Gear ratio recommendation

[fordguy]

Tirerack lists all the specs. Rubber Overdrive?

 

[.94 OR]

Tirerack lists all the specs. Rubber Overdrive?

Maybe like a wrinkle-walled slick where it grows in diameter proportionate to speed?

 

[ntsqd]

You're over thinking it, Just running a taller tire to try to reduce the RPM because the gears are too short.

 

[nvrstuk]

Swampers... must be an undersized guy running the company 'cause all their tires were over an inch smaller OD than advertised...

 

[nvrstuk]

Maybe like a wrinkle-walled slick where it grows in diameter proportionate to speed?

Yeah but they reeeally grow!

 

[nvrstuk]

Thanks, this is helpful. Im putting my 74 back together with a 4R70W with US shift. I think Im going no bigger than 33s and I want 2000 RPM in overdrive to produce 70mph. Its also getting a 427 CI engine. So replace the 3.50s with 4.11s.

427 chugging at 1800 to 2K is in a different league than a 302/351 chugging along at that same 18-2,000 rpm. Course every hill is different. Every day with or w/o a breeze is different. Loaded with gear & misc 600# of tools, people etc makes a difference.

Main thing is evey car/truck with OD shifts out of it for almost every hill.

The more gears in the trans the less rpm split there is & it is that much less noticeable. 10r's have 4 OD gears & are almost unnoticeable when its shifting. 4r is not but that's what it's designed to do!

 

[fordguy]

You're over thinking it, Just running a taller tire to try to reduce the RPM because the gears are too short.

That rubber overdrive reference
rolled right by me lol

 

[Broncobowsher]

Another way to look at it, the 4.10s are in the ballpark. Even if not perfect, it is a starting point. Since the front is already set that way, Put them in the back and give it a shot. If they are right (or right enough), you are done. don't even have to think about the front again. If it isn't the right choice, you won't know it until you drive it. Then you can try different options in the rear. Once that is dialed in, then worry about changing the front to match.

 

[msmith]

Another way to look at it, the 4.10s are in the ballpark. Even if not perfect, it is a starting point. Since the front is already set that way, Put them in the back and give it a shot. If they are right (or right enough), you are done. don't even have to think about the front again. If it isn't the right choice, you won't know it until you drive it. Then you can try different options in the rear. Once that is dialed in, then worry about changing the front to match.

From the information I've gleaned from this thread, this seems to be the way to go. "Close" is good enough for me. I was worried that the difference was going to be hateful and wanted to rectify it while I had everything apart. I have some time to make an engine decision, but now I'm intrigued with a 351. Thanks!

 

[.94 OR]

A 351 is still going to be happier with proper gearing.
I'm also leaning in that direction, but need to get my gearing figured out I'm not geared quite where I want to be for cruising in town at 25, 35 or pulling in the 50-65 mph range. More power would help but I need to get the power there through gearing as well.

 

[ntsqd]

Generally the flatter the torque curve the less finicky it will be about the gearing. You can be in the wrong gear and it will still pull w/o lugging or otherwise complaining. Easiest way to build a flat torque curve is the longest con-rod that reasonably fits. In a 302 the 289/Boss 302 con-rod length is slightly longer than the stock 302 rod length (5.155" vs. 5.090"). I'm talking about shaping the curve, not where it peaks as rod length is not known to me to influence that at all. Can not merely run normal 302 pistons on 289 rods, have to use pistons with the correct pin height. I've not looked in decades, that may be simple and it may be expensive.

Keep in mind that longer rods cause the piston to dwell at both TDC and BDC longer than a shorter rod, which is how it flattens that curve but it also makes such an engine more sensitive to too much ignition timing.

Of course with more stroke you have more torque and then the shape of the curve may not matter enough to worry about.

 

[DirtDonk]

Engine Masters did a con rod length comparo once.
Don’t remember the details, but they concluded, as you said, that the changes didn’t make a big effect on peaks. I just don’t remember if they found anything with the curve either.
Now I want to go search that one out again.

 

[ntsqd]

Years ago Circle Track magazine did a pretty involved dyno testing of long vs. short rods in an otherwise identical engine. They did it in a premier engine builder's shop. I've long ago lost any memory that I might have had as to when it was published. My two take-aways from their results were what I posted above. The long rod engine was more sensitive to ignition timing and had a flatter torque curve. The short rod engine had a very peaky torque curve and was not as sensitive to the ignition timing.

I've since experienced that a little. Patch was born with a short deck, short rod 22R engine. When it was about to expire I got a later tall deck, long rod engine to rebuild. Once the new engine was installed and broken in I noticed that the timing setting for the old engine was too much and I had to back it down about 2° and even then it was an easier engine to make rattle. These engines weren't as similar as CT's dyno test engine, but they weren't dramatically different either.

 

[DirtDonk]

I remember those circle track magazine articles very well! I devoured that magazine every month, including all the Smokey Yunick articles and everything else.
Even gleaning technical expertise from motorsports varieties that I had no interest in. But thought the info might come in handy someday.

 

[DirtDonk]

That’s one of the reasons I’ve wanted to experiment with my Ford 400 as much as I could.
Long rods and long stroke.
Always thought that would be an interesting combination to play with.

 

[nvrstuk]

Billboards (Broncos) need long rods to propel them... I had a great quick revving engine back in the early '80's... HP289 K code... if I was cruising at 3,500 rpm and burped the throttle it Barked and got to 7300rpm in about a second.

Add in some bore and you've got a winner. lol

 

[fordguy]

According to the calculator I used (project33.com) my bronco with 32.5 inch tires will be 1900ish @4.11s
and 2200ish@4.56s. Can I live at 1900 ish with o/d off? I think so, we are going to find out when I get a transmission delivered.

 

[ntsqd]

I suspect that 1900 from a 302 is going to be marginal. IME 302's have a resonance at around 1800 and you really don't want to dwell there as the exhaust tone will drive you out of the vehicle if the design of the exhaust system isn't well dialed-in. BT, DT, took several exhaust iterations and a consult with a tech guy from FlowMaster to kill that drone.

 

[fordguy]

I am running a single turbo, not a huge problem with drone, but only live data will tell. I have a new carrier and new 4.56 ring and pinion, but will. Run the 0old 4.10 first, since it is already installed.

 

[nvrstuk]

ivI agree, give what ya got a try and then dial it in the way you like it.


RPM calculators are good only to give a really basic ballpark rpm # as there are, as many of us are mentioning, many variables that make for a workable and comfortable rpm like Thom mentioned.

Engine torque, multiple OD gear selections, tire rolling resistance (believe it or not- Toyo's take more HP to roll and many of us don't run at full psi rating on the tire), wind aerodynamics of vehicle (everytime you lift it the aerodynamics are affected and gas mileage suffers, local or highway driving, exhaust note, vehicle weight and of course HILLS in your driving area. Living anywhere East of the Rockies makes gear selection different than living west of the Rockies (generally) since where I live I can't leave home (espcially coming back) going 3 of 4 different directions without a 1,000-2200+ foot elevation gain for miles.
This translates to do you mind going up hills in a gear lower than the typical 1:1 ratio of direct drive and having the engine spin a bit more - which has always been the standard until cars now with multiple OD gear selections.

Just tossing some factors out there as to why some on-line calculator is at best, a basic guide.