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Maths time, Compression, Fuels.....
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Author:  jbeenz [ Fri Oct 27, 2006 4:39 pm ]
Post subject:  Maths time, Compression, Fuels.....
Ok,so calculating my static compression and im a little worried, a few questions, firstly am i calculating this right?
so basicly swept volume / combustion volume
my engine is/will be a 1220, with a 940 head, flat top pistons, and they sit flush in the bores
so volume per cyl = 305cc
chamber volume = 21.5cc
allow 3cc for head gasket?? :?
and 2cc for small valve 'recesses' in pistons
and 2cc for valve pockets.

Total swept volume = 305cc
Total combustion volume = 28.5cc
=10.70 : 1
does that sound roughly right?

cheers
Author:  Blokeinamoke [ Fri Oct 27, 2006 4:49 pm ]
Post subject:  Re: Maths time, Compression, Fuels.....
jbeenz wrote:
Ok,so calculating my static compression and im a little worried, a few questions, firstly am i calculating this right?
so basicly swept volume / combustion volume
my engine is/will be a 1220, with a 940 head, flat top pistons, and they sit flush in the bores
so volume per cyl = 305cc
chamber volume = 21.5cc
allow 3cc for head gasket?? :?
and 2cc for small valve 'recesses' in pistons
and 2cc for valve pockets.

Total swept volume = 305cc
Total combustion volume = 28.5cc
=10.70 : 1
does that sound roughly right?

cheers


Image

b = cylinder bore (diameter)

s = piston stroke length

Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.
Author:  jbeenz [ Fri Oct 27, 2006 5:06 pm ]
Post subject: 
Cheers, that makes more like 11.7:1 assuming that Vc is arround 28.5cc
that will do nicley :D
Author:  Lillee [ Fri Oct 27, 2006 7:31 pm ]
Post subject: 
Can I ask how you know that your chamber is 21.5cc?
Author:  jbeenz [ Fri Oct 27, 2006 7:35 pm ]
Post subject: 
Lillee wrote:
Can I ask how you know that your chamber is 21.5cc?

It could be a little smaller, stock 12G940 is 21.4cc, all the other volumes are estimated though.
Author:  drmini in aust [ Fri Oct 27, 2006 10:21 pm ]
Post subject: 
With Imp pistons back in 2003 I allowed 1cc total for the 2 piston cutouts, and 0.6cc for upper ring land volume.
You will need to use a 1275 gasket. BK450 one is about 3.5cc I think.

11.7:1 is pretty high, what cam and fuel are you using? :P
Author:  jbeenz [ Sat Oct 28, 2006 4:56 am ]
Post subject: 
drmini in aust wrote:
With Imp pistons back in 2003 I allowed 1cc total for the 2 piston cutouts, and 0.6cc for upper ring land volume.
You will need to use a 1275 gasket. BK450 one is about 3.5cc I think.

11.7:1 is pretty high, what cam and fuel are you using? :P


yea, so 28.5 is arround right, accurate enough to know that its high.
im guessing itll be running 98 all the time, do you think ill need somthing stronger :D ?

I was thinking about using a stock a-plus cam, but thats not going to be anywhere near enough, ive still got my 8m73 mistery cam, which i assume to be fairly lumpy as it came from a pre65 motor...(pre65.org.nz).i should just get it measured!
This is why the engine isint together yet, deciding on cam.

Can the cam bearings be changed with the crank in?, as i havent replaced them, they are good, but if i buy a NEW cam, ill want to change them, and the crank is locktighted in :)
Author:  drmini in aust [ Sat Oct 28, 2006 7:40 am ]
Post subject: 
I reckon for 98 it's WAY too high with that cam. It'll ping its head off.
10:1 would be near the money.

But with a 270* cam you can get away with 10.6, with a 286 around 11:1 or a little more.

I'd get thee an RE13 or similar... :wink:

<edit> re the cam bearings- if good leave them. They hardly wear unless grit goes through the motor.
Bearings can't be changed in car, but cam can (small bore or Cooper S).
Author:  jbeenz [ Sat Oct 28, 2006 3:11 pm ]
Post subject: 
drmini in aust wrote:
I reckon for 98 it's WAY too high with that cam. It'll ping its head off.


The stock one that is right?
ill try get somthing arround the 286 mark(or an re 13)ock knock, or get the chamberes oppened up a little :P
whats GR's new cam like?
thanks for the help, id like use it as av gas only but the costs add up :lol:
Author:  slinkey inc [ Sat Oct 28, 2006 4:31 pm ]
Post subject: 
FOr racing Avgas isn't going to be that expensive really. At OG road speed shop (THE speed shop in adelaide) it was about $1.42 a litre, that was a few months ago but on that day unleaded was at about $1.32, I remember being amazed that Avgas was at the same price as 98 Vortex. So who knows?.... Avgas probably doesn't fluctuate in price nearly as much as commercial petrol??
Author:  drmini in aust [ Sat Oct 28, 2006 4:59 pm ]
Post subject: 
slinkey inc wrote:
FOr racing Avgas isn't going to be that expensive really. At OG road speed shop (THE speed shop in adelaide) it was about $1.42 a litre, that was a few months ago but on that day unleaded was at about $1.32, I remember being amazed that Avgas was at the same price as 98 Vortex. So who knows?.... Avgas probably doesn't fluctuate in price nearly as much as commercial petrol??

If they catch you running Avgas on the street in NSW it'll cost you $5K.
Probably the same penalty in other states of Oz, so watch it.
In UnZud, I have no idea... :wink:
Author:  slinkey inc [ Sat Oct 28, 2006 5:08 pm ]
Post subject: 
drmini in aust wrote:
slinkey inc wrote:
FOr racing Avgas isn't going to be that expensive really. At OG road speed shop (THE speed shop in adelaide) it was about $1.42 a litre, that was a few months ago but on that day unleaded was at about $1.32, I remember being amazed that Avgas was at the same price as 98 Vortex. So who knows?.... Avgas probably doesn't fluctuate in price nearly as much as commercial petrol??

If they catch you running Avgas on the street in NSW it'll cost you $5K.
Probably the same penalty in other states of Oz, so watch it.
In UnZud, I have no idea... :wink:


Yeah, it is illegal due to not complying with emmission standards or the like personally I reckon it's big business push that makes thing's illegal. Same as the stamping out of grey imports. Due to toyota claiming it affec ted their sales. Only with hilux's and the like...but a vehicle over 15 years old is NOT incompetition with a new one. I'm problably just synical about the government.... But yeah what is the reason for Avgas being illegal??
Author:  feralsprint [ Sat Oct 28, 2006 5:13 pm ]
Post subject: 
It still has lead in it because the planes that use it need it :D so you cannot buy it in NSW unless you have a fuel log book for a plane and only at the airport, you have not been able to buy avgas at a race circuit for some years now only 100 octane unleaded

Jon
Author:  jbeenz [ Sat Oct 28, 2006 5:18 pm ]
Post subject: 
its supposed to be a fast road motor :roll: heheh, its going to be fun!
Avgas is legal over here as far as i know, my uncle is a cop and he runs it in his XJc (s door jax xj6, pillarless 8) ) as do many members of his club as it dosent go off as fast? he said its sweet,
but thats beside the point, id like to be able to run 98 :D

Btw, if anyone knows what 8m73 is, let me know, its not an NZ grind as far as i can find out.
think ill try afford an re13 and join the club! :D
Author:  miniobsessed [ Sun Oct 29, 2006 6:06 am ]
Post subject: 
does anyone know the timing/lift etc... figures for this re13 cam that everyone keeps talking about?
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