Dished vs dome pistons?

Going over compression ratio and all the player's that can affect it. So I learned that a dome piston will bump up thr CR by creating a smaller combustion chamber. And a dished piston will in change drop compression making a larger combustion chamber..

So why would someone get a dished piston and what purpose does it have?
Beside it creating a safe place for the intake and exhaust valves to work.

Also, milling the heads down will bump up the compression too right.

And, how are some of you guys coming up with compression ratio number so fast. Is there a technique your using in your head or using a CR calculator online?

How does gas... ethinol, 87,89 amd 91 gas affect it. In my younger day i drove a sbc350 that the neighborhood car guy puy together, he told me to always use 91 and i did without understanding why.

 

dished pistons help create swirl.....air movement....inside the cylinder which can help mix the fuel and air together.

octane is actually basically a rating of anti knock. meaning the harder it is to light on its own.

keeping mind that normally high compression ratio require more octane due to increased cylinder pressures and temperatures, but things like camshaft profile, cylinder head shape and material, boost, timing, nitrous, etc also play into this. So compression ratio is not the only factor.

now you only need as much octane as needed to prevent knock.......there normally isn't much hp gain for running too much octane.....mean putting 110 race gas in you everyday caviler isn't going to much hp beyond it will allow the computer to run maximum timing safely

knock and detonation are not the same thing but do get used together.

there is alot more in depth we could go but this is basics

 

Interesting. I guess back in my day it was better to have a smaller combustion chamber with a flat or dish piston or a dish vs having a dome.

Regardless, I would still want the lightest piston possible

 

dishes and domes both get the air moving better than a flat top does, but combustion chamber design really effects piston profile. big chevys with larger 118ish chamber allow domes well........58cc stage3 or 65 cc stage 2buick dont have room to fit much dome

 

I see so what do our engines do good with if I was looking for a high compression street car?

 

Is CV cubic volume?

 

Larry the wizard has a link for a good compression calculator that he uses and it allows you to play with all the numbers. He will probably chime in soon!

 

https://uempistons.com/p-27-compression-ratio-calculator.html
This is the one Larry showed me, its been excellent for hours of playing around with numbers.

 

should had been cc, cubic centimeters, corrected above. never had a the pleasure of have a nailhead motor apart so dont know

 

I understand that. I must misconstrued your post in where it sounded like dome pistons was the major deal because of all the heads out thete.

 

i was always told a flat top piston was the way to go if you have the option, dome gets in the way. with a dish you could never really get a zero deck height, only around the edge. where am i going wrong ?

 

If you want the best dish get a spherical dish piston.
Are the eagle rods and pistons for 0 deck a spherical dish piston.
Mr. Wiz?

 

normally high compression and street engine in our motor architecture normally dont mix.

let's start with a few basic questions. do you plan to run power brakes? if so does that means if you wanting to power then solely from the engine vacuum. you either have to tone down the overlap on a hydraulic or solid cam, or spend the money to go to a roller cam setup, you will want to try to have near 13" vacuum to safely have enough.

now smaller overlap cams and higher static compression leads to higher dynamic compression which will require your higher octane to keep things happy

what heads are you running, aluminum heads will allow you to run more static compression, even though the aluminum heads have a smaller cc that iron head. they pull more heat from cylinder quicker than iron heads do.

how much power are you shooting for, having a solid end goal allows selection of the correct parts, that work well together. having parts that dont play well together hurts power and waste money.

take my race motor, I decided not to cut my heads or run the thinnest gaskets possible, I settled for close to 12:1 instead of pusing it towards 14. I didnt want to have to run 10,12,15 dollar a gal gas. I wanted to be able to run the same avgas we have been running. it's only 5 dollar a gal and has a little over over 100 octane and easy for me to get ahold of. some weekends we make 20-30 passes with test and tune and a full 3 days of racing if we do well each night, our car uses about a gal a pass from trailer to trailer. so saving 5 dollars a gal in fuel cost saves 100-150 bucks a weekend in total cost of a weekend race trip. I am only bracket racing so I don't need every last hp

I also didnt want a true 0 deck, we went .010 down to still give me room service the deck incase something happened. it's easier and worth it to give a little now to not have to build a new block later.
On my combo the difference between where I am at .010 down and .040 gaskets has me sitting at 12.09. going to 0 deck would it me at 12.42, going from there to .027 gaskets would be 12.88, going down towards 58cc from the 65 would be close to 14:1

also cutting the heads could of had me doing more machining to the intake also to get a proper seal. the 2 points in compression is probably worth 7ish% in total power.

I know it was a long read, but knowing what I needed and wanted and didnt want allowed me to target build our 464 to fit what we wanted to do. we didnt dyno our motor yet, but we should be 700ish maybe a little more

 

In a perfect world here is how it works out:

1. Take your block to the machine shop and have them mill the deck surfaces perpendicular to the crank. Often factory decks are not square leaving some pistons further in the holes vs other pistons. Only have enough material removed to square things off.
2. Pick your cylinder heads, have them rebuilt and then measure the cc head volume.
3. Pick your desired RPM range and desired fuel octane.
4. Pic your camshaft
5. Learn what static compression ratio is needed to put that cam in the happy zone for dynamic compression (cylinder pressure).
6. Order custom pistons to get your compression ratio to what you want. The custom piston should have a compression distance (height) to put the piston at or close to zero deck. The amount of dish or dome ends up being whatever the math works out to get your desired compression ratio. Of course you can mill your heads if you want, allowing a larger dish which some people prefer. Spherical dish is often thought to be preferred over flat tops as it promotes more complete burn patterns in the chamber. Custom valve reliefs are normally added to allow the valves to clear the pistons.
7. Personally I set my quench or squish zone to 35-40 thou. This is the distance between the cylinder head surface and the top of the piston. This area is where the air and fuel gets squished into the chamber before it’s ignited. Too tight of a quench distance and you make hit your pistons into your heads causing a fatality. Too loose of a quench distance and you loose power and efficiency and actually promote pre-ignition which is engine damaging... on stock engines the quench distance is so large that there no real quench action. Most gaskets are about 40 thou so many people do zero deck pistons and just use the gasket distance for the quench distance.