deck height vs. tdc - importance

[jimbob-mcgrew]

if you fitted either shorter pistons, or shorter rods, or a shorter crank, to effectively lower the compression of an engine quite a lot, would it matter that tdc is now 3mm below the deck height ?

does the piston crown have to be level with the deck, for a proper explosion to occur ? ... does it matter ?

is there any chance of damaging the deck face, or the head gasket with an initial explosion happening below the deck level ?

an example of what i mean. below deck level.

http://2.bp.blogspot.com/-4oeHs0EYg3E/UNXojunHBzI/AAAAAAAACCk/qtCHqOBkiWY/s1600/IMG_0770.JPG

[Royston]

I would expect it to work, but would have a significant effect on compression ratio, I assume you intend making this up elsewhere? are you fitting a turbo or supercharger? reducing the head combustion chambersize

You could machine the deck height to increase the CR if it calculated to be too low

There should be enough material around the liner top to support the forces. The bang will still force the piston down!!

I haven't read about this build, is there a thread......enlighten melol

[jimbob-mcgrew]

I would expect it to work, but would have a significant effect on compression ratio, I assume you intend making this up elsewhere? are you fitting a turbo or supercharger? reducing the head combustion chambersize

You could machine the deck height to increase the CR if it calculated to be too low

There should be enough material around the liner top to support the forces. The bang will still force the piston down!!

I haven't read about this build, is there a thread......enlighten melol

yeah, turbo was the plan.
i was just concerned about the lip-edge of the deck, being exposed. i guessed it might put it in more of a hot spot zone.

ive analysed a bunch of low comp piston heads, and they always seem to machine meat out of the crown, whilst keeping tdc at deck height, so it made me think it was necessary for safe running, but after a bit of searching, i found out the drag race boys in america, run this lowered down tdc height, and apparently its fine for them, and they put their engines through hell and back down the strip.

no thread or anything sorry.
all i have so far is a cylinder head and an exhaust manifold.
a friend of mine did an engine swap a few weeks ago tho, and i might buy his old bottom end for it, if he'll sell it.

i need to look into water spray too, as i hear its a good way to stop a turbo engine destroying itself.

what material are the vauxhall smallblock piston heads made out of, aluminium are they ? or steel ?

[Stuart]

fwiw, explosion = bad :p lol

anyway.
a shorter throw crank will not only reduce the compression ratio (I think), I'll reduce the capacity..... Not sure you want that?

[Iain]

I can't see how it differs from a decompression plate to be honest, but open to being proven why it's not ideal.

[jimbob-mcgrew]

fwiw, explosion = bad :p lol

anyway.
a shorter throw crank will not only reduce the compression ratio (I think), I'll reduce the capacity..... Not sure you want that?

uncontrollable explosions, are the way forward :)
the crank is staying put, i was just using that as an example.
the rods or pistons will deffo need changing in some way tho, or itll pop its top.


I can't see how it differs from a decompression plate to be honest, but open to being proven why it's not ideal.

yeah your probably right, those plates dont normally fry do they, they sometimes leak if the pressure gets too high tho, but i think thats more to do with the nature of their design, sandwiching double gaskets, rather than what there made of.
it should be ok shouldnt it.

-----------------------------------------

answer to my piston material / manufacture question :-

Material



Pistons are manufactured from alloys of aluminum and silicon. The amount of silicon incorporated into the alloy--which makes the alloy stronger--is dependent on the intended severity of duty of the engine, and also influences the expansion properties of the piston. Stronger alloys with greater silicon content allow smaller size and reduced weight.

Manufacturing Process



Molten aluminum alloy is formed into roughly shaped blanks through either casting or forging, and once piston blanks are created, they are machined to final specification.

Cast Pistons



Cast pistons are most common due to the simpler and less expensive manufacturing process; molten alloy is poured into a mold of the near-final shape. Once machined, they are suitable for standard performance applications.

Forged Pistons



These pistons are made from higher strength aluminum alloy billets which are pressed into their basic shape rather than poured. The process results in a piece that has more uniform metallurgical properties that better withstands severe duty.

[meritlover]

you need to do the sums and see what the static compression ratio actually is. Without knowing the volume of the head and the actual swept volume and volume between the crown and the deck, we will only have 3 pages of bullshit guesswork and speculation as to whether or not it will work.

as Stuart said, you dont want an explosion.

having a burn down in the bore wont do anything harmful. What you may loose out on is the burn pattern of your head. Normally heads have a squish area which the mixture is concentrated in (normally around a plug) then the burn propagates across the top of the piston to give you a controlled sweeping burn. But this is unlikely going to be much of an issue.

Pistons are aluminium.

[jimbob-mcgrew]

you need to do the sums and see what the static compression ratio actually is. Without knowing the volume of the head and the actual swept volume and volume between the crown and the deck, we will only have 3 pages of bullshit guesswork and speculation as to whether or not it will work.

as Stuart said, you dont want an explosion.

having a burn down in the bore wont do anything harmful. What you may loose out on is the burn pattern of your head. Normally heads have a squish area which the mixture is concentrated in (normally around a plug) then the burn propagates across the top of the piston to give you a controlled sweeping burn. But this is unlikely going to be much of an issue.

Pistons are aluminium.

ill liquid cc everything before i make any changes. :thumb:

i cant find any info on rod/crank journal sizes tho, which makes it hard to figure out what i can and cant use.
ive only seen that sort of info in haynes manuals, so maybe ill have to buy a few to check.

[Stuart]

Is his for a pikey build, or are you actually considering it properly?

i ask as you could simply keep the standard 10:1ish cr, use even the most basic of aftermarket management and get what you want/need.

you only need uber low cr if you are running the turbo in a very inefficient place to try and cram as much booooooost in, which isn't the same as airflow and real power

[jimbob-mcgrew]

im always broke as a joke, so the majority of bits will consist of cheapskate breakers yard components, and itll be pebble-dashed together the best i can, using whatever i can find.

ive never messed about with turbo's before, so this will be a cheap introduction to the world of boost for me, that will hopefully not be a total disaster :)

if it works and i like it, ill do a serious 2nd build later down the road.