They are very different options.... but they both check all the boxes for what needs to be done, yes a 4bt makes better low end tq for crawling, yes a l83 has a better rpm range for highway use but i'm not building exclusively for either of those.
I probaby should have put my background for both types of motors in my first post and really this post was made as and i'm bored poll to see what people would vote for. I've personally owned and driven either through my own vehicles or company cars 5 LS based cars and one GEN V truck (countless Gen I and Gen II chevy v8s also) and trucks and i'd say close to 15 diesel trucks and cars. Everything from a BEW VW, OM617, OM603, OM606 mercedes, 6.2 6.5 gm, 6.6 Duramax, 8.1 Hino, 12V and 24V cummins, 7.3 and 6.0 Fords, etc. I know the differences between diesel and gas engines.
I personally don't really care about resale value of my vehicles, never had or will, that's not why I buy them. It is a good point to put for other people though and I should have put that in my original post as this was more a what would you do post and I may have made it a more what would Al do post. 4bt swapped 80s traditionally sell for less for a few reasons, most people aren't ready for the noise and vibrations from one and don't do the smart thing and balance it before putting it in. Also a lot of older ones were done with 700R4s which are one of the worst trans ever made in my opinion. People use them because they are simple with no electronics and can be built for fairly cheap but even built they have issues and then let you down. Its only been recently that a 6l80e combo was even possible on a 4bt as no one had made a way to control it. That trans will handle a 4bt and give it much better street manners with its gear selections.
All what you say is valid though.
I do have around 200K miles of seat time with 4BT's in a few different vehicles. I would not characterize them as having tons of low end torque. 4BT is fairly gutless until boost. I have used a 4BT offroad and overlanding and (IMO) it's a poor choice for enjoying the outdoors. Especially if there's anything technical where you need to communicate with a spotter. Good luck hearing them over the engine.
The problem with 4BT's and autos has to do with how little power they make off-boost and how big the trade off is between low RPM boost and inefficiency at cruising speed. 4BT's do poorly with tighter converters. Because of the inherent imbalance of an inline 4 cylinder engine and how the angular accelerations of the crankshaft are amplified through the timing gears affecting the injection timing you get some really wicked idle vibes when you put any converter load on a 4BT at idle. They need a sloppier converter to get up in RPM to make boost, but then they also need a real stout transmission that can lock the converter and handle very high torque at low RPM for the times when you are lugging under boost. I've tuned a lot of them with autos using various turbos and fancy converters and I will never do another one, but I'm pickier than average when it comes to drivability. 700R4/4L60E would be horrible. I have used 4L80E (not strong enough), 47RH and Allison 1000.
Edited to add: One other major dilemma for tuning an auto behind a 4BT is how dramatically different the engine makes power cold VS hot. It is really hard to come up with an auto trans tune that works OK when the engine is cold and also performs great when up to temp. You need multiple shift maps corelated to engine temp, but you can't really do that yet. That's why stickshift works so much better with a 4BT.
4BT is good in a fullsize truck when it's not a family vehicle. Especially anything that works hard all the time and tows. I had one in an F-350 4x4 and it was a great setup. 25 MPG running average with a close ratio ZF5. With a large air-air intercooler and a super HX30 If I could pull a gear to 3000 I could upshift and it would pull it in the next gear regardless of load or grade. I pulled cabbage hill in East Oregon at 55 MPH grossing 19,000 with that little 4BT at 30 PSI. Impossible to beat the fuel economy of a well setup 4BT I'll give them that too.
The 4BT idle shakes are unrelated to the balance of the rotating assembly. The reason a 4BT shakes is because all inline 4 cylinder 4 strokes have a combustion event imbalance . The 4BT has more wicked shakes than just about any other engine for a few reasons. 1- They do not have a balance shaft to cancel out the imbalance. 2- They have real heavy rods and pistons and they used a lightweight stamped crank pulley. The high reciprocating mass and loss of rotational mass on the front of the crank increases the angular acceleration created from each combustion event. 3- The Camplates and camshafts used inside the Bosch injection pumps on the 4BT's have aggressive ramps for a very fast injection event. This is one of the reasons you can really turn up a Cummins fuel system without melting stuff and why Cummins have a distinctive sound, but it's also why engines like Isuzu, Hino, Mitsubishi and Toyota diesels aren't loud and shaky. They use lazy cams in their pumps for a gentler, longer injection event.
Weight matching and dynamically balancing a 4BT will be more noticeable above 2500 RPM than at idle. What makes more difference than balancing is installing a heavier crankshaft pulley and flywheel. The added rotating mass resists the combustion power pulses. The other thing is good, balanced injectors. They still shake though no matter what you do.
5.9 has none of these deficiencies BTW and pulls down about 90% of the same MPG's with tons more power. The difference in off idle torque from a 4BT to 6BT is staggering. It's like 100% more torque, not 33% more like you'd expect.
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