Supercharging, boost and compression
07-07-2006, 07:02 PM
#1
Racer
Thread Starter
Supercharging, boost and compression
I am going to drop my engine this autumn and do a complete rebuild. At the same time I am going to mount the TPC Supercharger kit. The TPC kit (with an intercooler) is a bolt-on kit with a low boost. Since I am doing a rebuild I have to option of lowering the compression and run with higher boost.
Anyone of you 3.6 tech. guys that have some valuable input? Any tips, suggestions or views would be highly appreciated.
Anyone of you 3.6 tech. guys that have some valuable input? Any tips, suggestions or views would be highly appreciated.
07-07-2006, 07:59 PM
#2
Rennlist Member
Some have said that putting boost to a 11:1 comp ratio engine is not a problem... I still have my doubts and am considering the same kit by TPC. Lowering the CR is a good move, IMHO.
Take a look at this thread from the 993 forum though. Not a glowing review of TPCs Customer service. https://rennlist.com/forums/showthre...ferrerid=35947
Great topic! I am looking forward to reading the replies.
Take a look at this thread from the 993 forum though. Not a glowing review of TPCs Customer service. https://rennlist.com/forums/showthre...ferrerid=35947
Great topic! I am looking forward to reading the replies.
07-07-2006, 08:19 PM
#3
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Pretty well no 964 has 11:1 static compression , more like 10.5 . The dynamic compression is around 9.5 :1.
For light boost of less than 0.5 bar I would take the static down to 9.0:1 .
That will lead to fewer compromises on ignition timing / torque.
I had an involvement with a 964 sc running 8.0:1 static and around 1.0 bar boost . It made a little over 450 bhp and massive torque . Great fun .
My involvment was to recalibrate the DME to allow running on either European 95 or 98 Octane fuel .
I believe the TPC system uses some kind of piggy back ECU with a 7th injector?
Probably done for US emmision regulations and not the correct way to get uniform fuelling on a low boost system . The mixture variation cyclinder to cylinder will make the motor more prone to detonation.
Hope that helps to kick off some input on your project .
Good luck
Geoff
For light boost of less than 0.5 bar I would take the static down to 9.0:1 .
That will lead to fewer compromises on ignition timing / torque.
I had an involvement with a 964 sc running 8.0:1 static and around 1.0 bar boost . It made a little over 450 bhp and massive torque . Great fun .
My involvment was to recalibrate the DME to allow running on either European 95 or 98 Octane fuel .
I believe the TPC system uses some kind of piggy back ECU with a 7th injector?
Probably done for US emmision regulations and not the correct way to get uniform fuelling on a low boost system . The mixture variation cyclinder to cylinder will make the motor more prone to detonation.
Hope that helps to kick off some input on your project .
Good luck
Geoff
07-07-2006, 08:31 PM
#4
Rennlist Member
Geoff,
Wow, guess I need to do some reading. Static vs Dynamic? Thought CR was what it was.
Published stats for 964 CR is 11.3:1... I always took that for it's face value.
Cam timing comes into play here?
Very interesting...
Wow, guess I need to do some reading. Static vs Dynamic? Thought CR was what it was.
Published stats for 964 CR is 11.3:1... I always took that for it's face value.
Cam timing comes into play here?
Very interesting...
07-07-2006, 09:33 PM
#5
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Mark,
A good starting read is Supercharged by Corky Bell. When you get hooked there is a lot of good stuff around on WW2 aircraft motors ! With 30 litres of motor to play with our little car stuff looks weedy !!!
Being serious , yes cam timing , the inlet closing point sets the dynamic compression . Thats why big cams need more static compression . Its not another fertility symbol !!!!!!!!!!!!!!
Have fun,
Geoff
A good starting read is Supercharged by Corky Bell. When you get hooked there is a lot of good stuff around on WW2 aircraft motors ! With 30 litres of motor to play with our little car stuff looks weedy !!!
Being serious , yes cam timing , the inlet closing point sets the dynamic compression . Thats why big cams need more static compression . Its not another fertility symbol !!!!!!!!!!!!!!
Have fun,
Geoff
07-08-2006, 11:43 AM
#6
Racer
Join Date: Mar 2006
Location: Joplin, Mo
Posts: 345
Likes: 0
Received 0 Likes
on
0 Posts
Geoff,
It would seem that dynamic rather than static compression is the key to running boost.If so dose that mean that you can change the cam rather than the piston or combustion chamber size when running boost.
It would seem that dynamic rather than static compression is the key to running boost.If so dose that mean that you can change the cam rather than the piston or combustion chamber size when running boost.
07-08-2006, 12:00 PM
#7
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Jeff522,
That has been done but not on a 964 for lowering the dynamic compression as far as I know .
The 964 inlet closing is reasonably late already, so for a street motor a very special cam grind would be needed. Probably cheaper to have the piston tops skimmed !
The other factor is that just fitting a pair of "bigger " cams will increase overlap and so lose low speed boost . Like all this stuff , there are so many factors to consider and all you can hope for is the best compromise !
All the best
Geoff
That has been done but not on a 964 for lowering the dynamic compression as far as I know .
The 964 inlet closing is reasonably late already, so for a street motor a very special cam grind would be needed. Probably cheaper to have the piston tops skimmed !
The other factor is that just fitting a pair of "bigger " cams will increase overlap and so lose low speed boost . Like all this stuff , there are so many factors to consider and all you can hope for is the best compromise !
All the best
Geoff
Trending Topics
07-08-2006, 12:39 PM
#9
Three Wheelin'
My SoK 964 runs 9.0:1 @ 8 psi w/custom cams & a big intercooler. I've got about 30k miles on it now w/about 10k of those being track miles. The set-up is over 10 years old so, needless to say, it's been very reliable 
Like mentioned, since you are going to be inside the engine anyway, I would lower the compression & go with the strongest components you can (pistons, rods, ARP bolts, etc.) Might as well get cams ground for the application as well.
Have fun with the project! You will love the results
Like mentioned, since you are going to be inside the engine anyway, I would lower the compression & go with the strongest components you can (pistons, rods, ARP bolts, etc.) Might as well get cams ground for the application as well.
Have fun with the project! You will love the results
07-08-2006, 02:06 PM
#10
Super Duper Moderator
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Originally Posted by Red rooster
...I believe the TPC system uses some kind of piggy back ECU with a 7th injector?Probably done for US emmision regulations and not the correct way to get uniform fuelling on a low boost system .
Marc
07-08-2006, 02:37 PM
#11
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Marc,
The 7th injector is there to provide the extra fuel that is required. Its needed if you dont change the standard injectors .
The problem with that approach is that at low boost it is pretty difficult to get the same fuelling levels for each cylinder . Just not enough hurricane to carry the fuel everywhere ! A lean running cylinder will get hot and detonate .
May seem strange but at low boost levels there are some serious decisions to be made as to whether an intercooler is worth having from a power perspective !
All good stuff
Geoff
The 7th injector is there to provide the extra fuel that is required. Its needed if you dont change the standard injectors .
The problem with that approach is that at low boost it is pretty difficult to get the same fuelling levels for each cylinder . Just not enough hurricane to carry the fuel everywhere ! A lean running cylinder will get hot and detonate .
May seem strange but at low boost levels there are some serious decisions to be made as to whether an intercooler is worth having from a power perspective !
All good stuff
Geoff
07-09-2006, 06:07 PM
#12
Addict
Rennlist
Site Sponsor
Rennlist
Site Sponsor
There are two ways to look at compression ratio when considering the design of your engine.
The traditional "static" compression ratio is a comparison of the difference in volume of the cylinder over the full stroke of the piston. As Geoff and others have mentioned before, although Porsche quote the 964/993 engines as 11.2:1 if you measure the compressed volume in the combustion chamber most of these engines are actually 10.5-10.6:1.
The second measure is effective or "dynamic" compression which compares the trapped volume in the cylinder from the point where the intake valve closes to the volume trapped at TDC. In my opinion dynamic compression is of far more interest in designing engines than static compression because if you collect data from a wide variety of engines (designed to run on normal pump fuel) you will find that the dynamic compression ratio is pretty much a constant somewhere around 7.5-8.0:1. This naturally explains why you can run race engines on long duration cams at a much higher static compression ratio because the intake valve closes a lot later in the cycle, so with a shorter effective stroke to work with there has to be a smaller combustion chamber to compress the mixture to the same level.
Another factor to take into account in the boost equation is Volumetric (cylinder filling) Efficiency, because irrespective of how much boost pressure is used in the manifold, how much air is in the cylinder is the only aspect that really matters. The outcome of VE (assuming optimum fuel mixture) is that the more air you cram into the cylinder, the greater the pressure generated on the piston after combustion, and hence the higher the measured Torque of the engine. The best engineering measure of these pressures between engine specifications is Brake Mean Effective Pressure, but let's not complicate matters more than necessary for now.
So to the question in hand. In my opinion you can simply regard any low boost supercharger as a method of achieving a better VE across the entire rpm range of the engine, from the the engines point of view it does not know how the cylinder is filled. From our dyno tests the best torque we have ever achieved from a modified 964RS n/a engine is around 310-320lbft (usually around 5000rpm), thus provided your supercharged engine does not exceed this torque and that the intake air is at or near to atmospheric temperature, you can safely use any boost required in order to achieve the same torque throughout the rpm range and the engine will not know any difference! The goal is simply to achieve a flat torque curve throughout the range, thus if you hold the peak torque to the redline you will make a lot more power than a n/a engine but without stressing the engine beyond its known limits.
The traditional "static" compression ratio is a comparison of the difference in volume of the cylinder over the full stroke of the piston. As Geoff and others have mentioned before, although Porsche quote the 964/993 engines as 11.2:1 if you measure the compressed volume in the combustion chamber most of these engines are actually 10.5-10.6:1.
The second measure is effective or "dynamic" compression which compares the trapped volume in the cylinder from the point where the intake valve closes to the volume trapped at TDC. In my opinion dynamic compression is of far more interest in designing engines than static compression because if you collect data from a wide variety of engines (designed to run on normal pump fuel) you will find that the dynamic compression ratio is pretty much a constant somewhere around 7.5-8.0:1. This naturally explains why you can run race engines on long duration cams at a much higher static compression ratio because the intake valve closes a lot later in the cycle, so with a shorter effective stroke to work with there has to be a smaller combustion chamber to compress the mixture to the same level.
Another factor to take into account in the boost equation is Volumetric (cylinder filling) Efficiency, because irrespective of how much boost pressure is used in the manifold, how much air is in the cylinder is the only aspect that really matters. The outcome of VE (assuming optimum fuel mixture) is that the more air you cram into the cylinder, the greater the pressure generated on the piston after combustion, and hence the higher the measured Torque of the engine. The best engineering measure of these pressures between engine specifications is Brake Mean Effective Pressure, but let's not complicate matters more than necessary for now.
So to the question in hand. In my opinion you can simply regard any low boost supercharger as a method of achieving a better VE across the entire rpm range of the engine, from the the engines point of view it does not know how the cylinder is filled. From our dyno tests the best torque we have ever achieved from a modified 964RS n/a engine is around 310-320lbft (usually around 5000rpm), thus provided your supercharged engine does not exceed this torque and that the intake air is at or near to atmospheric temperature, you can safely use any boost required in order to achieve the same torque throughout the rpm range and the engine will not know any difference! The goal is simply to achieve a flat torque curve throughout the range, thus if you hold the peak torque to the redline you will make a lot more power than a n/a engine but without stressing the engine beyond its known limits.
Last edited by NineMeister; 07-10-2006 at 08:48 AM.
07-09-2006, 06:32 PM
#13
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Colin,
Small typo ? Dynamic compression etc is from where the inlet closes to TDC ?
The other factor is detonation sensitivity which is at its highest around the n/a maximum torque point .
The linear compressors being considered almost need the boost lowered at around 5000 and then brought back up again .
From what I have seen screw compressors tend to be fairly inefficient and so heat the intake air a fair bit more than the heat rise due to compression. Problem is that an intercooler has an associated pressure drop which can get significant at low boost levels.
Doing the sums leads to the conclusion that a low boost screw compressor on a high static compression motor is a tricky set up to get right !
All the best
Geoff
Small typo ? Dynamic compression etc is from where the inlet closes to TDC ?
The other factor is detonation sensitivity which is at its highest around the n/a maximum torque point .
The linear compressors being considered almost need the boost lowered at around 5000 and then brought back up again .
From what I have seen screw compressors tend to be fairly inefficient and so heat the intake air a fair bit more than the heat rise due to compression. Problem is that an intercooler has an associated pressure drop which can get significant at low boost levels.
Doing the sums leads to the conclusion that a low boost screw compressor on a high static compression motor is a tricky set up to get right !
All the best
Geoff
07-09-2006, 06:36 PM
#14
Rennlist Member
Colin, thanks for the excellent info.
With a low boost SC on a stock 964 3.6, is detonation a concern? Particularly with 91 octane here in Ca, USA? Or will the "knock control" handle it for you?
With a low boost SC on a stock 964 3.6, is detonation a concern? Particularly with 91 octane here in Ca, USA? Or will the "knock control" handle it for you?
07-10-2006, 09:00 AM
#15
Addict
Rennlist
Site Sponsor
Rennlist
Site Sponsor
You were right Geoff, I meant intake valve - bit of a mental block after suffering throught the world Cup Final (corrected it now in the text).
You are right in that the torque/bmep peak at 5000rpm is the most ignition sensitive rpm point in atmo engines because this is where the minimum ignition timing is usually seen. Obviously if a supercharged engine sees the same bmep across the rev range it too needs to be mapped very carefully throughout to avoid detonation.
I would guess that on 91RON fuel a stock 964 engine will cope with 4-5psi of boost and will not detonate provided that the ignition timing is optimised and the fuelling is correct. Ideally the ecu should be remapped or in the TPC system a digi-box is used to pull back the timing. However you can definitely not rely on the Motronic knock control feature to do the job for you, ok?
You are right in that the torque/bmep peak at 5000rpm is the most ignition sensitive rpm point in atmo engines because this is where the minimum ignition timing is usually seen. Obviously if a supercharged engine sees the same bmep across the rev range it too needs to be mapped very carefully throughout to avoid detonation.
I would guess that on 91RON fuel a stock 964 engine will cope with 4-5psi of boost and will not detonate provided that the ignition timing is optimised and the fuelling is correct. Ideally the ecu should be remapped or in the TPC system a digi-box is used to pull back the timing. However you can definitely not rely on the Motronic knock control feature to do the job for you, ok?