Flat top or dome

[noodle]

Please excuse my ignorance, but can one of the Gurus tell me what the benifits or downfalls are of one piston over the other, if any? I did a search but couldn't find anything.

 

[Matt_E]

Flat top pistons will give lower port timing in a SJ engine, giving better bottom end power, and a bit more power throughout.

 

[noodle]

Cool, thanks that makes sense.

 

[yamaboy2000]

I have personally had 4 flat-top engines, most in the last few years and before that, way to many Yami domed/pumped engines..Flat-tops *might* produce like 4 Hp more(on a twin) and be a bit lighter by being shorter and forged weisco's, but I don't think the gain(if noticable) is worth the price, special set-up and general pain-in-the-azz..My current engine which was a flat-top lamey now uses a 4mil spacer plate between the case and cylinder so I can use domes OEM/pro-X pistons again and I am much happier..On a dyno flats will look better on the water and in your hands I doubt theres a noticeable difference, just more cost and hassle..:brap:

 

[noodle]

Ok so if I got hold of an 8mm Lamey with flat tops in it and wanted to replace them with dome tops on a rebuild, what size spacer plate would I need to keep the same port timing?:dunno:

 

[Roo]

Ok so if I got hold of an 8mm Lamey with flat tops in it and wanted to replace them with dome tops on a rebuild, what size spacer plate would I need to keep the same port timing?:dunno:

If it is already setup for Flaptops, why would you want to convert back to Domes? Just curious...

Maybe there was a point from the last post I missed.

Anyway, :brap: ON!

 

[sjetrider]

LUV the flattop power, IMO its well worth it if done correctly, but it is much more expensive to rebuild a FT top end unless your set up will allow kawi pistons (like lamey sleaves).

 

[egbrig]

Flattop or Kawi style pistons doesn't lower the port timming it increases it with the way the cylinder's are set up. The flattop's squish works better and the piston crown disapates heat better due to less surface area. I've heard from a very reliable sorce that there is about a 8 hp. gain in the bottom to mid for a yami engine. There are many reasons to use the flattop i.e: gain port timming without changing roof angle, lighter design piston and several others just look it up. As far as initial compression goes, there is only about a 1% increase in that so that doesn't come into effect.

 

[RideItLikeYouStoleIt]

Flattop or Kawi style pistons doesn't lower the port timming it increases it with the way the cylinder's are set up. The flattop's squish works better and the piston crown disapates heat better due to less surface area. I've heard from a very reliable sorce that there is about a 8 hp. gain in the bottom to mid for a yami engine. There are many reasons to use the flattop i.e: gain port timming without changing roof angle, lighter design piston and several others just look it up. As far as initial compression goes, there is only about a 1% increase in that so that doesn't come into effect.

I dont think that makes sense. The more surface area for a given material the higher the heat transfer. The only way that could make sense is that the piston top has less area to absorb the heat from the ignited gases. So really it must be the lower surface area doesnt absorb as much heat. I think its really minimal though.

 

[RideItLikeYouStoleIt]

Keep the info coming though. I am thinking about switching to flattop big bore setup and deciding if its worth the $$$.

 

[nikad58]

Piston price should make that decision for you.. Bang for buck !
Also im 99 percent sure that R&D stoped thier dominator piston line..

 

[Matt_E]

Flattop or Kawi style pistons doesn't lower the port timming it increases it with the way the cylinder's are set up.

Thanks for setting me straight. You are right - the domed piston crown is already even with the bottom of the transfer ports at BDC, so a flat top piston will sit below that. So, you can lower the floor of those ports and have higher duration while keeping the roofs the same.
However - the ports will close later than in a round dome setup.

 

[egbrig]

Matt,
I didn't mean to sound like a "know it all" DH. If you put a degree wheel on the engine and watch what happens when you just put in a FT piston then it will make sence. Yes, and Wayne, that was a poor gramatical statement I made, "it made sence in my mind" but not after I typed it out for all to read. The FT piston dome has less surface area and therfore doesn't absorb as much heat in the same time frame as a more domed piston. You can get away with a little more in a FT pison of 2' than a domed @ 14'. There are also flow issues that a FT has a little advantage over a domed, but thats at the opening of the port and is rpm dependent. There is information out there and my explans-ha ha, will be limited by my communication skills. But a domed piston is stronger because of the structure of a dome in compression on the power stroke. Once again go read. Its all what are you trying to achieve. But then the Polaris engines make as good of a power band as any FT Yami. SETUP, SETUP, SETUP
Erik

 

[1550]

how much flattop piston head differs from dome piston head?

Have anybody picture?

Can i modify mine protec girdle head domes for flat top? And how much is good dome size when using 86 pistons and want to use 99 ron & mon gasoline.

i have now 34cc 84 bore domes but i think i must make those fit for 86 pistons, but should i change some dimensions?

 

[bad93blaster]

Piston price should make that decision for you.. Bang for buck !
Also im 99 percent sure that R&D stoped thier dominator piston line..

they are made by wiseco for r&d. so if r&d stopped you should be able to get them through wiseco.

judge just got me a set of 85mm almost double the price of a domed piston

 

[duckman1]

The flat top also resist detonation better, thus one of the main reasons the factory teams used to use them.

 

[egbrig]

Yes, resistance to deto is one of the most important reasons for a FT piston and this is partly due to less surface area and the effect the squish design offered by a FT. I cut my domed pistons with a bandsaw and polish them with a belt sander. Is this the best method?

 

[#ZERO]

they are made by wiseco for r&d. so if r&d stopped you should be able to get them through wiseco.

judge just got me a set of 85mm almost double the price of a domed piston

I was told through R&D that they no longer carry the Dominator flat top pistons and if I wanted to order some there would be a two hundred count minimum order.

I'm pretty sure that Wiseco will have the same minimum order quantity.

 

[#ZERO]

Flat top pistons will give lower port timing in a SJ engine, giving better bottom end power, and a bit more power throughout.

Yep... the flat top pistons are 3.8mm lower then the standard pistons and give lower transfer port timing but the cylinders have to be milled down from 119mm to 115mm which raises the exhaust port timing roof.

Also… the flat top pistons are not completely flat they have about a two degree angle.

 

[egbrig]

#.0 I don't understand what you mean by lower tr. port timming. If you have to deck the base of the cylinder then you are doing so to lower the port timmings back to a workable range and when decking the base, doesn't that lower all ports the same amount?