Question re turbo and sustained top speed.

[Emission]

I'm brand-new to the world of turbos, and driving around this evening got me thinking. Theoretical here...

If I took my 930 to it's maximum speed in 4th, I assume it would need full boost from the turbo to keep it there.

Could I hold it at that speed for a significant amount of time, or would the turbo overheat (explode/whatever) after a few minutes at constant full boost?

What is the top end of a 930 without any boost? I would assume around 140-145 or so?

[beepbeep]

Yes, you can keep it there for prolonged time, just watch your oil temps. In warm climates you might be running pretty hot.

Top end without turbo? Hmm...who knows...why? :-)

930 engine has something like 160hp w/o turbo, so it wouldn't be to impressive i think.

[RarlyL8]

Someone posted a bit ago that their 930 registered 0.5bar at a sustained 140mph. The motor is built specifically to be turbocharged and will not perform well without it.
It would be my guess (I do not have a turbo so don't know) that the stock 4spd cannot redline in top gear. They are geared really high. At max sustained throttle the stock turbo would be pulling 0.7bar constantly. The high air speed might keep things cool.
Hopefully someone with a stock 930 that has actually done this will chime in.

[Tinker]

I have the car and the story, but not the firsthand experience. It was my former brother-in-laws 930, 77, non intercooled. He was `racing` a M6 out on the 5 on the way to Frisco in one of his moments. The way he told it: He was cruising at 80 - 100 and came up on the M6. The M6 gets on his rear wanting to play so my former bro drops it a gear and runs with the M6 in tow. He claimed he could walk away from the M6 up till about 150 - 155 indicated. I don`t know what the actual top speed was but he stated that it was all the car would give in fourth. At this speed, the M6 would catch and pass the 930. It pissed him off to no end. Any ways, he stated that they did this about three times (this was during the middle of a summer day when the ambient temps. were in the high 90`s) and on the third time the car lost power and became very audible (noisy). Pulled over and thought he had a miss. Now my former brother in law is a great guy, but was undeserving of this car and didn`t know the first thing about Porsches. Stops at a mechanic in Fresno (the car is barely running now) who though a plug blew out of the #3 cylinder. Wrong. Couldn`t correct the problem so towed it to a more knowledgeable shop who discovered the head had failed at the junction with the cylinder. Soooo. without mods I would not consider running a 930 at sustained boost. I read at the press intro in the 1970`s at the banked speedway in Texas they also wounded a few turbo car due to the same problem.

Tinker

[Formerly Steve Wilkinson]

Well, I was at that press intro, when I was the Editor of Car and Driver, and it was not in Texas but at the seven-mile (as I remember) oval at the Transportation Research Center in Ohio. I don't remember any of the cars there failing in any way, but I do remember driving one hands-off at an indicated 150 on the banking. Got a ticket from an Ohio Highway Patrolman on the highway leaving the track, too, though it wasn't for 150.

Stephan

[Emission]

All very good answers, and they align with my thinking.

I ask this question after hearing a rumor. The rumor said "...you could take a 911, change the oil, then run it Coast to Coast at redline without any problems."

I started to think that a Turbo may have problems at redline (under boost) for extended periods (especially Coast to Coast). I don't think it would be a engine issue - I assume it would be a turbo (overheat?) issue.

I know the 930 is heavier, and dirtier (aerodynamically) than the standard Carrera. With that in mind, I would assume it needs boost to even come within 10 mph of the top end of the standard car (standard Carrera does 145-150?). With boost, it should be able to sustain 155-160 (my car has a K-27 with 1 bar, so maybe I could get even more out of it).

I don't have the guts (or maybe I have too much brains) to drive a 16-year-old car over 130, but I'll take your word that it will do more!

Thanks for the other insight. I am especially impressed with a 150 mph run without hands!

[Doug Zielke]

Is this any different than a stationary engine, say as used in a diesel generator, running for hours at full load?

I've witnessed 100% generator load tests, where the turbos glowed like the sun. The tests could last for 12 hours with no complaints from the prime mover or it's turbos.

[GeorgeK]

I can relate there. I have an '83 930, with some tweaks.
It can run for LONG periods at sustained high speeds (above 140 mph), but what happens is that the oil temp slowly creeps upwards, and the boost pressure slowly drops.
I never had it near worrying temps, but in 20 minutes or so, I could see the temp needle creeping up and the boost wouldn't go above 0.8-0.9 Bar (mine can do 1.05) during reacceleration. For this activity of sustained high speed driving, you'd need a larger intercooler to retain full boost for any length of time.
GeorgeK

[sammyg2]

A couple things to consider:

a non intercooled turbo engine will make normal boost for a little while, until the entire intake tract develops heat soak. That is when the pipes etc. lose their ability to absorb heat because that are already hot. At that time, the boost charge temperature continues to climb above acceptable levels. That is not good for an engine.

Now lets look at an intercooled turbo.
The engine develops boost for a long time. If the intercooler is not up to the task, heat soak can again happen (stock intercooler).
With a larger aftermarket intercooler, the efficiency of the cooling can keep up with it and continue to cool the boost charge. unfortunately, most people who install an aftermarket intercooler also increase the boost pressures.
So..... assuming the oil cooling system is up to the challenge, the answer would be that if the boost levels are matched with the sustained intercooler efficiency you could maintain redline under boost.
Clear as mother's milk.

[GeorgeK]

Hi Sammy,
You hit the nail on the head.
In my case, I am in the situation where I need short bursts between hairpin turns in the mountain passes, so the boost isn't sustained for long periods. The situation of sustained boost is occasional at best. That is why I didn't go the larger intercooler way...yet
GeorgeK

[masraum]

I would think good intercooler and good oil cooler and it would be possible. Fortunately the way the turbos are geared the engine wouldn't be near redline when it topped out. I think I've seen somewhere that redline in a turbo would be around 200 if the power was there. Now an NA 3.2 car does top out at redline. I think a new Porsche engine if prepared/built to spec and properly broken in would easily be able to accomplish the task that you say. Didn't they do similar runs and break records on test tracks all over Europe?

Because of the reduced compression ratio of the turbo motors I doubt they would come close to a same year NA car if the turbo was removed or not functional.

[tobluforu]

I know this has nothing to do with porsches, but check this out. A 250 hp saab motor. Yikes!!

[911pcars]

So you want to keep your foot in it for a "prolonged" period of time. And you think it's going to be okay and not melt anything. And just because there's an intercooler and you watch the oil temp. every thing is going to be okay. And you also have faith. You can also do this if you have a large engine budget.

IMHO, more prudent types might want to afix an EGT/cylinder head temp gauge or equivalent to let you know what's going on inside the engine after mile 1 and proceed accordingly.

Sherwood Lee
http://members.rennlist.org/911pcars

[Emission]

Originally posted by 911pcars
So you want to keep your foot in it for a "prolonged" period of time....

Theoretical question. I never wanted/expected to do this, I just wanted to determine where the weak link is. As expected, it is not the Porsche flat-6 mechanicals, but the turbo and associated plumbing. That little air pump generates some SERIOUS heat!

It is apparent the turbo is designed for limited periods at full boost, not sustained full-boost high speed driving. With a larger intercooler, larger oil cooler, and proper instrumentation... hmmm....

[beepbeep]

Well i don't know about my 930 but i drove my ol' 240kmiles SAAB 9000T @ full chat for more than 15 minutes, doing around 230km/h w/o problems...popped the hood at gas station just to see how it looks like. Turbo and collector pipes were cherry red. Needles to say, i didn't shut off the engine while doing this...

Modern turbo engines can be driven at full boost for as long as you whish. Don't know about 930 but it has so low C/R that it should be practically unbreakable.

My Saab had 9.1:1 C/R and 1.1 bar of boost. Watercooling and big IC probably helps, tough.

[Mike the mechanic]

A turbo car uses its boost to accelerate (under load). It doesn't necessarily need full boost to sustain a constant speed, where the only load is wind resistance. I haven't tried this on a 930, but my own Carrera 3.2 turbo uses full boost to accelerate to top speed (5th gear rev limiter set at 7000rpm) but once the speed is reached, the load on the engine drops significantly and boost drops down to almost atmosperic. All my turbo cars were like this. The only cars that will run on full boost are cars that have undersized turbines on their turbos.
By the way, the 3.3 turbo intercooled makes about 135 hp without any boost.

Mike
86 Carrera t