Squash and squeeze my chamber?

How do I calculate the squish band width for my engine`s combustion chamber? I get asked this from time to time and usually end up doing the sums myself and supplying the answer. The actual equation involved is quite straight forward, but maths can be off-putting for some people so rather than have a go they just don`t do it! The equation here looks daunting but it is easy to work through, no experience necessary other than being able to use a calculator, or even long hand, just substitute your own numbers.

The example below is for a 50% band and for a 54mm bore.

W= S x D x ( 1- sqrt (1-S) )

Where: W= squish band width
D= bore diameter
S= squish factor as a fraction, ie 50% is .5, 60% is .6, and so on.
sqrt is for square root, on the calculator symbol, and inside of the brackets (parentheses) are
calculated first.

W= 0.5 x 54 x( 1-sqrt 0.5 )

W= 0.5 x 54 x ( 1-.707 )

W= 0.5 x 54 x .293

W= 27 x .293

W= 7.9mm

To give some confident authenticity to this number the Aprilia`s squish width is identical, so you will be in good company. The remainder of a complete combustion chamber design then comes from this initial calculation. Where the other parts can then be broken down into simple geometric shapes building up into the final volume.

Trevor

the formula is easy, so can ya explain what width is used for what kind of power band? Is like the 50% used on mid to high power band and the 60% used on low to mid range like for trials or?

Mutt,
The short answer is yes, but only to a certain limit, trials engines ostensibly operate at lowish rpm levels and as such combustion efficiency is not too high. Wider squish bands creates an extra touch of turbulence to enhance flame travel and improve combustion. Countering this is that for a given comp ratio the chamber will be deeper , so the flame travel will be longer, reducing the time available for a complete burn, not too much of a problem for a trialer, but the one negates the other to a certain extent.
Combustion in a typical air/fuel mixture is down to around 2mtrs/sec, left like this the engine might attain 1500rpm! Not a lot of good for a race engine.
So how is it possible for a Bantam engine running at 10,500rpm to complete it`s combustion cycle within 50* of crankshaft revolution, the answer is turbulence. Racking up the comp ratio, having a compact chamber profile with a 50% squish band and minimum band clearance all provide turbulence. The higher the flame speed, potentially the more power is available, race fuels offer some advantages with high octane and extra oxygen released at faster burning rate, but are forbidden for our use. No problem for you I guess?
Why 50% squish band, that percentage has been arrived at as the best compromise, and if you can get to a squish velocity of 35mtrs/sec by juggling all of the variables you can rock-n-roll!
The Aprilia engine`s combustion chamber, for running on un-leaded race gas, has the plug face just 7.5mm from the piston crown in a very compact chamber, one more reason why it pumps out well over 54hp!

There is probably a more in depth article on the forum, with a lot more specifics about all of this, so it will be worth spending a day or two ploughing through it all.

Trevor

Ok. I will see if I can find those articles. Can ya tell me what the minimum clearance is , between the squish band and the piston crown?

If my piston crown is 12º, should the squish band be the same or as I read some where on the net, it should be 4º more?

I hope y'all don't mind a mind a novice asking too many questions !!!!


No, all of our racing is "outlaw". The only thing the powers that be doesn't allow is a reed plate or exhaust valve, they say because it cost too much for the average racer to have done to their motors (but I thought that was what racing is all about, see who can get the fastest bike, regardless of what ya do to the motor.)

Ask as many questions as you like Mutt, every one will be better informed by doing so!

The squish clearance should be a close as you can get without piston contact at peak rpm. There are of course practical, mechanical reasons why you can`t go too close, in general terms 1% of crank stroke should be a starting point, but a 64x58 long rod engine will be ok at .040". Make the squish band parallel with the piston crown, a 4* extra taper will reduce the efficiency of the effect you are trying to achieve. If you are cutting your own cylinder head chamber profile then put NO radius on the blend between band and dome, just a gentle de-burring of the edge is all you need to do. The sharp edge will not over heat and will generate more turbulence, which is good, it works! The more turbulence you can create in the combustion chamber the less likely detonation will occur.If you have no residual mixture at the edges of the chamber then you have no scope for detonation of these end gasses to occur.

Trevor

Thanks, I'm just trying to learn. Not many forums around that go in-depth. I almost always do my own machine work and always do my own welding work, that way I have no one to blame but me if I louse it up !!! Most of the bikes at the track don't go over 60mph (1/2 mile oval grass track) I want to built something that will smoke all the competition (sure y'all know about that !!!)

So 12º / .040" it will be. So I got some work cut out for me.

The squish band is wider than the .369" it needs to be (if I remember right it was about .562") and, of course, the edge leading to the chamber is radius'd real good. The band is somewhat recessed into the head maybe .030" or .040". The piston edge is about .020" lower than the gasket surface of the barrel, Combined with the .060 gasket, as it stands now, I have near about .125" gap between the piston crown and squish band !!! Where's the best place to start in remedying these problems?

Rex, do ya got any real thin head gaskets?

Hi Mutt,
You might consider a spigot type head joint?
No gaskets and head concentric with bore?
I can send a photo via email to give an idea?
All the Best from Sunny Uk!...............................

Rex and Debb

I go along with that Rex...

We (when I was wearing my Deutz Diesel hat) had no head gasket on a spigot faced head joint. This was a cast-iron barrel with alloy head on air-cooled diesel engines with compression ratios ranging from 19:1 t0 23:1. They´d sometimes squeak when starting in a very cold ambient... but once warmed would seal perfectly.

Mutt -- where are you? -- with a half mile oval grass track? Sounds like real fun.

Squeezing oneself into a frog-like, bone-breaking contortium to propel onself in front of other uncomfortable Loons is a `Special´ sort of fun but cannot be compared with wide bars and that comfortable upright racing postion...

Is the grass track left-handed? If it is then the racing would be akin to a large Speedway track which, by just thinking about, has me getting a tingling feeling all over.

The 60 mph is insignifigant: in any form of racing the acceleration is more impoortant and you are doing the right thing to go for as much power as you can get. The difference between your Bantam -- as is -- and say, the current fastest road racing Bantam is about 4 times, so you have to get cracking and I wish you good luck....

To close, I had a few seasons of grass racing a 500Jap in Enfield Bullet farme, a 250 AJS engine in Bitza frame -- both on methanol dope.
A couple of times, when rain was forecast, I took along my 350 AJS road hack, made a late-entry at the circuit and had the fun of being competitive ...

... which does mean that with a `soft´*** motor it is possible to have a couple of `good´ races (rather than just a "Ride") but nothing can take off the moments of battling for the lead with maximum power, no matter what form of motorbike racing it is....

In any case, crushed- grass smells (and sometimes tastes...) better than warmed-up tarmac....

***the 350 AyJay was low compression on petrol and could probably be compared with your Bantam at the moment.

All the best,

Cheers,

John-Boy

There you go Mutt, a nice offer from Rex.

In the meantime you could have a look at the "Liquid or Gas" topic that you can find on page 3, there you can see the spigot arrangement I use on the liner/head location of my water-cooled engine. On the plan view photo you can see the faint line that the sealing "o" ring leaves, on the liner flange, no other jointing medium is used. From what you have described you have a complicated arrangement, simplifying this will help a lot, gaskets and stuff always impose a thermal conductivity barrier. Getting heat away on an air cooled engine always pays dividends!

Trevor

Rex, yes email me a pic please,

John I should have said a 1/2 mile dirt track, because there is no grass. I have a 5/16" mile oval track in my pasture to practice on.
Trevor, I'll have a look

Hey Mutt, that´s some back yard!  I am sick with envy ....

I was nearly as lucky. Living in `The Boonies´ of Essex,  my parent´s place was a short distance from the "River Fields"  -- also known as `common land´. This was a very large grass field  the length of three normal sized fields, alongside the river Roding which  used to flood every so often,  which meant  practice was sometimes somewhat muddy. No one ever complained of noise because it was about two miles away from houses, across farm fields.  I suppose riding a motorcycle across those fields might have been illegal....!

We used to say -- with cricket, that is -- "Rain stopped play" but it was never like that with grass tracking and road racing except for once that I knew of when weather stopped motor bikes racing. That was March 6th, 1968 road race meeting at Brands Hatch. It had to be cancelled  when a freak snow-storm filled the valley between Paddock Bend and Druid´s Elbow to a depth of 6 feet. Just no snow plows available....

All this use of Imperial inches and feet would not be understood by the Germans here
in Deutschland and their (Long) Grass Tracks -- are often a mile in length. The only one, in the UK -- I knew of -- to compare with that was Stokenchurch at High Wycombe, Bucks called a "Mountain Grass Track" which used two fields with a valldey in between. I was electronically timed on the straight there at 99.8 mph whilst racing my 500Jap/Enfield...
That sound rather grand but it was only fifth fastest:- Lew Coffin, Don Godden and two others (also pro Speedway stars) were quicker. If I remember aright Godden was fastest at 101 mph...

... but that was an experience never to be forgotten: coming off that timing straight was to dash into a right-hander where both wheels drifted out and then into another bit of straight where the gound dropped away alarmingly toward the ditch at the bottom of the valley...
I apologise for the sickly nostalgia ....

Take care,
cheers!

6 feet in a freak snow storm ? Wow, it would take 50 years for that much snow to fall where I'm at. There was a fella in the far south east here in 1952 that turned a 13.5 second 1/4 mile (standing start) on his 1948 125cc. I thought that was pretty impressive. It was nitro powered and his pipe was just the factory stock header pipe, cut off to about 18" long. Back then, they say ya just rubbed a line of your gal's lipstick down the center line of the pipe and cut the pipe off at the point where the lip stick didn't burn !!!

Weird how I can remember such things that happened nearly half-a-century ago and cannot recall the name of someone I met -- for the first time -- last week.
  This snow scene is so clear in my mind. Most of Brands Hatch circuit had a light sprinkling of snow which was melting with the morning sun (where the stretches of road were without shade). It was in the valley down between Paddock Bend and Druid´s Elbow that the fallen  snow had been piled up by driving wind during the night....

It was a National meeting with some of the world´s top stars wanting to use it as a warm-up for the big-time stuff and the events included all classes imaginable -- including a race for only 125  Bantams. Phil Read along with ( I think) Bill Ivy and Peter Willaims (I think?)  were all for hiring a bull-dozer to clear the track but a more down-to-earth Brands-Gofer pointed out that by time a dozer got there and moved the several tons of snow it would be mid-afternoon.  I´d gained so much experience by test development practice there -- over a year -- that I was devastated by not being able to race the Bantam....

Mutt, I dig that straight-thro pipe theory! Does your racing formula allow use of methanol-TEL or other doped  fuel?
The Australian Walsh Bantam was on 16;1 compression-ratio using such an exotic fuel  and with a megaphone exhaust was beating up 250s regularly with Maurice Quincey on board... I don´t know much more than that it had a single 60thou thick piston ring and the engine friction losses were greatly reduced...
   ... which ís a factor to take very serously in tuning.
Apparently Maurice used to change the ring between practice and races...

Maybe one of the Bantam-Lads will mention the gearbox*** mods  and the oils used....?

***One mod  is the replacing of plain bushes with ball bearings and the oil drag represents a power loss (to friction, of course)  when multiplied up by the drive ratios.

Cheers!

Yep, run any kind of fuel ya want. No stipulations, because EVERYONE can afford $12 a gallon fuel !!!! most people are running 110 AV gas and castrol bean oil. Smells lovely !!! My motor is all ball bearing and either double seal or double lip seals thru out. This winter I hope to do a complete teardown to install this stroker crank (61.11mm) with Pro-X Honda CR250R rod. It's massive compared to the stock rod that is in there now. I have a friend that manufacturers bearings. He set me up with a pair of really far out ceramic bearings. The balls are silicone carbide ceramic and hardened steel races and steel cages. They have 3 times the
rpm capability and will even run without oil (I wouldn't advise it) Max rpm 30.000 where as the standard roller bearing is 10 or 12g . I had to shave the insides of both webs .062 and enlarge the recess o.d., so the rod will fit right. That should get me to 198cc on a standard bore !!!!

Double-Lip seals Mutt?
Necessary? Twice the drag....!

Hopefully one of the Bantam Gang in the UK will give advice on that aspect.

As I understood it some lip seals provide more Friction Loss than others and it is surprising how much engine power is LOST to friction on things that don´t appear to present much drag.

I didn´t get an answer to my query which was made undder a `Nicasil Liners´ subject where I suggested a Piston being nicasilled on its thrust faces and the liner left as honed. Like you say the silicone carbon is very very hard and I would have thought having the piston-thrust faces treated this way would be advantageous.

Sorry to drag another subject into yours -- but really, it is the same -- DRAG....

Go well...