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How To Build a 475 Horsepower LT4 Powerhouse Engine, Part 2

Building The LT4, Part 2

Jim Smart Mar 29, 2016 0 Comment(s)
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In Part 1 of our LT4 engine build (click here), we inspected, machined, and assembled our LT1 turned LT4 short-block at Trans Am Racing under the watchful eye of Mark Jeffrey. Mark has a long-standing reputation for coaxing a lot of power from his engine builds. He has a mind for cam and head selection and knows how to build in power. Mark looked to the expertise of L&R Engine for both our LT1 block and great machining technique that would prove beneficial to horsepower. L&R Engine machined our block with several things in mind. The cylinder bores were machined to induce a minimum amount of friction while producing sufficient cylinder sealing. The same can be said for the main and rod journals in order to control both oil pressure and stability. The result was a solid foundation on which to build a 475hp/460 lb-ft of torque Gen II small-block.

Here in Part 2, we’re going to finish up our LT4 with some good old-fashioned hot rodding techniques. Our Chevrolet Performance LT4 cylinder heads from Summit Racing offer good performance right out of the box. However, they flow just marginally better than the LT1 castings. With this in mind, Mark decided to go to work on them with porting techniques designed to improve flow in and expedite exhaust scavenging out of the 54.4cc combustion chambers. Porting, coupled with our custom-grind camshaft from Comp Cams, brought us an additional 100 horsepower. Displacement and stroke are the remaining reasons for the substantial power (a whopping 75-horsepower gain) increase we experienced with our LT4. We’re convinced that with a little more dyno time and TLC we would have gone over 500 horsepower. We didn’t have the luxury of time and had to make deadline.

Because the Gen II LT1 and LT4 are essentially small-block Chevys, they fit just about anything that can accommodate the timeless Bow Tie V-8. Seems most LT1 and LT4 builds we’ve seen have successfully made 350-400 horsepower and an equivalent amount of torque. Trans Am Racing has taken our LT1 foundation a step further with a Coast High Performance 383ci stroker kit, an aggressive Comp Cams hydraulic roller camshaft, and LT4 heads and induction with common-sense port work. The results have been numbers better than we ever expected.

In Part 1, we built the Gen II LT4 short-block and buttoned it up with a reproduction small-block Chevy oil pan. When it was time to install it in a 1995 F-body, we discovered we had installed the wrong oil pan, which was lacking the low-oil sender provision. Don’t let this happen to you if you’re building an LT1/LT4 for a 1992-’97 GM car or truck. Although the absence of a low-oil sender won’t stop you from firing the engine, it’s nice to know it is there.

Top Job

Mark Jeffrey’s top-end prep is methodical in detail, bringing new meaning to blueprinting and engine building. Because the Gen II small-block is a wet-deck block, you’re going to want to use commercial-grade Teflon sealer on all cylinder head bolt threads.

2 Lt4 Engine Dog Bones 2/17
3 Lt4 Engine Retainer 3/17

01-02. Top-end wrapup begins with Comp Cams roller tappets, dog bones, and retainer—sometimes called a spider. Bolt threads should get Loctite 242 and be torqued to GM specifications. Lifters should have received a good soaking in 30-weight engine oil to fill the cavities for a good primed start-up. You can also prime the lifters in the engine with an oil pump priming, which is accomplished by driving the oil pump with a 1/2-inch drive drill prior to start-up.

4 Lt4 Engine Head Gasket 4/17

03. Block deck and head surfaces are checked for any debris, no matter how small, prior to head gasket installation. Do a wipe down with lacquer thinner and a tack rag, then install the head gaskets. One option from Fel-Pro is the Print-O-Seal head gaskets, which are not shown here. They seal incredibly well and are a nice option for your LT build.

5 Lt4 Engine Teflon Sealer 5/17

04. Cylinder head bolt threads are dressed with commercial-grade Teflon sealer because the Gen II LT1 is a wet-deck with boltholes straight into the water jackets.

6 Lt4 Engine Roller Rockers 6/17
7 Pushrod Checker 7/17

05- 06. Mark does a fit check for our Comp Cams Pro Magnum roller rockers. Comp Cams has a pushrod checker, which enables you to check rocker arm geometry before ordering 0.080-inch wall thickness one-piece pushrods. You’ll want the roller tip centered on the valve stem all through rocker arm travel. Any valve stem side-loading due to improper geometry will damage valve stems and guides quickly.

8 Lt4 Engine Check Valve Spring Coil Clearance 8/17

07. At peak lift, Mark checks for coil bind and clearance issues. Clearance here is around 0.021-inch, which is acceptable for safe operation.

11 Lt4 Engine Intake 9/17

08. The LT4 intake manifold is often thought of as an LT1 manifold in red powdercoat. However, it isn’t. The LT4 manifold is different than the LT1 thanks to improved port configuration and airflow. When you opt for the LT4 manifold, you must also go with the LT4 heads. The LT4 manifold and heads take a different intake manifold gasket due to a different port configuration.

12 Lt4 Engine Throttle Bodies 10/17
13 Lt4 Engine Throttle Body Gasket 11/17

09-10. To install the BBK 58mm throttle body, we needed parts from the smaller LT1 throttle body. Mark removes the hot water manifold from the original throttle body and transfers it to the BBK. We need clean surfaces on both the hot water manifold and throttle body for a leak-proof seal.

14 Lt4 Engine Idle Air Control Solenoid 12/17

11. Mark swaps the hot water manifold and idle air control solenoid to the BBK throttle body.

15 Lt4 Engine Throttle Body 13/17

12. BBK’s 58mm twin throttle body glides into place—a perfect fit. Summit Racing specified the 58mm piece to jibe with 383ci and a hot hydraulic roller cam. Take care to remember the hidden vacuum port located beneath the throttle body. We missed this and couldn’t get the engine to idle on the dyno—caused by a huge vacuum leak.

16 Lt4 Engine Fuel Injectors 14/17

13. Our factory fuel rail has been fitted with 30 lb/hr fuel injectors to maximize the performance gained from 383 ci, LT4 heads, and a more aggressive camshaft. Mark lubricates the O-ring seals with white grease and sets the fuel manifold in place. He gently sinks the injectors into place and tightens the rail bolts.

17 Lt4 Engine Valve Cover Gasket 15/17

14. With valve lash adjusted and the oiling system primed, we are ready for the Westech Performance dyno. Fel-Pro gasket technology is second to none. These silicone composition valve cover gaskets out-seal those old cork jobs, and without sealer.

To Tell The Truth …

Westech’s Steve Brule did a nice job for us dyno testing our LT4. Instead of a factory GM PCM system, we opted for a FAST system on the dyno from our friends at Comp Performance Group. We had teething problems getting our LT4 operational, including snapping two rocker arm studs, making it necessary to replace all of them. It turns out that the stock LT4 studs weren’t long enough to support proper valvetrain geometry. Mark installed 16 new ARP studs and torqued them to 45 ft-lb using Teflon sealer on the threads for proper sealing. He reinstalled and adjusted the Comp Cams Pro Magnum rocker arms to the 1/2- to 3/4-turn standard.

When it was time to fire the engine again, Mark decided to look to PCMForLess.com for a specially programmed PCM for our LT4. Provide PCMForLess.com with all of your LT1/LT4 vehicle’s specifics (vehicle type, weight, transmission type, axle ratio, tire size, with or without air conditioning, camshaft specifications, induction system type, injector size, displacement, and other specifics) and they will set you up with a ready-to-install PCM.

Dyno Results
RPM Horsepower Torque BSFC
2,000 133.7 351.1 0.631
2,500 182.8 384 0.476
3,000 243.8 426.9 0.425
3,500 287.3 431 0.419
4,000 336.4 441.6 0.429
4,500 394.9 460.9 0.421
5,000 435.7 457.7 0.442
5,500 461.5 440.7 0.459
6,000 475.2 411.6 0.491

We’re convinced our LT4 383ci stroker could have gone over the 500/500 mark with more extensive dyno tuning. The element of time was against us and the engine had to be shipped out of state. One overlooked item that wasn’t found until after our dyno session was a large vacuum leak (uncapped vacuum port) at the throttle body, which was not visible to us with the engine on the Westech Performance 902 dyno. This, perhaps, explains why we had problems getting this engine to idle.

Though we have performed dozens of dyno pulls over 30 years in the automotive publication business, mistakes will happen from time to time and we don’t get the results we had hoped for. Let the lesson we learned be your education to be mindful of all those pesky details when you’re building an engine and when you go to the dyno. There are no unimportant details.


9 Lt4 Engine 16/17
10 Lt4 Engine 17/17

Damper Differences

We learned something from our LT4 stroker build: There is a difference between the LT1 350 crank and the Coast High Performance 4340 383-inch stroker crank. The LT1 employs a press-on harmonic balancer with a taper fit. Most small-block Chevys have a Woodruff key arrangement. We had to have our factory LT1 balancer machined to work with the 383 crank. Ideally, our machine shop would have been able to cut a keyway into the balancer. However, they didn’t have the equipment necessary to cut the slot. Instead, they did some light machine work in order for the balancer to enjoy a snug fit. Expect to run into pulley alignment issues, which can be corrected via spacers and shims. This is only if you build an LT1/LT4 stroker. Best to check with your stroker kit supplier to see if it produces a crank designed specifically for the 1992-’97 LT1 engine.

What Sense In Replacing Sensors?

Whenever you’re building a fresh computer-controlled engine, it is strongly suggested you go with new sensors in the interest of safe, reliable engine operation. All of these sensors need to be fresh because they each give the PCM specific feedback on a particular engine dynamic. Without that feedback, performance suffers and engine damage can abound. We learned on the dyno that we needed a new IAC (Idle Air Control), which controls idle speed by creating an electronically controlled vacuum leak at the throttle body. When a faster idle is needed, the IAC creates a larger vacuum leak, which raises rpm. This happens when we turn on air conditioning, the engine runs hot, or we put the 4L60E automatic in gear. If you’re experiencing idle quality issues, most of the time it is the IAC or a vacuum leak.

Sources

Comp Cams
Memphis, TN 38118
800-999-0853
http://www.compcams.com
Summit Racing
Akron, OH
800-230-3030
http://www.summitracing.com
Westech Performance
Mira Loma, CA 91752
951-685-4767
www.westechperformance.com
Coast High Performance
Torrance, CA 90505
310-784-1010
www.coasthigh.com
Trans Am Racing
Gardena, CA 90249
310-323-5417
transamracing
Chevrolet Performance
888-924-9147
www.chevroletperformance.com
L&R Engine
Santa Fe Springs, CA 90670
562-802-0443
www.lnrengine.com/
PCM For Less
724.349.4431
pcmforless.com
MSD Ignition
915-857-5200
www.msdignition.com

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