Blown 400 And Six-Speed For Suburban?
Question: I have a '93 Chevy Suburban 1500 4x4 with a 350 V-8. The engine has 180,000 miles on it and is very tired. This summer I plan to replace the engine with a Roots-blown 400. This is pretty straightforward. However, I would also like to get rid of the Slush-O-Matic and install a six-speed manual. I know I will need a donor clutch-pedal assembly and a compatible bellhousing as well as a new driveshaft. Can you point me in the right direction for a good donor vehicle? What tranny should I run that will bolt to the small-block and transfer case?
Would it be better/easier to find a five-speed setup versus the six-speed? Are there any axle upgrades that would handle the power of the blown 400?
Stephen G. Dant
Answer: Chevy has used the ZFS6-650 six-speed transmission from 2001 in some applications both with gas and diesel engines. I believe that you will need the gas version, and there are not a lot of them around, especially those hooked to a transfer case which you will also need. It will be an expensive transmission. Maybe a better choice would be the NV4500 five-speed as there are a lot of those around.
A 1500-series Suburban may not be the right choice, axle-wise, for a Roots-blown 400-inch motor. That much torque and horsepower will make a lot of little pieces of metal out of the front and rear axles. I would suggest a Dana 60 swap up front and a 3/4- to 1-ton Corporate 12-bolt for the rear. The rear changeover is pretty straight forward, but the front will require a major suspension change. Cage Off Road (www.cageoffroad.com) and Off Road Unlimited (www.offroadunlimited.com) are two companies that come to mind that offer solid-axle conversion kits. There is a lot of information on the Internet if you do a search on "Chevy solid-axle conversions."
In reality, you would be better off finding a nice earlier 1-ton solid-axle Suburban as a platform to start out with. Some of these even had big-blocks in them, which would be a better engine for four-wheeling than a 400 with a blower, as that combination tends to have little low-end torque as well as overheating problems.
How To Measure Backspacing
Question: I have an '89 S-10 Blazer 4x4 that I want to put new rims on. I was going the cheap way and getting some steel wheels from Cragar, the 397 series in a 15x8 size. The only problem is that I don't know how to calculate the backspacing. I would like the tires to stick out from under the vehicle a little bit, but not too much. Can you help me figure out the spacing I would need so I can get the correct rims?
Answer: I have seen all sorts of so-called trick ways to figure the correct backspacing, and nothing beats trial and error. No, I don't mean you have to buy or borrow a lot of wheels and tires, but that would be the best way to do it. First off, why do you want the tires to stick out the fender? For one, the tires will throw trash over the sides of the truck, and second, in most states it is illegal. But perhaps you plan on running some type of aftermarket fender flare.
From the mechanical aspect of it, the less the backspacing (more tire sticking out), the more load on wheel bearings and suspension components. Plus, you can interfere with what is referred to as "scrub radius." Basically, the suspension is designed so that the tire will pivot off the center of the tire. More or less backspacing, and even a wider rim, will change the pivot point and cause the tire to "scrub" when turning. Usually it's no big deal other than a bit more tire wear.
Keep in mind that the 8-inch wheel with stock backspacing will also move the center of the tire outward about an inch (half the distance of the additional width of the new rims versus the old rims). You didn't say if you were going to use your present tire or go to a larger tire. A wider/taller tire will not only bulge out more to the outside but to the inside also. This may or may not cause more contact with the frame or fender opening. If you're keeping your present tires and just adding the wider rims and you have no tire-to-body contact, then most likely the stock backspacing is what you want to use.
Backspacing can easily be measured by placing a straight edge against the rim flange, and using a tape or ruler, measuring from the mounting surface of the wheel to the low side of the straight edge. It's important that your straight edge only makes contact with the rim and does not rest against the tire.