MPG and Manifold Vacuum

[Lugnutz65]

My T5 OD transmission gear is 0.76 with a 3.73 rear axle but I have not seen any improvement in MPG compared to the factory 3 speed with 1:1 final drive.

Others have suggested that I'm increasing throttle when in OD and that the Rochester B power valve kicks in - thus wasting fuel. I disagree since I do not lug the engine when driving, and I usually don't shift into OD unless I'm on the highway. Back road cruising in OD is tempting but manifold vacuum drops a lot, so I don't do that.

I hooked up a vacuum gauge to the manifold to check what happens when driving. If I'm on level highway and keep the throttle steady, manifold vacuum drops a little when I shift into OD. I think a slight drop in vacuum should be expected since the engine strokes that produce the vacuum are fewer than when in 1:1 and it's not a perfectly sealed system.

I hope this will remain an academic discussion. I know I can spend a lot of cash on EFI or just swap to LS, but my questions are more related to MPG and Overdrive transmissions.

Here are some details.
1963 230ci with a Rochester B carb. Engine is completely stock.
Oil pressure is 25 PSI at idle and near 40 when driving.
I have 135 PSI in all six cylinders and absolutely no smoke.
1963 C10 Fleet short bed.
Rochester B carb

[truckster]

So what kind of fuel mileage are you getting? What type of traffic? What speeds?

If we're talking highway speeds, I think that 230 is working pretty hard to push your truck through the air. Overdrive may not do much for you.

[Lugnutz65]

Drove 240 miles with 1:1 and 95% highway at 62 MPH. Got 14.5 MPG
Put in the T5.
Drive 700 miles with 0.76 OD. Again it was 95% highway and 62-65 MPH. Got 14.5MPG again.

[mongocanfly]

I would have been turning somersaults of joy if my manual 86 had got 14.5mpgs... I was lucky to get 11-12

[AcampoDave]

Even though the mpg's remained the same, it has to be putting less wear on the drivetrain in o.d. I'm curious, what were your highway rpms before and now?

[mongocanfly]

what size tires are you running Lug?

[YoungPup1977]

What MPG do you think you should be getting ? Is this your daily driver ?

[Lugnutz65]

Thanks for the questions. Responses are below.

I'm trying to understand more about manifold vacuum and fuel economy. At idle, manifold vacuum is high because the throttle plate is nearly horizontal and only a little air can get past the throttle plate. As you accelerate, the throttle plate tilts more vertical and more air flows and thus manifold vacuum drops. To increase speed, fuel is added by tilting the throttle plate more vertical which also adds air and causes a drop in manifold vacuum.
Once a certain speed is achieved the throttle is held steady again (i.e. cruising at 40MPH) and manifold vacuum begins to catch up again.

So 2 factors seem to be at play here that affect manifold vacuum. RPMs and throttle plate position.

Originally Posted by truckster

So what kind of fuel mileage are you getting? What type of traffic? What speeds?

If we're talking highway speeds, I think that 230 is working pretty hard to push your truck through the air. Overdrive may not do much for you.

I think the wind resistance plays a huge part in this, but I thought I would see a little improvement in MPG at a highway speed of only 62MPH.
I got 14.5 MPG with 1:1 final drive when I had the factory 3OTT.
I swapped in the T5 with 0.76 OD and nothing else was changed. I still got 14.5 MPG.

Originally Posted by mongocanfly

I would have been turning somersaults of joy if my manual 86 had got 14.5mpgs... I was lucky to get 11-12

While I'm happy to get double digit MPG, I hear others are getting MPG closer to 18 if they keep the pedal steady. I'm only going 62 MPH.

Originally Posted by AcampoDave

Even though the mpg's remained the same, it has to be putting less wear on the drivetrain in o.d. I'm curious, what were your highway rpms before and now?

RPMs with the 1:1 ratio were 2679 at 62MPH and a 29" diameter tire.
The 0.76 OD brings the RPMs down to 2036.

Originally Posted by mongocanfly

what size tires are you running Lug?

I have 235/75R15 tires which are 29" tall.

Originally Posted by YoungPup1977

What MPG do you think you should be getting ? Is this your daily driver ?

There are folks who get better than 16 MPG.

[TwoFiftyShifter]

Its that motor I think. At 2700 it is spinning a bit fast for economy, but its requiring very little throttle to push the truck. At 2000, its purring nicely but isnt making the same power so its working a bit harder. I bet with a 307 V8 and that T5, you would clear 15mpg. That said, 15 isn't terrible.

If you did a before and after at 75mph, you would see an increase I bet.

[AcampoDave]

Have you checked to see where your timing is at the new freeway rpm? Perhaps the reduction in rpm has also caused a reduction in the centrifugal/vacum advances.

[Lugnutz65]

Originally Posted by AcampoDave

Have you checked to see where your timing is at the new freeway rpm?

I have not checked that. I suppose I can just check total timing at 2100 RPMs when I’m in the driveway.

[rpmerf]

Along with timing, I am wondering what the fueling is like at that RPM and vacuum level, and if you might be able to lean it out a bit. Not sure if this carb has adjustable part throttle you can alter with different springs to give less fuel at your target vacuum level.

[Tom]

Sounds like the I6 doesn't have the power at lower revs to move the truck along without more throttle.

[Dead Parrot]

Power to move the truck at 65 should be about the same in either 3rd(1:1) or OD(.7:1). Aerodynamics don't care about rpms or gear ratios. Only savings is the difference in engine friction between 2700 and 2000rpm.

Depending on the mechanical timing curve, you are probably losing some advance between 2700 and 2000. Same for vacuum advance at the lower vacuum due to lower RPM. Might be worth seeing if you can adjust the timing curve for the mechanical without causing pinging.

If that carb has any type of part throttle mix adjustment, might see about adjusting it to make up for the lower vacuum with the OD.

To remove one possible variable, might do a 100 mile trip in 3rd and repeat the same trip in OD just in case the new transmission reads different then the old one on speed & miles traveled.

[cadillac_al]

Darn good info around here. I'm very very surprised that dropping rpms from 2700 to 2000 didn't save any fuel. It could be as simple as what Tom said but I think Dead Parrot may be on to something with the mechanical and vacuum advance theory. I think that timing curve needs more investigation.

[toolboxchev]

One question still remains unless I missed it, What vac reading are you getting @ 2700 rpm with the old transmission? And again what reading with the new transmission @ 2000 rpm?

This intrigues me due in part I am playing with the same issues without doing the trans swap. I am thinking of a rebuild with the most economy and hp, trq.

[Marty68]

following this.. planning gathering parts for t5 behing 283. I don't think 283 will have enough torque at 1600 .76 final drive to propel heavy 51

[LostMy65]

Updates?

[Lugnutz65]

I just got the 63 back on the road. Have not had a chance to play with timing or check vacuum readings.
Since the T5 is in the truck, I plan to run the following test.
1.) I’ll drive a distance of XXX miles in 4th gear (which is 1:1 and the same as 3rd gear when it had the factory tranny).
2.) I’ll repeat that exact drive using my OD gear when appropriate (mine us 0.76) and compare MPG.

If I get really ambitious, I’ll play with the timing curve to see if MPG improves. It all takes time, so be patient.

[Lugnutz65]

Originally Posted by Tom

Sounds like the I6 doesn't have the power at lower revs to move the truck along without more throttle.

There might be some truth to this. However, the HP and TORQUE graph for a 230 shows that there is plenty of power at 2000 RPMs. ( I’ll find the graph I have and post it.) My 230 has great compression in all cylinders and runs great with good oil pressure. So it’s not a tired old engine.

Originally Posted by Dead Parrot

Power to move the truck at 65 should be about the same in either 3rd(1:1) or OD(.7:1). Aerodynamics don't care about rpms or gear ratios. Only savings is the difference in engine friction between 2700 and 2000rpm.

Depending on the mechanical timing curve, you are probably losing some advance between 2700 and 2000. Same for vacuum advance at the lower vacuum due to lower RPM. Might be worth seeing if you can adjust the timing curve for the mechanical without causing pinging.

If that carb has any type of part throttle mix adjustment, might see about adjusting it to make up for the lower vacuum with the OD.

To remove one possible variable, might do a 100 mile trip in 3rd and repeat the same trip in OD just in case the new transmission reads different then the old one on speed & miles traveled.

I agree that the amount of advance will be less at 2000 compared to higher RPMs. What seems puzzling is this comparison.
Drive in 1:1 final drive at 2700 RPMs for XXX miles and then repeat that trip at 2000 RPMs, staying in 1:1 for both trips. We all learned that driving slower saves gas, but the change in timing advance would be exactly the same as when I’m in OD. Am I missing something?

[YoungPup1977]

I installed a 700r4 behind a I6 for a guy and it worked great. He also had a problem with distributor bushings being worn out. Just my 3 cents Good luck on your MPG adventure !
The carb that was on this guys vehicle I believe was a carter Y series (YF), I believe its a one barrel carb...he was getting 16 to 18

[rpmerf]

Originally Posted by Lugnutz65

I agree that the amount of advance will be less at 2000 compared to higher RPMs. What seems puzzling is this comparison.
Drive in 1:1 final drive at 2700 RPMs for XXX miles and then repeat that trip at 2000 RPMs, staying in 1:1 for both trips. We all learned that driving slower saves gas, but the change in timing advance would be exactly the same as when I’m in OD. Am I missing something?

The reason driving slower saves gas is wind resistance. I've heard the amount of drag doubles every 10 mph.

[scottofksu]

Looking at the the powerband for your engine, your putting out just 75 hp at 2000 rpm and we’re putting out 125 hp at 2700rpm. Your still fight the same drag and most of the same frictions only with 66% less power!

[scottofksu]

Looking at the the powerband for your engine, your putting out just 75 hp at 2000 rpm and we’re putting out 125 hp at 2700rpm. Your still fight the same drag and most of the same frictions only with 66% less power!

[mattfranklin]

In theory, lower speed and higher manifold pressure should get you better fuel economy, unless the carburetor is getting into an enrichment mode. There are other factors, like turbulence and maldistribution of fuel in the intake that might also make it worse. Modern cars downsize the engine or "downspeed" the engine for better fuel economy.

There is a way to sketch things out if you have a proper performance map for your engine. To do this right you'd need an efficiency map (also called performance map or engine map) for your engine and the load characteristics for your truck.

The EPA (surprise!) does some excellent work in this area, but unfortunately for us it's on newer engines. A couple of examples of efficiency maps are on page 531:
https://www.epa.gov/sites/production...tion-alpha.pdf

You'd get a map like this for your engine. Then you'd plot curves of your vehicle drag (converted through the transmission to engine torque) for a few different gears. Then you'd pick the gear that had the lowest BSFC (highest efficiency) at the vehicle speed you plan to run.

SAE has techniques to measure the load/drag of your vehicle in their manual. What you do is find a flat section of road and do a handful of coastdown tests. That plus the weight if your vehicle will give you enough to estimate that.

For you case, without an efficiency map, I'd recommend a wide-band O2 sensor to tell you what's really going on with your carb. Then tune accordingly. ANd definitely play with the timing to see how much it helps.