YB Slow?

Will you help out a sucker who purchased something that was made out to be something it wasn't?

I purchased a '69 Firebird and it is a decent driver. It had some driveability problems when I road-tested it, but I saw a few things right up front that I knew could be fixed. Where I was misled is regarding the hood and air cleaner, which are marked 400. I asked the seller if the engine was correct. He told me it was a '69 400 block with the application code YB.

After purchasing the Firebird and trying to get some head snap or tire smoke from what the 400 should do, I spent some time looking into the serial numbers. From what I can figure, I have a '74 350 code-YB block with No. 46 heads. The engine and drivetrain is all new or rebuilt, so I don't want to tear everything down to the rotating assembly unless I absolutely must. I also spent my budget on the car, so I don't have the funds to buy a 400 or a crate engine.

Can you recommend a good cylinder-head-and-camshaft combination to wake up this engine? The rest of the driveline includes a Turbo 350 with a stock converter, 3.23 axle ratio, and stock manifolds with dual exhaust. I want to keep my Firebird a driver and the engine pump-gas friendly, but some tire smoke or acceleration would be nice when I leave the car shows or during cruise nights.

Pete Benzinger
Via Internet

Rocky Rotella responds: Pete, I'm sorry to hear that the displacement of the V-8 in the Firebird you (and possibly the previous owner) bought wasn't accurately disclosed at the time of purchase. I can understand your frustration, but don't let it get you down. While the 350 does not perform quite as well as a similarly equipped 400, there's no mistaking that it can perform respectfully with the right combination of parts.

Pontiac used YB when coding several of its V-8s backed by automatic transmissions over the years. It specified 400/two-barrel applications during the late-'60s, and again in '70. In '72 only, YB was used for the 455 H.O. Pontiac, then for a 350/two-barrel application in '74 and a few production years after.

Like the YB code, the No. 46 cylinder head was used in at least two different applications. It was a 400-spec cylinder head in '69 and then a 350-spec cylinder head in '73-'74.

With the variety of code-YB blocks and No. 46 cylinder heads produced over the years, I strongly suggest positively identifying your Firebird's particular engine combination before making any modifications, if you haven't already. It's quite possible you have more than you think!

To properly identify your Firebird's block, search for its casting number, which is located on the ledge just behind the No. 8 cylinder. Because of its rarity (and your stated lack of performance), I believe it's highly unlikely that you have a 455, but if it happens to be the '72 H.O. application, you'll find it numbered 485428. It's much more likely to be a 400-inch block, number 9790071, or a 350, which is 488986.

The easiest way to determine the vintage of your No. 46 cylinder heads is the location of the coolant-temperature sending unit. If there's a large boss between the left and center exhaust ports to house the sender, then they're '73-'74 versions. If there is no sender provision—at which point it was located in the intake manifold—then they're '69 castings. You can also find the date code of the heads embossed near the valve covers.

Regardless of the model year, original No. 46 cylinder heads are equipped with 1.96/1.66-inch valves. In '69, these heads have combustion chambers that measure around 78-80 cc. In '73-'74, these heads displace about 96 cc. That's a significant variance, and if you find that your code-YB engine is a 350 with '74 vintage No. 46 heads, as you believe, then it's likely it won't have much more than the factory compression ratio of 7.6:1.

I think the '74 No. 46 casting is a decent cylinder head. The addition of larger, 2.11-inch intake valves really won't add a great deal of performance in your particular instance. It should also be equipped with threaded rocker-arm studs, which makes the conversion to an adjustable valvetrain a simple task.

The 96cc combustion chambers are the '74 No. 46 casting's largest detriment on a 350. You could certainly find early castings with smaller chamber volumes, but the cost to purchase and completely rebuild them may not be worth the result. Instead, I suggest having your No. 46s milled 0.060 inch to boost compression by about one point. I strongly recommend physically measuring the chambers first to determine the exact volume before any machining. In the end, if you can get them near 85 cc or slightly less, you'll end up somewhere between 8.5:1 and 9:1 on a 350 overbored by 0.030 inch. A ratio in that range still allows for reliable operation on premium-grade pump fuel.

Many excellent cams are available. On a budget, something like Summit Racing's 2800 hydraulic flat-tappet will provide excellent performance throughout the entire rev range with stock-replacement-type valve-springs. A hydraulic roller would certainly offer some benefit, but the cost of the total package can certainly exceed your budget.

The 3.23 axle ratio offers a good compromise of acceleration and highway cruising. If after the engine modifications you're still looking for more performance, but don't plan to venture out on the highway much and are willing to tolerate slightly more axle gearing, then a 3.55:1 set will definitely give your 350 more punch off the line. It will, however, make your engine rev higher while cruising, and that certainly impacts fuel economy. Be sure to contemplate all this before making a decision.

If you can perform the work yourself, you should be able to get your heads machined and the new cam installed for an investment of less than $1,000. The gear set and professional install may set you back about the same amount. Combined, you should notice a significant improvement in your 350's performance, especially if you take the time to optimize its carburetor and distributor.

In total, these modifications should buy you some time if you want to save for a 400 block, and maybe even an aftermarket stroker assembly that will produce 461-plus cubic inches down the road. If you're into performance, you won't look back!


Email questions to christopher.phillip@sorc.com