Hello all, question for you, what is better - flat or dished pistons assuming that I can get my target 9:1 compression ratio with either one by machining the block and heads as required? I have been told by a knowledgeable source that dished would provide better combustion, lower NOX, and less chance for pinging. Most of what I read, though, points to flat pistons as the better choice for any performance build. Or maybe it doesn't make much difference?

I am building up a ~300hp, 350 torque motor on a '77 with this cam:

http://tinyurl.com/mdrwclb

 

That is my understanding as well. I used 12cc dish pistons and 61 cc chambers on my build.
No pinging at 9.9:1 even at 86 octane and a 3.08 rear ratio with th350 tranny.
I did use aluminum heads as well which helps.
Can't remember where I read that, but I do remember discovering that in the planning of my engine.

 

A reverse dome or D-cup is the preferred shape for a dish type piston since it has a quench pad for squish. My preference on a 350 would be flat tops even thought most of my research points to the smaller the combustion chamber the better with the exception of a BBC..

 

I'd use flat tops if I could get the desired compression ratio.

 

Yes the d-dish pistons have a squish pad. Those are the ones to use. I used KB hypereutectics.

 

Which ever route you go, do the math for static compression ratio. Don't just take it on faith when someone sells you 9:1 pistons or heads. You need to accurately cc the chambers and calculate for head gasket volume and piston depth in the hole. On an L-82 that I had at one time, the 74cc heads were supposed to yield 8.5:1, it actually measured out to 7.9:1.

 

dochorsepower, that is exactly what i will do. The volumetric measurements will dictate the amount of material to take off the block and head (if necessary) and then the final dial in will be head gasket thickness. I forgot to mention that I am going to hang on to my 882 heads so 9:1 is as high as I want to go on 91 octane gas.

I thought I would get more response on the dished vs flat question, it is such a fundamental one. My "knowledgeable source" is a well seasoned mechanic who really loves this stuff. Here is his rationale for dished pistons over flat:

<<the deal with dished pistons is the propagation of the flame front being better. For a moment,think of the old chrysler hemi combustion chamber and how the charge would propagate in it.
a dished piston kinda works like that but in reverse. A bonus of the dished piston is that the largest part of the heat is kept AWAY from the cylinder walls and concentrated within the dish itself. a localized and concentrated swirl effect is also accomplished during the overlap phase of the valves that is superior to that of a flat top piston. Also,if a relief is called for in a flat top piston to accommodate valve clearances that very same relief will cause eddys in the intake charge that will destabilize the flame front propagation and leave the combustion chamber more prone to produce detonation. Remember the deal with removing sharp angles from the combustion chamber because of the inherent possibility of flame fronts colliding at opposing angles and causing detonation? Same principles apply here. This is why i go for dished pistons instead of flat top/relieved for valve clearances ones. it's all about efficiency. >>

Some food for thought....or at least stoke the conversation. The part about keeping heat away from the cylinder wall makes sense.

Comments welcome.

 

Not sure I'm on board with this part of the theory.
The piston is made of aluminum. It can only take so much heat.
An important function of the rings is to transfer heat to the cylinder walls away from the piston.
The cylinder walls have a water jacket around them to carry heat away.
So if anything the cylinder walls have the greater capacity for heat control that the piston by itself.
Get a piston to hot and you'll melt a hole through it.

Maybe he meant to say it focus' the energy of the combustion pressure on the piston better than a non dished design.

 

a choice between a D cup or a flat top? doesn't seem like much of a choice now does it

 

I'd only use dish pistons if i were supercharging the car, and needed low compression.

I'm not on board with your mechanic's reasoning either, you never see dished pistons in hi-perf normally aspirated engines for a reason..

 

REELAV8R - yes, the focusing of the energy is the main emphasis of his recommendation, for efficiency which should help to lower NOX and potential for pinging. Keep in mind that I am trying to get the most out of a basically stock L48 with cast iron heads and a mild cam upgrade. GM made a ton of these engines with dished pistons so I am sure they worked out the heat transfer to prevent localized overheating of the piston.

Airbuspilot, since I live in CA and will need to smog the car and run on 91 octane gas that plays into the logic. Engines with higher compression, aluminum heads, etc. I guess would be a different ball game.

7t9l82 - I had to think about that one for a minute...maybe I have been working on the car too much ;-)

 

You have 76CC heads. Flat tops are your only option to get to 9 to 1 compression. If you use a 5CC flat top with stock compression height, no decking, no head cut, Felpro 1094 gasket, 4.030 bore and the 256 cam you will be at 9.13 to 1 compression and 7.67 to 1 DCR with perfect .040 quench and no problem running 91 octane, likely can use 89 or 87. If you 0 deck it and use a .039 x 4.125 gasket your basically at the same place.

 

I agree with Mako63. Those 76 cc 882 heads are as much as 80 cc's in reality. So you may not get as much CR as you want. The smallest head chamber size I came up with on mine was 78.5 cc's.

Post #12 in this thread if you want to see how it's done.
http://forums.corvetteforum.com/c3-t...-my-cr-be.html

If at all possible different heads w/smaller chambers would be better.

 

REELAV8R: I guess great minds think alike ;-) your cc'ing procedure is pretty much what I have been doing although I have one advantage, I now have my pistons out and I did clean the carbon off one of them so using the plexiglass technique I can measure the dish volume and get an accurate measurement of the chamfer space. I cc'd the head last week and got 76cc on the button. Did it twice although I only measured 1 chamber. Not sure why yours are different. Here are my numbers:

Bore 4.030"
Stroke 3.48"
Deck clearance 0.038" (using a dial indicator and magnetic base)
Head gasket thickness 0.038"
Dish volume 11cc (measured with syringe, plexiglass etc)
Chamfer volume 2.55cc (calculated, chamfer is 5/32 x 5/32")

I have a compression calculating spreadsheet that I got from a Jag buddy, btw my other vintage car is a '67 E-Type Jag:

http://www.xkedata.com/cars/detail/?car=1E13653

The CR numbers from his spreadsheet agree with the Summit compression calculator so that is reassuring. So for my stock engine I am getting a CR of about 7.9:1, somewhat consistent with what you got.

If I am going to get 9:1 with dished pistons I will need to take some material off the heads, enough to get them down to about 70cc.

I still have a few days to work on this, my block goes into the machine shop next Saturday. We will first check the heads to make sure they are good, if they are then go into the block. If the heads are not good then I might have to consider a crate engine, new heads are not in the budget.

Thanks for all the great input, and please keep it coming. Haven't spent any money or cut any metal yet so I am still in the "talk is cheap" phase.

 

The fuel lights off in the head so that combustion chamber size and shape is very important . As far as flame travel or the mixture burn , A flat top would provide a better more evenly applied burn across the top of the piston in my opinion..

I am sure this topic can go both ways but my preference for street performance would lay with a flat top.

 

So this is not a stock motor. It sounds like those are "rebuilder" pistons with a lower compression height than stock. Stock down the bore should be .025 with stock compression height of 1.56". .038 down the bore and a .038 gasket gives you a .076 squish. Not a desirable distance.
Maybe those heads have been shaved once? Just to flatten them when it was rebuilt. Giving you a 76 cc measurement instead of the stock over 76 cc volume?
In any case the 4.030 bore with true 76 cc measurement does improve your situation.
If you can get a flat top piston with 6cc and a compression height of 1.56 (stock) then your CR would be 9.04 with the .015 felpro shim gasket.
I would go that route before shaving those heads any more. I've heard that the heads can become thin and crack down the road if you take too much material off. Ask at the machine shop if you got a good one, see what their opinion is.

Try this CR calculator it has more inputs than summits.
http://www.jeepstrokers.com/calculator/

 

Ok, that is all very good to know going in. Looks my machine shop guy and I will have a lot to think about, thanks.

 

Woops .038 + .038 is .076 squish (LOL). My math skills not so good eh?
In any case shoot for about .040" of squish (piston down the hole plus gasket thickness) for good quench and detonation resistance.
Could be your deck has been taken down a bit too. So you may have to select a CH that fits the deck or zero deck the block ( unless it already been done) and use a .040" thick head gasket to get the desired .040" squish.

 

I was wondering about that but figured you knew what you were talking about ;-) Actually I am not sure about that number, I was getting some funny measurements, it was kind of an average. Either way I am sure it will get a lot of attention as we sort through all the variables. We did talk about zero decking the block as an option. It will also be interesting to see it the heads have been shaved. You are right, I would be reluctant to do it again if that is the case.

Here is a photo that I took yesterday of the block, let's see if I did the attachment right...