AO

I firmly beleive the 928 engine can handle 15+ psi of boost (assuming everything is in good shape to begine with) as long as detonation can be eliminated - but that's not an easy task.

hacker-pschorr

Fuel igniting without ignition is a form of detonation called pre-ignition. This can happen at any boost level if the car is not tuned properly. This has nothing to do with boost level.
It’s the worst form of detonation since once the knock sensor picks it up, it’s usually too late.

I almost never see anyone talk about intake air temperatures. This is one of the #1 cause of pre-ignition, yet almost none of the boosted world (not just here) ever measure it. It was the first sensor I added to my car.
You have no idea how well (if at all) your IC is working without measuring intake air temp. Most skip this since doing it properly is not cheap. It either requires a very expensive gauge (mine cost $250) or the proper data logging equipment with a proper sensor. Your run of the mill temp sensor is not adequate (reacts too slow).

I'm still shocked how many boosted people are running stock spark plugs...

You will reach a detonation limit with pump gas before you reach any sort of boost limit on a 928 engine.


As for the 931...it's CIS with mechanical controlled ignition. Not a good comparison to a 928.

Dennis Wilson

Andrew,

Agree you could probably get by with 15 lbs (not 30-40) as long as there were no gotchas like a failing fuel pump, bad relay or corroded wires/fuses. I'm running 1 bar (14.5 lbs) on my 931 (designed for .6 bar) and 1 bad wire at the computer cost me $1,500 to repair the damage and that engine was built to turbo spec's.

i.e. There needs to be some redundancy built in like an IC or methanol injection if you run that much boost.

Dennis

AO

Don't get me wrong... 30psi is an achievable goal with the right equipment/tune. Like Hacker said, intake temps are critical, as are proper AFR's, ignition, etc. etc. The "recipe" needs to be right otherwise you'll end up with mush instead of a souffle.

Dennis Wilson

Hacker,

The series 2 931's are CIS but with the lambda fuel and computer controlled ignition. There is an air intake temp sensor, boost pressure sensor, A/F (O2) sensor and crank position sensor all feeding data to the computer (DITC). Wouldn't call that mechanical.

Dennis

Mongo

The biggest restriction I feel to any forced induction mod on any car is pump gas. It's also my fear if I ever want to go forced induction. 91 octane will not cut it with a supercharger. There are guys here with blown motors of course living in CA, but gas over scares the crap outta me

hacker-pschorr

Ok, I stand corrected. I still wouldn't compare that system to LH/EZK while using a sharktuner. Or better yet what Todd is using - Autronic.

atb

Andrew, this statement does not take into accout the BFSEUAAC (built for someone else's use at a charge) factor, which reduces the effective psi rating that an engine can consistently handle by 30%.

Z

There are some people that would disagree:

www.procharger.com/chevy_nova.shtml
http://youtube.com/watch?v=ZtaYdzoCzl8

Mongo

that depends on boost Z. I'm sure 5-7 can be done, but levels above, I'd be extremely cautious. These cars originally weren't designed for forced induction, hence my theory of "don't blow something that isn't meant to be blown."

hacker-pschorr

As long as the engine is tuned for whatever octane it will be using, you will be fine. If you take an engine (of any kind, boosted or not) that is tuned for 93 and dump in 87, bad things can happen.
Is your threshold lower with lower octane fuel? Absolutely it is, that doesn't mean you still cannot build a boosted motor.

Tune for what you have available.

I know people who tune their track cars on low octane fuel because some areas they run, good gas is not available.

Some of the motors I see are running such an awful tune on high octane gas, if they cleaned up the program they would probably run just fine on low grade.

Mongo

And Sharktuning an RMT 928 would get it dialed in right? Count me in then if this really works reliably!

AO

Too soon to tell, but if the tune is correct, then yes, you should be fine.

Z

I can't even begin to guess how many times I've seen the boost go to between 10-15psi on a number of different 100% stock 928 engines that were using only pump gas and nothing like methanol injection. That's if things are set up and tuned properly. Without knowing what you're doing, or not having things set up well and tuning correct, even normally aspirated 928 engines making stock horsepower have blown up.

AO

+1

I learned a lot last weekend. It's all in the tune (and making sure you do a proper sponge count). If you don't tune it right... you'll blow it up - even on a low boost engine.

If you can control detonation and pre-ignition, you can go a lot further with your mods and your car will be much better off as a result.

Just don't leave anything in your intake.. Saw a Mitsubishi 2.3l motor ingest something while making 650WHP. One piston is now heavier than the others, and missing a spark-plug electrode!