Anyone else notice the 800 pound gorilla?
#46
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1991 928 GTs were not designed to have "RMB Motorsports X Pipe, RMB, 310 RWHP". I guess you better revert it to stock ASAP or your head gaskets are at risk. Two RMBs is just too dangerous for any 928.
#48
There are quite a few factors that influence detonation. Way too often people only think about one, and completely disregard all others. The most common example of this I see is when people think that they'll be safe from detonation if they just run the mixture really rich, because "richer is safer". Lower compression as a one step cure-all is just about as bad though. There are factory boosted cars with higher compression than the 928 engines have that don't detonate, and there are engines with lower compression that do detonate and blow up as well. Thinking along the lines of one aspect being the complete solution, and not taking all factors into account, is a pretty good way to end up doing an engine rebuild sooner or later.
#49
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Z: My car has been knock-free on the Spanner. I'm not at all advocating no tuning. I'm just suggesting that the problem is greater with GT motors based on my casual observations and experience.
#50
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Yes, you are right Z. The new Mini cooper turbo runs a 10.5:1 compression ratio.
Another thing that can eliminate knock is to increase the engines cooling system capacity.
BTW, there is a great explanation of this in Corky Bell's book. He explains why doubling the power of an engine with 1 bar of boost doesn't double the stresses on the engine compnents. He also explains why detonation and pre-ignition will destroy the engine in short order. I am out of town and cannot post the pages. Maybe someone else can.
Edit: Here you go. This is a PDF copy of Maximum Boost by Corky Bell. The detailed explanation begins on page 5. This is a large download that encompases the entire book. If you just want to read the pages I mention you can stop the download after 20 seconds or so.
http://www.freetrust.be/piaa1/Maximum_boost.pdf
Another thing that can eliminate knock is to increase the engines cooling system capacity.
BTW, there is a great explanation of this in Corky Bell's book. He explains why doubling the power of an engine with 1 bar of boost doesn't double the stresses on the engine compnents. He also explains why detonation and pre-ignition will destroy the engine in short order. I am out of town and cannot post the pages. Maybe someone else can.
Edit: Here you go. This is a PDF copy of Maximum Boost by Corky Bell. The detailed explanation begins on page 5. This is a large download that encompases the entire book. If you just want to read the pages I mention you can stop the download after 20 seconds or so.
http://www.freetrust.be/piaa1/Maximum_boost.pdf
Last edited by Fabio421; 10-17-2007 at 05:44 PM.
#51
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Are the heads o-ringed? I'm guessing they are as well as forged internals, and that would be a major difference when comparing to a 928 motor.
Last edited by Abby Normal; 10-17-2007 at 05:59 PM.
#52
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Flying Dog,
It's common knowlege that an RMB adds AT LEAST 10 hp, therefore TWO rmbs will add 20 hp. However, since you're concerned I'll remove one.
(I can't believe I've never noticed the redundancy in my sig)
I also have reassurances from Carl that his X pipe is a jenyooine Porsche piece they "forgot" to add to my car.
It's common knowlege that an RMB adds AT LEAST 10 hp, therefore TWO rmbs will add 20 hp. However, since you're concerned I'll remove one.
(I can't believe I've never noticed the redundancy in my sig)
I also have reassurances from Carl that his X pipe is a jenyooine Porsche piece they "forgot" to add to my car.
#53
Some of the newer factory boosted engines are running the higher compression ratios, using head gaskets with no "O" ringing, using cast pistons, and are doing it with a 14.7:1 full throttle air/fuel ratio. I'm not talking about just the relatively new direct injection engines either.
#54
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Some of the newer factory boosted engines are running the higher compression ratios, using head gaskets with no "O" ringing, using cast pistons, and are doing it with a 14.7:1 full throttle air/fuel ratio. I'm not talking about just the relatively new direct injection engines either.
#57
Race Director
With modern engine controls and knock sensors most modern turbo or SC engines are at least 9.5CR....some are even higher.....
So a 10.0 CR 928 motor with its dual knock sensors should be fine once the timing and fueling is adjusted to the boost level....
With that said, the motor is still an open deck 4 bolt main design.....pleny strong for most NA applications...but a bit weak for a high boost or Nitrous scenario..... There is a reason many racing engines are closed decks with 6 bolt mains.... Chebby rates its new LSX 6 bolt main small block to 2500hp and 45psi!!!!!
So a 10.0 CR 928 motor with its dual knock sensors should be fine once the timing and fueling is adjusted to the boost level....
With that said, the motor is still an open deck 4 bolt main design.....pleny strong for most NA applications...but a bit weak for a high boost or Nitrous scenario..... There is a reason many racing engines are closed decks with 6 bolt mains.... Chebby rates its new LSX 6 bolt main small block to 2500hp and 45psi!!!!!
#58
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The problem here is we all have different opinions on what "high boost" is. To some it's any amount of boost, to others it's 20psi, in Z's eyes life doesn't begin until 2 bar over ambient
#59
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Also, Porsche overstated the CR on most 928s including S4s and GTs, which are closer to 9.3-9.4:1.
#60
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I am actually quite glad this thread came along. I have been wondering about adding a SC to our 84 16V and many of the above posts help articulate the vague concerns that have drifted around in my head.
I would love to feel the surge of a 300+ hp shark. But my gut tells me that not only does the car have to change when a blower is put in, the owner's sensibility has to change. That is, superchargers aren't simply for people who want to go faster. They are for people who want to go faster and who are inclined to do regular checks and tweaks to ensure proper ignition and performance. That to me, more than the cash outlay, is the buck for the bang.
I would love to feel the surge of a 300+ hp shark. But my gut tells me that not only does the car have to change when a blower is put in, the owner's sensibility has to change. That is, superchargers aren't simply for people who want to go faster. They are for people who want to go faster and who are inclined to do regular checks and tweaks to ensure proper ignition and performance. That to me, more than the cash outlay, is the buck for the bang.