The setup: S5 TII shortblock with S5 NA rotors
S5 HT18 snail with stock wastegate setup but manifold has been ported to lessen boost creep
S4 TII ECU and knock box, new knock sensor
S4 boost sensor
S5 coils
S4 TPS
S5 CAS
S5 AFM
S5 injectors
Single wire O2 sensor
2.5 inch exhaust all the way back (~10 feet) Dynomax muffler at the rear
NGK 7 leading/9 trailing plugs, new as are the plug wires.
Can't find a vacuum or intake leak anywhere.
Other than some mixing and matching everything's stock such as the FPR, it has the late pulsation damper. Using a MSD fuel pump. Fuel pressure is ~40 PSI at idle, don't know about acceleration, rules won't allow putting a fuel pressure gauge in cockpit without major $$ isolators etc. Boost is limited to ~7.5 PSI.
I run two filters: a big coarse element Fram before the fuel pump and after the pump a big fine element filter, about big as a fist. 93 octane with some 2 stroke mineral oil, ~1 oz/gallon and also running an S4 mechanical OMP.
Now that the engine is seated in a bit it runs great, with one exception: under boost/high load. Think 3rd gear WOT out of a medium speed corner, foot to the floor, boost coming up, doesn't happen under vacuum. It will intermittently jerk/stutter under these conditions, no backfires so I don't think its ignition related. Back off the throttle a bit, it smooths out. That sorta points toward fuel.
I'm concerned that it might be going lean and that won't do at all. But the plugs come out with sooty black negative electrodes and dark chocolate colored insulators.
Can't make it happen free rev, only under load and boost.
Weird.
Ideas?
Spark blowout. On top of carbon'd up plugs.
Is the airflow sensor moving freely? S5 has the plunger type (as opposed to the door) and maybe the spring is sticking a bit or something.
More ideas as they happen, it's been a long weekend.
Plunger moves freely, plugs and wires are new. It is a bit odd; I thought at one point maybe the knock sensor was kicking in and there is no way to test the damn thing.
Ranger50 wrote:
Spark blowout. On top of carbon'd up plugs.
Try this first. I battled with a similar problem on my miata then finally gapped the plugs tighter and it it's solved. I will eventually swap to better coils.
Hmm. Can't regap the plugs.
try another coil? what coil are you running?
i had this EXACT issue with a COMPLETELY different car. WOT, high load would stumble, let off just a little bit and it was smooth as glass. for me it was a ford, and the TFI module was weak, and a new one fixed me right up. i say all this to ask the following: on a rotary, what tells the coils to discharge? that item, whatever it may be (i know nothing about rotaries), could be your issue.
-J0N
I'm using the stock RX7 coils and igniter modules. There are swaps available but just haven't had the chance to dig deeply into it. The leading coil setup is no biggie to swap out, both plugs fire at the same time (waste spark setup) but the trailing is different, the ECM uses rising/falling 5v reference to decide which one to fire to complete the burn started by the leading plug. On turbo rotaries the apex seals are very sensitive to detonation so that's not something I want to screw around with until I know EXACTLY what to do.
Are those the correct plugs ?
Plug wires and/or lean.
Yep, that's the right plugs. I keep coming back to leaning out under boost and that concerns me. I bought a cheapo AFR gauge (an Innovate wideband is in the to do list) and I think I will add a 1k ohm resistor to the coolant temp circuit, fool the ECU into running a bit richer.
