Head squish

[ktm 550 mxc monster]

I have done alot of research and I wanted to throw out some numbers and get some feedback to make sure I am on the right track.
Bike 01 KTM 300exc
Flat top piston
Stock squish about 2.1mm
Idea's
Pump fuel reduce squish to 1.5mm
Race fuel around 1.1 mm
Theory: too much squish space is leaving fuel/air laying around where the flame front doesnt propagate well, leads to rich bottom end and finiky decel pinging/ low load knocking due to remaining fuel lighting off later in stroke.
Concerns:
Will squish width need to be changed?
Will volume need to be added back to the bowl?
Thanks in advance to anyone that wants to kick this around with me.

 

[ktm 550 mxc monster]

anyone?

 

[David Trustrum]

First thing to do in the absence of info for your model & I don’t mean the drivel they quote from the factory, would be measure the compression ratio to see if it will likely go too high from this change. (Piston at top, engine vertical syringe [big] in 2-stroke oil).

Next thing if you want to get fancy is calculate the squish velocity in case that gets too high. In that case you would likely reduce the squish area.

There are a few programs around to check MSV, some free but TSR is actually useful.

 

[bclapham]

monster, patience my man. i tried t pm you but your inbox is full

my yz250 had 1.8mm after porting, setup for pump. i went to 1.0mm for race gas but it ran a bit flat, it must have had too much comp, i had to run a thicker base gasket- it messed me up and i had to get some thicker base gaskets to get it back to 1.3mm.
ironically, the rm144 came to tight at around 0.5mm and it would deto plugs loose! ive had major headaches with all of this, my advice, be patient and just send the whole lot to eric. when you talk with him, ask him for the target head specs and write them down. when it comes back, then adjust for those specs- from what i see, port timing is the most important thing- once you get that set by eric decking the cylinder, then just take down the head in small increments, you might even want to measure the head volume- i think its real difficult to get the squish bang on when you only have a head and cylinder in your hands, so you have to build it then adjust.

how is the jetting on that bike???? if you run plenty of oil and a good race gas, you can really dial them on the money, even with 2.0mm (not that bad on a real big bike)- try james deans program for jetting, its awesome. i recon squish is much more important on a 125 than a open class bike.

good luck

bruce

 

[ericlachance]

whats this jetting program your talking about?

 

[bclapham]

http://shop.thumpertalk.com/Catalogs.asp?pdf=ttJDJet

 

[ktm 550 mxc monster]

Thanks for the replies guys!
Bruce, my inbox is full? I went and checked and it is empty and I am using 0%..maybe it is/was a program glitch that day.
Here is where I am at right now, I had a problem with when I set my "x" dimension at +.004 (spec is .000 to +.004) I heard a slight knock. using a thicker gasket would have put me out of spec, i was either out of spec or plus.004 when setting up the top end. I removed the head to look at some burn patterns and stuff and noticed that the intake side of the head was forming a burr and a small notch. the intake side of the piston was hitting it.
On the ktm the head pilots onto the cylinder via a counter bore to center it, the fit was good, so obviously when the head was machined the chamber was slightly off. looking at the size of the notch forming I decided it was about.005 off
While I was at it I noticed that the squish was around 2.2 mm. pretty large. I know that too much squish will reqire race gas due to poor chamber design,
ktm 300 owners are always chasing low throttle pings and decel pings, and having to richen the pilot jet beyond optimum usually to get it to stop.

The result is less than stellar off idle to 1/4 response and power and people who never reallys seem to be able to completely get rid of it on pump gas even though the compression seems pretty conservative to me.

Here is where I am now
opened the head chamber diameter .010
reduced squish to .056 from .086, I was aiming for .060, but the shop didn't have an insert really suited for alum at the rpm I was using for surface finish reasons, I forgot to use oil on my finish cut .001 and the alum stacked on the tool, wedged between the tool and my work and galled it, so I had to regrind the tool, remember to use lots of oil and had to take off .004 to clean it up from the galling, as well as taper from the alum stacking, stupid mistake.
cc is now 25, I didn't have my setup to meaure it beofre cutting as it was loaned out, i wish I had it the now just for notes at least.

With my lack of knowledge on thsi subject I have come up with these results using bimotion, although like I said I am no expert on answering the questions it ask and the instruction pdf is kinda sketchy. Using mm measurements of the exhaust port I have come up with these results.
SQUISH BAND 44%
COMPRESSION TRAPED 7.9
SWEPT 12.9
MAX SQUISH VELOCITY 23.7
@10.8 BEFORE TDC
TURBULENCE RATIO -.012

to be honest I don't even know what some of this means and what is "good", the instructions don't make mention either, I am trying to research this and learn what it all means now.

Seat of the pants the bike is stronger, lost no top end, and very very crisp with regards to throttle response, it revs much much quicker.
The air screw which seemed to be so unresponsive beofre is now very very sensative to as little as 1/4 turn, where as before it was about 3/4 to realy notice it had changed, not total out turns, just a slight turn from where you are now is very sensative, I wonder if this is due to not having a buch of air,fuel laying around not getting burned?

what else should add or research to make sure I am not making mistakes?
as always I welcome any opinion on the subject.
Thanks,
Clay

 

[LJW]

What angle is the squish band relative to the flat piston top?

 

[ktm 550 mxc monster]

same as stock no angle they run parallel. Is this not wise?

 

[LJW]

Originally posted by ktm 550 mxc monster
same as stock no angle they run parallel. Is this not wise?

Parallel or a 1 to 2 degree angle sounds like the hot setup.
My KX500 has a 15 degree squish angle with a flat top piston. This is not wise!This is high on the list of things to change.

 

[SFO]

I used to cut a lot of heads for my friends who tuned tz-250s.
We would cut 4 or 5 sets of heads at a time and he would want his chamber volumes to be within .2 cc of each other. We used the specific gravity of aluminum which is 2.2 grams per cc, if I remember right, to weigh the volume changes once we had a baseline via the burret.
Weighing was quicker than using the burret, anyways, I just wanted to add my 2 cents...