I'm going to go out on a limb here and say: Poor.

 

I'm going to go out on a limb here and say: Poor.

so, i guess my in-progress 496, 4 speed, 4.10, 67 SS will be something like uhhhh, "pi$$ poor"??? :yes:

 

with no OD trans, some where around 8-9mpg

aaron

 

Anthony 400 at the rear wheels is a healthy motor, like Todd said gas mileage is not going to be its strong suit. Don't worry about it, drive it, have fun, that's why you got it.....

 

Need all your specs for a good idea....carb size is important, trans, cam etc... I dont expect anything better than 6mpg with my ride. However when your building that much power, gas mileage shouldnt be a factor. If you want good gas MPG, OD trans or manual with some 3.55 gears. Can get you near 20MPG.

 

An OD makes a huge difference. See the mild 468 (still) in my signature? The unit delivered 21 mpg with 3.73's, a .64 OD, 28" tires... going exactly 60mph on a flat freeway with no horsing around. Wow!!! That being said, I have gotten 7mpg on a tank before

 

Has the 468 changed from the link below?

http://www.chevelles.com/forums/showpost.php?p=2767786&postcount=12

Great mileage. I know some have used a wideband O2 sensor to tune a carb. Did you do anything like that?

I have more less decided to go the LS route due in part to mileage. Some cruise nights are 40miles one way and with gas $2.70 for the cheap stuff. Plus I'd like to drive the car to Daytona Turkey Run from Georgia or Run Through the Hills in Pigeon Forge.

 

I have a 396+.060 = 408, with 670 Holley street avenger and Lunati 60202 cam, 063 heads, shorty headers, 3.42 gears, and an overdrive (700R4). With mixed but occasionally enthusiastic daily driving I get 12.5 MPG consistently (calculated fill-up to fill-up).

 

Our '69 has a stock L78 with headers and 4.10 gears. Jetting is #73 front, #76 rear in the 780 Holley. It runs about 12-13 mpg highway and that is with the bigger imperial gallon. A decently set up Q'jet will add at least 2-3 mpg to that. I've had a couple of L34 Chevelles over the years and they were typically in the 17-18 mpg (Imperial) range on the highway. The '66 had the 585 Holley and the '68 had the Q'jet. Proper carb set up and advance curve can sure help your gas mileage figures. Some times you can lean out the cruise circuit and help the gas mileage.

 

An overdrive and tightened down secondary spring could get you over 20 mpg.

 

lots of mixed areas. ill be running a 650cfm carb, and a 700r4 tranny if that helps

 

My stock 396 QJ got 10.6 to the gallon in my 70 Chevelle ss th400

 

We don't build them for gas mileage, drive it !

 

i got a 1971 396/402 and am going to make it around 400 hp/torque at the rear wheels.

400 hp at the rear wheels means a 500+ at the crank which is a pretty wild 396/402. That's not going to be a daily driver and you should expect less than 10 mpg.

 

OD is the way to go.

 

Anthony,

I think you are dreaming if you think that you are going to get 400hp at the wheels without some super build and still get decent mileage as a daily driver.
My completely stock '70 L34 with a Th400 and stock 3:31 rear gets 11.3 mpg and that's with level headed driving. What do you want? High horsepower or good gas mileage? You can't have both.

 

Exactly, you dont build a BB period and worry about mileage. OD trans is about the best thing you could do to help it out. When its empty refill the sucker.

 

ok i guess im shooting a little too high. how about 330/350 at the wheels? or is this still a longshot? i was hoping to get like 13-15 mpg :/

 

When I first got my 71 many years ago, it had a completely stock LS-3, 402 rated at 300 hp at the crank (which is probably 240-250 at best to the wheels), M-21 4 speed and 3.31 gears. It was my daily driver for a couple of years. Combined driving (city/highway) I would get 12-13 mpg with very sane driving.

 

Find a 200R4 trans, a tight converter, a Q Jet, a carefully spec'd motor (put some thought into the heads, maybe even vortecpro peanuts) and a good exhaust system. You might be pleasantly surprised. I'd bet 18 or maybe even more.

 

OMG if you want gas Milage get a Prius or a jeta LOL

 

My Dad's `66 396 wagon w/TH400 got 16-17ish on the hyway with fairly tight engine and a 390 HP 427 cam. Don't remember if it was a high performance cam for a 396. Pop's mileage dropped considerably because he always had his foot in it with the newer cam. Don't know what the rear gear was.

 

Ken, what were the specs on the 427, 390 h.p. cam? That's an interesting mod. My guess would be similar to the 396, 350 h.p. cam.

 

I think it's the same cam. 214/218 .460/.481 on 115 ls. That gives you a very little overlap and helps milage. But it's a very old degsign. If you run a tight converter, 3.08 rear and 28-29" tires and a spreadbore carb, like Rochester, and PP heads for good velocity, and 10:1 comp I think it's possible to get liveble highway milage. And good thourqe to move all yr lbs.:yes:

A good ignition and gap on yr sparkplugs will help also. Don't forget to use the vacum adv on yr distrobutor to adv the ignition when running with light right foot.

By the way, gas price here is close to 7usd/gallon :sad:

Good Luck!
Burt

 

Googling, I found this. Looking at the specs, I'd say this is it.

3883986 1965-
72 Hyd .460I/.488E .213I/.217E See Note 2

Note 2: 396 325/350hp L-34/L-35, 427 385/390/400hp L-36/L-68, 454
345/360/390hp LS-4/LS-5, good torque/HP

Looks like your right. I found the cam used in a 390/427. Put new lifters on it and never had any trouble.

Found specs here: Don't know if they are correct.

http://www.leverfamilysite.com/imag...ages/PDF_Files/Microsoft Word - Big Block Chevy Engine Parts Identification.pdf

I would bet one of UDHarold's mild VooDoo's would be a better choice.

Some years back, I built an 11:1 302 with pocket ported 461 heads with a good valve job, close quench, and a 929 stock cam. Engine was in a Camaro with 3.31 gears. I couldn't get over what a good street engine that made. It was torquey, got good mileage, ran strong until the lifters pumped up. What I discovered from that experience was the importance of better head flow, when used with short duration cams. Joe Sherman's winning EMC SBC, I think it was 366 CI, used a 224/224 cam, essentially, a 350 HP hydraulic.

Quote:bigjoe1 wrote:
The cam I used was in fact a 224-224 -108--I don't care what the magazine says= I did use the cam in another engine that I built for a friend some years later= The surprising thing was that the HP peak was up at 65-6600 RPM, probably because the heads I used were pretty big for the size of the engine


JOE SHERMAN RACING ENGINES Unquote.

One thing I'm not is "Big Joe"!!! But my experience makes me suggest smaller cams than most. AFR also suggest this approach on their website. If you want a REAL street engine, one that doesn't buck and will pull hyway gears, Duration in the low 220's MAX, and close LDA's, 106-108, (smaller engines) can make for good street performance. Of course, large engines NEED more cam timing. But you need to keep intended usage in mind, and not order your cam from the bottom of the page. Most important is knowing what you want to do with a specific engine. None of this advise applies to max performance engines. Unless your "Big Joe"!!!!

 

I once built an engine which had to pass smog so at 427 CI I used 049 ported heads, an Isky 270HL 214*@.050 w/ .510 lift on a 114 lsa with a chevy # 770 cfm holley and Rectangle chevy intake. Car ran 12.68 and passed emissions easily. It was a fun sleeper and idled dead smooth.

 

We need more posts like this. Good mileage can also be High Performance.

 

My 350 with T56 overdrive only gets 14mpg. It amazes me how well some peoples fuel economy is. I tuned cruise pretty lean, vac advance, drive calmly, etc. I mostly blame the cam. I think cam is the biggest killer of fuel economy. My rpms are low, ~2000rpm cruising in 6th at 70mph but I think its not very efficient because my cam. Vac is very low at high speeds and low rpm. So be careful what cam you choose.

 

Have you thougt about LSx engines??? I know they get blasted here sometimes because they are FI small blocks. But, you can get to 400HP pretty quick on these and get 20+ MPG. I'm getting 22 MPG avg. on my '66 LS1 conversion - 350HP/375tq. stock. When I was looking into doing a 383 stroker or the LS1, I did the math and in 2 years of 5000 miles driven a year at $3 gas - it would pretty much pay for itself.

And if you like sleepers, my car is ver unsuspecting until I hit the happy pedal

 

I have a 3 speed auto. Over 65mph it looks like it gets about 4-5 mpg lol