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79c25 · 09-17-2015, 09:40 PM

4bt, nv4500, np205 passenger drop. Should put you around 1800 rpm in 5th going 60. I know in my old 12 valve I got about 20 mpg going 65 everywhere, I think that the 4bt will do a little better. But, that's if you wanted to go with a manual. As far as an auto the 4L80E should be right about the same. But I'm interested in this swap as I'm looking to do the same in my 84 K5. As a side note the 12 valve does fit in the engine bay but I wouldn't use a 4L80E behind a 12 valve personally. That's my $0.02.

09-17-2015, 09:40 PM	#2
79c25 Registered User Join Date: Jul 2014 Location: Lovell WY Posts: 93	Re: 3.9L Cummins/Isuzu diesel swap in 89 Suburban 4bt, nv4500, np205 passenger drop. Should put you around 1800 rpm in 5th going 60. I know in my old 12 valve I got about 20 mpg going 65 everywhere, I think that the 4bt will do a little better. But, that's if you wanted to go with a manual. As far as an auto the 4L80E should be right about the same. But I'm interested in this swap as I'm looking to do the same in my 84 K5. As a side note the 12 valve does fit in the engine bay but I wouldn't use a 4L80E behind a 12 valve personally. That's my $0.02. __________________ 1979 GMC Sierra Grande Crew Cab C25, 454/TH400/4.10-Project truck/ DD 1985 Chevy K5 Blazer 700r4/4.10/8 lug- in search of new engine 1993 GMC Sierra K1500 4L60E 2007 Suzuki Forenza- her DD and the family vehicle