I was just running the numbers for the OD swap in the Imperial.
I will be using Edelbrock heads on a stock-ish cam 440 and shoving around a 5300 lb Imperial. It wears 3.23 gears out back.

Without OD I'm looking at 2500 rpm for 65 mph and 2900 rpm for 75 mph.
With OD I'm looking at 1700 rpm for 65 mph and 2000 rpm for 75 mph.

1700 rpm seems pretty low for a 440 to lug around that much weight. Is that too low for an efficient cruise? Stepping up to 3.55 gears would increase rpm by 250 or so.


Please stay away from the "engine should run at peak torque rpm blah blah blah" uneducated BS. We all know that's not true in the real world. If it were true, our cars would get amazing mileage at 4000 rpm.

On the up side of things, with the 3.23 and 518 if I could hit 6000 rpm the car would be cruising at 228 mph.
Do you think John Force has a spare engine I could borrow?

Eddy heads on a pIMP-mobile ? ... ...are you looking at competeing with that Cleveland IMP ...or is this going to be a cruiser that gets super MPG ?

In a non-towing combo ...I would look at a 516 head. And KEEP the 1.60 valve. And toss-in a 2.76 gear.

I know it's a big difference in rear gear ratios, but my brother had a 3-speed OD in a Cuda that he stuck a 2.76 gear in for a month long drive around the US and the AC would not work properly at 65mph because the compressor was running too slow. And I'm with Doc, big valves are gonna hurt performance at the kind of rpm you will be using the most, I would go stock 2.08/1.74 if possible.

What are the roads like in your part of the country? Flat or hilly?
If they are flat you could probably use that combo. If you were in upstate NY with hills and mountains, that engine would be lugging and the trans would probably be shifting in and out of o/d.

1700 sounds a little low. 2100 for mine at 70 and is a sweet spot. Don't have to shift down to pick up speed most times. Once 5000 pounds is moving, maybe 1700's OK. 440 Tremec 3:54.

My Imps had 2.70 gears.They all cruised allday.Never did get the RPMs but it was low.Rocky

Prior to adding the OD
you could try a few 'real world' experiments
by adding a vacuum gauge connected to the intake manifold
then driving in top gear at 1700 rpm at whatever mph the current ratios give,
up a progression of ever steeper hills,
to increase load on the engine.

When the vacuum drops below
the 6 to 8 inches of Mercury level,
you are getting out of the operating region of good fuel economy.

You can estimate your horsepower load
at different speeds and grades of hills
using the stuff found in this article;

http://web.archive.org/web/20061123075351/http://www.etrucker.com/content/downloads/ccj0302.pdf

notice it is in pdf format

Although the article is focusing on big trucks, the same applies to cars climbing hills

This admittedly old (1960s)
but 'universal'
BSFC graph from the Taylors at MIT for engines
shows the 'island' of good fuel economy:



Notice the bottom line of the graph is not rpm
but 'average piston speed' in feet per minute
which you get by converting your stroke from inches to feet, doubling it since it goes up and then down once per each revolution of the engine shaft,
then multiplying by rpm

Didn't some of the later '70s and '80s B/F/M/J bodies have 2.45 gears and 318s? I may be out in left field here, but seems like with a big block it would be a non-issue.

For many years I had 2.76s and a mild 383 in my 3800 lb RR (with a single plane manifold, no less) and it didn't seem to bother it a bit. Don't remember the rpms, but the math says 2250 rpm at 60 mph. Even with the 383, it felt like it needed another gear. (Maybe from all the years driving the DD Toyota 5-speed.)

My 454 powered dually runs a 4.10 gear, 32.25" tires and an overdrive ratio of .75. At 2500, I am moving along at 76MPH. That tank is mucho heavier than your Imperial and has absolutely no problem keeping up with in any situation. I believe that your 440 is a more effecient engine than the big block Chevy also giving it an edge. I would go to a 2.76 gear with an RV cam and some 27"-28" tires. The cam and small valves is the key to port velocity.
Just my $.02 worth.

Depends on your carburetor somewhat. You'll run rich and your gas mileage will suck if you don't get out of the idle circuit. Really need to be above 2000 rpm for best mileage.

Had a .64 OD and 3.55 gears on my Charger way back when. I was lugging along below 65 mph getting 12 mph. Wind it out to 75 and I'd get 16 mpg. Eventually I put in a set of 4.10's.

Bigger valves installed properly (deshrouded and good valve job)improves flow even through the same size port, in order for more air to flow through the same port it HAS to have MORE velocity, therefore a bigger valve will have more velocity in the port not less. Any one who beleives otherwise is it is very elementary physics not rocket science. Velocity at the valve will probably decrease but total flow will go up and that means you can make the same power with a smaller cam and a smaller cam will help MPG.

As for gearing, lug the snot out of it. My 5.9 in my 3/4 ton ram turns 1500 at 60 and gets much better MPG in OD than 3rd. You car is slightly more aero dynamic and probably lighter than a jacked up extended cab 4x4 3/4 ton and your engine is definately more powerfull even at those low RPM.

You definately do NOT need to be above 2000 for best MPG, mabey best passing power but it is bad for MPG.

360View, I crunched some numbers real quick...

Assuming a 4" stroker (), at 1700 rpms the piston speed is 1133 ft/sec, right in the sweet spot of that graph.

Assuming a BMEP of 100 psi, we can figure the needed torque to be roughly 275 ft-lbs, at that 1700 RPMs. bmep link

So, does it make sense that if we build an engine with that amount of torque at that rpm, adjust the rest of the gearing as needed to get a good highway speed at that rpm, that we should get the optimal mileage out of it?

Of course, depending on the air & rolling resistance, I guess we may still need more power to move along though...

Or am I way off

I think 2250 RPM @ 75 MPH sounds about right. 2000 RPM is low. I'd go with the 3.55's + OD. Real world fuel economy may be better... you won't need to kick it out of OD as often for headwinds, hills and gentle acceleration. Plus its a smoother ride without excessive shifting.

Some of the Buick Roadmaster's had 2.56 rear axles with an overdrive transmission. (There transmission's overdrive ratio is nearly identical to the 518's ratio) These cars had 350 engines and weighed about 4200 lbs. I think this gearing put them at around 1300 rpm at 60 mph. I know a guy who is a Roadmaster fanatic. He thinks the 2.56 was a little to much gear. He likes the 2.72(?) or 2.94(?)axle ratio's better.

It is very true that gearing the car so it will obtain best fuel economy WILL make the transmission shift in and out of overdrive more frequently. (particularly in the 40-60 mph range) When I'm driving a vehicle with real stiff gears, I manually shift the transmission into third gear until I'm on the highway, then shift into overdrive.

If an automatic transmission is making the engine lug, the shift points on the transmission should probably be adjusted. (or manually downshift it)

If you want to run real stiff gears a lock up torque converter will be necessary. Otherwise, the torque converter will slip more at low rpms. I believe some higher stall, lock up converters are available. These should help with both performace and economy.

Getting the idle mixture set lean will be critcal to achieving good fuel economy at real low rpms. The following article on how to tune a carb for economy is worth a read, even though the carb is on a chevy the same priciples would apply on a Mopar.

http://www.chevyhiperformance.com/tech/e...eage/index.html

Personally, I'd try a 2.76 to 3.23 ratio depending what I had on hand at the time. I'd guess a 2.92 with a higher stall, lock up converter would be best, but that's a guess, since I've never built anything like an Imperial. A 3.23 would certainly help performance, but would probably hurt highway fuel economy just a bit. It might help around town mileage though?

I'm quite interested in building a C-body as a cruiser / summertime driver. When you get this done, I'd be very interested in the results you achieve.

Thanks for the input so far. I forgot about the Diplomats and their 2.45 rear gears. I had one and it was a pig around town. It did cruise nicely on the highway.

I will most likely make the Power Tour with a carburetor. However, I will be reverting back to my old ways and stuffing this in the Imperial.



When I do that, I'll probably switch back to the smaller 900 cfm throttle body. No doubt throttle response would be much better than with the 1700 cfm unit I've currently got on it.

There is a slim chance it will be EFI'd for the Tour but that will depend on how the rest of the build is going.

Within a year carb tuning should not be an issue.

Had a Diplomat coupe years ago: 318 4bbl, lockup converter & OD, most likely a 2.45 gear, 235/70R15s. It was laaaazzy around town, but IIRC 75MPH was something like 17-1800RPM, and it was fine, even on small hills & headwinds. Something like California's Grapevine would have me either giving it some boot or pulling it down a gear.
For a carb I'd want a T-Quad for its small primaries, but why bother if your plans are to run the EFI/Turbo setup? With carbs, I get the best MPG driving with as little throttle as possible; 2 of my EFI cars pull better MPG at the lowest RPM they'll turn & still pull the car, which with a carb would man frequent large throttle openings & low vacuum to maintain speed. The 3rd (it's the least aerodynamic) gets better MPG @ 3000RPM than at 2700. Mystifying? Nope, aero.
My GMC 454 dually (5500 lbs, 4.10, TH400, Gear Vendors = 3.23ish gear) makes its max TQ @ 1700, but that doesn't equate to hwy speeds; 65-70 = 2700RPM. Needs more gear towing uphill, but unladen it's a freakin' locomotive.
Hopefully your turbo setup is sized right so it won't require much RPM. Seems like a really good application for a stroker motor.

Here's my experience with tall geared (don't start Doc) heavy vehicles.

I had a 79 F100 with a 300 inline 6, 3 on the tree with 2.75 gear and a 28" tire. Had a Clifford intake with a 600 4150 Holley with headers and 2.5" exhaust. 10 MPG hi-way that's it that's all. Vacuum gauge read 10" at speed. Changed the rear to 3.50's and it got 16+. Vacuum gauge now reads 16-18" at speed.

My take on it is this: A 300 makes lots of torque BUT is a bit shy in the HP dept. It takes HP to go fast and in order to make the HP number required to run 65 at 2150 RPM required a larger throttle opening than it did with 3.50's even tho it now ran 2750 to do the same speed. the vacuum gauge tells the tale. 10" vs 16+". 1" of vacuum is 1 MPG give or take so try it with a few different ratios if you have them to try. You might be surprised at what you find, I know I was.

Kevin