I took the new turbo for a flight yesterday.
Changes I made included:
New Hybrid TO4 turbo
New side intake scoop supplying cool air to turbo and last 1/4 of the rad.
Flush, clean and dry cooling system, change to Evans NPG coolant.
Oil change.
PSRU dissambly and inspection.
Just when I was feeling a little too happy about climbing through 2500 as I departed the pattern - temps well in the green, overtaking a Mooney (32" MAP and about 5100 RPM) - it happened. The RPM suddenly dropped to 4700, but still sounded smooth. That's plenty to keep flying, besides my climb angle was steeper than a glide angle at that point. Landed without incident, engine seemed to be running fine.
My initial thoughts were:
the new intake I installed had come undone
or the waste-gate actuator came off
or I had a leak in the intake system (like the pop-off valve)
or the turbo is too big and I was hitting compressor surge,
or, worse, ruined another turbo....
Was too frustrated to look into it yesterday, but today I checked it out:
waste gate actuator still in place.
new intake still in place.
Compression sounds and feels normal in both rotors.
turbo looks fine, no visible evidence of anything wrong, spins freely without end play.
adjusted pop-off valve to make sure it wasn't leaking - place a telltale(foil over the opening).
Several Ground runs yielded the following info:
Engine ran normally up to about 35" MAP when the RPM dropped again to 4700 (from 5100). MAP was still showing 35" measured at the engine block. . This pretty much rules out surge or intake problems.
As I backed down the MAP to about 28" it would 'catch' again the the RPM would go back up to where it should be. Increase the throttle again and somewhere between 31" and 35" the RPM will suddenly drop.
(Have run up to 45" MAP with the old turbo without any such problems)
Turning to full rich increases the RPM slightly but does not make it normal.
Fuel flow and fuel pressure remain normal.
oil pressure and all temps remain normal.
Occurs on controller A or B.
No change with disabling leading or trailing plugs.
It kinda feels like an old motorcycle I had where you couldn't advance the throttle too quickly or it would get weak and sound hollow...
No smoke, coolant, or oil leaks.
BTW, the new turbo sounds awesome! :-) with that nice whurr that I never heard with the stock turbo.
This could either be something new that I just created, or something that has been going on for many hours but I didn't know it because I have not been able to get above 28" while flying normally aspirated.
My current thoughts about cause are:
1) Timing: I advanced the timing a while back when I went N.A. and I seem to recall I set the timing retard to occur around 30". But a little bit of timing retard shouldn't hurt that much. Maybe the advance I gave it was too much and I am detonating?
2) The Evans coolant really cant keep the metal surfaces cool because it doesn't vaporize and I am getting pre-ignition. Unlikely because the race car guys don't report the problem and they are putting out much more
h.p.
3) Fuel - something... maybe a sticking injector. Is it harder for the injector to open when the fuel pressure is higher?
4) SAG - but that is a lot more than typical SAG and the plugs are only 20 hrs old.
5) ??? The answer probably lies in that list, but it all seems so unlikely...
Any thoughts?
Thanks for your help.
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