Attention: TDR Forum Junkies 
OK, first note- my truck, for those who dont read signatures, is a 11/90 build date, non IC, with 4. 10s and a Getrag (which is looking for luggage for its trip south... . ). I also have a PDR HX35 stuffed in there and afro engineered plumbing.
I ran for about 3 mo with the stock 18, got good MPGs, and it ran fine. EGTs got warm from time to time, but that is to be expected on a nonIC truck with a cranked pump. I had the chance last summer to swap with a guy who wanted a 21 and an 18 for a twins project he was working on, and had a 12 he'd trade for them. I jumped at it. I noticed an immediate increase in spoolup, a gain of about 8 psi peak boost, etc, etc. No REAL decrease in cruise or peak EGT, though it was cool to ba able to make 40+ psi with stock injectors, and peg the gauge in 3-5th gear with the EDMs. MPGs went down about 1. 5-2, though. I thought about that, stsarted driving like a bluehair on the way to church, tweaked the AFC, etc, etc. Then I read Forrest Nearring's post on the HX40-16 he had installed in his 96, and gaining MPGs. Theory was, the reduced backpressure let the engine breathe easier, thus the air in/ exhaust out was better accomodated, esp with lower gearing. Sounded ok to me, and would apply to my 4. 10s (which, BTW, are available for trade for 3. 54s or higher). I got the 18 yesterday via UPS from another guy, ported it to within an inch of its life, as I had ported the un-shrunken manifold when I put it on last month. Put it on this morning, and shipped the 12 to the guy who sent me the 18. Here are my preliminary observations:
the 18 DOES let the engine breathe better. I noticed a slight increase in EGT- maybe 50*, which can more than be attributed to weather. PEAK EGT is about the same, too- right around 1225-1250, but peak boost is lower. This is empty, but on the same stretch of road, similar weather. SOTP dyno shows no difference in pulling ability. Cooldown isnt appreciably different- down to 350 by the time I get to the red light; I can turn it off (<300) by the time it turns green again. BUT, I have gone from 8-10psi @ 60mph to 3-5psi @60mph. Peak boost was reduced by about 6psi, which is expected- 1 psi per square cm difference. There is a hair more lag, which can be adjusted by loosening the tension on the spring, to bring on the fuel at lower boost than with the 12, but I'll have to drive it a bit to see if it would be worth the difference. I DO like the lower drive pressure, decreased backpressure, though.
CONCLUSION- if you have the 2nd lowest final drive ratio (3. 54, no OD being lowest) , no intercooler, aftermarket injecotrs, and some pump tweaks, leave the 18 on there, or maybe go with a 16; the 12 is just too restrictive at higher boost/ RPM. If you have the highest final drive (Mom's truck- 3. 54 autoOD), and are at stock fuelling, the 12 might be a good upgrade, if you plan to stay stock. But, since you are reading this, I somehow doubt you'll stay stock for long.
Gone back to the 18 housing for better breathing...
Daniel
I ran for about 3 mo with the stock 18, got good MPGs, and it ran fine. EGTs got warm from time to time, but that is to be expected on a nonIC truck with a cranked pump. I had the chance last summer to swap with a guy who wanted a 21 and an 18 for a twins project he was working on, and had a 12 he'd trade for them. I jumped at it. I noticed an immediate increase in spoolup, a gain of about 8 psi peak boost, etc, etc. No REAL decrease in cruise or peak EGT, though it was cool to ba able to make 40+ psi with stock injectors, and peg the gauge in 3-5th gear with the EDMs. MPGs went down about 1. 5-2, though. I thought about that, stsarted driving like a bluehair on the way to church, tweaked the AFC, etc, etc. Then I read Forrest Nearring's post on the HX40-16 he had installed in his 96, and gaining MPGs. Theory was, the reduced backpressure let the engine breathe easier, thus the air in/ exhaust out was better accomodated, esp with lower gearing. Sounded ok to me, and would apply to my 4. 10s (which, BTW, are available for trade for 3. 54s or higher). I got the 18 yesterday via UPS from another guy, ported it to within an inch of its life, as I had ported the un-shrunken manifold when I put it on last month. Put it on this morning, and shipped the 12 to the guy who sent me the 18. Here are my preliminary observations:
the 18 DOES let the engine breathe better. I noticed a slight increase in EGT- maybe 50*, which can more than be attributed to weather. PEAK EGT is about the same, too- right around 1225-1250, but peak boost is lower. This is empty, but on the same stretch of road, similar weather. SOTP dyno shows no difference in pulling ability. Cooldown isnt appreciably different- down to 350 by the time I get to the red light; I can turn it off (<300) by the time it turns green again. BUT, I have gone from 8-10psi @ 60mph to 3-5psi @60mph. Peak boost was reduced by about 6psi, which is expected- 1 psi per square cm difference. There is a hair more lag, which can be adjusted by loosening the tension on the spring, to bring on the fuel at lower boost than with the 12, but I'll have to drive it a bit to see if it would be worth the difference. I DO like the lower drive pressure, decreased backpressure, though.
CONCLUSION- if you have the 2nd lowest final drive ratio (3. 54, no OD being lowest) , no intercooler, aftermarket injecotrs, and some pump tweaks, leave the 18 on there, or maybe go with a 16; the 12 is just too restrictive at higher boost/ RPM. If you have the highest final drive (Mom's truck- 3. 54 autoOD), and are at stock fuelling, the 12 might be a good upgrade, if you plan to stay stock. But, since you are reading this, I somehow doubt you'll stay stock for long.
Gone back to the 18 housing for better breathing...
Daniel
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I noticed how you said the dyno showed no difference in pulling power--i thought for sure the 18 would increase the pulling power 
. When I get the money, (read- win lottery), I plan on getting a 16 and in IC. 
to fix it. BUt, I noticed it didnt seem to work as hard to keep 65 up the hills, and it ran about the same temp once i got to speed. IF I had the money (or a female money bags, like some youngsters
