So as a fellow TB'er that has blown his freeze plugs twice....im going to ramble a bit.
3 stages to this problem.
1....bought 91 940 with 17x,xxx miles. stock balancer and stock "small" wp pulley. 15g turbo, IPD cam, AW71, blah, blah, blah. Mikes chips with 7200 revlimit. Car was fun, made 206whp and could rev wot throttle with noproblems. Safe to mention here that the tall gears of the aw71 limits how quickly the stock setup will rev imho. Also, found out later that the water pump had been replaced with an IPD unit that has the 6 stamped steel impeller blades like a chevy pump.
2...start the build of the 16v monster. I had a dales pulley, but wanted to keep the stock harmonic balancer as the dales pulley is just an alum pulley. I end up (by accident) with the larger early model wp pulley from the yard, have the block at the machine shop where they install some Dorman freeze plugs.
Get the whole thing together and about 50 miles on the motor, Im headed to lunch...motor is at 140* with 13lb radiator cap. (at this point during the build I had done a large 2 core alum radiator with standard american cap). I was having the to small of turbo syndrome and could not control boost....the motor would rev RFN and spike to 21psi (I also installed a gm 5 speed). At the 50 mile mark I blew 3 freeze plugs out of the block.
Start the fixing and wondering WTF!
I was able to put some retaining plates over the exhaust side, but since the intake side has the pcv box, I could only plate the forward 2. Drive car for a while and all is better. Didnt really do anything to fix the issue.
3...install better clutch and get a 57trim on there. To this point there have been no problems....now remember Im still tuning and keep the boost low at wg pressure for a while and slowly turn it up to 18psi, but all my runs are made in 3rd gear so the motor is loaded and does not rev out fast from a 20mph takeoff.....which gets us to the Dyno.
I was having boost control issues and could not build anymore than 15psi.....car gets taken off Dyno to do some street time.....get the boost issue solved and end up running the car with no lines to the WG.......pretty much revs out in first RFN, even more RFN than before and spikes to 30psi....wow! Got that fixed and was hitting 25psi on setting the Truboost was on......I make 2 more launches and blow them freeze plugs out that I could not plate on.
Go home and fix the problem...redo the pcv box and plate the final plugs in. Each time Im using loctite #1 for sealant and actually switched to a brass plug the second time around.
Now my remedies that have worked so far...plenty of pulls with no issues.
Kept the stock Harmonic Balancer......made a larger wp pulley. went from 5.375 to a 6.125. Its driven 1-1 now. Got rid of the heater conrol valve that the 9xx cars have thats pre-heater core. Now it flows like an old chevy does. And I install a 3/8 bypass from mid head to the forward side of the thermostat housing.
I should state now that when the motor was rebuilt i bought a Hepu pump that is a brass cast unit with built in cav plate, unlike the jap replacement which had bent impellers.
My findings to this point.
Car has trouble cooling at an idle now, so installed a larger electric fan on the radiator, and since the install is not "ideal", meaning the rad is not completly seal from the engine bay I was getting some hot air recircing to the front side with the hood down. So i installed fan infront the IC to feed the radiator with cool air. WORKS awsome. Tho i never had a problem cooling while driving, and none of my logs show any major coolant rises or splikes under any conditions.
If the 3/8 bypass is open, about takes forever for it to warm up.....................and if you open it up after its warm and drive at highway speeds, I got it to cool down to 160* before I closed it off again!
There is an electronic 3/8 solenoid in the bypass and is controlled by the ECU to open under 1psi of boost or more....so really any conditons that will cause the motor to rev to quickly, the loop is open.
Looking at a 16v head and an 8v head, the 16v already has all the passages opened up, and its just a minor flow restriction in the head gasket....while the 8v has small ports and some are not even present.
Im still leary of it happening, or at least pushing a hose or something, but nothing so far....very happy.
So its stock Harmonic, matching diameter wp pulley, valve gone on heater line to heater core, and a bypass (that could be activated by a nitrous switch or something) and my issues seem to be gone.