The cooling mod with the pump has been patchy described in these three posts ( you will need a google translate)
https://www.drive2.com/l/556895218838799299/https://www.drive2.com/l/558172301594460670/https://www.drive2.com/l/576524250173670433/?page=0#comments (see comments).
I do a summary for you:
1. Statement - stock pump has low efficiency especially at idle hence we have a slow coolant flow and local overheats. Solution - increase coolant flow which makes temperature evenly distributed across different parts of the engine, especially around Cyl4.
2. Proposed solution - slightly different from the original GDT tune. Pipes from Left head (added pipe) and Right head (existing pipe) connected together with a T connector , then the rest of T connector goes to BOSH pump then to INPUT to the heater (not to output from the heater as per original GDT tune). I could not get from these posts whether or not the added pipe from the left head still has a restrictor similar to the original GDT tune.
3. Results of this mod described as follows:
a) On idle coolant temperature difference between "output of the engine block" and heater's output is 2-3 degC when climate control is off, 6 degC when the heater is on max. Before the mod the difference was over 15C. That confirms increased coolant flow after the BOSH pump is installed.
b)IAM stays at 1.0 . Before the mod , overheated Cyl4 was susceptible to detonation earlier than other cylinders (and this is why we have our knock sensor set close to cyl4). Once the Cyl4 got cooled better, we have less detonation conditions there and that improved the IAM value. That info is also related to the original GDT tune.
c) A slight change to the exhaust sound - it becomes more even with a sort of less spikes and even tones (pardon my description, I am not a musician). Could be a result of an even EGT across cylinders.
My notes to the above and GDT tune:
1) I had an experience when after a good run on load I have bubbles in the coolant tank, on 2 subarus, both stock no tune, 1 turbo and 1 NA. Multiple times and multiple methods tested - not a head gasket for sure. Now it can be explained as follows: Heavy load -> Residual heat collected in block /heads -> Vehicle stops, runs in idle -> Low coolant flow due to inefficient stock pump -> Local coolant overheating and bubbles -> Slow gradual cooling - > All back to normal. Also that can contribute to our radiator cracking as such boiling can cause coolant pressure spikes.
2) The articles above mentioned a pipe connector 21170АА020 from bi-turbo Legacy. It will not work alone for the original GDM tune as a restriction of flow from the left head is required. You can use that 21170АА020 if you add a separate restrictor of, as said, 8-10 mm internal diameter. 10mm has been tested ok as per the links above and the heater still works for a mild winter. I do not know what is the size for the original original GDT tune. I think the goal of GDT is to get equal temperature for the left and right heads under load both coolant and EGT, that may or may not be an equal coolant flow to achieve that. Hence GDT said they did a lot of tests to define the required restrictor diameter. There is no similar test mentioned in the links above, so I would say, 8-10 mm internal diameter is a good guess.
3) Subaru did not do that for a purpose? To make one cylinder worth than others and to a knock tuning by a single bad cylinder? New engines have 2 knock sensors AFAIK. GDT says after their mod the 4th cyl is still the hottest but with much smaller difference to others, so even a single knock sensor would work.
4) Re original GDT tune on GenV: The kits that GDT sells will not fit - GenV has an engine mount and at least will require longer pipe to get aground it. I tried to look but cannot see the head's plug behind the mount. May be we also need a right angle connector or so as the head is to close to the mount. SO it is a pain to get there and install the kit. I see the point and like to do it, but it is a JOB for sure for GenV. Others seems to be much simpler.
5) on installing the BOSH pump, I would add a bracket to mount the pump to the firewall. I do not think a heater's pipe is strong enough to take load from a pump weight, especially when the pump may also move and resonate on uneven roads, turns etc.
Hope that helps. Let me know if you have more questions for the abovementioned articles, I'll read them for you.
Regards, ilib11
