I'd like to up my gas mileage a bit and wonder if anyone can guess what is the best rpm for hyway drivng.

Car Specs:
66 El Camino, has a canopy
355
SR Torquer heads stock out of box
Generic 204/214 cam (Melling, 278/288 SAE dur, .420/.442 lift, 112 LSA, 107 ICL)
1.6 rockers on both for .448"/.471" lift (put them on before I knew intakes only was best)
Cast pistons, quench is .083"
HEI w/Accel coil, Taylor wires, NGK UR6 plugs
16º initial, 36º total, 10º vacuum adv, manifold vacuum on the VA for 26º at idle
Edlebrock 600 manual choke jetted 8% lean from factory. I don't what recall which rods are in it (I'm at work)
Eldebrock Performer Plus manifold (2101)
200-4R trans, stock converter
3.36 gears, 245/60-15 BFG T/A radials
Stock manifold with 2.25" pipes and Super Turbos

This engine runs great. It has well over 100,000 miles on it and no oil consumption. It tows my bracket car with relative ease.

With the converter lock in OD, 60 MPH is 1750 rpm, 65 is 2000 rpm. If I'm not in a hurry, I drive at 55 MPH, the engine is at 1500 rpm.

AFAIK, all dyno programs are for WOT use so the torque curve/peak shown might not be applicable to cruise speeds. BTW, the odmeter doesn't work so calculating MPG is iffy at best. Going by highway mileage signs, I got ~15.88 mpg between Tucson and Phoenix, while towing the trailered 67 Elky, last October.

Any ideas on which rpm would be best for best mileage. If 65 mph gives the best mileage, I'll drive that, if 55 is best, I'll stay there. TIA

Put a vacuum gauge in the cockpit. Whatever gives the best vacuum, gives the best mpg. Sounds like you've already leaned it out during cruise.

Good idea, I hadn't thought of that.

From the studies i have seen in the trucking industry. 55 mph is where you will get the best mpg it goes down after that.

Pat,

I think it is the point of max. torque which should also be the max. suck point (i.e., vacuum point) too?

I ran your combination guessing about some stuff and it looks like your best torque point lies between 3000 and 3500 rpm now. (I guessed your CR. at 10 to 1?? And used 2.02"/1.60" valves?? 204/214 cam)...

This tells me that your cam is too big so I dropped back to a 184/194 duration at .050" lift and now it is at 2500rpm! But torque goes south at 3500 and hp at 5000 rpm after that..

I think this exercise is telling me why tall geared stock cars have such small cams!!

I may be too high on your CR.??

I also changed from duration at .050" to advertised so I could try a CC 240/248 HE type cam and lowered the lift but kept the same LCA and ICA..

Now it is t at 25 to 3000 and hp at 4000 rpm and both falling off.

Interesting to say the least!! I just donno??

pdq67

The cam is a wierd one, a very old design. Notice the quite long adv dur and the short .050" dur. The engine has about a 8.6:1 CR. Melling sells the cam as a torque cam, and it does quite well as one. If I were to do it again, I'd go with a much shorter duration (about 248º) with about the same .050" (204/214). I might even try a tight LSA of 108º installed 6º advanced with a DCR around 7.72.

I'm sure the numbers you ran are for WOT. I don't know if they apply to cruising on the highway.

Ive also heard the peak torque rpm = best mpg.
Mitch

A few things that may help. Do you have a Wide Band O2 sensor. It can really help tune you cruise and idle. Plus the transfer between the two.

Do you have a wind deflector on the back of the canopy? It can really help gas mileage.

Then there is always EFI. Even a perfect carb can not match a well tuned EFI setup.

Steve

According to Engine Analizer 3, the peak torque is at 3500 rpm. I don't want to run the engine that high. That's why I put in an overdrive trans. With the TH350 70 mph was about 3300 rpm but gas mileage was abysmal, barely 10 mpg. To run 60 mph at 3500 rpm I'd need a 4.34 final drive. With the OD trans (.67:1 OD) to get that RPM in OD, I'd need 6.48 gears. Just not in the cards smile.gif .

Steve, I have a set of shorty 1.5" headers with O2 bung in them. I'd like to get them on but the PS bracketry has me puzzled. I have to do the entire install in one session as I haven't another car to use. I'll check into the deflector. It might look funny but that's OK if it helps. I've thought about EFI, but I don't think the cost/benefit ratio works out too well.

You have a mildly built 66 and you are looking for gas mileage??? Spend $1000 and go buy a import beater. I did an engine swap on my civic recently, running good, and is getting anywhere from 43 - 48 mpg. I'm certainly not complaining. lol OTHERWISE drive a bit slower, take it easy on the gas, do the vac gauge deal, maybe retard the timing a little, buy some cheap tall tires. just my .02

I like to see an import beater pull a trailor with a 67 el camino,that would be a funny sight! smile.gif

15.88 mpg between Tucson and Phoenix, while towing the trailered 67 Elky

Pat I hope you're not complaining, that's pretty dang good. What do you get without the trailer?
A set of long 1.5 headers & play with timing & fuel mix. I'd say keep RPMs as low as possible.

Hey Pat, if you could swap to a long water pump set up cheaply, the PS is easy to do with a long water pump. We've done two by simply taking the PS bracket off of a 6cylinder car and flip it width wise, and it makes a perfect bracket...

But again, I only know this works for long water pumps (at least, i think 70's have long water pumps...either way, whatever style pump the 70 has).

71 Chev, I'm going to try the vacuum gauge thing. That does sound like a good idea. Running more advance usually helps mileage. That's the reason for vacuum advance.

77 crusier, I'm pretty happy with the mileage on that trip. I don't know how accurate it is though, with the non-operating odometer.

Enrique, The long pump would require moving the alternator to the other side. Do able, but I don't know if it is worth the hassle. I read all the post about mounting the PS pump with headers. However, I wish I had archived them. I'm sure when I go to install the headers, they will be long gone and I'll have to start another thread.

Thanks for all the replies, keep them coming smile.gif .

I don't know about all this theory with max torque/hp and rpm equaling best milage but I don't think that's the way to approach the issue.

If you can idle down the road at 70 mph you will get the best milage. The more the throttle plates open the more fuel goes through.

I use a el cheapo O2 sensor and a vacuum gage to tune the carb and keep one eye on the vacuum gage when driving to help keep my foot off the pedal. Gearing(engine rpm)is the whole trick.

With a 2.73 gear and 700R4 I was getting almost 20mpg on interstate driving cruising at about 1800 rpm at 70mph. WhenI changed to a 3.31 gear the milage dropped to 17 when cruising 70mph at about 2100 rpm.

I'm sure my 454's peak torque/hp would be higher than 2100rpm but I bet if I gear to run 3000 rpm at 70mph(which would be closer to my peak torque/hp)I will not get better milage.

Just my pennies worth.

If the car is cammed properly then the peak torque will definitely get the best gas milage. My former girl friend had a Honda Accord with a 2500 RPM torque peak which routinely got 31 MPG at 55-60 MPH. On a trip we held it between 2400-2600 which was (72-75 MPH) and got 45.1 MPG, this was on a strech of road between Las Vegas and Reno, Nevada. My old Buick got better gas milage at 70 (14.2 MPG) than it did at 55 MPH (12.5 MPG). I'd say at nearly 16 MPG while towing your getting about all that can be expected from anything but a diesel and i doubt that would be much better.

The wind deflector idea probably has merit. The motor in a steady state cruise will get better mileage with a lot of timing, not less. Most distributors are arranged backwards to stay away from detonation problems. I heard somewhere a long time ago that someone arranged a distributor so the vacuum advance retarded the timing when accelerating and then set the timing at something like 50 degrees and got substantially better gas mileage. Increasing the compression ratio helps also. On a fuel study done once it was printed that raising the compression ratio by from 9 to 1 to 12 to 1 resulted in a 9 percent power increase but the test engine got 19 percent better milage. Feuling, (i can't remember his first name), has done a lot of gas milage research and even designed his own Big Block Chevy high compression head for towing with a big block. One more thing, towing is always supposed to be done with the transmission (out of O.D.) according to the manufacturers. Something about killing the transmission prematurely if you tow in O.D..

A 66 El Camino with a diesel engine,now there's something to think about,and you were concerned about headers and bracket alignment!! :D

I'm into Mr. Fueling's high compressioned BB head/very eff. combustion chamber head/87 octane gasoline theory stuff but his company doesn't sell a SB head, or at least the last time I got into it's web site.

I figure 60 cc, 283 "Power-pack" heads with the fel-pro, PN 1094 .015" shim headgaskets would get Pat up to 11.2 to 1 CR. or so if he wanted to try Mr. Fuelings theory out?? Drop in the smallest cam you can find along with an old obsolete SP-2P E-brock, dual plane intake AND max. her out for LOW RPM suck. I'm talking about 1800 rpm suck....

Try a Crane 305 Compucam PN 114102 if they still sell it?

244/184/254/194, 104/104, .378"/.401" lift hy- cam.. Crane say's it's basic rpm is 500 to 3500 rpm!! I figure it will cruise below 1800rpm so is what Pat is looking for..

Dynamic compression ratio should be around 9.95 so she will definately test Mr. fueling's high compression/87 octane gas engine combination theory, imho...

OR find an old 8.5 to 1 CR, low compressioned stock 350 motor and shoot for a 7.54 DCR number?????

Only problem I see is that we are screwing with his daily driver motor so I don't know if it is worth messing with or not b/c I wish you had a spare 350 to play with and after you got her put together switch them out so if theory didn't work in the "REAL-WORLD", you wouldn't be out your original motor......

Just thinking out loud is all, take it with a grain of salt, A BIG GRAIN, he, he!!

As alway's, just old pdq67 mouthing off.....

Pat,

Don't worry about trying to run the Elky at its torque peak for best mileage. The torque peak is simply the most efficient point in the engine's operating range, not the point that it uses the least amount of fuel. To put in another way, at the torque peak, the engine is converting the potential energy of the incoming fuel at a higher percentage than at any other RPM.

There are ways to calculate the theoretical fuel consumption of a vehicle, but you need to know the fuel flow rate at a particular steady-state RPM. Otherwise, all you can do is test. A lot of the ideas you've been given are good. Anything you can do to reduce engine RPM when cruising will yield you an improvement. My dad's 99 Camaro Z28 can get 30 MPG at 85 mph in sixth gear (0.5:1) because it's only at 1700 or 1800 RPM. That is definitely not an LS1's torque peak, but it's a point at which fuel consumption is very low, while still making just enough power to move the car at the desired speed. So unless your torque peak is between 1500 to 2000 RPM, running at your torque peak RPM wouldn't give you any better results than just running at a lower RPM.

I hope some of that makes sense and helps. It sounds like you've already done quite a bit and your mileage is very impressive! You could also look into a TPI or throttle body injection system. They're not too expensive (all said and done) and you might not gain a whole lot on the highway (1-2 MPG), but your around town mileage would jump.

Ryan

These are some great ideas, thanks. I agree with those who say keeping the rpm down so the carb is just barely open. I heard it only takes 15-25 RWH hp to maintain 60 mph on a level road. Not so much an issue in my case is overlap. With a lot, low rpm could hurt mileage.

Harold, intersting idea on the distributor. I might look around for an old distributor to experiment with. Compression is a definite help. It seems to help just about everything as long as there is no detonation. The next engine, hopefully several years away, with be high compression with proper quench. I only tow in OD on level stretches or down grades. When there are hills I drop to 3rd and lock the converter. Hopefully, that will not hurt the trans.

A diesel, huh. LOL!!

The OD trans has helped mileage a lot. I think it will pay for itself in a couple years. So far, I figure a better than 50% improvement.

I'm going to hook up a vacuum gauge today and do some testing. I'll look for maintaining the highest vacuum. Might be a while before I know what any improvement might be.

Thanks again,

I ran the vacuum test but it was somewhat inconclusive. The freeway isn't all that flat, ups and downs abound. Nothing steep but enough that you need to move the throttle to maintain speed.

At 55, I got about 20" of vacuum, 60 was about 19.5", and 65 was about 19". Even though these number are not all that accurate, it appears that 55 yields the highest vacuum. I have no idea what this would translate into gas mileage. Approximately, 1" of vacuum doesn't seem like it would make a big diference, but it might. I wish I had thought of testing in 3rd gear with the converter unlocked. It would be interesting to see what the vacuum readings were with the TH350 trans. I'm going to leave the gauge hooked up for a few days and keep an eye on it. BTW, idle vacuum is 20.5" neutral, 19" in gear.

Cool Pat..

pdq67

Definately don't want to cruise at WOT peak torque. All that means is peak we AT WOT.

You want a point that has an efficient part throttle fuel consumption. Lean it out until this part throttle efficiency goes down, run as much timing until this efficiency goes down. This has nothing to do with WOT.

A big cam doesn't like low, low RPM and you have a small cam.

You basically want the point that consumes the least fuel which has to do with throttle postion and the BSFC at that point as well at part throttle. Towing a car will require more power and could change what you'd want vs not towing.

As was said, 16 mpg towing is pretty good.

Going faster really won't get better economy, unless it puts you at an RPM and throttle postion that is a lot more efficient and overcomes the larger drag at higher speeds.

Unless it is a big cam, going to a lower cruise RPM usually is a very good idea, but towing could change that a bit.

A carb could get better fuel economy that EFI potentially.

Yeah, I was going to comment too ... all the comment of max torque is usually where max BSFC occurs. Basically the most power per unit of fuel.

I'm surpised nobody has mentioned yet ... a good Q'Jet might get you another 1-2MPG! The primary side of a Q'Jet is the most effecient carb I know of. And forget EFI, a good Q'Jet can be tuned just as well ... not always as easily though.

As far as speed, you can try different one's ... but you will probably find out the slower the better, just don't go too slow and get ran over. It has nothing to do with the motor, but your elky is a boat ... and wind resistance is not linear to speed, but actually exponetial.

Check out the bonneville racers for ideas, though the gains will likely be minimal. Try a front chin spoiler first, lower the car, block off as much of the radiator as you can afford, and aerodynamic changes UNDER the car influence it also.

A couple engine ideas (though I'm not an expert here) ... maybe try a thinner head gasket, a modern cam, mild porting on heads.

Just remember that I assume you are looking for better MPG to save $$ ... so calculate how many miles that $300 investment will take at a 1MPG gain? ... And, remember that any change to premium fuel comes at a cost too.

Take care, Neal

Ya a Qjet will definitely improve things. If you select the right combination of power spring, rod, and main jet, you can tune it to run really lean at cruise, but still have proper ratio for best power when you dip into the throttle.
What vacuum advance can are you running? The leaner you go the more advance you need.

Then the next step would be to hook up an EGT meter, and/or a wide band O2 meter, but if you are pulling 20" at your cruise, I think you'll find there's not alot left to gain. Once you get over 50mph it's more about frontal area, and the hp required to overcome the resistance, rather than rpm. If you want to do some reading on the effects of EGT and mixture, do some aircraft related searches. They have this stuff down to an exact science, and have for a very long time.

For ex; http://www.sdsefi.com/techegt.htm

http://www.americanaviationinc.com/effectsCessnas.html

By the way, once you add the trailer to the equation, you will see a much larger variation in vacuum. It will become clear pretty quickly what your most efficient towing speed will be.

I've seen boats outfitted with a 2" diameter digital gauge that reads gph fuel flow, and can log total gallons used. A small flow meter is installed inline in the fuel line and wired to the gauge. You can probably purchase such a guage through boatersworld.com or cabellas.

Highest manifold vacuum will not necessarily provide highest MPG. Highest BSFC at a given vehicle speed will provide the best MPG. While dynoing an engine once, I noticed that the BSFC at idle was over 1.0. At part throttle it dropped to 0.7-0.8. We only achieved the typical 0.5-0.55 at wide open throttle.

I have a heavy Caprice with a 350 similar to Pat's: 9.3:1 compression, 64cc 1968 327 heads, 0.048" quench, 262/272 204/214 # 0.050" Summit cam, (7.8:1 DCR I believe) Edelbrock Performer, extensively reworked Q-Jet, Turbo 350 and 2.56 gears.

I run a Crane adjustable vacuum advance and the Q-Jet is modified to allow simple adjustment of the primary cruise air fuel ratio. I've fiddled extensively with cruise mixture and vacuum advance and could never beat 18 MPG. This was achieved with 20°+ of vacuum advance and really lean A/F ratio.

This set-up would ping at both part and full throttle, even using 92 octane. The car runs the best with richer A/F ratio and about 10° less vacuum advance. I still get 17 MPG with no ping at part or full throttle.

I think that running ultra lean mixtures and lots of vacuum advance gets the combustion chambers so hot that the engine is more susceptible to full throttle ping.

I don't think there's much more than 0.5 mpg diffenence running 60-80 mph with my combo.

Like others have suggested, I'd also recommend trying a later model non computer controlled Qjet if you're interested in maximum economy. I've found 71 primary jets, #41 rods, and a power piston spring that starts at 10" HG and is fully extended at 6" HG performs best.

Lowest brake specific fuel comsumption number..

And I figure that max. torque peak at as low an rpm as you can make it will allow max. suck which will allow backing off the throttle and still achieve ALMOST a max. suck.

It's the old, "make her more powerful/eff. so she can use the same amount of fuel but run faster or use LESS fuel to run the same as you aready are" sorta deal..

VERY good post!! keep it all coming guy's..

pdq67

EFI can be done for pretty cheap. It depends on how much time you want to spend researching on how to do it. A GM 454 TBI system (should work well on your motor) could probably be done for 500$ with some junkyard searching. But then you need to learn how to burn your own chips, or go with something like the Megasquirt system.

The wind deflector should help quite a bit. 1-2 mpg is my guess. There's a reason GM put them on almost every wagon and SUV made. And it helps keep the back window cleaner too. :D

Just curious what Doug F ment by A carb could get better fuel economy that EFI potentially I will agree that a perfectly tuned carb could equal EFI, but it would need constant attention every day. A few degrees change in temperature will change a carb slightly, while EFI adjusts automatically.

Steve

Wow those fuel meters are $$$. I have instantaneous mpg readouts in my Crown Vic, so it must incorporate a fuel flow sender somewhere. I'll look into it and see how they are measuring it. Could be a hot rodders solution to measuring fuel flow.

What I meant is that when running a closed loop narrow band setup, you run around 14.7:1 A/F ratio. If a carb is well designed and is tuned leaner than this, and the engine is more efficient at say a 15.3:1 A/F you could get slightly better fuel economy. Now if it is wideband closed loop you can run at any A/F.

I am an EFI guy and will NEVER run a carb on my stuff, but a good carb, tuned for a specific application, can run good. You just have to know how to tune it.

It takes "x" amount of HP at the wheels to go down the road. The engine that uses less fuel to do this will get better economy, period.

I didn't read every post here, but I've done quite a few experiments with fuel economy.

I found that a Carter AFB can get better, more consistent fuel economy than a Holley vac. secondary.

But the thing that surprised me the most is that when I have my vacuum advance on the HEI set to advance to it's maximum (without detonation), I get the best mileage. Period.

When I put the MSD Billet distributor in there, it wouldn't allow enough vacuum advance to do any good at cruising speeds. It's a difference of 5-7 mpg with the Holley on there.

And I've also found that at 55-60 mph, I only lose 2mpg by running in third gear (an extra 1000 RPM), where I lose a great deal more running at 70-80 mpg (as compared to running in 4th gear with a 700R4 and lockup converter).

Remember, I'm running a mild solid cam with low lift and moderate duration. It gets much better gas mileage than the hydraulic I had in there before which had nearly the same seat-to-seat duration and a little less lift.

So I think more lift "under the curve" equals a more efficient engine. I'm sure this is why todays pushrod cars come with roller cams.

I hope to incorporate a good EFI setup one day soon.

Great responses here. Seems that quite a few of you have looked into what it takes to improve mileage.

EFI isn't going to happen, at least in the foreseeable future. A Q-Jet might.

The way I see it, lowering the RPM, running as lean as possible, and reducing drag are the main avenues to better mileage. I've played with tire pressures but running at the tire's max isn't always economical since the center of the tread can wear out quite early requiring early replacement.

Bomber 67 did a test running the engine at high temperature, about 210º+ or so. I don't recall the numbers, but his mileage improved quite a bit. I'm hesitant to run a hotter stat. I'm like most here, I like to see 180-190º. When it gets over 200º or, I get concerned.

TJC, I've seen aftermarket automotive trip calculators/computers for sale. I'd think they would do the same as your factory unit. I'll look into that.

Doug, EFI would certainly be more effective for mileage but the payback time is very long given the cost and and that the increase in mileage isn't extremely high. I'm still working on the payback for the extra $700-$800 the 200-4R trans cost over a TH350.

Chad, I'm glad you chimed in. I know you have paid close attention to your mileage. No doubt max advance helps mileage. My current setup pings slightly under a heavy load and heavy throttle. I'm not sure I can run more. Part of the problem is the horrid quench distance the engine has of .063" even with a .018" shim gasket (damn cast pistons). That hurts, restricting the advance I can run, but will have to stay for now. I'm surprised at the 2 mpg change between 3rd and 4th. I think, but am not sure, that I picked about 5-6 mpg between the TH350 and the 200 R. BTW, I setup my trans so I can lock the converter in 3rd around town. I don't know for sure that it helps mileage but it seems to. To avoid detonation and lugging in traffic, I mounted a fingertip switch on the turn signal stem.

Thanks for all the great responses. This might be a good thread to archive.

Hi Pat, I think whoever suggested the Quadrajet carb was right about it's being the best for gas mileage. Care must be taken to see that the secondary air flapper valve is tightened some so it doesn't try to open at low throttle settings though, as this will kill the mileage in a hurry.

I was driving a newer Taurus once with a digital MPG readout on the dash. At 65 mph on a level freeway, it lost 5 mpg dropping from OD into 3rd gear with the cruise conrol on.

Harold, no doubt the Q-Jet is the piece too have. I'm going see if I can round one up. New ones are way too pricey and would take a long time to pay for with gas savings.

Roadknee, That pretty much jives with my experence.

Pat if you find some that don't look worth a crap, get those too cause the xtra metering rods jets & hangers are nice to have. As long as the price is right.

Would it be possible to get workable results using a chassis dyno?

Originally posted by 67 GTO:
Would it be possible to get workable results using a chassis dyno? I don't thnk so. I've only seen them used at full throttle.


Jim, good idea.

Originally posted by TJC:
Wow those fuel meters are $$$. I have instantaneous mpg readouts in my Crown Vic, so it must incorporate a fuel flow sender somewhere. I'll look into it and see how they are measuring it. Could be a hot rodders solution to measuring fuel flow. Ok, I scanned through my Ford EEC-4 manual and it looks like they use the air meter and vehicle speed to determin instantaneous fuel economy. So no joy in finding a fuel flow sensor off of one of these cars.