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Help me understand ratios

76moneypit

Jr. Member
Joined
Oct 7, 2008
Messages
69
Loc.
Lander, Wyoming
I'm looking down the road at my final build for my CB and have a few things in mind for the engine, transmission, and wheels and tires but as I read other posts alot of you guys talk about ratios...with the above mentioned items; I need help understanding.

This is what I'm looking to do. rebuilt 320 with 250-300HP, NV3550, Dana 20, 3.50 rear end and 16" rims with 235/85/16" tires.

I looking for a way to make sure this all works well. My driving is 80% town daily driving and 20 trail. I live in Rocky mountains and really like cookie cutter tires for snow and ice.

What are your thoughts on the combinations of above and it someone is so inclined to tell me more about ratios I'd be open to learn.

I was under the understanding that if you were in say 5th gear going down the raod you would want the to keep the RPM's low as possible for better fuel economy...

I know this is alot but your thoughts are welcome....thanks again.
 

Heus33

Sponsor/Vendor
Bronco Guru
Joined
Jun 1, 2005
Messages
7,404
mmmm cookie cutter wheels. Do you mean pizza cutters? Tall and skinny or chewy and chocolatey? ;D

Its gonna depend largely on the size of your tires. What are you planning on running?

I have 33's with 3.50's and its fine for my street mobile.
 

Doyle

Bronco Guru
Joined
Jun 12, 2006
Messages
1,015
Check out this table On 4Lo.com http://www.4lo.com/calc/geartable.htm ,there are also some nice calculators on that site, if you know final drive ratio of tranny and overall tire diameter. Your 3.50 gears may lug the engine with the overdrive tranny, you'll probably be below 1800 RPM at 55 MPH.
 

WyleCoyote

Bronco Guru
Joined
May 2, 2002
Messages
1,739
Randy's ring and pinion has some good calculators on it too. For example I used the calc on there to figure out your tires are 30.7 inches. With a .78 overdrive you will be at 1780 rpms at 60mph.
 

WyleCoyote

Bronco Guru
Joined
May 2, 2002
Messages
1,739
If you are trying to make the engine work at an RPM that is too low it will bog down or "Lug" - Not working efficiently or generating enough power to do what you are asking of it. Like using a cordless drill with a large drill bit and leaning on it really hard - you can hear it trying but just can't do it!

Same thing if you run too high of a gear ratio. With the 3.50 and 0.78 overdrive you would have to be going 80mph to be in the proper RPM range. If you got taller tires the speed would go even higher. Basically you won't need the overdrive much, but that transmission has a pretty low 1st gear so you should be fine on that end. Check out this tech article about the NV3550 for more info.

235/85/R16 tires are 30.7 inches tall - that is just the conversion. The calculators I used want the height in Inches.
 
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76moneypit

Jr. Member
Joined
Oct 7, 2008
Messages
69
Loc.
Lander, Wyoming
Boy these charts are good and I think I get the engine lag thing. I can see that with the setup I was thinking of it may be hard to achieve the correct RPM's for the engine.

So if I put in 4.10 rear end I would satisfy the RMP's????
 

needabronco

Bronco Guru
Joined
Jul 2, 2004
Messages
6,411
Loc.
Prescott/Farmington
You also need to understand where your engine makes power and is 'happy'. If you know your camshaft spec's you'll know where your engine makes it's torque. You want your cruising rpm's to be well into your torque curve, that will minimize 'lugging'. Most 4x4's and trucks are 'happy' at or near 2000 rmps, due to the lower rpm range of their cams. Now if you have a hot rod cam your torque curve could very well be at much higher rpms. I had a VW Scirocco that was very happy at 2800 rpms, and would run all day and get 28mps of coarse it red lined at 6500 rpms. My bronco doesn't hardly ever see more than 3000 rpms, and likes between 1800 and 2400 rpms. Just more food for thought.
 

trailpsycho

Bronco Guru
Joined
Dec 11, 2003
Messages
4,857
Depending on your cam, plan your axle gear ratio to get you about 2500 rpms at about 72-75 mph...thats where the wind-drag really requires more torque from the motor to keep your speed and be able to pull grades without having to stick your foot in it too much--where you need some bottom end. Most cams will be approaching/building peak torque in the mid-to-late 2K rpms.

Knowing that your in the mountains and will needing to pull some steep grades...if you plan to drive it on the Interstates, I would advise a gear ratio that puts you a little above 2600-2700 rpms at 75.

For example, with my setup--35" tires (34.8"), 4.86 axle gears and 0.72 overdrive (NV4500), I am running 75 mph at 2500 rpms...which is great for most cruising and driving. However, if I am going up a steep, long grade it really helps to be doing 78-80 mph, the few extra rpms are in a better torque (power) band to pull me up the grade without lugging the engine. If I attempt to run the same steep grade at 65-70 mph, it will do it, but I have to give it gas and be pro-active and anticipate the grade.

For a half year, I installed/ran an NV3550 with 31"s, 3.54 gears...so very similar to what you are looking at...and it was a dog on the interstate as far as at the cruising speeds. Towing, strong head winds, mild grades, etc at 70 mph really made the Bronco feel anemic. Now, if I got it up to 80+ it felt better, but the gearing was definitely not optimal. I would recommend 4.11s at a minimum, 4.33s or 4.56s would be better. If you might consider going up a tire size down the road to a 33" tire, then go ahead and put 4.56s in...you might really like that gearing with your setup . If you dont plan on running 75 or 80 mph often, then go ahead and put the 4.56s in. You will still be running low enough rpms for the interstate at sane speeds of 65-75 and you will have plenty of power at all times...especially around town, off-road or towing.
 
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76moneypit

Jr. Member
Joined
Oct 7, 2008
Messages
69
Loc.
Lander, Wyoming
Trailpsycho,
again you guys are a wealth of knowledge...I was thinking about the 4.10 and now you are lightening me with bigger rear end ratio...

two questions for you just for fun...what do you think gas mileage would be from 3.50 to 4.33's at 65MPD cruise? Next what cam do you have and do you like it? Sounds like the power band you would look for in a cam is 2000-2500? Or am I way off???
 

needabronco

Bronco Guru
Joined
Jul 2, 2004
Messages
6,411
Loc.
Prescott/Farmington
Moneypit, cams in our heavy/non aerodynamic rigs are very particular to what you want to do. I think as a gross generalization, most of us run a RV/performer type cam, and the rest run a high performance street cam. with that said if your only consideration is highway cruizing you could use a higher rpm'd cam. The rest of us that 'idle' around in 4 low, 2nd or 3rd gear have a low rpm cam and compesnate the best we can on the highway. For example I'm planning an AOD swap (.67 overdrive), 4.88 gears, and 35's. I plan to run about 2300-2500 rpms on the highway (70-75mph). I'm willing to sacrifice a little on the high end for better control on the low end. Others with my same setup are happy with 4.56's, and will have lower rpms on the highway and a little less on the low side. It's all a trade off.

Do you know what cam you have? I would try and determine what you have first and then decide if a cam swap is in order. It would be silly to spend $2-400 swapping a cam to discover you already had something close to it...
 

trailpsycho

Bronco Guru
Joined
Dec 11, 2003
Messages
4,857
Funny you ask about gas mileage...it will mostly be a wash. I used to think with the proper gears, OD and tire set-up that I would be seeing mid to upper teens... I was fooling myself. Again, my setup is 5.5" of SL, 1.5" BL, 35" MT MTZs, 4.86 gears, NV4500 (0.72 OD)...I get about 12 around town in what I would call "spirited" driving--others might call it aggressive driving. On our trip to the FL Keys last fall, we averaged about 13-13.5 for the 3K+ miles. There were some tanks where we were cruising, running upper 60s to lower 70s and saw 14 mpg. We had one tank at 15 mpg--cruising across the everglades with very few stops, lights, running 62-66 mph.

Recently--two weeks ago--I made a trip to BHM, AL (~500+ mi each way) running 75-80+ mph ont he Interstate and saw mileage from 12.5-13 mpg. If I run the back roads and smaller highways at 60, I think I would make the middle teen-mark. But, typically, I am looking to get from point A to B and while not necessarily in a hurry...its difficult to run less than 75 on most Interstates without being the "problem vehicle fouling up the flow"...inevitably you get cock-blocked behind a semi if you drive too conservative...so I find it more suitable to run in the upper 70s to about 80. I can take my hand(s) off the wheel and it tracks nice and straight.

But getting back to mileage and economy...these are BRONCOs...they are 4-5000 lb iron boxes-depending on trim (cages, tools, extras), oftentimes, jacked up in the air (like mine) damn near 3 ft; you cant expect it to get great fuel economy. As well, I run most of the time a safari top only. I dont seal my soft-top around the windows even when I do have them in...this all increases the drag. I would love to think that a Bronco modestly lifted (<=2.5"), hard-topped, EFI, OD-tranny, on smaller tires could easily see the mid to upper teens on 30s or 31s. I ran that way (minus the EFI and OD and hard top-seasonally) for years and saw 14 mpgs for similar highway trips (slightly lower speeds).

Ultimately, you should be thinking about performance and utility. What do you want (performance)? What do you need (utility)? What kind of driving will you most likely do? Configure around that.

I wanted to build something that I could drive a few hundred or a few thousand miles, get whereever it was that I wanted to see or do while I was there and then get back home. Cruising to Alaska and or Central or South America kind of trips. Colorado, Utah to wheel or going back home down south to the beach to surf or haul a couple of canoes to the Boundary Waters... Thats the beauty of the Bronco...it does all that with style.

The only reason I mention going a little larger in the axles...is that you will probably spend alot more of your time at lower highway speeds, so you dont need to and probably wouldnt really want to--and it wouldnt do so great anyway--70 mph @ 2000rpm. If your running 60 mph in the mountains, you will not be able to use your OD, cause it probably wont want to pull those grades at 1800 RPM. If you do run up to 80, its probably to pass or b/c you have to be somewhere--hence fuel economy may not be very relevant...so if you can find a ratio (4.33s) that gets you 80 at a little under/over 3K than thats probably pretty sweet. FYI, 2600 would put you at 71.5 mph (w/4.33s)...good power, not alot of gas, plenty of ass...just my opinion. If you think you will go with 33s down the road, definitely do the 4.56s. And remember you have to gear both axles roughly the same (4.88 vs 4.86 or 4.31 vs 4.33 or 4.54 vs 4.56), but I am sure you already know that. Expense-wise, its best to pick the right gear the first time...and anticipate growing pains.

As far as my cam, I am running a 2020 grind from Crane (PN 444211--actually its a 444212 which is the kit with cam and valve springs) for EFI controlled 5.0s. I am pretty happy with it.

Good luck. I am sorry to be so long-winded.
 
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76moneypit

Jr. Member
Joined
Oct 7, 2008
Messages
69
Loc.
Lander, Wyoming
I like it when you guys are winded on the responses..I learn alot. Like I had on clue that I needed to match both differentials. but when you think about it, it make perfect since.

On the thought on power verse fuel economy...I'm a power guy. I've had this CB for 3 years and have never once checked the mileage I'm getting. I know I should just to keep tabs on it but I have a hard time knowing I'm getting 10-15 MPG. I drive for a living and have to stay on top of MPG all the time...When I'm in the CB I don't care, I just live to drive it.

So I think my engine had never been rebuilt (96,000 miles) and is strong for a 302. I've got 120 pounds of pressure per cylinder. So I think I'm running a stock cam.

I don't think that I will go with a bigger lift and tires...but you never know. this CB is family/DD/fun ride, but I think a RV cam will be in my future when I do do a rebuild, a few years away.

I can't imagine running a CB down the HWY at 75mph...I guess I need my sterring box, tires worked on to do that. I'm two hands on the wheel at 65. Not that I wouldn't like to feel comfortable at that speed.

so the RV cam power band is at a lower RPM than a street cam? Thus the power for the 4 wheeling? I'm not sure which way I will go with the cam, to get anywhere from where I am is at least 60 miles on the HWY...

Thanks again for your input...great info
 
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