Talking Squish

[outasite]

Putting a drag together for the old stream.Its got factory 21cc heads,at the price of race gas,the 21s gotta go on the shelf.I have a cherry pair of 18787-C4 with 2 1/8" top hat chambers.My plan is to convert them To orings, this will nesatate cutting a squish recess of around .035 my question is how do Mercs respond to a taper squish band.I was thinking of around a two degree angle,And a slight radius at the pocket.Im wondering if the increase resulting from the angle cut is a bad thing.The squish clearance near the pocket would be .050 or more.When done I'm looking for 175/180 psi

[eli]

am having mine done his winter 2 degree angle and dome work ,the roundy round F1 motors have angle squish ,so do many cycle engines ,i left you a p.m with some info .j

[texasvector]

eli ....didnt you have a thread on this not long ago??

[eli]

yes

[dwilfong]

leave the edge at the chamber sharp. It will help swirl the gases in the chamber. I have found 3 deg squash angle worked the beset on the testing I have done on small 2 stroke eng.
Also the MSV(maximum squash velocity)is important to help swirl the gasses in the chamber.

[outasite]

I am a little concerned of a sharp edge lighting off the charge.

[dwilfong]

I am a little concerned of a sharp edge lighting off the charge.

The testing I have done was on small Nitro engs. thy are very prone to detonation. the 3 deg angle dose not make a large small space to trap the fuel to detonate. the sharp edges will not create a hot spot as the fuel is washing over it on its way to the chamber.
The squash velocity helps this also as it forces fuel over the edge at a faster speed.
There are calculators on the web to find out what the squash velocity is at a certain RPM.

[outasite]

Bobbie,these heads are 33cc uncut.My plan is to check both,and make a very light cut to clean up and make equal.Then cut the oring groove,then a .035 pocket cut for a squish,then a two degree angle to the chamber,and finish off with a slight radius around the chamber.hopeing to end up with an oring head equal to a 28-30 cc gasket style head.

[Forkin' Crazy]

RC engines on nitro are totally different from a 2.5 on pump gas. The physics between the two is not even a comparison..... plus the fuel, etc, etc. I think a bevel or a radius would be best.

[racer]

What I found on OMC engines on the dyno which was later confirmed by one of the race group. 2 Degree's works best for power with a slight radius, however a sharp edge is less detonation prone.

[Snotrekie]

I've done extensive testing in the field and on the dyno with regards to this topic. The truth of the matter is that head, pipe, fuel combo is just that.. a combo. For this conversation 2 or 3 degrees is safe and leave it sharp. In outboards I found the biggest problem is the exhaust is limiting

[dwilfong]

What % of the bore is the squash band?

[outasite]

Bore 3.500 dome 2.125 squish .750x2=1.500 puts it in the 40/45% of bore. Actually the squish is closer to .725 so around 40%

[rude tim]

Squish area is directly related to RPM, 50% would be a 6000 rpm engine 40% for a 10,000 rpm engine. This information came from a factory MX Yamaha engineer. He also suggested a radius leading into the dome. Like Al said the radius makes more power but it is easier to have detonation.

[dwilfong]

The reason the % makes a difference is the MSV changes with RPM. The less % of area reduces the MSV.
The MSV increases with RPM. So a smaller % keeps it with in range at higher RPMS
MSV is what it is all about....................................
This changes with angle and % of the squash band and chamber cc. in conjunction with RPM.
MSV is the constant when designing a head. All adjustments should revolve around this for tuning the chambers.