924Board.org

[Kenodog]

Can someone tell me if there is a formula for calculating how much boost I can safely run with an intercooler or do I really need a knock sensor to figure it out? Like if ambiant temp "A" is divided by intake temp "B" etc.? Lizard is trying to tell me that I can now run 15 psi safely on an everyday basis but i'm very skeptical of that.I imagine 10-12 on my "high" setting for the boost controller is about max with an everyday "low" setting of 8-9 psi. This won't cause excessive wear on the turbo or engine and will still be alot more fun than running at the stock 7psi. Am I wrong ? Can I SAFELY run 15 psi at sea level on a daily basis without drastically shortening my engine's life ?
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1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[flosho]

I would say you could run 10psi all day without worry. When I get my IC installed and even when I get my car back without the IC installed I will be running 10psi. Just put in 93 octane and you wont have to worry.

With an intercooler 12-15psi shouldn't be a problem and its still in the turbo's efficiency range so you'd be fine that way.

What kind of boost controller are you running?
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[This Space For Rent]

[Kenodog]

A TurboSmart two stage controller.Works awsome,you can switch from low to high on the fly with a switch on the dash.

You are telling me what Lizard is telling me about boost levels although neither of you can quantify your answers.I'm looking for hard facts not general suppositions.Thanks tho......
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1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[Smoothie]

Do you have the book "Turbochargers" by Hugh MacInnes (HP Books)? Interesting stuff.. I'd imagine if the formula you're looking for exists, it'd be in a book like that, but I'm not finding it there. Probably too many variables like piston design, fuel octane, timing curve, boost pressure, ambient temp, engine temp, barometric pressure, intercooler temperature drop as well as pressure drop, etc. that would need to be considered to calculate a knock point. My guess is that the Poindexters in Porsche engineering don't even try to calculate it with a formula, but instead rely on knock sensors and testing to find when it happens.
From my own experience here at sea level I can tell you that without intercooling and without water injection, my engine will run without knock (the humanly audible type of engine knock) at 12psi with 93-94 octane, but at 12psi and 87 octane it will knock. Then with my water injection in service and using low octane (87) it doesn't knock at 12psi - and I'm sure I could turn it up to 15psi or higher with water injection, but just haven't tried it.

Of course you'll want to get your fuel situation corrected before upping the boost any.
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[Lizard]

this is the way I look at this,

the 80 931 runs 7.5:1 CR, and in ROW cars they run 10 PSI STOCK,
the 81/82 931 ran 8:1 CR, and again ran 10PSI STOCK in ROW cars.
the 1980 CGT ran 8.5:1 CR, with a small imo inefficent IC (which was right above alot of heat) with 12PSI of boost.

there are also alot of 931 owners that I am aware of that run a 1 bar boost spring from a 930 without any issues, though I do not know what altitude they run there cars at.

the only formula out there is a book by Corky Bell which you will need to purchase and read to get that formula, (I plan on getting this book soon myself)

but I see absolutely NO reason that you should not be able to run 1 bar, (around 14-14.7 is what I have stated) with the IC being that it is seeming to be a fairly efficent IC setup.
esp if other cars are running that boost without an IC
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3 928s,

[flosho]

[81turbo]

I run 10psi on my non-intercooled car. It is just fine. I do however have an air-fuel gauge to keep tabs on the situation. I live in the bay area and am at about the same elevation as you.

I say stop being a pussy and run 15psi if you have that intercooler hooked up. I would!

[Kenodog]

All I ever use for fuel is Chevron's 94 octane.Every couple of months i'll throw in a bottle of Techron also.
I thought that the intake temp was wrong also but I just did a little test.
I have a VERY expensive,calibrated thermometer that I lay right on top of the I/C tube for half an hour and then went into the car and turned on the temp gauge and .....they read exactly the same,60 degrees F on both.(ambient air temp) This however makes me shake my head alittle because IF it is right and all is well with the gauge then the other day when it was around 75 F outside I was driving down the highway in 5th gear,120 kms/hour,just cruising no boost,and the gauge was reading 35 F going into the intake and 85 F coming from the turbo itself.It seems ah,well,ah,REALLY efficient.Almost too efficient if you get my drift and that's with no water injection (yet). Has anyone else actually gotten an intake temp that's LOWER than the ambient air temperature ??
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[Lizard]

also 81Turbo and smoothie have 81/82 model with 8:1 CR,

I believe that the ratio is 2 PSI of boost = .5 of 1 CR, but I could be wrong on that,
with the IC and high octane (which I know you run) w/ 15PSI should be fine.


and in Corcky bells book he does have a forumla for finding/calculating detonation point,

one other point is that the 931 head is a good design for boost and that piston shape doesnt have any sharp edges (valve reliefs) and those things are what lowers the detonation thershold drastically.

I also forgot to mention that there are 928 guys with S4 engines running 12 PSI of boost IC at a 10:1 CR and they do HAVE relief cutouts on the pistons, there are some that run 7 PSI on 10:1 without an IC without any issues. and most of those engines have over 200 miles on the clock, vs yours which has just been rebuilt.
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3 928s,

[Smoothie]

[numbbers]

OK guys, as most of you know, I probably run as much boost as anyone on the site who is still running a stock CIS fuel set-up. I run 17 PSI, without an intercooler, and have been running it for about 15 years. I just slowly started to increase the boost, waiting for detonation to occur. It never happened. However, I am in the Denver metro area, the mile high city. So, my advice is to slowly increase your boost level, until you get detonation, and then back it off a little. Just getting a little detonation once or twice is not going to blow your head gasket. But, if you jack your boost up too fast, you may get into the damage zone. Also, if you continue to run with just a little detonation, you will still damage your engine. I.E., any detonation is too much, so be sure to back it off. This is the same methoud that is used to determine the maximum you can advance your timing.
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1980 924 Turbo

[Kenodog]

No there unfortunately isn't supposed to be a 1 in front.The gauge starts at 30 then 50 then 100,200,300 and 400.In the books you say they talk about turbo temps being 200,250 & 300 degrees F but wouldn't those be larger compressors than ours and running higher boost levels ? I haven't measured the temperature at 15 psi yet so maybe it would be around the 200 deg mark who knows but because i'm only at 7-8 psi it reads much cooler.Also I haven't really driven it hard yet at higher rpms and boost levels so as quickly as it heats up it cools right down again.All I know FOR SURE is that when the car is cool it reads the same as an independent,outside source.I'm gonna go for a spin and do a little experiment,i'll be back.

And 81turbo i'm not a pussy i'm just cautious about wrecking something i've just spent $16,000,countless hours and a girlfriend on.
All in good time my friend,in good time.
Damned shame about the gf too ,she was a hottie.
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140

[john h]

I run around about 0.8 bar (11 pounds) most of the time with GT intercooler, and 96 octane gas (unleaded) - I've done this fro many years (14+) and no problems - for racing I run to 1 bar and 102 octane leaded gas again no problems and it's been raced at this boost for about the same number of years
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Remember a Porsche is not just for Christmas,

if you take it to pieces slowly it can provide anguish all year long!

[CBass]

[Kenodog]

K I just went for a rip and after switching the tc wires as suggested by the dealer .I do get a reading of 150 (F ?)at 11 psi from the turbo and 120 (F ?) from the ic.I'm gonna have a different aluminum heat exchanger made up this week from a buddy that owns a rad shop.This will signifigantly lower the water temperature.I think my problem is definitly not enough flow through the heat ex.Still 150 F at 11psi seems kinda low..It MUST be C !!!
That would be 302 F in and 248 out,sounds right to me.Gotta talk to the dealer again tomorrow.
Anyone want a 931,cheap?
_________________
1979 Euro 931, Olive
1981 931, Sabine
1991 Ford Ranger XLT 4x4, Ricky
1996 Ford E-350 ex-FedEx Van
2014 Mazda CX-5 (Kinderwagon)
2019 KTM 790 Adventure
2024 KLX300
2024 KLX140