Please give me your advice!
#21
10-05-2006, 12:14 AM
RE: Please give me your advice!
ORIGINAL: Lee Willis
No doubt people can figure I I love this stuff: I used to design these things for a living.
I view it a bit differently than the cylinder only handling so much pressure. The basic LS engines can take only so much cylinder pressure, but a rebuilt one can take much more, yet there are still limits to a combination of high compression and high boost.
When you get down to it, an amount of power, say 650 HP at 5000 RPM, will take the same amount of cylinder pressure upon combustion regardless of how the power is made: turbo on non-turbo, high or low compression, etc. Once rebuilt, the LS engine can take boost up to about 28 lbs at 8:1, or 1300 HP.
The real issue with high compression and high boost has to do with the speed of combustion inside the cylinder and the pre-disposition of the fuel-air mixture in the cylinder to pre-ignite (i.e., explode when it wants to, not when you want it to). When things work like they should, a lot of air-fuel mixture makes its way into the cylinder and then explodes when the spark tells it to. The explosion may look and feel instantaneous to the naked eye, but it actually takes a split second, about 1/3000 of a second, to move outward in a wave of flame from the spark plug down through the cylinder. As it do4es it produces a steady pressure, for the entire power cycle, on the top of the piston: torque. And the more air and fuel you can cram into the cylinder, the bigger the bang and the more pressure and the more torque and the more HP.
But air-fuel can misbehave. If you boost too much air into a cylinder (high boost) and then "squish" it too fast (high compression), it does not explode outward from the spark plug in 1/3000oth second, but instead all of it explodes simultaneously througout the cylinder in 1/50000th second. This is detonation, or pinging, or knock. It produces next to no power (there is no wave of steaqdy pressure and torque) and will, sometimes, blow a motor apart (the sharp bang caves in the top of the piston).
You can avoid this only through any of four methods (unless you can re-design the basic engine, then you have other choices, too).
First, you can adjust the timing, firing the mixture before it explodes itself, but to do this effectively you have to advance the timing so early (before the mixture detonates, as to where you get little power: this is self defeating, and sometimes does not work at all.
Second, you can use higher octane fuel, which won't detonate until a higher temperature-pressure level. This is why people use race gas in really hi-po cars: its higher octane permits more compression ratio or more boost or both, hence more power (the gas itself produces no more power unless you do one or both).
Or, you can push less air into the cylinder (lower boost) and still have your high compression.
Or, you can lower the compression ratio and go ahead and cram more air with with the SCr high boost. If you do this last approachr, you get about half what you give up: lowering compression ratio cuts power but it permits more boost -- enough to more than make up for what you lost. So, an engine with say, 6:1 compression ratio can take gobs of boost (50 lbs) whereas one with 11:1 can take only five.
It is always a compromise. High compression means high efficincy: good economy. But it limits the boost you can get. My compromise was the following. I built my C5R with 10:1 compression ratio, giving up a bit of around town fuel economy (about 2 mpg) but with a target of one thing: at 10:1, my engine can take enough extra boost that no stock, LS7 C6, with its 11.5:1 ratio, would be able to SC enough boost and beat it in RWHP or the 1/4 mile: the LS7 stock compresison ratio does not permit enough boost. An LS7 will have to be rebuilt - rather a costly effort, to match or beat me. A rather childish goal, perhaps, but that's why I did it.
No doubt people can figure I I love this stuff: I used to design these things for a living.
I view it a bit differently than the cylinder only handling so much pressure. The basic LS engines can take only so much cylinder pressure, but a rebuilt one can take much more, yet there are still limits to a combination of high compression and high boost.
When you get down to it, an amount of power, say 650 HP at 5000 RPM, will take the same amount of cylinder pressure upon combustion regardless of how the power is made: turbo on non-turbo, high or low compression, etc. Once rebuilt, the LS engine can take boost up to about 28 lbs at 8:1, or 1300 HP.
The real issue with high compression and high boost has to do with the speed of combustion inside the cylinder and the pre-disposition of the fuel-air mixture in the cylinder to pre-ignite (i.e., explode when it wants to, not when you want it to). When things work like they should, a lot of air-fuel mixture makes its way into the cylinder and then explodes when the spark tells it to. The explosion may look and feel instantaneous to the naked eye, but it actually takes a split second, about 1/3000 of a second, to move outward in a wave of flame from the spark plug down through the cylinder. As it do4es it produces a steady pressure, for the entire power cycle, on the top of the piston: torque. And the more air and fuel you can cram into the cylinder, the bigger the bang and the more pressure and the more torque and the more HP.
But air-fuel can misbehave. If you boost too much air into a cylinder (high boost) and then "squish" it too fast (high compression), it does not explode outward from the spark plug in 1/3000oth second, but instead all of it explodes simultaneously througout the cylinder in 1/50000th second. This is detonation, or pinging, or knock. It produces next to no power (there is no wave of steaqdy pressure and torque) and will, sometimes, blow a motor apart (the sharp bang caves in the top of the piston).
You can avoid this only through any of four methods (unless you can re-design the basic engine, then you have other choices, too).
First, you can adjust the timing, firing the mixture before it explodes itself, but to do this effectively you have to advance the timing so early (before the mixture detonates, as to where you get little power: this is self defeating, and sometimes does not work at all.
Second, you can use higher octane fuel, which won't detonate until a higher temperature-pressure level. This is why people use race gas in really hi-po cars: its higher octane permits more compression ratio or more boost or both, hence more power (the gas itself produces no more power unless you do one or both).
Or, you can push less air into the cylinder (lower boost) and still have your high compression.
Or, you can lower the compression ratio and go ahead and cram more air with with the SCr high boost. If you do this last approachr, you get about half what you give up: lowering compression ratio cuts power but it permits more boost -- enough to more than make up for what you lost. So, an engine with say, 6:1 compression ratio can take gobs of boost (50 lbs) whereas one with 11:1 can take only five.
It is always a compromise. High compression means high efficincy: good economy. But it limits the boost you can get. My compromise was the following. I built my C5R with 10:1 compression ratio, giving up a bit of around town fuel economy (about 2 mpg) but with a target of one thing: at 10:1, my engine can take enough extra boost that no stock, LS7 C6, with its 11.5:1 ratio, would be able to SC enough boost and beat it in RWHP or the 1/4 mile: the LS7 stock compresison ratio does not permit enough boost. An LS7 will have to be rebuilt - rather a costly effort, to match or beat me. A rather childish goal, perhaps, but that's why I did it.
#22
10-05-2006, 01:26 AM
RE: Please give me your advice!
Hey, thanks Lee!!!
Those are the lines I was thinkin' along, but wasn't sure.
So....... with 8 lbs. of boost, and 9.5:1 comp, I'm lookin' at right around a 200 horse gain. No need for twin turbos, right? I'd just pay twice the money for the same results, and only runnin' 4 lbs. of boost off each turbo, or 8 lbs. to each bank with a split intake manifold. ( I can see where this would be a nightmare to tune)
Am I on the right page here?
Those are the lines I was thinkin' along, but wasn't sure.
So....... with 8 lbs. of boost, and 9.5:1 comp, I'm lookin' at right around a 200 horse gain. No need for twin turbos, right? I'd just pay twice the money for the same results, and only runnin' 4 lbs. of boost off each turbo, or 8 lbs. to each bank with a split intake manifold. ( I can see where this would be a nightmare to tune)
Am I on the right page here?
#23
10-05-2006, 01:52 AM
RE: Please give me your advice!
Very true, the final compression ratio of the mixture plays a role in the power output, but it goes much further than that. You will always be able to get more power out of a turbocharged low-compression engine compared to a turbocharged high-compression engine. The final compression ratio is not why correctly turbocharged engines can make extremely huge hp increases with only relatively small increase in peak chamber pressure. A simplified explanation: When the mixture combusts in a high-compression NA engine, it makes a quick big push on the crank. When a turbocharged, low-compression engine combusts, the explosion is slower and it pushes longer on the crank. A turbocharged engine can generate 4 times the pressure on the crank after 50 degrees of crankshaft rotation. This longer period of high-compression is what is making the huge power increases.
Think of it this way (again simplified for the sake of discussion), we will say there is a static limit to the amount of pressure a cylinder can hold (be that because the head will lift, or the rings will let the mixture by, etc.). A high-compression engine will only be able to take in say 10 units of air/fuel because it will compress that say 4 times to arrive at the same final pressure that a low compression engine takes in 20 units of air/fuel compressed 2 times. They both have the exact same final pressure now, but one of them has twice the amount of air/fuel in the cylinder. Which is going to make more power here? Obviously the low compression engine with the larger amount of fuel/air.
Also heat plays a factor in the combustion process. As you described, pre-detonation occurs when the heat generated by compression reaches the heat needed for combustion (again not this cut and dry, but generally speaking this is the case). Not only do people use high-octane race fuel to combat this, but water injection is also used. Another way to reduce the amount of heat in the intake charge is to use a larger turbo. The smaller turbo's compressor wheel generates a lot more heat to compress the air to say 20psi than does a larger turbo with a larger compressor wheel. This is because there is a limit to efficiency of the turbocharger itself. At a certain point to increase the boost it takes a huge magnitude increase in energy to do so. It's not uncommon to use very large turbochargers and spray to get them spooled.
Compressing the air outside the combustion chamber (especially when using an intercooler) also reduces the chance of pre-detonation as the heat caused by compression is dissipated outside the combustion chamber. This also gives you more oxygen per volume unit. So for the same amount of pressure, you not only have more air, you have more oxygen.
You must also watch out for running too lean a mixture when using forced induction, while a leaner mixture can help spool the turbo early on, you can easily melt a head gasket, or piston at higher boost.
Reactor2.
Think of it this way (again simplified for the sake of discussion), we will say there is a static limit to the amount of pressure a cylinder can hold (be that because the head will lift, or the rings will let the mixture by, etc.). A high-compression engine will only be able to take in say 10 units of air/fuel because it will compress that say 4 times to arrive at the same final pressure that a low compression engine takes in 20 units of air/fuel compressed 2 times. They both have the exact same final pressure now, but one of them has twice the amount of air/fuel in the cylinder. Which is going to make more power here? Obviously the low compression engine with the larger amount of fuel/air.
Also heat plays a factor in the combustion process. As you described, pre-detonation occurs when the heat generated by compression reaches the heat needed for combustion (again not this cut and dry, but generally speaking this is the case). Not only do people use high-octane race fuel to combat this, but water injection is also used. Another way to reduce the amount of heat in the intake charge is to use a larger turbo. The smaller turbo's compressor wheel generates a lot more heat to compress the air to say 20psi than does a larger turbo with a larger compressor wheel. This is because there is a limit to efficiency of the turbocharger itself. At a certain point to increase the boost it takes a huge magnitude increase in energy to do so. It's not uncommon to use very large turbochargers and spray to get them spooled.
Compressing the air outside the combustion chamber (especially when using an intercooler) also reduces the chance of pre-detonation as the heat caused by compression is dissipated outside the combustion chamber. This also gives you more oxygen per volume unit. So for the same amount of pressure, you not only have more air, you have more oxygen.
You must also watch out for running too lean a mixture when using forced induction, while a leaner mixture can help spool the turbo early on, you can easily melt a head gasket, or piston at higher boost.
Reactor2.
#24
10-05-2006, 01:58 AM
RE: Please give me your advice!
So....... with 8 lbs. of boost, and 9.5:1 comp, I'm lookin' at right around a 200 horse gain. No need for twin turbos, right? I'd just pay twice the money for the same results, and only runnin' 4 lbs. of boost off each turbo, or 8 lbs. to each bank with a split intake manifold. ( I can see where this would be a nightmare to tune)
Am I on the right page here?
Am I on the right page here?
#26
10-05-2006, 03:19 AM
RE: Please give me your advice!
This thread is a keeper for sure. Buying a C3 with 454/4speed. lots of good stuff here. "build your own beast" great idea. Can't wait to get it. I got the fever now and there ain't no antidote but raw HP.
#27
10-05-2006, 08:47 PM
RE: Please give me your advice!
The major advantage two small turbos have over one large one is resopnse time -- the two can spool up to boost faster. One is simpler and easier, but will have more lag before coming on boost. If I only wanted a bit of boost I would try a SCr instead (less hassle) but if you have carbs on the engine you need a "Pressure box" approach.
By the way, a point about all I said before: aluminum heads are worth about 1 full point of compression ratio. An aluminum block doesn't have any affect, only what the heads are made of. Aluminum conducts heat so much more effectively that the coimbustion chambers run cooler and the air fuel mixture can be denser. If you are going to use iron heads on that 454, Starship, you need to subtract one full point (9.5 becomes 8.5:1) from the ratios I spoke about above. Your 9.5:1 would be good for somewhere around 5.5-6 lbs max, which would give you, I think about 200 extra HP (you'd get a bit more than 30 HP per lb boost on a displacement that large, I'd hope).
By the way, a point about all I said before: aluminum heads are worth about 1 full point of compression ratio. An aluminum block doesn't have any affect, only what the heads are made of. Aluminum conducts heat so much more effectively that the coimbustion chambers run cooler and the air fuel mixture can be denser. If you are going to use iron heads on that 454, Starship, you need to subtract one full point (9.5 becomes 8.5:1) from the ratios I spoke about above. Your 9.5:1 would be good for somewhere around 5.5-6 lbs max, which would give you, I think about 200 extra HP (you'd get a bit more than 30 HP per lb boost on a displacement that large, I'd hope).
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