KZ Flat top vs Crowned piston

I wanted to know if anyone’s had experience switching from the stock 4 degree crowned pistons on their TM KZ or ICC motors to the aftermarket(Vertex, Meteor, etc) flat top pistons?

Theoretically there should be gains with the flat top but has anyone noticed an appreciable benefit? What about the lighter pistons offered, or the combination of the two?

How does the flat top affect the squish band? If ~1mm (.039”) is a good starting point for the crowned piston what would it be for a flat op?

Finally, I know the head insert/combustion chamber has to be changed out or reworked as well, but I would think the the actual cylinder head would remain stock as it doesn’t have an effect on the shape or volume of the combustion chamber.

Thanks for sharing your thoughts and experiences

Hiya,

Flat top pistons have better top end run out vs 4 degree. Seems most have gone that way now.

Can’t comment on anything else though sorry I’m not a builder

Not an engine builder either, but wouldn’t the crowned piston compress the air/fuel mixture more, because of the rounded cylinder head?

The squish gap will be the same, but with a crowned piston, it may compress the fuel more, since the gap between the top of the cylinder head and the piston is smaller in the middle than with a flat piston.
But then again, less squish gap sometimes means more top-end performance.

I don’t know anything about it, but that would be my theory.

Generally the head (Well combustion chamber) is machined to match the profile of the piston crown along with squish clearance and velocity being considered.

On flat vs dome, it’s a fairly nuanced thing from what I noticed. But yes ideally you will want head inserts to match the profile and make sure the head volume is above the minimum for KZ.

Thanks for the responses guys.

@ohasha …Thanks, I wonder if the improved top end is mostly due to the flat top being lighter. The stock pistons on all the TM KZs up the R1 are 4 deg, but I suppose if you are looking for higher peak RPM at the expense of some mid range the flat top is the way to go.

@Alex… you are right, tighter squish will increase compression, so the motor will be run a little flatter at high RPM but will be a little punchier in the mid range. I don’t think the crowned piston reduces combustion chamber volume and increases compression because, if nothing else changes, the ratio of CC volume from BDC to TDC remains fixed.

@James… I haven’t come across minimum CC vol before inthe regs for any of the local clubs or in the homologation for my engines. Where can I find that info?

Have come across squish velocity before on the forum but don’t know much about it and how yiu would calculate it.

Fortunately I can source the matching CC insert for a flat top piston for my KZ10, I presume the squish band is machined to 0 deg to match the piston. I’ve done many top ends but have never changed the insert. Is it a press fit? Anything else you need to know thr change the head insert?

Thanks again guys

If you’re running KZ and it’s expected to run KZ regs then there’s a number of regulations on:

Carb must be VHSH 30
Exhaust as homologated
Ignition analogic only (ie no CDI)
Exhaust port timing 199deg max
Head volume 11cc minimum

CIK Regulations for KZ2 here: https://www.fiakarting.com/sites/default/files/2021-04/6.0_RT2021.pdf

Although they are CIK regs, they are pretty universally adopted whereever those classes are in place.

Now if it’s an open class that allows you to run a KZ engine however you want, then it’s open season. That said, it’s hard to squeeze more on-track performance out of a KZ even without those restrictions on a sprint track. My personal first choice would be ignition controlled via throttle position with a basic standalone ECU.

Port timing and or bigger carb can make the engine peaky which hurts chassis balance.
Compression kills your rolling entry speed.
While those will give gains on the dyno they can be challenging to tune around.

Thanks James, before I saw this I pulled up the CIK regs but gave up looking for the CC vol, thanks for finding it.

In the local clubs there here in western Canada and US NW it’s typically another 10-15 lbs for mod vs stock KZ, and by default the Stock KZ class should conform to the CIK regs or homologation for a given engine .

Interesting point about compression and rolling entry speed. I think the increased engine braking from higher compression would be an advantage if you can trailbrake as deep as possible into the corner, but in practice this is hard to do with only the left hand on the steering as you are downshifting. With paddle shifters I think you could probably take advantage of the added engine braking.