Repost - octane booster question - more details

Bob McElroy rmcelroy at midcom.anza.com
Wed Jan 8 17:05:40 GMT 1997


<snip>
The car is a 1977 BMW 530i. The engine was rebuilt by a hi-po mechanic=20
<snip>
Going over the invoice for the engine build, I've discovered it has =
9.5:1=20
pistons, and a BMW 733i cam. It was also bored out a little, and ported =
and=20
polished. Stock pistons are 8.5:1, I think.

The engine compression is at 150-157 psi, right at the upper limits of=20
factory settings for a stock engine, in the BMW manual. I have recently=20
<snip>
replaced. WR8DC spark plugs (stock recommedation) at .25 gap. Fuel =
injection=20
is early Bosch L-jet, without o2 sensor, knock sensor or computer =
controlled=20
electrical advance. Distributor has mechanical advance, points replaced =
with=20
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=20
<snip>
Problems at present: With 87 octane, pings under load. With 93 octane, =
seems=20
to choke and hesitate under load, at 4-4.5 rpm with stock advance. Will =
run=20
upto higher rpm with more advance, but then idle and emmissions are =
lousy.
<snip>
Samir Shah
gofast at ix.netcom.com
Stamford, CT

_____________

Now I feel more comfortable stating my opinion (remember, this is my =
opinion and I am not an automotive mechanic.)  I think you need to =
upgrade your ignition system to take full advantage of the other =
upgrades you've done.  First, get a higher voltage ignition system (I'm =
sorry, I don't have enough experience to recommend anything.)  A spark =
plug gap of 0.025 sounds small by modern standards (your post stated a =
plug gap of 0.25, which is much too large for anything short of probably =
a top-fuel dragster.)  AFAIK, the correct gap for a GM HEI is .045, last =
I checked.  The size of the spark plug gap is directly related to the =
amount of spark voltage your system has got.  The more voltage you have, =
the larger the gap can be.  The larger the gap is, the quicker the flame =
kernel can expand and initiate combustion.

Next, adjust your advance curve to give more advance at the top end, =
while not affecting low end advance settings.  If you do install an =
aftermarket ignition upgrade, don't mess with the advance curve until =
the new ignition system is functional, otherwise you might end up =
needing to adjust the advance curve again.  The mechanical advance =
relies entirely on springs and weights in the distributor.  By replacing =
the weights and springs with different weights and different spring =
rates, you can control the rate of change (the slope of the advance =
curve) that the ignition timing advances.

When this is done, I would then try using 87 octane gas again to see if =
it still pings.  If it still pings, but the  pinging is very slight, you =
can try retarding the base ignition timing a little.  If this doesn't =
fix it, then you will need to use the 93 octane stuff.  If the engine =
pings on the 93 octane gas, you have the ignition timing too far =
advanced.  Back it down and slowly start increasing timing.  Sorry, I =
don't know of a good way to determine how far you can advance the =
timing.  I would suggest finding an experienced mechanic or tuner who =
can hook up a temporary knock sensor to your engine to set the amount of =
timing advance.  Some aftermarket ignition upgrades can be used with a =
knock sensor (you don't have to have an existing knock sensor.  I would =
highly recommend investing in a good ignition system with knock sensing =
capability, if it is within your budget.)

I read your post again before sending this and realized you may not be =
able to upgrade the ignition separately from the fuel injection system.  =
I told you I don't have much practical experience, and I don't think =
I've even seen a Bosch L-Jetronic fuel-injection system, let alone tried =
to understand exactly what it does and controls.  My advice is only =
applicable if you do *not* have an integrated control system (i.e.. fuel =
& ignition controlled by the same system.)  If you cannot upgrade the =
ignition separately, someone else will need to offer some suggestions.

I hope I've been able to help out, and not completely confuse everyone.



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"Two things are infinite, the universe and human stupidity"
"When everyone is in agreement, it signifies that no one thought very =
hard."
							       - Albert Einstein

Bob McElroy
Manufacturing Engineer
Midcom, Inc.
Waverly, IA
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