F5's 300 head mod thread

[(F5)]

Well I might as well start a new thread for this, there have been a couple before but not with quantitative figures that I have seen.

Clearly if this gets beyond you there are plenty of tuners that will take your money for their well earned development time & expertise machining stuff.

This is my take doing it myself using fairly simple well trodden paths. I've done this a heap of times on small cc roadrace bikes and the odd larger roadbike & dirtbike. The principles are the same although the application requires a different approach for best results. -read: roadrace application lots of revs, lower MSV, top end power over acceleration.

Anyhoo this is a playbike to me, not a racebike. one of the big considerations is being able to kick it over in snotty trails & I'm 5'8". The advantage here is there is a compression hole drilled above the exhaust port to reduce the compression ratio, but it is only ~4mm so while it will help at kicking speeds it shouldn't have much affect at high rpm. If you were going to Motard this you'd block it up.

That & the easy compression ratio is why the 300 is as easy to kick over as the 200. I intend to keep the comm the same to retain the same level of ease. That said. maybe I'll be a little greedy and crank it 1/2 a ratio & see how it goes, I can easily pull a bit more out if needs be. Easier than putting metal back in.

OK to start with here are a couple of pics & I'll add how I got there next post. You can see the mandrel which screws into the spark plug hole. It needs to be flat/straight to be worth while.

[(F5)]

OK my bike is an '07 EC300. The principles will remain the same, but I can't tell you where other models may have altered in spec.

3 years ago I bought it & I fired a new piston in it 2 ago. I sadly onl get to ride this bike once a month on average. When I measured it I was a bit surprised, but its taken me till now to do something about it as time has been tight & racebikes take priority over perfectly serviceable playbikes.

Compression ratio was about 12.5:1, which is easy on a 300, esp a square one at 72/72 (maybe new ones grew 0.5 with a quick google). The squish clearance was a mammoth 2mm.

Squish areas have long been used to reduce risk of detonation by creating a thin boundary layer of cool gas that doesn't burn & promotes good cooling to the piston from the cool head.

Also by reducing the spark lead (Length the flame has to travel from the sparkplug) and thus combustion time. As a point the piston is not driven down by an explosion, the spark ignites & burns the fuel to increase the pressure; Boyles law in a fixed volume, except the fixed volume, has a piston on one side that is pushed to create a bigger volume, reducing pressure as it does so.

On top of that it increases the MSV but I'll get to that later.

Any space in the squish is a lost batch of combustible gas. Of course you need to have some space as the clearance gets used up at high revs. But as long as it never touches, you're sweet.

Manufacturers always leave this gap huge so if they get a batch of long rods, tall pistons & short barrels, there is still a gap. However if you just shave the head to reduce this to a decent gap the compression ratio always seems to go too high, so its a bit of a faff.

The queer thing is my bike had a 4*(degree) difference between the piston & the head squish area. This increases the area of unburnt gas which the flame can't effectively get to and decreases the pressure that the mixture is squished & squirted into the burning part of the chamber.

As the piston squishes the last bit of area of a mixture (that has sparked a few degrees earlier) & flame is propagating, increasing pressure, the mixture burn efficiency can be affected by the turbulence smashing the fuel into smaller particles supposedly increasing surface area, though I struggle with that, but helping mix the gas for sure. Anyhoo the speed that this mixture is squirted out of the squish area can be calculated & is called the Maximum squish velocity. the idea is that a certain MSV will suit your application, ours being dirtbike type power at comparatively low revs.

There is software to calculate this, however some uber tuners have started to poopoo this as it ignores the fact that the volume in this squish area decreases as the engine revs higher so the numbers are false. Either way the numbers give a reasonable indication of where you should head. People moved to Mota and then to EngMod, but I can't be with the $400 & considerable time investment to learn to drive it rather than monkey enter some data & get peanuts results.

This aside a dirtbike running somewhere around 50% of chamber area to squish area with a close gap & reasonable compression should be in the ballpark so I might not give a rats. I've lost my sw on this PC so I'll try find it at home & give it a bash for giggles.

[(F5)]

Back to my bike; so I found the squish went from 2mm at the outside edge to 3mm at the chamber edge. The squish area seemed to be 13*(degrees). The piston measured 9* hence the wedge shaped space.

I was going to reduce the gap down to 1mm (safe as houses on the 300, I run my 50 at 0.5mm, but it has less mass to stretch/displace). Mail order Tuners probably add a tickle of extra clearance as they can't trust that an owner will have measured the squish properly (esp hard when it is wedge shaped), or that they will do it again when they rebuild the bike.

But I decided to flatten the squish area out to a almost parallel shape. Having a closing shape is a no-no so I got the squish cut to 10*. Yep I paid someone $50 to do it as I mounted it on my 1940s? lathe & decided there was too much movement to create a flat surface. Well I got scared.

I wanted a 3.3mm step from the chamber to the sealing surface to get the gap right (it was 2.4 before, heck maybe I'll be conservative & it will end up 1.1mm, but the chamber was 0.05 off left to right). Anyhoo, once the squish was at 10* (leaving a little at the inside as a starting point) there was only a slither of the original step showing shown by the anodising visible (see outside picture 2). Then cut it down a bit to achieve where I want to be. Note to self, I'll have to ovalise the head stay mounts a little now I think of it.

So that's where I'm at. 1mm is about 4cc so 1.7mm cut is more like 7cc from the original 25.7ish which will take me to ~17:1 which is clearly waay high.

Next step is to shave some from the chamber to reduce the ratio down to <13:1, but I'll bolt it on first to check the clearance to be sure. I have a curved cutter to remove material & as it isn't a seal bearing surface my lathe will be well accurate enough. I can remove area from the chamber area or I can encroach into the squish area to make the chamber diameter wider. This would reduce the MSV if that was desired. roadrace bikes typically have much smaller squish areas for this reason.

As I take cuts out I will then have to place the head back on the bike & remeasure the volume with the piston at TDC. Its a bit boring but it is free & better than watching some renovating program on TV. I'll prop the bike up so the engine is level & lightly grease/wipe the piston & lock at TDC with some chain grips on the flywheel (the magnets like to move it off TDC). I'll then use some fork oil in a burette available from an online lab supply place for very little for a plastic one.

I fill the spark hole till 2 threads up from bottom to account for spark plug internal shape. I perform this a few times till I get consistent measurements, it takes a while to get your technique in. Head off & wipe between trials.


The end result I hope will be a better running bike that carburettes better at lower revs & is working more efficiently. Did wonders for my 200.

Oh yeah I never bother with compression tool readings. They are convenient, but hopelessly inaccurate on a 2 stroke as they can be affected by oil flicked up from the crank. As an example when I did my 200 I used a meter between modifications. As the compression got pretty high there was little or the wrong way numbers when I had removed material & had a psi higher reading. Rubbish even on my fair quality German guage.

I'm racing this weekend so maybe next week I'll get this dusted ready for first ride that sunday.

[twowheels]

Alright, I'm ready to declare (F5) is to head mods what Jake is to jetting! Great write up ... more pics as the project progresses please.

[(F5)]

Well I still have to end up with a quantifiable improvement yet.

Surely with this much dikn round I should end up with 200hp more power, 10mpg better mileage, longer floating wheelies & of course get more chicks.


. . .actually I don't want to get more chicks, the wife would complain & besides I ran over an eel in a shallow puddle once & I still feel guilty about it.

[Jakobi]

Great write up F5. I think I find all the cc'ing and measuring of the trapped volume good too, but don't have the access to machines or knowledge to use them to make it happen.

Having recently had my 2013 250 head setup, I can confirm, the newer ones still run around 3 degrees difference between the piston and head. On my 2010 S3 head we left it as is. On the new one had the band recut to match the dome.

[StuJohnson]

Thanks Jake for that.
Very interesting setup. Where did you find that info? Would love to read more.


Sent from my iPhone using Tapatalk

[Jakobi]

There is not a whole lot more to read. Just a jetting thread on another forum where Dave chimed in as he'd just set one up on his dyno.

Now also keep in mind thats on Australian fuels and for some reason I have noticed we typically end up a bit leaner than US. The mods he has done are probably very similar as what F5 has done. Cut the band back to reduce the angle, reduce the squish clearance, and then open up the chamber to correct the volume. I'd ask Dave to chime in here in the thread but he's been seriously busy with work. Its hard enough getting a few messages back and forwards on the phone. Its also on the newer designed cylinder with the G stamped into the side of it.

[(F5)]

ooh, interermisting,

I've found a hole in my Keihin jet range below 178 & above 165. I might just have to cash up & buy those jets & find an old tyre & dyno it.

38 on the Pilot? I've got a 40 but atm I've got idle wound in all way so I was going to try pulling the slide down so less influenced by start of needle, but I thought a 42 would have been the direction to compensate. I'm on a CEL (or was it CCL?).

[Jakobi]

Thats all on the newer AS2 carb with the shallow bowl F5. Seems to like much leaner jetting in general. They have a newer model AS2 on the 2012/2013 from what I can see too. My new bike has a leak jet. I have a pic of it somewhere but can't recall what number it is. The NEDx/NECx needles also supply a decent amount of fuel towards wot. They taper right down like a toothpick, where comparitively the N3xx needles are much thicker towards the pointy end.

Even with the two engines I have setup very similar in terms of base gasket and head mods, the port layout is different, the way they run is a little different too (same jetting specs, both running in sunshine mode). Slight differences in carb, different airbox, different silencers.