5.7L Third-Gen Engine Build - Balance Of Power :Decision '07

Like most election fodder, many issues that lead to poor consequences with engines stem from small details and missed opportunities. In our case, a $7 thermostat caused the failure of a 40-cent freeze plug on the back of the block (inside the bell housing) and toasted an otherwise perfectly serviceable L-98 350 TPI engine in our '88 Trans Am. Since we were destined to either replace or rebuild the smoldering remains of our engine, we needed to research all of our candidates' qualifications, sorting through so many campaign promises for the hard information before we elected an engine combination that we could live with in the long-term. We weighed the major party platforms, which included cut our losses and rebuild stock, go with a crate engine, or do a complete performance build. We chose the latter.

Since making these choices can be a daunting task, we decided to take the reader through our decision-making process and offer solutions for many situations that hobbyists may face while taking into account price and fitment issues specific to the L98 powerplant.

The first step for making these complicated decisions is to list priorities, and thus the options begin to sort themselves out according to your wants and needs. As with political candidates, you must do some digging to find the important details.

Soon, a number of pitfalls and obscure fitment issues hinging on the intake manifold/heads arose to repudiate our original budget limit, thus requiring an increase of financial and other resources we had not intended. Follow along as our detours lead us to a more expensive and hopefully much more fruitful plan and build up.

The priority going in was to see if we could find 350 rwhp while not exceeding $5,000 total on the entire package. Though we have a few decent solutions that could bring those results, they would come at a compromise to the quality of parts in the bottom end, which wasn't acceptable.

Our other objective was to try and improve efficiency, as our previous exhaust upgrade had delivered just over 200 rwhp but at a marginal 16 mpg. We wanted to squeeze closer to 20 mpg out of the old girl. This will be a challenge with the 700-R4 automatic transmission, so our priorities rank exactly in that order: power, budget, then mileage.

The Incumbent
Our '88 Pontiac GTA's 350 TPI engine has been tinkered with during several previous HPP fact-finding missions, and we found a lot to like and dislike about this candidate. Powered by the first major redesign of the venerable Chevy 350, this is a one-piece rear main-seal block and has longer lifter bores and cast-in mounting bosses for the hydraulic-roller cam spider, so the block and crank don't interchange with older small-block Chevy units.

This incarnation also includes heads with center-mount bolts for the valve covers, and the center two bolts on the intake manifold to cylinder head are at a 72-degree angle (as opposed to 90 degrees for all earlier heads). Although this is a distinction of the '87-'95 engines, an earlier head can be used on this block with an earlier manifold, or the later '96-'02 Vortec heads will also bolt on with a Vortec-pattern intake manifold. So, even though we have a small-block Chevy to work with, in reality, there are just enough differences that most of the key components won't interchange.

Although the L98 was regarded as a performance engine in it's day, GM missed the boat on a performance head by a wide margin. Unlike the Corvette L98's aluminum heads, the F-bodies came with cast-iron heads featuring press-in rocker-arm studs and no guideplates. They instead employed machined slots to guide the pushrods, resulting in pushrod interference with any rocker arms over about 1.60:1, even with the stock cam. These iron L-98 heads have only adequate ports and Fred Flintstone-era combustion chambers.

Since our estimate for upgraded valves, springs, and related hardware; a three-angle valve job; and a port and polish of these heads came in at approximately $900, it seemed logical to push the budget and ditch the incumbents for some state-of-the-art performance heads that would give us substantially more power. However, this will require the replacement of the also-restrictive TPI manifold base-not just because it becomes the power bottleneck, but it also won't fit most of the good cylinder heads due to the strange 72-degree angle of the center two intake-to-head bolts on either side.

The block is a decent part and will hold up fine at our target power rating of about 420 flywheel horsepower without needing any special treatment apart from some girdling of the main area, so it will be retained. Though the nodular-iron crank would also be adequate for our power target range, upon evaluation of our other options, we found that we can upgrade to a new 3.75-inch cast stroker crank (stock is 3.480 inches) for this application quite economically. We would put about $50 into the stocker in machining anyway, so it's hard to argue against an additional 28 ci for about $150-$370, depending on the crank selection.

It's money well spent from a power/torque standpoint, but this will substantially increase the load on the bottom end and increase piston speed. The bargain crank, rebuilt stock rods, and cast pistons would be OK if we can keep the rpm down, but higher rpm is what we need to make power, so we'll need to upgrade in this area as well. Since the parts we are considering seem to be headed over our budget, we considered swapping in a decent crate motor.