617 RPM

[billva]

I'm in the middle of putting a 617.951 with a 722.3 trans from a 81 300SD into a 1966 Ford F100 pick up. Have the new front suspension in, set the engine in the frame to make sure there would be no clearance problems, drive shaft, exhaust and motor mount options. Everything looks good, as the 617 is almost the same size as the 6 cylinder I took out.

I got my tires and rims I'm going to use, now it's time to change the rear axle to match the front track width and bolt pattern.

My question is what rpm do I want to be running around 60mph. The truck weighs about 3100 lbs compared to the approx 4000 lb 300sd.

The truck will be a back road cruiser. The land here is mostly flat with a few rolling hills, 55mph speed limits. A few of the main roads are 60 mph speed limit.

I see the 300sd came with a 3.07 gear ratio, but don't think I want to use that ratio to calculate what I want to use this truck for.

Any help will be appreciated. Thanks.

[Stretch]

That looks great!

Why don't you want to use a differential close to the original 3.07?

Are you planning on hauling lots of weight about? Or will this just be a cool ride?

FYI

The OM617a was eventually mated to a 2.88 differential

The non turbo OM617s had 3.69 diffs

The OM616s (non turbo 4 cylinder engines) had 3.46 diffs

EDIT:- the benefit of keeping the existing differential ratio (if at all possible) is that it will be easier to set up your 722.3 transmission - the data in the FSM transfers straight across

[cho]

'

lighter car,flat roads... 2.24 (gasser) diff is made for your project IMHO.

if you go manual trans = 2.47 or 2.65

you can calculate rpm's from there.....

cheers
ChO

.

[billva]

Like cho said, it's a much lighter vehicle, and what I'm using it for is different than what the 300sd was intended to be I believe.

I have never driven a Mercedes, so for me it's a guessing.

I'm just looking for opinions from someone who has more experience than I do.

From what I calculated, the 300sd would be running at 2642 rpm at 60mph with the stock 195/70-14 tires.

Again, thanks.

I have a full size GMC that I use for the towing and heavier loads. I won't be putting more than a few hundred pounds in it.

[cho]

handy tool

Engine RPM Calculator

.

[Shortsguy1]

The thread below relates to a totally different vehicle, but I put up some graphs related to your engine (posts 47 and 48). Perhaps they will be helpful. For instance, if you want to minimize fuel consumption at 60 mph, you would choose a diff to ensure that the engine was turning at 2400 rpm. That is the minimum Brake Specific Fuel Consumption for the 617A. Most of the original gearing choices on these cars were to maximize top speed. So if that is not important to you, perhaps a taller top gear ratio would be preferable.

Diff Ratio and Gearing Analysis

[billva]

Good info. Lots of things to consider to make my decision.

Or indecision,should I say.

[Stretch]

I forgot about that thread shortsguy1!

@billva if you can fit the original differential (3.07) / get one that has a similar final drive ratio I'd give that a go first and see how you get on. You might find in time that you want something different anyway - these kind of modifications give you a sense of freedom that you might not have if you were playing with a more stock vehicle. There's plenty of time for choice and tuning - first step has got be getting it all rolling along hasn't it?

What ever you decide to do please keep us all updated with your progress - I for one am a fan of American pick ups.

[t walgamuth]

I agree with stretch. You prolly have taller tires than in the SD so you'll have an effectivly taller final drive. I'd start with around the 307 gear. your truck will have more frontal area so a much taller gear would probably be a bit much to push the boxy ford through the air.

[Walkenvol]

Originally Posted by billva

I have never driven a Mercedes, so for me it's a guessing.

I'm about 2 years into my Mercedes diesel journey. My initial reaction was that the motor was always reving too high until I became accustomed to the "feel" of the vehicle.

Obviously if you're capable of this type of project you know to factor comparative tire height into the diff gearing.

While 2400 rpm's may be the most economical rpm, I'd suspect you would be down shifting on every grade if you set your differential to cruise at that ratio. I'd set it up to go 70 mph at apx 3150 rpms so it would be useable on an interstate if you ever need to drive there. This would have you at a comfortable 2700 rpms at 60 mph for your local roads you mentioned. This is about 15-20% higher rpm's than you might expect to set up a comparable gas motor and 30-40% higher than a DI diesel. But to me, this is just how these motors roll.

Even though your truck is lighter, I'd suspect the aerodynamics at 60+ mph would offset the weight gain compared to the donor vehicle. At lower speeds the lighter weight should be felt in the seat of the pants.

I'd definately can the slush box though and mate it up to an AX-15 5-speed (cheap adaptor plates available) if it were me in order to get the max out of the power plant.

By the way, your project is awesome!

[kmaser]

I Have a 617a in a Mercedes g-wagen and they have the aerodynamics of a brick wall. It isn't made for speed but just for comparison the factory tires are I believe 28" and the gearing is 4:88 and people drive them all day long at 4000rpm. I would be looking at 3:73's at least in a 1/2ton.

[Dan Stokes]

The OM617 has a pretty high RPM range for a Diesel (at least compared to what I'm used to). Mine redlined at 4500 RPM stock and I'm hoping to get 5K out of it. There are TON'S of MPH/RPM calculators on line and I'm sure all are about equal.

Your Ford ought to have a 9" Ford differential and that has a drop-out center section - easy to change gear ratios. I don't have all the available rations committed to memory but there are probably more gear sets for the 9" than any other diff on the planet. Land Speed Racers use them a LOT for just that reason. In fact, it's common to have a couple of carriers set up with different ratios for different purposes. There are a few different series of carriers for this rear so be sue that your gear set matches the carrier style. Any reputable vendor will check on this before selling you a gear set.

The 9" has pretty high internal friction and if you want to reduce that you might want to go with a 8.8 Ford rear as found under 5.0 Mustangs, Crown Vics, and other bigger Ford vehicles. It does NOT have a drop out center section but they're cheap, many have rear disc brakes, and come in a variety of widths so you ought to find something that will work for you. Most 8.8s will need spring perches welded on for your truck but that really isn't too big of a deal.

Good Luck!

Dan

[billva]

Dan, my rear is about 6" narrower than what I need to match the front, and bolt battern is different also.

I can install a Crown Vic rear to match my front suspension that has 3.27 gears. Has discs, but will be require removing spring and trailing arm brackets, but that's an easy fix for me.

80-83 F100's come with correct bolt pattern and width, available with 3.50, and I believe 4.11's, but that may be a little harder to find. Would have to get a kit to add the rear discs that I want.

Using my tire diameter(26.75), I come up with

3.27-2604 rpm @ 60mph

3.50-2778 rpm @ 60mph

I'm leaning towards the 3.27 rear. Have to go search the junk yards to see what I can find.

Thanks for all the replies. Nice to be on a site where you can get help instead of criticism.

Bill

[Dan Stokes]

The Crown Vic rear sounds like a nice option for you.

Dan

[vstech]

... the stock weight of an 1st gen 126 SD is 3200lbs... approximately the same weight as the empty truck... put anything in the bed, and it's much heavier than the "modern aerodynamic" sedan!
the much taller tires and barn door front end, I'd stick with the 3.07 or go with a 3.46 even...
if you can get an overdrive transmission to go with the truck, I'd put in the 3.69 or find a 4.10 rear gear...