domed vs flat

[JointTech]

i know for you old hats this has probably been beat to death. Im about to start the adventure of building my first engine and as I research I come up with domed and flat top pistons.
i understand the principle of a dome making the combustion chamber smaller thereby increasing the compression ratio.
what I dont understand is why you would use domed on a new build when you can choose the chamber size of your heads during the build.
is there some reason aside from raising the compression level which could be done in the head anyway?

[Marv D]

Flat / top small chamber is WAY more efficient than a big ol mountian sticking up in the way of the flame front. Remember we live in a wonderful era where we have all sorts of aftermarket configurations of heads and pistons available. In the 'good ol days' we had GM castings,, PERIOD. Only way to make more compression was to stick a dome in the chamber. Today you set compression by choosing a part # from a catalogue.

[kevin9574]

What Marv D said. Flat or dished has it's advantages over domed at the same compression, but you can only make a chamber so small. Too small a chamber kills the flame travel too. Some designs work better with a big chamber and a dome. BBC is one of them, that's why all aftermarket heads are open chambered. Now a SBF works really well with a dish and a small chamber. Clear as mud huh? lol

[hotrod 80]

I like a smaller chamber and a dish ( think Hemi) . Also on a small block a smaller chamber head can be moved to a smaller flattop motor in some cases and work.

[JointTech]

If i go with the edelbrock topend kit and want to have 9.5-10.1 compression I need about a 6cc dome. I think its like finding a needle in a haystack.
the edelbrock heads are 110cc chamber.
the standard flat top that comes with most stroker kits are -3cc with the valve relief.
that puts me at 9.26:1.
if I swap to a dome kit they are mostly +13cc or more. which puts me way over 10.5:1
any thoughts or help with my math.


I can get a headgasket that compresses to .022 and get to 9.5 but is there a better way to do it? Im really trying for about 9.8 -10.

[Super73]

Carefull with an ultra thin gasket, you could be asking for piston to valve clearance issues. Remember, in most cases, your intake valve will be opening before top dead center and your exhaust will be closing after top dead center. You could put a se t on and check PTV.

But the other thing to then think about is quench. Does your piston sit in the cyl or slightly above the cyl when at top dead center? To run a .020" gasket, I'd want to see the highest part of the piston .02 in the hole to give you .04 quench. Keep in mind, a piston does rock in the bore transitioning from before TDC to after TDC.
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[Marv D]

I am not sure why your not looking at the normal option of milling the heads. Depending on if it's a open or closed chamber BBC head, it only takes 5-7 thousandths of flat milling to loose 1cc of chamv=ber volume. Most of the aftermarket aluminum heads have a buttload of deck thickness (especially the street heads) loosing 20-30 thou to reduce chamber usually isn't a biggie. I don't have he spread sheet here to calc it but mess with chamber size and see how quickly it sets compression exactly where you want.

I mean Edelbrock's are pretty clean, but I wouldnt put any head on a motor of mine without a total disassembly, inspection, and cleaning. While it's part a trip to the machine shot fopr a 0.020 or 0.040" cut sounds like it would set your compression with a shelf stock piston. You may still be fighting VtP issues mocing the valve closer to the piston,, but thats 'normal' headaches to work through.

[JointTech]

im not looking at the "normal" option because none of this is normal to me ;P im learning.

ill play with the Chamber volume and see what happens.

[JointTech]

well taking it down to 100cc chamber with a -3cc piston get me to the 9.9:1 that I'm looking for.
Edelbrock does have a 100cc high compression head buts not an open chamber, some kind of hybrid.
Questions: 100cc is smaller than any of the after market heads. what am I losing?
I think but am not positive the 100cc head is one of the options in the top end kit. ( i dont want to screwup the combo edelbrock created and tested)

Can I figure out the deck clearance or is that something that has to be measured during the build? Is there a general number I can put in for the testing?
what are they actually asking for there?

[Marv D]

Your machinist will square the block to the mains and bring it back to flat (and measure it for you) ot take it to a zero deck height,,, whatever you want.If the short block is together it's pretty easy with a good dial caliper just measure at the pin centerline at TDC

[gofastnut]

[QUOTE=Marv D;6133412] Depending on if it's a open or closed chamber BBC head, it only takes 5-7 thousandths of flat milling to loose 1cc of chamv=ber volume. Most of the aftermarket aluminum heads have a buttload of deck thickness (especially the street heads) loosing 20-30 thou to reduce chamber usually isn't a biggie.QUOTE]

But doesn't that issues for intake manifold fitment? Is there a limit(I'm sure there is) to how much you can mill off and still have OK manifold to head fitment and port alignment?
I've always been told it's a no-no over .010", but that's from a lot of people who I've found were more talk than actual knowledge.

[Marv D]

Any good machine shop will correct the intake face and the bolt hole bosses if the angle mill is enough to require it.,, and its a simple mill to correct the intake. My machinist charges $30 to cut a intake if it's necessary. 'Most' intakes have smaller ports than the port in the head (a step up to a big hole in the head is OK, a step down to a smaller hole in the head is bad )
A verticle shift of 0.050" is typically not enough to create a 'dam' at the transition between intake and head. UNLESS someone has port matched,, then you have a whole new ball game. You just mill the intake face and re-port match both the head and intake,,, or weld a quick bead of aluminum to blend to a new gasket match.

Then too.... depends on what your doing. I have a old Holley ProDominator that is probably got a POUND of epoxy in it. A guy that was crew chief for Stan Mizel in the 80's spent HOURS screwing with this thing and a flow bench trying to improve the flow through the intake. Did he accomplish anything other than spending buttloads of time on it???,, and adding a pound to the weight. Have no idea but it sure looks purdy inside LOL

I guess point is, correcting mis-mathed intakes and ports isn't a big deal unless your messing with iron heads,,, then you can put your faith in todays epoxies if your froggy.

[gofastnut]

Thank you Marv!
I've wondered about that.

[C20-67_N_MO]

Wow!

You guys really know your engines. To me it sounds like a good forged flat top and small chamber heads will bump up your compression ratio just fine. Milling the heads is always an option and insures a true and straight surface.