Flat Top Vs. Round Top

[BROSS]

I know this has probably been talked about time and time again, but I want some advise from all of you out there on if Flat Top pitons are worth the money vs Round Top. Is it worth the money and how about the reliability of a Flat Top, and which is better for torque. Just wondering.....

 

[oxnard111]

I too have wondered when it is appropriate when to use a flat top piston.

 

[BROSS]

I'm riding on a budget so this is very important to me if I should spend extra cash on Flat Top.

 

[FL-cracker]

I was running a 85.5mm flattop (kawi Pistons/TLR ported). It ran awsome! Much more power then any ported dome top, stock stroke motor I have ever ridden. (my boat is fairly light though as well).

You will gain about 10% hp with flattops over round tops.... IMO If you run good oil, and always let your motor warm up, it will be just as reliable as roundtops, but you may need to rebuild/re-bore a little sooner than roundtops because of the shorter piston will slop around a little more... But the pistons are lighter, so probably a little less stress on the crank and rods.

Any motor I build from now on will have Kawi Flattops, they are much cheaper then the R@D' pistons.

I put probably 60hrs on mine, and the cylinder/pistons/rings are still very clean! It could have gone much longer Im sure.... unfortunetly I sunk it in the surf, and by the time I got it in I lost my airbox, and sand got in the motor, had to tear it down.

 

[crammit442]

I was running a 85.5mm flattop (kawi Pistons/TLR ported). It ran awsome! Much more power then any ported dome top, stock stroke motor I have ever ridden. (my boat is fairly light though as well).

You will gain about 10% hp with flattops over round tops.... IMO If you run good oil, and always let your motor warm up, it will be just as reliable as roundtops, but you may need to rebuild/re-bore a little sooner than roundtops because of the shorter piston will slop around a little more... But the pistons are lighter, so probably a little less stress on the crank and rods.

Any motor I build from now on will have Kawi Flattops, they are much cheaper then the R@D' pistons.

I put probably 60hrs on mine, and the cylinder/pistons/rings are still very clean! It could have gone much longer Im sure.... unfortunetly I sunk it in the surf, and by the time I got it in I lost my airbox, and sand got in the motor, had to tear it down.

That HP claim is a very broad statement that will be difficult to prove through anything other than anecdotal evidence. I've "heard" 2%, but have never personally seen any improvement over domes. The "dyno" reports tossed around here have been thoroughly discredited, so we can't use those claims. A well set up domed motor run back to back with a similar flattop motor will be VERY similar in performance. I've got a magazine with Hedlund saying that there was a small HP increase, but the reason they used them was for their slightly better resistance to deto in hi-comp triples. Well setup motors(particularly in freestyle boats) on appropriate fuel don't normally have deto problems, so that isn't much of a benefit. I suspect what you actually had was a better than average motor and THAT, not the flat tops was the difference. There are far more average or below average motor setups out there than really good ones. The additional hassle and expense of R&D or Riva pistons and potential ring gap issues with Kawi pistons keeps them from being the preffered setup for me. Also, just because your motor was "clean" doesn't mean it isn't worn out. Motors can look perfect even while the piston skirts have collapsed considerably, especially w/Wiseco. No way to tell w/o measuring. Point is, just because you have a good running flattop doesn't mean it has anything to do with it being a flatop. It might just be a good running motor.:veryhappy:

 

[FL-cracker]

Thanks for that info Crammit!

Hopefully I can borrow a micrometer from work tomorrow... but I bet the #'s are on.

That TLR cylinder is beautiful. Some of the cleanest porting I have ever seen.

Kevin Collins at legdrag did the case porting.

And I brought the weight of the Ski down to 270lbs.

The boat works great for flatwater, but usually i am beating it in the surf, and the motor takes it all.... It has been sunk in the surf on two ocasions, and cylinders full of water alot.

But sounds like the price to go flattop is not worth it.

Maybe my boat feels so strong because all the other boats I ride weight 30+ pounds more. :biggrin:

 

[BROSS]

Thanks for the info fellas, its awsome. I am thinking about round top for the money and reliability...

 

[kurtwolff]

Bill, are you also considering the 66E setup? I'll bet the big guys would give you a steal on a 800 boat this winter, part it out and keep the motor. I actually know of a complete XLT800 in Kernersville that I may pick up if you are interested!

 

[SUPERTUNE]

For the ease (a lot less expensive) of building a GOOD running engine, stay with std Yamaha dome pistons. Now if budget isn't a problem, GO BIG w/ my X-SCREAM 1000cc:eek2:

 

[BROSS]

Bill, are you also considering the 66E setup? I'll bet the big guys would give you a steal on a 800 boat this winter, part it out and keep the motor. I actually know of a complete XLT800 in Kernersville that I may pick up if you are interested!

Thanks for looking out for me, but the 800 is out and the Big Bore 61X is in swing for the winter. Too much weight in the 800, and too many questions. I am going to do all the work myself to save the big bucks.... Hope its worth it.

 

[WDKfan]

Kurt, hit me up I may be interested int the XLT 800 if the price is right.
Brian

 

[Big Kahuna]

That HP claim is a very broad statement that will be difficult to prove through anything other than anecdotal evidence. I've "heard" 2%, but have never personally seen any improvement over domes. The "dyno" reports tossed around here have been thoroughly discredited, so we can't use those claims. A well set up domed motor run back to back with a similar flattop motor will be VERY similar in performance. I've got a magazine with Hedlund saying that there was a small HP increase, but the reason they used them was for their slightly better resistance to deto in hi-comp triples. Well setup motors(particularly in freestyle boats) on appropriate fuel don't normally have deto problems, so that isn't much of a benefit. I suspect what you actually had was a better than average motor and THAT, not the flat tops was the difference. There are far more average or below average motor setups out there than really good ones. The additional hassle and expense of R&D or Riva pistons and potential ring gap issues with Kawi pistons keeps them from being the preffered setup for me. Also, just because your motor was "clean" doesn't mean it isn't worn out. Motors can look perfect even while the piston skirts have collapsed considerably, especially w/Wiseco. No way to tell w/o measuring. Point is, just because you have a good running flattop doesn't mean it has anything to do with it being a flatop. It might just be a good running motor.:veryhappy:

Every builder has his niche or specialty, TLR has an incredible Flattop Big Bore package. Probably one of the best out there....... He fine tuned that porting piston package.

But like Charles said, in the long run you are better off staying with Domed pistons........ Also, FLAT TOP motors are designed to be run on race gas and with a controllable/programable ignition so you can really tweek the timing on it to maximize the benefit of the pistons.

 

[SuperJETT]

I've heard numbers tossed around before, but never really seen proof.

The theory is the flame front propagates quicker and the force is more direct, but the difference between a domed and flat piston is very little in terms of how much dome there is, so the increase is not going to be huge IMO.

True testing to see an increase would be very hard since when you go to a flat top setup, you change the port timing since the piston is typically shorter. You couldn't just swap out pistons/domes and say it's the same engine with only different style pistons.