CPR Takes us Through Building a 550 HP Street 383 Stroker LS

By Jeff Smith/Photos By Jeff Smith and CPR-Engines

It seems that the only engines that get any media exposure anymore in the popular media are 1,000-plus horsepower “street” engines. The problem with those big-power stories is that they do not represent what’s really going on. A turbo motor that can make 2,000 hp isn’t what the average street guy is all about. So we thought we’d follow along on a real street engine that was intended for a daily-driven ‘04 Corvette. Our pal Dan Livezey has been autocross racing for more decades that he cares to remember. Recently he picked up an ‘04 LS1 Corvette that supposedly had been rebuilt, but after a week or so, the motor started knocking heavily.

A subsequent teardown revealed that the LS1 had spun the Number 7 and 8-rod bearings. Dan took the dead LS1 to Martin Marinov at Custom Performance Racing Engines (CPR-E) with a plan to resurrect his engine. The combination they created would bump the displacement with more stroke, a touch more compression with forged pistons, some CPR-Enhanced CNC porting on the stock heads, a mild cam, and a better intake. Of course all of this was hinged on one other critical factor – they had to keep this engine at least appear to satisfy the smog police since Livezey lives in Southern California.

All of these points were essential factors for our build since loads of compression and a big cam just don’t play well when it comes to smog testing. It turns out that Marinov has some experience in this area and together he and Livezey hatched a plan that – because you’ve already cheated and jumped ahead to our dyno test – revealed this 383c.i. combination made an honest 550-plus hp on the dyno. Let’s see how they pulled this off.

Concept and Execution

The heads sport 2.02/1.60-inch stainless SI valves. CPR-E also milled the heads to bring the chambers down to 63cc from the stock 67cc.

The heads sport 2.02/1.60-inch stainless SI valves. CPR-E also milled the heads to bring the chambers down to 63cc from the stock 67cc.

The easiest way to make more power – even with an emission engine – is to pump the displacement. The only way to do that with any LS1 aluminum block engine is with stroke. The iron cylinder liners barely allow a 3.908-inch diameter bore so Marinov ordered a 4.00-inch stroke crank from nearby Scat to replace the original 3.62-inch version. Retaining its 24x reluctor count, Marinov then added a set of dished AutoTec 4032 alloy forged pistons designed to accommodate a set of 6.125-inch long RPM H-beam rods. The 4032 alloy contains a little shot of silicon to limit piston growth which allows a tighter piston-to-wall clearance, making these pistons much quieter than a typical 2618 alloy piston more commonly found in race engines. These pistons combined with a minor cleanup on the heads produce a 10.8:1 compression, which is pretty close to ideal for a pump gas LS engine.

CPR-E also does all its own machine work, which means the block was subjected to a line hone, simple decking, and a mild honing procedure using Rottler machines to make the block ready for assembly. Final assembly began with a set of King rod and main bearings and DuraBond cam bearings that can take the abuse of the mild increase in spring pressure without pushing out. Once the bearing clearances were set, CPR loaded up the COMP hydraulic roller camshaft and Rollmaster timing set to make sure the cam was where it should be. They also verified the valve-to-piston clearance since this cam is capable of over 0.600-inch valve lift on both the intake and exhaust.

Over their short period of time, CPR has developed a CNC porting package for the cathedral port heads that is pretty impressive. They start by adding a set of 2.02/1.60-inch stainless SI valves, machine the seats to this larger size along with their own multi-angle valve job again using Rottler equipment and then hand-blend the seats to the CNC porting to come up with some pretty impressive flow numbers. We’ve included a cylinder head flow chart that you can study at your leisure with some impressive intake and exhaust flow numbers. When you can squeak out over 300 cfm from a 227cc cathedral port head (200cc is stock), you’re achieving great flow plus maintaining excellent flow velocity, which usually pays off with great mid-range torque numbers as you will see. This LS head upgrade also includes of PAC springs set up with 130 pounds of load on the seat and 370 pounds of open pressure, just to make sure the valves stay where they are directed.

Testing

With the heads done, CPR-E wrapped up the engine build and bolted the engine on their in-house engine dyno. They began the test with the factory LS6 intake, stock 75mm throttle body and a pair of 1 7/8-inch primary pipe headers through open exhaust. As you can see from the power numbers, CPR-E’s very mild 383 made some fierce torque, which was exactly the plan. Even down at 3,000 rpm, the 383 thumped 446 lb.-ft of torque with a peak of 516 at 4,800 rpm. The 529 peak horsepower arrived at 6,100 rpm. That alone would have been newsworthy, but then CPR-E bolted on a FAST LSXr cathedral port intake manifold and a 104mm FAST throttle body. Both the new LSXr and the original factory LS6 intakes mounted a set of FAST 46 lb./hr. injectors to make sure the engine didn’t run out of fuel, since the stock injectors promised to be a little on the small side to feed this much power.

With the FAST manifold and larger throttle body bolted on, the 383 again pushed through the power curve and after a little WOT-tuning on the stock ECU, the numbers surged. All you have to do is look at the graph to see how the FAST manifold bumped the power curve up across the entire rpm span from 3,000 to 6,400 rpm. There aren’t too many aftermarket parts that can pull of that kind of broad power magic across a 3,400-rpm span. As for the specific numbers – the most important really isn’t the peak torque at 528 lb.-ft at 4,800 or even the 556 peak horsepower number. The most impressive number is the average 14 lb.-ft improvement across the entire power curve. Add to this a minimum of 500-plus lb.-ft from 3,800 rpm to 5,600 rpm and the fact that the torque never dropped below 450 lb.-ft over the entire curve and those are some outstanding numbers. True, all this comes at a price. The induction package comes to more than $2,000 for the manifold, throttle body, fuel rails and injectors. But short of a supercharger or nitrous, it’s hard to come up with something that can add power across such a broad power band.

That means this engine will deliver excellent drivability and fantastic acceleration while still delivering near-stock idle characteristics. This engine should also be able to pass a California emissions test even with the LSXr manifold in place since it has a California Executive Order (E.O.) number, making it a legal manifold with the smog police. Owner Livezey is currently hunting for a set of headers that also offer the same E.O. clemency. Of course, there’s bound to be a minor power loss when the engine is bolted in the car since it will have to breathe through the Corvette’s street exhaust system but that should present only a minor decrease. Frankly, the torque will probably suffer the least, and that’s exactly what Livezey intends to rely on the most. As we said, Livezey is an autocross racer, so you can expect to see his ’04 Corvette at more than its share of local Los Angeles autocross challenges. Be prepared to discover he is quick behind the wheel!

CPR-E Flow Numbers

This chart compares the stock 243 LS1 cathedral port head with CPR-E’s CNC-ported , 227cc version with 2.02/ 2.160-inch valves, a CPR-E valve job and some minor hand blending. The E/I column is the exhaust-to-intake flow relationship. Generally, the higher the percentage of exhaust flow compared to the intake, less additional exhaust lobe duration is required to help the engine make horsepower.

Valve Lift Stock Intake Stock Exh. CPR-E Intake CPR-E Exh. E/I
0.100   64 55 68 61 90%
0.200 139 102 131 114 87%
0.300 193 138 193 161 83%
0.400 215 175 243 193 79%
0.500 228 189 279 219 78%
0.600 236 199 302 230 76%
0.625 304

Cam Specs Chart

Camshaft Dur. at 0.050” lift Valve Lift (inches) Lobe Sep.Angle
Stock ’02 LS1 Intake 196 0.479 116
Stock LS1 Exhaust 201 0.467
COMP LSr Intake 219 0.607 112
COMP LSr Exhaust 227 0.614

Power Curve

RPM TQ1 HP1 TQ2 HP2 TQ + HP +
3,000 446 255 456 260 10 5
3,200 464 282 471 287 7 5
3,400 477 308 483 312 6 4
3,600 486 333 494 339 8 6
3,800 494 358 502 363 8 5
4,000 495 377 504 384 9 7
4,200 499 399 510 408 11 9
4,400 507 425 519 435 12 10
4,600 513 449 526 461 13 12
4,800 516 472 528 483 12 11
5,000 514 490 525 500 11 10
5,200 508 503 522 517 14 14
5,400 500 514 519 534 19 20
5,600 490 523 512 546 22 23
5,800 477 527 498 550 21 23
6,000 463 529 485 554 22 25
6,200 448 528 471 556 23 28
6,400 429 523 454 553 25 30
Peak 516 529 528 556 12 27
Avg. 484.8 433.1 498.8 446.8 14.0 13.7

The two tests plot the power difference between the stock LS6 intake and the major torque and horsepower gains offered by the FAST LSXr intake and 102mm throttle body. But don’t overlook how much torque this 383 makes. How many street 383’s on pump gas have you seen that make 500 lb.-ft of torque at 3,800 rpm?

Parts List

Description PN Source Price
Scat 4340 steel crank, 24x 4LS140062 Summit Racing $1,097.97
RPM LS H-beam rod, 6.125-inch LG3-6125LSH RPM $388.00
AutoTec pistons, 1000622 CPR-E $Call
AutoTec ring pack. 1.5/1.5/3.0mm AT4905 CPR-E $Call
King main bearings MB5013XP Summit Racing $135.00
King rod bearings CR807XPN Summit Racing $  67.50
DuraBond cam bearings CH10 Summit Racing $  20.97
COMP 269 LSr hyd. roller 54-456-11 Summit Racing $373.97
GM  LS hyd. roller lifters 12499225 Summit Racing $124.97
COMP rocker trunnion kit/tool 13702-KIT Summit Racing $139.90
SI Valves, 2.02” Intake SL-2095 SI Valves N/A
SI Valves, 1.60” Exhaust SL-1168 SI Valves N/A
PAC valve springs 1211X Summit Racing $243.82
Romac Rollmaster timing set CS1135 CPR-E $135.00
Melling Oil pump 10295 Summit Racing $128.97
FAST Intake, LSXr 146302B Summit Racing $951.97
FAST 102mm 54102 Summit Racing $509.97
FAST LS6 fuel rails, black 146032B Summit Racing $247.97
FAST fuel line kit 54028KIT Summit Racing $124.97
FAST fuel injectors, 46 lb-hr 30462-8 Summit Racing $410-.97
Fel-Pro head gasket, left, 0.041 1161L041 Summit Racing $67.97
Fel-Pro head gasket, right, 0.041 1161R041 Summit Racing $65.97
ARP main studs 234-5608 Summit Racing $266.38
ARP head bolts – different lengths 134-3609 Summit Racing $199.93

Sources

COMP Cams
compcams.com

Custom Performance Racing Engines (CPR-E)
cprengines.com

Fuel Air Spark Technology (FAST)
fuelairspark.com

King Bearings
kingbearings.com

Melling Automotive Products
melling.com

PAC Racing Springs
racingsprings.com

RaceTec Pistons (Auto-Tec)
racetecpistons.com

Racing Parts Maximum (RPM)
racingpartsmaximum.com

Scat Enterprises
scatenterprises.com

SI Valves
sivalves.com

 

About Jeff Smith

A clue into how long Jeff Smith has been writing technical automotive stories might be his following of second generation readers. Writing continuously for nearly 40 years, his focus with Xceleration covers all things technical. His collection of cars includes a bevy of Chevelles and El Caminos. When not writing about cars, he likes to spend time with his wife Valerye, children Amber and Graham, and granddaughter Celeste.