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Old 03-28-2008, 07:44 AM
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400 Chevy vs. 305

And just after I had my mind all made up, too....
I had just decided that due to lack of cash flow I was going to stick with a tranny replacement on my 78 Impala coupe this summer, since the 305 2v runs smoothly and doesn't burn much (if any) oil. I was planning an upgrade to a 4v intake and quadrajet, however. When the 305 pooped out, I was planning a 350 with vortec heads, performer intake, towing cam, etc, for some reliable, real world HP.
BUT, my brother just called... apparently he has a lead on a good running 454, so he is going to put it in his truck, which leaves his 400 small block just sitting on the garage floor. He wants to know if I want to swap it into my Impala, since I'll have it all torn apart for the tranny replacement anyway.
Herein lies my dilemma. I'm a first year teacher and have big student loans and very little income: I NEED to get at least 20-22mpg on the highway, 18-19 minimum, or else I won't be able to afford to buy gas and I'll have to sell the Impala and buy another Geo. I was pretty sure I could get that out of the 305 (we had an 84 Delta 88 with a 307 that would pull 22-24 on the highway) but I'm not sure if the 400 will return any mileage with another 95cid to feed. I understand that the bigger motor won't have to work as hard to make the car move, so it might balance out, but I don't know. Also, I'm swapping in a 700R4 tranny and planning on rebuilding/replacing my rear end gearing, so crusing RPM can be adjusted as needed, and that stuff was already in my budget for this summer.
So, what do you guys think? The only experience I have with 400s is getting 8mpg out of my bro's 4x4... in a car that weighs less with more highway friendly gearing, what could I expect for mileage? The added horsepower would be nice, but if I can't afford it I can't afford it. I've got a few more questions but I'll start with this. Let me know! Thanks!

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Old 03-28-2008, 07:49 AM
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really no comparison between the two
the 400 is twice the motor.
305 is really a low compresion smog motor that isnt worth much.
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Old 03-28-2008, 07:56 AM
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You need to go with 350 because it will get better gas mileage over the 400.
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Old 03-28-2008, 08:34 AM
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Wow, quick responses! Thanks!
I would love to go with a 350, but I can't afford to build one (have one sitting in the garage in need of a rebuild). Since I get the "family discount" (basically free) on the 400, I can afford to swap it in without rebuilding (runs fine as is anyway), or I can afford to keep my 305 without rebuilding it. I would also love to have the extra power of the 400, so long as I can pull 20+ mpg on the highway. Is it doable? I don't know any specs on the 400, but I'm just going to assume it is cammed for low end grunt since it is in a pickup.
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Old 03-28-2008, 11:29 AM
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Quote:
Originally Posted by JonRunaas
And just after I had my mind all made up, too....
I had just decided that due to lack of cash flow I was going to stick with a tranny replacement on my 78 Impala coupe this summer, since the 305 2v runs smoothly and doesn't burn much (if any) oil. I was planning an upgrade to a 4v intake and quadrajet, however. When the 305 pooped out, I was planning a 350 with vortec heads, performer intake, towing cam, etc, for some reliable, real world HP.
BUT, my brother just called... apparently he has a lead on a good running 454, so he is going to put it in his truck, which leaves his 400 small block just sitting on the garage floor. He wants to know if I want to swap it into my Impala, since I'll have it all torn apart for the tranny replacement anyway.
Herein lies my dilemma. I'm a first year teacher and have big student loans and very little income: I NEED to get at least 20-22mpg on the highway, 18-19 minimum, or else I won't be able to afford to buy gas and I'll have to sell the Impala and buy another Geo. I was pretty sure I could get that out of the 305 (we had an 84 Delta 88 with a 307 that would pull 22-24 on the highway) but I'm not sure if the 400 will return any mileage with another 95cid to feed. I understand that the bigger motor won't have to work as hard to make the car move, so it might balance out, but I don't know. Also, I'm swapping in a 700R4 tranny and planning on rebuilding/replacing my rear end gearing, so crusing RPM can be adjusted as needed, and that stuff was already in my budget for this summer.
So, what do you guys think? The only experience I have with 400s is getting 8mpg out of my bro's 4x4... in a car that weighs less with more highway friendly gearing, what could I expect for mileage? The added horsepower would be nice, but if I can't afford it I can't afford it. I've got a few more questions but I'll start with this. Let me know! Thanks!
The 700R4 with a functioning converter lock up would be a major asset toward getting good mileage from the 400, but 20-22 would be hard to get to without fuel injection and modern pistons and heads. The same would be true of the 350.

I would guess (educated but a guess) that with a carb, period heads, and the 700R4 that 12 to 15 would be about what you could see out of a 400 or 350. Improved combustion chamber design with "D" dish pistons and Vortec type combustion chambers would allow more compression against the fuel's octane (even 87) this would allow a compression increase from the low 8s to the lower to mid 9s which would greatly improve cruise mileage and satisfy that occasional urge to get on it. This change would add a couple miles per gallon. Fuel injection would do another 10 to maybe as much as 20% but the cost and complexity factors are pretty high unless you can score a complete OBD-I set up off a late 80s thru mid 90s car. That's the fuel tank, plumbing, computer, wire harnesses and injection system inclusive of manifolds in and out. OBD II systems are aven better at being stingy with gas on big engines but they are more complicated and more expensive, still something to at least look at.

Bogie
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Old 03-28-2008, 11:45 AM
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I got a question. How much does it take to fill up the Impala? I have a 1995 suburban that takes $150 to fill it. But I also pay 110.7 a litre for 87 octane fuel. Im sure you can manage at 12-15 mpg I know I do and I work for 10 bucks an hour.
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Old 03-28-2008, 01:55 PM
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Oldbogie: You answered a my next question without me asking it I was just going to ask if Vortec heads would fit the 400. I remember there being some difference between 350 heads and 400 heads (something about the water passages being different?) but I couldn't remember exactly and wasn't sure if it would be an issue. Whatever engine I finally end up with, I want it to be as efficient as possible, and modern heads make a lot of sense to me.

400smb_10: I'm sure you spend more money on gas than I do. If you would like a "yep, you sure are a better car guy than I am for spending so much on gas" you can insert it here. Gas is a huge percentage of my disposable income, and when you drive a lot of miles a week like I do, little increases in fuel mileage really add up. Please forgive me if that sounded rude, it wasn't meant to, but please don't put me down because I have to live on a budget.

Any second opinions on mileage? Any other ideas?
Thanks!
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Old 03-28-2008, 02:07 PM
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why don't you take what you have now, and buy the geo, then do the impala with the savings in gas.

sam-missle
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Old 03-28-2008, 02:16 PM
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go ahead and get the 400.....then save your pennies and dimes and build that 350 up a bit at a time if you don't like the mileage outta the 400.

I'll just about bet that that 400 will only be a little worse that that gas guzzling 305

the volume efficiency on the 305 is WAY down.

plus the impala is a HEAVY car and the added low end torque of the 400 will get it rolling easier, with alot less throttle than that 305.
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Old 03-29-2008, 10:26 AM
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Did a lot of thinking last night about my long term plans, and I think I'm going to risk the mileage drop and go with the 400 (assuming my bro does in fact give it to me). If mileage completely sucks I can always spend a weekend and put the 305 back in. I really appreciate the input on this; I know very few car guys in this area so when I need advice this message board is a great asset.
I just wanted to say one more thing to 400smb_s10. I thought about what you said, and decided you are completely right. The difference in gas mileage between 15 and 20mpg is about $10 a week, which, you are right, I can afford. But $10 a week is $520 a year, and that is a lot of money to me right now. If I can spend the cash on something else besides smoke out my tailpipe it would really help me out. I just wanted to apologize for being rude to you and let you know where I was coming from.
Thanks for the opinions guys, I'll dig this thread up in while and let you know how it turned out!
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Old 03-29-2008, 12:41 PM
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once you put the 400 in youll never go back to the 350!
as long as you dont have a heavy foot the 400 will be fine.
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Old 03-29-2008, 12:56 PM
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You'll never get 20 mpg from the 400; not without a huge investment in some pretty custom stuff. It will require EFI, higher compression, and a very careful tune. Put it this way, the LT1 in my 96 Impala barely squeezes out 18-20 mpg on the highway with 10.25:1 compression, EFI, and all the computer tricks.

On a stock 400, expect 12 mpg maybe. If you went to great lengths with head work, cam research, EFI, and other helps you could probably get your goal; but it would be a $10,000 engine in a 78 Impala.
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Old 03-29-2008, 04:02 PM
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Maybe 20mpg is an unrealistic goal, then. I don't have a lot of real world experience (hey, I'm only 23!) so I guess I just didn't know what to expect. This has brought up a bunch of questions for me, however, so I think I'm going to start a new thread on what makes an efficient motor, just for my own intellectual benefit.
Thanks guys!
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Old 03-29-2008, 04:27 PM
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20MPG out of a 400 is not unrealistic. Just go with a stock TPI setup, a crane 252 cam, vortec heads, long tube headders, and 9.5:1 compression. If you can't hit 20 MPG's you must be towing a boat and the dock its tied off to.
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Old 03-29-2008, 06:54 PM
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[QUOTE=Holder350]go ahead and get the 400.....then save your pennies and dimes and build that 350 up a bit at a time if you don't like the mileage outta the 400.



the volume efficiency on the 305 is WAY down.
QUOTE]


Holder350:

What is volume efficiency and how is it measured/calculated?
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