99Hawk120

I'm getting ready to start building one of my 400 SBCs, and I'm curious about how to pick a compression ratio and what's the best way to go about achieving it.

I'll be using iron heads on this motor. I'd prefer it to run regular gasoline, but premium pump gas (92-93 octane aroudn here) is acceptable. I don't plan on using a real radical cam in this one; something along the lines of a ZZ4 cam (appropriately sized up for a 400). The engine will be carbureted; right now it has an older mechanical Quadrajet on it.

First of all, what's the best compression ratio to shoot for with those specs? Two separate numbers would be fine (one for regular and one for premium).

Second, what's the consensus on the best way to achieve a moderate compression ratio 400? I've done some calculations and noticed that the 3.75" stroke tends to make the CR a little on the high side if you aren't careful. Is it better to use flat top pistons and a larger combustion chamber in the head, or to use heads with average-sized combustion chambers and dished pistons?

firechicken_3

iTrader: (4)

IMHO,

for the piston question, i would go ahead with the flat tops, and use a large chamber cylinder head, because if in the future you wanted more compression, you would only need to mill and or change the heads, as opposed to a set of pistons that probly have alot of life left in them.

i have a friend facing a similar issue with a 400 that he is planning to build, he wants to keep it streetable but 90% of the pistons availible, make for a 12-13 to 1 compression and thats just too far, he's using a set of old double hump 461's though to, that are believed to have been milled a little already, thus making a small chamber even smaller

99Hawk120

Yes, but some preliminary calculations show that I need a REALLY big combustion chamber to get a sane compression ratio.

For example, a stock bore and stroke 400, zero decked, with a 0.041" thick head gasket (pulled at random from a Fel-Pro product list), is slightly over 11:1 with a 72cc chamber. No way are you going to run pump gas with iron heads that way!

Even an 80cc chamber only gets you down to about 10.2:1, and I don't know if that's low enough to run pump premium with iron heads. 88cc is required to get down to 9.5:1. Where are you going to find heads with chambers that big? And, do you REALLY want them?

If I go the other way, and assume I can't really get more than a 72cc head, I only need a piston with a 10cc dish to drop the CR down to 10:1.

See what I mean? It's almost impossible to get a reasonable CR for a 400 with flat pistons. You either have to A) not deck the block, B) find a HUGE combustion chamber cylinder head, or C) run dished pistons.

From everything I've read about octane tolerance and ping resistance, you want a tight squish area (although I don't know how tight you actually want, yet). This means decking the block and using a thin head gasket. That leaves with the choice of running dished pistons or huge combustion chambers. Which is better in terms of power and efficiency? I don't know--I was hoping someone on here could tell me

RB83L69

All the motor builders these days are using dished pistons and small chambers. That's what my 400 is too: 64cc heads, about 12cc dish, comes out around 10.4:1 CR which is just about high enough for pump gas. It requires premium which sux, but runs OK with 12° static and 36° total which seems adequate with the heads I have (old lovingly massaged double-humps, the good ones with the squish feature & bolt holes, not 461s).

My pistons are round dish; for really optimum results you'd want a D dish instead. That would get your squish back. I have no idea whether that's available off the shelf, you're probably into custom piston land with a configuration like that. If you go that route, get your dish to mirror your chamber.

99Hawk120

Thanks, RB, I figured at least someone on here would know which approach is "better"!

I see at least one piston that's designed for 5.7" rods, with an 18, 22, or 30cc D-shaped dish. That looks more or less like what I would want. Quick calcs with a 64cc head and 18cc dish indicate around 10.0-10.1:1 CR, depending on gasket thickness and deck height.

Thanks for the help! Now it's just a matter of getting the block machined and doing some number crunching on the CR to pick out the parts.

Oh yeah, and I'll need heads, eventually.

Bort62

Ill agree with RB on this one.

Everything I have heard points to a Dished piston being desirable over a Flat top In terms of power potential and detonation resistance.

So Run your Dish and Small Chamber heads. Will make head selection easer, too.

99Hawk120

Excellent, thanks for the help, all!

Now I just need to decide between the Dart Iron Eagle heads and the Pro Lightning Iron 23* heads. The prices arecomparable and the flow numbers on the Pro Lightning heads are slightly better especially at low lift.

Damon

Yeah, D-dish pistons are my preferred way of going. Keith Black makes several hypereutectics to chose from of this description.

One thing to consider when choosiing the chamber size of your heads..... although the pro builders like small chambers, an out-of the box head will have less valve shrouding around the intake with the LARGER chambers. I can tell you from just looking at 64 vs. 72cc Iron Eagles that things certainly "look" tighter around the intake valve to my eye. Pro builders modify, open up and unshroud the intake on any heads they use, but are YOU going to do that?

I have no direct data to prove this, just my own eyballs and some experience. Something to think about, anyway. I use 72cc Iron Eagles or 72cc Pro1 heads on most of my recent 383+ci motors recently, right outta the box, and they work very well.

91L98Z28

i'm using keith black 22cc dished pistons with the fastburn heads (62cc chamber) in my 406 for my 71 camaro...
roughly 9.6:1 compression ratio....

dished pistons, small chambers, and squish (quench area) is definately the way to go!

lukerene

i am building right now a 1976 400,,with a new set of vortecs,,on top,, they are 64cc,,,rather than ,,destroying the combustion chamber,, to get my desired 8.5 9.0:1 mAximum pump gas ratio,,,(premium unleaded is not always available,,,here in some areas,outside major citys,),,,we have elected to mill the top off the pistons,,to get the desired ratio,,,reason we aren,t just puting in the right pistons to start,,,i bought a reconditioned 400,out of a wreck, and its 0.40 thou,,,!!,,by keeping it simple,mild compresion,a mild cam,say XE280comp cams,a good set of 1.52 rollers and a nice edelbrock and headers,,the 400 i think is the way to go,,,if compresion ,is up higher than 10.1 and its a hassle,,,in an every day driver,as fuel isn,t like it used to be,,an engine will still make good HP around 8.5 9.0;1 comp ratio

91L98Z28

a while back the piston milling thing came up, i am kind of opposed to it in principal (thinning an area exposed directly to the combustion process, making it more sensitive to detonation and whatnot) but some other guy on here had done it and said it was fine....

why not lightly hone the cylinders and get some 0.040 over pistons?

otherwise i like your plan.

Bort62

Problem with Milling the top off the piston as I see it ( apart from possible structural integrity issues ) Is that your loosing your squish, your basically De-decking the block.

So while it would be effective In decreasing Compression ratio, It would be IMO the least desirable Method.

AlexJH

I thought I'd add to make sure the cam you use likes the compression ratio you pick.

91L98Z28

good point! loosing squish (quench area) is bad, it will actually make the engine more prone to detonation and poor combustion (all else being equal). the quench area increases turbulance in the chamber and helps eliminate hot spots and blend the mixture better.

lukerene

living in australia,,,we havn,t got the same ,,,,hi performance shops on every corner,,deal,,,so we learn real quick,,how to overcome ,,that problem,,,we are forced to,,make ,do with what we have,,,i can get pistons here at $400,,a set for KB ,or JE,,or arias,,,, ,the engine had ,,or has near new dished pistons in it now,,,the engine looks like it was reconditioned maybe 5000 miles ago,,,,we will incorperate the right head gaskets,,,mill a smidgen off the pistons,,,,and cc the whole thing,,,in the 1980,s a friend was importing 14.1:1 ross methanole pistons,,,he bought 70 sets in america,at an auction,,no one wanted these things,,,we were milling the tops off,, and getting el,supremo,,pistons for $90 a set,,,these engines went on to win many powerboat races,at international levels,and boy ,,we got a bargain,,,,,,hand made pistons @$90 a set,,,i milled a set to run 9.0;1,yeh its safe,,,very little will be milled,,off my 400 dished pistons,,dont let the guy in the local gas station do it,,,any good engine shop will do it,,,AND,the new vortecs are rough out of the box,,and need to be debured around the edge of the combustion chamber to remove the sharp edge ,,that can glow red hot and cause detination,,,these new vortecs are rough,,,,dont bolt them things on ,,,as is ,,,the valve seats are flame hardened and the valves are not lapped in ,,,from the factory,,the seats are in some cases were black and blue from heat,,,these are brand new heads,,,,we will get 400hp on pump gas,,,,we will get it,,,we done this before,,,thanks mate,,, steve,the aussie

Bort62

Steve,

Milling the dome off a piston designed to run a 14.1:1 compression ration w/ a XX cc head is a Differant story than Milling a Flattop or Dished piston Down to increase compressed Volume at TDC.

When you remove or reduce the dome, You are not Changing Quench area.

But Obviously its a trade off, and when in a situation with limited options, You need to Make do with What you have.

99Hawk120

Good call!

Here's some specific data from Pro Topline, 200cc intake, Iron Lightning heads:


Lift ---- 72cc chamber --- 64cc chamber
.100 --- 70.74 ----------- 68.50
.200 --- 146.41 ----------- 134.32
.300 --- 203.72 ----------- 191.63
.400 --- 249.84 ----------- 235.96
.500 --- 260.98 ----------- 260.98
.600 --- 268.51 ----------- 268.51
.700 --- 276.41 ----------- 278.41


As you can see, the 72cc chamber is better than the 64cc up to .400, the same up to .600, and slightly worse at .700.

I know I'm not going to run a cam with a .600 lift, so the 72cc head has better flow numbers at any number I care about.

I do find it interesting they actually flowed the two versions of the heads. It shows a nice attention to detail.

Damon

Thanks, Hawk. I gotta take out an insurance policy on my "calibrated eyeballs!" It's getting scary that I can just "see" what's gonna flow more without ever measuring it on a flow bench!

OK, I'll get my ego in check now. It was just an educated guess based on a little expereince. I could well have been wrong and it it may not apply to all cylinder head designs, obviously.

99Hawk120

I know how you feel. I can set a choke on a Qjet just by eyeball and by feeling the tension. Usually if it's not working 95% right on the second or third try, it turns out to be the choke is defective (just happened to me on my Oldsmobile, in fact--I condemned it based on the feel of the spring when the choke was partly open).

Back to the subject at hand, you're right, it might not apply to all cylinder heads. But without having a set in front of me, it's something that I wouldn't have stopped to think about at ALL, and it stands to reason that it might be the case with quite a few heads, now that you've brought it up.

Since I'm still in the design change, I can easily decide to go with the larger chambered heads and adjust the CR elsewhere to compensate, whereas before I would have just picked whichever chamber size made the calculations nicest.