Advice on Built 355 / th400

[Hutch998]

I have a built 355 with a edelbrock 650 carb, and oversized cam. It's sitting in a 86' chevy k20 Custom. 35" tires and a 4in lift. I bought the engine after the build, and am not sure what else is done to it. It has a th400 with a shift kit, and 4.10 front and rear. I'm wanting to make this a daily driver, however, at 3.65 mpg its not possible. I'm curious as how to make it better on gas? I've tried tuning the carb to no avail, and I'm avoiding "De-building" the engine. Any advice would be great.

[VileZambonie]

Sounds like you don't have that engine dialed in right. How aggressive is the cam? Stock torque converter? Have you thought about swapping in an overdrive trans?

[Gwtt]

35's and good gas mileage wont mix. you could get a gear vendors or look into updating that turbo 400 with a overdrive transmission. make sure your tires are properly aired.

[VileZambonie]

I had/have 35"s on my trucks with decent mileage. 350/700R4 with TBI is your best bet

[Hutch998]

I've got an 86' k20 with a 4inch lift, 35's, a th400 and 4.11 gears front and rear. It currently has a built 355 in it that gets 3.65 mpg, which I got an offer to sell. I'm wondering what engine I should get that could make this a decent daily driver? As far as it goes my choices could be a tbi 350 or 454, a 350 with tpi, or a 5.3? I'm unsure of what would be most beneficial for power and fuel mileage? Any advice would be great!

[LTZ C20]

Of the 4 options you stated, a 5.3 is the most fuel efficient. A 6.0 would probably be a little better for low end torque to get a truck like that moving. But with a lift, big tires, a 3 speed transmission and tall gears, fuel economy is not in your vocabulary. An LS 6.0 or 6.2 with a 6 speed automatic transmission would be much better suited for fuel economy. I see customer trucks all the time with big lifts and tires that still get decent milage because of the efficiency of the LS engines, the 6 speeds transmissions amd the rear end gears are much lower than 4:11, they are down in the 3's, like 3:73. Performance wise, do you mean putting go fast parts on like intakes, headers or are you talking about reliability and everyday performance.

[Hutch998]

Basically what I'd like is a big truck to drive daily, that can really blow the doors off the typical truck. Kind-of a hard balance, but there's gotta be a way to get around 10mpg with a powerful truck. I am also a fan of swapping in an nv4500 5 speed. So the th400 is just a for now thing. I'm only 17 so my knowledge and funding is very limited. From what I understand, the tbi trucks aren't horrible for power, but are nothing compared to tpi? Obviously LS is the way to go, but with a tight budget that may be too much to hope for. How much trouble would it be to swap a 350 or 454 tbi into my truck?

Also if I were to go ahead with the manual swap, how much work is needed for that? is it as simple as taking the parts off the donor, mounting them and cutting a hole for the stick? or do I need to rebuild the whole floor of the truck to fit the transmission? If these are "rookie" questions, I apologize, I'd just like to know as much as possible to get it right the first time.

[Irish_Alley]

heres my take if youre getting 3.65 miles per gal then something is wrong with the math youre using or something is wrong with your right foot. running 38s and a 350 i was able to get 8-10 out of my 79 with a sm465 (3 speed trans) even if you put a LS motor with those low numbers you might be able to get 6. when in reality you should be getting 11-15. but dont be so hard on the gas pedal and keep the speed 60 and below.

[Captain Swampy]

heres my take if youre getting 3.65 miles per gal then something is wrong with the math youre using or something is wrong with your right foot. running 38s and a 350 i was able to get 8-10 out of my 79 with a sm465 (3 speed trans) even if you put a LS motor with those low numbers you might be able to get 6. when in reality you should be getting 11-15. but dont be so hard on the gas pedal and keep the speed 60 and below.

I agree, either your math is wrong, your foot is really heavy, or you have a serious carburetor problem. Fix that problem first and then get a 700R4 so you have overdrive and a lock up torque converter. I get get 6 MPG with my Kenworth pulling 80,000 pounds.

[LTZ C20]

If you want a jacked up big truck with a 454 that will blow the doors off any other normal truck, then say good bye to fuel economy cuz your not gonna get it. The tight budget is gonna get even tighter if that's what you want just from fuel costs alone. You should look into what Irish said.

[rich weyand]

Your mileage with what you have is stupidly low.  Something is not set up right.  Even if you drive like a complete moron, you should be doing better than that unless you are generating 700 hp, and that's not possible with a normally aspirated 355.  So I have some questions.

Is that 355 carbed?  Which carb?  What tune are you running in the carb (jets and rods)?

What is your timing set at?  What kind of vacuum advance can are you running?  How much mechanical advance?

A "built 355" is just a 30 over 350 with someone's idea of fancy parts.  What kind of fancy parts are in it?  In particular, what pistons and heads?  What is your compression ratio?  What kind of gas are you running?

[Stewart G Griffin]

You cannot get "performance" and mileage with a 4x4 truck with 35" tires unless you spend $$$$ on a turbodiesel.  In which case it would be cheaper to stay with the engine you have even with the high fuel consumption rate.

So, i would suggest nitrous in your case.  Although that's probably not good advice to be giving a 17 yr old.......

[Irish_Alley]

rich i was also thinking about how bad the carb could be tuned to get that low of millage but i cant compute the ideal that the only way it could be that bad off is if it was blowing black smoke all the time and someone would think thats ok

[rich weyand]

Yeah, agreed.  But let's see what the answers to my questions are.  He should be able to get 7-8 mpg with almost anything under the hood, but it could be as bad as he says with a bad tune.  If he has to put a lot of pedal into it, for example because the timing is way retarded, the mixture could even be close, and no black smoke, just a lot of burned gas being wasted.

[Irish_Alley]

i can only hope its something like a bad tune, i think even when i would spin tires at every light i would only see 6 mpgs