Repost - octane booster question - more details

[gofast at ix.netcom.com]

Wow! Thanks for all the responses to my earlier post (which I've included at 
the bottom. I think I need to be more descriptive about my car and what I've 
checked so far:

The car is a 1977 BMW 530i. The engine was rebuilt by a hi-po mechanic 
before I knew anything about cars, and suffered being in storage for many 
years after the original car was crashed. I've recently transplanted it into 
another 77 body, and am now trying to make it run well. The engine probably 
has a total of about 30-40k miles on it.

Going over the invoice for the engine build, I've discovered it has 9.5:1 
pistons, and a BMW 733i cam. It was also bored out a little, and ported and 
polished. Stock pistons are 8.5:1, I think.

The engine compression is at 150-157 psi, right at the upper limits of 
factory settings for a stock engine, in the BMW manual. I have recently 
'rebuilt' the intake system and replaced all seals, gaskets and hoses, and 
put in new fuel injectors (stock).  Also did a valve adjustment. Fuel filter 
replaced. WR8DC spark plugs (stock recommedation) at .25 gap. Fuel injection 
is early Bosch L-jet, without o2 sensor, knock sensor or computer controlled 
electrical advance. Distributor has mechanical advance, points replaced with 
Allison XR700 optical trigger. Stock Distb advance is 22 deg BTDC @ 1700 
rpm, goes to 32 degress at 3500 rpm (I think). Fuel pressure is 32 psi at 
idle, 41 psi without vacuum. Stock is 2.5 psi FPR, 36 psi +- 3 psi, so I'm 
only a little short. Plug wires are stock, about 1.5 yrs old. Pulling each 
plug does cause a change in running, so all plugs are firing, at least at 
idle. All smog devices removed.

Problems at present: With 87 octane, pings under load. With 93 octane, seems 
to choke and hesitate under load, at 4-4.5 rpm with stock advance. Will run 
upto higher rpm with more advance, but then idle and emmissions are lousy.

What I've done so far: I also have a 76 530i, which runs well, so I've been 
moving parts back and forth. There has been no change in performance with 
the coil, distb, AFM, or computer from the other car, suggesting those 
components are working. Went back to points; no change, so allison is 
probably OK. Changed the spark plugs, no change. Replaced all the intake 
seals, no change. Tightened AFM spring - lost some power. (Need to reloosen 
AFM spring and try that).

Earlier, I thought I might be running out of gas at higher rpms, as 
suggested by archives and the Alfa l-jet site (I dont have the address handy 
at present to give the appropriate credit), but have not been able to find a 
rising rate fuel pressure regulator at an affordable price.

I read in a book put out by the BMW club, in an article by Jim Rowe of 
Metric Mechanics (BMW Enthusiasts Handbook?) that 9.0 was max p iston 
compression recommended for use with 2002 with pump gas, or 'you'd be 
buying gas at the airport'. Which is why I posted the question below, in 
case I had an engine that was not meant to be run with pump gas. The folks 
on the BMW digest have been very helpful, but they are nowhere near as 
technical as this group, and only a handful have any experience with the 
older cars.

BTW, can an o2 sensor be used as a sniffer at the tailpipe? Or is the 
exhaust too cool by then. What if I use a 3-wire and heat it by connecting 
all 3 wires?

Thanks again, for the space, time, and feedback. You've already made a 
number of very useful comments, dont know if this info provides any more 
insights, but I'll appreciate any further thoughts.

The original post:
Hi guys. I *think* I am running out of octane in my BMW 530i - I have high 
compression pistons, and the car is choking at high rpms. Have checked most 
of 
the obvious things like fuel pressure, filter, etc.

I read here that xylene or Toluene is the way to get more octane. How much 
should one add to each tank?

What is the minimum octane I would need with 9.5:1 pistons?

Thanks in advance.


Samir Shah
gofast at ix.netcom.com
Stamford, CT