Ci...rl_Power_2.jpg
http://www.theoldone.com/articles/Ci...rl_Power_3.jpg
http://www.theoldone.com/articles/Ci...rl_Power_4.jpg
http://www.theoldone.com/articles/Ci...rl_Power_5.jpg


http://www.theoldone.com/articles/Ho...oft_Head_1.jpg
http://www.theoldone.com/articles/Ho...oft_Head_2.jpg
http://www.theoldone.com/articles/Ho...oft_Head_3.jpg

[Billzilla]

Flat top is generally better, as Ed mentions the flame propogtion path is easier.
FWIW the new 4A I'm building will have 11.2:1 compression and the pistons are actually dished a bit - I'm milling a full 2mm off the head though.

[kingmick]

Bill go 12.5 if i can do it with a chev on pulp you can do it with a 4a no prob! sorry if that was a hint and didnt go into detail

[Billzilla]

Bill go 12.5 if i can do it with a chev on pulp you can do it with a 4a no prob! sorry if that was a hint and didnt go into detail

Nah, the cylinder filling on my engine is going to be much better than a ratty old Chevy engine mate.
I also want to just be able to drive it anywhere and buy fuel like a normal car.

Just got back from the sports car nationals, the new engine has a wee bit more squirt than the old one, it was the second fastest car in a straight line (all bar the Chiron) and hooked up too well on the line on the first race. Blew the gearbox up big time and turned one of the trailing arms into a nice banana shape.

[kingmick]

i promise you the filling wont be even close to the chev all thing being considered!

and it will be fine on high octane unleaded or a bottle of booster

[ed]

please keep the dick pulling out of this thread

[kingmick]

sorry, i thought people would have liked to know the comp you can run on a 4a, from experiance not whats in books or the net that is 10-20 years out of date

[ed]

oh for sure...
just keeping it civil thats all

[kingmick]

its Mick and Bill talking, not Rod and Bill
all civil here. thats not saying Rod isnt civil, just saying they like the toe to toe, not me hehe

[mrshin]

Well, if someone can tell me how I might be able to get 12.5 comp into my 1UZ without getting some ugly (and expensive!) domed pistons made up, I'm all ears...

[ed]

big cams

hmm, sorry thats 'how' youd get it to run

'how' youd actually get the 12.5:1 ratio without a piston swap? errrr.....

[mrshin]

Big cams is a given...

Fairly big comp though I'm seeing little option but some unthinkably expensive pistons..

[ed]

yup, wanna go halvsies in 2 sets with me?


*shudders at upcoming bills*

[mrshin]

Would consider it, but I somehow don't see the bill being all that much smaller!