Peak RPM for best torque, fuel efficiency?

[bud45]

I have a 1965 283 transplanted into my 1969 GMC longbed p/u, which still uses three-on-the-tree original transmission and (I think) 3.73 rear gears, with 265.60x15 tires, pumped to 36-38 psi, cold...and have peaked & tuned on it, but I haven't found one thing that interests me: at what RPM would the engine be the happiest for peak torque for best M.P.G?

I've googled for the specs on these engines and haven't found anything yet- ideas, please?

[ed2552]

That is impossible to know. There are so many other variables that the only way to know is to try as many as you can and see which one gives you the best results.

[Longhorn Man]

While agreeing with Ed, I would say that with the aerodynamics, you wouldn't want to drop the RPMs much under 2200 RPM at a modoate cruising speed.

[SCOTI]

Find a local chassis dyno & see where the dyno says your power peaks are. Using that info should help guide your decisions. Torquey V8's like the 2-2200rpm area (the higher the torque, the lower you can get away w/for cruise rpm's).

[mnunn454]

Originally Posted by bud45

I have a 1965 283 transplanted into my 1969 GMC longbed p/u, which still uses three-on-the-tree original transmission and (I think) 3.73 rear gears, with 265.60x15 tires, pumped to 36-38 psi, cold...and have peaked & tuned on it, but I haven't found one thing that interests me: at what RPM would the engine be the happiest for peak torque for best M.P.G?

I had a stock 283/3 speed in a 61 Apache once. It had a 3.90 rear gears, a stock Rochester 4 barrel carb and manifold, tiny headers and 36" tall military tires pumped to 40 PSI. I got 25 MPG by taking off real slow at traffic lights and keeping the speed under 55 MPH. No tach though, but by the sound of it, I'd say it was turning about 2200 to 2500 RPMS at 55 MPH.

My tires would have been much taller than yours, but the R&P difference (3.73 vs 3.90) would make up about 10% of that amount. So I'd guess you're turning about 2700 to 3000 RPMs at 55 MPH?

Seems to me that's about 500 RPM above the "sweet spot" for a stock 283.

I agree that a chassis dyno trip is in order to get a baseline for comparison.

1st thing I'd do is get the RPMs down by going with taller, narrower tires. Then if funds allow, a 3.08:1 R&P. The two together should net you about a 25% drop in RPM. (guessing 3000 down to 2200?) That should still leave you in a decent torque range as long as the speed is kept low. (refer to chassis dyno results) You might try using a vacuum gauge too. If the normal vacuum level drops off while cruising then you may have gone a little too far and need to shorten the tires some.

Assuming the above gets you into the right RPM range, then to further optimize fuel mileage, I'd get a tiny 4 barrel (or a Q-jet with tiny primaries) and a wide band O2 sensor and tweak the primary jetting 'till you get close to 14.7:1 air/fuel mixture while cruising at highway speed. Or you could go back to the chassis dyno for another run (but then you wouldn't have your own neat little wide band sensor to do more testing and tuning)

Other thing I'd do is force myself not to drive any faster than I could safely get away with. With our trucks having the [aforementioned] aerodynamics of a brick, anything you can do to keep the speed down will be money in your pocket.

Good luck.

[prg machine]

just do a 383 and it's all peak torque at any RPM!

[bud45]

Thanks, all...at 62 mph, I seem to get the best MPG, and the engine doesn't sound like it's in a strain...going to get a tach in a couple of weeks (been buying ammo for my shooting hobby) and see where it's revving @ 62 MPH...plus, lots of people here have slowed down just about 60-65 MPH, so I'm staying in the flow of traffic...although some are still screaming by in the "show-off lane"...they must have more $$$ than sense!!!!

The idea of 2200 RPM sounds like the "sweet spot" I've been looking for...thanks again....

[Longhorn Man]

speed limit is 70... probably against the law to go much slower than what you are talkin about running.

[subalternnavert]

In al, minimum speed for the highway is 40mph. I cruised in the slow lane at 60 and the worst I got were nasty looks from all the chavs in stickered up rice burners.

[piecesparts]

With a GM small block, you might try to get your combination at a point where the engine will carry the vehicle up and down hills without a lot of extra application of gas pedal. I have found that my GMC truck (383 stroker motor) 700 R4 tranny and 3.73 gears is the happiest around 2500 RPM. My DD 91 Silverado 350/700R4/3.42 gears is hppiest around 2200 RPM. Many people think that a lower RPM is ALWAYS best, that is far from the truth. There is a point that you can reach where the RPMs will cause the engine to have to apply more fuel to achieve the climb on those hills and change with driving conditions. I drive my vehicles around 75 MPH for the most part and I get decent mileage for driving bricks. The 383 stroked motor gets 15 MPG and the 91 silverado DD gets 20 MPG. It all lies with tire size gear ratio, and tranny output ratios. My son's truck used to get 12 MPG with a 350 SBC, 3.08 geared rear end, and a700 R4 OD tranny. That RPM combo was too low for efficiency. We installed a 3.73 geared rear end and got the RPMs up around 2500 and he gained 2.5 MPG.

There are calculations that will help you achieve the combination that you want. Note the formulas below are the top items are divided by the bottom items.

RPM = rear axle ratio X MPH X trans output ratio X 336
Tire diameter

MPH = RPM X Tire Diameter
Axle Ratio X Trans ouput Ratio X 336

Trans Output Ratio = RPM X Tire Diameter
Rear Axle Ratio X MPH X 336

[bud45]

Ah, sounds decent...I get high 12's and low 13's MPG.

Working for Cummins Engine Co., I see where our engines are rated at xxxHP @ xxxxRPM, with peak torque xxxxft/lbs @ xxxxRPM...usually lower than the max RPM. The peak torque at the lower RPM stage is what I was looking for.

I also drive so that I only have to feather the throttle...minimum speed limits here on the Interstate is 45 MPH. I also take routes that have very few stops or varying speed limits on the way to & from work.

Well, maybe I can figure something out...thanks again!

[piecesparts]

I think that you can gain on this by getting a focus on what RPM that you want to run at. Pick a speed that you are comfortable driving with and calculate the tire size and gear combination you would need to achieve it. I chose the 75 MPH for my goal and worked to get my engine set where I wanted it. I am attaching a website that will allow for an easy calculation of tire sizes and comparisons. All you have to do is put the size into the slots and press the button and you have the tire diameter and width available to you.

http://www.discounttire.com/dtcs/infoTireMath.dos

[bud45]

Pretty cool link, pieces! Thanks, gonna be interesting...and my bro-in-law owns & runs a tire store where I have "sweat equity", to boot! I'll bet I can get some used tires to experiment with over the next few weekends!

Problem is, the cops in our little town and the adjacent town don't play, so I'll have to watch my speeds! Don't want to be paying my hard-earned gas money for fines, you know!

[piecesparts]

Get a stopwatch or a good watch with a second hand and start timing your speed on the highway. It is simple---measure the number of seconds in between a number of mile markers and then average it out. Divide that number into 3600 and it will give you your speed. EXAMPLE: 3600 divided by 60 seconds = 60 MPH. It is the same process, that the cops uses. I made up a sheet that has all of the numbers from 80 MPH down to 30 in seconds, so that I can now do it without a calculator in hand. As you swap out different tires, then set your self up for a speed and see where you fall on RPMS.

[deepnsteep]

Good discussion....so are there any clues that a torque curve can give that would help a person find the "sweet spot" rpm? I've been giving this thought lately in hopes to come up with a good tranny, t-case, axle ratio combo that would max highway mileage....

[oldgold70c10]

I have been asking these questions for a while know, ever since I got my truck.
I eventually decided to keep the three-speed automatic, since they are so reliable and almost "bulletproof", and change the rear ratio to 3.08:1. This gives engine speeds from 2250 to 2700 rpm from 60 to 70 mph. I felt this would be just right to keep from lugging the carbed engine while trying to pull a brick-shaped object through the air at those speeds.
Currently I am running a tired 305 with an edelbrock carb, wieand stealth intake, and a comp cams # 12-238-2 cam with 218/224 @.050 duration and 110-degree seperation. I installed these upgrades last winter as a band-aid fix until I could rebuild my 350, and so far, the 305 has suprised me. I have been consistantly been getting 15-17mpg, despite having leaky valves and barely any compression left.

When I rebuild the 350 I plan to go to a Q-jet carb, a 206/206 @ .050 & 110-degrees seperation cam (comp cams # 12-205-2), 9.0:1 compression ratio, and hope to get at least 20mpg.

[andresco50]

If you want more MPG you have to maintain the rpm's between 1200-2500.. that's all man