gfd263

iTrader: (4)

I have a 1979 RX7 with a 12a turbo running megasquirt. I have 3 factory turbos. ht18-bm and ht18s-bm. Love having peak torque at 3700 rpm but looking for more. Ht18 compressor is 63 mm where the ht18s is 56mm. Is swapping to the big compressor while retaining the smaller turbine a viable option?
Running the HT18S, I made 148 rwhp @ 7200 and 149 trq. My datalog did not record on the last run, but it started losing boost at 5500 rpm on the other pulls. 4000rpms I had 7.5 psi and fininshed around 5 psi at 7800. Not sure if this was a waste gate bleeding boost or the compressor being so small.
I can not find anything on these turbos (a/r ratio, pressure ratio, airflow).

gfd263

iTrader: (4)

forgot to mention. pinching the wastegate hose had no effect wich tells me the turbo is doing all it can.

GrossPolluter

iTrader: (1)

I'm not sure which turbo you are running. I have had s4 and s5 t2 ht18 turbos and they are all the same size. Weither they are hybrid or not, they should be getting boost, even with the wastegate hose pinched, which sounds deadly to me. Maybe you are having a boost leak somewhere else in the intake piping, intercooler, bov, or somewhere else.

I would just remove the turbo and inspect it. Also make sure the engine has decent compression. I have had countless problems with buying used and even getting the turbos rebuilt. 87-91, these turbos are getting old, and add the exhaust heat and pulse of a rotary to it, these old hitachi turbos are getting used to a point where its hard to find a good usable, or a good rebuildable core.

I almost went with a full turbo upgrade, but bnr offered me a hybrid with everything new except the exhaust housing. If it wasn't for that, I would have went with a complete turbo upgrade because of the countless turbo rebuilds I have been through.

bumpstart

he is running the 12a turbo.. he mentions both 18- BM and 18s -BM.. one is the original 4 bolt rear turbo and the other is the smaller later 12a "impact" turbo with a 3 bolt rear not unlike the later FC turbos
( makes more power than the larger version )

options for these turbo include similar compressor upgrades as s4/5 turbo hybrids

but reality is the turbo is badly hamstrung by the size of the turbine housing,, and while making for instant boost hold back even the stock compressors potential up top

the best move is to go to a custom exhaust manifold and use the s5 13b hybrid turbo options
then the rear is much better sized with the potential of the engine
( and will bring wet cooled core that wont require turbo timers to cool down )

PS.. pretty sure i have a BM ( no "S" ) front on one of the highflows here .. they take the same compressor hi fi upgrades as the FC turbos .. this one i think has an S trim to4 wheel

GrossPolluter

iTrader: (1)

Oh, thanks for clearing that up.

gfd263

iTrader: (4)

yes, this is the 12a turbo as sold in Japan and Australia. When I ran it on the dyno, I was very impressed by the low end. It died off soooo quickly, though. I am trying to figure out what to do and all input is appreciated.