The 10-Bolt Bop
Phil Blake, Ortonville, MN: I didn't see in your article ("Build a 10-bolt on a Budget," Mar. '06) anything about what size gears would fit in what size housings. I have been wondering what to do with a 10-bolt Posi out of a '71 LeMans and was told I can't put 3.42 gears in it because it has 2.70-somethings in it now. I've been told I would have to start with at least a 3.08 housing. The rearend I have looks like the one in your story with the cutouts in the cover.
I also have half a dozen or so BOP 12-bolts out of Cutlasses laying around and have yet to find one that actually has 12 bolts on the ring gear (haven't checked them all). Only one of these so-called 12-bolts is a 3.08. I would be more than happy to sell any of these fine rearends to you California boys for that $500 figure you mentioned. (I know, it's not out of a Camaro.)
Jeff Smith: You bring up a few interesting points worth illumination, Phil. Let's start with the basics. GM began building 10-bolt, Salisbury-style integral rearend housings in 1964 (as opposed to the drop-out style used from '55-'64) for Chevelles and later Camaros and Novas. These were 8.2-inch ring-gear-diameter housings. The Buick-Olds-Pontiac (BOP) stuff also used 8.2-inch-diameter 10-bolt ring gears, but the internal parts, including the gearset, are not interchangeable with the Chevy-style axle. The 12-bolt Chevy rears were 8.875 inches in diameter. In these rearends, the number of bolts on the rear cover is the same as the bolts used to attach the ring gear to the differential.
From '68 to '70, Oldsmobile used an axlehousing with a 12-bolt rear cover, but internally, it employed an 8.5-inch-diameter ring gear with 10 attaching bolts. You mentioned that you were looking through your 12-bolt Olds housings for a 12-bolt carrier; you can stop looking because you won't find one-they're all 10-bolts. This is still an 8.5-inch-diameter carrier, so it is stronger than the 8.2-inch 10-bolts, but it is somewhat rare, as its internal parts are not the same as other "corporate" 8.5-inch housings used in later GM models. Richmond Gear does make 3.42, 3.90, and 4.10 gearsets for this Olds-only (not BOP) axle assembly, and according to Richmond's catalog, those gears will fit a 2.93 through 3.23 ratio differential. The basic rule of thumb is that a 2-series differential will only work with gear ratios in the 2 series, as in 2.56 or 2.78. The other ratio series follow a similar path. This is not a hard-and-fast rule, however. For example, one Buick/Pontiac 8.2-inch rearend differential will mount 2.87 through 3.31 gears, while another will accommodate 3.36 and deeper ratios well into the 4s. So you need to be careful here when matching a differential with a given ring-and-pinion. Just to make it more confusing, many aftermarket gear companies offer, for example, a 4.10 gear for a 12-bolt Chevy housing that will mount to a 4-series carrier as you might expect. But some companies also make a 4.10 gear that will mount on a 3.08 through 3.73 (3-series) differential by making the ring gear thicker.
To answer your question about swapping from your 2.79 gear in your '71 Pontiac to a 3.42 gear, the information you received is correct. Unfortunately, you will need to come up with a 3.36 or deeper (higher numerically) style differential to use that 3.42 gear. Keep in mind that the additional ratio of a deeper gear will make it much easier to spin the tires. This means that a limited slip (also known generically as a "posi" which comes from the GM term Posi-traction) is a great idea to add in at the time you add the gears. Auburn, Precision Gear, and others offer limited-slip differentials that will bolt right into your 8.2 housing. The price hovers around $400 to $450.
Ask Anything is the portion of What's Your Problem where readers send questions for industry bigwigs, and we get the answers. So pick a hero, come up with good questions, and send them to CarCraft@primedia.com.
Steve Rambone, Fairfield, CT: I have a '70 Chevy Impala that I've been working on for about a year and a half. It's a coupe with a small-block 400, TH350 trans, and a 10-bolt rear, and it's mostly stock, save for the four-barrel carb and intake I added and the dual exhaust I had a shop fabricate. I got the car from a relative, who had owned it since the early '70s-when the relative got older, the car sat in a garage most of the time. It has about 95,000 original miles, and the body is in pretty good shape, and now that I'm finishing the mechanical restoration of the brakes and suspension, I want to drive it more than just around the neighborhood. Trouble is, if I leave the battery connected, it goes dead in about a day. I didn't really notice the problem before because the car would sit in my garage for several days or even weeks between starts, and I would always disconnect the battery when I parked it. This car has factory power windows, rear defog, A/C, and some other options, so I'm a little overwhelmed when I think about trying to diagnose the source of the drain. How can I narrow it down?
Joe Armstrong, Technical Expert, Painless Performance: This is a common malady found on older and late-model cars alike. Often it is the result of an aftermarket alteration to the factory wiring harness, but sometimes it can be caused by a faulty switch or relay. The following information will help diagnose this issue.
For this test you will need the following: a 12-volt test light and whatever tools are required to remove the positive battery cable, the wires from the charging system, and the wires from the ignition switch. You should also have a sheet of paper and a pencil. Make sure the battery is fully charged before you begin.
1) Remove the positive battery cable and put the test light in line between the positive battery-cable terminal and the positive post on the battery. This will measure the current flow from the battery to the electrical system. Look at the test light and take note of the brightness before you change anything.
2) Disconnect the test light from the positive post on the battery. Make sure the positive battery cable does not touch the positive post on the battery. Remove the wiring to the charging system. Be sure to insulate the ends of the wires, as these are battery hot-circuits, and you do not want them to short to ground. Reconnect the test light to the positive post on the battery. Take note of the brightness. If you see a dramatic change, the charging system is causing a drain. Your alternator or generator will need to be tested, rebuilt or replaced. If there is little or no change, go to step three.
3) Remove the test light, and replace the positive battery cable on the positive post. Close the door to your car, and make sure the dome light goes off when the door is shut. If the dome light works properly, remove the dome light bulb (be sure to remove any courtesy light bulbs as well). Using the test light, make a list of each fuse to determine which fuses have power with the ignition switch in the off position. Disconnect the positive battery cable from the positive post on the battery. Put the test light back in line as described in step one. Remove each fuse on your list one at a time. Check the test light after you remove each fuse. If removing a fuse causes a dramatic drop in the brightness of the test light, that circuit is causing the drain. You will need to identify what is connected to that circuit (we can help identify what could be connected to each circuit if you determine which fuse has the drain). If no fuse causes the drain, continue to step four.
4) Disconnect the test light from the positive post on the battery. Make sure the positive battery cable does not touch the positive post. Either unplug or remove the battery circuit from the ignition switch. Be sure to insulate the end of the wire, as it is a battery hot-circuit, and you do not want it to short to ground. Reconnect the test light to the positive post on the battery. Take note of the brightness. If you see a dramatic change, a circuit from the ignition switch is causing a drain and more extensive diagnosis is required. If there is little or no change, go to step five.
5) At this point we have checked every hot-circuit with the key off. One or more of these circuits should have made a difference. If none of these circuits had any effect on the brightness of the test light, have the state of the battery checked. Be sure the battery is fully charged before checking its state.
Oil Cooler Cure
Scott Meyer, Bensalem, PA: I have a '95 Trans Am with the original LT1 engine and a six-speed trans. The car has what appears to be a factory oil cooler that uses an adapter between the oil filter and the block to source oil for the cooler feed and return lines. The problem is that I have a persistent oil leak at this junction, and even though I've tried to tighten the two Allen-head bolts that hold the adapter to the block, oil keeps leaking. I pulled it off and changed the O-ring, but it still leaks. What's the deal?
Terry McGean: Engine oil coolers became fairly common on GM trucks and performance cars in the late '80s, and most use similar adapters and lines. The main difference seems to be with the type of cooler-some use an oil-to-water cooler integrated with the radiator tank (like a trans cooler), and some use an external oil-to-air cooler mounted ahead of the radiator. The engine adapter is the same in most cases regardless of cooler type. It's also common for these setups to leak, often from the fluid-line assemblies, which use rubber flex hoses coupled to aluminum hard lines with crimped junctions. The fluid-line leaks are usually at the crimped junctions, though sometimes the hoses themselves split. Either way, repairing them means replacement. But the leak you describe does indeed sound like it's coming from between the block and the adapter. The O-ring you changed is only part of the sealing for that arrangement-there should also be a gasket between the block and the adapter, though from what we've read, some vehicles seem to have been assembled without the gasket. Even those that have it can tend to leak, and usually, once the leaking begins, tightening the Allen-head-adapter mounting bolts won't cure the problem. The gasket and O-ring seem to have been updated by GM, which now offers them together in a seal kit (PN 88893990). The new O-ring is black, replacing the orange factory piece, and the new gasket has a metal reinforcement plate sandwiched between the gasket material. The seal kit should cost less than five bucks at the dealer. We recently went though this process on our '96 police-spec Caprice and were warned by a dealer tech that getting the old gasket off would be a chore ... he was right. We used some aerosol gasket remover to soften it up and continued to scrape it with a hand-held razor blade. The Allen bolts should be torqued to 17 lb-ft when reinstalled. Ours seems to have stopped leaking as a result.
A Question of Octane
Lee from Alabama, via CarCraft.com: I noticed that Chevrolet recommends 92-octane gas for both its 454 engine with 8.75:1 compression and its 350 engine with a 10.0:1 compression ratio. I was always under the impression the high-compression engines needed higher-octane fuel. Why the same octane for both of these engines?
Jeff Smith: That's a great question, Lee. Static compression ratio is just one part of the much-more-important point of dynamic cylinder pressure. This is an area where the intake-valve closing point has a dramatic effect on the dynamics of cylinder pressure. One way to test for dynamic cylinder pressure is with a simple cranking compression test. Have you ever wondered why engines with different static compression ratios will have similar cranking pressures? Let's take an 11:1 big-block Chevy equipped with a long-duration camshaft as an example. Let's say the cam has something like 250 degrees at 0.050 inch of tappet lift. In a cranking cylinder-pressure test, the engine may only have 180 psi-yet this is an 11:1 motor. This big cam closes the intake valve later than a cam with shorter duration. The later the intake valve closes, the less air there is trapped in the cylinder. At high engine speeds of say 6,000 rpm, there is less time to fill the cylinder, so the longer the intake valve stays open, the more "time" there is to fill the cylinder. Conversely, if we take the same engine, change the cam to a really short, RV-style cam with a duration figure of 210 degrees at 0.050, and check the cranking cylinder pressure, it might be something like 220 or 230 psi-very high! This motor will detonate its brains out because the low-speed cylinder pressure is way too high. Roughly 190 psi of cranking pressure is about the limit for 92-octane pump gas unless the engine has an exceptionally good combustion-chamber design.
We looked up the engines in the GM Performance Parts (GMPP) catalog, and the small-block with 10:1 compression is the ZZ4 350 engine equipped with a 208-degrees at 0.050 hydraulic roller camshaft. The 454 engine with 8.75:1 compression actually uses a longer-duration 211 degrees at 0.050 roller cam, which probably closes the intake valve a little later than the small-block. GMPP is probably playing it safe by recommending 92 octane for the 454. Combustion-chamber design is also a part of this equation, and the ZZ4 engine enjoys a slightly better chamber design than the older big-block. Plus, the iron heads on the big-block tend to hold a little more heat that can make the big-block rattle. It's possible you could run 89 octane on the Rat in light-duty applications with that cam. It would come down to testing the combination to see how the engine reacts to the lower-octane fuel.
Not So Olds
John Griffin, Cour D'Alene, ID: With gas being so expensive, I am looking for a way to get big torque with good mileage. Is there a way to get close to 600 lb-ft of torque at reasonably low rpm and get good gas mileage? I am using an Art Carr TH2004R overdrive automatic.
I am building an Olds motor and have a choice of blocks-either the 425 or the 455. The 455 Olds came stock from the factory with 500 lb-ft of torque and the 425's came with 480. In 1968, the two-barrel 455 with small heads made 500 lb-ft at 2,400 rpm!
So, 40 years later, can I improve on this torque and keep the rpm in the range so the overdrive will give good gas mileage? I'd like to get high-12s in the quarter-mile with more than 17 mpg, but I'd settle for mid-13s and 15 mpg or so. I ran these motors in the '70s and managed 15 mpg-but gas was only 29 cents a gallon. Thanks for your help.
Jeff Smith: Let's start with some basics before we get into the details. For the most torque, you'll want to go with the biggest displacement you can create. But for the best mileage, the smaller engine will be more efficient. Both the 455 and the 425 use the same bore, which means the 425's stroke is much shorter (3.975 versus 4.250). From here, you'll have to decide which is more important to you, economy or power. The engine with the shorter stroke generates less piston travel and therefore better economy, but of course, it gives up some torque. In this case, we'd say go for the bigger engine, since the difference in fuel economy will probably be marginal, and Olds 455s are a lot easier to find than 425s, which were only produced for three model years.
You also mentioned you want to run high-12s or low-13s; we'll assume the 455 is going into an A-body like a Cutlass and not some 5,000-pound boat. To run high-12s, you also must make horsepower. Torque will help accelerate the car, but you'll also need decent horsepower for mph. This will mean not much gear, which is good because you can then use the overdrive to run the engine slowly enough (under 2,000 rpm) on the freeway to get decent mileage. The TH2004R uses a 0.67 overdrive, so with a 3.23 rear gear and a lockup torque converter, at 70 mph with a 26-inch-tall rear tire, the engine should only be spinning 1,960 rpm. Spinning the engine slowly will help mileage.
You mentioned using stock heads. Pay particular attention to a quality, three-angle valve job by changing the intake seat angle from 30 degrees to a combination of 30-45-60 degrees and back-cutting both the intake and exhaust valves with 30-degree angles. Any minor intake or exhaust throat-pocket porting that will help low-lift flow will improve power and mileage. The chambers on these heads are not very efficient, so make sure you have a tight piston-to-head clearance of around 0.040 inch. This will keep the quench active and improve torque, horsepower, and mileage. Piston coatings are nice, but I doubt you'd see a mileage improvement by using them. Compression helps all these as well, but keep it under 9.5:1.
Cam timing has a huge impact on engine performance. For mileage, a wider lobe-separation angle improves idle quality but hurts mid-range torque. Crane's mild cams often use a lobe-separation angle of 112 to 114 degrees compared with 108 to 110 degrees. For the Olds 455 with a 39-degree lifter-bank angle, a Crane PowerMax 204/216 duration at 0.050-inch tappet lift with 0.456/0.484-inch lift would work well. Crane also offers one cam smaller and several larger. A smaller cam helps mileage but hurts horsepower, while a longer-duration cam will help overall power but will sacrifice mileage. Do you begin to see the quandary?
Go with a Performer intake, and we'd suggest using a Q-jet carburetor. The carb flows 800 cfm yet offers tiny primaries that provide excellent control over part-throttle fuel. What you'll discover, John, is that if you can lean out the idle circuit, that will do more to help part-throttle cruise mileage than anything else you do. When the engine is cruising at light throttle on the freeway, it is actually running on the idle circuit, not the main metering circuit. Talk to Q-jet experts like JET or Sean Murphy (SMI) for the mods you'll need to the Q-jet.
Next, pay attention to initial timing, use a quick mechanical advance curve, and experiment with adjustable vacuum-advance canisters. This may give you something like 40 or perhaps even 45 degrees of timing at part-throttle. Don't worry, it won't rattle because there's very little cylinder pressure under those conditions. Crane makes an adjustable-value vacuum-advance can that might help mileage. All this will also improve throttle response and create excellent driveability. It's possible to get 15 to perhaps 17 mpg on the highway with this big motor if you tune the Olds right to the edge.
Auburn Gear Inc
2501 Ludelle St.
Sean Murphy Induction
17662 Metzler Ln. #B, Dept. SC
Jet Performance Products