16v vs 8v head turbo application - max static compression ?
01-19-2011, 01:29 PM
#108
Nordschleife Master
01-20-2011, 11:04 AM
#109
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First – let me just say that since I put away the train that was under the Christmas tree I can no longer wear my engineer’s hat…..
On to the ‘bench race engineering’ – things to consider:
A vast majority of the heat transfer in the block takes place in the top 2” of the cylinder. Our engines have a stroke of 78-88mm. That’s the total area inside the cylinder that is exposed to the heat of combustion. Half of that is obscured by the piston for 50% of the cycle. So the top half of the stroke does the vast majority of the heat rejection – that’s about 1 ½” to the nonmetric thinkers.
The 3.0/2.7 liter block has a much higher floor than the 2.5 block. This is great for rigidity – especially it you fill the block to cylinder cavity under the floor with the correct structural epoxy. It makes no difference to the ability to transfer heat – there is no heat source in that area.
The amount of coolant inside the block is irrelevant. In fact I would not be surprised to see that the coolant in the bottom area of the 2.5 block is fairly stagnant. There is no reason for it to flow since the coolant has to flow into the head to return back to the radiator. This is especially true for the lower area around cylinder 4. Flow rate (measured in volume) is what matters and that is determined by the pump and RPM.
Pressure can be an important design parameter. I am not talking about the simplistic system pressure (what your radiator cap sees). It the localized pressure that matters. I have seen a highly instrumented small block Chevy that was getting 50psi of coolant pressure in the block. This is done using the right restrictions in the cooling passages between the block and head. The purpose was to control boiling in the block – higher pressure + higher boiling point. The key is to match the pressure to flow rate to get the best boiling protection along with the best hear transfer rate.
Cylinder head heat transfer is another interesting issue. If you are trying to get the heat to transfer through the cylinder head you may end up with a hotter combustion chamber. If you apply a heat rejection coating to the combustion chamber you may get a cooler combustion chamber and more heat energy in the exhaust. These things need to be balanced correctly. (BTW – important to note that a lot of heat is transferred from the valve head to the valve seat – this is important to keep the valve cooled down enough so that it won’t fail. This is a strong plus for the 16v engines since the delta between valve area and valve diameter is not a linear equation).
All that being said, the 3.0/2.7 blocks (and yes, they are the same) have more than enough cooling flow. Be careful that you don’t misuse the term capacity – it can refer to volume or ability. The 3.0 block has less volume of coolant but the approximate equal cooling capacity as the 2.5 block. If you look at instrumented tests you will see that the delta between the coolant temp in vs out is about the same on a 2.5 and 3.0.
Oh yeah – there was a post about external coolant pumps. Its an interesting idea (I have a test system in the works) but the failing of the electrical pump is volume vs pressure. Externals electric pumps can move large volumes of coolant - but the curve falls off rapidly when the pump has to work against pressure. A mechanical pump driven at a decent rpm can move a metric crap load of fluid and generate very high pressures. A system would have to be optimized to work with an electrical coolant pump vs a mechanical pump. It also becomes a lot easier to play with flow direction with an outboard pump…..
Oh yeah….Toot, toot….I be an engineer….I drive a train!
On to the ‘bench race engineering’ – things to consider:
A vast majority of the heat transfer in the block takes place in the top 2” of the cylinder. Our engines have a stroke of 78-88mm. That’s the total area inside the cylinder that is exposed to the heat of combustion. Half of that is obscured by the piston for 50% of the cycle. So the top half of the stroke does the vast majority of the heat rejection – that’s about 1 ½” to the nonmetric thinkers.
The 3.0/2.7 liter block has a much higher floor than the 2.5 block. This is great for rigidity – especially it you fill the block to cylinder cavity under the floor with the correct structural epoxy. It makes no difference to the ability to transfer heat – there is no heat source in that area.
The amount of coolant inside the block is irrelevant. In fact I would not be surprised to see that the coolant in the bottom area of the 2.5 block is fairly stagnant. There is no reason for it to flow since the coolant has to flow into the head to return back to the radiator. This is especially true for the lower area around cylinder 4. Flow rate (measured in volume) is what matters and that is determined by the pump and RPM.
Pressure can be an important design parameter. I am not talking about the simplistic system pressure (what your radiator cap sees). It the localized pressure that matters. I have seen a highly instrumented small block Chevy that was getting 50psi of coolant pressure in the block. This is done using the right restrictions in the cooling passages between the block and head. The purpose was to control boiling in the block – higher pressure + higher boiling point. The key is to match the pressure to flow rate to get the best boiling protection along with the best hear transfer rate.
Cylinder head heat transfer is another interesting issue. If you are trying to get the heat to transfer through the cylinder head you may end up with a hotter combustion chamber. If you apply a heat rejection coating to the combustion chamber you may get a cooler combustion chamber and more heat energy in the exhaust. These things need to be balanced correctly. (BTW – important to note that a lot of heat is transferred from the valve head to the valve seat – this is important to keep the valve cooled down enough so that it won’t fail. This is a strong plus for the 16v engines since the delta between valve area and valve diameter is not a linear equation).
All that being said, the 3.0/2.7 blocks (and yes, they are the same) have more than enough cooling flow. Be careful that you don’t misuse the term capacity – it can refer to volume or ability. The 3.0 block has less volume of coolant but the approximate equal cooling capacity as the 2.5 block. If you look at instrumented tests you will see that the delta between the coolant temp in vs out is about the same on a 2.5 and 3.0.
Oh yeah – there was a post about external coolant pumps. Its an interesting idea (I have a test system in the works) but the failing of the electrical pump is volume vs pressure. Externals electric pumps can move large volumes of coolant - but the curve falls off rapidly when the pump has to work against pressure. A mechanical pump driven at a decent rpm can move a metric crap load of fluid and generate very high pressures. A system would have to be optimized to work with an electrical coolant pump vs a mechanical pump. It also becomes a lot easier to play with flow direction with an outboard pump…..
Oh yeah….Toot, toot….I be an engineer….I drive a train!
01-20-2011, 11:36 AM
#111
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na Corleone just had multi stem vent kit not reverse flow . and i expect his set up would have incresed coolant flowas the coolant boiling onthe back of each combustion chamber was able to escape easor
I'm going for electric reverse flow on one of mine wich I'm hoping will be a another way i can cheat convention relating to compression ratio+boost
I'm going for electric reverse flow on one of mine wich I'm hoping will be a another way i can cheat convention relating to compression ratio+boost
01-20-2011, 11:45 AM
#112
Nordschleife Master
na Corleone just had multi stem vent kit not reverse flow . and i expect his set up would have incresed coolant flowas the coolant boiling onthe back of each combustion chamber was able to escape easor
I'm going for electric reverse flow on one of mine wich I'm hoping will be a another way i can cheat convention relating to compression ratio+boost
I'm going for electric reverse flow on one of mine wich I'm hoping will be a another way i can cheat convention relating to compression ratio+boost
01-20-2011, 12:09 PM
#113
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So, if lowering static compression from 10.9:1 down to 9.5:1, how much shorter does the rod have to be (or how much thicker does the head gasket have to be?) A quick back of the envelope suggest only about 2mm or so...
01-20-2011, 02:21 PM
#114
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The rods and gasket should remain that same dimension – just get some custom pistons and do it the right way! In reality it’s not that much more expensive and you would have new pistons and rings. It’s a lot less than custom rods.
01-20-2011, 02:54 PM
#115
Rainman
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na Corleone just had multi stem vent kit not reverse flow . and i expect his set up would have incresed coolant flowas the coolant boiling onthe back of each combustion chamber was able to escape easor
I'm going for electric reverse flow on one of mine wich I'm hoping will be a another way i can cheat convention relating to compression ratio+boost
I'm going for electric reverse flow on one of mine wich I'm hoping will be a another way i can cheat convention relating to compression ratio+boost
01-20-2011, 07:34 PM
#116
Drifting
First – let me just say that since I put away the train that was under the Christmas tree I can no longer wear my engineer’s hat…..
Chris, you crack me up!
Chris, you crack me up!
01-20-2011, 11:22 PM
#117
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It doesn't sound like you've packed the train away completely yet...
So what budget would you suggest for new pistons, and what additional price for uprated stronger/lighter rods (if I was going to get on the slippery slope)?
01-21-2011, 06:18 AM
#118
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It gets complicated – if we are apliing the conversion factor to a 944 turbo engine bay you first have to divide by .5 because you have already stuffed 10lbs of crap in a 5lb bag….
I solved the DME problem a long time ago, standalones don’t need a DME relay….its a little more expensive than keeping a spare DME relay but there are a few other benefits besides the relay delete!
I solved the DME problem a long time ago, standalones don’t need a DME relay….its a little more expensive than keeping a spare DME relay but there are a few other benefits besides the relay delete!
01-22-2011, 09:47 PM
#120
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an electric pump is speed controlled according to engine temp not engine speed
the pump is not over sped and cavitating like crazy as you hit redline.
the pump is not running to slow when your stationary. idling.
.
extra pumps and messing with flow direction to try get uniform head temps looks like it will be easier.
the pump is not over sped and cavitating like crazy as you hit redline.
the pump is not running to slow when your stationary. idling.
.
extra pumps and messing with flow direction to try get uniform head temps looks like it will be easier.