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Maximum Brake Torque Timing Advance - Linear to Cruise?

 
  #1  
Old 04-15-2019, 07:32 PM
FrankGRUN
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Maximum Brake Torque Timing Advance - Linear to Cruise?

I tried to trigger a discussion of this topic on the 460Ford Forum but could elicit no responses. I thought I'd try here and see if there is more technical interest.

Significant changes to my 460 prompts me to ask some more involved questions than usual about the details of the optimal timing curve for my motorhome version of the engine. To repeat the details of my configuration, about 10,000 miles ago I had to replace the stock engine with a ProMar crate remanufactured stock engine with cast iron heads, high temperature valve seats, and a Melling MTF-3 cam with double roller chain and cam timing set to 2 degrees advanced. The Melling MTF-3 specs are 292 degrees exhaust duration and 282 intake, exhaust lift is 214 and the intake lift is 204, lobe separation is 112. Smog equipment has been removed, the intake is an Edelbrock 2166 non-EGR Performer, current carburetor is an Edelbrock 1406 (tuned by Bob’s Carburetor Engineering), Hedman long tube headers go to 3” single exhaust feeding a 3” inlet/outlet 6” diameter and 30” long Magnaflow muffler (no catalyst). The ignition is DuraSpark II (recurved and rebuilt by Scotty) with a Crane Hi-6S multi firing CDI box and 0-20 degree variable retard, and Crane Fireball LX-91 coil. The transmission is a C6 automatic with Gear Vendors Overdrive and Hughes towing converter. The 158” wheelbase, 24 ft. motorhome has a 11,000 pound GVWR. Rear axle is 4.56:1 and the tires are 8.75R16.5LT giving a measured 2340 rpm at 55 mph, 2550 at 60 and 2780 at 65. Therefore, the engine cruising rpm is the band from 2300 to 2800 rpm and I am especially interested in maximizing the fuel economy of this configuration!

The unique new opportunity that presents itself is the acquisition of a Fitech Street Go EFI 400HP system to replace the existing Edelbrock 1406 and a CB Performance Black Box programmable ignition controller. The Fitech makes it possible to tune the AFR during cruise to 15.0:1 and the programmable ignition controller, with its MAF vacuum sensor, gives full control of the ignition timing. In addition, the Crane Hi-6S fires a series of spark pulses for each ignition event at cruise rpm that gives a total spark duration of 20 degrees (crankshaft).

Now for sometime I have been studying a number of engine design papers and articles about the relationship of timing at lean cruise conditions for maximum economy. As most of you know, lean conditions require additional advance to fully consume the charge before the appropriate point after TDC for a clean power stroke. The amount of advance depends on compression ratio, spark burn rate, combustion chamber design, intake and exhaust flow parameters and temperature (and probably the lunar phase). So, I think I have an opportunity to tune the timing advance in this important operating range of a more or less stock 460.

But several questions have bothered me. The Duraspark II system curved by Scotty for this engine is 32 degrees total, all in at 3200 rpm. The vacuum advance is 12 degrees. My idle vacuum is 15” at 5,200 ft. altitude and the manifold vacuum at cruise is between 14 and 16 inches Hg. With carburetor the system is just fine with overall mpg between 8.5 and 9.8 mpg. But, the mechanical advance is just a linear increase with rpm from the initial value. The total advance is clearly to give the best advance at high load and high rpm just shy of detonation. The vacuum advance is the wild card to account for the wide range of engine load variations. But when I look at engine dyno results (all I can find are SBC and BBC engines) that look at maximum power at different rpm’s the timing curve is NOT a simple linear function. It appears to change rapidly from 1000 to 2000, then slowly taper to 4000, and then change very slowly if at all at still higher rpm. Unfortunately, nowhere can I find a BSFC curve for the 460 engine with any type of cylinder head design.

So, I wonder if others in this group have looked into the “Optimal Timing Curve” for the cast iron head, low compression, towing cam version of the 460 with their dyno results?

I have thought that I might address this (without the dyno) by taking advantage of the Variable Timing Retard feature of the Crane Hi-6S. The Retard Control is mounted in the E350 cab and quite accessible while driving. Its dial is marked in 1 degree increments. I have calibrated the numbers while monitoring the timing at idle speeds. For the experiment, I would also have to calibrate the vacuum advance canister. Then, I could add, for example, 10 degrees of advance to the initial (idle) timing - and then remove those 10 degrees by retarding with the Hi-6S control. Then, finding a flat section of extended highway ( probably the Hwy to Burning Man/ Black Rock Desert) and fixing the Fitech AFR at 15:1, and cruise control at 2350 rpm, I could vary the timing with the Hi-6S while monitoring the manifold vacuum. Finally, I have a high gain electronic stethoscope to monitor for the onset of engine knock, should I venture into that territory. Running the experiment at several rpm levels might answer the question of best timing advance at a fixed AFR. Additionally, It would answer the question of whether or not the maximum vacuum level can be used to optimize the low-load timing curve.

If I can get these numbers, I will load the digital spark map into the “Black Box” and put the instruments away!

Any comments (short of outrage) welcomed! As always, too long…
 
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Old 04-15-2019, 07:49 PM
FrankGRUN
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Additional Data

In the hope to stimulate some discussion, I have decided to add some of the information I have gathered on the subject. I'll add proper references and pdf's of papers if requested.

As I indicated above, with a locked distributor (I'm going to use the Ford TFi distributor from the 88-96 460's using the Hall sensor trigger output) and the CB Performance Black Box Programmable Ignition, I have the ability to add a full Spark Timing Map. Typically, these are generated on a Dyno, but I don't have access to a Motorhome compatible one in Reno, so I have to develop the full tune on the road. Best would be to get a Dyno tune for the 460 with similar use application, camshaft, heads, etc. Fortunately, I have found one timing map for a build similar to mine.

But first to the background data. A variety of classic engineering texts demonstrate the "optimal" timing for a cast iron head, 4-stroke, gasoline engine using data from the GM Tech Center on experimental engines.

The enclosed figure labeled Torque vs. Spark Advance at WOT. As you can see, the the torque max is measured at wide open throttle by varying the spark timing at a constant engine rpm.
A second important figure, shows torque measured against spark advance for several different constant RPM values but also noting the onset of light knock. From this figure we can take the KL-MBT value (Knock-limited Maximum Brake Torque) at a given RPM. Finally, for this set, the next Figure plots the torque for an optimal spark advance curve vs. the normal curve, showing that approximately 15% of the possible torque is left at the table by the non-optimal curve. BTW, the optimal curve is calculated by the measured KL-MBT value and subtracting 2 degrees for a knock margin.

Now, in the next Figure I have taken a data set from another V8 engine (Not Ford 460) that studied MBT spark advance for various loads represented by manifold pressure in KPa units and added the data from Scotty's recommended curve for my engine (12 initial and 32 total in at 3200 rpm) to the plot.
I have added a second curve for Scotty's recommendation and added 12 degrees for maximum vacuum advance. Notice how closely Scotty's no-vacuum curve parallels the 100KPa curve and the Mad Porter + Vac curve parallels most of the 50KPa curve from the detailed study.


The next Figure summarizes the data from a comprehensive Thesis that studied a SAAB 2.0L Turbo 4 cylinder. The Spark advance curve is the KL-MBT value less 2 degree knock margin. I have also added the Scotty derived curves to this plot. Notice that the + Vac curve closely parallels the 2 degree knock margin curve save below 1500 rpm and just 3 degrees advance separates these curves above 2500 rpm.

Finally, as I mentioned above, I found a full timing map, optimized from the starting base map, for a "Mild" 460 build with very similar parameters to mine. In the last Figure, I have plotted the spark timing values for three different load values, 10 MPa, 30 MPa and 98 MPa which corresponds to WOT. I have also included the Scotty-derived curves as well.


To continue, here is the plot from the Dyno-optimized runs for the "Mild 460 Build" that I referred to above. Again you see that the Scotty-derived curves at +Vac, closely approximate the 10 KPa plot from 2000 to 3500. However, in a situation where degrees are significant, there are substantial differences. In particular, there are surprising differences between the curves for 10 and 30 KPa (and for the 20 KPa, 40 KPa, 50 KPa and 60 KPa curves that I haven't plotted here) in the region below 2300 rpm. Also, the 30 KPa curve shows less advance that the WOT curve in the region from 2000 to 2800 rpm.

I'll also add that I'm nervous about adding a bit of onset detonation stress to my engine. I'll first try to study the effect od timing advance at a specific AFR and constant RPM on the measured manifold vacuum. I've setup a crude data logger system to help. Finally, I'll be adding the GM ESC anti-knock system with module and knock sensor tuned to the BBC 454 stock truck engine. The ground signal will be used to trigger the Crane retard by 2-4 degrees as needed.

As before, any comments or data strongly solicited.My point in all of this is that I have no interest in maximum power, just access to the full available torque from idle to 3000 rpm. I still plan tp manipulate the AFR at 2-3000 low load conditions to see if the lean conditions are further optimized by spark advance.
 
  #3  
Old 04-15-2019, 08:31 PM
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Skimming through all of that.. There are too many verables that come into play. What works for one won't seem to work for another. In general, you always want all the timing it will take as fast as it will take it in any situation without detonation. With your set up that mileage isn't horribly bad considering what your asking for a big block gas engine to do. If you use it a lot you might consider a diesel swap.
 
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Old 04-15-2019, 09:05 PM
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Mark, Thanks for the comment. Early on I considered a Diesel but the swap was beyond my backyard capabilities. I looked long at a Cummins Turbo Diesel swap but it was even more intimidating.

What motivates here is the combination of the self learning and AFR adjustable EFI fuel system, combined with the fully programmable CB Black Box ignition. Since my engine is working in the 1500 to 3000 rpm range, I wanted to grab the opportunity of pulling all the available torque just shy of detonation in that range. By my understanding, the maximum torque timing point (vs. load) represents the best fuel efficiency for the design. Many studies of the SBC and BBC series of engines have looked at the best timing advance (generally starting at 3000 rpm) at WOT, but I can find nothing but theory to look at timing vs. load and AFR in the cruising range (2-3000 rpm). I was hoping some individuals with a mild or towing build and Dyno results would share the below 3000 rpm data.

I'm also concerned with the possibility of engine damage while trying to road tune by listening for the onset of knock detonation.I have an ESC (Electronic Spare Control) module designed by GM during the TBI engine phase. It has a built-in set of electronic frequency filters tuned to the knock signal generated the knock sensor designed for the BBC 454 block. When it detects a knock, it triggers a ground signal. I will use this ground signal to trigger the variable retard capability in the Crane Hi-6S. GM designed the module and ESC system to retard the ignition by 5 degrees when detonation is detected. The time to stop detonation with a retarded timing varies with the engine design, so I could choose anything from 2 degrees to 6 or so. With cast iron heads, several papers suggest 2-3 degrees will be adequate.
 
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Old 04-16-2019, 12:06 PM
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The quality of the fuel and the weight of the vehicle are important things to consider. Are the two systems interfaced ? If not you might be wasting time to no avail. You can always get more timing with better fuel but your after keeping cost down and you use so much of it, I know it's got to be hard not to put the crap fuel in when highway driving.
I wouldn't worry too much about a little knocking here and there while your tuning it, just don't stay in there for long. As with tuning anything, it's all trial and error and takes time but at the end of the day I just don't think the gas engine is ever going to pay off for you.
 

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