Head cc's or dish cc's: any difference?
01-02-2011 | 09:26 PM
#1
Head cc's or dish cc's: any difference? I was wondering something lately. I'm building an engine and since my heads' cc's were a little on the smallish size, I opted for pistons with a larger dish to get the compression ratio I wanted. This gets me to the question- Is there any real difference between making the cc's in the head or in the piston's dish? Is there any advantage one way or the other?
My guess is that with a dished piston I might not be getting the flame pattern needed if the chamber was designed for a flat-top piston. Any ideas on this guys?
My guess is that with a dished piston I might not be getting the flame pattern needed if the chamber was designed for a flat-top piston. Any ideas on this guys?
01-02-2011 | 11:15 PM
#2
Thats actually a pretty good question. Also one I haven't ever thought about til you brought it up. Technically, its the same volume just relocated to a different spot. It's all in the name of clearance though. Lets see if someone has had a real world test or knows if its a difference at all. I'm pretty curious too.
I bet it doesnt make too much of a difference since its probably not like a -60 cut.
I bet it doesnt make too much of a difference since its probably not like a -60 cut.
01-02-2011 | 11:37 PM
#3
It all comes down to flame travel valve shrouding and hot spots. A flat top piston will have a cleaner flame travel and create less hot spots than a dished piston with peaks and valleys from the dish and valve releifs. Also when using a larger chamber over a dished piston, you have less valve shrouding with a more open chamber rather than one just big enough for the valves to reside in.
01-03-2011 | 12:09 AM
#4
If your pistons are dished to still allow maximum quench you'll be fine.
Look at the print of the head gasket on your heads. You will have areas of the head inside this fire ring that are flat. Does the flat area of your piston match the flat area of your head? If they do all is good. You need to get the final clearance between the piston and head between .030-.040 to really take advantage of the quench action.
I'd have done maximum valve deshrouding on the heads (this opens up sections of the comb chamber) and re-cc the heads before choosing a piston.
A flat top piston has some advantages. They usually weigh less than a dish and can be stronger. Usually cheaper as well.
Unless you have DEEP dishes you won't effect the flame pattern. Domes are what give the biggest troubles with flame front.
A " physics perfect" combustion chamber would be 100% spherical in shape with the ignition occuring dead center. this would allow the flame front to expand evenly in all directions and completely cross the combustion chamber in the shortest amount of time. This allows reduced ignition timing. Sadly this shape is not the easiest to put good ports into and getting that ignition dead center will be a real trick. Ignition arcing from the electode in the head to the top of the piston (the ground) is being played with now in R/D labs so this trick may well be possible in the future. This design would place 1/2 of the total cc in the piston. Dished pistons are not inherently bad.
Years ago I saw pictures of Ford designed Jaguar research and development engine. They were testing for maximum compression before detonation using the best normal pump fuel available in europe. They were able to run 14 to 1 using a weird looking set up. Most of the chamber was flat with the intake valve right on the bottom of the head. The combustion chamber looked like a childs dixie cup impression and the exhaust valve was at the bottom of this cup (substantially different valve installed depth between the two). The dixie cup diameter was 1/3 of the diameter of the chamber and the spark plug was in the side of this depression. A matching depression, less deep, was in the top of the piston. Ford used info they gained from this engine in revamping the Jag engine line. Ever seen a little tag on the back of a Jag that says "HE" (high efficiency)? The above test engine is why. I wish I still had the pictures. Looked like a strange diesel head.
One distinct aspect that dishing could help would be in piston to valve clearance. Concentrate dishing to this area if possible.
If your dished pistons have a large area of the dish where the head is flat then the piston designer should be drawn and quartered using Shetland ponies (takes longer/more painful).
This is just my opinion-not a fact or a suggestion.
Reducing the quench area never helps and almost always hurts (increases detonation tendancies). The only reasonable reason I've seen to do this is when the quench pad on the head overshrouds the valve and the detonation caracteristics of the head allowed the removal. It needed more flow and was not prone to detonation anyway. (4 valve rb26 Nissan engine in a high boost application running race gas).
IMHO any dishing of the quench area is a poor design. Dishing in the valve contact area is a good design as long as you can minimize weight gain and still have the strength you need in the piston top.
Enough bench racing for me tonight.
Time to go rebuild a Holley.
Look at the print of the head gasket on your heads. You will have areas of the head inside this fire ring that are flat. Does the flat area of your piston match the flat area of your head? If they do all is good. You need to get the final clearance between the piston and head between .030-.040 to really take advantage of the quench action.
I'd have done maximum valve deshrouding on the heads (this opens up sections of the comb chamber) and re-cc the heads before choosing a piston.
A flat top piston has some advantages. They usually weigh less than a dish and can be stronger. Usually cheaper as well.
Unless you have DEEP dishes you won't effect the flame pattern. Domes are what give the biggest troubles with flame front.
A " physics perfect" combustion chamber would be 100% spherical in shape with the ignition occuring dead center. this would allow the flame front to expand evenly in all directions and completely cross the combustion chamber in the shortest amount of time. This allows reduced ignition timing. Sadly this shape is not the easiest to put good ports into and getting that ignition dead center will be a real trick. Ignition arcing from the electode in the head to the top of the piston (the ground) is being played with now in R/D labs so this trick may well be possible in the future. This design would place 1/2 of the total cc in the piston. Dished pistons are not inherently bad.
Years ago I saw pictures of Ford designed Jaguar research and development engine. They were testing for maximum compression before detonation using the best normal pump fuel available in europe. They were able to run 14 to 1 using a weird looking set up. Most of the chamber was flat with the intake valve right on the bottom of the head. The combustion chamber looked like a childs dixie cup impression and the exhaust valve was at the bottom of this cup (substantially different valve installed depth between the two). The dixie cup diameter was 1/3 of the diameter of the chamber and the spark plug was in the side of this depression. A matching depression, less deep, was in the top of the piston. Ford used info they gained from this engine in revamping the Jag engine line. Ever seen a little tag on the back of a Jag that says "HE" (high efficiency)? The above test engine is why. I wish I still had the pictures. Looked like a strange diesel head.
One distinct aspect that dishing could help would be in piston to valve clearance. Concentrate dishing to this area if possible.
If your dished pistons have a large area of the dish where the head is flat then the piston designer should be drawn and quartered using Shetland ponies (takes longer/more painful).
This is just my opinion-not a fact or a suggestion.
Reducing the quench area never helps and almost always hurts (increases detonation tendancies). The only reasonable reason I've seen to do this is when the quench pad on the head overshrouds the valve and the detonation caracteristics of the head allowed the removal. It needed more flow and was not prone to detonation anyway. (4 valve rb26 Nissan engine in a high boost application running race gas).
IMHO any dishing of the quench area is a poor design. Dishing in the valve contact area is a good design as long as you can minimize weight gain and still have the strength you need in the piston top.
Enough bench racing for me tonight.
Time to go rebuild a Holley.
01-03-2011 | 10:09 AM
#5
Lots of good info there, thanks.
Here's the dish of the pistons I'm using. I like that there is plenty of thickness at the top ring land. It's a -18cc dish. These will be used on a boosted engine.
Here's the dish of the pistons I'm using. I like that there is plenty of thickness at the top ring land. It's a -18cc dish. These will be used on a boosted engine.
01-04-2011 | 07:27 AM
#7
They're forged CP pistons, 3.905" -18.2cc CH. They are going into a forged iron 383 with a 2300 Magnusen supercharger with around 14lbs boost, 9.6:1 compression and 62cc CNC ported 243 heads with 2.02/1.57 valves hopefully on pump gas. It should be done by summer.
