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danllew

11 Posts
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Afternoon all,

I've just been doing some quick calcs to try and see if it would be possible to use a head with a standard chamber volume (21.4cc) when running a turbo.

The idea was to not modify the head but use the pistons to give the decrease in compression ratio.

So the pistons I looked at were the 18cc dish forged Omegas.

But working out the compression ratio give 8.2:1 which is a bit on the low side.

Are there any pistons available between the 10cc cast Omegas and the 18cc forged ones?

Thanks in advance

Dan


Barrieri

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To my knowledge in the past there used to be the 15cc Omegas but I guess there aren't being produced anymore. I had the same problem when buying mine for the twin cam build.


theoneeyedlizard

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You could have a small amount skimmed from the head.

In the 13's at last!.. Just


danllew

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Member #: 11468
Member

Yep I guess so, or deck the block slightly. I'd rather not touch the head as its a nicely ported N/A head.

It just seems a bit strange that no-one offers a piston that would give you a good compression ratio while keeping the standard deck height and combustion chamber volume.

I've just redone the calc as I'd forgotten to include the gasket volume.

10cc Cast Omegas give ~9.1:1 (too high)
18cc Forged Omegas give ~7.75:1 (too low)

For the same deck height a 13cc dish would give 8.5:1.


Callum

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Get some more material removed from the 10cc omegas?


theoneeyedlizard

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Ah. I'd definitely get the pistons flush with the deck.

In the 13's at last!.. Just


shane

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Lowestoft, Suffolk.

The 10cc omegas can be opened up to give 14cc, also there is a difference in compressed volume of the bk450 and the af470, only miniamal but this may help.

Shane

Edited by shane on 13th Sep, 2017.


Brett

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reading between the lines on what you have put is sounds like you already know this but im going to write it anyway

the forged omegas sit way way down the block

without looking back over my notes the last engine i built had 1.5mm skimmed off the head and block combined

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shane

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Lowestoft, Suffolk.

Someone may want to double check this but based on ;
+020",bore std stroke,
.025" deck to piston,
.037 for your gasket,
21cc head,
using a set of cast 10cc omegas machined to 14cc gets you in the region of 8.8:1
I think the trade off will be the cost of forged 18cc pistons and a little machining of the deck or head VS cast pistons and machining to gain extra cc's and possible machining of the block?

Shane


danllew

11 Posts
Member #: 11468
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Cheers for the replies.

I'm really struggling to see the point of the 18cc pistons. Why design and manufacture a piston that requires the block to be decked to get any sort of useful compression ratio? Who knows.

I've done the same calcs but with a deck to piston of 0.040". With a piston dish of 13cc I get 8.5:1 which would be perfect.

I've had a look but has anyone got any details of how much meat can be taken out of either the cast omegas or 21253's?


Joe C

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Carlos Fandango

Burnham-on-Crouch, Essex

block decking is irrelevent, any motor should be decked to get the pistons pretty much flush.

On 28th Aug, 2011 Kean said:
At the risk of being sigged...

Joe, do you have a photo of your tool?



http://www.turbominis.co.uk/forums/index.p...9064&lastpost=1

http://s95.photobucket.com/albums/l154/min...2006/?start=all



danllew

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Member #: 11468
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Hi Joe,

Pardon my ignorance.

Whats the reason for decking the block?

Cheers,

Dan


Joe C

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Carlos Fandango

Burnham-on-Crouch, Essex

basically squish band as below,

https://en.wikipedia.org/wiki/Squish_(piston_engine)

std gasket is 0.8mm which is pretty much right whereteh squish wants to be with the pistons flush to the block.

I tend to build with pistons 5-10 though down the block so theres room for a skim should it ever be needed.

this is why de comp plates are bad, they open up the squish band so the mixture isnt agitated in the cylinder as much which hurts efficiency, especially at lower rpms and throttle openings. recently I cam across somone racing a K head mini who uses just 1 thin steel shim as a gasket ans swears by it.

On 28th Aug, 2011 Kean said:
At the risk of being sigged...

Joe, do you have a photo of your tool?



http://www.turbominis.co.uk/forums/index.p...9064&lastpost=1

http://s95.photobucket.com/albums/l154/min...2006/?start=all



danllew

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Member #: 11468
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Ok that all makes sense.

So based on calculations, if I want to keep the standard head volume.

Using the 18cc pistons I'll need to deck the block to achieve around 10 thou deck/piston clearance.

That'll give around 8.3:1.

Cheers for the input guys.


TurboDave16V
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SouthPark, Colorado

Good points bought up here, but I can't help wonder if getting the pistons flush with the deck is good (as we all know is considered "optimal"), is having them proud - say half the gasket thickness - better? Basically, can you have too thin of a squish thickness? Maybe these 18cc pistons with their very small radial width of squish band (about 3-4mm) would benfit more than a piston with a wider land?

On 17th Nov, 2014 Tom Fenton said:
Sorry to say My Herpes are no better


Who remembers this first time around: https://www.youtube.com/watch?v=JF0a5aoEGsw


Barrieri

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So then with a standard stroke and standard deck, how much will the 18cc forged omegas sit below the deck ?


Joe C

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Carlos Fandango

Burnham-on-Crouch, Essex

ummmm a fair bit, pretty sure I had 50 thou taken off my block, but that could have been the build before.

On 28th Aug, 2011 Kean said:
At the risk of being sigged...

Joe, do you have a photo of your tool?



http://www.turbominis.co.uk/forums/index.p...9064&lastpost=1

http://s95.photobucket.com/albums/l154/min...2006/?start=all



Barrieri

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50thou ?? Thats a lot !!


Sprocket

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Good squish also promotes 'Quench'. Basically, the space is so narrow the charge is difficult to ignite due to the boundary layers of the piston and head. Quench is good since detonation tends to start around the squish band. Charge in this area of the cylinder is usually known as the 'end gas' (the charge that tends to burn towards the end of the burn initiated by the spark). If the charge wont burn in this area, the cylinder less detonation sensitive. This also leads on to when detonation occurs. Detonation is reasonably predictable in most cases in that it occurs somewhere around 14 degrees after TDC. Basically the piston is starting its travel down the bore and this 'squish' or 'quench' area is getting taller, if it is tall enough by the mechanical make up of the fitted parts, that allows the charge in this area of the cylinder to burn, detonation can occur more easily. This is not to say that it prevents detonation, just that it makes the cylinder less detonation sensitive.

Is that a bit to complicated for this topic? *oh well*

On 26th Oct, 2004 TurboDave16v said:
Is it A-Series only? I think it should be...
So when some joey comes on here about how his 16v turbo vauxhall is great compared to ours, he can be given the 'bird'...


On 26th Oct, 2004 Tom Fenton said:
Yep I agree with TD........


Barrieri

251 Posts
Member #: 11231
Senior Member

Good info sprocket. There is a golden book on this subject really. It's called "Internal Combustion Engine Fundanentals"


Barrieri

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Senior Member

Now thatyou started the subject I'm a bit wooried as I didn't know about this problem that the 18cc sit do low below the deck. I've handed the block over to the machinist this week. So I guess when I'll get it back I will check the piston to deck clearance and off it goes again to have a little bit chopped off the deck. I'm aiming for 9:1 but with the 50thou Joe mentioned, that'll get me down to around 8:1.


Sprocket

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On 14th Sep, 2017 Barrieri said:
Now thatyou started the subject I'm a bit wooried as I didn't know about this problem that the 18cc sit do low below the deck. I've handed the block over to the machinist this week. So I guess when I'll get it back I will check the piston to deck clearance and off it goes again to have a little bit chopped off the deck. I'm aiming for 9:1 but with the 50thou Joe mentioned, that'll get me down to around 8:1.


Surely you mean 5 thou or 0.005"?

On 26th Oct, 2004 TurboDave16v said:
Is it A-Series only? I think it should be...
So when some joey comes on here about how his 16v turbo vauxhall is great compared to ours, he can be given the 'bird'...


On 26th Oct, 2004 Tom Fenton said:
Yep I agree with TD........


Barrieri

251 Posts
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Is it 5 Thou or 50 Thou Joe ?


TurboDave16V
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SouthPark, Colorado

I suspect it's 50 thou - stock na Pistons are about 30 thou down iirc, and I recall hearing these big cc pistons are a way down.

On 17th Nov, 2014 Tom Fenton said:
Sorry to say My Herpes are no better


Who remembers this first time around: https://www.youtube.com/watch?v=JF0a5aoEGsw


Nick
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On 14th Sep, 2017 danllew said:


I've had a look but has anyone got any details of how much meat can be taken out of either the cast omegas or 21253's?



I've seen the 21253's out at 14.7cc before, I ran them at 11cc

On 20th Oct, 2015 Tom Fenton said:

Well here is the news, you are not welcome here, FUCK OFF.

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