F5's 300 head mod thread

[zeuszuki]

Hey I've been racing small roadrace motorcycles. I got 2nd in our national 50cc class yesterday. That saddened me as I wanted to win,

Worse I got duffed up by a 19yr old. . . . Which I suppose isn't as bad as getting a 19yr old up the duff, but makes you feel old nevertheless.

Trail ride on next weekend so back on the GG - I'm pulling the pv out before that if all goes well & can inspect, clean, repair, compare in short order. Next step is PV mods for more preload but lets get back to std.

Lol, I am the 50c class National champ from 1988 GT50
You race against my mate Chris Cattermole ?

 

[(F5)]

Erm, think that is Nudez, but he hasn't raced for some while. There is the open 50cc GPs but that wasn't run for about a decade & then last run in 2008 when I won it at Ruapuna. Had to give that one back but there were some interesting names on it early on. Erm, who was the guy with the electric screen engine in the GSXR, GPmechanicy guy? think he was on it, someone like that.

But there has been F5 (hence the name which has no connection to GGs) run every year (except 12) back to when I started racing in erm, about '87 at Ohakea & then other tracks. I've won it 5 times. We had a field of 10 this year & one bike had Pete Sales Watercooled GT50 engine in it still going fast.

 

[(F5)]

Baah. Ok well all apart, cleaned off carbon. Not too bad but maybe its been an issue. Buy some more PV bearing & will be back together tonight. tightened a few things including lathing up a stepped collar for holding the kickstart lever on tighter. New tyre for front to go on but its midnight so call it a night.

Look at forecast. Poos. Think we might get the tail end of the tropical cyclone that is hammering the poor islanders in Pacific atm. But I wanna ride. Well its a few days off.

 

[zeuszuki]

Erm, think that is Nudez, but he hasn't raced for some while. There is the open 50cc GPs but that wasn't run for about a decade & then last run in 2008 when I won it at Ruapuna. Had to give that one back but there were some interesting names on it early on. Erm, who was the guy with the electric screen engine in the GSXR, GPmechanicy guy? think he was on it, someone like that.

But there has been F5 (hence the name which has no connection to GGs) run every year (except 12) back to when I started racing in erm, about '87 at Ohakea & then other tracks. I've won it 5 times. We had a field of 10 this year & one bike had Pete Sales Watercooled GT50 engine in it still going fast.

Yep, Nudez. Won it on the old Wigram circuit, now that was a race track, especially on the big bikes
Pete Sales , haha still got the plans for his pipe.
Small world.

 

[(F5)]

Ok well finally got out again. result; seems great. If there's a dip its not not noticeable, bike ran fine & was really tractable yet revved out on some of the uphill sections pulling fine. Tractable was useful as the 2nd 1/2 of the 2nd loop I got a rear puncture so was pottering to finish the loop without damaging the rim & tyre.

Job Jobbed.

 

[motopsycho87]

Did you manage to get a decent dyno reading out of it?

 

[(F5)]

that would mean getting the roadtyre back on it & to be honest I'm over tyre changes. Works well, love riding it.

Spent last night rebuilding the muffler which involved a fair bit of remanufacture, welding, cutting & drilling. Other bikes are back up the queue again.

 

[spooly]

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.

I read this post and it gave me a bit of a headache lol but I got the jist of it... iv been messing around with base gaskets and squish and jetting since Christmas. ..I ran a speed event last weekend with a .15 base gasket and 42 poilit I think. . I couldn't get a correct squish measurement.. I only learning atm how to jet and tune a 2t.. I thought id like a lot of torque at the bottom but I didn't realise how much it took from the top.. so yesterday I put a .3 base gasket on I haven't tried it yet but after reading your post I measured the top of the piston with the bottom of the exhaust port at bdc and I make it out to be .5.. so if I want to get the most from my engine should I add a .5 gasket and get the head machined. ? Now another thing is the shity pump fule im using is probably making it even harder to jet especially wit just the .15 gasket. ..so this weekend im going to try some octane booster with the new set up (.15/.3) .. but form doing some research in the long run I might get the head machined locally and add the .5 gasket. .would this be the way to go and would this method make it very difficult to jet... whats your opinion? ?

 

[(F5)]

I run 95 RON pump gas,- wiki will tell you how that compares with your local (RON or MON).

You may have to run less comm if octane is much lower. Ethanol fuel can be a drag apparently for response. I'd tend to leave it in std height but depends on your use. Don't obsess about piston at bottom of ex port.

But def optomise the squish by machining which will help it run nicer and jet. But this will make the compression too high so you have to machine the bowl too.

 

[VilluEST]

Hi! Great thread! As I'm also planning to do the head mod and it's impossible to find someone in my area that has done thing like that before. So I'm planning to learn everything I can and then assist someone on machining the head. It's been a great topic to learn from! However I'm a bit unclear about one thing: in order to get the compression ratio correct again, you are shaving the chamber curve (e.g. make it wider on the top) as shown on the picture (area painted red)? And in order to decrease the squish clearance you have to cut the head where the outer red ring is drawn on the pic?

PS: sorry I used your pic, I hope it's okay

 

[Jakobi]

Hi! Great thread! As I'm also planning to do the head mod and it's impossible to find someone in my area that has done thing like that before. So I'm planning to learn everything I can and then assist someone on machining the head. It's been a great topic to learn from! However I'm a bit unclear about one thing: in order to get the compression ratio correct again, you are shaving the chamber curve (e.g. make it wider on the top) as shown on the picture (area painted red)? And in order to decrease the squish clearance you have to cut the head where the outer red ring is drawn on the pic?

PS: sorry I used your pic, I hope it's okay

Affirmative.

Cutting the outer area will allow the rest to sit down further into the cylinder, decreasing piston to head clearance (squish), and also increasing compression by reducing volume. To re adjust the volume you need to remove it from the combustion chamber, as you have marked also.

See these pictures.

S3 High comp head - squish set a 1.25mm with 0.5mm base gasket. No material required to be removed for squish clearance adjustment. Compression too high though.

Reshape the compression chamber to correct compression ratio.

 

[VilluEST]

Thanks! So the shape of the compression chamber after modification is not so important as long as I keep it's height the same? And of course I need to keep the squish width.

 

[Neil E.]

The height is not critical (you don't want to interfere with the plug threads or get close to a water jacket). What you are doing is increasing the volume of the head dome to reduce the compression ratio that was increased when the entire head assembly moved closer to the piston as a result of machining to reduce the squish distance.

EDIT: I am referring to the height of the dome. Once the squish is corrected, the height of the head relative to the top of the cylinder must be kept the same.

 

[(F5)]

Yes and yes. I also encroached into the squish area to gain more chamber area, but only about the same amount that was still black after machining the squish angle shown in pic.

The pics blown up with the measuring template of original and the gap left will give you a good starting point to see how much to remove from the chamber. But do buy a cheap burette online and check to be sure.

The end result was well worth it, and despite a coincidental tailspin at the time I wanted to dyno, it is still working well. I changed the in coil and cleaned the PV and other issues went away, can't remember which was decicive.

 

[motopsycho87]

Righty. I'm a bit confused by all this now. I moved my cylinder up to 1.5mm with gaskets. The head I modded before was far too low compression now, measuring 28cc at TDC to the first plug thread. So I bought a new head, I've machined it to 1.1 - 1.3mm squish. I checked the volume of the head and it's 24cc (rounded). This gives me a static figure of

(293.15+24)/24=13.21

I've given it a few stomps with the compression tester and I'm at around 185psi. Just wondering why the combustion chamber cut if the finished ratio is bang on the money? Yet to run the bike, but if figures are to be believed, it should be perfect... Are newer heads different or something?

 

[Jakobi]

Is 24cc the volume of the head alone? If so, you need to rework and calculate the trapped volumed. The dome of the piston reduces the volume of the head once at TDC.

 

[motopsycho87]

I'll get the measuring flask out tomorrow. Just has confused me a bit. I measured the trapped volume with the old head, 28cc (I know, way too much). I measured the difference between the heads on the bench and it was indeed 4cc, so in theory when I measure it should be about 24cc. Compression tester reads nearly spot on what is expected, but I will double check anyway. I kept an extra 0.2mm on the squish just to be safe, more on the compression ratio side than piston contact.

Whats the highest comp on these without adverse affects?

 

[motopsycho87]

Checked it this morning with atf (a bit thick) and running between 23.5-25.4cc so on the cusp of 14:1, will measure again with diesel later for a bit more accurate. TDC measured with vernier.

 

[Jakobi]

Interesting.

When I cc'd my 300 (S3 head and High High Insert) I had the following.

1.3mm base gasket stack
25.5cc trapped volume

however 1.95mm squish.

So if you were to skim another .7mm off the head, you'd be removing 2.85cc of trapped volume.

So say 22.65 trapped would give you 13.95:1 UCCR. Pretty high comp for a big slug, but if you're willing to give away some top end and run decent octane fuel it should be OK!

The 250cc is set up much differently and runs a load more compression out of the box.. around 15:1 .. I brought mine down to 13.5-14:1

Also note, volume of piston dome is approx 6.74cc from my records. Can't seem to recall cc'ing the actual 300 head alone. Definitely did on the 250..

 

[motopsycho87]

So my finding is roughly 24cc. Which seems to be the limit for premium, will give it a ride soon when my new camera turns up and return results