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.

 

[(F5)]

Well had a few min spare to put the head on the barrel & measure squish. 1mm at the cylinder wall & 1.1ish at the chamber which is pretty much exactly where I wanted to be & its the same the back as the front which is an improvement over stock.

Here it now waits forlornly in my Sheldon c/w old style tool holder for everyone to laugh at, but it still works fine for this sort of stuff. The hand cut head tool is pretty small as I made it for doing heads of 40-50mm dia. A bigger one would be easier but my tool holder ain't that big. I'll just take a few cuts and work carefully with blending radius's.

I had a quick play with some old TSR sw & calculated the head currently in the 40s which with a suggestion to stay within 15-25m/s range is pretty high, so first thing I'll do is take off the inner anodised section which should take my squished area from 56% down to 50 & lower that substantially.


I'll need to remove a fair bit of material but I'll do it in a few steps & measure cause even though that is tedious, its easier than putting material back in.


I've taken the head stays off to slot them a bit more than std to account for about 2mm of movement down.

 

[(F5)]

Just a couple of snaps, bike held so cylinder vertical with some straps & front wheel on stand. Excuse filthy bike.

Flywheel held still at TDC, easy as had a weight to grab, but carefully can put on flywheel itself. Won't stay at TDC by itself, the magnets pull it around.

Burette down spark hole with bright torch so you can see it actually coming up lug hole. I use 2 turns from bottom to account for plug inner recess. Take your time to practice technique. I've done this a few times but it takes me a while to get my eye in.

 

[(F5)]

ok here we are after 5 cuts & measures. I should have taken some more heavy cuts at the start, but as I said; harder to put metal back on. Good thing I'm not charging myself for this.

This leaves me with 24.0cc measured on the bike so full stroke compression ratio of 13.3:1. This is still higher than my target but it got to my self imposed midnight curfew (any later in the garage & the wife starts to consider coming down to see I'm not trapped under a bike or anything).

I could run it like this on decent gas, but kickstarting might get slightly harder & it might take me another 3 years to get around to taking some more out.

I think I'll pull another cut & get to 13 or just under tonight. I ran a bit of sand paper in it but it cut quite nicely from the tool.

Pic 2(edit snafu, pic 1) shows how much metal has been removed. Its quite a bit, but simple math & measurement says we chopped out about a 2mm donut of space from the outer edges of the cylinder, it will take more out of a smaller circumferenced area to replace it.

You may note the un-radiused edge on the squishband edge. Conventional tuning wisdom tells us to remove that, but advice from the research of the uber tuners debunks that practice.

I might make a template (a finger of kneedit pushed into the chamber left to right & sectioned in 1/2) so I can do a friends one with little measuring if it turns out all right.

 

[Jakobi]

Keep up the good work! Love all your spares clipped/zipped around the bike chasis.

 

[(F5)]

Ha, yeah lost a gear lever a while back so had to complete a loop in 3rd which was tedious, fortunately no killer hills. Hence, a gear & brake, clutch lever. Split link, the tube in front of PV cover is a looped 4' length of gas tube. Black thing at top of pic is small bicycle pump tied to rad hose. All items very light.

 

[twowheels]

That's great work. Here's a question though - from your previous work with 2T engines does the shape of the chamber (taller shoulders to achieve volume) make much difference in how the engine will idle? Rev? Torque?

I'm getting excited to fire up the lathe and have a go first at cutting a mandrel and then at modifying a head.

 

[(F5)]

Most of my work has been on high rpm roadrace bikes so the power you want is a good spread before & past peak power to cope with gear changes without falling from power. We try to make the chamber real shallow. There has been a trend to toroidal shapes which are more bathtub, skinny squish areas & with flat top pistons even moving the sparkplug closer to the piston so it is protruding from the chamber ceiling. Makes measuring volume like I've done problematic.

Dirtbikes, esp Enduro/trail bikes you want a considerably wider spread. To be honest I don't know whether the same extreme shallow shape would help or hinder. What I do know is that the squish area is larger so the chamber will naturally be taller anyway. The most influence will be in the effect of the squish area. Get that working properly & you should be ahead of the game. I think this will have some reasonable effect at lower revs, lower throttle position & idle. But there should be less gas wasted in an unburning area. this should mean more power.

The std chamber is a bit odd in that it has a flat section by the spark plug. I think the reason they have such an angle on the squish area is to stave off detonation as uncertain which fuel the customer will use (who reads the manual huh?).

Anyhoo, I made another cut & that ended up as 13.1:1 & I decided to call it a night, slapped it back together & ready to ride sunday. Hopefully it is worth the effort, proof in the pudding & all.

 

[byost]

F5's head mods

First off thank you for sharing your knowledge. I am a rookie when it comes to rebuilding modern 2 strokes but I enjoy tinkering with machine tools. I was re-reading your first post and I am confused about the 4mm hole above the exhaust port you were describing. Is this stock or something you have added? I don't remember ever seeing a hole in any pictures I have seen. Also, I always assumed that the top of the piston was flat. Is it actually domed? You said that there was a 4 degree difference between the piston and the squish band. I am just trying to picture this in my head. I am also wondering how wide the squish band is from the outside edge of the cylinder to where the dome starts? Once again, thanks for sharing!

 

[byost]

Knowledge

Ok, I found the technical article and the excellent diagram of the squish band shape by RB. It's all starting to make sense now, like I said , I'm a rookie. Now I want to go home and check my squish. I love this forum!

 

[Fred1956]

I was re-reading your first post and I am confused about the 4mm hole above the exhaust port you were describing. Is this stock or something you have added? I don't remember ever seeing a hole in any pictures I have seen.

That hole is on the 300 cylinder only, it is supposed to be a compression 'release' to aid kickstarting. From what I have read you want that hole there when kickstarting a 300. I have no experience with it, just what I've read on it

 

[twowheels]

So, just for the fun of it, where in the stroke do you start your displaced volume calculation?

Is it full stroke, so BDC? When you close the exhaust port? When you pass the compression bleed hole (on the 300)?

 

[(F5)]

Full stroke. That's what matters when specifying a com safe for the gas you will use. When the engine is running the pipe is pushing mixture back in when in its range. when out of range the effective compression will alter, but the power valve is moving changing that point so so called 'trapped ratio' is a moving target anyway.

If people still argue point out that foul strokes stated compression ratio never mention when the cam opens or closes the valve. Why should the position of the hole - side of bore vs top of head make a difference.

The hole in the 300 (wasn't one in my 200, haven't pulled apart a 250) helps kickstarting as it does lower the compression ratio as the engine is turning over so slowly & cycle not started yet.


Actually I hadn't read that tech article (or at least might have skimmed it many years ago). It is well written & his picture looks quite a bit like my end result, but tiny picture. He also seems to have polished the heck out of it which looks nice. I don't agree with everything he says, but not enough to argue. $75US is a very fair price to pay (about $400nz kidding, -actually our exchange rate is much better, about $95NZ) is a good deal for that existing development. Cost more in freight & time for me & I like doing things myself, but if yer stateside it would make good sense, he's not being greedy.

 

[Jakobi]

No comp bleed on the 250's.

 

[(F5)]

Well I'm impressed. At first I was wondering if the bike was just running great but it was what I love about a big bore 2 stroke. Maybe I've made noi change and I'm just making it up as I've done some work. As I went on I realised it had woken up at low revs and low throttle positions. It's much like my old GG trials bike.

So after a loop (37km), I swap bikes with my mate, also a 07 EC300. The difference is stark. I find myself blapping. We change back. He's impressed, more bottom end and sound different he sez. Mine feels like it's burning more cleanly.

Fuel consumption is identical though.

Might have to experiment with mainjet. Feels altered in requirements but not sure which way. Not getting on it for long today. Also idle is lowered to the point it will stall. Must try altering idle jet but it feels so perfect off bottom I'm not inclined to mess with it.