Optimum EGT for Merc Drag

[CI STV]

You have the diverters installed!?
In order to go on the limit you would want to equalize as much as possible
Cooling, injectors, ignition
This way when reading piston tops everything should look as equal as possible
Then decide if you want the bottom cylinders to run richer

Do you mean the in-block water diverters? If so, my motor was built by Diamond, so I’m sure that they did what needed to be done with those. I’ve also added a PPE water flow kit that takes the water form the top of the block to the lower part of the heads, which should help to equalize temps between the cylinders.

[RBT]

Thanks. I’m using a Brucato ACU and the motor was dyno tuned at Diamond, with factory 21cc heads. I changed those out to PPE heads so that I could increase the compression.
mom thinking of getting the Steamwheel, so that I can make simple small adjustments, without opening the ECU.
Rather than messing with that, when I change the heads, I’ll probably bump the fuel pressure up a pound or two to fatten up fuel delivery and then dial it in from there.

leave the fuel psi static, change in the map only

[CI STV]

leave the fuel psi static, change in the map only

Okay. Is that so that it doesn’t go lean in the middle?

[RBT]

Okay. Is that so that it doesn’t go lean in the middle?

No, the ecu has voltage compensation changing the psi doesn't give a linear return on fuel delivery

[Stoker boy]

RBT is correct leave the fuel pressure alone. I’m not familiar with Brucato,I’ve been told two different things. The steam wheel adjusts the percentage of the overall fuel curve. I’ve been told if you want to add or take away fuel at between eg. 5000 and 7000 RPM to either lean or richen the mixture you can. If you can I guess you have to open up the box and cannot do it from the dash? Being able to tune the boat from the dash really makes things easier.

[JCSmile]

No, the ecu has voltage compensation changing the psi doesn't give a linear return on fuel delivery

Can you expand on this? What is voltage compensation? I know some of the prostock guys lower their psi to ~50lbs as a way to pull fuel without using the map, but perhaps they aren't concerned with it not being linear.

[CI STV]

Voltage compensation, as I understand it, is the same as “voltage offset”, where the pulse width of the injectors is increased very slightly (based on data programmed into the ECU) where the ECU detects a reduction in voltage.
The logic here is to keep fuel delivery for a specific RPM/load constant, regardless of voltage, which influences the injectors’ performance. By that, I mean that, since an injector is controlled by a electro-magnetic coil, increasing the voltage to the injectors can have an effect on the injectors’ pulsing, and, consequently, fuel delivery.
So, without voltage compensation, all other things remaining constant, your engine would run richer when the battery voltage is at 14.6 volts (charging state) than when it is at, say 12.8 volts.
I’m familiar with this from tuning my own vehicles using HP Tuners software, where you have tables in the ECU fuel mapping (“the tune”) that are required to be populated with very specific data for different injectors.
I was thinking of buying the Brucato software and trying my hand at that, but I don’t feel ambitious enough to risk my motor, since 2.5 Drag motors are practically unobtainable now, whereas GM LS motors are a dime a dozen, comparatively speaking.
Having gone through all of that mental masturbation, if I have interpreted voltage compensation correctly, I don’t see how fuel pressure and voltage compensation are directly related though, because the ECU doesn’t read or respond to fuel pressure adjustment?
I’ve struggled with this issue before myself, since some GM vehicles have fuel pressure sensors and fuel pump control modules (and have specific tables for controlling the same) and some (like my C6 Vette) don’t have a factory fuel sensor or control module, so there’s no relevant tables for those functions.
By comparison, a Brucato ACU is stone-age tech to the late model GM ECUs.
In that vein, two of the things I wish I could log on my boat:
(1) throttle position, so that when I read a log I could tell exactly where I went WOT and when and to what I backed off at the end of a run. But, with the ACU relying only on MAP data to determine throttle position, it’s harder to discern that from a datalog, and;
(2) exhaust gas content via a wideband. With the newer Bosch 4.9 wideband sensors, I think this would be more accurate than reading and trying to interpret exhaust temps via a couple of thermocouples, which is what provoked me to post this thread.
On the other hand, I do like the simplicity of these motors and, in any event, there is no fancy equipment that works better than reading plugs and piston tops, IMV… as long as you know what to look for. On that note, big thanks to Jay Smith for sharing his tuning technique with us.

[CI STV]

Is it normal to see a 30-40* difference on EGT between the left and right bank at WOT? I know it’s fairly normal at idle, since the exhaust gas is moving slower and one bank is higher than the other, but I would’ve thought that the two banks would be about even at WOT?
If that’s normal, which one normally runs a higher temp? I would think the higher bank (right) would be cooler, no?
At ~9700 I’m now seeing 1072* on the left bank and 1112* on the right (I need to check to see if the pyrometer leads aren’t switched around).

[PanRonnie]

I am down to 10 fahrenheit but i spend a lot of time flow matching my injectors
The second set is from my motor in break in
So the fuel pressure is set higher as currently i am running an A6 and an A32 promax 225 ecu's

So 30 to 40 fahrenheit could be from injector flow difference

[CI STV]

I am down to 10 fahrenheit but i spend a lot of time flow matching my injectors
The second set is from my motor in break in
So the fuel pressure is set higher as currently i am running an A6 and an A32 promax 225 ecu's

So 30 to 40 fahrenheit could be from injector flow difference

Thanks . That makes sense. That’s where mine was at first too: ~1050 and about 10 degrees apart.

I recently swapped to 17cc chambers in the heads and at first I thought that’s what caused it, but I’ve gone back and checked my datalogs and realized that it simply changed on its own one day.

I’ve also since leaned the main circuit by 6%, so I need to check to make sure that’s it’s not a single injector that’s leaning out, before I toast a cylinder.

The chambers are custom cut for the deck height of each specific cylinder, so I would expect that, if anything, that would promote more equal EGTs?

Maybe I’ve gotten a bit of dirt in one of the injectors, although I have a 10 micron microglass filter downstream from the pump. I’ll swap the injectors around and see if the EGTs follow suit.

[CI STV]

Just to update this thread and edit for accuracy, I went back and checked my datalogs and realized that the only thing I did before the bigger difference in EGTs showed up was I dropped the fuel pressure from 57.5psi (which is where I set it when I first set up the boat) to ~56 psi.
I had not yet changed the heads, or adjusted the ECU, but when I ran the boat, the left side EGT went up slightly by about 10 degrees and the right bank went up by about 30 degrees.
So the weird thing is that a fuel pressure adjustment alone would cause a significantly different rise in EGT on one bank??
After that, I turned the fuel pressure back up and then changed the chambers to 17cc, and I didn’t notice any significant change in EGTs, but the stagger between right and left bank stayed basically the same.
I subsequently leaned the main circuit down by 6% in the ECU and that produced the EGTs I posted before (1072* left bank, 1112* right bank).
So, I’m now convinced it’s a dirty or lazy injector, because I haven’t done anything that should cause a ~60* rise in EGT on one side and only 20* on the other.