Optimum squish for a 12 300

[Diggs345]

Hi all

Iv have done a search and seen several figures mentioned , so whats the optimum measurement for the head mod , there is a guy local to me who will do it next week , off the top of his head he said 1.8 , iv seen anywhere from 1.2 to 2.0 mentioned on here , so can somebody please inform me of the optimum please.

Chris

 

[GMP]

Ron's have always come back around 1.2. Also, its about a COSISTANT 1.2mm clearance across the width of the band (matching piston dome). This is why the solder sample is important. He will also have to scoop the bowl out to readjust compression.

 

[Diggs345]

Ron's have always come back around 1.2. Also, its about a COSISTANT 1.2mm clearance across the width of the band (matching piston dome). This is why the solder sample is important. He will also have to scoop the bowl out to readjust compression.

Thanks glenn , im taking him a solder sample too , he is a well known 2 stroke tuner in the uk , he makes handmade pipes for road race bikes too http://mickabbeytuning.co.uk/hydropipe.shtml
Thats quite an artform how he makes them

 

[Diggs345]

Glenn would that measurement alter any for the 300 , or for the octane rating or does it stay pretty much the same.

 

[Jakobi]

The octane rating is usually determined by the compression ratio Diggs. Super high compression requires better fuel. If you tell them you still want to be able to run on crap fuels when required and ask for a stock CR they will just remove more material from the bowl lowering the CR. I agree with Glenn too re squish height, 1.2mm is good.

 

[GMP]

Not sure what UK fuel is like, but Ron works with 200 PSI max for USA pump premium. My 250s measure 192 PSI. 300s are a bit less.

 

[Diggs345]

Our low grade fuel is 95 ron but where i can i run 98 or 99 ron , and thanks for the help guys

 

[Diggs345]

Just been swatting up , your super unleaded is much the same rating as ours take a point or so

 

[Jakobi]

Yeah the US uses a RON/MON rating I think where as we use a straight RON. I had my 250 set with an uncorrected compression ratio of 14:1 and have no issues running it on 98RON. I haven't tried it on 95 but have no doubt it would run fine.

On a 300 around 13:1 will favour bottom end while still letting the bike rev out.

 

[Vindicator]

Could the same squish or close be achived with the any of the S3 Stars Head insert ? Anyone knows ?

 

[Jakobi]

Could the same squish or close be achived with the any of the S3 Stars Head insert ? Anyone knows ?

The S3 Stars head looks nice. That is all. The outer head itself mounts flush to the cylinder. The insert determines the squish height and compression ratio. All 3 inserts are the same volume (and also the same volume as the stock head), but hang down further into the cylinder (smaller squish, increased compression). So the High High / Black insert is essentially the same as shaving a stock head without having the compression ratio corrected.

I've got 2 x S3 heads, 250 inserts, 300 inserts, and a stock 300 head. Have also compared S3 250 volume to engine tuners records of stock 250 volume.

 

[Diggs345]

Does the piston have to line up with the bottom of the exhaust port on bottom dead centre , I'd say mine is about 1.5 to 2mm above the port

 

[Diggs345]

Just talked with the guy who is going to do the head , like glenn refered to in another thread setting the deck hieght to suit the exhaust port and piston at bdc can possibly throw out the transfer port timing , he did ask me if i knew the transfer port timings , but thats a bit above my skillset lol,he is going to design a program to cut the head when i take it in tomorrow

 

[Diggs345]

Sqush was 2.1 to 2.9 with a 0.5mm base gasget

 

[Jakobi]

You'll probably find a gasket stack of 1.3mm will bring the piston flush with the base of the exhaust port. Its always hard to measure the amount it sits above and eyeballing is only guessing.

Setting the piston top flush with the base of the ports will allow for max peak power. As the piston sits further into the cylinder you will move the power curve down to give more bottom to mid at the expense of peak power. If you go too far you will cut your top end out. Remember that the compression ratio also has an effect on power delivery too.

Here is a simulation my engine tuner ran when cutting my 250 (from thread http://www.gasgasrider.org/forum/showthread.php?t=11970)

So i got the cylidner on Friday arvo... just had a chance to map it out this morning and compare it to the last stock '11 EC250 i worked on...

Some obvious things about the S3 cylidner...
- it is the exact same basic cylidner as a stock unit, obviously cast from the same batch as the standard cylidners, it just has S3's logo on the side.
- its had some extra work done to the ports over the stock unit and the finish on the ports is MUCH better than the last stock one i had (which was terrible)
- the aux EX ports have been widened
- Main Ex port has been widened and raised to match the timing of the aux EX ports (stock is staggered with Main Ex lower)
- Main and Aux transfers have been widened
- Boost port has not been touched

All pretty standard stuff aimed at more top end power, most likely at the expense of bottom (as to be expected)...

I ran it through the sim software and got the following graph...

BLACK is the S3 cylidner with a 1.3mm base gasket stack
RED is the S3 cylidner with a 0.5mm base gasket stack
GREEN is the stock EC250 cylidner with a 0.5mm base gasket stack (how the last stock bike that i worked on was set up from the factory)

So the S3 produces significantly more power up top than the stocker due to the larger ports, but you pay the price in reduced bottom end. When i did the last one i faced the same issue and decided to leave the 0.5mm base gasket stack in order to retain some bottom... it was pretty much a half way measure and the final results were very very similar to the RED line on this graph (that person had asked for an increase in top end without a drop in bottom if possible).

So Jake the choice is yours...

- run a 1.3mm base gasket stack and have the port floor flush with the piston at BDC (remembering its only there for a fraction of a second)... this will give the strongest top end but less bottom/mid and the head will have to be shaved 0.8mm to get squish clearance to 1.25mm (current SQ with the 1.3 gasket stack is 2.05mm)

OR

- run a 0.5mm base gasket stack and have the piston a bit above the port floor at BDC (remembering its above the port floor for the rest of the crank rotation anyway)... this will give less up top but keep more down low. The head wont need to be shaved because the cylidner is 0.8mm lower (dome volume will need to be sorted out still though, yet to do the calcs on that)

Of course you can run anything else between 1.3 and 0.5 if you want and its intuitively obvious how the power will be affected.

Aside from the drop in bottom end power the real issue is how important is is to have the piston flush with the port floor at BDC anyway?? In my opinion (and there are many others way more qualified to answer this) the port floor issue is the lesser of two evils... for normal trail riding id rather retain some bottom/mid and live with the fact that my port floors arent flush for the ~0.0001 of a second they otherwise would be at 8000rpm.


Another issue with the EC250 is that the fact that the powervalve snaps open very fast causing a drop in the mid range with the larger exhaust port... if you could keep the same opening start point (about 4500) but extend the opening period out to 7000+ rpm (currently fully open by ~6000) it would work better with the bigger ports... this is obvious in this graph how the GREEN line doesnt drop at all at 6500 but the other two do... its timed to work with the smaller stock ports.

Plenty of food for thought.