Squish adjustment

[Marcão]

Hello All,

I'm right now spending some time to improve the visual and performance of my EC (it's originally a 2005 250 but converted into 300 a few years ago).

So I've reached the point of modifying the head to get a better combustion and easy jetting. I have access to a Lathe and some machining skills, so I will be doing the squish adjustment by myself. My current squish is around 2,4 mm which is quite normal for these bikes according to what I've seen in many posts here.

We have down here in Brazil 20% ethanol in all our gasolines, both in regular or premium. The premium one should be around 98 octanes (or 95 anti detonation index) and is the gas I probably will be using most of the time.

Following my doubt:

- Is it necessary to go into 1,2 mm squish to get the benefits ? Or if I adjust it for ~ 1,6 mm for instance I will see real benefits ???

I ask because I will not be able to machining the combustion chamber, only the "face" of the head, and I'm a little bit afraid that reducing to 1,2 mm will bring the compression ratio to a dangerous borderline. I'm using only one gasket of 0,5 mm on the cylinder and don't want to touch this point now.

I would appreciate all your comments !!!

 

[Jakobi]

If you can cc the chamber and confirm the volume I can crunch some numbers and work out the uncorrected compression ratio for you. I just went through this with another member in Australia on a 2011 SD300. At 1.3mm squish and the combustion chamber volume (stock) of 25.4cc the trapped volume is 24cc which gives an uncorrected compression ratio of 13.25:1

His originally had a 2.3mm squish band and was 11.5:1 so gained a nice healthy boost to bottom end.

Tightening the squish band a little will show improvements over stock. Obviously tighter is better though up until the point that the piston and head start to make contact.

 

[Marcão]

Jakobi,

Unfortunatelly I cannot CC rhe chamber right now. Assuming the stock volume and imagining that I reduce the squish band to 1,6 mm, is it possible for you to estimate the future compression ratio ?

 

[Jakobi]

No worries mate.

Combustion Chamber Volume = 25.4cc
1.6mm Squish band/Deck height volume = 6.52cc
Piston Dome Height = 3.3mm, Diameter 72mm, Volume estimated 6.74cc

Trapped Volume = (Combustion Chamber + Deck Height Volume) - Piston Dome Volume
= (25.4 + 6.52) - 6.74
= 25.18cc

UCCR = (Trapped + Cylinder Volume) / Trapped
= (25.18 + 293) / 25.18
= 12.64:1

In saying all this it would only take an extra couple of minutes to actually cc the complete trapped volume on the bike if you were going to pull the cylinder. If you are just doing the head you could also easily cc the chamber volume to confirm its 25.4cc.

 

[Jakobi]

Note that all the above is based on you doing this to the 300 cylinder and head. If you were going to do the 250 the figures change.

 

[Marcão]

Hi, Jakobi.

Thanks a lot for taking your time for such a calculation.

I've machined the head today and mesured the new squish. It's now 1,65 mm. For sure the compression increased (because I didn't reshaped the combustion chamber) but I hope withnin the limit. Considering that I'll be using premium pump fuel and that our gas has a mix of 20% ethanol, I don't think higher compression should be an issue...

As soon as I ride the bike, I'll post the results...

 

[Jakobi]

No worries. I'm not sure what effect the 20% ethanol will have on the engine, however you are generally good using 92 octane up to 13:1 from what I have read. Look forward to the ride report.

 

[stainlesscycle]

- Is it necessary to go into 1,2 mm squish to get the benefits ? Or if I adjust it for ~ 1,6 mm for instance I will see real benefits ???

i've done 1.2 without adjustment of head - compression goes up significantly... i had to play with timing and run 112 octane to make it run right/never ping. i don't think i would recommend it unless you can get and run 112 always (and jet for it). 1.6 squish runs on 92 or better (i run 50/50 92/112) and have not had any problems with stock timing. i have run straight 92 at 1.6 and had no problems, but i'm jetted for race mix, so it wasn't optimal. you will feel a significant difference in hp, power delivery, and fuel economy at 1.6. it makes a big difference on ease of jetting also...... very worthwhile...

 

[Jakobi]

Good info Stainless. One thing to note with all of this is that I am basing it on the volume of the head I cc'd and an estimate of the piston dome volume (using a spherical calc based on dome height I measured). I should take the time to cc the piston dome volume for the 2 pistons that I have laying around.

A piston with a larger dome, or if the dome is a few cc larger than my estimate, it'll also push the compression ratio. A couple of cc can make a significant difference.

 

[Marcão]

Guys,

Today I've finished the job... The squish now is exactly at 1,6 mm (on the outside border of the squish band and reaching 2,00 mm on the inside border). The jetting is now as follow:

175 Main
42 Pilot
CCK Needle @ 3rd slot
Slider #7
Air screw 1 3/4 out

I haven't had the opportunity to ride on the trails, just gone up and down on the pavement. But the difference in performance is noticeable !!! But I've the impression that the engine is working now hotter than in the past... Maybe just a impression... Does it make sense ? (my previous jetting was N1EF @ 2, 178 MJ, 42 PJ ...)

 

[Jakobi]

You've changed the jetting and the new settings are much leaner. Will run hotter and probably better as well. Ideally you'd run the bike with the same jetting to grasp the difference with less variables.

 

[Jakobi]

Any updates Marcalo?

 

[Leo]

I planning to change my piston (OEM dome top) to one from Honda CR (flat top) because CR pistons are less than a half price here.
My bike is an EC 250 2011

The idea is to face the head to keep the same compression ratio. But reading this post I realize that I must (or can) modify the chamber profile.

Someone already done that? This is a good idea?

 

[Jakobi]

The squish band itself is also set up to work with a domed piston so that would need to be addressed too. However..

If using a flat-top piston available for this motor, you could then run a toroidal head insert, and in so doing, run a higher compression, with a smaller cc head, and liberate more power again.

Pic attached shows profile of toroid - this is from our TSS500 engine, which runs 73mm bores. (http://www.twostrokeshop.com)

Toroids and flat-tops came into universal use in the 125/ 250/ 500 GP bikes when ULP was introduced in the late '90s ... as, being the most deto-resistant combination, you couldn't afford not to run them.

This info was provided by the TSS which is located in the town where I live. I had enquired about having my head work done with them when I first got the gasser. They really know their stuff.

 

[Eric]

I planning to change my piston (OEM dome top) to one from Honda CR (flat top) because CR pistons are less than a half price here.
My bike is an EC 250 2011

The idea is to face the head to keep the same compression ratio. But reading this post I realize that I must (or can) modify the chamber profile.

Someone already done that? This is a good idea?

What piston are you getting?

You need the 86-96 honda cr piston its the only honda one that fits but even its 0.01 of a mm bigger so you may need to replate, atleast check the clearances it may fit. It has a small notch in the bottom of the intake skirt too but that doesnt seam to afect anything.

I have a prox honda cr piston in my 03 it needed a replate anyway... the dome was the same as the wiesco gasgas piston I removed and the pics of the wiseco cr piston show it as domed too.

 

[(F5)]

. . .
This info was provided by the TSS which is located in the town where I live. I had enquired about having my head work done with them when I first got the gasser. They really know their stuff.

Do a google earth search of TSS address. yup, that's his suburban house, doesn't even have a decent garage from what I can see (although I have a mate who's been there & was under whelmed). The Brains of the show was Wobbly & they had a handbags at dawn fall out. He does read an awful lot, but he's a Journalist, not an engineer & its just him. No factory, no shop. Posh website & some reasonably informed articles looks very convincing.

 

[Jakobi]

Hmm.. Very strange F5 as Mr Robby Madisons old man had nothing but good things to say about them/him..

 

[(F5)]

yeah I'm not trying to pull him down, but it does irk me that he portrays an image of some big arrangement. He is sourcing some great stuff, but just be aware. Sounds like the chap you are working with has his head on his shoulders.

 

[Jakobi]

I hear you F5. Haven't met the bloke in person either. I must say I'm very pleased with Daves (dmcca) work and his work ethics. Shame he lives at the other end of the country.

 

[(F5)]

Meh, my pipe builder lives in S.Af if you like inconvenient.