Pistons
Turbo Mini Forums

Hi all,

I'm currently running 11cc Dished Omega Pistons (cast) on my 1380 build, and wondered if they are ok to use in my turbo build?

Also, any ideas on a decent head to use with these for a strong turbo setup would be appreciated.

Cheers, Tim

http://www.metropower.co.uk/portal/default.asp?id=75&mnu=75

For a head of the kind of chamber size you need, you're best just having a word with Stuart Gurr...

Something that i would suggest though, is that you don't go overboard on the port sizes. Spend your money getting the chamber large and the short-side radius / area around the guide opened out. I can't see the benefit in paying money for large ports on a turbo... Just my opinion though, i have no data to back this up, just theory..

Edited by turbodave16v on 9th Sep, 2004.

Tim,

regarding the Omegas, yes. I ve run them for about 10 years and currently running them at 15 psi.

My head has 35 cc volume, for a CR of about 7,8:1. In UK Stuart is probably the only one that supplies large volume heads.

Jukka

Forgot one thing, my head still retains the beak in the chamber (although cut back a lot), retaining the std ish shape. Stuart, have you ever run back to back tests to see if there´s a difference in off boost output between the heads with/wihtout the beak ?

Jukka

hi
no, its just a theory , but the beak type chamber is ideal with a flat top piston and minimum deck height, when there is a round chamber below i think its advantage is less , the turbo solution in my mind is a channel machined thru a flat top piston and mirrored chamber shape to give a good area of squish front and back to suppress detonation , its just thery but i have to say i havent yet melted a piston using it , squish over beak is the prefference !

Am I right in thinking that droplet size increaces with centrifuging and with low octane, naturally aspirated engines, droplets are more likely to combust when in contact with the hot exhaust valve. (Low octane has a lower flashpoint and this enabled efficiency to be maximised using crappy 2 star). So, whilst a beak is important to reduce centrifuging (droplet size) which gives a more efficient burn, it isn't super critical unless the fuel you are using is prone to flashing up. (I think that there is a difference between detonationability and flash point, not quite sure though). I have also noticed in some (MED?) high boost heads, the spark plug boss forms a pseudo-beak the other side, I imagine this has a similar effect. How the beak would perform in higher densities, I am not sure. It would be interesting to find out. I expect that the fuel would be more likely to condense (wrong word) if there was centrifuging and that would influence your burn rate due to the closer proximity of droplets. Absolutely no clue how the beak works, the above are what I reckon. I'd love to know the proper answer, anyone.??

stu

Bugger off, I'm getting there.

A bit like this head chamber shape??

http://www.turbominis.co.uk/photos/index.p...mg=dscf0068.jpg

Yep, that's the one.

One thing I find really odd with the A series is the placing of the spark plug relative to the inlet valve (near to it). Legend has it that engines are much better with it near the exhaust. Harry Weslake was smart enough to go with the beaked chamber which on smaller engines is pretty necessary but the exhaust valve thing, that is odd. Much have been a reason for it. I have an old head which I have on my desk (big ported useless thing) and looking at it, I would have thought that the beak had very little effect on the inlet charge as it was drawn in, as the compression happens, looking up towards the valves, the mixture is rotating towards the spark plug from the edge of the inlet valve. As the piston moves up, this rotation speeds up and then the beak deflects the spinning mixture towards the spark plug. What really happens to the movement and density of the charge and how it burns is pretty difficult to figure. I imagine without specialist equipment, it is virtually impossible. But, having the beak 'impersonator' on the other side might throw the charge where a local factor is different, again difficult to predict and god knows what happens.

I am interested in how Jimsters circular beakless head works as that must surely be a measure of the effectiveness of a beak.

I had a good book by an old motorbike tuner, his point was this simple. The object of the excercise is to create the right burn in the cylinder head. What influences this was octane (burn speed) effective compression and the homogeneity of the charge (how big the drops are). So for a given fuel and effective compression, you need a chamber to allow complete burning of the fuel, this means it will not create pockets of unburnt fuel, droplets which will burn incompletely with the highest effective compression allowing maximum thermal efficiency.

How you go from these criteria (and probably some I missed) to a chamber shape is probably the product of years on the dyno.

As turbos are running at least a vastly different density of charge I expect that they would require a different chamber shape, how it would differ, I am not sure. This is also compromised by the head casting and how much meat you need to remove (loads). I suppose the best compromise will come out in due course.

Ken Elders 8 port head seems to have a really deep bath shaped chamber which might provide an interesting raw material for sculpture.

One thing which would make it a lot more simple for me to understand is........."If you have low octane fuel, it has a high speed of burn/flame front, if you have a high compressed high octane fuel equally near to detonation, is the burn speed equally high?"

From that you can gather that a Low Compression/Low Octane head will share similar combustion chamber dimensions, if this relationship is different, ie the blown engine is close to detonation but has a higher/lower flame speed, it will be necessary to alter the dimensions accordingly. As the density has gone up, I would expect the burn speed to be really high in a turbo engine, so favouring a spatious chamber, as flame speed decreases, the chamber should be made more bath shape (sausage shape distorted by the valve sizes). This is further influenced by the bore-stroke ratio, head, cam, etc.

I have a semi-downdraft 8 port head design which I have pretty much finalised. The design of the combustion chamber has left me pretty clueless as to what to do. So, I will start with a small deep chamber and experiment.

So, Jim, does the beakless gubbins work or not?

stu (aka mr fulloshite)

just clarifying my ideas, if anyone can add something (relevant), I would be very grateful

Bugger off, I'm getting there.

Cool thread guys...

Some good ideas and points there Stu!!

SOme of my thoughts....

"homogeneity of the charge (how big the drops are" I thought homogeneity was the distribution of droplets rather than their size?

While thinking about this... is droplet size somthing we need to be worrying about? In our engines we typically have much higher Air Charge Temps and so the fuel is more likely to vapourise? Or have I missed the point?

Anyway, back to combustion chamber desgin!

The design of the chamber effects ABOSOLUTLY everything, from power and emissions to cooling and durability. Its not only the basic design but also things like the angles of the walls and of course the "beak".

A-series are blessed with very good swirl... reprted to be as good as some F1 engine from the mid 90s (of course I can't back up that claim!) But the beak provides the main guide to the swirl once the inlet valve is shut. There are both advantages and disadvantages to swirl (like everything!) but generally its a good thing for complete combustion.

The question comes from what the effect on swirl is when we alter/remove the beak, will it help with out right power on a turbo charged engine?!

Alex

AlexF

wow , too many people reading books , i cant cope after a day at work !
swirl on the mini engine is created by the intake valve being offsett from the center and the port turning before entry to the cylinder (not the beak) as most 8 valve engines - note 16v not as efficient because of low swirl , but more power due to incresed flow area -
the beak works because the chamber volume needs to be near the spark plug to give a quick efficient burn ie if the beak wasn't there the burn would take longer to get to that 'dead' area,
its that shape because its the only shape it can be(with in reason) without shrouding the valves
look at the ford cvh head , original and lean burn shapes maximum advance on timing reduced by 4 degrees with a beak - more power lower emmissions
port finnish plays an important part to droplet size , to smooth incourages fuel to 'adhere' to suface further reducing flow by surface tension textured surface can be quite rough without effecting flow ie smooth golf ball verses more efficient dimpled one and current racing yatch hull finnishes
higher charge temps increase total saturation of air with fuel but decrease power due to lower total densisty
for ideal chamber and plug location look at two stroke kart engine center plug, small circular chamber smallest distance for burn with maximum squish , things just have to be comprimised when valves have to be in the equation
brian ache !!!

So, efficient burn=more of a compact chamber which would imply that the charge is cooler/higher octane/richer and enables more welly to be got out of the head with those size valves.
An economy version would have more of a spread out chamber allowing the higher burn speed to be got from hotter/lower octane/leaner setups. ????????

I'm just trying to put the bits into a pattern here.

When I was interested in exhaust design, I had the misfortune of reading a comprehensive 2 stroke book. That melted a fair old bit of my motherboard......I'm about ready to have another crack at it.

Am I right in thinking the object of the excercise is to maintain as high effective compression without detonation, getting the maximum advance for the maximum power?

So for low octane/economy you have more spread shapes and for a high octane/performance bias, you have a more compact slow burn shape. (fitting all of it around the valves)...........

Strewth!

Bugger off, I'm getting there.

maximum efficiency ,usually gives maximum power ,
lower octane fuels burn quicker as they are less refined so more volatile but cannot be compressed as much as higher octane,
high octane fuels give more power in a higher compression engine through its refining and therefore resistance to pre igniting
forced induction engines having a denser charge have a quicker burn and so need less ignition advance - more boost less advance (assuming you have a cool enough charge , octane of the fuel is high enough and the engine is good)-
less total advance is an indication of efficiency of burn , i have tuned lots of race minis on the rolling road , one stands out in my mind because it produced max power at 27 degrees as to the norm of 30/32 indicating quick burn ,
less advance means more more crank degrees of pushing the piston down and more power
also if you start burning oil from worn bores ,rings guides or turbo the oil will lower the octane of the fuel increase chances of pre ignition and give higher combustion temperature and bang !

Most people forget how much of a problem oil in the charge is!

Old engines do nothing for power...

Then again mines an old bottom end but still works lol

alex

AlexF

Good info stu. I'll add that to my arsenal of useful information. Tuning books generally tell you what to do and what happens but not why. That's what I'm interested in. Thanks.stu

Bugger off, I'm getting there.