V-Twin Forum banner

1 - 11 of 11 Posts

·
Registered
Joined
·
42 Posts
Discussion Starter · #1 ·
Assuming that the same compression would be build - say 9.7:1 - with flat top pistons versus domed pistons.
What would be the difference between these 2 ? Any reason to choose one above the other - besides cost ?

Jef
 

·
Registered
Joined
·
1,700 Posts
valve clearance , also worth considering is are they cast or forged.
 

·
Registered
Joined
·
487 Posts
flat tops offer a more efficient chamber as there is nothing disturbing the turbulence of the mixture from the squish area around the outer surface of the chamber where as the pop up dome actual gets in the way of this depending on the shape of the outer edge and how close to the chamber edge of the pop up could actually shut this off but at 9.7:1 the diference would be minor
 

·
Registered
Joined
·
6,983 Posts
Otto said:
flat tops offer a more efficient chamber as there is nothing disturbing the turbulence of the mixture from the squish area around the outer surface of the chamber where as the pop up dome actual gets in the way of this depending on the shape of the outer edge and how close to the chamber edge of the pop up could actually shut this off but at 9.7:1 the diference would be minor
Yes, and I would add that flat tops offer a more even burn across the face of the piston.
 

·
Registered
Joined
·
9,150 Posts
Interesting thread...

nidan, you mention cast versus forged. My dealer recommended that I go with the cast high compression pistons instead of forged because they were not as noisy and the fact that I wasn't going to get into head work.

He put in: 10.25:1 Cast High Compression 95 cu. in. Pistons 22661-99A

What are you guys opinion... seems like the dreaded chirp started for me about the time they put them in. I have since gone to the Wood TW-6G cam.

Hope this question still works in your thread.
 

·
Administrator
Joined
·
11,285 Posts
Flat tops are considered a better performance design because it doesn't interfere with the flame front and it doesn't obstruct the area around the quench (squish). In the current HD motors, the combustion chamber design is different than the older motor. In the older motors a domed piston or pop-up piston would litterally divide the combustion chamber into two part. The flame front would actually have to climb over the piston and this would cost power and cause other issues. Look at the old ironhead sporty pistons for a prime example. This made dual plug heads more desireable. In the Twin Cam design the domed pistons (hi comp) don't actually divide the combustion chambers. This doesn't block the flame front and it doesn't have to climb the piston. I wouldn't hesitate to use a domed piston in a conservitive preformance build. If you are drag racing and need every ounce of torque and power, that might be a different story.

As for cast vs forged. If you don't plan to exceed 6200 RPM's or 10.5:1 compression or add any type of boost, there is no reason to use forged pistons in a street build. IMO

In the photo below you can see a flat top piston on the left, shovel piston in the middle and a sportster piston on the right. The TC HC pistons just aren't the same.

 

·
Registered
Joined
·
465 Posts
Flat-tops also minimize surface area for heat transfer, minimizing heat transmission into pistons. Less heat leaving the burning gasses means more energy for making power. Probably a very minor effect, though. I think uninhibited flame front propagation is the major benefit of flat-tops, followed by less quench interference. I don't think valve clearance is much of an issue with Evos and TCs because the pop-up areas aren't in the way of the valves like they were with the hemi style iron sportys and shovels.

I've read that a well designed cast piston will have more sophisticated geometries to improve thermal expansion into a nice right cylinder. However, one would have to sell a crapload of pistons to make the casting investment worthwile. So forging is a less expensive alternative to a lower volume enterprise, or one that must make many variations. With a forged billet one can make all varieties while stocking identical aluminum billets. The expense of forging is more raw material, machining time, and CNC program investment. The advantage is manufacturing flexibility and a simpler, lower tech process. I know the mfg pimp CNC machining from billet as the zenith of technology but they're full of crap. Hitting cycle start is simple, once the programming is completed (also not that hard). Investment casting a complex shape for a demanding environment takes skill.

No, I haven't said which is better. Depends on application, I think. Like someone typed, only a real firebreather NEEDS forged pistons. Other than that I think cast are superior. But you may have limited options with cast pistons.

Chilly
 

·
Registered
Joined
·
755 Posts
Nicely done, Springer. :thumbsup:
 
1 - 11 of 11 Posts
Top