Big Issues with Cylinder Pressure (DCR)

[digger]

F-Bird 88, are you stating that an aluminum block expands enough at operating temperatures to significantly change it's detonation characteristics?

It changes the CR. This is just 1 of the differences between a aluminum block and a cast iron block.

how much does it change the compression ratio?

peanuts

[Tom Walker]

I am not saying that the expansion of the aluminum block does not have some affect on compression ratio, but I am saying, all else being equal, that the affect is inconsequential as far as detonation is concerned.

[digger]

I am not saying that the expansion of the aluminum block does not have some affect on compression ratio, but I am saying, all else being equal, that the affect is inconsequential as far as detonation is concerned.

if the bore expands more and the total chamber size stays the same relatively speaking will the CR go up or down..........

[Tom Walker]

Up or down, I don't think it matters because, it probably has a .0000000123456789 per cent affect. We need David R. to do the math on this.

[F-BIRD'88]

F-Bird 88, are you stating that an aluminum block expands enough at operating temperatures to significantly change it's detonation characteristics?

It changes the CR. This is just 1 of the differences between a aluminum block and a cast iron block.

how much does it change the compression ratio?

Enough to matter.

[David Redszus]

Up or down, I don't think it matters because, it probably has a .0000000123456789 per cent affect. We need David R. to do the math on this.

Tricky math dept eh?

Assume 100.0000mm (3.937) bore and a SCR of 10.0 at 70F.

At 250F an iron block would become 100.1067 mm (3.9412) with a new SCR of 10.02

At 250F an aluminum block would become 100.1867mm (3.9440) with a new SCR of 10.04

For iron that is a growth of 0.004", and for aluminum it is 0.007".

Thermal block expansion does not impact compression substantially but does have an effect on clearances.

[Geoff2]

And while we are talking about miniscule amounts, the top of the piston will be higher in an iron block at operating temperature, raising CR a smidge.......

[digger]

Up or down, I don't think it matters because, it probably has a .0000000123456789 per cent affect. We need David R. to do the math on this.

Tricky math dept eh?

Assume 100.0000mm (3.937) bore and a SCR of 10.0 at 70F.

At 250F an iron block would become 100.1067 mm (3.9412) with a new SCR of 10.02

At 250F an aluminum block would become 100.1867mm (3.9440) with a new SCR of 10.04

For iron that is a growth of 0.004", and for aluminum it is 0.007".

Thermal block expansion does not impact compression substantially but does have an effect on clearances.

who would have thought the aluminium would result in higher compression i thought higher static CR would give less detonation resistance @ FF

[F-BIRD'88]

When the block heats up it grows taller. The compression the ratio drops because the block grows taller more than the rotating assembly does (aluminum vs iron)
Same as the net valve lash grows larger as a aluminum block heats up on a pushrod engine.. .008" to .012" is typical on a aluminum block V8.. The CR DROPS BY ABOUT .1/4 to 1/3 RATIO.. When running at WOT VS not running at 70°F

[mt-engines]

When the block heats up it grows taller. The compression the ratio drops because the block grows taller more than the rotating assembly does (aluminum vs iron)
Same as the net valve lash grows larger as a aluminum block heats up on a pushrod engine.. .008" to .012" is typical on a aluminum block V8.. The CR DROPS BY ABOUT .1/3 RATIO.. When running at WOT VS not running at 70°F

WTF CARES

[David Redszus]

It is wise to remember that engines don't care about ratios. They only respond to actual physical effects like
pressures, temperatures, vibrations, etc.

And they don't make horsepower either. They only make torque; we calculate the horsepower for comparison sake.

[David Redszus]

When the block heats up it grows taller. The compression the ratio drops because the block grows taller more than the rotating assembly does (aluminum vs iron)
Same as the net valve lash grows larger as a aluminum block heats up on a pushrod engine.. .008" to .012" is typical on a aluminum block V8.. The CR DROPS BY ABOUT .1/4 to 1/3 RATIO.. When running at WOT VS not running at 70°F

Yes, thermal expansion does cause the block height to grow. But the water quickly become a bit muddy.

Consider a block height of 150.0 mm (5.9055") at 250F.
An iron block would grow to 5.9118" a change of 0.006".
An aluminum block would grow to 5.9282" a change of 0.023"

But the rod and piston would also grow by some small amount which must be subtracted from block height.
Compression would be reduced by the net growth mount.
And, as earlier noted, the area would also grow increasing compression.

What is the net result on compression? While I have not calculated it (lack of realistic data) it is probably negligible.

But just for shits and grins:
Linear expansion
Linear % change = L * C * dTc

Area expansion
Area % change = A * 2C * dTc

Volume expansion
Volume % change = V * 3C * dTc

dTc = change in temperature in Centigrade

[Tom Walker]

Yes David, for me, the math is tricky, but this forum has so many different people with so many talents and abilities. I was being half facetious, but I was secretly hoping for you to display your abilities with the numbers, the preciseness that the math brings to the conversations on this site is so informative and brings a clarity of thought that I appreciate.
Well done and thanks for the math, numbers speak so clear and precise.

[gmrocket]

When the block heats up it grows taller. The compression the ratio drops because the block grows taller more than the rotating assembly does (aluminum vs iron)
Same as the net valve lash grows larger as a aluminum block heats up on a pushrod engine.. .008" to .012" is typical on a aluminum block V8.. The CR DROPS BY ABOUT .1/3 RATIO.. When running at WOT VS not running at 70°F

WTF CARES

I wonder why in his block growth scenario he doesn’t mention the crank , rod or piston growth at running temp?

Or the stretch at max rpm ?

[F-BIRD'88]

The crank rod piston is included in the net difference.
Rotating assembly
Stretch caused by rpm is not. Its the same on both motors.
Aluminum block head engines are typically built with 1 whole CR increase to make same power as a identical cast iron engine does. The difference in net all in block growth is just 1 factor in that.
Do not equate the practical compression ratio is of say a all aluminum LS3 vette engine to what is a practical compression ratio on a cast block gen 1 sbc engine.
Cast block -aluminum block different beasts.
Keep it relevent to this cast block gen 1 sbc engine.