Cummins 4bt or 6bt? (1 Viewer)

[RockJock82]

If you could have a 4 or 6bt cummins in your 60 series which would you opt for? I know both have been done. 4bt is lighter and smaller but can rattle a bit. 6bt has more power, no rattle but larger in size and weight. Curious on people's thoughts.

 

[dbleon]

6...

 

[GeologistFelix]

4BT takes a fairly large amount of lift (SOA with stock springs is considered the minimum 'safe' lift without extensive oilpan mods to clear the front steering components and axle), and it's loud and vibrates a lot, and they're quite expensive, but you can probably use your stock t-case and rear axle if you're not doing crazy things to considerably more than the factory torque out of it.

6BT takes a similar amount of lift, probably a fair bit of front frame reinforcement, and almost certainly a beefier t-case and a bigger rear axle. Quieter than the 4BT, though, and possible to get a good 12-valve one for cheaper than an equivalent-condition 4BT.

So, it really depends on what you're going to do with the rig. If you want to tow a boat over the highway, maybe involving mountain passes, I'd pick the 6BT. If you're mostly going to wheel it and not tow anything, the 4BT would probably be better.

Personally, I'd pick an Isuzu 4BD1T any day of the week for a 60-series, however. You can get most of the benefits of the 4BT without the drawbacks (engine height, noise/vibration, crazy inflated prices), so long as you're happy to only tune it up to ~200 HP/275 ft/lbs. For similar money and less labor than a properly set up 6BT swap, you could also get an almost bolt-in Toyota diesel like a 1HZ and stick a turbo on it for similar torque/HP numbers.

But, really, it depends on what you're planning to do with it, and how much you want to modify the rig.

 

[dbleon]

you could also get an almost bolt-in Toyota diesel like a 1HZ

+1000

 

[RockJock82]

Yeah Im not going to throw a Toyota Diesel in this rig... It already has a nv4500 in it so I will stick with that and a Cummins. Originally it had a ISB 4cyl Cummins in it but it was removed to be installed in a friends 80 series in the near future (SeaDrifter). Monday I will likely be picking up a 4BT though Im on the fence of just putting a 6BT.... Cost is about equal on the 6bt vs 4bt in AZ

 

[dohcdelsol93]

i can find 6bt's up to 1500 less than the average price of 4bt's locally.

4bt will require some tweaking to get to the power level that most people would be happy with as an on road vehicle, mostly just a turbo upgrade and/or compounds.

a 6bt will make pretty much more than most would use right out of the crate. It weighs more but who cares.

So long as you don't drive the 6bt like you stole it the stock transfer case should hold up fine. Many people put extremely powerful built v8s in their fj40/60 and do well with the stock drive train, i just don't recommend you rev it and drop the clutch at every red light in an attempt to out run a civic.

 

[The Phoenician]

Sorry for the rushed reply, I'll write a better one when I have chance.
I have a 4bt with nv4500 stock transfer case and stock axles. Mine doesn't rattle like most people claim, perhaps because it's a 4btaa or I'm too used to military vehicles. I know a couple of people who have the 6bt with stock transfer case, but theirs' is in an 80. The 4bt has plenty of power for anything you want your 60 to do, but I understand the seduction of a 6bt. Just be prepared to spend on axle and frame reinforcements (possibly a beefier transfer case) on what you thought you saved in getting a 6bt.

 

[RockJock82]

Yes the motor I am looking at using is a 4bta also with the inline pump (P-Pump). Probably try out the hx30 turbo first and if she isnt enough then bump up to an HE341 or hy35. I will also increase timing, fuel plate, gov spring the usual stuff... Should be a pretty fun truck to run once I get it done. Phoenician are you running the liquid isolators for mounts? For cost I cant find a 6bt in AZ for less than 3k but 4BT's 2500 all day long

 

[White Stripe]

Im sure if you don't have lockers and didn't tow a whole bunch of weight the stock tcase and axles would hold up for the most part to a 6bt. Install lockers and I would worry. If you are only using it for mild wheeling, I would go the 6bt. More heavy wheeling do the 4bt. Me personally, if I see a 60 on craigs with a 4bt I don't really care, if I ever saw one witha 6bt, I would check my bank account for funds. I would love to have one.

 

[The Phoenician]

Yes the motor I am looking at using is a 4bta also with the inline pump (P-Pump). Probably try out the hx30 turbo first and if she isnt enough then bump up to an HE341 or hy35. I will also increase timing, fuel plate, gov spring the usual stuff... Should be a pretty fun truck to run once I get it done. Phoenician are you running the liquid isolators for mounts? For cost I cant find a 6bt in AZ for less than 3k but 4BT's 2500 all day long

Yea the p pump is what you want. The hx30 spools up pretty quickly as well. I changed the governor to a 4k governor spring, makes a big difference. I'm not entirely sure about the mounts, They were put in while I was in Iraq but I think they are liquid. I wanted to clarify one thing, I changed out the rear axle to an 80 axle. I did it to get a full floater not because the 60 axle couldn't handle it.

 

[White Stripe]

Yea the p pump is what you want. The hx30 spools up pretty quickly as well. I changed the governor to a 4k governor spring, makes a big difference. I'm not entirely sure about the isolators, They were put in while I was in Iraq but I think they are liquid. I wanted to clarify one thing, I changed out the rear she to an 80 axle. I did it to get a full floater not because the 60 axle couldn't handle it.

Sounds cool, how bout some pics!

 

[The Phoenician]

I'm at drill, about to go in for the day. Search for big red toy, there's a few pics there.

 

[Kurtis]

What is the deal with the 4bt's and everyone having to be careful with their egts? I would be interested but It always concerns me how much thought everyone seems to put into driving their 4bt. Has anyone got one of these setup so you can drive it like a v8 cruiser with no concern how you drive whatsoever? That is what might push me to a 6bt.

Even in AZ with the gasser in mine It didn't matter how I drove or even if there was 5k behind it going up the mountains you could put the peddle down and not worry about anything.

 

[GeologistFelix]

Most people turn the fuel up pretty aggressively with 4BTs to get more than the stock 100-125 horsepower out of them, and as a side effect, the exhaust can get stupid hot and do bad things if you choose the wrong gear and speed for a long climb, especially with a load behind you. The same danger exists for any diesel you turn the fuel up on, but the 4BT is so beefy that you can turn it up ridiculously high without breaking rods/cranks/etc., so the heat can really get out of control.

But, back to your point, I think people who drop a ton of money into a 4BT (or other diesel) swap don't want to destroy their investment. The same would go for a V8, but they aren't normally modified to make 30-100% more power than when they left the factory, and they don't have a turbo to roast immediately downstream of the exhaust manifold.

 

[Elbert]

it you "hot-rod" on any of the diesels by increasing fuel delivery and boost on the turbo you get into the land of a narrow operating band respective to EGT. Stock you should just drive it without issue for the most part....when you hot rod on them....and you get into long periods of load on the engine or heavy throttle.....exhaust temps can go crazy. Before you entertain swapping engines you should go drive one with a 4bt or 6bt or yota diesel before you spend big $$.

 

[RockJock82]

This 60 will be getting a mild 4BT and I have no doubt when I am done with it power will not be an issue. Now Im not going to get crazy and add twins.

 

[GLTHFJ60]

Yes the motor I am looking at using is a 4bta also with the inline pump (P-Pump). Probably try out the hx30 turbo first and if she isnt enough then bump up to an HE341 or hy35. I will also increase timing, fuel plate, gov spring the usual stuff... Should be a pretty fun truck to run once I get it done. Phoenician are you running the liquid isolators for mounts? For cost I cant find a 6bt in AZ for less than 3k but 4BT's 2500 all day long

The HE341 or HY35 would not be an improvement over the HX30w. Many, many, many on-road tests and even more discussions over on 4BTSwaps have proven this.

As others have said, EGTs are always a concern for turbo diesels, and more so for 4BTs because people want to get more than 105 or 120hp out of them. (@GeologistFelix hit the nail on the head)

However, none of the 4BTs came with an air-air intercooler from the factory. They had air-water aftercoolers, aka the 4BTA (A stands for Aftercooled). Plan an intercooler into your 4BT swap and you will have to triple the power output of the 4BT, with the appropriate turbo or turbos, before you run into EGT issues. 6BTs don't typically have EGT issues because all of them have come with an air-air intercooler from the factory since 1991 IIRC. We can get into a discussion about air-air and air-water intercooler ups and downs if you'd like.

Another reason why EGTs are of such concern with the 4BT is because of the turbo statement I made above. The old paradigm with the 4BT was to stick a turbo from a 6BT on there and give it hell. 6BT snails such as the HX35, HY35, HE341 and HE351 are not matched to the flow of the 4BT (displacement is 2/3 that of a 6BT), so boost comes late (boost lag), which equates to skyrocketing EGTs. The IDEAL turbo (as a single or high pressure turbo in a compound set) we've found thus far for the 4BT is a tossup, between the HE221w6 and the HX30w6. You make 20psi by 2000 rpm easily with either turbo and EGTs are nice and cool, because there is enough air to burn all that fuel.


With tuning diesels, one important thing is the opposite from a gasoline motor. The richer you make the combustion mixture, the hotter your diesel will run. A rich gasoline motor will run cool. Therefore, all of your smoke blowing diesel guys all have high EGTs. Put a turbo on that can pump enough air in (if the current one is insufficient) and set your max fueling, as well as your boost-compensated fueling, appropriately, and you will not only have good power, but also clean exhaust and cool EGTs.



Cummins motors are more robust than Toyota Diesels when it comes to higher than stock rated power output levels. Longevity is probably pretty close between them in stock form, but you can put 40+ psi through a Cummins with nothing more than head studs and run it for a quarter million miles like that. Try doing more than 15 in a Toyota diesel.



Finally, having put 25,000+ miles on my 60 with a 4BT, that I did myself, I would recommend a 4BT over a 6BT, for multiple reasons, but foremost for me is that I have tons of engine bay room for accessories (intercooler included) as well as plenty of room to work on it.

 

[RockJock82]

I wouldnt ever think about not using a charge air cooler with a diesel, this 60 came with one. On the turbo note I would disagree with you. The HE341 and HY35 is a fine turbo to run with mods done to the motor. For a stock setup the hx30 is plenty though I plan on trying out the little guy (hx30w) and see how it does with minor mods. If my egts are too high I will add a bigger turbo.

 

[White Stripe]

I know many think differently. For a 60 I personally like the idea of a 6bt for a couple main reasons. Since you are looking at the diesel route your probably not looking to do KOH races or mud drags. Diesels do favor expedition type rigs and pulling. So I would say go with the 6bt. Whats an extra 400lbs? Put light weight bumpers on and get a trailer rather than a roof rack, to help lighten total weight. The axles will handle it as long as you don't run lockers. If you do run lockers, put a 14 bolt in and that should get you by. Similar reasoning to ppl considering either a 35" or 37" tires, in they end their always happier they went with the bigger. If you go with the 4bt I think youll be content cuz thats what you will have. But I bet if you go 6bt, you won't wish you did the 4bt. And how much fun would it be to keep up with diesel trucks up the mountain passes!

 

[GLTHFJ60]

If my egts are too high I will add a bigger turbo.

I know everyone is entitled to their own opinions, but this is flat out wrong. Putting a "bigger" turbo on a motor with EGT issues will not correct those EGT issues, it will exacerbate them, and produce more black smoke.

EDIT:

I should clarify what I interpret as "mods". I interpret that as a 3200rpm or 3800rpm governor spring, turning up the full power screw 2 or 3 turns, a Denny T fuel pin and maybe a dynamic advance spacer. With the correct turbo and proper tuning, this is capable of 250-300hp with a single HX30/HE221, and maybe more, with twins.

If your "mods" involve a fire-ringed head, huge injectors, a worked injection pump and a host of other things, then maybe a HX35w is appropriate, MAYBE, but you're still going to have bad EGT issues until that thing spools up.