Rocker Ratio and Rear Gear Concerns for a 455 H.O.

I really enjoy the Engine Assembly Do’s and Don’ts series (HPP, May-July ’13); it’s what I need before I finally start my summer rebuild. I’m a novice engine builder on a budget and have been collecting parts for years. I’m not a weekend warrior—I’m certain if I can get the combo below to work, it will be more than fast enough for cruise nights. However, I want to be sure all that I’ve collected is compatible and I’m not building Frankenmotor.

The parts include a low-mile, standard-bore ’72 WM-code 455 H.O.; a Torker II intake; a Holley 750-cfm 4150 carb; a Crane blueprint R/A-IV cam; Hooker Super Comp headers; a stock distributor with a Pertronix unit; and finally, a set of 0.030-over 8-L2394F pistons.

First, I do not have rockers yet, and I love the sound of 1.65:1 rockers and a R/A-IV cam. Will 1.65s work or will I simply make metal confetti? Next, all this will reside in a ’71 GTO convertible with an M-22 and a 12-bolt limited-slip. The 12-bolt currently has non-highway-friendly 4.56:1 gears. I’d like to lower the gear to a 3.90 or even 3.55.

Will all this work or should I just take it all to a swap meet? What else should I consider before diving in?

John Coker - Via Internet

John, you did well! It sounds like you have a well-thought-out plan, and arming yourself with information is the best thing you can ever do before jumping in.

You didn’t specifically mention if you plan to rebuild your 455 H.O., but based on the fact that you have new pistons, I’d guess so. Making the assumption that the engine will be completely disassembled for machining, there are a couple things I would do differently if I was building such an engine for my car. The L2394F piston from Speed-Pro is an excellent unit that’s domed to significantly increase a 455’s compression ratio when using larger-chamber cylinder heads like those from a 455 H.O. or SD-455. It is, however, rather heavy, and its crown must be machined to achieve the compression ratio you desire.

In my opinion, installing a L2359F flat-top piston would be much simpler. You won’t need to perform any additional machining beyond boring and honing the cylinders. By milling your cylinder heads to achieve a combustion-chamber volume measuring around 100 cc (approximately 0.060-inch if the heads are previously unmilled), you can attain a compression ratio in the low 9s. That range is ideal for the 91-to-93 octane premium-grade pump gas available today.

Stock-cast connecting rods are quite durable, especially in a street engine, but yours are more than 40 years old and will need proper reconditioning for your rebuild. For the cost involved, I strongly suggest a stock-length connecting rod constructed of forged 5140-steel. They are much stronger than stock and can be purchased for less than $300 from any popular Pontiac engine builder or parts supplier. If the budget allows, a 4340-steel rod is even more durable, but it may not be required for your combo. The “041” cam will perform quite well in your application. It contains 0.470-inch lift on the intake and exhaust with 1.5:1 rockers, and that increases to nearly 0.520-inch with 1.65:1 units. I don’t know if 1.65:1 rockers are needed in your instance, however. In my experience, the ’72 455 H.O. No. 7F6 casting peaks at about 225 cfm at 0.480-inch lift. Another point to consider with the 1.65:1 rockers on the No. 7F6 is that its stock valvespring package has a rather short installation height. When using stock-length valves, too much lift can cause the retainer to hit the valve seal and/or guide. Assuming your machinist takes that into consideration during your rebuild, the extra lift associated with the 1.65:1 rocker likely won’t hurt anything. So if you wish or plan to run 1.65:1 rocker arms, have him/her check the valvespring package for sufficient clearance and elongate the pushrod guide holes to prevent any binding while the cylinder heads are off. Your 455 should have no issues running with a rear-axle gear ratio of 3.55:1. Depending upon what’s available, you may even find that a 3.31:1 gear set will provide you with the best combination of off-the-line punch and highway cruising ability.

Beyond those simple suggestions, I feel as if everything else you’ve selected for your 455 will complement each other well. If I had to guess, I think you’ll find it producing somewhere between 375 and 425 hp, and that will certainly feel like a monster on the street.

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