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theory on more boost
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16 w/porsche931  
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PostPosted: Sat Nov 10, 2001 5:22 am    Post subject: Reply with quote

i have a 80' 931 that is in the process of rebuilding. i am planing on putting on a larger radiator. and a intercooler to keep temps down. i want more boost so how does this sound. i was going to put a one bar spring in the waste gate and then put a presure regulator between the presure pipe and the waste gate( it will be pretty much a boost controler) so now i am getting more boost from the turbo. but what about the pop off valve. it releases presure in the intake pipe of the car if it gets over 10-11 psi if i read correctly. so you can boost 20 psi but the engine will never see it so my theory is the shim up the popoff valve spring( or close it completly and put a after market blow off valve there). then the engine will see the boost and will not lag cause it will still pop off just be able to hold more boost. just a theory love to know what you think. nate vincent
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cs  
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PostPosted: Sat Nov 10, 2001 6:51 am    Post subject: Reply with quote

i thought the pop off valve was set at closer to 20 psi. there's a few people on here running up around 15-17. i think rick maclaren and numbers. i would be more worried about the fuel cut switch, they don't seem to be that consistant and i wouldn't want to be running full throttle in third gear just to find out where the fuel pump cuts off at. my brothers merkur's wastegate malfunctioned in a way that sent the boost building to the 20+ range and when the fuel went it felt like we had be rear-ended.
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John Mueller  
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PostPosted: Sat Nov 10, 2001 2:11 pm    Post subject: Reply with quote

I thought the pop off was to protect the butterfly from breaking from getting hit with the boost when you jump off the throttle???
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cs  
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PostPosted: Sat Nov 10, 2001 5:40 pm    Post subject: Reply with quote

the popular reason to have one is to let the turbo keep boosting between shifts to lessen lag. but it also serves in protecting the intake from blowing hoses, gaskets or whatever.
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John Mueller  
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PostPosted: Sun Nov 11, 2001 3:44 am    Post subject: Reply with quote

I just read my post. I must have been asleep!
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16 w/porsche931  
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PostPosted: Sun Nov 11, 2001 12:22 pm    Post subject: Reply with quote

is the pop off valve set at 20 psi or so cause if that is the case than it is all good. also about the fuel cut off i heard somwhere that it is only on second series turbo 931. the ones with the new smaller turbo. and last does anyone know where to get after market universal intercoolers. i was thinking of getting one out of a eagle talon,mitubishi eclipes, or a crysler conquest. i think all these cars are intercooled. thanks
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cs  
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PostPosted: Sun Nov 11, 2001 7:56 pm    Post subject: Reply with quote

the talon, eclipse, lasers are all intercooled. they aren't that big but they flow decent and are good for the 931 in any practical situation. there are people running up to 300hp with them on the talon. they would probably work well if you moved your oil cooler and window washer tank and mounted it like it was originally. the conquest is a front mount intercooler if you were going to go through that much trouble i would use the 944 cooler instead they both cost about the same on ebay, but you can get the sidemount talon cooler for well less than a hundred most of the time.

[ This Message was edited by: cs on 2001-11-11 20:58 ]
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John H  
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PostPosted: Mon Nov 12, 2001 9:29 am    Post subject: Reply with quote

16w/porsche 931,
If you’re rebuilding your motor, a couple of hints.

Have the motor balanced, I had mine done in 1984 and it is now up to about 260,000 with out a rebuild. The motor has been raced for 14 years and runs up to 1.1 to 1.2 bar boost. (15 to 18 lbs) regularly and has run up to 1.4 (20 lbs). The only problem with running the higher boost is the fuel pump pressure switch cuts in the moment the boost hits the 1.4 bar mark.

I’ve found the safest way to turn the boost up is to mount a pressure reducing valve in a line from the inlet manifold and feed this to the chamber on the top of the waste gate.
What happens is the boost turned up by feeding pressurised air into the spring chamber. In order for the waste gate to open the valve has to overcome the resistance of the spring and then the additional resistance of the boosted air feed into the chamber. If you have the waste gate off the car and feed air into the unit you will see that it starts to open at about 5 lbs. With the air feed in the other side it will not open until the pressure on the spring side is less than the original inlet.
Also this method means that if you encounter low octane fuel you can easily turn the boost down and still run the motor without risk of denotation. Another safety feature is that if you have a “old motor” using a regulator in the original line you run the risk of getting oil and such like down the line and overtime it can cause a blockage – hence the motor can over boost.

I also make inter coolers for the 931’s using a core of a Japanese turboed car. These mount where the GT inter cooler is fitted and depending upon the core I can normally have them fabricated for around the US $500 mark plus freight from New Zealand to US. Most of the ones I’ve made have been for the series II cars. But I’ve done a couple for the Series I- these need modifications to fit the mounting brackets and to clear the vacuum mechanism on the distributor – I can’t do these as I have yet to see a Series I in New Zealand to make the brackets to fit. If you look at the following site you can see an example of a Series II intercooler. http://communities.msn.co.nz/924CarreraGTInfoSite/intercoolers.msnw.

The pop off valve in the turbo should have no real effect on the raising of boost as this is mainly to keep the turbo spooling while you change gear etc.
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TroyDest  
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PostPosted: Tue Nov 13, 2001 7:00 am    Post subject: Reply with quote

Nate,

Stop worrying about the pop-off valve. It doesn't operate like the pop-off valves you see on aftermarket turbos. The valve on the 931 is actually a bypass valve that only opens when there is vacuum (negative boost) in the intake manifold from closing the throttle. More boost has no effect on it whatsoever. By the way, replace the gaskets at either end of the valve when you have the turbo off. They disintegrate, leak, and cause a rough idle. I used viton o-rings (#227) on mine instead of gaskets and they are working great.

If you are going to run an adjustable boost controller, don't use the 1 bar spring. Stick with the stock spring so you have more adjustability.
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numbers  
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PostPosted: Tue Nov 13, 2001 8:52 am    Post subject: Reply with quote

You will have problems with your overpressure switch if you run boost over 15 PSI. I replace mine with a new one, and it still opened at about 15. This is the switch located at the front of your upper turbo duct, about half way between the lower duct flange and the throttle body. It has a single wire goint to it. This wire supplies ground to the fuel pump. If you are going to run more than 15 PSI, you will need to ground this wire to the head. I used the fitting where the switch mounts for the line to my boost guage.
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larso  
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PostPosted: Thu Nov 15, 2001 3:31 pm    Post subject: Reply with quote

Yeah i think there is a bad and good batch of those cut off switches. Some of them cut off at 21 PSI like they are supposed to, but some cut off at 15 PSI.

They are on ALL 931s tho if that's what you were asking.
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Patrick  
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PostPosted: Wed Jan 16, 2002 6:51 pm    Post subject: Reply with quote

Hey all!

I would just like to say that you can just unplug the switch and ground the cable.

No more worries!


Patrick
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Rick MacLaren  
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PostPosted: Thu Jan 17, 2002 3:54 am    Post subject: Reply with quote

Been there, done it.

Whoooooosh...hooo haaa...BOOOM!

DOH!

_________________
1980 Porsche 931 Bitched.
1979 Porsche 924 Tweaked.

[ This Message was edited by: Rick MacLaren on 2002-01-17 04:55 ]
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924RACR  



Joined: 29 Jul 2001
Posts: 8242
Location: Royal Oak, MI, USA

PostPosted: Thu Jan 17, 2002 6:40 am    Post subject: Reply with quote

Heh heh...

I gotta wonder if there's a way we can work in a replacement switch and circuitry if necessary as well...

Evil thoughts, I should shut up, I still have work to do on the racecar, and have yet to finish rebuilding the chassis on the turbo...

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Vaughan Scott
Webmeister
'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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Zuffen  



Joined: 31 Jul 2001
Posts: 1424
Location: Owasso, Oklahoma 74055

PostPosted: Thu Jan 17, 2002 7:30 am    Post subject: Reply with quote

I shouldn't say anything but you can remove the switch and replace with a pressure sensor(similar to a oil sender), then feed the sensor to a gauge so you can see what you got and calibrate, and then you can add a multiselector switch (1-5 for example) and have it be fed by the original sensor so that depending on what you electrical range read it has it can trigger a simple relay hooked to the same lines as the original switch, it can trigger the fuel pump to shut off at a preset value.

Lars or one of you electrical whizzes draw it up and tell us what radio shack parts to buy....Maybe that Auto Atlanta Oil/boost gauge could be used.



_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951
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