Pistons - flat or domed?

I have seen in pics here that it looks like you fast guys like those flat top pistons.
What are the advantages?
Has technology changed that much or has information gathering just gotten that much better?

Growing up when and where I did, Compression Ratio was huge when building a hot engine. 8.5 wasn't going to cut it.
I've had Powermax pistons in my bike for the past 15 years (street use) and never had a problem.

Do you racers still use the Omega or Hemi type pistons?
Just wondering.


ANOTHER QUESTION:
Do any of you guys still use the Hepolite Powermax pistons or am I really that far behind the times?

My simplistic explaination is that the Commando combustion chambers are a pretty good shape and the ports reasonably good too. Later tuning trends see improvements to the inlet, but overall its a good 2 valve head. Flat top pistons allow a good flame spread and even pressure on the piston. Increased compression is achieved by decking the head i.e. the 750 Combat heads to 10:1 compression. On the other hand, Triumphs raised compression by fiting big dome pistons. These have some limitations in mixture flow and flame propagation. You may also note, the Commando full advance ignites at 28 degrees where other big twins would be upwards of another 10 degrees. That implies a more efficient combustion environment.

Mick

From my experience Powermax pistons actually make pretty good power with their mild dome. They are a bit fragile for racing use because they don't handle high temperature as well since they are a cast piston. I have seen better power using flat-top forged pistons if you can keep the compression ratio up. Jim

My simplistic explaination is that the Commando combustion chambers are a pretty good shape and the ports reasonably good too. Later tuning trends see improvements to the inlet, but overall its a good 2 valve head. Flat top pistons allow a good flame spread and even pressure on the piston. Increased compression is achieved by decking the head i.e. the 750 Combat heads to 10:1 compression. On the other hand, Triumphs raised compression by fiting big dome pistons. These have some limitations in mixture flow and flame propagation. You may also note, the Commando full advance ignites at 28 degrees where other big twins would be upwards of another 10 degrees. That implies a more efficient combustion environment. Mick

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MIke made it plain and simple above, when maxing out push and shove, flat tops for faster burn in chambers rule. If not maxing out then why worry, the differences don't show up till pressing detonation zone, where ignition timing for max torque pressure is the goal. Less advance in a fast burn chamber has less initial combustion pressure to work against before TDC beside more shove just past TDC.

What kind of CRs were you running in your race bikes, Jim? As I recall, it was pretty high.

Mark, you can get the stock 750 up to a pretty respectabale CR with a flat top piston, but it takes a bit of fitting. I used to take .060" off the top of the cylinder, no base gasket, stock flame ring head gasket, deeper valve reliefs in the Hepolite pistons, and a little bit off the head. The highest CR I managed that way was 11.4:1. That was with a standard 750 head with only slightly larger intake valves, and no combustion chamber mods. It's also pretty easy to get a high CR with flat top pistons on the 850 engine. Of course it's really easy with a 920 with standard 850 head. The Commando combustion chamber design with squish bands really likes a flat top piston. The problems start when you go to the shorter strokes and larger valve heads. Norton used a domed Omega piston in the short stroke 750, and still had a pretty low CR, because the big valve head had a larger combustion chamber. When you re-angle the valves to allow larger ones, and fully sphere the combustion chamber, you add a lot of volume.

With a bit of work, and pistons with a stepped dome, you can get over 13:1 CRs. My experience with them is that the extra CR gives great drive out of the corners, but looses a little top end power on a long track like Daytona, where you hold the throttle open for a long time. I ran my 920 there with a large valve head that limited the CR to just over 10:1 with a flat top piston, and it It seemed really strong the whole length of the straights and banking (right up until it broke the cases, that is). You can also get a high CR with flat top pistons and still have big valves by welding the head into a bathtub sort of configuration. I don't have enough experience with those to know how well they work.

Ken

The best on my racebike was 11.5. Beyond that I would see an increase in midrange and a loss up higher, along with cooling problems. I did a bathtub chamber but I couldn't get good high rpm power . The best chamber I had was a flat-top piston and about 1/4 inch of metal added to the chamber opposite the spark plug. Jim

Metal added across from the plug??
That's where I find the carbon blasted off the chamber in a thumb print size area. I assume d/t the intake velocity out the valve, not combustion blast, but don't know. Was the extra metal to take up volume, add squish surface or put trip up turbulence in the mix? I've really eyeballed that thumb size spot thinking about the textured surface heads I've seen.

In some race engines they have to retard spark some at higher rpm to get a head start on the slower flame front to get the max pressure push. Maybe might apply to extremists Nortons?

Reshaping a combustion chamber is way beyond me!
but..... If I was to have it done, I would look to comnoz.

If I remember the specs correctly, the Powermax pistons bumped my 850Mk3 (8.5 CR) up to a more normal 9.1.

Thanks to you guys for your replies!
The amount of expertise gathered here is what keeps me coming back.

http://www.mez.co.uk/mezporting/page6.html

hi steve, mez,or george mansfield has not been with us for a few years now

Oh, sorry to learn of another soul lost to us. I never contacted him over last decade but refer to his site to ponder what bathtub and D shaped ports are about.
Also IIRC had only example I've ever seen of two spark plug per chamber head.

I don't think I've ever seen domed pistons for Commandos, do they even exist?

Hepolite powermax and Weisco were both domed for Commandoes. I think I still have a couple pair stashed away. The metal I added to the chamber was mainly just added squish band. When I tried to add metal near the valve it just ruined the airflow and killed the high rpm. Jim

Ok now I know two brands of domed pistons to avoid in my meager needs. : )
I really only want to be a pilot, but depend on experimenters like you so I don't have to find out the hard way - with what ever time left. I've studied Singh groove forums a few years, in regards to anti-detonation, but can't see it applying to hemi's with small squish rim. Not much effect to them unless engine operated in otherwise detonation zone by lugging or low octane or extra boost. One thing interesting was how they monitored the groove effects by observing the darker to cleaner areas on piston and chamber and valves. They liked more cleaner areas.

Ken Canaga, milled Peels Combat CHO head so 920 pistons stick up in there. IIRC volume come out around 43 ml. Forgot what gasket thickness he assumed.
Smallish ratio chamber? Was pleased to see this gave 1/4" to 3/8" wide squish band. Just enough to ponder grooves again, but not doing that experiment.

hi all,domed pistons,why not,every guy and his dog knows that flat topped pistons are the way to go power wise providing you have a shallow combustion chamber with steeply angled valves,both of which our nortons dont have,ask tim joyce{phantom 309} or any fast trumpet rider if they are using flat topped pistons,i know what the answer will be

chris plant said:

hi all,domed pistons,why not,every guy and his dog knows that flat topped pistons are the way to go power wise providing you have a shallow combustion chamber with steeply angled valves,both of which our nortons dont have,ask tim joyce{phantom 309} or any fast trumpet rider if they are using flat topped pistons,i know what the answer will be

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Any Norton I know of has shallow combustion chambers in comparison to a Triumph. Triumph racers need domed pistons to make enough compression to race. Norton motors do not. Jim

Boy howdy dome pistons sure don't seem a hindrance to Tim's Triumph power!
Wonder whats going on with his engines over Nortons?

There's all kinds of interesting things going on as spark hits and then pistons approach TDC. Vapors get as dense as honey to slow swirl, compression waves get pushed ahead of piston rise, jets of fluid getting squirted here and there, then reverse -suck back directions after TDC. I like flat tops as easier to clean off.

Triumphs and Nortons are pretty similar in their power producing capability. Triumphs with their steeper valve angles and taller chambers have a breathing advantage over a Norton but a Norton's shallower chamber has better swirl and combustion efficiency. Tim' s Triumph goes very well mainly because Tim is a sharp tuner and has spent many hours getting it right. Jim

Thanks for more insights on the two ancient rivals JIm. Tim's blue/white Triumph we saw at Barber's was so much faster than Kenny and 2 others all running together neck and neck down the straights, quite a ways behind Tim - i'm still confused if Tim's Turnip missile was in another race class. But no one else even close so confused it Tim was the only one running a bike in his class as sure out classed rest of the field. Tim even had to back off quick in the initial take off 100 yd or so from start because his bike pulled a rather high sudden pop up wheelie. I assume either topping out in 1st or hitting power band in 2nd. Kenny made no mention of Tim running away from them, just the 3 way battle of Kenny and 2 others he lost a postion he said d/t a missed shift. So makes me think Tim's Triumph has some other features that put him in class Kenny was not competing with. Why am I confused at Tim's dramatic advantage? You say good tuning but I can't see how it'd make so much difference w/o some hardware differences not allowed in Kenny's class.

Prior to the Barber event Tim was on here teasing Kenny and others about his Triumph eating Nortons - I though it just fun bluffing till Tim dusted them.

The biggest difference is in development. Tim has spent many hours on the dyno playing with his bike to get the most from it. Time has not been spared.

Kenny's bike has not had that advantage. I built Kenny's engine with off the shelf parts and just assembled it with care to make sure it would be durable. Then I put it in a box and sent it to NY. I have never seen it run. Kenny's time on his dyno was to break it in and set the fuel mixture and timing. The fact that Kenny's Norton can be competitive with Tim's Triumph is a fine testament to the people all around the world who designed the parts well enough that they could simply be bolted together and do as well as they do.
Kenny also does one hell of a job of riding the wheels off it. Jim

Oh yes I'm left tingling by Kenny's proven performance too, but a little bit upset he's had so many tests of his skill to recover an over done turn or missed shift or component breakage. So I'm a bit more impressed by Kenny's skill than just his potent 'Commando' . Racer MacRae blows me away even more across the scope d/t racing a isolastic chassis and more than holding his own among the solid mounts.

I assume Tim's time is spent, head porting, dialing in cam degree and exhaust then diddling with spark and fuel to keep up? Any who I can no longer view Triumphs potential as second class to Nortons, though I await Kenny and others further refinements to close the gap.

For sure Triumph racers are as anal as Norton'rs, one telling me he'd keep plugs wires packed in white rice inside deep freeze to pull out extra dry for a heat.