19 April 2006, 05:26
Lukas,
How much SCR you can run with is partly dependent on how much boost you want to run.
Yes, the old school of thought was that to run 10 PSI of boost, you needed to have the SCR below 8:1. That was how our B21ETs and B21FTs were set up: low SCR and some moderate boost levels.
But things have changed. Look at the B230FTs at 8.7:1; and at the B230ETs with 9.7:1 SCR. Part of what made those higher SCRs possible was the use of the knock sensors, and computerized ignition timing control.
And there is something else that can also help out a lot to reduce the possibility of detonation: tight squish.
If you will have your machine shop surface the block deck so as to end up with the piston crowns topping out above the deck about 0.017in to 0.018in, then when you torque the Elring headgasket down, the pistons will come within 0.030in [0,75mm] of the cylinder head.
How does that work? The Elring headgasket for either a B21 or a B230 has a compressed thickness of 1,2mm [0.047in]. Take that 0.047in, and subtract the height of the piston crown above the block...0.017in....and the piston to cylinder head clearance is 0.030in [0,75mm].
If you will set up your motor to have that tight squish clearance of 0,75mm, then you can run a higher SCR and more boost than you could if you do not go with tight squish.
To answer your question directly: yes, running a lower SCR [such as less than 8.5:1] is NOT as efficient as would be running with a moderate SCR [~9.0:1 to 9.5:1]. The higher the SCR used, the better the off-boost [NA] performance.
And it isn't actually a matter of if you run with 10:1 SCR, then your turbo can't or won't work as well. It is more a matter of that with a higher SCR [such as 10:1], then the turbo may not be able to make as much boost before you run into pinging problems.
And that gets into the three types of compression ratio that you have to be concerned with in a boost motor:
...the static compression ratio; which is determined by the volume of the cylinder at TDC versus the volume of the cylinder at BDC [bottom dead center].
...the dynamic compression ratio; which is controlled by the camshaft...WHEN the intake valve closes. DCR [dynamic compression ratio] is always lower than static compression ratio.
...the effective compression ratio; which is a sum of the SCR and the boost level...how much air you are actully forcing into the cylinder, and then compressing.
The ECR [effective compression ratio] is limited by the octane rating of the fuel. Higher octane fuel will withstand higher pressures [and temperatures] before it auto-ignites [detonates]. The higher the octane, the higher the ECR you can run before you run into detonation [spark knock, or pinging].
How you reach the ECR limits of your fuel is what you are deciding when you choose a SCR and a Boost level.
In the old days, people ran low SCR so that they could run more boost before they hit the octane limit [ECR limit] of their fuel.
With better spark timing controls, and with knock sensors and computers to retard timing when detonation is detected, people can run more SCR and more boost before they run into that solid brick wall that is the octane limit of the fuel. That octane limit is REAL. Every grade of gasoline has its limits as to how much pressure and temperature it can take before it self-ignites and explodes.
But there are ways to 'cheat'. If you can find ways to cool things down inside the combustion chamber, then you can run more pressure before you hit the wall. Think of intercoolers. Think of water injection.
And there is another way to 'cheat': known as speeding up the burn so that the fuel does not have time to get super hot and super compressed and then detonate. That is called "fastburn". And one of the ways to achieve a faster burn is to increase the turbulence inside the combustion chamber. Which is one of the things that tight squish does.
*****
...that is the short version of the answer to your question. I could write a hundred pages on the subject. [I know this: I have written over 100 pages on the subject]. And there is a lot of information to be found and read on the subjects of tight squish, quench, fast burn, or fastburn.
If you utilize the capabilities that tight squish can give you, then you can run a higher SCR, and a higher boost level, before you hit the octane limit wall.
I describe it this way: tight squish is an octane booster that you only have to pay for once: when you build your motor.
Thomas Fritz
...the stealth FTi
PS: Which motor did you decide to go with?
How much SCR you can run with is partly dependent on how much boost you want to run.
Yes, the old school of thought was that to run 10 PSI of boost, you needed to have the SCR below 8:1. That was how our B21ETs and B21FTs were set up: low SCR and some moderate boost levels.
But things have changed. Look at the B230FTs at 8.7:1; and at the B230ETs with 9.7:1 SCR. Part of what made those higher SCRs possible was the use of the knock sensors, and computerized ignition timing control.
And there is something else that can also help out a lot to reduce the possibility of detonation: tight squish.
If you will have your machine shop surface the block deck so as to end up with the piston crowns topping out above the deck about 0.017in to 0.018in, then when you torque the Elring headgasket down, the pistons will come within 0.030in [0,75mm] of the cylinder head.
How does that work? The Elring headgasket for either a B21 or a B230 has a compressed thickness of 1,2mm [0.047in]. Take that 0.047in, and subtract the height of the piston crown above the block...0.017in....and the piston to cylinder head clearance is 0.030in [0,75mm].
If you will set up your motor to have that tight squish clearance of 0,75mm, then you can run a higher SCR and more boost than you could if you do not go with tight squish.
To answer your question directly: yes, running a lower SCR [such as less than 8.5:1] is NOT as efficient as would be running with a moderate SCR [~9.0:1 to 9.5:1]. The higher the SCR used, the better the off-boost [NA] performance.
And it isn't actually a matter of if you run with 10:1 SCR, then your turbo can't or won't work as well. It is more a matter of that with a higher SCR [such as 10:1], then the turbo may not be able to make as much boost before you run into pinging problems.
And that gets into the three types of compression ratio that you have to be concerned with in a boost motor:
...the static compression ratio; which is determined by the volume of the cylinder at TDC versus the volume of the cylinder at BDC [bottom dead center].
...the dynamic compression ratio; which is controlled by the camshaft...WHEN the intake valve closes. DCR [dynamic compression ratio] is always lower than static compression ratio.
...the effective compression ratio; which is a sum of the SCR and the boost level...how much air you are actully forcing into the cylinder, and then compressing.
The ECR [effective compression ratio] is limited by the octane rating of the fuel. Higher octane fuel will withstand higher pressures [and temperatures] before it auto-ignites [detonates]. The higher the octane, the higher the ECR you can run before you run into detonation [spark knock, or pinging].
How you reach the ECR limits of your fuel is what you are deciding when you choose a SCR and a Boost level.
In the old days, people ran low SCR so that they could run more boost before they hit the octane limit [ECR limit] of their fuel.
With better spark timing controls, and with knock sensors and computers to retard timing when detonation is detected, people can run more SCR and more boost before they run into that solid brick wall that is the octane limit of the fuel. That octane limit is REAL. Every grade of gasoline has its limits as to how much pressure and temperature it can take before it self-ignites and explodes.
But there are ways to 'cheat'. If you can find ways to cool things down inside the combustion chamber, then you can run more pressure before you hit the wall. Think of intercoolers. Think of water injection.
And there is another way to 'cheat': known as speeding up the burn so that the fuel does not have time to get super hot and super compressed and then detonate. That is called "fastburn". And one of the ways to achieve a faster burn is to increase the turbulence inside the combustion chamber. Which is one of the things that tight squish does.
*****
...that is the short version of the answer to your question. I could write a hundred pages on the subject. [I know this: I have written over 100 pages on the subject]. And there is a lot of information to be found and read on the subjects of tight squish, quench, fast burn, or fastburn.
If you utilize the capabilities that tight squish can give you, then you can run a higher SCR, and a higher boost level, before you hit the octane limit wall.
I describe it this way: tight squish is an octane booster that you only have to pay for once: when you build your motor.
Thomas Fritz
...the stealth FTi
PS: Which motor did you decide to go with?