Don Terrill’s Speed-Talk

Given all being equal how much is a flat top piston worth, as far as hp, over a domed piston. (If any at all?)

ie. = 14:1 to with a flat top and a 50cc chamber on a sbc or 14:1 with a dome and a 64cc chamber.

Sorry if this has been beat to death but I screwed around in search and couldn't find squat.

Thanks.

BTW the reason I ask is this. Have dart 64cc chamber heads and I am switching to a dart block over the winter so I going with a bigger bore so I am trying to figure out....plane the heads and run a flat top piston or leave the heads be and run a dome piston

FWIW, I've always made more power with a dish or flat top than any dome. I don't use domes any more if there is an alternative.

thnx, jack vines.

Part of the reason why an exact answer isn't practical is that some engines will suffer more from a dome, not only because the dome masks flow and flame travel (duh), but also that it changes the effective overlap, so if the cam was highly developed for the engine before and you go to a flat dome your overlap may now be excessive (higher, narrower powerband). Of course, if the cam was mild before it may help here!

panic wrote:Part of the reason why an exact answer isn't practical is that some engines will suffer more from a dome, not only because the dome masks flow and flame travel (duh), but also that it changes the effective overlap, so if the cam was highly developed for the engine before and you go to a flat dome your overlap may now be excessive (higher, narrower powerband). Of course, if the cam was mild before it may help here!

I am assuming that it changes overlap characteristics due to the dome affecting air flow?

Every real big horsepower engine I have ever made had a dome of some sort. I do not feel that the dome is bad for high horsepower. I do realize there are bad dome shapes, but when it takes high compression to get the power up where you want it. I go for the domes every time.

JOE SHERMAN RACING

Ive seen the same thing as Joe. All our high powered engines have had domes on them to get compression up. We have never done a 16:1 flat top engine though. But I suspect that If we ever have a combo that will make 16:1 with a flat top we will just put a dome on it for 17:1. I have NEVER added compression and lost power.

Couple things to be aware of for your deal. If you surface your heads a ton take into consideration the effect it will have on valve pocket depths. The pocket depths will dictate top ring placement and piston weight. By the time you surface 14cc's off your heads (probably close to .085) the piston design may be seriously compromised. Also, surfacing heads a bunch sometimes hurts air flow.

Theres almost always a give and take thing going on when you are doing a high power engine project.

bigjoe1 wrote:Every real big horsepower engine I have ever made had a dome of some sort. I do not feel that the dome is bad for high horsepower. I do realize there are bad dome shapes, but when it takes high compression to get the power up where you want it. I go for the domes every time.

I agree with Joe.

The introduction of a hemi combustion chamber shape allowed larger valves to be used for a given bore size. Since the valve were angled, it became very difficult to obtain the required compression ratio. This was solved by the use of domed pistons. Other popup piston crown shapes (like pyramid shapes for pent roof chambers) have been used as well.

At very high compression ratios, there is very little combustion volume remaining and the valve relief pockects become the combustion chambers.

So all the latest best designs are wrong? Haven't all the OEMs and the Cup engines gone for high compression with shallow chambers and sometimes a dished piston? Do we see many domed pistons on anything but the older engine/head designs with large combustion chambers? Sometimes it's the only way, but is a dome the best way when there is an alternative?

thnx, jack vines

Cup engines had domes until the compression ratio was dropped.

We have an 11:01 rule Circle track and flat tops only no dome the rule says and we used a dish piston and made better power then a flat top piston but teck got us on calling the pistons a reverse dome piston.

robert1 wrote:Cup engines had domes until the compression ratio was dropped.

That was along time ago. Even though they are limited to 12-1 today instead of the open comp (15-1+) of yesteryear, they are up 100hp from the open comp/dome day's. Basicly all other rules being the same. There have been other changes that have led to the power increase, but the smaller combustion chamber/no dome is a big factor. Comp ratios being equal, unless spark plug location is compromised getting to the desired comp (over rolling due to angle milling ect.), a flat top or dish will always run better. IMHO

On the last few builds we have done we have angle milled the heads to gain compression points and used either flat tops or dished pistons. I am interested in your thoughts on this method of gaining compression and if this also helps slightly with the vavle angle.

What about the effects of domes on ignition advance and limits due to detonation?

If you are pushing 16:1 and the limitation of "normal" combustion it seems to me that the more compact chamber of a flat top of dish would need less timing and be less subject to detonation limited ignition timing.

Thinking about this if I was going to rev the piss out of an engine at over 9000 rpm for 4+hours straight, along with allowing for light weight rods, small wrist pins, Honda journal, etc..etc.. I might want the lightest piston I could get in there (dish)…while still keeping my desired cp ratio(Dome..extra weight?)

stealth wrote:Thinking about this if I was going to rev the piss out of an engine at over 9000 rpm for 4+hours straight, along with allowing for light weight rods, small wrist pins, Honda journal, etc..etc.. I might want the lightest piston I could get in there (dish)…while still keeping my desired cp ratio(Dome..extra weight?)

Cup engines have min weights. 400gm piston

Crown thickness is still pretty thick on a dish piston. I think the gain has more to do with controlling the flame kernal and pushing straight down on the piston with the dish rather then rocking the piston by working up over the dome.