Words and Photos by Richard Holdener
The phrase, “I’d rather be lucky than good,” comes to mind, but every bit as appropriate is, “Even a blind squirrel finds a nut”. What do we mean by these and what do they have to do with improving the performance of your average Tune-Port Chevy motor?
This story began with a simple search to locate a suitable Tune-Port test motor. The wrecking yard route is sometimes difficult, especially when looking to locate something so specific (and now decades old). Lucky for us, Chevy made a lot of Tune-Port motors back in the day, and there were still usable examples to be had in the bone yards.
We went looking for what we thought would be nothing more glamorous than an LB9 TPI 305, but what we ran across was actually an L98 350. Score! Our confusion stemmed from the fact that said L98 was the early (1985-86) iron-headed version, which looked all the world like the iron-headed, TPI-equipped 305.
Aluminum heads would be a dead give away, but imagine our surprise when we got back to the shop to reveal the increased displacement (block casting number). Though either motor would work for our test, the larger 350 is always more desirable than the small-bore 305!
Our plan for the L98 was to upgrade it with a cam, springs and then nitrous. Though significant power gains would ultimately be limited by the stock heads and trick-looking, factory induction system, we knew the combination would produce enough torque to torture the tires on any IROC. As if the L98 itself wasn’t enough of a score, everything on the well-used 350 turned out to be in working (meaning dyno-ready) condition.
Our pre-run, leak-down and cranking compression revealed eight happy holes. The same can be said of the factory fuel injectors, as a quick test on the old ASNU injector machine revealed equal flow rates and spray patterns. Knowing nitrous was in the cards, we replaced the factory spark plugs with colder units, then filled the crankcase with five fresh quarts of 5W-30 synthetic.
In order to hook up the FAST XFI engine management system, we replaced the factory HEI distributor with an MSD billet unit. Finishing touches included an electric water pump, long-tube headers and connecting the throttle linkage. Run in this configuration, the TPI L98 produced 312 hp at 4,600 rpm and 412 lbs. ft. of torque at 3,400 rpm.
Given the fact that the factory power rating for the L98s were rated from 225-245 hp (depending on year), we were more than a little surprised to see this thing put down over 300 flywheel hp. The factory power rating was measured with full accessories, exhaust and air intake system, using the (conservative) factory tune. Part of the power difference came from the fact that our test motor had none of these, and even included long-tube headers, but we still wondered whether this L98 was sporting something stronger than the factory camshaft. Regardless, we installed our Xtreme Energy XR270HR from Comp cams.
Healthy for the TPI motor, the XR270HR cam featured a .495/.502 lift split, a 218/224-degree duration split and a 110-degree lobe separation angle. A better choice might be the smaller XR264HR grind designed specifically for the TPI motor. The 264 cam offered a 488/.495 lift split, a 212/218 duration split and a more desirable (for an EFI application) 112-degree lobe separation angle.
We also installed a set of 26918 (beehive) springs to work with the new cam. After getting the new combo all dialed in, the TPI L98 responded with 323 hp at 4,600 rpm and 414 lb-ft of torque. The Comp cam offered gains as much as 25 hp higher in the rev range, but this TPI motor still needed more power.
Rather than swap the heads or TPI induction system, we decided to push the easy button and just add some Zex nitrous. Sure, a blower or turbo will also work, but no other mods are as simple, effective and inexpensive as a dedicated nitrous kit. Given the iron heads, impressive torque output and low activation rpm needed for the TPI motor, we chose to hit the L98 with a simple 75-hp shot. The math told us that an extra 75 hp might put us right at the magical 400-hp mark.
Every bit as important was the fact that the nitrous-injected TPI motor would thump out some serious torque. Looking at the already impressive torque out of over 400 lb-ft, we fully expected the Zex-enhanced TPI 350 to exceed 500 lb-ft. For the L98, we chose a universal wet fogger kit from Zex that included a single fogger to simultaneously provide both fuel and nitrous.
The fogger nozzle was quickly installed into the factory rubber inlet elbow. After programming the controller, heating the bottle and purging the system, we engaged the nitrous. After dropping the fuel pressure to the nitrous slightly (and retarding the timing by 2 degrees), we were eventually rewarded with peak numbers of 402 hp and 519 lbs. ft. of torque. With the right cam and a little juice, TPI motors can still get it done!
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