Repost - octane booster question - more details

[kleenair at ix.netcom.com]

gofast at ix.netcom.com wrote:
> 
> 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.
> 

Your best bet is to put the sensor closer to the engine.  An alternative 
would be to use a CO / HC analyzer.  I would also mount a fuel pressure 
gauge to monitor WHILE ITS ACTING UP.  Rigging up a timing light just to 
check for plug firing would also be useful.   Also,  did you already try 
swapping coils between the two cars?

Best Regards, Mazda