700R4 fuel mileage

[Chevcaddy41]

This is only my second post, first was over a year ago on the old forum. Just got finished with a summer long test of a 700R4 behind my 70-500. Results are very disappointing, Cadillacs are NOT Chevys. My car is a 41 Chevy coupe with a 70 ElDorado engine with an Edelbrock intake, Holley carb and headers, thru flowmaster mufflers, 9" Ford rear. Last year I drove the car to Cumberland Md. for the Western Maryland Street Rod Roundup and got 20 miles to the gallon with the old turbo 400 and 2:73 gears. This year when I really needed the mileage I got 15 with 700R4 and 3:00 gears, 129 miles one way. Mileage was even worse going to Hershey with the 700R4 and 3:73 gears, I think around 13.8, then went to Columbus with 3:50 gears, got 14.4 miles per gallon. Guess who just sold a rebuilt 700R4 to some Chevy nut for cheap? Got home from Cumberland Sunday night, the 700R4 was out Monday afternoon, Turbo 400 got put in Tuesday, just got back from test drive with the old 400. I won't be changing trannys again, 400's are the only way to go. Thanks for letting this old Mountaineer vent!!

[Vern]

Interesting. I'm not a fan of the 700R4 either but it is surprising. What stall converter did you run in each? Did you have a lock up converter with a manual lock up? How much does your car weigh?

I can get 17mpg on the interestate with a 3800lb including me Cutlass with a 2004R, lock up convereter, 3.42s, 28" tires, 8.5 rear, and q-jet on 89 octane. This is despite a 230@50 duration cam. It runs mid 12s at 109.

My buddy has a cab & half full size chevy truck with a 2004R tranny, lock up converter, q-jet and 230@50duration cam in a 454. His mileage sucked bad as in 10mpg in town barely 14 on the hwy. But his converter was never right. It would start to noticeably slip going up a grade if he gave it any gas. I think the truck was to heavy for his converter.

[Chevcaddy41]

Vern, the converter and trans were a package from TCI. Tranny and converter were what they recomended for my car. The converter did have a manual lockup. The car is a 41 Chevy coupe, 1970 ElDorado 500 engine, 9" ford rear, 28 inch tall tires, weights about 3400 pounds. Now you got to remember when I'm coming out from home I'm on I-79 in WVA. and I-68 and there ain't many level spots, this is real mountains down here. Anyway the turbo 400 is back in and the 700R4 is sold.

[Chevcaddy41]

Vern, 17mpg on the interstate isn't that great, I got 20 last year with the turbo 400. I can get as much as 23 going to Columbus once I get thru Pa and get on some flat roads and the rice burners stay out of my way. Mid 12s are kool for a street car tho.

[Modly]

That is quite some fuel economy you are pulling out of that thing. I don't think I've ever heard anybody getting 20 out of a Cadillac engine, without it being a complete dog.

I'm trying to figure out what would be wrong with that 700r4. My best guess is that it never locked up, or it had too much throttle. I know with my old truck (Chevy powered, but had a 700r4), If I went over 75 or so MPH, it would downshift to third gear (Which 700r4s do, when you give it more than a certain amount of throttle). Without an aftermarket modulator, it would just up and downshift like nuts at a certain speed, which killed economy. Assuming it was doing this up/downshift process over and over, and with that higher (numerically) gear ratio, it is pretty reasonable to see why it got lower fuel economy.

Honestly with the torque I'd get out of a 500, I'd want a 4L80 if I wanted a 4 speed tranny.

[Vern]

No 17 is not great but I think its not bad considering the cam and that I can get it while doing 75mph. I think if I was useing a th400 with the same everything else it would have gotten maybe 12 and turned a lot more rpms. Your mileage with the th400 & 500 Cad is the best I have ever heard of. I am a fan of the th400 behind the big Cad for most applications and like I said not much of a fan of the 700r4. But I suspect something was wrong that it did worse than your th400.

[CadillacPat]

Chevcaddy41,
This topic has been brought up before on the old board. According to Al, a 500 in a '76 Sedan DeVille with his MT3 cam (rebuilt of course) will get 22 mpg on the highway. You have to understand that this cam is more technologically advanced than the cam of the day. It is made for very low speed torque (not over 4000 rpm). His MT5 cam is a modern version of the old '72/'73 Caddy cam and makes more hp and torque, with slightly lower mileage (than the MT3; if you can still get about 20 mpg with this cam, the extra power is worth it if you are not for all out fuel economy).
As far as the 700, gearing is everything. The stock Caddy cam likes an engine speed of about 2600 rpm-it's torque peak. So, with a 400, you need about a 2.93 rear, which came stock with 472/500's in the day. On my car, with a stock '73 500 and 400 ('84 Coupe with a 2.41 rear), the car gets it's best mileage around 70 mph-about 2200 rpm. With a lower rear like a 2.93, the car is supposed to get better mileage. This is because the engine WOULD be running at a more efficient speed. Also, with the 2.41 rear, and running over 70 mph, the car has much more drag to overcome. This is why a slightly lower gear at 70 mph should net more mileage. Currently, I get about 15 mpg at 70, but, the engine is in serious need of a rebuild. I am in the process of rebuilding either a 425 or 472 before winter, and will rebuilt a 500 next spring, probably at tax time.
Consequenly, this is why you got your best mileage with the 400 and 2.73's. With the 700 and 3.00's, you had an effective rear-end ratio of about 2.10. With the 700 and 3.73's, about 2.61. While the effective ratio of 2.10 is way too low, especially with a 230-degree cam, the 2.61 should be close enough to the 400's with 2.73's effective ratio of 2.73. a 3.50 rear would also be bad. Since you are running a 230-degree cam, which raises the engine's torque peak, a ratio of about 3.00 should work well as a compromise. You also have to remember that for most of us, around town driving is the most common, and the higher engine speed should net a slightly higher city mileage. While it may seem a bit high, the engine would turn about 2500 rpm at 65 mph with these gears (my tires are about 28" tall; 235/70/15).

[CadillacPat]

Chevcaddy41,
This topic has been brought up before on the old board. According to Al, a 500 in a '76 Sedan DeVille with his MT3 cam (rebuilt of course) will get 22 mpg on the highway. You have to understand that this cam is more technologically advanced than the cam of the day. It is made for very low speed torque (not over 4000 rpm). His MT5 cam is a modern version of the old '72/'73 Caddy cam and makes more hp and torque, with slightly lower mileage (than the MT3; if you can still get about 20 mpg with this cam, the extra power is worth it if you are not for all out fuel economy).
As far as the 700, gearing is everything. The stock Caddy cam likes an engine speed of about 2600 rpm-it's torque peak. So, with a 400, you need about a 2.93 rear, which came stock with 472/500's in the day. On my car, with a stock '73 500 and 400 ('84 Coupe with a 2.41 rear), the car gets it's best mileage around 70 mph-about 2200 rpm. With a lower rear like a 2.93, the car is supposed to get better mileage. This is because the engine WOULD be running at a more efficient speed. Also, with the 2.41 rear, and running over 70 mph, the car has much more drag to overcome. This is why a slightly lower gear at 70 mph should net more mileage. Currently, I get about 15 mpg at 70, but, the engine is in seriously need of a rebuild. I am in the process of rebuilding either a 425 or 472 before winter, and will rebuilt a 500 next spring, probably at tax time.

[CadillacPat]

Chevcaddy41,
This topic has been brought up before on the old board. According to Al, a 500 in a '76 Sedan DeVille with his MT3 cam (rebuilt of course) will get 22 mpg on the highway. You have to understand that this cam is more technologically advanced than the cam of the day. It is made for very low speed torque (not over 4000 rpm). His MT5 cam is a modern version of the old '72/'73 Caddy cam and makes more hp and torque, with slightly lower mileage (than the MT3; if you can still get about 20 mpg with this cam, the extra power is worth it if you are not for all out fuel economy).
As far as the 700, gearing is everything. The stock Caddy cam likes an engine speed of about 2600 rpm-it's torque peak. So, with a 400, you need about a 2.93 rear, which came stock with 472/500's in the day. On my car, with a stock '73 500 and 400 ('84 Coupe with a 2.41 rear), the car gets it's best mileage around 70 mph-about 2200 rpm. With a lower rear like a 2.93, the car is supposed to get better mileage. This is because the engine WOULD be running at a more efficient speed. Also, with the 2.41 rear, and running over 70 mph, the car has much more drag to overcome. This is why a slightly lower gear at 70 mph should net more mileage. Currently, I get about 15 mpg at 70, but, the engine is in seriously need of a rebuild. I am in the process of rebuilding either a 425 or 472 before winter, and will rebuilt a 500 next spring, probably at tax time.

[zenish]

chevcaddy.did you try leaving the 700r4 in drive and check the mileage?in drive it has the same gear ratio as the turbo 400.

[CadillacPat]

chevcaddy41,
Well, I just completed a trip to the Detriot area, around the city and back (interstate most of the way). I topped off my tank (rocking the car to fill it completey each time) and went 195.4 miles on 12 gallons on the way to Detroit. This works out to 16.3 mpg. Next, I drove about the city 55.5 miles (with a light foot), and used 5.5 gallons of gas. This works out to 10.1 mpg. Finally, I drove filled up at a gas station on the way home traveling 180.9 miles and used 11.1 gallons, which works out to 16.3 mpg.
As I have previously determined, my car gets it's best mileage around 65-70 mph, which I drove at for most of the trip while on the interstate.
While I don't know the mileage on the '73 500 in my car, I have put about 35,00 miles on it over the last five years. Previously, the car the engine was in had been sitting for about 2 1/2 years. Also, I know the engine is very well worn as it uses about a quart of oil every 500 miles. This is why I am in the process of acquiring the parts necessary to rebuild at least one engine.
Finally, when I first got this 500, it ran much better, and certainly didn't have the slight "oil burning" smell it has when towing. While the car doesn't "blow" oil out while driving steady, under heavy acceleration, it is slightly smoky. It also smokes slightly when cold.

[Chevcaddy41]

CadillacPat, I just got back from Canfield, Oh swap meet. I didn't check going up, but checked coming home. Its 212 miles from where I filled up this evening to where I filled up when I got home, the 41 took 11.1 gallons, figures out to 19 miles to the gallon. Altho most of it is interstate there was about 40 miles of 2 lane roads. The 41 seems to like 75 mph for the best mileage. Theres not too much level ground between here and Canfield, so the mileage is a little off. I expect to get over 20 when I head for Indy later this month to visit the kid. My 500 was freshened up about 3 years ago, mild cam, pocket porting, Edelbrock intake, Holley spread bore carb and big block chevy headers with home made flanges. The engine is nothing special just a good highway engine. I haven't drag raced for 30 years, but I'd say it might turn mid to hi 13s with the 2:73 gears, feels real strong. Wow this is the most I've typed ever, my finger is tired.

[CadillacPat]

Chevcaddy41,
Considering all the mods to your engine, especially the cam, headers, intake and pocket porting, and, the TH400, 19 mpg is about the best that can be obtained (over 20 mpg is possible, but you would need Al's MT3 cam, which is a true "mileage" cam). You also have to consider the reasonable 75 mph speed you're traveling at.

[Chevcaddy41]

CadillacPat, I'm hoping for better than 20mpg next week when I go to Indy to visit the kid. When I get past St. Clairsville, OH I'll fill up and go till its close to empty. I-70 is mostly flat from there to Indy, should get over 20. Last year going to Good-Guys Columbus I got 20 checking from WV.

[CadillacPat]

Chevcaddy41,
Certainly, the grade of, and the changing of road grade affects mileage. For the most part, on level grade, road friction (the tires rolling on the road, and, wider tires produce more) constitutes the vast majority of horsepower to move a vehicle up to about 45 mph (and probably higher speeds with today's "jellybean" bodystyles). Above this speed, the Cd (coefficient of drag), which is caused by wind resistance (and more so if you are traveling into the wind), increases the amount of power required.

[Chevcaddy41]

CadillacPat, I'm leaving in the AM for Indy. I'll let you know how the changing road grade affects real world fuel mileage. Once I get on the level on I-70 West it will show weather I was right in changing back to the turbo 400. So far around home its a little better with the 400, fuel mileage that is. I figure if I get over 20 on this trip it was well worth changing back. Got to get extra sleep tonight, I want to be past Columbus ohio before 8 am to miss the rust hour traffic. I'll try to post the results tomorrow evening if my Grandson will let me use his laptop.

[Chevcaddy41]

OK, here gos, got to Indy around noon, total of 432.5 miles to my grandsons home. I filled up outside of St Clairsville, Oh, then filled up again when I got to Indy, 312.6 miles. I used 14.3 gallons in 312.6 miles, all interstate, works out to 21.8 miles to the gallon. I drove at 75 to 80 mph most of the way, the 500 and turbo 400 seem to like the higher speeds, me too. I sure am glad I got rid of the 700R4 and put the turbo 400 back in. I'll be doing some driving around here this week, might check it around town, I'll be happy to get 14 around town. Been a long day, but very satisfying.

[CadillacPat]

Chevcaddy41,
Great to hear! I was out of town taking my daughter and her girlfriend to see a concert in the Detroit area. While there, the computer for my Holley Projection, and, was stolen that same night. I had to ask my sister-in-law to take to a junkyard to find a Q-Jet. I USED to keep one in my trunk on trips just in case something serious happened with the Holley, but it had worked fairly well for the last year I have had it in it's current car. While the car was stranded near the State Theater in Detroit, it was vandalized. The vandal (s) broke out the passenger side window and stole anything of value-the computer for the Holley system (not much use to anyone since it didn't work), 4 of the 8 Bosch Platium spark plugs I had recently purchased (they didn't see the other four partially hidden under the seat) and a Snap-On 1/2" drive breaker bar. They also partially pulled out the back seat trying to get into the trunk. Fortunately, I have spare glass and will try and repair the seat.
As far as the Holley computer, I was planning on upgrading to an Ultra Mega Squirt programmable computer. It requires a custom home made wiring harness, but I can reuse most of the connectors and sensors along with the throttle body.

[Terrible One]

Man, that is horrible. I'm truely sorry to hear about your misfortunes.


I'm really contemplating whether or not to go the Megasquirt route...

[Chevcaddy41]

CadillacPat, sorry to hear about your problems, haven't had a chance to use my grandsons computer, but I just got back to WV. Don't you just wish you could have been there with a gun. Damn people too lazy to work so they have to rip people off. My trip was pretty good, found out I made the right move putting the turbo 400 back in. Gas mileage around town was good, 14.5 stop N go traffic, I figure thats OK for now. I'm going to start tearing the 41 down tomorrow, going to get around to finishing the body work. I'm tired of the black, red & gray primer, time to get some paint on this beast, well sorta, if you call flat black, paint. CadillacPat I hope you've had a better week since your last post.