[gobrdgo]

I’m working on my 428 with a 455 crank and 96 head d port head engine. I’d like to achieve as close to 9.5 to 1 comp ratio as I can- given my understanding CA 91 will work without detonation. I plan on buying new pistons for the engine and custom is an option. Does anyone have experience with custom pistons that have a small dome in the spark plug side of the combustion chamber only? As in the small dome just enough to get the proper CC from the spark plug going away from the valves. Then I start to wonder if my efforts to unshroud the valves in the combustion chamber would be for negated with the dome? What do you think? I’m just a lineman not an engineer.

[gobrdgo]

Of course milling the head is an option to get it right but I think that will lead to having to machine tri power manifold also. Which is a bummer but I plan on divorcing the water passage anyway so once I do that I guess I could machine it without guilt. But I wouldn’t have to divorce it if the ports end up lining up nicely. We’ll have to see about that.

[tom s]

I think you will need a small dish with the 4.21 stroke.I have a set of 96s on a 434 and run on our 91 crap gas.Tom

[gobrdgo]

So my math is off. I need to re work that as I was coming up with like 9.37 to 1 with a .030 head gasket

[Tom Vaught]

Post up your numbers using Wallace's Compression Ratio Calculator.

http://www.wallaceracing.com/cr_test2.php

Tom V.

I find it to be pretty close if you put in the correct numbers.

[Half-Inch Stud]

I think a dome-feature surrounding the Plug scallop may counter the squish in a positive way, and improve flame-travel.

Yet/but there may be a dead zone behind the dome-feature along the ring land that will be wet-gas, with tar ills.

[tom s]

I think the 96s have to be actually CCed to be sure of what you need?Tom

[Stan Weiss]

Originally Posted by gobrdgo

I’m working on my 428 with a 455 crank and 96 head d port head engine. I’d like to achieve as close to 9.5 to 1 comp ratio as I can- given my understanding CA 91 will work without detonation. I plan on buying new pistons for the engine and custom is an option. Does anyone have experience with custom pistons that have a small dome in the spark plug side of the combustion chamber only? As in the small dome just enough to get the proper CC from the spark plug going away from the valves. Then I start to wonder if my efforts to unshroud the valves in the combustion chamber would be for negated with the dome? What do you think? I’m just a lineman not an engineer.

Originally Posted by gobrdgo

So my math is off. I need to re work that as I was coming up with like 9.37 to 1 with a .030 head gasket

Some one help me out here. What a I missing? Just what is going from 9.37:1 to 9.5:1 going to do? To cause the need $$$ for custom pistons.

Stan

[tom s]

Stan,im just asking if they have been CCed to actually know what the CR is?Looks like some chamber work has been done,no idea if have been cut etc.Tom

[gobrdgo]

The head in the picture is a 4x we’ve been using as a test mule. We went from 114 to 120cc after unshrouding the valve. It will be a different amount on the 96 heads. For sure. Yes we will be cc’ing those. I’m mostly interested in the dome idea to maximize the efficiency of the chamber and it can perfect the compression ratio at the same time

[tom s]

I would not worry about it until you know the CCs!You might need a dish instead of a dome.Tom

[tom s]

IF the heads are 96CC and your 428 needs 30 over and a 455 crank I have you at 9.21 with a 30 head gasket.Not worth trying to get too 9.5 with any kind of custom dome IMO.Tom

[steve25]

Did you check your deck clearance with the stock rotating assembly ?
95 % of 455's I have worked on had a .013" deck clearance.
Most 455's also seem to need to be decked .006" to get things the way they should be in terms of being square and to even out the deck.

If you deck the block.013" for zero deck height then you have 2 more CCs to play with.

You also have the option with the custom piston of having it made with a plus .010" deck height.

I see you unshrouded the shallow side of the chamber lip, what gains or losses did that work produce on a flow test?

[tom s]

Steve,he has a 428 block!Tom

[Tom Vaught]

Originally Posted by gobrdgo

The head in the picture is a 4x we’ve been using as a test mule. We went from 114 to 120cc after unshrouding the valve. It will be a different amount on the 96 heads. For sure. Yes we will be cc’ing those. I’m mostly interested in the dome idea to maximize the efficiency of the chamber and it can perfect the compression ratio at the same time

I find it interesting that you are going to maximize the efficiency of the chamber and it "perfect the compression ratio" at the same time.

Typically it takes over a thousand hours of testing to determine the best efficiency of a chamber, High Swirl, Low Swirl, Max Tumble, Low Tumble, Flame Front "Average Speed" vs Compression Ratio, Spark Plug Position, etc.

And you are trying to do this on a #96 Pontiac cast iron head?

I wish you a "Semi" truckload of patience and globs of money for actual Dyno Time using Kistler Cylinder Pressure Equipment to get close to your assumptions based on a very very old design head when GM and Pontiac did not even have a flow bench to test their efforts.
Talk to Wade Congdon @ BOP as he was selling the Kistler equipment to the OEMs at one time. Assume $$$$$

Tom V.

[67Fbird]

really....9.4 vs 9.5....obviously more money than I.
hit it get it and be done.

[gobrdgo]

Originally Posted by Tom Vaught

I find it interesting that you are going to maximize the efficiency of the chamber and it "perfect the compression ratio" at the same time.

Typically it takes over a thousand hours of testing to determine the best efficiency of a chamber, High Swirl, Low Swirl, Max Tumble, Low Tumble, Flame Front "Average Speed" vs Compression Ratio, Spark Plug Position, etc.

And you are trying to do this on a #96 Pontiac cast iron head?

I wish you a "Semi" truckload of patience and globs of money for actual Dyno Time using Kistler Cylinder Pressure Equipment to get close to your assumptions based on a very very old design head when GM and Pontiac did not even have a flow bench to test their efforts.
Talk to Wade Congdon @ BOP as he was selling the Kistler equipment to the OEMs at one time. Assume $$$$$

Tom V.

Was I not careful enough with my verbage? I didn’t mean to sound pompous. I was wondering if anyone thought a dome on the spark plug side only would be a a possible benefit. I take it no one thinks it’s worth chasing.

[Tom Vaught]

"Ideally" you would like a flat top piston surface or a dish (if designed correctly for flame front and flame speed across the chamber). Lots and lots of time and effort to get the flame across the chamber to do a really nice job on the smoothest flame front and a complete burn of the fuel and air in the chamber.

Residual gases from the previous thermal expansion need to be minimizes because they take up chamber volume and screw up the combustion process.

Trying to get a extra .2 on the compression of a race engine might be worth the bank for the buck but on a cast iron head engine going from say 9.2 to 9.5 is normally not worth the effort as MANY HERE have posted. BUT IT IS YOUR ENGINE SO DO WHATEVER MAKES YOU HAPPY.

"I was wondering if anyone thought a dome on the spark plug side only would be a possible benefit. I take it no one thinks it’s worth chasing."

Again, in the "movies" you have a nice clean burn across the chamber. In real life, you might have initially some tumble when the air first enters the chamber and then some
swirl (depending on the chamber design), BUT usually when you have two sprinters for example and one has a clear track and the other has to go over a hurdle (your dome),
the clear track guy is faster. The burn rate has to match the rpm of the engine for best efficiency and power. It is well documented that a dome (unless you have a hemi chamber) is typically is an impediment to a clean burn across the chamber.

"Impediment" meaning screws up the actual power at the end of the day.

Tom V.

[Tom Vaught]

"Ideally" you would like a flat top piston surface or a dish (if designed correctly for flame front and flame speed across the chamber). Lots and lots of time and effort to get the flame across the chamber to do a really nice job on the smoothest flame front and a complete burn of the fuel and air in the chamber.

Residual gases from the previous thermal expansion need to be minimizes because they take up chamber volume and screw up the combustion process.

Trying to get a extra .2 on the compression of a race engine might be worth the bank for the buck but on a cast iron head engine going from say 9.2 to 9.5 is normally not worth the effort as MANY HERE have posted. BUT IT IS YOUR ENGINE SO DO WHATEVER MAKES YOU HAPPY.

"I was wondering if anyone thought a dome on the spark plug side only would be a possible benefit. I take it no one thinks it’s worth chasing."

Again, in the "movies" you have a nice clean burn across the chamber. In real life, you might have initially some tumble when the air first enters the chamber and then some swirl (depending on the chamber design), BUT usually when you have two sprinters for example and one has a clear track and the other has to go over a hurdle (your dome), the clear track guy is faster. The burn rate has to match the rpm of the engine for best efficiency and power. It is well documented that a dome (unless you have a hemi chamber) is typically is an impediment to a clean burn across the chamber.

"Impediment" meaning screws up the actual power at the end of the day.

Tom V.

[Half-Inch Stud]

Tom, You ignorant slut. I happen to agree with all your points.

But, you know, a fella has to try, and find out newness on their own.