What the little LT1-based L99 4.3L really needed was some Tune Port torque! (Image/Richard Holdener)

What the heck is an L99? We know GM uses these alphanumeric designations to label its many engine configurations, some of which were more notable than others. For instance, most Chevy fans recognize the famous ones, like DZ302, L88, or LS6—and some of the real diehards even know the difference between the LT-1 and LT1 (first and second gen performance 350 motors).

But who here knows about the L99?

Note: We’re NOT talking about the LS-based L99 here either.

What is the “Baby LT1” Chevy L99 Engine?

Not quite as legendary as its larger brethren, the L99 was actually a diminutive 4.3L V8.

What? A 4.3L V8? Don’t you mean the 4.3L V6?

Nope, the L99 was a gen-2, LT1-based 4.3L V8 offered in the (of all things) full-size Caprice Classic!

Why did GM use the smallest small block in one of the largest passenger vehicles? More importantly, why did GM use the torque-lacking, short-runner, LT1 intake on the smallest V8 ever offered in the biggest package?

What the little V8 needed more than anything was additional torque. And adding torque to the little 4.3L is what this test is all about!

Swapping Intakes to Make More Torque

The most obvious answer to adding torque was with displacement, and GM did exactly that by offering a larger, more powerful 5.7L version of the LT1.

But what about the 4.3L folks? Besides, we have always wanted to compare the previous generation, long-runner, LB9/L98 Tune Port intake to the later, short-runner LT1 intake, and this was a perfect opportunity.

The big hurdle for this test was the fact that the Gen-2 LT1 intake and heads featured a different bolt pattern than the previous (conventional) SBC intakes. The Gen-2 LT1 featured reverse cooling, meaning (among other things), dedicated head casting with different intake bolt patterns and no coolant passages in the intake. The lack of water flow through the intake presented no problem for the intake swap, but the bolt-hole alignment required attention.

Lucky for us, Jim Hall and the boys over at Tune Port Injection Specialties (TPIS) welded, milled, and drilled on the LB9/L98 lower intake until the holes matched the bolt pattern on the LT1 heads. It was necessary to stuff rags or paper towels into the water passages to stop oil from the lifter valley from finding its way out (the LT1 gaskets don’t cover the water passage opening on the L98 intake), but this was an easy fix. 

Our L99 Test Engine

With the TPI intake now swap ready, we installed our modified (junkyard) L99 up on the dyno. Prior to running, the factory LT1 was modified as well, by drilling a hole in the back to accept a conventional distributor.

Unfortunately, the Holley HP ECU would not accept the trigger pattern offered by the factory OptiSpark, so we pushed the easy button and took out the hole saw! After drilling and tapping a hole for the distributor hold down, we were in business.

The little L99 had been previously modified (lots of testing) with a mild cam (.410″/.427″ lift, 207°/214° duration, 117° LSA), a valve spring upgrade, and a set of Comp 1.6-ratio (guided) roller rockers.

The 5.7L LT1 was offered with both iron and aluminum heads, depending on the application, but the smaller L99 received dedicated (small-chamber and valves) iron heads. The iron heads used on the LT1 featured both larger valves, larger combustion chambers and increased flow. The iron LT1 heads are said to be on par, or even slightly better than, the aluminum LT1 heads used on the F-body and Corvette applications. The smaller L99 heads were down on flow by 10 to 15 cfm (depending on valve lift), but the smaller chambers (49cc vs. 57 to 58cc) meant a head swap might not yield the desired power results.

Testing the L99 Intakes

The 4.3L L99 was first run with the stock LT1 intake, stock throttle body and 1-3/4 inch long-tube headers. Smaller 1-5/8 inch headers were run in a later test and improved the power and torque by five to six horsepower and a like amount of torque through the whole curve. After optimizing the AF and timing curves (30 degrees total), the LT1 intake offered peak numbers of 277 hp at 5,900 rpm and 279 lb.-ft. of torque at 4,100 rpm.

After installation of the LB9/L98 Tune Port Injection intake, the peak power dropped to 252 hp (at 5,000 rpm), but peak torque jumped to 293 lb-ft at 4,100 rpm.

True to form, the TPI system improved torque production up to 4,800 rpm, but the short-runner LT1 intake offered improve top-end out to 6,000 rpm. The question (as always) becomes, where do you want your power production?

The ideal situation would be to have both, but short of a dual-runner intake, that would be a difficult task, unless you just stepped up to the larger 5.7L. Of course, you could always combine the 4.3L crank and rods with the pistons and block from the 5.7L LT1, and make yourself a (de-stroked) Gen-2 DZ302. The other option is to combine the crank and rods from the 5.7L LT1 with the pistons and block from the 4.3L to produce a 305 (stroker 4.3L).

So much cool testing to do, but for now, know long runners do add torque, but the penalty is almost always peak power!       

For this test we ran the LT1-based, 4.3L L99 with two different induction systems. Thinking that the short-runner LT1 intake used on the small-displacement 4.3L V8 could use some extra torque, we decided to compare it to a factory GM LB9/L98 Tune Port intake. Because the LT1 heads featured a different intake bolt pattern (with no intake water passages), this required modifications to the TPI intake. TPIS performed the required mods to facilitate the swap. Run with the LT1 intake, the mild (small cam) L99 produced 277 hp and 279 lb.-ft. of torque. After installation of the TPI intake, the 4.3L produced just 252 hp, but torque jumped to 293 lb.-ft. The TPI enhanced low-speed torque up to 4,800 rpm, then the short-runner intake took over from there. Which would you rather have? (Chart/Richard Holdener)
Originally a junkyard motor, the 4.3L L99 test motor was first pulled from the yard and run on the dyno in stock trim to ensure the high-mileage motor was still in fighting trim. (Image/Richard Holdener)
The stock 4.3L pistons featured valve reliefs to allow us to install something a little healthier than the stock L99 cam. (Image/Richard Holdener)
To improve the power output of the 4.3L, one of the many cams we ran included a unit from TPIS that offered a .410″/.427″ lift split, a 207/214-degree duration split and 117-degree LSA. (Image/Richard Holdener)
Unlike the larger 5.7L LT1, the smaller 4.3L was equipped with small-chamber 49cc iron LT1-based (reverse cooling) heads. (Image/Richard Holdener)
To work with the mild TPIS cam, we installed a set of Z/28 valve springs on the stock iron heads. (Image/Richard Holdener)
We also took the liberty of installing a set of guided (no guide plates) Comp 1.6-ratio stainless roller rockers. (Image/Richard Holdener)
The L99 shared the same, short-runner, LT1 intake used on the larger 5.7L motors. (Image/Richard Holdener)
To allow use of a conventional distributor, the author dilled a hole in the LT1 intake! The ECU would not recognize the trigger pattern supplied by the factory OptiSpark ignition. (Image/Richard Holdener)
With our opening in the intake, we installed this MSD billet distributor. It was necessary to remove the factory oil pump drive prior to installation of the distributor. (Image/Richard Holdener)
The little L99 was run with a set of 1-3/4 inch long tube dyno headers. (Image/Richard Holdener)
To measure timing, we positioned the motor at TDC, then marked the damper and whipped up a custom timing pointer. (Image/Richard Holdener)
Both EFI combos were run with the same factory L99 injectors. (Image/Richard Holdener)
We dialed in the AF and timing curves for each combination (on 91 pump gas) using a Holley HP management system. (Image/Richard Holdener)
Run on the dyno first with the factory, short-runner LT1 intake, the mild L99 produced peak numbers of 277 hp at 5,900 rpm and 277 lb.-ft. of torque at 4,100 rpm. (Image/Richard Holdener)
Off came the LT1 induction system to make room for the TPI combo. (Image/Richard Holdener)
Because the TPI was designed for a conventional small block intake bolt pattern, TPIS welded, milled, and drilled the lower manifold to allow installation on the bolt pattern used on the LT1 heads—which is different than the Vortec as well. (Image/Richard Holdener)
Installed on the L99, the TPI induction looked like it was made for the little 4.3L! (Image/Richard Holdener)
On the dyno, the TPI showed its torque-producing nature by increasing torque production on the 4.3L to 293 lb.-ft. Since there was no free lunch, the long-runner limited power production past 4,800 rpm, with a peak of 251 hp. A big, heavy Caprice Classic might want the extra torque, but would you be willing to give up the top end? (Image/Richard Holdener)

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Richard Holdener is a technical editor with over 25 years of hands-on experience in the automotive industry. He's authored several books on performance engine building and written numerous articles for publications like Hot Rod, Car Craft, Super Chevy, Power & Performance, GM High Tech, and many others.