Compression Ratio With Pump Fuel

[bruce]

Would need to know the head volume to work it out. Stock is around 63cc. Did your machinist measure it?

You could measure it with a clear sheet of plastic (like a CD cover with a hole in it) to cover over the chamber, and a pipette or syringe to introduce metho or turps etc., like here:

 

http://www.rollaclub...attach_id=87204

 

Then whatever milliliters fits into the chamber gives the cubic centimeters.

 

There is a program that takes the piston ring height into account, but you could do a quick calculation with a pen and paper and it should be fine. Or here are some online calculators

 

http://www.csgnetwor...m/compcalc.html

 

http://www.rbracing-rsr.com/compstaticcalc.html

Edited July 6, 2012 by bruce

[agius]

yea the head people said they measured it and got 63cc after they shaved it

[bruce]

OK, even shaved down close to the erg holes. Mine was shaved 1mm and it still measured 63cc as well. Measured it a few times. Also measured an unshaven head as well and it was the same. That was without the clear plastic sheet though, so thought I couldn't measure the difference. Maybe because the chamber is a spherical shape it needs alot shaved off to notice the reduction in volume.

 

So perhaps the head gasket, piston dome and oversized pistons would be more effective in increasing the compression ratio.

Using this calculator:

 

http://www.csgnetwork.com/compcalc.html

 

with these values:

 

Bore: 85mm

Stroke: 78mm

gasket thickness:1.5mm

dome: 20.6cc (from Wiseco catalog)

 

gives a compression of 9.69

 

with 1mm

 

gives a compression of 10.2

 

with a 30 thou gasket (0.762mm)

 

gives a compression of 10.47

 

What piston dome volume did you use in the calculations? That needs to be known accurately. Could imprint the piston dome on some clay or plasticine and measure the volume, similar to the cylinder head chamber volume measurement.

[agius]

Its making alot more sence now,

 

i never made any calculations till now, but i belive when the head place made the calculations, piston dome value was worked out as 20.0cc

 

Head gasket was worked out to 1.0mm and dome as 63.0cc

 

Thus giving me the rough number as 10.1 static compression

 

Guess they got lazy and decided to work it out in whole numbers

 

Unluss there programs do so..

 

4 posts ago if i read what I'm writing now i wouldnt know what I'm on about, ive learnt so much it just came all together

 

The edges of the cumbustion chambers of the head must be almost dead streight for atleast over 40thou to shave 1mm off and have the exact same cc volume as completly stock

 

Of corse the cc is going to decrease, but not in way if the edges where curved dramaticly to the sphere right from the very start..

 

Ive honed my bores coz they were still in great condition and ive already purchased standard size rings

 

Thats why ive never botherd about high comp pistons...

 

I think the best option is just chuk the motor all together put up with the low compression meanwhile shave a 2t head to the right spec.. but hopefully the motor runs in correctly with the low comp ratio hrmm..

[agius]

i don't want to bother about the thin head gasket with the 3t head

 

i rather save it for the 2t head so i can save that extra 1mm on the 2t head just incase god forbid it ever overheats or i ever need to reco it or something ya know

[philbey]

http://cochise.uia.net/pkelley2/DynamicCR.html

 

This is the best dynamic comp ratio calculator I found. It's very handy. I ended up with nearly 12:1 by accident on my 5k and I got away with it running a big ass cam, premium only.

 

If I run stock fuel, as I had to once, it's not good at all.

[agius]

http://cochise.uia.n.../DynamicCR.html

 

This is the best dynamic comp ratio calculator I found. It's very handy. I ended up with nearly 12:1 by accident on my 5k and I got away with it running a big ass cam, premium only.

 

If I run stock fuel, as I had to once, it's not good at all.

 

thanks for that philbey

[agius]

so i got the head back, ive taken a few photos

 

there is hardly any meat left between the valve and the edge of cumbustion chamber

 

 

EGR holes i was talking about shaved almost to the thread

 

 

 

[bruce]

Looks like the compressed head gasket thickness is 1.3mm, probably a permaseal one?

 

Then the effective dome volume: dome volume - piston reliefs = 30-8 = 22cc

Head has been shaved 30 thou (0.762mm).

Looks like they've taken the piston ring height into account in the compression calculation.

Edited July 11, 2012 by bruce