AlwaysWave

This has me baffled, and I hope you guys have the answer.

I’ve put about 3000 miles on my 72 since we installed a BTO 200r4 trans in the car, replacing the TH400. There is not a bit of difference in the gas mileage. I was getting about 16 ½ mpg before and am still getting the same. The driving habits haven’t changed at all. Most tanks it’s half city and half highway. Even on an all highway tank it’s still in the 16 mpg area.

Here’s what the car has:

2002 put in a new 250 HP crate motor from Scroggin Dickey. Basically an L48 clone. 8.5 advertised compression.
Edelbrock Performer manifold
Edelbrock 600 cfm carb.
Edelbrock aluminum water pump
DeWitt’s radiator and Spal fans
MSD 6A box and Suprconductor wires.
Pertronics Ignitor
Rapid Fire plugs gapped at 040
1 5/8 long tube headers
2 ½ exhaust pipes with H-pipe
Synthetic oil in engine, trans, and differential
VB&P front and rear mono-leaf fiberglass springs
3 x 14 K&N air filter
K&N sub-stack on the lower filter base.
Powermaster 100 amp alternator
Airconditioning 100% removed
3.08 gearing
200r4 tranny with a 1600 stall converter.

It has no lean surge, nor is the exhaust sooty, The plugs show a light tan color, and there’s never any hesitation or bog problems, so I think the carb is well adjusted.

The timing is at 14 initial and 36 in at 2700 rpm, and using manifold vacuum to the advance can.

There are no vacuum leaks, It has 21 inches at idle and the same at cruise.

I expected the highway mileage to be around 21-22 mpg.

Why do you think the mileage didn’t change?

Mike Mercury

oh great. I'm just about to do this upgrade to the 1973, and now didn't need to hear this

Seems everytime I'm about to order the complete OD package... something comes along and throws a wrench into the works

I am VERY interested in others experience with this.

CrossedFlags77

We were discussing this in my drivetrain class at school. The question was asked, ''What is the reason for more gears in a transmission.'' One of the people said, better gas mileage.

My teacher explained to us that there is still a debate whether more gears does get you better gas mileage. Its more of how much throttle is needed to stay at a certain speed. You could ultimately be using more throttle in 5th than you would at 4th in order to stay at a certain cruising speed. That's probably not what you want to hear, but that is how it was explained to me.

turtlevette

At one time i had a 700R4 with 3.08 rear and 283 motor with mild cam and Qjet.

The best i could do going to work at a leisurely pace was 17mpg.

I had always got 13-15 mpg with the Turbo 400.

thats when i threw my hands up and installed the 4.56 rear. Much more fun to drive. I have the goodwrench 350 with 208/212 cam and L-98 aluminum heads and still get around 15mpg on the highway.

I guess you need fuel injection to get the decent mileage. I also wonder about how much drag the brakes add. Modern cars tend to pull the caliper piston back enough to prevent rotor drag. Ever try to push a c3? You can tell something is dragging.

Also someone here mentioned the C3 has much more aero drag than newer cars adding to highway inefficiency. Thats something you can never change.

I guess we need to see what these guys that claim their getting mid 20s are doing. I bet most of them have fuel injection. The rest of them may tend to exaggerate or they are math challenged.

zwede

Crossedflags: I think your teacher needs a refresher course... Lower rpm will definetely improve mileage simply because less energy is lost to internal engine friction. Another important thing to realize about the internal combustion engine is that it is the most efficient at full throttle. A higher gear means more throttle and thus higher efficiency.

Now to Alwayswave's issue: There are a few things I would check:

First you need to make sure the carb is not going into power enrichment in OD cruise. As the load is higher you will have less vacuum. The carb may be set to start the enrichment too early. If I remember the edelbrock carb right the vacuum where enrichment starts is easily adjustable by changing the step-up springs.

Next you need to verify your advance curve. If it is sticking or has springs that are too stiff you may not have enough advance at the new (lower) cruise rpm. On a low compression motor you can probably run 40-45 degrees total (including vacuum) timing @2000 rpm and go to 48-50 dgr @2800.

What about the torque converter? What is your cruise rpm? If it is a non-lockup the rpm might be low enough to where it is slipping.

Finally, I think your 22 mpg goal is realistic. I get over 20 mpg with a 454 and 5-speed. I wouldn't be surprised to see you getting 24-25.

kona

1980 350+200r4 getting 24 Hwy 19 in town.

Kona

jdmick

I don't think 16 MPG with a carbureted 350 is too bad to begin with but I wonder if the engine isn't lugging a little with the 3.08 rears. If it was me I'd try some 3.55s or 3.70s.

mandm1200

That's also my thought. Once the car is crusing at a steady rate, all gas being burned is to overcome friction; internal engine friction, wind resistance, drivetrain, etc. Shifting to a higher gear will lower the rpms and the internal friction of the engine. Everything else stays the same except for engine effeciency. I have no idea if effeciency is better at 1800 rpms or 3200rpms.

turtlevette

fuel injection or carb?

some more details please. Us MPG challenged need some help.

kona

I don't know about efficienty but the drive is much more enjoyable doing 75mph at 1800 RPM's than 3000+ w/o an overdrive setup.

Kona

1980 Last year with a Carb.

BerniesVette

If I am not mistaken the 400R and the 2004R are both overdrive transmissions. I don't know if they both have lockup convertors but I know the 200 does. This being the case the gas mileage difference will be close.
Bernie

markdtn

A few questions:
1) are you sure the converter is locking?
2) what is your before and after rpm at say 65 mph?

My guess is that you are lugging the engine with 3.08 gears and are in an inefficient spot on the torque curve. 3.36 or 3.55 would probably give better mileage and better performance in this case.

zwede

Depends on camshaft profile. Long duration cams often have soo much overlap that they get better mileage at 2500+ rpm. A stock cam or a mild performance grind will use less gas at 1800 than 3200.

Clint's C3

This may not be the entire reason, but headers can actually hurt low end performance and mileage. The highway mileage dropped about 4 mpg (28 to 24) after I installed headers on our 6 speed LT1 Z-28. Something about exhaust velocity and efficient scavenging. It runs great above 2500 rpm but lugs along at 1500 rpm in 6th gear at 70 mph.

GrandSportC3

If you do a longer road trip (highway only), you'll notice a nice difference...

page62

Ditch the Edelbrock 1406 and put a QJet back on there.

zwede

I don't see how headers would decrease mileage... I would suspect a header leak if I saw a drop in mileage after a header install. A header leak will suck air into the exhaust during the low pressure phase of the exhaust pulse and that makes the O2 sensors read lean. The computer compensates by adding fuel resulting in a mileage drop.

gkull

If you want better mileage. Make the motor more efficient. With a high 300 hp 355 ci TH350 and 3.55 gearing I got @21 mpg on a 3000 miles test. I usually drove @20 mph over the speed limit. So if it said 65 mph I was driving 85-90

With the same motor and 700R4 with lockup OD and 2.86 overall gearing with my 4.11 rearend the mileage was nearly equal on the same 3000 mile test with @21.4 mpg

Even though I was doing less rpm - 700's I've been told use 20 more hp than and hopped up TH350.

I think that a low HP motor with OD and a 3.08 rear must barely even be able to run in lockup OD. You would be better off with 3.73 or 3.90 gearing. I've been driving 4.11 gears since the late 80's with OD.

AlwaysWave

2000 RPM = 75 mph
1800 RPM = 67 mph
1500 RPM = 56 mph

zwede, I had checked the vacuum at cruise speeds by bringing the gauge inside. At cruise the gauge reads 20-21 inches. At light throttle acceleration it dropped to 12 inches, and at WOT it read 3 inches. The carb rod springs are supposedly rated to release at 5 inches.

I had checked the advance curve, and near as I can tell by revving in the garage everything is functioning OK. I have 52 degrees in with full vacuum at about 2700 rpm.

The stall converter is a lock-up. As a matter of fact, I still don’t have it adjusted well enough because it won’t come out of lock-up unless the brake is applied real hard.

Jdmick and markdtn, I was wondering about the gearing, but this brings another question or two. How do I determine if the engine is “lugging”? Another thing is, C4’s, C5’s, and the C6 manual trannnys are double overdrive. They do what? Like 1200 rpm at 70 mph?

GrandSportC3, I have taken a few long trips. This has actually been a strange year for me, but I’ve made 4 trips to Detroit in the car, at about 500 round trip miles each time. (Usually I do 8,000-12,000 miles a year.)

Page62, My performance and mileage went up when I traded the Edelbrock for the Q-Jet. When I first got the car, it was 100% stock and ran at 9 mpg

bobs77vet

with my l48 dog and the th350 I got 12.5 miles per gallon aound town.....i upgraded to a 350 HP 350 and a 700r4 and i get 14 MPG around town.....on my one long extended hiway jaunt that was at 70-75 mph i figured i was in the low 20s. around town i just don't get into 4th enough to make a difference. so i doubled my horse power and increased gas mileage also, not a bad deal.