More usable power, better gearing &/or better aerodynamics IDK?
Something's gotta give...
The M46 1st is a stump puller.
That was common for late 70s/80s eurotrash cars, had to be able to pull a trailer/loaded car up swiss grades in 1st/granny-low or something crazy by law to sell cars in Europe & pass emissions for the USA market with the same # of gears/existing platform at no major cost increase.
Can't have a bunch of windage/oil/faster spinning motor stirring up a bunch of crankcase HCs &/or burning real lean with a bunch of NOX on a high compression motor in the late 70s emissions era.
So they really drastically lowered/compromised the compression on most cars, gave them emissions cams, taller axles for 55mph cruise etc & horrible garbage truck gearbox ratios to be able to get going fully loaded.
Volvos actually fared a lot better than most though;
most other cars had:
-mellions of vac lines
-3-speed transmissions
-500 cubic inches pushing like 125HP.
-8:1 compression N/A
-EGR
-Air pumps.
-zillions of vacuum solenoids.
-Trying to get "progressive" carbs to burn cleanly
-Vacuum retard dists.
etc etc of horrific band-aids on existing non-compliant platforms sold by most companies.
'79+ the B21F got flat top pistons (-'78 are 8.5:1 compression dished/even more gutless), no EGR, updated bearings M46 w/fine spline input shaft that lasted much better, lambda trimming etc, so be somewhat grateful for what you DO have in the 40 year old car of
that era all told?
The Volvo is a tractor, but it won't really "accelerate" at all with a 3.54 or 3.31s or be able to pull grades in OD except at 70+ MPH, maybe near sea level with original wagon tires under ideal conditions, sometimes.
3.73 for 70+ mph or convert to 2.3 &/or turbo power?
B23 under k-jet head is kinda nice, though all B21F heads are large coolant passage/right next to the fire ring on a B23 headgasket.
& figuring out the spark curve for that or getting it to pass emissions/not detonate on cheap 87 octane gas would be its own challenge.
Volvo sold the B23E for Canada in either K or A-cam flavor, but it for sure WON'T pass USA emissions & all have air injection/BCP heads (bit of a buzz kill) & no lambda trim/constant idling speed system/electronic idle control (kinda nice, when it works).
& all those cars rusted out/died 20 years ago, so you'd be sifting thru old wet-shelf cores/junk we never got in the USA market.
Lots of engineering compromises to make cars pass lighting/safety/emissions laws/hammer coming down hard in 1974+ on the new big-$$$ low octane no-lead fuel/OPEC (1973) on a shoe-string budget/car companies not wanting to fully redesign existing platforms/band-aiding them then.
At least the little obscure car company Volvo cars was conveniently way more 'ahead' of it than most & less exposed having only 1-2 car platforms commonly sold that basically met the criteria already for lighting/safety/born ugly staid styling (not a sports car wearing giant diving board bumpers at the wrong/new federal mandate bumper heights)/fuel economy per ton hauled and didn't totally have to ruin their cars, just morphed them a little with some cautious engineering compromises & didn't have to re-market them to their probable buyers much (nerds/dorks in the USA).
'74-'77 was a real rough patch for car companies, and '74-'77 140/240s are the lesser/shallow end of the Volvo gene pool too, probably why most TBers/extreme volvo owner dorks get them for dirt cheap/fall in love with them?
They did what they could and still be legal/cost effective without making some really terrible compromises, especially '79+ after the first wave of "struggling to comply" 1973+.
3.73s are dirt common, but 3.54 isn't in 1031 (if that matters/you need longevity carrying weight with some torque).
Funny though, on the cold/damp fog NE/upper PNW cars running around with toothpaste rear axle grease that never gets warm/has condensation in it/leaking pinion seals, even the 1031s are worn out on some, by now...
Probably best to just find a surviving 3.73 out of a late car & appropriate speedometer drive gear for the trans...
Even though the auto trans is gentler on the rear axle generally speaking, I've seen quite a few noisy 3.73s in abused late cars now.
3.31s the pinion is giant & most of those gears get warm/a ton of fuel efficient highway miles is my theory.
Leaks get addressed and the gear oil actually gets warm enough to steam off all the condensate/sweat in the housing.
Auto-tragics with steeper axle ratios have smaller pinion spinning faster getting few miles/slammed around the city on cold/condensate toothpaste for gear oil that never warms up.
Grab a 3.73 from a late auto from a dry warm weather/long highway/hot valley commute area car like Hell.A/inland empire/Phoenix/Dallas/similar spinning fast under light load/cruise most of the time with no moisture?
IDK what to say?
Drive a Volvo that's pre 74 & '71-'73 D-jet with good ratios, but doesn't have to contend with the exhaust emissions constraint, yet & 3:1 1st and 4.10s/4.30s?
Or 1982-1983+ and EFI/computer controlled ignition/10:1 compression after they figured out how to comply with smog/get some more torque/really broad torque curve to pull the thing along with added displacement/compression and still pass emissions/get going up hills fully loaded (garbage truck ratios/same # of gears) with good economy, but it for sure won't rev very much/happily at all with 3.31s.
T-cam is better than the M/will play nice with all the existing management/wont' fry the kitty, but it won't pass emissions quite as squeaky clean for NOX.
4-speed auto by then sourced from Toyota/Aisin/copied from Borg Warner 03 design that the 2.3 could actually pull along helped too.
Or?
??? try to mix the junk bin around @ .17?/hr to find the compromise (as volvo sorta did, but was stuck with K-jet) between the two above options?
The guys in white lab coats at volvo did what they could for ya...
If you can bend a little on the emissions constraint for NOX or keeping the car totally original, and want to put more of your time in than just buying an '83+ car or -'72-'73 model would cost...
'79 is pretty good tho...pre wiring harness screwup by one year (chlorinated plastic being banned on most euro-trash cars then with such (existing) toxic manufacturing chemicals/processes), post horrific emissions era contraptions/compression kill now with lambda-sond & MUCH better updated fine-spline M46 by 1 year!
Could be worse?
Can't say they weren't trying/didn't do pretty well compared to other car companies or even compared to their '74-'77 "automobile dark-age" selves and where they were headed EFI+, but weren't there yet/at least they gave you something better in the inter-rim?
Could be american with terrible use of space/interior volume, fuel economy, rusts out in 2-3 years, tons of emissions contraptions, & big heavy engine with about the same amount of power as a B21F?
Are they the greatest ever? No.
But made it 40 years/still return decent economy/safety/emissions/didn't rust out instantly/can haul a lot in/on a wagon.