In a stock long block L31 street use carbureted Vortec, would changing the factory circular pistons to a reverse dome 18cc or so “D cup” piston be at all advantageous?

The tighter quench, better swirl, & detonation resistance of d cup pistons could be an improvement, with the heart shaped chamber, but how much torque, even greater detonation resistance, & horsepower do you think you could gain? (.015” or .034” head gaskets?)

Not pretending to be a Chevy drive train engineer, just asking.

I’ m sure I’m leaving out some critical info for sofa, fast355, Orr etc.

 

I would, if I were to build another Gen 1 SBC, and depending on my compression ratio targets, opt for a D-shaped piston dish for the reasons you mentioned.
I wouldn't do it as a one for one replacement (if nothing else is being done) as the cost vs benefit isn't there.
Case in point: I've plans for a 383 utilizing my 65 cc heads. A dished piston is mandatory for me to keep to the CR were I need it. This is were the D-shaped dish (with the Vortec style heads) would pay off.

 

Skinny z......
Thank you!

 

I will just state my experience. I have never used the D-shape but have run the GM 350 and 383 pistons. Neither produce a good quench effect. Using the same cam and heads both had better results using flat top pitsons and 0.40-0.42" quench. Neither were carb engines but even at 10.5:1 my last iron head 383 ran 30° of timing advance on 93 pump gas without detonation. My current aluminum head 383 calculates to 11:1. Have not run it yet but not expecting to have a detonation issue either. Quench ended up at 0.041".

 

Years back I had a 355 built for my T/A. Speed Pro flat tops, decked block, Dart aluminum heads. Quench was in the .035/.039 range. Had a super L98 profile cam from TPIS, as well as a custom burned chip from them. Compression calculated to around 10.0:1. I’m not sure what the total timing was that was programmed, but I do know the thing detonated with whatever premium gas I put in it. In fact, the only time it wouldn’t detonate is when I would fill half and half with premium and 108 octane race gas. Night and day difference. Nothing in the engine should have caused this, my guess is a lot comes down to the tuning of it.

 

My understanding is that for an efficient combustion chamber, the shape of the head notwithstanding, the flat topped piston provides the best overall configuration. No obstructions to deal with and a better flame front (etc, etc) all around. It also allows for an effective quench and all the benefits it offers. That said, when the CR target is outside that which a flat top piston and in this case, a 64 cc chamber can provide, some method of relief is needed. This is where the D-shaped piston top, with the larger portion of the piston top still effectively flat where it meets the cylinder head face, allows for the requisite decrease in CR while still maintaining much of that quench, or perhaps all of it. This would be my choice (or will be ) when I take a go at 383 CID and a 65 cc head with 10:1 SCR. Putting the piston within .040" of the head will require about a 12 cc dish. I'm a proponent of the tight quench so that D-shape is the go to profile as far as I'm concerned.
Now all of that said, what's the net result? Hard for to say really other than knowing that a full point of compression often nets only a 4% increase in output (all else being equal). There's more to it than that obviously but the values are there to inspect.

It's all in the tune.
I've not been able to approach the results with respect to CR and timing that Fast355 has. I qualify that by saying he tunes electronically and has far better control over the results than I do what with having to deal with a distributor that has mechanical and vacuum advance mechanisms to deal with.

 

Now this is of interest to me as I plan the next engine. Having lost my machine shop partners after I made the move cross country, I find myself in a tough place when it comes to getting something custom built that's going to be 1) what I asked for and 2) not crush me financially.
One choice I have is GM's HT383 shortblock. A few of the drawbacks to this particular block, besides the external balance and the short rods (at 5.7" plus they're PM rather than forged) is the piston design. Plain old dish at 18cc (I think) and with what I want to do, quench is high up on the list. That is'nt going to happen with that particular shortblock. I'm still in the process of trying to get things lined up but it's not easy with no real car connections.

 

The D cup (hahaha) retains all the "quench" and "squish" properties of flat-tops, but allow lower compression. Ideally the cup in the piston should always be a reflection of the head chamber shape.

For a 350 w L31 heads, I'd stick w flat-tops. For a larger motor, I'd go with D cups to get the compression down to the desired level, if necessary. Standard round dishes totally defeat the "quench" or "squish" principle and should be avoided if at all possible in ANY motor.

If I were building a 383, I would use 6" rods, a crank that allows STOCK balance (AVOID the words "internal" and "external" when talking about 1-pc RMS motors as those just muddy the waters), and custom pistons with narrow and narrowly-spaced rings and a D dish if necessary. If the cam I would need to use for the RPM range and all that that I want ended up less than 235° on the intake or thereabouts, or if it was for any kind of boost or nitrous, I'd have the pistons made with a D cup of appropriate volume to bring the compression to where I want it. I would be VERY careful about having a dish cut into flat-tops: the thickness of the head of the piston might not allow for it, without seriously weakening the end result.

 

Which is precisely why I posted the following (although I fully understand the distinction between "internal' and "external" balance regardless of the route getting there).

Although the OP has appeared to have moved on, it's interesting to have some dialogue with respect to this particular platform.
Yes, an all forged assembly, with perhaps the exception of the pistons, with a 6" rod and the balance contained within the engine (the weighted flexplate for a 1 pc RMS block notwithstanding) would be my preferred approach. With a 65 cc head, to keep the CR inline with my cam spec and RPM ceiling, a dished piston is a must. And a D shaped dish would be my preference. That in itself will probably mean that I have to piecemeal this thing together and take on the cost of a balance job as well. The thin ring pack would be something more that falls into what would be available as I'd either have to buy a complete rotating assembly ready to go (and be at the mercy of the manufacturer) or spec what I want with a local machine shop (and be at their mercy).
It sucks being unconnected.