squish on flat tops

[Goose]

on flat top pistons im hearing .015 is safe to run is this true?

 

[Rickster]

I'm running .055 on my ski's and I think Waterfreak likes his around .045 on his flat top motors.

We do not do WOT runs with these setups...

.015 is way too tight IMO...:sad6:

 

[waterfreak]

I'm running .055 on my ski's and I think Waterfreak likes his around .045 on his flat top motors.

We do not do WOT runs with these setups...

.015 is way too tight IMO...:sad6:

that's for sure!! at .015, your pistons will probably hit he domes. I've heard people running as tight as .035 but I would run .050 to be safe.

 

[Goose]

thanks i just been hearing from some people that you can run .015.but thanks alot guys

 

[yamaboy2000]

I just went thru this last year in great detail with DASA...if running race gas and the engine is for comp only and engine life isn't that important .035 ----if the engine is on pump gas(correct compression of course) & is for hi-perf/hard core recreation .050-.055---So some where between these #'s for your application and intended purpose...

 

[Goose]

no im confused.from what i understood you can run less squish with a flat top.now im hearing i need more than what i would normaly run on a dome piston.i figured with the flat top dealing with deto better that you can run less squish and still be safe

 

[norcal ex]

I thought you could run more compression on flat tops...making them better?

 

[#ZERO]

Your squish band clearance depends on a number of different factors. Most pump gas engines tend to like a couple of degrees of squish angle with wide squish band widths and smaller diameter domes with more volume. Race gas engines like narrow squish band widths and larger diameter domes with less volume. When checking your piston squish the smallest area will be at the edge of the piston because the squish band width has a bending radius toward the dome. If you're using ADA flat top domes they have a .015" squish relief at 86mm with a 4° angle. If you're using Riva flat top domes they have a .005" squish relief at 85.5mm with a 1.5° angle. Also Riva/R&D flat top pistons are completely flat while the Kawasaki pistons have a 2° angle this need to be calculated in when checking your squish band radius.

What type of flat top engine are you building, stroke, bore, fuel, pistons, porting, ignition, dome size, etc?

 

[Goose]

gonna be a bigbore with la sleeves 5187b and 84.5 wiseco pistons that are cut at a 4degree angle across the top of the piston to make it a flat top.probly run 100 octane ported cases and cyls ex port will be 70%of the bore size
port timing will be
ex ports open at 89.5 degrees
transfers open at 122.5 degrees
185 duration on ex
119 duration on intake
33degrees blowdown
intake side of the sleeves will be cut about 10mm off and cut off about 12mm of the sleeves on the transfers
on 62t cases

 

[yamaboy2000]

what kind of pistons are you cutting 10mm of the intake side?? are you creating an eyebrow shape or U shape or straight cutting 10mm off and why? just curious....my understanding of flat-tops is really that you can get away with a bit more compression when compaired to a domed unit without detno as the heat is spread across a larger surface not a raised dome top that heats more quickly...I would recommend a target squish of .045 for you and compression, who knows with race 100+ race gas the sky could be the limit:dunno:???

 

[Goose]

im only cutting the top of the piston im not cutting the skirts of the piston.im only cutting teh sleeves and ill cut it straight across on the transfers just to open them up

 

[egbrig]

The squish band design depends on several key factors. You can figure out MSV, cut the dome, then try it out. I don't know where some of the info on squish is coming from but #.0 your parameters are incorrect. I can tell you that the best factory yama. motors run exactly the reverse of what you stated. If seen guys run .35" squish but they had a good billit crank. Also do you know how to check squish properly so you don't include deflection of the piston? The guy who said .15" is ok, don't listen to anything he has to say because he doesn't know a thing about how a engine works. Just checking cleareance isn't the only factor of what type of gas you can run with a squish cut. What about squish angle? Width, and what effects the width and angle have. There are a few guys on this board who build engines professionally, get there #'s and call, or maybe they'll chime in. Also flattop squish differs from dome squish design.

 

[#ZERO]

I wouldn't recommend cutting the Wiseco piston tops because they're too thin already. I've seen more then a few cases where the center or the pistons dome collapses from overheating and making them thinner would make this problem worse. Also if you're going to use 4 degrees on your piston tops you'll need to custom machine your domes with a 1.5 ~ 2.0 degree bending radius gap toward the center making your domes with a 5.5 ~ 6.0 degree angle. If you're going to build a low budget flat top engine just use the Kawasaki pistons and chamfer the edges of your auxiliary transfer ports well and lightly chamfer the ring end gaps so there won't be an issue with the ring snag. If you're going to install the sleeves lower and modify the transfer ports, an angle rotary table works great on a milling machining to ensure accuracy. It's very important to keep the height as close as possible because each .015" in height change is equal to about one degree in port timing. One of the main reasons why flat top engines are less prone to detonation damage is because of the smaller piston top surface area with less heat buildup and most flat top domes have a narrow squish band radius with faster reaction times.

The squish band design depends on several key factors. You can figure out MSV, cut the dome, then try it out. I don't know where some of the info on squish is coming from but #.0 your parameters are incorrect. I can tell you that the best factory yama. motors run exactly the reverse of what you stated. If seen guys run .35" squish but they had a good billit crank. Also do you know how to check squish properly so you don't include deflection of the piston? The guy who said .15" is ok, don't listen to anything he has to say because he doesn't know a thing about how a engine works. Just checking cleareance isn't the only factor of what type of gas you can run with a squish cut. What about squish angle? Width, and what effects the width and angle have. There are a few guys on this board who build engines professionally, get there #'s and call, or maybe they'll chime in. Also flattop squish differs from dome squish design.

What's wrong with my parameters, are you still cutting your domed pistons with a bandsaw and polishing them with a belt sander?

 

[Goose]

so is their a great advantage to the flat top design or not much gain for the work thats invovled?

 

[egbrig]

I meant cut the squish band (combustion dome). I've cut a few heads in my day and played with different angles and band widths. 50% is a standered band ratio and I will tell you that the best factory yama. and kawi engines have had more, I've seen it for myself. If you want to get technical you don't need to set the squish angle to more than the piston dome it can be run as a paralell angle. Yes I have seen the best of Teams Kawi and Yam. Its depentant on the rest of the setup but it not an absolute.

 

[Goose]

how would the above setup run with regular yam domed pistons

 

[crammit442]

so is their a great advantage to the flat top design or not much gain for the work thats invovled?

In the prime of Moto GP some factory teams used domed pistons, some used flat tops. I personally don't believe flat tops are worth the extra hassle and expense. I also haven't experienced the extra performance they're supposed to provide. I don't have issues w/deto so I don't need their claimed resistance to it. I've ridden really good running motors with both types and haven't seen any trend proving their value on a seat of the pants level. A well setup dome motor will spank a run of the mill FT motor. Just my opinion.

 

[#ZERO]

so is their a great advantage to the flat top design or not much gain for the work thats invovled?

Just run the dome pistons spend your money in other areas, the extra machine work is not worth the power gains. IMO

I meant cut the squish band (combustion dome). I've cut a few heads in my day and played with different angles and band widths. 50% is a standered band ratio and I will tell you that the best factory yama. and kawi engines have had more, I've seen it for myself. If you want to get technical you don't need to set the squish angle to more than the piston dome it can be run as a paralell angle. Yes I have seen the best of Teams Kawi and Yam. Its depentant on the rest of the setup but it not an absolute.

Two stroke engines tend to like about 1.5 ~ 2.0 degrees of tapered squish angle. This helps with setting up turbulence under the dome and ensures a complete burn of the fuel air mixture effectively. On a stock restricted-exhaust engine, it's much easier and more common to run parallel squish but on highly-tuned performance engines running parallel squish can be asking for trouble.

how would the above setup run with regular yam domed pistons

Those sleeves have the 760 port timing so it should run great in the mid range but probably won't hit off idle like the 61x sleeves.

 

[BROSS]

He was talking of lowering them also.

 

[Goose]

the bottom of the cylinders are cut and the port timing is within 1 degree of a 61x