squish/piston wash

**baja200merk** · 09-01-2014, 09:37 PM

Yes sir I completely agree. This motor was ported and assembled with the intention of never turning it over 7k, with a 1.62 sporty bolted to the bottom which would make short rod survive just fine. Well we let it turn up and the inevitable happened.

We have a fresh 225 in its place as of today and GOD IT IS GUTLESS..... The stock carb jets are 14 sizes smaller in the stock motor but beef still managed to run it out of gas today

**Mr. Demeanor** · 09-01-2014, 10:22 PM

So the only way to reduce squish and keep the same compression would be to CC, deck the head, and then mill the pocket to get the CC back correct?
I just cut 50 thou off a stock set of heads and the squish is .07 now. I have domed pistons with about a 7 degree angle on the squish band to match the piston dome Compression will be about 145 but that squish still seems large by comparison to what you guys are doing.
Would there be worthwhile performance increase reducing the squish band clearance but keeping the compression at 145psi ?

**baja200merk** · 09-01-2014, 10:32 PM

A stock 99 225hp measured 55-60 last time I did a quick check. We cut the pistons flat so had no dome to worry about cutting back into the heads. It is a mutt though, The heads are from an 87 225 bolted to a 07 e-tec block with carb'ed 99 front half, 97ish intake, 87 225 carbs, 89 Yamaha ignition and the list goes on....

**Chaz** · 09-02-2014, 03:42 PM

Originally Posted by Mr. Demeanor

So the only way to reduce squish and keep the same compression would be to CC, deck the head, and then mill the pocket to get the CC back correct?

It's not the only way , but it is the most popular . No matter how you get there , you still need the chamber to hold the same volume.

I just cut 50 thou off a stock set of heads and the squish is .07 now. I have domed pistons with about a 7 degree angle on the squish band to match the piston dome Compression will be about 145 but that squish still seems large by comparison to what you guys are doing.

I shoot for .038 - .040 around the outer edge with a ten degree cutter .

Would there be worthwhile performance increase reducing the squish band clearance but keeping the compression at 145psi ?

Any time you can convert more of the charge within the cyl , power will go up. I dont think cast pistons are a big fan of the added crown heat that comes along with it . And if you don't get off the throttle during those wake jumping times when the prop is out of the water , you stand a good chance of "printing" the head .. other than that ... yes it's worth a couple of ponies ..

**dwilfong** · 09-02-2014, 05:06 PM

what is the thinnest gasket worth a dam not cheep junk for a 2.5 Merk?

**Tom Foley** · 09-02-2014, 05:55 PM

Originally Posted by dwilfong

what is the thinnest gasket worth a dam not cheep junk for a 2.5 Merk?

That would be MERC , spelling counts here !!!

**Chaz** · 09-02-2014, 06:32 PM

Originally Posted by Tom Foley

That would be MERC , spelling counts here !!!

As long as fong can "spull" God Father ...

I could do an ol lead~sled Merc ... 600 inch SVO ProCharger deal ..

**dwilfong** · 09-02-2014, 07:01 PM

WOW I have not bin called Fong since high school.
Well what is the thinest Merk gasket now that we got the spelling Bee over with.
I'm a Pontiac man my self.

**Mr. Demeanor** · 09-04-2014, 12:16 PM

Im pretty close to seeing if it makes a difference on my motor. What Monty told me was that less squish with the same compression is safer in terms of detonation. Ive leaned on thisthings pretty hard though and willing to try a more aggressive squish with same 145psi and see what happens.

**texasvector** · 09-04-2014, 03:07 PM

Originally Posted by Mr. Demeanor

What Monty told me was that less squish with the same compression is safer in terms of detonation..

This is what I wanted to know/hear.

My compression is on the high side for pump gas safe.....maybe the looser squish will be safer.

**dwilfong** · 09-04-2014, 04:28 PM

I have found on the two stroke eng I have had the most experience with the closer the squash the less detonation with the CR being the same.
Have also found that a flat squash compared to a squash band with a 3 deg angle to the piston top also helps stop detonation.
I have found you can raise the CR with theses to things combined.
You don't have to run as lean to get good throttle response with a good squash design.
You lean on the eng because you cant burn the fuel. If you can burn the fuel on hand you do not have to run as lean.
You have to account for the chamber burning more fuel if you make the changes. so if you do not richen it up it will detonate and you will think you have gone the wrong direction.
You are stuck with a fuel map on a ECU eng so you need to be able to make changes to the fuel delivery to see what is what.
The combustion chamber design also changes how the fuel burns.

The flat top flat sided chambers have yielded the best results on the eng I have played with.
The percentage of squash area to boar also play a part in the equation.

**Mr. Demeanor** · 09-04-2014, 04:51 PM

Im intersted to see if my loose squish was why I have had to run such enormous jets. I am way bigger than what anyone of the Yamaha gurus and even Gordon would have guessed. If it goes super fat first time out I will know.