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Maximum Compression Ratio On Olds 455 - What's Your Problem?

Rat Chronicles
Doug Razze; Franklinville, NJ: I bought my first 454! Little did I know, nobody knows anything about Gen V engines. Can you help me bring this boat anchor back to life on a budget? Some say I can use old 454 heads, and some say use marine heads. Please help! This is a good running motor but not worthy of my '70 Camaro.

Jeff Smith: The 454 big-block Chevy went through a series of significant changes in the '90s that directly affect parts interchangeability. We'll hit the high points and then clue you in on some affordable changes that will help this aging Rat make some power. The first-generation big-block from '65 to '90 is commonly referred to as the Mark IV, and all the parts interchange. GM's '91 revision, known as the Gen V, made drastic changes. Starting with a one-piece rear main seal, the new Rat retained the stock head bolt pattern, but changes to the upper coolant holes in the block made converting to older Mk IV heads not impossible, but certainly problematic. The Gen V also eliminated the mechanical fuel pump boss, moved the main oil gallery alongside the camshaft, and enlarged the diameter of the freeze plugs. While the Gen V retained a flat-tappet hydraulic camshaft, the valvetrain was changed to a net lash system that eliminated the rocker arm stud, replacing it with bolts that did not allow valve lash adjustment. Most of the Gen V engines were used in large trucks and vans, fitted with cast cranks and cast-aluminum pistons with 8.0:1 compression and 118cc chambers. Then in 1996, GM revised the big-block yet again, giving it a hydraulic roller cam and multipoint fuel injection. The block coolant passages were changed slightly, making a conversion to Mk IV heads easier. The best way to tell a Gen VI engine is by its six-bolt cast-aluminum timing cover and large multipoint EFI manifold. Finally, GM ruined Rat interchangeability by converting to an 8.1L (496ci) version that suffered from so many revisions (including a completely different head bolt pattern) that no parts interchange, effectively killing this Rat as a performance platform.

It sounds like you're on a budget, so the easiest way to pump a little iron into this Rat is with a cam and breathing. We'll assume the short-block is in decent shape. While many enthusiasts still claim you can only make power with a set of rectangle-port heads, the reality is the factory oval-port castings are perfectly suited for street performance. If you are willing to spend a little money, the best move is to remove the heads and have your machine shop mill the net valve lash stands and drill and tap the original 3/8-inch boltholes for 7/16-inch head studs and guideplates. That will allow you to run a decent performance camshaft and good aftermarket rocker arms. You might also consider milling the heads to improve the static compression. Milling the chambers to 110 cc will bump the lame 8.0:1 compression up to 8.5:1. Also make sure the valveguides are in good shape. Even if the valves are in decent shape, consider increasing the valve size from the stock oval-port 2.06/1.72 inches up to at least the stock rectangle-port head 2.19/1.88-inch valve dimensions.

Because the compression is so low, you have to be conservative with cam timing using a mild hydraulic flat-tappet. The Lunati Bare Bones big-block Chevy version specs out at 214/224 with 0.501/ 0.527-inch lift combining the cam and a set of lifters for a mere $115.95 from Summit Racing. Top this off with an Edelbrock Performer RPM dual-plane intake (PN 7161, $219.95, Summit Racing), a 750-cfm Holley carburetor, and a set of 13/4-inch headers and you're on your way. This package should make about 450 hp and might push upwards of 500 lb-ft.

More Info:
Torrance, CA

Olive Branch, MS

Summit Racing:
Akron, OH

Cop Car Coilovers?
In this month's Speed Shop, we mentioned the coilovers on our Crown Victoria. Here is a picture. They are QA1 shocks and springs modified by Naake Suspension to fit in the stock attaching point in the lower control arm of '03 and '04 Mercury Marauders and '03-and-later Crown Victorias. Go to to see the kit.

CC Quickes
Vintage Draw-Through
We recently took a trip to the Bonneville Salt Flats and spotted this engine in a stretched Model A. It is a small-block Chevy with a crank-driven Potvin supercharger being fed by eight carbs. We didn't hear it run, so it might just be art.

It Feels Faster, I Swear
In other cop car news, we are planning some 4.6L, two-valve, bolt-on articles, so here's our car at Racers Edge Tuning in Downey, California, for baseline numbers. The bad news: 208 hp at the wheels. The good news: It can only get better.

Mile-Long Timing Chains
Fans of the DOHC 4.6, don't fret. We are also working on some 4V build articles using this '95 Lincoln Mk. VIII long block we bought from Midwest Mustang in Lawrence, Kansas. We just dropped it off at QMP Racing Engines in Chatsworth for inspection and machining. Look for articles starting in a few months.

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El Segundo, CA 90245

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