928 Motorsports supercharger mods
09-16-2011, 01:07 PM
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928 Motorsports supercharger mods
I am starting this thread to run down what I have done with the 928 Motorsports supercharger and where I plan to go next with it. I have not had much time to work on the system for the last few months but am interested in getting back into it.
What I have done so far:
Moved the MAF farther from the SC to reduce backflow over the MAF element. This fixed the bouncing idle and CELs I was getting from having the MAF located as per the instructions.
I fitted a grooved reversing idler to better manage the belt location
I have also tested other pulleys with various results:
Currently running the knurled Stage I pulley which seems to work. My concern with this pulley is that if the belt slips it will be quickly wiped smooth, and thus the belt needs to be kept tight and checked often. The first indication of belt slip is pretty subtle loss of high RPM boost.
The Stage 1 pulley is also not producing Stage II boost, which is more fun.
I was able to get stage II boost with the stock stage II pulley with a tight belt that had been roughed up a bit on the backside, but belt material buildup suggested there was still some slip. Racers also found the slip was destroying belts in this configuration.
I added another idler pulley to allow use of a longer double sided accessory belt and a grooved SC pulley. This modification had it's ups and downs, belt tension was harder to set due to my location for the extra idler, which reduced the effective adjustment. This issue can be corrected by moving the extra idler to a location that replaces the extra idler that comes with the kit, but locates it farther inboard to use up some more belt.
With the grooved SC pulley I was surprised to still see belt slip, which puzzles me. It seemed that regardless of the size of the pulley and grooved or ungrooved, the system hit a wall at a boost level of about 5psi and the change in pulley size just moved the slip up or down the RPM band. My assumption had been that with a small pulley I had less traction and should see more slip but more boost, and as I went larger the slip would be reduced with the boost. and Ultimately I would get above Stage II levels of boost with no slip due to the large increase in belt to pulley contact patch. Oddly what I saw instead was that I could get stage I boost without obvious slip, or Stage II with a little slip which was more or less what I had before. I could get stage II boost at much lower RPM if I wanted but that was it.
I did however find that the car was lots of fun to drive with a smaller pulley that produced fatter midrange and the tail off of high RPM boost was not a big deal on the street.
Belt slip however is bad news and needs to be avoided.
Going forward:
This work suggested to me that a viable and fun configuration would be to limit boost to stay below the belt traction threshold but build more midrange boost.
This could be accomplished with a restrictor plate on the SC inlet or with a boost limiting valve and a pulley that builds boost down low but does not overspeed the blower at high RPM. And does not slip (I think slip is not a big deal if boost is limited however.)
A big concern I have is the internal belt destruction some racers have seen, and the internal SC head bearing failure that has destroyed impellers and heads. My theory on why this has occurred is that the recirculation valve is too slow to react to a sudden throttle chop (like a transition from WOT to hard braking on a race track) and when the throttle plate closes the pressure between the compressor and the throttle plate spikes way up. With low traction between the drive belt and drive pulley, the belt slips and saves the day. However if that drive belt will not easily slip, something has to give. The impact might then be seen on the internal toothed belt and internal bearings. This pressure could be seen as pushing the compressor wheel away from the motor (loading the bearings) and also making it harder to turn (loading the toothed belt.) This spike would not be seen on a typical boost gauge fitted after the throttle plate. A boost gauge would need to be fitted between the outlet and the throttle body.
A boost limiter could be located before the throttle plate, or it might simply be a matter of plumbing the recirculation valve in as close as possible to the manifold rather than plumbing it in after of the RRFPR as it is, which I imagine slows down it's reaction time.
I have thought about fitting a cheap ebay BOV in parallel with the stocker, and connecting it via a one-way valve to vacuum. Then adjust it's spring pressure to bleed boost over, say 5psi or whatever threshold maintains belt traction. Hopefully this device would react very quickly to a boost spike and prevent high SC head loading.
Detecting belt slip at this point becomes problematic because the loss of boost no longer tells the story, but I have and idea about how to address that.
The larger crank pulley is somewhat interesting but adding more force might just make the system a more efficient killer of SC heads if that issue is not addressed. On the plus side the larger crank pulley would get you closer to fitting a double sided belt with less fuss, and that would open up the options for SC drive pulleys as well as provide better traction without a knurled pulley. Knurled pulleys provide lots of traction but again I worry that they can easily be eaten by belt slip while a grooved pulley just gets a polish (which seems to happen even w/o slip.)
-Joel.
What I have done so far:
Moved the MAF farther from the SC to reduce backflow over the MAF element. This fixed the bouncing idle and CELs I was getting from having the MAF located as per the instructions.
I fitted a grooved reversing idler to better manage the belt location
I have also tested other pulleys with various results:
Currently running the knurled Stage I pulley which seems to work. My concern with this pulley is that if the belt slips it will be quickly wiped smooth, and thus the belt needs to be kept tight and checked often. The first indication of belt slip is pretty subtle loss of high RPM boost.
The Stage 1 pulley is also not producing Stage II boost, which is more fun.
I was able to get stage II boost with the stock stage II pulley with a tight belt that had been roughed up a bit on the backside, but belt material buildup suggested there was still some slip. Racers also found the slip was destroying belts in this configuration.
I added another idler pulley to allow use of a longer double sided accessory belt and a grooved SC pulley. This modification had it's ups and downs, belt tension was harder to set due to my location for the extra idler, which reduced the effective adjustment. This issue can be corrected by moving the extra idler to a location that replaces the extra idler that comes with the kit, but locates it farther inboard to use up some more belt.
With the grooved SC pulley I was surprised to still see belt slip, which puzzles me. It seemed that regardless of the size of the pulley and grooved or ungrooved, the system hit a wall at a boost level of about 5psi and the change in pulley size just moved the slip up or down the RPM band. My assumption had been that with a small pulley I had less traction and should see more slip but more boost, and as I went larger the slip would be reduced with the boost. and Ultimately I would get above Stage II levels of boost with no slip due to the large increase in belt to pulley contact patch. Oddly what I saw instead was that I could get stage I boost without obvious slip, or Stage II with a little slip which was more or less what I had before. I could get stage II boost at much lower RPM if I wanted but that was it.
I did however find that the car was lots of fun to drive with a smaller pulley that produced fatter midrange and the tail off of high RPM boost was not a big deal on the street.
Belt slip however is bad news and needs to be avoided.
Going forward:
This work suggested to me that a viable and fun configuration would be to limit boost to stay below the belt traction threshold but build more midrange boost.
This could be accomplished with a restrictor plate on the SC inlet or with a boost limiting valve and a pulley that builds boost down low but does not overspeed the blower at high RPM. And does not slip (I think slip is not a big deal if boost is limited however.)
A big concern I have is the internal belt destruction some racers have seen, and the internal SC head bearing failure that has destroyed impellers and heads. My theory on why this has occurred is that the recirculation valve is too slow to react to a sudden throttle chop (like a transition from WOT to hard braking on a race track) and when the throttle plate closes the pressure between the compressor and the throttle plate spikes way up. With low traction between the drive belt and drive pulley, the belt slips and saves the day. However if that drive belt will not easily slip, something has to give. The impact might then be seen on the internal toothed belt and internal bearings. This pressure could be seen as pushing the compressor wheel away from the motor (loading the bearings) and also making it harder to turn (loading the toothed belt.) This spike would not be seen on a typical boost gauge fitted after the throttle plate. A boost gauge would need to be fitted between the outlet and the throttle body.
A boost limiter could be located before the throttle plate, or it might simply be a matter of plumbing the recirculation valve in as close as possible to the manifold rather than plumbing it in after of the RRFPR as it is, which I imagine slows down it's reaction time.
I have thought about fitting a cheap ebay BOV in parallel with the stocker, and connecting it via a one-way valve to vacuum. Then adjust it's spring pressure to bleed boost over, say 5psi or whatever threshold maintains belt traction. Hopefully this device would react very quickly to a boost spike and prevent high SC head loading.
Detecting belt slip at this point becomes problematic because the loss of boost no longer tells the story, but I have and idea about how to address that.
The larger crank pulley is somewhat interesting but adding more force might just make the system a more efficient killer of SC heads if that issue is not addressed. On the plus side the larger crank pulley would get you closer to fitting a double sided belt with less fuss, and that would open up the options for SC drive pulleys as well as provide better traction without a knurled pulley. Knurled pulleys provide lots of traction but again I worry that they can easily be eaten by belt slip while a grooved pulley just gets a polish (which seems to happen even w/o slip.)
-Joel.
09-18-2011, 04:49 AM
#2
Three Wheelin'
Joel,
Thanks for an interesting read. I am about to embark on a similar journey, albeit using a self-designed kit. The 5psi "wall" is an interesting issue, and one that is difficult to address without the technical data that Raptor seem so unwilling to provide (IP concerns - seems unfounded as if you were serious about copying the design the lack of datasheets would be no obstacle). Hence my choice is a Rotrex unit.
I have also looked into using a restrictor plate or boost limiting valve near the TB. At this stage my preference is to install a restrictor plate at the SC inlet. This will allow running maximum SC revs, and therefore maximum midrange boost. Calcs suggest Somewhere around 45mm seems appropriate for the 60mm inlet diameter of the Rotrex. The only downside is additional heat because of a reduction in efficiency - but I am going to run with an IC so no big deal.
As for your concern about pressure waves caused by an abrupt close of the throttle, or by an abrupt opening (tip in) I have given some though to this as well. One solution as you suggest is to plumb in a BOV valve in addition to the recirc valve. The problem with this of course is you are dumping metered air, but hopefully only instantaneously when the throttle is shut. If you get an adjustable BOV, you might get it to work. The other solution, although more complicated, is to use two TBs, one stock and one before the SC inlet. The advantage of this is there is no sudden pressure wave on tip in which can cause issues with detonation, surging, etc. This is quite a common modification with the Miata/MX5 supercharging crowd.
http://forum.miata.net/vb/archive/in.../t-367392.html
It may not be simple, but I can definitely see advantages. It may also have a secondary advantage of restricting the inlet and improving mid range boost similar to a restrictor plate.
I can see why 928M used the RRFPR, as it is a simple method of providing the additional fuel and has been in use since the 70s or 80s. However, as an improvement to the kit, I would also suggest you ditch the RRFPR and get a custom tune done using Ostrich/TunerPro. If your RRFPR fails or goes out of adjustment (and it will eventually) you will blow the engine.
http://autospeed.com/cms/title_The-V...7/article.html
Furthermore, the old gen injectors (like the original ones) do not flow very well at the higher pressures as the spray breaks down. The reason the newer injectors work better on the kit is because they can handle the additional pressure and not affect the spray pattern. Also, peak efficiency (VE) occurs at peak torque, and hence you actually want a longer injector duration at about 4000-5000 rpm as per the stock code. The RRFPR does not allow this to occur as it is of course a linear increase in boost.
I am surprised that the ignition timing does not require retarding also. Perhaps the kit relies on the knock sensor to retard timing at high temp/boost levels. For a SC running 6psi at the outlet, the temp gains are at least 40 deg C. So on a hot day 100C intake temps would be likely.
Good luck.
PS - I have ditched the A/C, so the install is much simpler and I can use a standard single sided micro-v belt. Once I get this kit going I'll start a thread - most likely after November.
Thanks for an interesting read. I am about to embark on a similar journey, albeit using a self-designed kit. The 5psi "wall" is an interesting issue, and one that is difficult to address without the technical data that Raptor seem so unwilling to provide (IP concerns - seems unfounded as if you were serious about copying the design the lack of datasheets would be no obstacle). Hence my choice is a Rotrex unit.
I have also looked into using a restrictor plate or boost limiting valve near the TB. At this stage my preference is to install a restrictor plate at the SC inlet. This will allow running maximum SC revs, and therefore maximum midrange boost. Calcs suggest Somewhere around 45mm seems appropriate for the 60mm inlet diameter of the Rotrex. The only downside is additional heat because of a reduction in efficiency - but I am going to run with an IC so no big deal.
As for your concern about pressure waves caused by an abrupt close of the throttle, or by an abrupt opening (tip in) I have given some though to this as well. One solution as you suggest is to plumb in a BOV valve in addition to the recirc valve. The problem with this of course is you are dumping metered air, but hopefully only instantaneously when the throttle is shut. If you get an adjustable BOV, you might get it to work. The other solution, although more complicated, is to use two TBs, one stock and one before the SC inlet. The advantage of this is there is no sudden pressure wave on tip in which can cause issues with detonation, surging, etc. This is quite a common modification with the Miata/MX5 supercharging crowd.
http://forum.miata.net/vb/archive/in.../t-367392.html
It may not be simple, but I can definitely see advantages. It may also have a secondary advantage of restricting the inlet and improving mid range boost similar to a restrictor plate.
I can see why 928M used the RRFPR, as it is a simple method of providing the additional fuel and has been in use since the 70s or 80s. However, as an improvement to the kit, I would also suggest you ditch the RRFPR and get a custom tune done using Ostrich/TunerPro. If your RRFPR fails or goes out of adjustment (and it will eventually) you will blow the engine.
http://autospeed.com/cms/title_The-V...7/article.html
Furthermore, the old gen injectors (like the original ones) do not flow very well at the higher pressures as the spray breaks down. The reason the newer injectors work better on the kit is because they can handle the additional pressure and not affect the spray pattern. Also, peak efficiency (VE) occurs at peak torque, and hence you actually want a longer injector duration at about 4000-5000 rpm as per the stock code. The RRFPR does not allow this to occur as it is of course a linear increase in boost.
I am surprised that the ignition timing does not require retarding also. Perhaps the kit relies on the knock sensor to retard timing at high temp/boost levels. For a SC running 6psi at the outlet, the temp gains are at least 40 deg C. So on a hot day 100C intake temps would be likely.
Good luck.
PS - I have ditched the A/C, so the install is much simpler and I can use a standard single sided micro-v belt. Once I get this kit going I'll start a thread - most likely after November.
09-18-2011, 11:00 AM
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Thanks Eric,
I thought about a TB ahead of the SC head, that solves a lot of problems. Even a DBW unit could be used and controlled with some aftermarket engine management. However I am not sure it is needed as the problems might be solvable without one and at some point cost and complexity of an SC system would push me straight to a V8 conversion.
Tip-in is another problem and my first thought on that was to try to devise a way to keep the BOV or recirculation valve from closing quickly. this could be done with a OW valve that allows vacuum to hit the valve full force, and then and bypass the OW valve with a circuit that uses a small orifice or needle valve that allows pressure to leak through slowly. Setting the needle valve to make the BOV react in 250-500ms would cause boost to build more slowly and allow the ECU time to react.
In this configuration I would want two BOVs. If I mess around with the speed of the signal to one, however slightly, I want one plumed to open as fast as possible to deal with a chopped throttle.
An air to air intercooler should mitigate these problems by adding considerable volume to the intake between the SC and the throttle plate.
*Interesting point - Tim had an IC but still shredded his internal SC belt, maybe hitting the "5psi wall" is what gets these belts and not a lift-throttle boost spike.*
The RRFPR seems to work OK with this kit for tuning but you have a good point. Should it fail, bad things would happen. My setup is using the Stage II injectors which should do OK at high working pressure. In general I am a fan of high fuel system pressures and would tend towards a 4.5 bar fuel system before using larger injectors if I was getting serious with tuning. One of the reasons I like the RRFPR is that the car can be changed over to non-SC operation pretty easily for emergencies, smog testing or perhaps a long highway trip or whatever. Tuning with a switchable map would solve this problem but again at some point I would be thinking I am tuning the wrong motor.
-Joel.
I thought about a TB ahead of the SC head, that solves a lot of problems. Even a DBW unit could be used and controlled with some aftermarket engine management. However I am not sure it is needed as the problems might be solvable without one and at some point cost and complexity of an SC system would push me straight to a V8 conversion.
Tip-in is another problem and my first thought on that was to try to devise a way to keep the BOV or recirculation valve from closing quickly. this could be done with a OW valve that allows vacuum to hit the valve full force, and then and bypass the OW valve with a circuit that uses a small orifice or needle valve that allows pressure to leak through slowly. Setting the needle valve to make the BOV react in 250-500ms would cause boost to build more slowly and allow the ECU time to react.
In this configuration I would want two BOVs. If I mess around with the speed of the signal to one, however slightly, I want one plumed to open as fast as possible to deal with a chopped throttle.
An air to air intercooler should mitigate these problems by adding considerable volume to the intake between the SC and the throttle plate.
*Interesting point - Tim had an IC but still shredded his internal SC belt, maybe hitting the "5psi wall" is what gets these belts and not a lift-throttle boost spike.*
The RRFPR seems to work OK with this kit for tuning but you have a good point. Should it fail, bad things would happen. My setup is using the Stage II injectors which should do OK at high working pressure. In general I am a fan of high fuel system pressures and would tend towards a 4.5 bar fuel system before using larger injectors if I was getting serious with tuning. One of the reasons I like the RRFPR is that the car can be changed over to non-SC operation pretty easily for emergencies, smog testing or perhaps a long highway trip or whatever. Tuning with a switchable map would solve this problem but again at some point I would be thinking I am tuning the wrong motor.
-Joel.
09-19-2011, 07:42 AM
#4
Three Wheelin'
Joel,
I haven't searched your previous threads, but do you have data logging showing your AFRs? I know of one 928M kit buyer that has ditched everything in the kit except the head unit and bracket. The tuner went to stand-alone and couldn't tune with the RRFPR to get decent AFRs. Just curious more than anything.
Keep us informed of how you go. Another alternative may be to ditch the cat and run open loop. At least that way you will be able to enrichen the PT maps. You can do this by simply adding a coding plug (1kohm resistor) to the DME in the wiring harness. From there you can easily enrich the PT maps to lower the AFRs. This will at least give better protection to tip in detonation, but will of course not help alleviate the pressure wave. I'm actually thinking about deleting the cat and running open loop. No emissions testing here in Australia, so it is really only checked when you sell the car.
I haven't searched your previous threads, but do you have data logging showing your AFRs? I know of one 928M kit buyer that has ditched everything in the kit except the head unit and bracket. The tuner went to stand-alone and couldn't tune with the RRFPR to get decent AFRs. Just curious more than anything.
Keep us informed of how you go. Another alternative may be to ditch the cat and run open loop. At least that way you will be able to enrichen the PT maps. You can do this by simply adding a coding plug (1kohm resistor) to the DME in the wiring harness. From there you can easily enrich the PT maps to lower the AFRs. This will at least give better protection to tip in detonation, but will of course not help alleviate the pressure wave. I'm actually thinking about deleting the cat and running open loop. No emissions testing here in Australia, so it is really only checked when you sell the car.
09-19-2011, 11:49 AM
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I have not done anything about tuning as yet, I set the RRFPR to full rich for now. I am unconcerned with tuning until I feel I can get the boost I want without belt slip or SC head destruction.
I'd have to be convinced that the closed loop operation is not tuning itself decently and would not want to go open loop and skew the PT map rich just to avoid issues with tip-in.
I'd have to be convinced that the closed loop operation is not tuning itself decently and would not want to go open loop and skew the PT map rich just to avoid issues with tip-in.
09-19-2011, 06:43 PM
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Just a quick note - take a look at how OEM does it... Pretty much every OEM supercharged car runs the throttle-body before the supercharger. This eliminates the need for a blow/bypass valve, as well as the chance of pressure spikes harming the supercharger.
The only real downside is the added tip-in latency due to moving the throttle, but I doubt it would be significant, and might not even be noticeable at all...
The only real downside is the added tip-in latency due to moving the throttle, but I doubt it would be significant, and might not even be noticeable at all...
10-07-2011, 12:57 PM
#7
Nordschleife Master
An air to air intercooler should mitigate these problems by adding considerable volume to the intake between the SC and the throttle plate.
*Interesting point - Tim had an IC but still shredded his internal SC belt, maybe hitting the "5psi wall" is what gets these belts and not a lift-throttle boost spike.*
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10-11-2011, 06:52 AM
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^^Interesting about heat killing the supercharger internal belts on the 928 kit. I wondered why you gave up on the SC development, (although you are probably having way more fun with the V8 anyway 
)!
I guess it highlights the criticality of picking the right head unit design for high output applications. The C2 procharger on mine runs 10psi and has never had any issues whatsoever despite constant racing in the Dubai heat. Belt slip is another story...PITA to solve
One thing to investigate with the belt slip, apart from the larger pulleys/more ribs which really jumped me forward, is the rigidity of the mounting brackets. I know the 350Z guys with the procharger kits unanimously hit a problems around 10psi with belts breaking and only those who fitted various types of fabricated braces to stop the head units flexing forward were able to get higher boost with any consistency. Their belts tended to break because they were cog belts, but the same flexing could account for aggravated belt slipping with rib belts. Its something I am looking into with mine, don't know how it applies to your own setups?
)! I guess it highlights the criticality of picking the right head unit design for high output applications. The C2 procharger on mine runs 10psi and has never had any issues whatsoever despite constant racing in the Dubai heat. Belt slip is another story...PITA to solve
One thing to investigate with the belt slip, apart from the larger pulleys/more ribs which really jumped me forward, is the rigidity of the mounting brackets. I know the 350Z guys with the procharger kits unanimously hit a problems around 10psi with belts breaking and only those who fitted various types of fabricated braces to stop the head units flexing forward were able to get higher boost with any consistency. Their belts tended to break because they were cog belts, but the same flexing could account for aggravated belt slipping with rib belts. Its something I am looking into with mine, don't know how it applies to your own setups?
Last edited by Dubai944; 10-11-2011 at 08:39 AM.
10-11-2011, 10:19 AM
#11
Nordschleife Master
My SC setup is for sale if anyone is interested, $2,000 plus shipping and insurance. I have all major pieces, just missing some of the rubber intake stuff, wires for extending the MAF, and the aluminum foil air smoother.
