11-26-2009, 02:49 PM
A lot of talk has been had about Forced Induction. This has been going on as long as the car has been around. I thought it was time to discuss the varying approaches, how they differ and compare, their benefits and liabilities, costs, and realities. It is a very complicated subject, and I won't attempt to go into too much detail, but some broad strokes seemed in order. I also won't get into any opinions or specifics regarding anybody's kits. We all have our own preferences for our own reasons.
The most important thing to remember is that there is no free lunch when dealing with Forced Induction. There are always things to consider. The following is not intended to scare anyone away from Forced Induction. In fact, it is quite the opposite. Forced Induction can provide the level of power that brings the 968 into the realm of the Supercar. I only want to make sure that anyone considering such a thing, no matter which route, vendor, or level of power, is fully informed, and makes the decisions that will provide for the most fun, and hopefully avoid some pitfalls.
Before considering any Forced Induction modification, or frankly any performance modification, you should always first make sure that the car is in tip-top running condition. Modifying a worn out car is like polishing a turd. Inspect carefully the brakes, hoses, bushings, tires, and any component that could see stress under load. Replace anything suspect. Remember that this is a 15 year old car. So are the parts in it. Any original rubber part is likely due for renewal. The head gaskets are particularly suspect on this car too. They are very well known to disintegrate over time, regardless of mileage. This should be renewed if it is more than 5 years old. The head could also need work. You should have a Compression Test and Leakdown Test done prior to any such modification, to determine your wear level. Any significant wear should be addressed. The cooling system will be working hard, so be absolutely sure that it is in perfect running order, with no clogging in the radiator.
<b>More power means more speed</b>
Whenever you start considering more power, you can assume that the power will be used, and that means the car is going to be going faster. If it is going faster, it will need to be slowed down. That means you will need more brakes to handle the higher speeds. The stock brakes, when in tip-top condition are adequate for daily driving, but if one is considering DE events or track use, or is the type to take the car to the edge, better brakes are in order. If you are adding Forced Induction, this becomes even more important. The more power added, the more brakes needed.
It is far more likely that you will be going faster through a corner too. That means that you really need to consider the suspension. It is highly recommended that this be uprated. The stock suspension of the 968 is NOT up to the task of handling the speeds that a car with even moderate Forced Induction power can generate. At a bare minimum, larger sway bars, stiffer springs, with shocks and struts to control them, should be fitted.
Larger tires are also indicated. This gets doubly complicated as now the stickier compounds and heavier weights play into the viability of the suspension. Too big and sticky and the components will begin to show their weakness.
The chassis is also not nearly as stiff or strong as some would think it is. There is very measurable flex under load. As the 968 is a Uni-Body car, this is actually not altogether a bad thing. A certain amount of weight transfer across the body is good. However, where it is not good is in the front end of the car. The lower suspension mounting points are the first to show flex. The upper strut tower points are next. After that, the front frame rails twist relative to each other. You are likely to never see this last one though unless you have over 400hp and 300ft/lb of torque and/or are on the track.
Axles and input shafts take a heavy beating. The G44 gearbox is fairly stout, but is not particularly strong in the lower gears. Like all transmissions, this is where the most torque can be applied. This is the weak spot.
<b>Porsche's Own</b>
This leads me to the factory cars. We all know that Porsche did their own Forced Induction conversion. They did it in 3 forms. they made the Turbo S with 305hp, the Turbo RS with 337hp, and a hopped up Le Mans version of the RS with 350hp. They determined that 305hp was as far as they could go with reliability given the components they chose. The RS version, while extremely fast, was not streetable and ran poorly down low.
When they did these, they took into account a LOT of factors. They looked at the components which were suspect and beefed them up. They took into account the higher speeds the car was capable of and improved things to accommodate that. They considered the purpose of the car, and defined the power accordingly. To give you an idea of some of the things they did, even at the very modest power levels of their cars:
They beefed up the transaxle to handle the increased loads. Axles were uprated, as were the input shafts.
They beefed up the suspension with M030 components (hubs, spindles, springs, shocks, struts).
They beefed up the brakes with the larger and thicker M030 brakes.
They increased tire sizes.
They added a strut tower brace.
<b>Which one is for me?</b>
There are basically 3 types of Forced Induction. These are Turbocharging, Centrifugal Supercharging, and Roots (or Positive Displacement) Supercharging. Each has distinct advantages and disadvantages over the others. No system does it all, and which one to choose is very subjective. Some people want the bragging rights of big numbers. Some want daily driveablity. Some want exclusivity. There is no right answer, and there is no answer that gives them all.
Turbocharging can easily make far more power than either of the other two forms. It is a well known and accepted method that has been shown to work well in the factory form, and has been used on countless cars over the years. It has its drawbacks though. Like anything, to get the most gains, you have to invest the most. It is by far the most expensive route. It is also the most complicated. It requires a lot of changes to the engine in order to work. This is not for the timid or the amateur, and is really best left up to the expert installer. Tuning is a huge requirement here. Turbocharging generates a LOT of heat that must be dealt with. This often results in detonation and damage. This is something that requires very expensive tools to tune for. This also almost always means an Intercooler, which usually means modifying the car to fit it, and adds to the cost. There is also a traditional "lag" before a turbocharger can build up pressure and come up on boost. There is traditionally a whistle sound on boost. It also will not pass any currently required emissions testing. However, this is the only way to get to the big numbers.
Centifugal Supercharging has been around for decades too. McCulloh (which became Paxton, which became Vortech, which became Paxton....) was doing this on the Pierce Golden Hawk back in the 50s. It is essentially a belt driven Turbocharger. It is not as prevalent in the Forced Induction market due primarily to the fact that it does not make the big numbers that advertise well. In fact, if probably makes the lowest maximum numbers of the 3 types of Forced Induction. Why then would anyone consider it? This is simple. Being belt driven, there is no lag before boost comes on, which means it achieves boost earlier and more smoothly, but then behaves like a turbo from there up. It is limited though by the turbine speed which is determined by the crank speed. It also generates the least amount of heat of the 3 types, and in low boost form requires no Intercooler. It is generally the easiest to install, often done in an afternoon by your average ShadeTree mechanic, and is the lowest cost of the 3 types. The newer units are generally silent.
Roots type systems are the ones you've seen for decades sticking out of the hoods of hot rods. These are the least efficient of the 3, but can generate the most torque down low. These are frequently used on SUVs and trucks. They make big flat torque curves over a wide band. This is of a huge advantage to a drag strip driver or someone towing. However they tend to flatten out and taper off early, which is not desirable to a road race driver. They generate the most heat in producing their charge, are fairly complicated to install, weigh the most, and also will not pass emissions. They are well known for their distinctive and ever-present albeit albeit loud gear whine. They are fairly inexpensive in components though.
<b>The Spoil Sports and Legal Mumbo Jumbo</b>
There are some things to think about regardless of which system design you would choose. This varies from state to state, but the concepts are consistent.
Many insurance companies will void your policy or refuse to pay a claim if you have modified the car in this fashion. Collector or Stated Value policies are prime examples of this. Check your own policy for such exclusions. Do not assume that "if you don't tell them, they won't find out". If you are in an accident and the car is towed and then subsequently inspected by the adjuster, they could easily find it. You can believe that they will take any opportunity to not pay a claim. They are in business to make money, not pay claims. Be sure you don't end up in a situation that can cost you your house.
Emissions laws are changing. For modified street cars this is a very big deal. As a part of the federal Clean Air Act all states are required to tighten up their standards. Many of them have been pushing back until the last minute their required compliance with the federal standards. California was the first to comply, and has always been the most strict, but other states have been coming on line one by one over the last few years. This has been a gradual process, but is about to accelerate, as the current deadline is 2010. It is based upon a measured air quality specific to a given area, which will be continually reviewed, and testing standards in that area adjusted accordingly. Many areas which currently enjoy exemptions will no longer have that privilege. What this means is that if you add Forced Induction now, in an area where it might be exempt, next year it might not be. This means you won't be able to register your car or get insurance. Only kits which have passed 50 states legal emissions testing will avoid this. Currently the only 50 state testing is C.A.R.B. If your kit is approved in California, it will be approved in all 50 states. Other states may begin this approval process as the standards tighten, but there has been no notification of this as yet.
<b>Deciding what to do</b>
Making a decision on what to do is a very subjective one. You have to consider what the car will be used for. I have always adhered to the philosophy that one should make any decision about the car, whether it be tires, turbos, or stereos, on the 80% use of the car. I recommend choosing something that you will be happiest with 80% of the time, rather than something that might please you a bit more 20% of the time, but less the remaining 80%. Only you can determine what is important to you. This could be bragging rights of big numbers, daily reliability, cost, uniqueness, looks, whatever. There is no right or wrong answer. The best recommendation I can make is to check out the options as thoroughly as you can. Talk to your friends. Talk to people who have done the conversions. Get as much information as you can. If at all possible, drive a car with the conversion you are considering. Don't listen to the hype, and see for yourself. Take your time and be sure.
The most important thing to remember is that there is no free lunch when dealing with Forced Induction. There are always things to consider. The following is not intended to scare anyone away from Forced Induction. In fact, it is quite the opposite. Forced Induction can provide the level of power that brings the 968 into the realm of the Supercar. I only want to make sure that anyone considering such a thing, no matter which route, vendor, or level of power, is fully informed, and makes the decisions that will provide for the most fun, and hopefully avoid some pitfalls.
Before considering any Forced Induction modification, or frankly any performance modification, you should always first make sure that the car is in tip-top running condition. Modifying a worn out car is like polishing a turd. Inspect carefully the brakes, hoses, bushings, tires, and any component that could see stress under load. Replace anything suspect. Remember that this is a 15 year old car. So are the parts in it. Any original rubber part is likely due for renewal. The head gaskets are particularly suspect on this car too. They are very well known to disintegrate over time, regardless of mileage. This should be renewed if it is more than 5 years old. The head could also need work. You should have a Compression Test and Leakdown Test done prior to any such modification, to determine your wear level. Any significant wear should be addressed. The cooling system will be working hard, so be absolutely sure that it is in perfect running order, with no clogging in the radiator.
<b>More power means more speed</b>
Whenever you start considering more power, you can assume that the power will be used, and that means the car is going to be going faster. If it is going faster, it will need to be slowed down. That means you will need more brakes to handle the higher speeds. The stock brakes, when in tip-top condition are adequate for daily driving, but if one is considering DE events or track use, or is the type to take the car to the edge, better brakes are in order. If you are adding Forced Induction, this becomes even more important. The more power added, the more brakes needed.
It is far more likely that you will be going faster through a corner too. That means that you really need to consider the suspension. It is highly recommended that this be uprated. The stock suspension of the 968 is NOT up to the task of handling the speeds that a car with even moderate Forced Induction power can generate. At a bare minimum, larger sway bars, stiffer springs, with shocks and struts to control them, should be fitted.
Larger tires are also indicated. This gets doubly complicated as now the stickier compounds and heavier weights play into the viability of the suspension. Too big and sticky and the components will begin to show their weakness.
The chassis is also not nearly as stiff or strong as some would think it is. There is very measurable flex under load. As the 968 is a Uni-Body car, this is actually not altogether a bad thing. A certain amount of weight transfer across the body is good. However, where it is not good is in the front end of the car. The lower suspension mounting points are the first to show flex. The upper strut tower points are next. After that, the front frame rails twist relative to each other. You are likely to never see this last one though unless you have over 400hp and 300ft/lb of torque and/or are on the track.
Axles and input shafts take a heavy beating. The G44 gearbox is fairly stout, but is not particularly strong in the lower gears. Like all transmissions, this is where the most torque can be applied. This is the weak spot.
<b>Porsche's Own</b>
This leads me to the factory cars. We all know that Porsche did their own Forced Induction conversion. They did it in 3 forms. they made the Turbo S with 305hp, the Turbo RS with 337hp, and a hopped up Le Mans version of the RS with 350hp. They determined that 305hp was as far as they could go with reliability given the components they chose. The RS version, while extremely fast, was not streetable and ran poorly down low.
When they did these, they took into account a LOT of factors. They looked at the components which were suspect and beefed them up. They took into account the higher speeds the car was capable of and improved things to accommodate that. They considered the purpose of the car, and defined the power accordingly. To give you an idea of some of the things they did, even at the very modest power levels of their cars:
They beefed up the transaxle to handle the increased loads. Axles were uprated, as were the input shafts.
They beefed up the suspension with M030 components (hubs, spindles, springs, shocks, struts).
They beefed up the brakes with the larger and thicker M030 brakes.
They increased tire sizes.
They added a strut tower brace.
<b>Which one is for me?</b>
There are basically 3 types of Forced Induction. These are Turbocharging, Centrifugal Supercharging, and Roots (or Positive Displacement) Supercharging. Each has distinct advantages and disadvantages over the others. No system does it all, and which one to choose is very subjective. Some people want the bragging rights of big numbers. Some want daily driveablity. Some want exclusivity. There is no right answer, and there is no answer that gives them all.
Turbocharging can easily make far more power than either of the other two forms. It is a well known and accepted method that has been shown to work well in the factory form, and has been used on countless cars over the years. It has its drawbacks though. Like anything, to get the most gains, you have to invest the most. It is by far the most expensive route. It is also the most complicated. It requires a lot of changes to the engine in order to work. This is not for the timid or the amateur, and is really best left up to the expert installer. Tuning is a huge requirement here. Turbocharging generates a LOT of heat that must be dealt with. This often results in detonation and damage. This is something that requires very expensive tools to tune for. This also almost always means an Intercooler, which usually means modifying the car to fit it, and adds to the cost. There is also a traditional "lag" before a turbocharger can build up pressure and come up on boost. There is traditionally a whistle sound on boost. It also will not pass any currently required emissions testing. However, this is the only way to get to the big numbers.
Centifugal Supercharging has been around for decades too. McCulloh (which became Paxton, which became Vortech, which became Paxton....) was doing this on the Pierce Golden Hawk back in the 50s. It is essentially a belt driven Turbocharger. It is not as prevalent in the Forced Induction market due primarily to the fact that it does not make the big numbers that advertise well. In fact, if probably makes the lowest maximum numbers of the 3 types of Forced Induction. Why then would anyone consider it? This is simple. Being belt driven, there is no lag before boost comes on, which means it achieves boost earlier and more smoothly, but then behaves like a turbo from there up. It is limited though by the turbine speed which is determined by the crank speed. It also generates the least amount of heat of the 3 types, and in low boost form requires no Intercooler. It is generally the easiest to install, often done in an afternoon by your average ShadeTree mechanic, and is the lowest cost of the 3 types. The newer units are generally silent.
Roots type systems are the ones you've seen for decades sticking out of the hoods of hot rods. These are the least efficient of the 3, but can generate the most torque down low. These are frequently used on SUVs and trucks. They make big flat torque curves over a wide band. This is of a huge advantage to a drag strip driver or someone towing. However they tend to flatten out and taper off early, which is not desirable to a road race driver. They generate the most heat in producing their charge, are fairly complicated to install, weigh the most, and also will not pass emissions. They are well known for their distinctive and ever-present albeit albeit loud gear whine. They are fairly inexpensive in components though.
<b>The Spoil Sports and Legal Mumbo Jumbo</b>
There are some things to think about regardless of which system design you would choose. This varies from state to state, but the concepts are consistent.
Many insurance companies will void your policy or refuse to pay a claim if you have modified the car in this fashion. Collector or Stated Value policies are prime examples of this. Check your own policy for such exclusions. Do not assume that "if you don't tell them, they won't find out". If you are in an accident and the car is towed and then subsequently inspected by the adjuster, they could easily find it. You can believe that they will take any opportunity to not pay a claim. They are in business to make money, not pay claims. Be sure you don't end up in a situation that can cost you your house.
Emissions laws are changing. For modified street cars this is a very big deal. As a part of the federal Clean Air Act all states are required to tighten up their standards. Many of them have been pushing back until the last minute their required compliance with the federal standards. California was the first to comply, and has always been the most strict, but other states have been coming on line one by one over the last few years. This has been a gradual process, but is about to accelerate, as the current deadline is 2010. It is based upon a measured air quality specific to a given area, which will be continually reviewed, and testing standards in that area adjusted accordingly. Many areas which currently enjoy exemptions will no longer have that privilege. What this means is that if you add Forced Induction now, in an area where it might be exempt, next year it might not be. This means you won't be able to register your car or get insurance. Only kits which have passed 50 states legal emissions testing will avoid this. Currently the only 50 state testing is C.A.R.B. If your kit is approved in California, it will be approved in all 50 states. Other states may begin this approval process as the standards tighten, but there has been no notification of this as yet.
<b>Deciding what to do</b>
Making a decision on what to do is a very subjective one. You have to consider what the car will be used for. I have always adhered to the philosophy that one should make any decision about the car, whether it be tires, turbos, or stereos, on the 80% use of the car. I recommend choosing something that you will be happiest with 80% of the time, rather than something that might please you a bit more 20% of the time, but less the remaining 80%. Only you can determine what is important to you. This could be bragging rights of big numbers, daily reliability, cost, uniqueness, looks, whatever. There is no right or wrong answer. The best recommendation I can make is to check out the options as thoroughly as you can. Talk to your friends. Talk to people who have done the conversions. Get as much information as you can. If at all possible, drive a car with the conversion you are considering. Don't listen to the hype, and see for yourself. Take your time and be sure.
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating www.968forums.com
"It isn't nearly as expensive to do it right as it is to do it wrong."
(This post was last modified: 11-26-2009, 06:51 PM by flash.)
