Switching from a 5.7L to a 6.5L

98silvz71 · January 4, 2006

I was looking into buying a 98 6.5L Turbo Diesel. I was just wondering if anyone had any comments on how much better they are. I have 5.7L Vortec and it has ok power but I was wanting some more. Does the diesel get pretty good gas mileage? I wouldn't really tow anything really big so I was looking at the old 98 1/2 ton models with the 6.5L. I would really appriciate any comments on the subject.

Shawn T · January 5, 2006

I was looking into buying a 98 6.5L Turbo Diesel. I was just wondering if anyone had any comments on how much better they are. I have 5.7L Vortec and it has ok power but I was wanting some more. Does the diesel get pretty good gas mileage? I wouldn't really tow anything really big so I was looking at the old 98 1/2 ton models with the 6.5L. I would really appriciate any comments on the subject.

<{POST_SNAPBACK}>

I stoled this link from another post 99 specs

Keep in mind these specs are for Heavy Duties and Hummers with the 4L80E.

If I remember correctly the C/K 1500's used the 4L60E and were derated about 10 HP and 15 FT-LBS of torque.

With 255 HP, your VORTEC 350 should run circles around the diesel unloaded. You mentioned that you were not towing much. I don't know exact fuel mileage, but I'm sure it is better than the gasser. I think that was GM's approach at the half-ton market, because they offered them in Blazer's back in the early 90's.

16gaSxS · January 6, 2006

I was looking into buying a 98 6.5L Turbo Diesel. I was just wondering if anyone had any comments on how much better they are. I have 5.7L Vortec and it has ok power but I was wanting some more. Does the diesel get pretty good gas mileage? I wouldn't really tow anything really big so I was looking at the old 98 1/2 ton models with the 6.5L. I would really appriciate any comments on the subject.

<{POST_SNAPBACK}>

That would have a "S" engine or L56 190 hp vs the "P" with 195 hp and the torque is 10 ftlbs less 385 vs 395 . The auto would be 4L80E , same as in my truck. Turbin Doc in Mississippi has simular set up with some power enhancments and he will tow up to 18,000 lbs for short distances. The big thing that make a difference with these 6.5TD is gearing. I went from 3.73 to 3.42 and I tow 5,000lbs about 175-200 days a year for the last 8 years. I have towed up to ~11,000 lbs and it does okay but would not want to do it regualrly,I'd go back to the 3.73 if I did. I have towed Travel Trailers from Yuma, AZ to Central Montana, right up the middle of the Rocky Mountains half dozen times, it's about a 1300 mile run. The Trailers have been from 23 foot to 27 foot and my truck does that real well. at 75mph with cruise running empty summer #2 I get 19.5-19.9 mpg towing a

23-27ft TT 65 mph 10.5-14.5 mpg. Empty if I slow down to 65mph I get 21.5-22 mpg. At 55 mph 24 mpg. Best crusie is at 1800 RPM (peak torque). rule of thumb for ever 5 mph faster you drive over the speed that gives you 1800 rpm you lose 1 mpg. MPG drop with winter fuel as it's "cut" with #1 because there is fewer BTU's per gallon of #1. This done to prevent fuel from gelling and fuel filters from pluging which start to happen at about 0 F with straight #2. With 30% #1 that drops to -15F straight #1 -40F (orC).

Now I have had mpg drop before I changed Fuel injectors, they will still work but not very well after about 100,000 miles. I changed the OEM injectors out at 156,000 and mpg jumped back up. A lot of things affect your MPG on these trucks.

98silvz71 · January 6, 2006

I was looking into buying a 98 6.5L Turbo Diesel. I was just wondering if anyone had any comments on how much better they are. I have 5.7L Vortec and it has ok power but I was wanting some more. Does the diesel get pretty good gas mileage? I wouldn't really tow anything really big so I was looking at the old 98 1/2 ton models with the 6.5L. I would really appriciate any comments on the subject.

<{POST_SNAPBACK}>

That would have a "S" engine or L56 190 hp vs the "P" with 195 hp and the torque is 10 ftlbs less 385 vs 395 . The auto would be 4L80E , same as in my truck. Turbin Doc in Mississippi has simular set up with some power enhancments and he will tow up to 18,000 lbs for short distances. The big thing that make a difference with these 6.5TD is gearing. I went from 3.73 to 3.42 and I tow 5,000lbs about 175-200 days a year for the last 8 years. I have towed up to ~11,000 lbs and it does okay but would not want to do it regualrly,I'd go back to the 3.73 if I did. I have towed Travel Trailers from Yuma, AZ to Central Montana, right up the middle of the Rocky Mountains half dozen times, it's about a 1300 mile run. The Trailers have been from 23 foot to 27 foot and my truck does that real well. at 75mph with cruise running empty summer #2 I get 19.5-19.9 mpg towing a

23-27ft TT 65 mph 10.5-14.5 mpg. Empty if I slow down to 65mph I get 21.5-22 mpg. At 55 mph 24 mpg. Best crusie is at 1800 RPM (peak torque). rule of thumb for ever 5 mph faster you drive over the speed that gives you 1800 rpm you lose 1 mpg. MPG drop with winter fuel as it's "cut" with #1 because there is fewer BTU's per gallon of #1. This done to prevent fuel from gelling and fuel filters from pluging which start to happen at about 0 F with straight #2. With 30% #1 that drops to -15F straight #1 -40F (orC).

Now I have had mpg drop before I changed Fuel injectors, they will still work but not very well after about 100,000 miles. I changed the OEM injectors out at 156,000 and mpg jumped back up. A lot of things affect your MPG on these trucks.

<{POST_SNAPBACK}>

Thanks for the info. You really helped out. I only pull a 16 foot covered trailer. I was wondering if anyone had towed with the 5.7 and the 6.5TD and which one they liked the best.

silverado3500 · January 11, 2006

I haven't towed a lot with the 5.7, but I have done a good bit with the 6.5 and with the (7.4) 454. You are looking at a lot of work to convert from the 5.7 to the 6.5. Plan on changing both the dash harness, and the engine harne,ss, the entire fuel system, exhaust will be different because of the turbo. It probably would be easier just to buy a truck with the 6.5, or find a complete wrecked truck with the 6.5. IMO the 6.5 would be a little better than the 350, but not my much. The 6.5 was designed as a fuel economy engine. Not a powerhouse like the newer diesels are. That said, I know several people who own 6.5td and all of them are getting well over 20mpg unloaded. Towing, the mileage isn't as good (obviously). There are performance upgrades that can be done, but nowhere to the extent that is offered with the newer engines.

Personally, I'd take a 6.5 over the 5.7 any day. However, if its sheer power that you are after(and you don't care about fuel economy), put a 454 in there with 4.10 gears. A truck with that setup would pass everything (but a gas station). The advantage with that option would be that there aren't as many parts to change (mainly the fuel system)