Modern Vespa

Finally go around to finishing a spreadsheet I'd been working on to calculate the squish velocity. I'd started this when working on Jess' GT200 landspeed bike and finally got around to making it more general for use on 2T as well.

For the longest time I just followed the "1-2mm" advice for how much clearance there should be between the piston and squish band. But really you want to size that gap so that the gasses are exiting it at a max of 15-20 m/s. With the spreadsheet I can adjust the gap till the velocity is where I want it. It also gives me the compression ratio and a few other important bits. There are a few more things I'd like to add, like allowing for an offset of the bore axis to the crank but that is seen more in 4T engines.


Squish velocity as a function of crank angle at 7000 rpm

MotoBi cylinder head, the motivation for finishing the spreadsheet.

I've always gone by the 1 - 2 mm squish guideline.

I understand what squish is but I'm not understanding "Squish Velocity" as shown on the plot. Some may also need an explanation of Crank Angle BTDC...not me of course....you know..."others"

little above my head, I understood the squish band to be important in retaining unburnt petrol/oil for lubrication purpose. Hence the importance of de-coking.

Why would the speed be a factor?

A squish band works in two ways.

1. it prevents the initial flame front from extending to the edge of the piston allowing the close gap between the piston and head to transfer heat better. Gordon Jennings covered this part of things well.

2. More importantly it creates turbulence that speeds up combustion and prevents detonation. As the piston and head come close the gas trapped between them is forced out into the rest of the combustion chamber. The velocity is a function of the area of the squish band, the gap between the head and piston, and the compression ratio.

The calculation is done by imagining there is a curtain from the edge of the squish band falling down to the face of the piston. When the crank moves through one degree of rotation the piston moves up a given amount and the gas under the squish band rises in pressure more than that under the combustion chamber. If we assume that the pressure remains constant, then a given mass of gas must move from under the squish into the space under the combustion chamber. We know the amount of the gas, the area of the curtain, and the time interval and from that we can calculate the velocity.

OK....squish velocity related to crank angle makes sense now. I need to look at this more closley now to get a better understanding but I really like the concept of the plot.

I do think a unit of measurement should be created for squish velocity....maybe OCTs?

Patrick,

What's the best way to go about reprofiling a head to match a piston? Just dump it on the machinist or are there measurements to do before?

So what would happen if that gap were even smaller, say under 1mm? Does that mean they exit too fast?

The wider the squish band and the smaller the gap, the higher the velocity.

So, with a given head the squish band will be fixed and the gap is the only thing you can really play with. On this head I get the following max velocities: Whenever I've had a head reprofiled I've just given the piston and head to the machinist. In some cases the curve of piston is small enough that the squish can just be cut as a simple cone. It's not hard to measure the curve of the piston and if you have a radius cutter (or CNC) it's not too hard to cut it to match the piston.

Patrick is what you're saying then is that the squish would be best between .71 and 1.0 to put the velocity at that optimum speed 15-20 m/s you mentioned up top of the thread? Or is that just for that particular head?

For this head the gap would be best between 0.71 and 1.0. For other heads or even other max RPM the gap will be different. Point is that there is a method to figure out what it should be to hit the target velocity.

copy that.
Thanks

So how do you model a different head in your spreadsheet? Do you need to create a 3D model of it first? Or just feed in measurements of the combustion chamber?

Patrick

For a while now I have been indexing my plugs.

I only do it because my dad used to do it on a couple machines.

I doubt it's doing anything more than stroking my 'wanna be tinker brain'.

I like your talk of squish and how it relates to making things more efficient.

Does indexing a plug play in any of this?

Here's the inputs on the left and some calculated values on the right. My spreadsheet also has another table where it does the degree by degree calculations.

Because I started this for a 4T that had a dished piston and several options for gasket thickness the total clearance is built up from those values. In this case the piston has a negative volume because it takes up some of the head volume. So I modeled the head as if it had a bit of glass placed across it and figured the piston taking up some of that room. This way I can do a test fit with no gaskets and then make a gasket in the right thickness to get it just right.

The spreadsheet is done in Apple Numbers and does not translate well to excel.

I've placed a copy in icloud and made it editable (via the browser) so you can play with the values and see the effects. If you want to keep others from messing up your own settings then save off a copy (even if it's just to your own icloud account)

I've also added a small table called "Spherical Cap" that can be used to calculate the volumes of the piston crown, and sections of the head.

https://www.icloud.com/iw/#numbers/BAIpwdoPL5_t5wrNaeCBsRMsi7Vj4hVriG2F/Squish-2T

That is pretty cool Patrick! Did you write that yourself?

Of course, now you realise you'll have to explain it...

Head volume - I assume mm^3 = mm³ ?
Piston Volume - is that the calculation of the 'spherical cap' bit?

Of course I have a few more but have to go... thanks for putting this up!

I wrote the spreadsheet but it's based of the method outlined in Design and Simulation of Two Stroke Engines

The piston volume sticks up into the space of the head so its volume is negative. The head is comprised of three spherical caps, one that is the same as the piston crown, one that is the shape of the combustion chamber, and one that is the overlap of the two.

I find the spherical cap formula the easy way to go for the piston, but for the head filling it with water is better. The formula is also useful if working from scratch on a new head to get the general size and shape right.

One mm3 = 1 millilitre? And just use a syringe? Surface tension can throw it out a little, is it worth worrying about?

No doubt in the book it says which velocity is best for which situation... what is your understanding of it? And if it's ideal for 8,000rpm, is it also good for 3,500rpm?

1 milliliter (ml) = 1 cubic centimeter (cc)

The book gives the guideline of 15-20m/s at the max power RPM.

To measure the head volume I use a burette and a plastic plate over the head. A bit of laundry soap in the water takes care of the surface tension, mostly.

It's just that I see the figure 11091 in the field 'V_h Head Volume (mm^3)', that doesn't seem to fit? And if it's actually cc's that you're referring to, why not use (cc) instead? I'm sure I'm just missing something here.

The head volume is 11.091ml = 11.091cc = 11091mm^3 (cubic millimeters)
This is measured from the gasket face up.

Thanks. Makes sense now... I will have to give this a try next time I have the top end off.

This is way too cool, I'm barely able to keep up with the discussion & theory. This is because of the increased compression/reduced space created by the coking, right ?? So if I run my non-competition scoot a bit rich (both fuel & oil) and the plug still looks good (no soot) and has stock CHT temps .... then I'd still be wise to pull the head just to check if I'm getting build-up?

Sorry if this question is too basic, I'm trying to best apply what all this means to an unmodified Stella 2T (other than a PX exhaust & air filter) that I use mainly @ 35-50 mph (it's not an urban scoot). Thanks much, I appreciate you more technical guys.

You can just contact Hot Rd Al who understands all the complex activities involved and advise him of your setup. He will provide a properly profiled head for a reasonable price.

Yup, that works too, but it's nice to be able to verify values sometimes.

Consider that many times a stock P200 has a squish gap to 3-4mm and that a Malossi 133 often has a gap of 2mm or more. This is normally a result of manufacturers making sure there is never too little clearance. Clearly too much in both cases, but by how much.

my small frame 135 malossi came with a squish of 4mm
I fixed that to 1.25mm

way better power

I love this shit.
What kind of timing would that project need to run?!
And i guess a very high compression head?
Has anyone measured the cc of the different stock 200 heads?

I seem to remember Magg measuring some. But that may have been on Scooter Central.


Edit: The one I was thinking of you already know Bholinath... http://www.scootercentral.net/forums/viewtopic.php?f=15&t=17073&hilit

thanks Ginch
thats so cool... but my memory isn't !!

thats a good read that i will need to have another read up on...
but i still dont know what/if there are differences (in size) between the early p200/rally head and the later EFL type head.

(I did a lot of research getting the pinasco head right, turned out mine is almost identical to the VRH head they started to produce)?!

Sorry to dig up an old post but I had some questions. I have a dished piston, so the piston volume would be positive, correct? Also, the dish isn't circular but elliptical, so I used an average of the smallest dia x2 + the largest dia /3 since the ellipse is elongated, which gave me 46.1mm. Should have just used the smallest dia as the dish dia? And for the RPM, is that max RPM or RPM at the power peak? I used 8000 which is the highest I rev when accelerating but my power peak is lower than that. With everything punched in I got .045"/1.143mm for squish clearance to give me 19.7 M/s at 8000 rpm.

Here's a good thread talking about why too high a squish velocity can be bad: http://speedtalk.com/forum/viewtopic.php?f=1&t=39725

And I just got a call from the cylinder head shop I dropped the jug off at, they couldn't safely hold the jug in the mill so now I need to find another shop, I've already tried several machine shops who wouldn't/couldn't do it. Or I can try using a thinner base gasket which won't get me to .045" but it will get me close, I already tried no base gasket but every sealant I tried still seeped oil. I've heard thick paper, like brown/kraft paper will work if coated with a gasket sealer.

Way to necro a 3 year old Vintage 2-stroke vespa thread to talk about a modern four stroke Genuine. Bravo. Well done.

Maybe you've suffered some brain damage from all that 2T smoke you've been inhaling and your reading comprehension has suffered.

Why don't you get off your high horse, it's only a 2 stroke Italian shopping bike for God's sakes. Get back to me when you get a real 2 stroke motorcycle.

If i wanted a bigger 2-stroke motorcycle i would fucking get one. And then i would take my ass to a bigger 2-stroke motorcycle forum, where people wanted to talk about such things, rather that gumming up the works with my interloper bullshit. But hey, that's just me.

Well, by golly, I guess asking a question about squish in a thread about squish to a guy who knows his shit concerning both two strokes and four strokes is considered interloping. I guess asking about my Piaggio designed clutch or transmission would be interloping too, I guess not asking about oil pumps and valves wasn't safe enough.

But you own and created this website so you get to decide what constitutes interloping. Oh, wait, you don't, that would be Jess who rides one of those modern Vespas (hey, that's the name of the website) you despise. Who was generous enough to create this subforum for vintage Vespas since all the other vintage Vespa forums died because everyone wanted to be dicks and no one wanted to actually run a website.

The whole reason I asked about squish on this thread is the fact that Oopsclunkthud had one of the most coherent and thought out reasons for a certain squish clearance that he backed up with data and a fancy calculator. All I could find was to make the squish as tight as mechanically possible, but no one had any kind of data to back it up. Armed with Oops' info I was able to dig deeper and figure out that excessive squish velocity can increase the risk of detonation, though this seems more of a problem on high reving, high compression 2 strokes.

And seriously, other than shooting shit, I ask questions where I'll get the best answers. Anything related to the Piaggio designed parts of my scooter, I ask here because this is where the knowledgeable vintage Vespa folks hang. I don't ask question here pertaining to stuff like head porting or valve timing because I don't expect this community to have sufficient knowledge to advise me, so I go to someplace that does.

And after all this time, I'm finally starting to make the replacement head for the MotoBi.

For those who found this thread, hoping to find details about squish, below are links that I've found useful.

http://www.scootercentral.net/forums/viewtopic.php?t=16888


http://www.mbscooters.co.uk/info/setting-up-*-engine-carbs-3/setting-up-*-squish+167.html

Finally getting around to trying this out. I recently found this article by Darryl Taylor in Scootering. He's saying that the figure to aim for is 20 - 30 m/s. Patrick, can you please have a look and see what you think?

https://www.scootering.com/scootering-classics-squish-velocity-piece-3-of-the-power-puzzle/

At the moment it's adding up to 27.5 m/s, but it's giving me a squish clearance of 2mm. Should I be adjusting that or leave that side alone? My actual squish is 1.05.

I've not seen values that high, but just did a search and found a reference from a voice I trust. (warning: this thread will suck you in as there are several people at the very top of 2T engine development that contribute a lot)

https://www.kiwibiker.co.nz/forums/showthread.php/86554-ESE-s-works-engine-tuner?p=1130119659#post1130119659

A few things to note though is those values are most likely for 100+ octane fuel, and they are using an ignition that retards at higher rpm. I think I'll stick to the higher end of 20m/s on most my stuff given the air cooled and lack of control of the ignition curve.

Even at the 15-20 m/s MSV expect to retard the timing, as the time it takes for the flame front to move through the combustion chamber is reduced a lot.

Can you share your setup values?

going to drop this link here as well, as there is good info on head design:

https://www.kiwibiker.co.nz/forums/showthread.php/86554-ESE-s-works-engine-tuner?p=1131051281#post1131051281