Couple of good questions there

1. Max VE in fact corresponds to max Torque, which is also typically near the max
thermal efficiency for an engine. The best VE rpm is often the best 'cruise speed' (as
used in boat or aircraft engines) for travelling the greatest distance on the least fuel.

Horsepower is derived from the equation (Torque X rpm) / 5252. In the olden days
that was foot-pounds /min which gave you (force X distance) / time - also known as
power. The best horsepower point is a function of airflow, as influenced by the
plumbing around the engine plus the shape of the cam and the size of the valves/
pistons/etc.

GM introduced the small block V8 in 1955. And many of the parts from that first
engine would bolt right up to yours today without change at all. Thus there is quite
a history of literature that speaks to modifying the thing - and by WAY better engineers
than me. A couple of very good authors/builders/racers include John Lingenfelter and
Dave Vizard. I feel sure their books have been translated into Oz-speak by somebody...

Unlike me, those guys knew what they were talking about. They even calculate stuff
in grams and meters and furlongs, and other such heathen talk.


2. Regarding air flow, there is another factor you neglected - manifold pressure. The MAF
meter is looking at atmospheric air; which is throttled by the butterfly valve to lower press.
The air density (and hence mass flow) is proportional to its Absolute Pressure as well as rpm.

Thus the pumping curves for the engine are compound curves, on the axes of rpm and manif
press. Volumetric Efficiency is the 3rd axis.

The base pulse width GM contemplated is centered around 14.7 to 1 wt of air to wt of fuel.
The 256 bits in the tables are in fact scalers, to modify fuel (inj pulse width) to match the ACTUAL
air flow at any given operating point. The base point is typically in the center of a scale, allowing
fuel to be added/subtracted to keep the AFR steady. The base pulse width can be scaled up or
down by +/- 128 counts - that often corresponds to +/- 15%, which is the rangability of the GM
stock control design. By modifying the basic shape of the fuel map, tuners are able to accomodate
larger cams, valves, intakes, etc - while keeping the design flexibility of the origninal GM
control scheme.

How that is done in a MAF table I leave to somebody else to explain. When I look at what GM
did to write that program my brain starts to hurt. Somebody way back when thought GM
developed it from the original Bosch Jet-Tronic hardware, or some such. Which would explain
why it's so complicated.


3. By way of contrast, MAP fueling is quite elegant - at least for a Chem E. A brief discussion is here:
http://forums.corvetteforum.com/c4-s...-equation.html


To learn more about your ECM and program you'll need to look up a commented hack of your BIN.
Check around over on the 3rd Gen PROM board to locate one...


4. Final Thought: Evetrybody worries about fueling curves, partly because they are easier to
visualize. And as you get into major cam and head changes you DO need to heed the fuel map.
But what really affects power and smooth operation is the spark map. And the only way to sort
that out is by MUCH road testing/measuring, or a LOT of time on a variable-load dyno.

Go now, and blaspheme no more against Isaac Newton by confusing work and power ...

 

I think you are talking about the Block Learn Multipliers (BLMs) here. There are 16 BLM cells which automatically recalibrate the fuel maps when any mods are made to the engine. Pity they didn't apply the same thinking to the spark. Then there wouldn't be any need for this trial and error tuning.

 

1. I may not have clearly explained fuel control. The ECM can adjust fuel rate ~ +/- 15%
from the map that matched GMs STOCK engine design. If a new engine condition exceeds
that range of control, the ECM will not do you any good AT ALL unless you change the
base map (or some actual hardware).

BLMs adjust inj PW about a DESIGN POINT - They can't recalculate how the engine was
built, or fuel quality, or injector size, nor anything else. The ECM is a computer from the
early 1980s, not from the Starship Enterprise.

Try it this way - the analog signals in the ECM are all 0-5 volt. Sending the ECM a 10 volt
signal won't make the car run twice as fast, 'cause it has no idea what 10 volts means.


2. Regarding timing, the point was made in a previous reference that ideal spark for an
engine produces max cylinder pressure @ ~15* ATDC (in general terms). Achieving
the optimum spark time changes with atmospheric conditions, fuel quality, engine
condition, RPM and load. There is no instrumentation on most cars to MEASURE cyl
pressure. Lacking a reference point and a feedback loop, there is no way then to make
the timing map correct itself, save the very crude 'knock sensor' that began this dialog.

I understand that some new cars are built with per-cylinder DSP knock sensors or ion-
detectors(?) that endeavor to adjust spark timing on-the-fly. You will not find this on
C4s.


3. We really have come to the end of my memory on C4 programming details here. It's
been years since I tracked all this stuff down for a car that I bought 20 years ago.
Electronic gizmos and computers moved on since then, and so has my car collection.


I recall that GM runs a PID controller for fuel adjustment, with updates from the EGO
device @ 80 / sec. The integrator function (at least on later ECMs) is kinda in two layers:
a free-running integrator count for short-term corrections, which triggered changes to
the long term fuel trim (the BLM) every two seconds or so. The PID had programmable
reset rates, and proportional gains for both positive and negative corrections to PW.
There was even a time delay to account for transport of the exhaust gas between the
cylinder port and EGO sensor...

Pretty much all the tables in Assembly work from a midpoint (offset)of 128, since it can
not deal in numbers less than 0. To subtract fuel (or timing) you had to start from an
offset point that was greater than any subtraction to be made.


Sorry I can't be of more help.

good luck

 

Thanks for your very insightful comments. There is not a lot written on the fuel control system that I can find so much appreciated.

The 16 BLMs and the INT are the closed loop controls with an adjustable AFR target of 14.7:1. I think you implied that the non-adjustable AFR target during open loop mode is 10:1. I have had the EGR deleted so not sure about your comment on that in relation to the INT.

I understand completely that control systems have their limitations and work within a range. Mods, fuel and atmospheric conditions generally have to stay within those ranges to ensure tunability. I have seen on the forum for instance that modded higher horsepower motors are exceeding the MAF limits / range at WOT and have to be tuned using other fuel enrichment tables.

As far as timing goes I would be more than happy for the c4 ecm (if it was able) to continuously monitor and adjust the spark map based on the knock retard I have experienced as per the graphs above. It would save me having to do all thoses driving tests, datalogs, analysis and re-burning chips all the time.

My comment about 'total' self-tuning engines was not directed at the C4. Control systems and computers have come a long way since then.

 

This is the latest result from all advance being zeroed from the above table:



I still have a little bit of knock happening during WOT so I am going to have to find another table to retard the timing.

 

Finally I have managed to stop the knocking at wot by taking a bit of timing out of the main spark advance table for the high LV8 column.

 

How Much Ignition Timing (BTDC/before top dead center CRANKSHAFT DEGREES + INITIAL BASE TIMING AT DISTRIBUTOR) are You able to Use Without picking up knock counts-detonation At WOT ?

From a tested RPM Range of 2,000 - 5,500- 5,700 RPM's.

Is Your L-98 still Mechanically close to stock ?

Engine still retain the factory GM "128" Aluminum Cylinder Heads?

Still using the basic GM TPI system ?

Have a 1987 Roadster like You do.

I have been experimenting with factory ESC / Electronic Spark Control Module and Knock Sensor on my car for last couple of weeks.

I have found the factory ESC on 87 corvetes to be over sensitive at times to false spark knock and retarding timing immediatley.

 

My 87 is stock but the 128 Al heads have been shaved and I am running the higher octane pump gas. I was getting knock at WOT so I have backed off the aggressive timing in the ARAP 89 bin I am running to address the issue. The timing curve at WOT ranges from 22 to 30 degrees total advance as below:

 

What does the bottom row of numbers represent ?

Is it the length of time in seconds the engine has running at WOT ?

I have seen very similar spark timing results on my 87 Vert lately driving around with my Snap On Scanner.

What Octane of Gasoline did You make Your tests with ?

How much was shaved off Your Cylinder Heads ? In inches, Millimeters,
or CC's measured with a 100CC Glass Burette.

Was it 87, 89, 91, 93, or 100+ Octane Race Fuel ?

Do they add 10% ethanol alcohol to the gasoline in Australia too ?

They do in most parts of the USA, and do add 10% alcohol to the pump gas here in Illinois State everywhere I drive to.

Its hard to find 100% pure gasoline here unless you purchase 100 octane race fuel.

Wondering if the 10% alcohol is causing us Premature Knock counts and thus the timing is quickly and automatically retarded.

In other words, the 10% alcohol pump gasoline has very poor Anti-Knock Qualities. Compared to 100 % pure gasoline of the same Motor Octane Rating.

Our C4 Corvettes were Originally designed to run on 100% pure 91 or 93 Motor Octane gasoline.

And also the spark map tables programmed into the ECM - PROM setup with Aggressive Spark Curves were meant only for 100% Pure gasoline of Proper Corvette Recommendations.

Goodnight.

BR.

 

The bottom row of numbers in the graph are sample numbers. There is about 7 samples per second so the WOT event is about 5 sec.

Fuel used is the best pump gas in Australia 98 Octane which I think is equivalent to about 93 in the way the US does its octane ratings. I don't believe there is any alcohol in it:

"BP ULTIMATE OCTANE 98 minimum
BP Ultimate is the highest octane retail fuel that BP has on the market. It has an octane rating between 98 and 100 and does not contain ethanol. It is suitable for all cars but older pre 1986 vehicles designed for leaded fuel may need to add an additive to prevent valve seat recession. BP Ultimate also contains a high dose of additive to protect the fuel system and to remove deposits that foul intake valves and the combustion chamber causing poor combustion and knock. BP Ultimate is better than required by the Australian Fuels Standards of 2000 for Premium Unleaded Petrol. Cars with knock sensors and engine management systems can take advantage of the higher octane to develop more power and performance. Older cars that were originally using super petrol were designed for 98 octane fuel and so can be tuned back to original performance with BP Ultimate."

The shave to the heads was basically as a result of servicing the heads. Not sure how much but it seems to be significant. My cranking pressures are up as follows:

cyl1: 185
cyl2: 190
cyl3: 192
cyl4: 181
cyl5: 190
cyl6: 182
cyl7: 190
cyl8: 205

 

Your Engine is in Very Healthy Condition. Excellent Cranking Compression Results.

If they just resurfaced your cylinder heads during a valvejob - rebuild,

Likely only about .005 - .010 " inches was removed. You would have about 2 or 3 cc's removed from the Original Combustion Chamber Volume.

Might see about a .10 or 1/10 compression increase over the factory rating. Not enough to cause tuning issues with aluminum cylinder heads I believe.

In the USA, they rate gasoline at the service station pumps using the R.M./2 method. Its the average octane result using the Research Octane Stated and Motor Octane.

Whenever I purchased Racing Gasoline (100 octane or higher), I just watched for the Motor Octane tested result.

Its much more accurate listing for the Anti-knock quality of that particular gasoline.

What time is it in Australia right now ? Its 12:12 AM here, Central- Chicago time. Date is 10-10-2010.

I am pretty tired, been a busy week repairing cars and trucks, and into the late night last couple of evenings.

I will think about our Corvettes some more in my sleep.

Goodnight Or Good Morning There in Australia !

BR

 

1:12pm Sunday. You are 13 hours behind.

 

When you set timing for no knock you are setting it to keep the worst cylinder quiet, robbing timing from the others.

 

 

If You have a chance this week, Purchase some more of the BP "ULTIMATE 98" from your local gas station that you fill up your 87 Roadster with usually.

Bring a Clean like new gas container and fill it up with the BP Ultimate 98.

When you get home find a Clean glass container like an old Jelly/jam Jar with a screw on lid.

Take the BP ULTIMATE 98 gasoline and fill the glass container to about 2/3rds. full and screw the lid on.

Don't be smoking a Cigarette !

Now shake the gasoline filled glass jar a few times and set it down a table. Watch for the next 10 or 15 minutes.

If there is alcohol in the gasoline, like here in the USA,

the alcohol will gradually separate from the gasoline.

You will see the alcohol floating above the gasoline in a proportioned amount.

I have been told that the alcohol is 100% soluable with gasoline, but whenever I have checked using a glass jar with a sample of GASOHOL,

I HAVE SEEN THE ALCOHOL AND GASOLINE SEPARATE OVER 10 TO 15 MINUTES.

Perhaps because Ethanol Alcohol, wood grain alcohol, and methanol alcohol absorb water humidity so readily from the air.

My gut feeling is that the 10 % blended gasohol gasoline here in the USA is not very Knock Resistant Fuel like the manufactures of it claims it to be.

Might be the same way in Australia where you purchase gasoline for your Corvette.

My Corvette still has the factory GM precats and the Main Undercarriage catalytic converter.

Otherwise I would have tried 100LL Aviation gasoline or 110 Octane Leaded gasoline a while back to see if that eliminates all knock counts and timing retards I see on my Snap On scanner.

I think Australia is rating the BP Ultimate 98 using the "Research Method"

Research numbers are much higher than MOTOR OCTANE OR THE RM/2 METHOD USED IN THE USA.

Going to remove - eliminate all 3 catalytic converts soon from my car, using a 1985 Corvette headpipe with no precats unlike the 1986-1991 L-98 Corvettes use. And eliminate the Main Converter too

I live far enough away from Chicago now, I don't have to worry about emission/smog checks.

Hopefully pick up a little more midrange and upper end WOT power too.

Empiricle testing is what I use at times, or trial error with your knowledge and experience from the past.

BR

 

I have a jerrycan with 98 in it already so I can test it as you suggest.

I have just removed the stock exhaust manifolds complete with modified y pipe with precats removed to put the LT Headers on. If you were near by I could just give that to you as I have no use for them. Probably not worth shipping it across the world.

 

I appreciate the You giving me your modified headpipe from your car.

It would cost more than it worth $ to ship to the USA.

I have a 1985 Corvette Headpipe with no precats already, purchased for $20.00 and in very good condition.

Test the gasoline as I described.

I am curious if they blend alcohol into the gasoline in Australia too.

Let me know what you see or find.

 

The company statement above specifically states that there is no ethanol in it so if I did find some it would become national news. I will check it out anyway.

 

I have learned from You too,

reading your posts.

Shell 93 RM/2 gasoline is what I usually use in my Corvette. Runs the best overall with it.

The BP/Amoco 93 RM/2 also allows my Corvette to run Very Good too.

Good Evening !