facboy

what might actually happen with the LBE set to 10 psi on an otherwise stock '88 951? i read that it's not a good idea, that you might blow the head gasket etc if you run too lean at high rpms, etc.?

my question is, will i know there is a problem because the overboost protection cuts the fuel (eg every now and then), or might other things go awry without this happening?

i've only had the LBE in for a couple of days, and i've dialed it down to 9 psi atm. at 10 the stock boost gauge would hit 2 bar at WOT around 4000-5500rpm (guessing), and would tail off after that to around 1.6-1.8. At 9 psi it hits around 1.9 bar or so instead, and i think it tails off still (haven't really caned it since i dumbed it down).

my mechanic says that it should be fine at 10psi with everything else stock, and he is generally regarded as being one of the best Porsche mechanics in this city (melbourne, aust). never heard a bad word about him, and he's had pretty extensive experience with the LBE as well in several other customer's cars (not sure if they are stock though). he runs one in a chipped '86 951 (among other things) and has it set to around 14 psi.

lindsey racing, however, recommend that i set it to 7 psi. presumably, this would result in the boost maxing at 1.6-1.8 again. would the fact that in aust we run 98 octane fuel make a difference either way (my understanding is that the best fuel available in US in some parts is only 91 or 92 octane)?

what are the thoughts of the experts at rennlist?

cheers,

chris.

Danno

Yes, with stock chips you want the LBE set at 7psi to give you stock boost. The issue is with getting a proper air-fuel ratio, regardless of your octane rating.

facboy

okies. sorry for being obtuse (i'm a bit of a car newbie), but having an incorrect fuel-air ratio means i will be running too lean, yes? what kinds of problems does this cause? detonation, i assume, but is there anything else.

soz, but just trying to understand what kinds of factors are involved when you modify bits and pieces.

Danno

Yes, lean mean there's more oxygen in the chamber than hydrocarbons to burn with it. As the mixture is burning, this extra oxygen causes the mixture to burn faster and hotter. The extra heat can overwhelm the knock/detonation limit of your fuel. And the extra oxygen combined with the extra heat will also try to react with anything else that's available, like your headgasket and valves!

Danno

darn duplicate posts again... <img src="confused.gif" border="0">

facboy

are these things (detonation, anything else nasty) that the overboost protection (or anything else) in the DME (KLR?) picks up? what exactly does the overboost protection detect?

i was running the LBE at 10 psi for about a day, and the extra performance was quite astonishing. this was on the street (purely a street car atm), so afaik i didn't do things like rev to 5500-6000 in 3rd gear, which would have put me quite a way over our rather draconian speed limits (did have WOT though). i didn't notice any pinging or cut-out from overboosting, or anything at all really. apart from the increased performance, obviously.

if i were to keep running it at 10 (i've dialled it back for now), is it likely that things would start to go wrong over time? i guess what i'm asking is, if i'm over-stressing various bits and pieces atm, will i know about it now, or will it just deteriorate slowly over time without my knowledge until it all falls in a screaming heap one day?

btw, regarding detonation b/c of extra heat/fast burn caused by running lean etc - my understanding was that higher octane fuel is something that might prevent detonation that would otherwise occur. is this right?

adrial

If you leave it at 10, your car will probably end up blowing up.

Yes thats exagerated, but without chips it is very unsafe to run the LBE set to 10psi.

If you get aftermarket chips and set the LBE to 10psi, you'll be good to go...

facboy

sorry again for being thick, so to be perfectly clear, the car will not be smart enough to tell that something is wrong, and there are no obvious signs that something is wrong, until something is REALLY WRONG?

getting on to aftermarket chips - i read somewhere that u should get chips specific to the country you are in (in this case australia), as differing fuel quality/air quality etc can mean that what works nicely in one environment ends up being quite bad elsewhere. is this true? i believe there are local outfits that will do a custom fuel remap or something with a piggy-back ECU (i think) - i think it takes about 4 hours. something like they do a dyno measurement and calculate correct a/f ratios every 200 rpm or so.

Beamz

Hey facboy have been reading your posts, i am in Melbourne too, and at the moment have my 86 951 at the shop getting the air con fixed. BUT on its return i have a new LBE in its box ready to go on. IM running a stock everything so dialling down sounds like the go. My car was Purchased and is serviced by WELTMEISTER porsche. Would like to hear of any outcomes you may encounter with the BOOSTER.
CHEERS!

Danno

" i read somewhere that u should get chips specific to the country you are in (in this case australia), as differing fuel quality/air quality etc can mean that what works nicely in one environment ends up being quite bad elsewhere. is this true?"

Well, I think air-density in Oz is the same as anywhere else on the globe, so the fuel-maps don't have to be different. The only thing you need to adjust is the ignition maps. There are chips out there that gives you 4 user-selectable ignition settings to make the most of the worse pump gas in the world all the way to 110-octane race fuel...

facboy

hmm okies...well i have decided to play it safe for now and dial it down another couple of notches until i chip the car. btw, i thought that varying humidity and temperature between different locations would change the effective air density (how much O2 is going in)?