The Quest for 600 bhp - Part 2 - The "Charger".
Discussion
OK, so here is the second installment in the "Quest" mini series, entitled The "Charger".
I will try and explain to both the beginner and the more knowledgable, how and why I have selected the particular Charger that I have. I have called it this because in the very beginning, I was looking at both Turbo"Chargers" and Super"Chargers".
For the beginners, a very brief description of the differences:
Turbochargers use wasted exhaust gas energy to produce boost by spinning a compressor from the exhaust gases running through the exhaust system using a turbine. Power is almost free!
Superchargers use an engine driven belt to spin a compressor, usually from the crankshaft front pulley. It takes power from the engine to spin the supercharger when producing boost, but only a small percentage of the extra horsepower it is giving.
Turbochargers can be used singly or in multiples, usually twin. Twin turbo's can be arranged in three seperate ways, parallel, staged or sequential.
Twin parallel turbo's are generally used where there is insufficient room to install a single large turbocharger. This is most convenient on the "V" engines. Both turbo's will be identical.
Staged twin turbo's, as on the new 330i and 335D use a small turbo for low rpm boost and a larger turbo that takes over at higher rpm, when the smaller turbo has reached maximum flow. This is a very good arrangement as lag is very low and yet high boost levels are not compromised. 2 different sized turbo's are used in this arrangement.
Finally, the sequential twin turbo uses a similar arrangement to the staged, except it uses one turbo to produce boost, which is then fed directly into the second turbo to produce very high boost pressure levels which are beyond the capability of the pressure ratio of just one unit. In these installs, boost pressures of 60 psi to 100 psi are typical. Not for the faint hearted!
In a single turbo install, many believe that twin turbo's produce less lag because of the inertia in the 2 is less. This is minimal because in the case of an 850 for example, each turbo is only being spun by 2.5 litres whereas a single would receive all 5.0 litres. An advancement of the single turbo such as the twin entry or VGT will surpass this lag.
A twin entry turbo uses two seperate ports to introduce the exhaust gases onto the turbine, therefore changing the force and gas volume to spin it up early without restricting it at higher flow rates, the VGT or "Variable Geometry Turbocharger" does the same thing by varying the angle the gases enter the turbine.
And no, twin turbos DO NOT produce twice the power of single turbo's, although the marketing men would have you believe so!
The supercharger can also be installed singly or in multiples. Again, twin superchargers can be smaller than a single and may be far easier to install spacewise, but will require 2 drives from the crank, although these may be possible from the same belt, generally 2 separate drives are used.
There are a number of supercharger designs, the most common being the positive displacement and the centrifugal types. Most people associate with the positive displacement type which will produce boost ON IDLE speed and has ZERO lag because it pumps air into the engine at any speed at a ratio dependent on supercharger drive ratio.
A centrifugal supercharger is almost a hybrid between a supercharger and a turbocharger and can be described basically as a belt driven turbocharger, producing no boost and little load on the engine at low rpm, but producing boost at higher rpm again dependant on supercharger drive ratio.
Examining all of the above, and considering them all, I decided against the option of the single large turbo due to mounting, relocation of underbonnet components and heat management issues, the twin turbo's involved doubling the cost of a single unit, involves two oil robbing supplies from the engine lubrication system, 2 drains into the sump and glowing red hot turbo's within millimeters of the chassis and wiring components, as well as the manufacture of a pair of exhaust manifolds only suitable for right hand drive cars and difficult maintenance issues around the turbo's.
Remember though that all high power output Japanese tuned cars invariably replace their twin turbo set-ups with a single large turbo.
As far as twin superchargers were concerned, this was only necessary if a single unit could not flow sufficiently, and as many GMC positive displacement superchargers can flow to 6000 bhp and more, I researched into what would be required for my mere 600 bhp. I found the most popular size/ flow superchargers to be the Vortec range of centrifugal superchargers. With severe space limitations in the "V" above the engine, this pretty much ruled out the use of a positive displacement type, and also whilst boost from idle is an awesome experience, there had to be SOME consideration on fuel economy, and the stress on the engine at all times, not just when required.(90% of the time he he)
Further research here in the UK showed a development being made by Turbo-Technics on a fluid drive high speed centrifugal supercharger, and yet more research showed a Danish company called Rotrex that were already producing these units. I investigated further, only to reveal that they do a unit rated to 640 bhp thats the size of an alternator! And guess what engine runs a second alternator on the Japanese Spec engine?
O YES, That'll be an 850 BMW then!
Well, here we had a supercharger small enough to fit, capable of running to 600 bhp, you only needed one, with a spare mounting area on the car. All that was required was a mounting arrangement and a drive set-up. I guess a few J-Spec owners will be deleting that 2nd alternator then?
The only further options are the trim of the supercharger to use and the drive ratio, and close examination of the flow charts should at least get you close to the correct one for each. I have chosen, so only time will tell.
Please note that there are exception to most rules so my descriptions above are a generalization for the benefit of the novice reader. I will be happy to discuss any detailed questions time permitting as long as it is of general interest to the board. This was turning into a book length post anyway........and I am a one-finger typist!
Next post.........the fuel system. Stay tuned petrolheads.
8Tech.
I will try and explain to both the beginner and the more knowledgable, how and why I have selected the particular Charger that I have. I have called it this because in the very beginning, I was looking at both Turbo"Chargers" and Super"Chargers".
For the beginners, a very brief description of the differences:
Turbochargers use wasted exhaust gas energy to produce boost by spinning a compressor from the exhaust gases running through the exhaust system using a turbine. Power is almost free!
Superchargers use an engine driven belt to spin a compressor, usually from the crankshaft front pulley. It takes power from the engine to spin the supercharger when producing boost, but only a small percentage of the extra horsepower it is giving.
Turbochargers can be used singly or in multiples, usually twin. Twin turbo's can be arranged in three seperate ways, parallel, staged or sequential.
Twin parallel turbo's are generally used where there is insufficient room to install a single large turbocharger. This is most convenient on the "V" engines. Both turbo's will be identical.
Staged twin turbo's, as on the new 330i and 335D use a small turbo for low rpm boost and a larger turbo that takes over at higher rpm, when the smaller turbo has reached maximum flow. This is a very good arrangement as lag is very low and yet high boost levels are not compromised. 2 different sized turbo's are used in this arrangement.
Finally, the sequential twin turbo uses a similar arrangement to the staged, except it uses one turbo to produce boost, which is then fed directly into the second turbo to produce very high boost pressure levels which are beyond the capability of the pressure ratio of just one unit. In these installs, boost pressures of 60 psi to 100 psi are typical. Not for the faint hearted!
In a single turbo install, many believe that twin turbo's produce less lag because of the inertia in the 2 is less. This is minimal because in the case of an 850 for example, each turbo is only being spun by 2.5 litres whereas a single would receive all 5.0 litres. An advancement of the single turbo such as the twin entry or VGT will surpass this lag.
A twin entry turbo uses two seperate ports to introduce the exhaust gases onto the turbine, therefore changing the force and gas volume to spin it up early without restricting it at higher flow rates, the VGT or "Variable Geometry Turbocharger" does the same thing by varying the angle the gases enter the turbine.
And no, twin turbos DO NOT produce twice the power of single turbo's, although the marketing men would have you believe so!
The supercharger can also be installed singly or in multiples. Again, twin superchargers can be smaller than a single and may be far easier to install spacewise, but will require 2 drives from the crank, although these may be possible from the same belt, generally 2 separate drives are used.
There are a number of supercharger designs, the most common being the positive displacement and the centrifugal types. Most people associate with the positive displacement type which will produce boost ON IDLE speed and has ZERO lag because it pumps air into the engine at any speed at a ratio dependent on supercharger drive ratio.
A centrifugal supercharger is almost a hybrid between a supercharger and a turbocharger and can be described basically as a belt driven turbocharger, producing no boost and little load on the engine at low rpm, but producing boost at higher rpm again dependant on supercharger drive ratio.
Examining all of the above, and considering them all, I decided against the option of the single large turbo due to mounting, relocation of underbonnet components and heat management issues, the twin turbo's involved doubling the cost of a single unit, involves two oil robbing supplies from the engine lubrication system, 2 drains into the sump and glowing red hot turbo's within millimeters of the chassis and wiring components, as well as the manufacture of a pair of exhaust manifolds only suitable for right hand drive cars and difficult maintenance issues around the turbo's.
Remember though that all high power output Japanese tuned cars invariably replace their twin turbo set-ups with a single large turbo.
As far as twin superchargers were concerned, this was only necessary if a single unit could not flow sufficiently, and as many GMC positive displacement superchargers can flow to 6000 bhp and more, I researched into what would be required for my mere 600 bhp. I found the most popular size/ flow superchargers to be the Vortec range of centrifugal superchargers. With severe space limitations in the "V" above the engine, this pretty much ruled out the use of a positive displacement type, and also whilst boost from idle is an awesome experience, there had to be SOME consideration on fuel economy, and the stress on the engine at all times, not just when required.(90% of the time he he)
Further research here in the UK showed a development being made by Turbo-Technics on a fluid drive high speed centrifugal supercharger, and yet more research showed a Danish company called Rotrex that were already producing these units. I investigated further, only to reveal that they do a unit rated to 640 bhp thats the size of an alternator! And guess what engine runs a second alternator on the Japanese Spec engine?
O YES, That'll be an 850 BMW then!
Well, here we had a supercharger small enough to fit, capable of running to 600 bhp, you only needed one, with a spare mounting area on the car. All that was required was a mounting arrangement and a drive set-up. I guess a few J-Spec owners will be deleting that 2nd alternator then?
The only further options are the trim of the supercharger to use and the drive ratio, and close examination of the flow charts should at least get you close to the correct one for each. I have chosen, so only time will tell.
Please note that there are exception to most rules so my descriptions above are a generalization for the benefit of the novice reader. I will be happy to discuss any detailed questions time permitting as long as it is of general interest to the board. This was turning into a book length post anyway........and I am a one-finger typist!
Next post.........the fuel system. Stay tuned petrolheads.
8Tech.
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