FlyingDog

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.

Abby Normal

Getting better by the page!

Z

You mean it only had audible pinging once, which is definitely not the same as saying it only had detonation once. The biggest reason for problems with these or any forced induction installation is that people think that if they don't hear knocking, or if they have knock sensors, everything is great, and there's no need for any kind of further tuning or modification. Then when something goes wrong, blanket statements like "supercharging causes head gasket failures" get made. If anyone wants to volunteer their stock normally aspirated car, I can show you how poor tuning can blow a head gasket on it in probably one trip around the block, with just some poor tuning and maybe some basic setup differences done. There might even be some broken pistons, a cracked block, or cracked heads as an added bonus. No boost needed.

You're also running non-intercooled with a roots type supercharger, which is the least efficient kind of forced induction compressor in common use. I absolutely guarantee that there are both boosted and non-boosted cars out there right now with engines that are damaged from poor tuning, possibly along with poor system design, and their owners don't know it yet, and think everything is just great. Just because it doesn't go bang in a big dramatic fashion right away doesn't mean it's that it's okay, and that damage isn't being done. It's the equivalent of someone thinking that their rod bearings will always be fine when tracking the car because they just finished one 15 minute track session and the rod bearings didn't catastrophically fail. I'm not posting this to try to scare anyone, rather I'm doing it to try to get people to stop doing things that will cause damage, and to start thinking and being a little smarter about it.

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.

Bill Ball

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.

Fabio421

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

Abby Normal

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.

ErnestSw

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.

Z

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.

FlyingDog

SAAB has been using CRs of 9.5:1 and over for 10 years. Some of the new turbo Audis are 11.4:1 IIRC.

928drvr86.5

Here is an article on examining head gasket failures as taken from the Corky Bell Supercharged! book.

928drvr86.5

the second half

IcemanG17

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!!!!!

hacker-pschorr

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

FlyingDog

Compared to an advanced boost-aware ECM like SAAB Trionic, the 928's knock sensing ability is crap. Since the computer isn't smart enough, the tuner has to be.

Also, Porsche overstated the CR on most 928s including S4s and GTs, which are closer to 9.3-9.4:1.

85fortheDrive

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.