Genesis98

Domed vs. Flat

Can somebody explain the differences, as well as advantages and disadvantages?


Oh and one more thing If a block is shipped with a bore of 4.125" and is said to be able to clear a stroke of 4" than should I look for 4.125" pistons and setup a stroke of 4" ?

ede

well flat pistons are just that and domed pistons have a raised top. you never ask but you might be able to figure out dished pistons have a bit of a recess i nthe tops. domed give the most compression and dish the least all else being equall. dome pistons will impede flame travel more than flat. never done it but i've always thought the next engine i build would be with dished piston because of several supposed flame travel advantages.

you'd definitaly want to buy pistons to match your bore but you don't need a 4" stroke if you don't want. the 4" stroke would be the max stroke it can take without block clearance issue. the longer stroke will give you more displacement and bigger is better.

Genesis98

Thanks ede.

If you don't mind I have another question so when designing how much stroke you want in your engine you do this by selecting the proper Connecting Rod and crank setup correct?

If this is correct what would the displacement of a Chevy 400 series block with a 4.125" bore and 4" stroke be?

sofakingdom

iTrader: (1)

Connecting rod length doesn't influence displacement; only bore and stroke.

pi x (bore/2)squared x stroke = displacement

4.125" x 4" is about 427.65 cu in, commonly referred to as a 427.

Most people build those after the first overbore, of .030"; 4.155" x 4" = 433.89 cu in, called a 434.

The sum of ½ the stroke, plus the rod length, plus the piston's compression height (distance from center of pin to top surface), plus the deck clearance, = the block's deck height.

In a stock SBC, stock deck height is 9.025" -.000" + .020" more or less. So for a 400 for example, stroke is 3.75", rod length is 5.565", compression height is 1.56", deck clearance is .025" + or -. In a 350 or a 305, you'll have the same compression height, deck height and deck clearance; but rod length = 5.7" and stroke = 3.48".

1.875" + 5.565" + 1.56" + .025" = 9.025" (400)
1.74" + 5.7" + 1.56" + .025" = 9.025" (350 and 305)

You choose domed / flat / dished pistons AFTER you select heads, to give the correct compression ratio. All else being equal, a small-chambered head with dished pistons will run slightly better than a medium-chambered head with flat-tops which will run FAR better in most cases than a large chamber with domed pistons. Assuming of course that ALL ELSE IS EQUAL: same CID, same compression, same deck clearance, same valve size, same head flow, etc.

Rod length and piston compression height must be selected together to produce the correct deck height (after all, the pistons must come just to the top of the bore, and not stick out. Again, all else being equal, a motor with long rods and short pistons will produce slightly more power with slightly less side loading on the cyl walls and slightly higher efficiency due to the piston dwelling at the top and bottom of the stroke for longer; but it will tend to be shorter lived, because the ring package usually has to suffer a compromise to allow the piston pin to move up. Also, in long-stroke motors, the rods MUST BE long in order to allow for larger counterweights on the crank. Generally one would run the longest rod one dares, given the degree to which one can tolerate ring difficulty (or, the degree to which one is willing to finesse the bore and rings to make them as perfect as possible). Crank mfrs usually specify the shortest rod that their counterweights will fit under.

Sonix

iTrader: (1)

the amount of times he says "slightly" is something to keep in mind...

ya newfie! (salt water huderite!), is this some aftermarket block I take it? Best to use dished pistons and small heads. This not only works very well in the beginning, but just in the rare case that you decide to go forced induction in the future (not like THAT will ever happen right? ) then the dished pistons make it easy to go that route as well, since you can simply go to larger chamber heads, and a different cam - ie, at least you don't have to change the rotating assy.

since it wasn't mentioned yet (was it?) --quench .
There's a keyword to take note of - reverse dome (like dished, but not the cheesy factory dish style), or flat tops, actually make use of quench. Google quench, or search on this forum, for more info on it.

Genesis98

I think us newfie's have a small army poised to take over Alberta. ....

But all things aside I do have an aftermarket block in mind but I don't think it will materialize. Right now my plans are to buy a refurbished 350 and put the Vortec heads on it that I have. That will atleast get the car running for now.

My dreams of a block with 400 mains are a few years away I guess I'm just planning for the future. As for forced induction I'd like to get throw a bunch of cash at a 400 series some day and get it over the 600HP mark but again many years away.