Working on the SC car again
06-09-2012, 01:47 AM
#1
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Working on the SC car again
After a long hiatus, mostly due to work and other distractions, I started messing with the 928MS SC car again today. Last year I had added an idler between the AC and crank that allowed me to use double sided belts from Volvo and Corvette applications that I felt should have solved the belt slip problem. Instead I found that I was hitting a 'wall' at about 5psi and was not able to boost above that, try as I might. I have a collection of pulleys and was only able to move the point where I hit the 'wall' up or down. My feeling at that point was that I was getting belt whip from my extra pulley turn, and still getting slip at the crank or SC.
I was out of ideas and with other diversions I stopped work on the project.
In the meantime a canny Welshman found that there was a double sided belt for the Euro-only Vw Phaeton V6 that would fit perfectly without the need for the extra idler. Trouble was it has 7 ribs instead of 6, but this could, I was told, be addressed with fortitude and a sharp Stanley knife. No parts dealer in the USA I talked to could obtain this part, and I found no Euro parts dealers who would ship to the Colonies.
One application of the puppy-dog-eyes and a rousing rendition of "Hen Wlad Fy Nhadau" later I exchanged some of the American "money" for one of these belts and shipping from this country (that I would swear was imaginary had I not visited in 2008) to my rural compound and workshop in the American West. It's fun to get a package like that. Wales forever.
It took a few months for me to finally get out the straightedge and utility knife to razor off a rib from this belt that cost me some dignity and the better part of 50 quid, but cutting off a rib turned out to be easy and it is clear that the factory belts are made in much the same manner.
Today, while prepping to paint some bits of my Audi A8 I decided to move the SC car out of the shop to clear a space, and why not do that with the new belt fitted? So I whipped the stage 1 setup off and put on a 50mm grooved SC pulley and the new double sided belt, which fit perfectly. I took if for a drive, expecting 8psi or more.
...and I hit the 5psi wall again.
It's now time to question my assumptions. What can cause this boost limit that moves around wrt RPM if I change the size of the SC pulley?
* Belt slip at a certain resistance level for the SC head
* A cracked hose that pops open at 5psi
* The workings of Satan
* A bad recirculation valve, or low boost signal to the valve
* ?
At this point I am kicking myself as I assume this is either the work of Satan or the recirculation valve and I could have solved this problem months ago and been boosting madly with a Corvette or Volvo belt. I wonder what it'll turn out to be? The suspense is killing me (I hope it lasts.)
Maybe I will find out tomorrow if I get the paintwork done early.
-Joel.
I was out of ideas and with other diversions I stopped work on the project.
In the meantime a canny Welshman found that there was a double sided belt for the Euro-only Vw Phaeton V6 that would fit perfectly without the need for the extra idler. Trouble was it has 7 ribs instead of 6, but this could, I was told, be addressed with fortitude and a sharp Stanley knife. No parts dealer in the USA I talked to could obtain this part, and I found no Euro parts dealers who would ship to the Colonies.
One application of the puppy-dog-eyes and a rousing rendition of "Hen Wlad Fy Nhadau" later I exchanged some of the American "money" for one of these belts and shipping from this country (that I would swear was imaginary had I not visited in 2008) to my rural compound and workshop in the American West. It's fun to get a package like that. Wales forever.
It took a few months for me to finally get out the straightedge and utility knife to razor off a rib from this belt that cost me some dignity and the better part of 50 quid, but cutting off a rib turned out to be easy and it is clear that the factory belts are made in much the same manner.
Today, while prepping to paint some bits of my Audi A8 I decided to move the SC car out of the shop to clear a space, and why not do that with the new belt fitted? So I whipped the stage 1 setup off and put on a 50mm grooved SC pulley and the new double sided belt, which fit perfectly. I took if for a drive, expecting 8psi or more.
...and I hit the 5psi wall again.
It's now time to question my assumptions. What can cause this boost limit that moves around wrt RPM if I change the size of the SC pulley?
* Belt slip at a certain resistance level for the SC head
* A cracked hose that pops open at 5psi
* The workings of Satan
* A bad recirculation valve, or low boost signal to the valve
* ?
At this point I am kicking myself as I assume this is either the work of Satan or the recirculation valve and I could have solved this problem months ago and been boosting madly with a Corvette or Volvo belt. I wonder what it'll turn out to be? The suspense is killing me (I hope it lasts.)
Maybe I will find out tomorrow if I get the paintwork done early.
-Joel.
06-09-2012, 05:07 AM
#2
Three Wheelin'
This 5psi limit appears to be a common issue. The bosch bov valves are good for 10 psi+. You can test by blanking off the outlet. You could apply some belt grip spray and see if this helps. How tight is the belt?
If the above make no difference, it sounds like a limitation of the head unit to me.
If the above make no difference, it sounds like a limitation of the head unit to me.
06-09-2012, 09:33 AM
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A head unit limitation is possible but I was getting higher boost before with single sided belts and the early stage II pulley. Unless the head unit clearances have loosened up or something I do not know why that would change.
I agree about the Bosch recirculation valves being good, I have used them for years on many different cars without issues. I am going to check this one and plumb in a different boost/vac signal and see if that helps. I have a brand new one and a good used one laying around also.
The belt is not particularly tight, I was running it for the first time. That might be a factor but this is the same behavior I saw with my other double sided setups which had been run in and the belts run quite tight. With a slipping belt I recalled more of a problem with boost tailing off at high RPM and some flutter. I'd see the boost go up to 6 or 7 or whatever and then drop to 1 or 2 when the belt slipped.
-Joel.
I agree about the Bosch recirculation valves being good, I have used them for years on many different cars without issues. I am going to check this one and plumb in a different boost/vac signal and see if that helps. I have a brand new one and a good used one laying around also.
The belt is not particularly tight, I was running it for the first time. That might be a factor but this is the same behavior I saw with my other double sided setups which had been run in and the belts run quite tight. With a slipping belt I recalled more of a problem with boost tailing off at high RPM and some flutter. I'd see the boost go up to 6 or 7 or whatever and then drop to 1 or 2 when the belt slipped.
-Joel.
06-13-2012, 10:15 PM
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I found a blown-off vacuum line that was probably causing low vacuum or boost to the recirculation valve. I am getting about 7psi now with a 50mm pulley. Good boost in the 3000-4500 range. A touch over 5psi at 5000 RPM. Brisk performance. No sign of belt slip.
It's possible this issue was my problem last year when I was working with the longer belts, but the Euro Phaeton belt obviates the need for the additional idler and that is pretty sweet.
-Joel.
It's possible this issue was my problem last year when I was working with the longer belts, but the Euro Phaeton belt obviates the need for the additional idler and that is pretty sweet.
-Joel.
06-14-2012, 01:18 PM
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Yes, stock engine management with RRFPR for extra fuel under boost. I can probably use water injection and get away with the timing map. I also want to look into slowing the boost tip-in to avoid tip-in detonation. On hot days I believe I can feel a bog caused by a rush of boost to an engine that is not quite in the WOT map yet.
06-15-2012, 12:34 AM
#7
Three Wheelin'
Yes - that is the problem with the stock maps and a supercharger. You could run an Ostrich Emulator and reduce the higher load PT ignition timing. That is what I have done and seems to work quite well. It isn't very hard to accomplish. Suggest you also get a knock counter as well.
The knock is very susceptible to intake temperature. The stock ECU doesn't pull much timing for intake temps and probably does not really compensate enough with a supercharger. At 7psi and on a hot day, there is a risk that the engine will not be able to pull enough timing.
On my S2 I am pulling back timing up to about 5 degrees at 7psi/6000rpm to minimise knock. I think the DME can only pull 6 psi (not sure) so it is probably running a fine line.
The knock is very susceptible to intake temperature. The stock ECU doesn't pull much timing for intake temps and probably does not really compensate enough with a supercharger. At 7psi and on a hot day, there is a risk that the engine will not be able to pull enough timing.
On my S2 I am pulling back timing up to about 5 degrees at 7psi/6000rpm to minimise knock. I think the DME can only pull 6 psi (not sure) so it is probably running a fine line.
