Q&A

Mailbag: EFI and Rear-Axle Swap Recommendations for a 500 C.I.D. Caddy-Powered ’84 Bonneville

 
8.5 inch rear end

(Image/Hot Rod Network)

Q: My car is a 1984 Pontiac Bonneville with a 500 cubic-inch Cadillac engine.

The engine is bored .030″ over and uses flattop pistons and Cadillac 425 heads for a compression ratio of 10.0:1. The camshaft is a COMP Cams Xtreme Energy grind with a 206°/214° duration @ .050 and .507″/.525″ lift.

The intake is an Edelbrock Performer, as is the 750 cfm carburetor. The exhaust system is comprised of 2-¼” mandrel bent tubing merging into a 3″ diameter intermediate pipe and a Flowmaster Delta Flow 50 Series muffler. The transmission is a TH-350C and the rear-end is a 7.5″ unit with 2.41 gears.

With this combo, the engine speed is 2,200 rpm at 70 mph and the vehicle delivers around 21 miles per gallon.

I’m thinking of changing the induction system to an EFI throttle body for more performance, better throttle response, and increased fuel economy. I’ve also considered installing an overdrive transmission and lower rear gears. What do you think?

A: Swapping in an overdrive transmission is a great idea.

A 3.42 or 3.55 rear gear combined with the overdrive of a TH-200-4R will increase the vehicle’s acceleration while maintaining the same cruise rpm and fuel mileage.

An EFI swap is also a good idea.

Holley’s Terminator EFI system will give you total control over fuel and timing curves, but the self-learning system will have your engine delivering optimized power and fuel economy in no time. These are the same throttle body air entries that you’ll find on every NASCAR Sprint Cup Series race car.

However, we do want to caution you about the 7.5″ rear axle—it’s a weak link. With the mountain of torque a 500 Caddy engine produced, we’re amazed the rear-end hasn’t given you any problems yet.

An 8.5″ rear axle from a Buick Grand National is considerably stronger and will swap right in. Plus, Grand National rear-ends are factory equipped with 3.42 gear so you wouldn’t have to do a gear swap.

Good luck!

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One Comment

  1. thomas jarrett says:

    just out of curiosity would that rear end fit/bolt up to my 62 Impala without any problems?

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