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Old 02-22-2010, 10:18 AM
truckpoor truckpoor is offline
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Default Fuel mileage?

I have an 79 F250 with C6 and gear vendor overdrive. Running E rated tires, and taller than originals but not much. The tires are street tread. I bought the truck not long ago, and it had sat a lot for some time. I have as yet to check the rearend ratio, and I have yet to determine the exact engine that it is, as the engine was replaced in 1989 by the now deceased previous owner. Paperwork left with the truck says it was a 460. However, I was talking to a freind at a local garage said the owner stated it had a 429 crank. When I bought the truck, it would start but had carb problems, and would not stay running. PProblem turned out to be a gasket in the carb was on wrong and stoped accelerater from working. How much damage was done to the engine with someone fighting this issue I can't tell. The carb was new ( holley 650 vacuum secondary ), and the engine now has a little over 80K miles on it. After I fixed the truck so it would run, I noticed it had a burnt valve. I ran a compression test, and it had one cylinder at 100 pounds, and several at 130 and one at 150. Since the truck ahd other issues withe sagging springs and needed front shocks, I decided to let the motor go, untill I had it road worthy. Finaly took it out for a long road test and drove to the Cape Giraredaue MO. swap meet this weekend. Runs better now after taking it on the road cruise, but still not perfect on take offs. I ran the truck hard the first 65 miles, then filled up and ran hard for the next 50 miles 70 to 80 mph, then drove home at around 65 most of the way. I am rather upset by fuel mileage. I ran 173 miles and used 23.5 gallons of gas. I was running empty but did have a topper on the truck which seems heavily made, plus it has a sleeper setup inside the topper made of plywood covered by carpet. The engine had a stage two RV cam installed back when, and runs like a scalded dog once off the line. I don't know what pistons are in the motor nor the compression ratio, but it does have the D3V heads. I would like to do another compression test to see if it has improved after the drive, but it's hard to do by yourself,and not that easy on the bad cylinder with help. Number 8 on the drivers side, behind power brake booster etc. If I can't get the milage up to 10 to 12mpg, I most likely will sell it. Valve job is in it's future, but wondering if this will be enough. I am wondering if a boost in compression would get it into the 10 to 12 range on fuel mileage? Also wondering how much weak valves affect the fuel mileage? Mostly just the one cylinder would be my guess but who knows.

Rich
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  #2  
Old 02-22-2010, 10:49 AM
DaveMcLain DaveMcLain is offline
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If the engine has a low cylinder this will effect the mileage by some amount. It's tough to say but 10% or so would not be unrealistic.

I doubt any damage was done to the engine by having the carburetor not opening all the way etc.

I have a '79 1 ton Ford pickup with a 460 and it'll get about 10mpg or so towing my trailer and not much better running empty. Mine has a 4.10 rear axle, 33 inch tall tires and no overdrive. How fast I run down the highway makes a BIG difference in economy because the truck is really overgeared. If I remember correctly it'll turn over 3000rpm at 70mph. Running along down the road it'll generally run with about 18 to 20 inches of vacuum.

All D3VE heads did NOT come with induction hardened seats just depending on the original application. So it's certainly possible that your engine simply has some sunk exhaust seats and this is hurting cylinder pressure/compression.
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Old 02-22-2010, 11:13 AM
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Something def. doesn't sound right. My old highboy used to get about 12-14hwy and it was running 38" tires w/ no overdrive (about 2400 rpm@60)and a healthy 390.
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Old 02-22-2010, 11:39 AM
lance70 lance70 is offline
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I have a 77 F250 with a stock 460 running a Q-Jet,C6 and a dana 61 w/3.07 gears,265x75x16 tires and get 10-12 mpg's.
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Old 02-22-2010, 01:22 PM
truckpoor truckpoor is offline
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Default Thank you for your replies.

I had a 70 mercury convertable with a 429 that got a good 16 mpg, and this is why I asked the question. I could not believe the gas mileage was up to par. I think with the overdrive this truck should be capable of 12 mpg, but who knows. I knew that i had to do someothing about the heads. but not know which engine is actually is, uncertain as how to procede. I do have a set of C8VY heads as well as a set of C9Vy's as well. The C8's go with another engine I have, but the C9's have no valves etc, and I have not been to pick these up since vatted. (very rusty) I also have a set ove D3's in the vat as well also rusty, but complete. I would rather be able to just have what I need built and ready to go on, but uncertain which crank and what pistons I have at present. Makes it tricky on what chompression I build for. I want to run pump gas with no problems, and not have to run a premioum , as it is not that easy to find around here. Armpit of the world don't you know. Smalltown USA southern IL.. I most likely need to pull the engine to be sure I get it right rering rebore etc. if neccesary. Major PITA, as I did not want to get that far into this engine. I thought about just building heads and slapping on when ready, but If I want decent mileage, I need to aim for about 9.5 compression ratio to get the best I can out of the poor stuff behind sold to us at exhorbitant prices. With the overdrive, I would think 12 to 14 mpg should be attainable with judicious ddriving, ;and 12 without holding my foot back. With the question of whetther I will want to keep the truck once mileage is in, I'm tempted to just do a bore scope and see what pistons I have, and what heads would give me best compression. Of course, If I don't do a rebuild, then I will wonder if it isn't the rings that also need replacing. Again a vicious circle. Damned if you damned if you don't. Should I get the gas mileage above 13mpg, then I most likely would keep the truck, as it is a great driver. Original interior in good shape exterior is good but lots of door pops and will need a paint job in the next few years to keep it pristine. Unfortunately it has the Explorer package with stick on stripes, and I doubt if these are anywhere to be found. Only one rust spot coming thru on the left rear fender well. about 2 inches long. I might mention , that at present own about 15 pickups and a few one tons, plus 5 cars counting the wife's. I really don't need the truck and have plenty more waiting in the wings to be liscenced. This in itself is becoming a problem as Illinois has jumped liscence renewal another 20$ this year. I at present have 5 liscenced vehicles. More than enough for the twoof us.

Rich
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Old 02-22-2010, 02:04 PM
BryanR BryanR is offline
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A few things you posted makes me think that your may have some valves sticking. The fact an older guy owned it, it sat for a long time, and it seemed to run better after you drove it hard for awhile.
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Old 02-22-2010, 02:21 PM
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The best I could ever get out my stock 460 with a 650 was 12 mpg and it didn't matter city or highway put a 750 vac secondary on it and went down 2 jet sizes from stock and got 17 city and 19 highway. This was in a 3500 pound car with 2.73's and 28 inch tires. Didn't lose any throttle response either.
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Old 02-22-2010, 02:55 PM
Rlzegers Rlzegers is offline
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I've got a 1978 F-350 2WD with a Gear Vendors unit behind my NP435 4 speed and I get around 12 mpg running empty on the hiway with my fresh 460 with a mild build. Normal driving around home I get close to 10. Towing my 10,000 fifth wheel it drops to around 7. You might lose a little mileage with the C6 instead of the manual tranny but not to the degree you've experienced.
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Old 02-22-2010, 06:07 PM
truckpoor truckpoor is offline
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Default 750 carb change.

Again, I appreciate the feedback and supporting information from all. I had thought the 750 carb might help, as it gives a bigger opening for air under normal circumstances and normal driving. Less vacuum at the carb should mean less fuel being sucked thru the venturies, was my thinking. From the experience described this could be very true.
Sticking valves had also crossed my mind along with carbon buildup. I did the ice water thru the intake a couple of times, and also automatic transmission fluid thru it. However, I think the real problem was being dogged around to much in more resent years, and seldom being driven because of fuel mileage. I have yet to check rearend ratio, but feel it is overgeared for around town. I will try and go out either this evening or in the morning and report what I find.

Rich
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Old 02-22-2010, 10:07 PM
OldRedFord OldRedFord is offline
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I thought D3 heads did have hardened seats?
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Old 02-22-2010, 10:24 PM
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Quote:
Originally Posted by truckpoor View Post
I ran the truck hard the first 65 miles, then filled up and ran hard for the next 50 miles 70 to 80 mph, then drove home at around 65 most of the way. I am rather upset by fuel mileage. I ran 173 miles and used 23.5 gallons of gas.
Well, it looks like you got around 7.3 mpg.

What was the rpm when you cruising at 65 and 70 to 80 mph?

Dave
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Old 02-23-2010, 09:40 AM
truckpoor truckpoor is offline
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Default RPM unidentified

I have a tack on the truck but it is not functioning right. I haven't gotten around to finding out why as of yet. It functioned soemtimes but I am not certain it was correct. It appears that it was below 3K at those speeds. Again, this is guesswork on my part as it would only be there momentarily and bounced around. At higher speed it quit functioning at all after wahile. It's a sun tack and I have not checked the wiring out. It may or may not be fixable. Hopefuly, I can find the problem. If not I will replace it, in the future. To many fish to fry at the moment. I still have to replace the head gaskets on the 75 OLDS 88, pull two 292 yblocks and tear these down to replace cranks and I'm rebuilding the 460 on my engine stand. Not the one for this truck though, to high a compression. Ballanced engine, it is going into a different truck, as soon as I decide which one.

Rich
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