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Old 09-05-2019, 09:48 AM   #1
Bricktothefuture
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Default Tall rear end in an M46 Kjet car?

My '79 originally had a 3.91 in it, was toast when I bought it. Had a tech install a used 3.73 from a 93 that's been loudly singing me the song if its people for a while now. Have tried fluid flushes, thicker stuff, synthetic, 50% lucas, etc. No luck.

Even with the 3.73 I still find myself wanting a slightly taller gearing - I use this car as a regular driver and rarely take it over 4K RPM. My early M46 has the ridiculously short 1st gear that does little more than get the car going from a stop. Would like to address that and lower my revs at highway speeds. I'm at 3.1K revs @ 70MPH.

Have thought about a 3.54 or even 3.31 but have seen some concerns that a kjet car doesn't have the power to handle taller gears very well - is the acceleration really that much of an issue? Climbing hills still doable? Or should I just get a quiet 3.73?
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Old 09-05-2019, 12:25 PM   #2
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The thing is a 79 B21F is designed to rev up a bit more than the B230F. So for any performance with a tall rear you will need to use the rpms and lower gears to get the power. That said if you are just tooling around. The 3.54 is probably the best compromise gearing.

I have the 3.31 in my 93 wagon and it is good for cruising. Adding a bit more power helped. That way if I need power I can rev it up and wind out the lower gears for performance. First is still fairly useless though. Your early first is actually better so that should help you out.

Don't forget you can also adjust things with tire diameter. I noticed many on here use taller tires than stock. With a 3.73 that can help wiht cruising. To help out my 93 wagon I use shorter than stock tires. That helps give me a good compromise for performance vs cruising.
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Old 09-05-2019, 12:28 PM   #3
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The thing is a 79 B21F is designed to rev up a bit more than the B230F.
First I've ever heard someone say this, very cool. I spent a decent amount to rebuild the head, go first oversize flat top, etc. I know I -should- rev the hell out of it but there's that built in fear of blowing up a 40 year old engine. Need to get over that.
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Old 09-05-2019, 12:45 PM   #4
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I had a 79 with a 3.73 rear in it. I would go to 4k with most every shift. 1st through 3rd. I used slightly taller tires like 195-70-14. Still was over 3k when going highway speed. But the engine didn't care.

With the B230F in my 93 wagon. It is about 2500rpms at 70mph. So the 3.31 will give you a lot less highway rpms. If you can find a 3.54 that is a good compromise to help keep some power available with the B21. When my wagon B230 was stock. It sucked climbing any real hill and would need a downshift. Now with the V15 cam and chipped ezk along with an exhaust it is fine except for the big hills. It has power at all rpms which is nice because it will pick right up if you do need to downshift for a big hill.
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Old 09-05-2019, 01:14 PM   #5
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What's odd is that I can't find a straight up walkthrough on swapping the rear axle anywhere. Lots of videos/articles on welding your diff, etc, but no clear step by step stuff.
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Old 09-05-2019, 02:21 PM   #6
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You mean swapping the whole axle or just the center diff? The while axle is a bolt in. Brake cables, brake lines and driveshaft flange and all the bolts holding the shocks and suspension arms to it. It's straight forward to swap the whole thing. If you have an under the axle exhaust it is a bit of work to deal with or just remove it while doing the install. Just be sure to have the correct size flange on the pinion for the driveshaft.
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Old 09-05-2019, 02:30 PM   #7
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Unless you swap in a T cam for some torque I'd reccomend sticking to the readily available 373:1's.
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Old 09-05-2019, 02:34 PM   #8
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Unless you swap in a T cam for some torque I'd reccomend sticking to the readily available 373:1's.
Running a B w/adjustable gear.
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Old 09-05-2019, 02:42 PM   #9
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3.54 is a bit tall for a B21F.

& a LOT harder to find in 1031 than 3.73/virtually impossible to find in 1031 in 200 series housing, if that matters.

Are you running comparable size tires to the 185R14 245s came with unloaded?
Or is it a sedan/coupe w/185-70-14 tirettes on it?

Even unloaded, with the 3.31s & a T-cam, the healthiest high compression '83 stripper/light weight/no options/no A/C 245 w/B23F can barely pull gradual interstate grades in OD/5th at slow Oregon speed limits (50-65) with 185R14 wagon tires, but will.

Fuel econ is unlikely to be good in the brick at 60-65+mph, whatever way you slice it.

With the B21F/B-cam, they had it kinda right with 3.91s...

140s had 4.10s & 4.30s in a lighter weight car before they had to pass emissions with the same amount of power, 3.73/3.91 are already kinda an emissions era compromise.

That said, the M400/M40/M410/41 has a 2.95/3:1 1st gear in the 140, which is nice.
With EFI/a very very healthy torquey B20E, the 1800E automatic 3.91 is kinda nice/welcome with M41/one step down.

Analogously, 3.73 w/B21F even with original height 185R14 tires probably isn't "bad."
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Old 09-05-2019, 02:45 PM   #10
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By the time you overwork the engine with 354's you won't see any fuel savings, in fact I'd dare say the all around fuel economy will actually suffer even retarding the B cam.
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Old 09-05-2019, 02:53 PM   #11
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Hmm. I know a 3.91 would mean I'd be around 3.3kish at 70 Mph. How would it affect my short first gear? From what I understand it would make it even less useful, right? Total admission that the gear ratio stuff sometimes gets a bit confusing.
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Old 09-05-2019, 02:54 PM   #12
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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.

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Old 09-05-2019, 09:17 PM   #13
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B21F-MPG cars had 3.54:1 rear axles with the M46. They also had an L cam that produced the torque at 500 more rpm than the B cam. So, I highly doubt that a 107 hp B21F is going to suffer with a 3.54:1 rear axle behind it. A 1031 axle would be nice, but not necessary with 107 hp.

Swedishiron.com ran with a 3.54 in his old "beige puzzle" '79 245 for years. It only felt slightly slower than my '81 242DL, which had the 3.73:1 rear axle.
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Old 09-05-2019, 09:22 PM   #14
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IDK what to say?
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Old 09-06-2019, 02:10 AM   #15
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B21F-MPG cars had 3.54:1 rear axles with the M46. They also had an L cam that produced the torque at 500 more rpm than the B cam. So, I highly doubt that a 107 hp B21F is going to suffer with a 3.54:1 rear axle behind it. A 1031 axle would be nice, but not necessary with 107 hp.

Swedishiron.com ran with a 3.54 in his old "beige puzzle" '79 245 for years. It only felt slightly slower than my '81 242DL, which had the 3.73:1 rear axle.
L-cam produces M-cam power curve with a B21 is all, some of us actually do these things, dyno them or degree wheel them.
It may well happen 500RPM later wide open on the dyno, but with all the overlap of the B at the lower range, it's going to suck more fuel if you lug it with a tall axle ratio anyway, negating all theoretical economy benefits that only appear on those 65mph ideal conditions downhill runs or with sedan tires maybe or some other such improbable idealization/ TB tweak fantasy rather than broad no-fuss civilian use.
Part throttle the B-cam overlap is basically acting as a form of EGR, negating going to the trouble of having to install an additional EGR device, but also will use more fuel if lugged/loaded with a taller axle.
May well make a little more power a little sooner full load/wide open quoted dyno test (what doesn't suck less than an M or L for power curve?...even the T is better/decent for what it is, especially in the turbo car as-is, but won't pass NOX emissions as clean as M or L, except on the more load at cruise/lower static compression turbo car), but that's not the whole story.

Having had an '82 cali B21F LH with the L, not strangled by K-jet (and the K-jet car with the L-cam emissions/gas-mileage hipermiler too), but leaner burning with the LH limiting power about equally to the N/A k-jet strangulation, sure, it's got a 3.54, but it doesn't really "accelerate" at all, more just gains speed with the gutless L making almost the same exact shape power curve as the M-cam 2.3s, which got 3.31s, having *just enough* torque to pull themselves along ok for emissions/economy at cruise compared to their slightly lower displacement (& equally importantly, compression) 2.1s.

It matches the power curve of the L-cam B21F well...in the same way the 3.54 matches the power curve of the D24 very well...it'd have to it's so gutless/the revs cut off so soon!
Has to use all the power in what band there is(n't) & return the goods on economy as the engineering compromise, and pass emissions on the L-cam emissions gasser...diesel...save a bit of fuel at cruise and limit the smoke show?

They're all slow, but with the B, good luck actually increasing the economy measurably, just makes the car slower/shift points worse with the garbage truck ratios than they were before.

But sure, go pull the 3.54 outta the '82 stick or diesel stick if you wanna.
Works with the L in the hipermiler well, diesel & turbo driver with a bit more compression/healthy to help it out.
Lots of the 1030 on the NE cars and city driven PNW cars are all hooped out now, more than absolute strength being the issue.
Go as new as you can/get down the road far as you can with one of these old buckets at this late date, as the OP already found out was the only thought.
Pull a known good rear axle out of a living highway driven maintained late LH car that actually works/fresh kill, not some ball of rust k-jet dead rat full of 40 years of toothpaste/condensate?


Volvo got it pretty right, they're already compromising for emissions compared to the lighter weight D-cam B20E cars w/4.10s & 3:1 1st.
Any taller than 3.73 with wagon tires and the B is unlikely to help & in likely real world scenarios, hurts.


INB4 the 79-82 3.71 1st instead of 4.0whatever.

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Old 09-06-2019, 02:21 AM   #16
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^

The most miserably slow car in a long line of miserably slow cars.

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Old 09-06-2019, 08:33 AM   #17
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Thanks for all the feedback. Thinking I'll end up going with another 3.73. I want to be able to enjoy this car, and the whine really gets to you once you're an hour in to a long drive.

On a kind of related note, after what dl242gt said about revving these engines I decided to try not being so conservative with everything. Had a little fun getting up to 5.5k in third on my way home. Advanced my cam gear from 2 to 6 degrees to get my power band to around 3K and was really happy on my first few test drives...
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Old 09-06-2019, 08:35 AM   #18
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^

The most miserably slow car in a long line of miserably slow cars.

Also worth mentioning when I rebuilt the engine a few years ago I had the shop go first oversize pistons w/flat tops and take .040 off the top. Running k-jet juuust a little rich, ignition timing around 14 degrees advance. Cam at 6 degrees advance. She's not quite an old dog.
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Old 09-06-2019, 01:26 PM   #19
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Nice to see you are enjoying it a bit more and it's not such a slowbie.

BTW long ago I had an 81 wagon with M46 and 3.31 rear. That was an early M type emissions cam in the poor kjet B21F. That car was so god awful slow it gave you trouble from people wanting to hit you from behind accelerating from a lite. Even with the flat flywheel to help. The cam and high rear axle combo killed any performance. Never mind the tall wagon tires.

So when I got a similar combo with my 93 wagon which is B230F, M47, 3.31 rear. I added the V15 cam, chipped ezk, and simons sport exhaust to at least help it out and that has worked very well. Not fast but keeps up with traffic now.

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Old 09-06-2019, 02:00 PM   #20
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L-cam produces M-cam power curve with a B21 is all, some of us actually do these things, dyno them or degree wheel them.
It may well happen 500RPM later wide open on the dyno, but with all the overlap of the B at the lower range, it's going to suck more fuel if you lug it with a tall axle ratio anyway, negating all theoretical economy benefits that only appear on those 65mph ideal conditions downhill runs or with sedan tires maybe or some other such improbable idealization/ TB tweak fantasy rather than broad no-fuss civilian use.
Part throttle the B-cam overlap is basically acting as a form of EGR, negating going to the trouble of having to install an additional EGR device, but also will use more fuel if lugged/loaded with a taller axle.
May well make a little more power a little sooner full load/wide open quoted dyno test (what doesn't suck less than an M or L for power curve?...even the T is better/decent for what it is, especially in the turbo car as-is, but won't pass NOX emissions as clean as M or L, except on the more load at cruise/lower static compression turbo car), but that's not the whole story.

Having had an '82 cali B21F LH with the L, not strangled by K-jet (and the K-jet car with the L-cam emissions/gas-mileage hipermiler too), but leaner burning with the LH limiting power about equally to the N/A k-jet strangulation, sure, it's got a 3.54, but it doesn't really "accelerate" at all, more just gains speed with the gutless L making almost the same exact shape power curve as the M-cam 2.3s, which got 3.31s, having *just enough* torque to pull themselves along ok for emissions/economy at cruise compared to their slightly lower displacement (& equally importantly, compression) 2.1s.

It matches the power curve of the L-cam B21F well...in the same way the 3.54 matches the power curve of the D24 very well...it'd have to it's so gutless/the revs cut off so soon!
Has to use all the power in what band there is(n't) & return the goods on economy as the engineering compromise, and pass emissions on the L-cam emissions gasser...diesel...save a bit of fuel at cruise and limit the smoke show?

They're all slow, but with the B, good luck actually increasing the economy measurably, just makes the car slower/shift points worse with the garbage truck ratios than they were before.

But sure, go pull the 3.54 outta the '82 stick or diesel stick if you wanna.
Works with the L in the hipermiler well, diesel & turbo driver with a bit more compression/healthy to help it out.
Lots of the 1030 on the NE cars and city driven PNW cars are all hooped out now, more than absolute strength being the issue.
Go as new as you can/get down the road far as you can with one of these old buckets at this late date, as the OP already found out was the only thought.
Pull a known good rear axle out of a living highway driven maintained late LH car that actually works/fresh kill, not some ball of rust k-jet dead rat full of 40 years of toothpaste/condensate?


Volvo got it pretty right, they're already compromising for emissions compared to the lighter weight D-cam B20E cars w/4.10s & 3:1 1st.
Any taller than 3.73 with wagon tires and the B is unlikely to help & in likely real world scenarios, hurts.


INB4 the 79-80 3.whatever 1st instead of 4.0whatever.

Whatever is right.


If the OP wants decent acceleration, he might want to go with a 4.30 or shorter. Even a 3.91 causes a stock B21F engined car to slowly accumulate speed. lol Even a 3.73 equipped B23E or F car feels slow. Want decent acceleration? B21FT, B23FT, B230FT, etc. Want fast? LS or 2JZ...



Re: your telling me to go read the manuals, go do it yourself. You will discover that, yes, the MPG version had a higher torque peak and produced less torque than the equivalent B cammed engine, when both were new and bolted to an engine dyno.


Most 240 rear axles are hooped by now, even the 1992-93 versions. Not very many super low-mile 240s out there anymore. Those that are, are either headed to Europe, where they belong, or aren't affordable on a TB budget and only get parted when the insurance company totals it because they live in wholesaleland. So, the OP should probably just get the one he has rebuilt. I wouldn't even bother getting a replacement axle, unless it's from an AZ or TX car that was wrecked when grandma didn't see the car in front of her on her weekly trip to the church. Something with 75K miles or less.
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Old 09-06-2019, 02:45 PM   #21
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Point is, the B-cam car won't get any better economy with a 3.54, you'll just lug it/get reversion with the overlap EGR B-cam & stock wagon sized tires in normal use.

.40 over and a little more compression might pull it with the 3.54 a bit though.
3.73 are common/came in some of the B-cam B21F cars, more than that it's down the tweak rabbit hole and trying to make it something it kinda isn't.
L-cam you can lug like the M & sip fuel/pass emissions.
T-cam you can lug like the L/M with almost as much power as the B, but can't pass NOX emissions quite as clean.
But then you get to try to get the timing & fuel right.

Overlap, limited lift+low RPM+ partially closed throttle=reversion/EGR effect, good for NOX and mid-range power like the B, or peak torque mid range wide open, but more fuel use part throttle lugging, which is what happens with M46 ratios+tall axle minus a turbo to help pull it along and still be able to burn lean at part load/under 3psi in the case of the B21FT cars.

More torque wide open as a line item in the book, yes, but the time spent wide open is rare in normal driving, and with the longer rear axle, lugging it in some vacuum it'll just burn more fuel for equivalent moderate acceleration/cruise (most driving).

Made the car pretty useless...slower/even more inept to tow/drive loaded down and using more fuel most of the time!
More carbon on the intake valves unless you really beat on it all the time.

Slightly quieter for ideal conditions 70mph cruise, but that's rare, and the whole old car is noisy (engine tractor noises and inevitable 240 wind/chassis tractor noise) (less bad on the garaged super low mile 89-93 cars with more goop/seals/plastic all still soft/like new & cloth seats to help absorb the noise a little more).
Economy basically goes down the toilet with the brick in the wind 70+ MPH whatever way you slice it anyway.

Most of the 1031s that get highway miles/warm are surviving enough to work with in the high production year EFI / galvanized body & newer cars that actually work for broad spectrum use by average people instead of the n/a k-jet emissions era uninspiring cars most tbers like as a cheap date to molest from that late 70s/ very early 80s (pre EFI) lost decade make good donors, but none come with a 3.54 really.
Something still living with not quite so many years on it & maybe even the late magnet+screen filter with the alloy cover.
City/damp weather/leaking pinion seal seems to be the deadly combo more than miles.

Last edited by Kjets On a Plane; 09-06-2019 at 03:21 PM..
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