What does Ld Var for scaling LV8 do?
03-29-2016, 04:31 PM
#1
Burning Brakes
Thread Starter
What does Ld Var for scaling LV8 do?
I'm messing with the prom from a Guldstrand/Traco 388 build done in 92, and they seem to have done some weird stuff in the tune. Incidentally, the handwriting and labels on the memcal look to be done by the same person that did the one in my '87 Callaway, but that's a different story!
Anyway, the car has stock Buick GN injectors (29.7 lbs @ 45 psi or 29.2 @43). However, the injector constant is set at 37.63 lbs. But, I also notice that the Ld Var for scaling LV8 is set to 68 instead of the normal 80. Seems the scaling factor difference would offset the overstated injector flow rate, but why would someone do that in the tune?
Compared to the callaway prom, which basically only backs off some timing in a few places, this Guldstrand prom (also $32b) has a TON of changes compared to a stock prom, even some in "undocumented" areas.
Anyway, the car has stock Buick GN injectors (29.7 lbs @ 45 psi or 29.2 @43). However, the injector constant is set at 37.63 lbs. But, I also notice that the Ld Var for scaling LV8 is set to 68 instead of the normal 80. Seems the scaling factor difference would offset the overstated injector flow rate, but why would someone do that in the tune?
Compared to the callaway prom, which basically only backs off some timing in a few places, this Guldstrand prom (also $32b) has a TON of changes compared to a stock prom, even some in "undocumented" areas.
04-02-2016, 02:41 PM
#2
Melting Slicks
The load scalar can be adjusted for several reasons and varies between applications as a function of the number of cylinders, presumably due to the differing relationship between drp and rpm.
One reason to reduce the scalar would be to increase the effective range of the signal before saturation, but this has additional implications.
Load essentially reflects the air mass in terms of gm/cylinder, but basically boils down to (gm/sec)/actual rpm * x, with x being the internal scalar multiplied by 60.
Background info borrowed from RBob's description (assuming a V8 engine):
DRP = 983040 / RPM
LV8 = (((DRP * gms/sec) / 256) * Scalar) / 64
Since 983040/256/64 = 60 and Scalar = 80 for V8 tpi applications:
80 * 60 = 4800.
Simply put: LV8 = ((gm/sec)/rpm) * 4800.
This is easy to remember and produces same result as the formal calculations from above.
Another way to express by removing the element of time is: LV8 = gm/rev * 80 or even gm/rev/4 intake strokes/rev * 320.
So, your scalar of 68 instead means gm/rev * 68.
The real significance of 80 is still not obvious, but it is what it is. It may simply be a factor without actual physical significance.
A larger cylinder can hold more air so reducing the scalar may allow the load signal to better reflect the cylinder filling in some situations.
With stock settings, the load for a given flow rate will always decrease above 4800 rpm, since the ratio of (scalar * 60)/actual rpm will be < 1 and decrease with respect to rpm.
A setting of 68 will cause the load for a given flow rate to be ~15% lower than stock then begin to decrease earlier above 4080 rpm.
Reducing the load, generally increases the spark advance. Could be good or bad, depending upon what is wanted. Keep in mind we're generally working with a limited range for both MAF and load signals, especially on modified engines.
If we take the simple case using a pegged MAF as an example. Lets say you have an engine that can peg the MAF at 255 gm/sec above 4500 rpm:
with stock settings (255/4500)*4800, the load will be pegged at 255 at 4500 rpm and remain there until actual rpm exceeds 4800...by 6000 rpm, the load will have fallen to 204, or a 20% decrease from the 4500 rpm value.
with the alternate settings (255/4500)*4080, the load will only be 231 at 4500 rpm and already be on its way down as rpm increases further...by 6000 rpm, the load will have fallen to 173, or a 25.1% decrease from the 4500 rpm value.
When set properly and not limited by signal range, the load signal will reflect the engine's torque curve. Too low and the signal will decrease prematurely with rpm, too high and the signal will be clipped before the natural peak. For best flexibility, the scalar should be set near the peak flow rpm assuming adequate signal range. Once signals are pegged as a function of rpm, its a compromise. For blower and turbo applications I like to scale both the load and maf signals in such a way to better distinguish between the boosted and vacuum regions.
One reason to reduce the scalar would be to increase the effective range of the signal before saturation, but this has additional implications.
Load essentially reflects the air mass in terms of gm/cylinder, but basically boils down to (gm/sec)/actual rpm * x, with x being the internal scalar multiplied by 60.
Background info borrowed from RBob's description (assuming a V8 engine):
DRP = 983040 / RPM
LV8 = (((DRP * gms/sec) / 256) * Scalar) / 64
Since 983040/256/64 = 60 and Scalar = 80 for V8 tpi applications:
80 * 60 = 4800.
Simply put: LV8 = ((gm/sec)/rpm) * 4800.
This is easy to remember and produces same result as the formal calculations from above.
Another way to express by removing the element of time is: LV8 = gm/rev * 80 or even gm/rev/4 intake strokes/rev * 320.
So, your scalar of 68 instead means gm/rev * 68.
The real significance of 80 is still not obvious, but it is what it is. It may simply be a factor without actual physical significance.
A larger cylinder can hold more air so reducing the scalar may allow the load signal to better reflect the cylinder filling in some situations.
With stock settings, the load for a given flow rate will always decrease above 4800 rpm, since the ratio of (scalar * 60)/actual rpm will be < 1 and decrease with respect to rpm.
A setting of 68 will cause the load for a given flow rate to be ~15% lower than stock then begin to decrease earlier above 4080 rpm.
Reducing the load, generally increases the spark advance. Could be good or bad, depending upon what is wanted. Keep in mind we're generally working with a limited range for both MAF and load signals, especially on modified engines.
If we take the simple case using a pegged MAF as an example. Lets say you have an engine that can peg the MAF at 255 gm/sec above 4500 rpm:
with stock settings (255/4500)*4800, the load will be pegged at 255 at 4500 rpm and remain there until actual rpm exceeds 4800...by 6000 rpm, the load will have fallen to 204, or a 20% decrease from the 4500 rpm value.
with the alternate settings (255/4500)*4080, the load will only be 231 at 4500 rpm and already be on its way down as rpm increases further...by 6000 rpm, the load will have fallen to 173, or a 25.1% decrease from the 4500 rpm value.
When set properly and not limited by signal range, the load signal will reflect the engine's torque curve. Too low and the signal will decrease prematurely with rpm, too high and the signal will be clipped before the natural peak. For best flexibility, the scalar should be set near the peak flow rpm assuming adequate signal range. Once signals are pegged as a function of rpm, its a compromise. For blower and turbo applications I like to scale both the load and maf signals in such a way to better distinguish between the boosted and vacuum regions.
Last edited by tequilaboy; 04-06-2016 at 07:12 PM.
04-08-2016, 05:47 PM
#3
Burning Brakes
Thread Starter
Thanks for the thorough reply! It's going to take me a little while to wrap my head around this, and I'll probably need to spend some quality time with the source disassembly to get it.
I think where I was leading myself astray is in thinking that LV8 (or load) is directly proportional to total fuel required, and it's not - it's RPM dependent. From your explanation, load seems more like a way of imputing vacuum (or MAP) without directly measuring it. gm of air entering a cylinder of known volume is essentially VE as used in SD tuning.
In this particular case, the tune is for a high-revving, solid lifter, single-plane EFI application (7300 rpm max). The spark tables are very shifted from what I've seen before, and this makes sense according to your description of the LV8 scalar.
I think it's real cool to see this tune from 1992 done with a lot of stuff the 'community' didn't learn about until a decade later. No doubt Guldstrand had factory help!
Now, I just need to understand the fueling side more and figure out why the injector constant would be overstated as well...
I think where I was leading myself astray is in thinking that LV8 (or load) is directly proportional to total fuel required, and it's not - it's RPM dependent. From your explanation, load seems more like a way of imputing vacuum (or MAP) without directly measuring it. gm of air entering a cylinder of known volume is essentially VE as used in SD tuning.
In this particular case, the tune is for a high-revving, solid lifter, single-plane EFI application (7300 rpm max). The spark tables are very shifted from what I've seen before, and this makes sense according to your description of the LV8 scalar.
I think it's real cool to see this tune from 1992 done with a lot of stuff the 'community' didn't learn about until a decade later. No doubt Guldstrand had factory help!
Now, I just need to understand the fueling side more and figure out why the injector constant would be overstated as well...
Last edited by 89onlyZ51; 04-08-2016 at 05:48 PM.
04-09-2016, 03:32 PM
#4
Melting Slicks
The elevated injector constant would tend to lean things out globally.
I suspect they were running very high fuel pressure to offset the pw cap limitation and then used the high injector constant setting to lean things out again.
As you surmised, load is analogous to map (kpa) in S/D applications. For a given target afr, the pw is generally proportional to load.
In $32 and $32B, you have the option to use the load vs pw table directly, instead of calculating the pw. You can clearly see that trend that the pw varies directly with load in this table.
I suspect they were running very high fuel pressure to offset the pw cap limitation and then used the high injector constant setting to lean things out again.
As you surmised, load is analogous to map (kpa) in S/D applications. For a given target afr, the pw is generally proportional to load.
In $32 and $32B, you have the option to use the load vs pw table directly, instead of calculating the pw. You can clearly see that trend that the pw varies directly with load in this table.