Your Tech Questions Answered
Q What is a crawl ratio? How can you figure it out? What’s the best ratio? I’m trying to build an S10, so I want to do it right.
A Your crawl ratio is the transmission First gear ratio times your transfer case low-range ratio times your axle ring-and-pinion gear ratio. I have no idea what your factory parts are, so I’ll guess that your S-10 has a 700R4 automatic transmission with a 3.06 First gear. Behind that it may have an NP231 with a 2.72 low range. And your axle gears may be 3.50 for argument’s sake:
3.06 x 2.72 x 3.50 = 29.13
Your crawl ratio is 29.13:1. Not very good if you’re looking for slow crawling control and torque multiplication for big tires off-road. That said, the automatic uses a torque converter so it can make up for the dismal number, but lower would be better. If you have a small engine then you’ll definitely want a lower crawl ratio.
My Toyota has a four-cylinder engine and 39-inch-tall tires, but by using a R151F transmission with a 4.31 First gear, dual Toyota cases with two 2.28 low-range options in series, and Dana axles with 7.17 ring-and-pinion ratios, I have a megalow 160.64:1 crawl ratio (4.31 x 2.28 x 2.28 x 7.17). Plenty low for big tires and a small engine.
Without knowing your current drivetrain it is difficult to know what ratio to send you toward. Bigger engines with more torque don’t need as low a crawl ratio; smaller engines do. Let’s say you have a 4.3L and a 700R4 auto and want a lower crawl ratio. You can go to a 4:1 low range in an NP231 if that is the transfer case you currently have. Multiply that with an axle ratios around 4.88 and you’re now at 59.73, twice your current crawl ratio. If you have a 4.3L Vortec V-6 and 33- to 35-inch tires I think you’d be pretty good for off-roading and still daily driving.
The first step is identifying what you have for transmission, transfer case, and axle gearing. Then determine how big a tire you want to go to and what type of terrain you want to wheel (rocks will want a lower crawl ratio, sand and mud not so much). An automatic will make up for a higher crawl ratio to a point with the torque converter, while a manual is best with as low a ratio as you can get. Remember, you can always shift up if the crawl ratio is extremely low.
Q I have an ’90 GMC Stepside z71, mostly stock. I have had this truck since 1993. The truck is in good condition, except northeast Pennsylvania has been pretty tough on the frame. I have had it patched a few times, but I am waiting for the inspection station to finally say no some year. I can’t seem to part with her. What is the easiest way to fix rust?
A Wow! How quickly I forget the dreaded Pennsylvania rust. I am a native Pennsylvanian who now lives in California, where I could find a truck like yours but rust-free for $2,000-$5,000 all day long in the classifieds. Fixing frame rust isn’t going to be easy other than sandblasting, grinding, and welding in new patch panels (see “Costly Corrosion,” May ’12). If your frame is rotten then I assume all the fasteners and body panels are not far behind, so doing a frame swap isn’t a great option either; the work wouldn’t be worth it. You could do these repairs, but rust doesn’t rest and eventually you’ll be repairing more and more of the frame than is worth fixing.
My best advice: Keep driving it until the frame fails or the inspector gives you the thumbs-down, and then search Craigslist or eBay Motors for a similar truck to yours but from the Southwest. Buy it, fly out here, and drive it home. You’ll get to see America, have an adventure, have a new rust-free truck, and save yourself the time and expense of replacing your frame and all the other rotten parts.
Nuts, I’m Confused
Flipped & Lifted?
Q I have a ’79 Chevy Blazer that has 4.56 gears, a 360-horse 350 V-8, TH350 transmission, and a 205 transfer case. It has a 21⁄2-inch Rancho suspension lift and a 3-inch body lift. I have the steering problems narrowed down and taken care of. I want to know what the benefit of a rear shackle flip kit is, and if it would affect the drivability of the truck. If I put this kit on, would I be able to get rid of the rear blocks? I was also wondering how I would go about getting rid of the 3-inch body lift on the truck. It is not cool on a windy day!
A Your Blazer sounds like it needs a quick suspension revamp. How about replacing your 21⁄2-inch lift and a 3-inch body lift with a 4-inch front spring and an Offroad Design (ORD) rear shackle flip? The shackle flip allows you to delete the lift block, keep the smooth-riding stock leaf pack, and gain enough clearance for a 37-inch tire with slight trimming. It works just fine for street driving and off-road use. ORD also has a 21⁄2-inch shackle flip, but the 4-inch seems to work best on Blazers that are often tail-low.
ORD offers various front leaf springs for a 4-inch lift, or you could keep your Rancho 21⁄2 and run the lower 21⁄2 flip kit. You could remove the 3-inch body lift (not a bad idea in my view), but without knowing your tire diameter it is hard to determine between the 21⁄2- and 4-inch. I’d say go up to the 4-inch leaf pack and 4-inch shackle flip, and ditch the 3-inch body lift if you’re running tires 35 inches or taller tires.
Since you’re looking into proper suspension upgrades and away from extremely tall body lifts (I don’t care for body lifts over 2 inches) I think you deserve this month’s Nuts, I’m Confused prize. I spoke to the crew at ORD and they’d like to send you a set of sway bar disconnects, perfect for an on-road GM 4x4 that also likes to twist it up off-road. These bolt on easily and not only allow more movement when disconnected, but also correct the geometry of your sway bar to still control your axle movement without overly binding the suspension. The ORD guys are experts on your square-body Chevy trucks and Blazers. They offer everything from bumpers and steering components to suspension and gearing modifications. Find out more at www.offroaddesign.com or 970.945.7777.