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Discussion Starter #1
I'm on the fence here but leaning. From you that know the 6.5L's I want your opinion. I love my 93 Dually but the old 6.5 is not going to last forever. I've been doing some reading about the Cummins 4BT which can be tweaked to over 300HP safely with little work and make great mileage to boot. My truck has 4.10 gears & currently running 235/85 16's which spin 62 mph @ 2000 rpms getting around 18 MPG unloaded. I'm guessing the 4BT will do 25 MPG or better under the same conditions from what I've read. I'm looking for more power and better mileage than my 6.5L can deliver, so is there a way I can have my cake and eat it too with the 6.5 or should I spend my rebuild money on a Cummins swap? :confuzeld
 

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I don't think you would be happy with a 4BT swap. I thought about it for a '78 FJ40, but for your truck, I don't think so. Many have done 6 cyl. cummins swps with great results. Look through the 6.5 forum, this was recently discussed. If you have some fab skills and the desire/ability to do a hybrid, the cummins swap can be a great idea. For many, plug and play with a new 6.5 is the way to go...
 

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the cummins 6cyl is way to go for power and less maintence. The 6.5 is great if you keep it happy but are limited in the power generating area. Depending on how much $$$ you want to throw at it! But would drive your 6.5 into the ground before swaping, they're good for plenty of miles!
 

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Discussion Starter #4 (Edited)
Here are some numbers I've been looking at.

GMC 6.5L Stock
187 hp @ 3400
379 ft-lbs torque @ 1700
Redline 3500 RPM's
engine weight about 700 LBs

Cummins 3.9L Stock
120 hp @ 2200
320 ft/lbs torque @ 1800
Redline 3000 RPM's
engine weight about 700 LBs

Are these numbers correct? I've been told the 4BT can make 275-300 HP @ 2300 with 25 PSI boost and IP adjustments and still give great MPG's while unloaded. Can I expect the same out of my 6.5L and not worry about failure? From what I've read the Cummins is virtually bullet proof. If I go with the Cummins I plan on running a Gear Vendors to help out on the Redline RPM difference.

I know the 5.9's conversions work, but for me I feel the 5.9 is just overkill with an added 400+ pounds of front-end weight, less fuel economy unloaded and being longer having to deal with interference problems. With the 4BT I can keep the 4L80E and have plenty of room for other stuff up front like a monster intercooler. :cool:
 

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I would have done the 4bt swap on my 6.2 half ton 4x4 pu when it needed replacement a couple of years ago . but i did not know they made a road version of the motor until shortly after i had allready stuck in the 6.5. it has more power but less mileage than the 6.2. a friend of mine knows of two local rigs that have 4bts and one with a 6bt. have yet to see them with my own eyes. will take photo's if I ever get the chance. saw a 79 bread van with a 4bt and turbo 400 in perfect shape on craigslist for 6500 thought about pulling motor and trans and sticking a gasser in it and passing it on. I would like to do a mid 80's suburban.
 

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And one thing I think you are forgetting stingray, power comes from fuel. If the motor makes 300 hp it's not coming from plain air. I think it would be easier and cleaner to just get the 6.5 to do what you want and use gearing to get the mileage.
 

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Discussion Starter #8
And one thing I think you are forgetting stingray, power comes from fuel. If the motor makes 300 hp it's not coming from plain air. I think it would be easier and cleaner to just get the 6.5 to do what you want and use gearing to get the mileage.
I agree that power comes from more fuel but only when you need it right? (Under Boost). It seems to me that the 6.5 unloaded normally aspirated would need to use more fuel spinning 8 cyl's continuously while cruising @ 65 MPH vs. the 3.9 only satisfying 4 cyl's at the same speed. My question is, do you think the 6.5 could safely make as much peak HP (without allot of expensive mods) as the 3.9 Cummins and still give the same MPG's unloaded? That is with drive train being optimized and equal on both. I would think the Cummins could safely work the turbo much harder to achieve max WOT HP while the IP curve could be tuned to match what ever max boost given. Right?



6.5L 20MPG @ 65 ----------------- ?Max HP WOT
3.9L 35MPG @ 65 ----------------- ?Max HP WOT

Turbine Doc, thanks for the links reading now...
 

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I don't know the 3.9, but if a swap is in your heart of heats, I think you would be better served with a 5.9 6 cyl Cummins swap, still not as inexpensive/easy as you think.

If you have fabbing skills you can build you own IC from ebay parts which would be your biggest expense if buying a prebuilt kit, you have a mechanical IP so just turn up the wick for fuel, if you are towing heavy find a set of 3:73 which will help mpg and still be stout enough to do some work with it. I've got maybe $4K in stuff one needs to make more power, reflash (you won't need), PMD cooler (you won't need), IC (will need if HD towing is required), Balance flow kit (still testing that one but I think a good one for a towing vehicle), H/O injectors (try without them and upgrade later if turning up wick doesn't do it for you)

Maintainability will be another consideration lots of "used but good" & performance 6.5 stuff, also some 6.2 stuff will work. For the 5.9 option plenty of that as well, if I wanted a project the 5.9 swap would be way I went. You might want to PM D Cam the lead mod for the 6.2 forum he has made several swaps and can give you some been there done that advise.
 

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Discussion Starter #10
I don't know the 3.9, but if a swap is in your heart of heats, I think you would be better served with a 5.9 6 cyl Cummins swap, still not as inexpensive/easy as you think.
The 3.9L Cummins I'm referring to was in the GM HD AeroVans. They came mated to 4L80E's. I think the only fab work required would be the motor mount relocation. Someone that knows tell me if I'm wrong. I'm planning to get the whole truck and rob the parts including the Cummins optimized 4L80E computer and torque converter. I was thinking about a cowl induction hood to clear the tall Cummins instead of doing a body lift. As for the 6.5L how lean can they made to run unloaded without drive ability issues?
 

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Discussion Starter #11
Here's a pic of the GM AeroVan doner I'm talking about. Comes with a Cummins 4B 3.9L and 4L80E Transmission.
 

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when it comes to making things pencil out. stock is always the way to go . but when you want something alittle out of the ordinary make yourself wait six months and see if you still have the bug to convert to a cummins and in the mean time do as you are doing . collect info. I have had people tell me I am nuts on several projects that I have done thats what makes me, me. my buddy says his contact with the 4bt in a burb gets close to 28 empty but when he loads his three horse slant and heads for the mountain pass he is doing 40 to 45 mph instead of ? i have pulled with stock 6.2 since 84 and still use them today. i can live with not going 70 at the top of the hill trust me we drive an 84 mercedes turbo diesel i would be luck empty at the top of the pass to be making 45mph. keep us posted with thoughts i am absorbing as much as possible.
 

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You won't have any of the "issues" you most often read about with a mechanical IP, remember it's not all doom and gloom forums mostly are populated by folks looking for help, and are just but a small sampling of the population as a whole that don't have problems (see FAQ my truck is running great) Mine other than the PMD and an alternator has had zero problems, (Drat now I'm in for it).

Evidently you got a good block I'd rebuild that, that many miles have the IP freshened up new injectors and upgrade the cooling with a retuned fan clutch, and maybe a dual thermostat H/O pump(not sure of pump rotaton will allow that on yours), good cleaning of the radiator, extra trans cooler, deep pan and balance flow. Opened exhaust & gauges is a must do IC if needing that last reserve in the 6.5

I've been in those cube vans, with 6.2, though no pick up at all IMO, I'm guessing the 3.9 is like a clydesdale horse same 1HP as 1/4 horse but you ain't going to get there fast.

For the hiway IMO you need a proper balance of grunt and performance unless you really have a targeted goal in mind, 300 HP and hi mileage reduced performance mite work for you, (this is an assumption I'ver never been in a 3.9 powered truck). My tweaked 6.5 is only capable of pulling 18K loaded GN trailer at 75mph @ 12-13 mpg up a small hill :D

So I'll still stick by my recommendation to freshen up your 6.5 and you will be better served, but hey best I can recall nobody I've heard of has done a 3.9 swap will be a neat conversion to watch.
 

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Discussion Starter #14
when it comes to making things pencil out. stock is always the way to go . but when you want something alittle out of the ordinary make yourself wait six months and see if you still have the bug to convert to a cummins and in the mean time do as you are doing . collect info.
:exactly:

I haven't made up my mind for sure yet, just trying to collect as much info as possible. But I'm definitely leaning toward the Cummins for economy and reliable performance. I know I could get the 6.5 built to handle 300 HP with no problem with the help of Turbine Doc and others here but I would still only get 20 MPG at best while crusing. My buddy that does my machine work said the 3.9L Cummins would be cheaper to build and would be a more reliable engine under high boost apps. I mean the things a beast @ 700lbs for only a 4 cyl. He also said if I wanted to keep my 4.10 gears the Gear Vendors OD would be a requirement to reduce highway RPM's since the 3.9L makes most of it's torque down under 2000 RPM's. :crazy:
 

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Here are some numbers I've been looking at.

GMC 6.5L Stock
187 hp @ 3400
379 ft-lbs torque @ 1700
Redline 3500 RPM's
engine weight about 700 LBs

Are these numbers correct?
Splitting hairs here, but those numbers are for the non-HO 6.5, or the "S" engines. The "F" 6.5's make a little more power: 195hp, and 430 ft. lbs of torque. If you bought a new crate 6.5 TD from GM, it would be the same "F" engine power levels.

I've been told the 4BT can make 275-300 HP @ 2300 with 25 PSI boost and IP adjustments and still give great MPG's while unloaded. Can I expect the same out of my 6.5L and not worry about failure?
With the stock 6.5 compression ratio, I would be concerned about longevity at 300hp with the stock pistons and crank. The weak link in the 6.5L in terms of cranking the boost up for more power is the very high compression ratio of I think 22:1.

On a separate but related note: I came across this website: http://www.62-65-**********.com/news2.htm

Sorry guys while it's good info we don't own rights to it so we can't post it Edited by TD

Does this mean my '99 Suburban has this lower 19.5:1 compression ratio?? This is the first I've heard of this change. I thought they were all 22:1.

My truck does have an 8 quart oil capacity. Not sure about the composite fan though - I'll have to check. I thought all the hp of the "F" engines was 195 - this is the first I've heard of it being 215. Can anyone confirm this article?
 

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Discussion Starter #17
With the stock 6.5 compression ratio, I would be concerned about longevity at 300hp with the stock pistons and crank. The weak link in the 6.5L in terms of cranking the boost up for more power is the very high compression ratio of I think 22:1
That’s why I plan not to mess with this 6.5L and just build something on the side that would be a quick drop in this summer. As far as rods and bottom ends go, check this pic out for a comparison. Man that’s a big rod...

As far as compression goes, I've read that somewhere before but mine is 22.5:1 for sure...
 

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Discussion Starter #18
Um ya probably shouldn't post articles from The Page since it's subscription based,,,,
Sorry bout that...

Heres where to look for a very good artical about 6.5L economy...

The diesel page .com/mileage.htm
 

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I strongly feel like you have to be a hard core Cummins fanatic in order to be a canadate for a conversion, however if that describes you there is nothing more rewarding than getting in you Chevy and firing up a half dozen pistons!!! (in your case 4) I have done several conversions and I thiink they are the sweetest thing since honey comb. As far a mpg I got 21 mpg with this list of negative factors. Pregnant wife, 2 small children, slept for over an hour while idling, punched it every time I took off and had a pretty big boost leak in my turbo system, so I think there is some really good mileage to be had from even the 6bta. I think I am aloud to say you might check out diesel truck resource or tdr for more info.
 

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can't seem to view any page there.
I think if you register with them you can access the site for general viewing & forum posting, to get full access the site you have to pay membership, but direct copy of their stuff here can't be allowed so myself or one of the other mods will have to delete it as I did above.
 
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