Negative Squish Angle on 2 Stroke

**ostsideend** · 12-16-2018, 04:37 PM

The original 3HP 2stroke Yamaha Malta does have a negative squish angle (I checked 5 of them)
So it is not done by mistake from the Japanese. They wanted them like it is.
I read that a negative squish angle in most cases does not make sense. But when it does make sense?

Due to that i cant find an answer on internet i'll try and ask you guys!
Does anybody know?

TX

Oliver

**Dave S** · 12-16-2018, 06:00 PM

Might be for piston clearance....when piston rocks?

**FMP** · 12-16-2018, 06:19 PM

With taper opening to the center of the crown, likely tries to creates a tumble of the front. Minimum clearance on the the outside circumference would probably need to be even tighter. Just speculating .
Did you measure the piston height verses the deck at TDC?

**Instigator** · 12-16-2018, 06:19 PM

Great guess.
Ive never heard of it.

Makes zero sense in terms of power.

Originally Posted by Dave S

Might be for piston clearance....when piston rocks?

**Motv18** · 12-16-2018, 10:11 PM

Some older motors have bifurcated pistons, to promote flow. Negative squish is the same idea.

It it can be bad on a 4s but on two strokes it promotes flow out the exhaust to keep it from going out the intake.

**Chaz** · 12-17-2018, 11:58 PM

Originally Posted by ostsideend

The original 3HP 2stroke Yamaha Malta does have a negative squish angle (I checked 5 of them)
So it is not done by mistake from the Japanese. They wanted them like it is.
I read that a negative squish angle in most cases does not make sense. But when it does make sense?

Due to that i cant find an answer on internet i'll try and ask you guys!
Does anybody know?

TX

Oliver

Yes, A negative squish angle will increase charge velocity in low compression, low horsepower, low RPM, applications like 3 hp Yamaha's, which would tend to foul spark plugs easier without it.

Their thinking is, if you want more power ... buy a bigger motor ...

If your dead set on building the baddest *(^$^&*&(% 3 horse on the planet ..
then go 1 to 3 degrees positive . It will be less prone to detonation and will have a few hundred rpm over rev compared to a parallel squish/crown .

**ostsideend** · 12-18-2018, 04:04 PM

Thank you guys for your answers!
i already changed one head to a small positive angle!

**Chaz** · 12-18-2018, 08:08 PM

That alone will lower compression. But it does open the door to cutting the head to raise compression not only to where it was before, but a little higher ( depending on gas quality ) to pick up a little power.

**ostsideend** · 12-19-2018, 01:53 AM

Fully agree. Head does give 11:1 ratio now. Fuel is 102 Oktane.
When i found the negative angle, i thought the japanese have wide tolerances or the pre owner put in a different piston for some reason. Later i read about existing negative angle designs and could not explain it to myself.

TX again!