Earlier this year we bolted a cam and headers on a 5.3L Gen III called Little Bro and made 458 hp at a screaming 6,600 rpm. That engine revealed the potential of a lesser-known family of iron-block GM engines with varying power levels and displacements using the Gen III nomenclature. The largest of the group is the LQ4 6.0L found in the fullsize GM truck line starting with the '99 model year and the LQ9 that appeared in 2000 with more compression and aluminum heads. While the 5.3L proved to be a great engine, the bore is 3.780, much like the Gen I 305. The 4.000-inch bore found in the 6.0L has the breathing room for performance applications and long rods from the factory, making it a perfect candidate for a stroker short-block.
SLP Performance Parts knows all of this and has exploited the availability of the iron blocks to develop an affordable 402-inch short-block stroker assembly that has succeeded in making 580 hp. It's arguable that the rest of the bolt-ons to complete the engine adds considerable expense, but with time and energy spent searching and carefully selecting components from the GMPP catalog and the internet, it's possible to put together an affordable screamer.
The ZL-402 PackageThe basis for the 402 is the LQ4 6.0L iron block. From the factory it has a 4.00-inch bore and 3.622 stroke with a "long" 6.098 rod. SLP decided to leave the bore alone to save the expense of boring and fitting oversized pistons and went directly for the 4.00-inch throw crank to get the extra displacement. As a guideline, the guys at SLP say the Gen III block is only able to grow to 4.060 without a sleeve, so in this case, they'll leave you with at least two rebuilds by leaving the bore alone.
Because all the LQ engines are equipped with a 6-inch rod from the factory, there is no clearancing of the block required to use a longer rod. Their choice was a 6.125 I-beam rod and a shorter Ross and Manely 4.000-inch forged piston to further reduce rod angularity and side thrust. The 4.000-inch stroke crank drops in with this combination of parts.
The Gen III ComponentsThe best bet here is swapping this into your late F-body or Corvette that already has the good heads and accessories, but if you're going for a street machine application, there are a couple of things you should know. The guys at SLP used the LS6 ported heads that have a 65cc combustion chamber, placing the compression ratio at 11.2:1. If this seems a tad high for a street engine, you're right. For the big pull they used a total of 28 degrees of advance in order to employ pump gas so they're giving up a little power to avoid detonation. One alternative is using the LQ9 or LQ4 head with the 71cc chamber volume to drop the compression, then dialing in the timing or using a stand-alone EFI control unit to run separate "pump gas" and "race gas" maps for street/strip cars. They also offer a forced induction version with a nominal ratio of 9:0:1 for which they recommend an aftermarket EFI control unit.
You should also note that because all the Gen III engine components are interchangeable, there is a forest of pulleys, pans, and intake options, and none of them are inexpensive. For the street machine builder, it is important to pay attention to parts selection because even the intakes differ between the car and truck engines causing clearance issues at every turn.
Go Get OneThis is just the beginning for the Gen III series as prices drop. Five minutes on the internet turned up a set of bare LQ4 heads for $75 and a new F.A.S.T intake and 90mm throttle-body for $800. We'd start with the short-block assembly and use as many less expensive truck parts as possible to complete the package. We'd use SLP's catalog of parts for stuff that proved hard to find, then drop the whole thing into a '81 Z28 with T-tops and a six-speed for a freeway destroyer. What do you think?
Carcraft.ComSee a dyno pull at carcraft.com-it's unbelievable!
