Ad Radar
Car Craft
Click here to find out more!

325 Chevy Small Block Engine - Bolt On A Cam And Heads And Add 120+HP

The GM Gen III 5.3L truck engines are inexpensive and make great power, especially when we show you how to get another 120 hp.

Photography by

With all the hype about the Gen III LS1 and 6.0L engines, a little 5.3L engine has appeared in well over a million GM trucks since 1998 that's been overlooked in the rush to make horsepower. This engine displaces a mere 325 ci, but what it lacks in displacement, it more than makes up for in horsepower efficiency. Over the years, we've heard about several low-dollar 5.3L truck engines that made excellent power with only a few minor changes, so we thought we'd try our own version and see what happened. We were pleasantly surprised when this basically stock engine made well over 430 hp with just a cam change. But there's much more to this story, so let's get to it.

The Truck 5.3L
The basic 5.3L Gen III engine began its production life in 1999. In its base form, it comes as either an iron-block LM7 5.3L or an LM4 aluminum blockversion appearing in GM light duty pickups, Suburbans, Yukons, and vans, which means there are literally thousands of these used engines now in boneyards. The original LM7 5.3 is basically an iron-block LS1 engine with a smaller 3.78-inch bore (the 5.7 is 3.89-inch) rated at between 285 and 295 hp and 325 to 335 lb-ft of torque in stock GM trim. These engines also share the 5.7L's 3.622-inch stroke, and some enterprising car crafters have built an iron-block 5.7 merely by boring the block to the 5.7 bore dimension and using a 5.7L rotating assembly (the cranks are different due to piston weight). The stock 5.3L comes with a decent 9.5:1 compression ratio right out of the box and uses essentially the same cylinder head as its larger LS1 cousin, but with a smaller combustion chamber.

GM also built a smaller number of LM4 5.3L engines that are aluminum- block duplicates of the LM7. Thisversion got an inkling of recognition as the original engine for the '04 SSR truck. Otherwise, it is exactly the same as the iron-block 5.3L engine and you can expect to pay more if you find one. Then, just to make things interesting, GM upgraded these engines to a Gen IV configuration in 2005, although they still shared much of the same Gen III architecture. The most plentiful Gen IV iron-block 5.3L is the LH5. There's also the LY5 aluminum-block variation. This engine is rated at 320 hp stock with 340 lb-ft of torque.

Identifying these engines is easy. Look first for the tall plastic intake manifold and then find the 5.3 cast into the block on a pad near the back. But be careful. Many light-duty trucks came with smaller 4.8L (293ci) engines. If the engine is still in the vehicle, look for the emissions decal under the hood. But even that isn't a guarantee. The beauty of the 5.3 is that there are so many of these engines out there, buying one with minimal mileage in good shape for an affordable price shouldn't be difficult. We found ours at a junkyard and landed the engine for $550 without the plastic truck intake manifold and the accessory drive. We also got the starter motor, water pump, coil packs, and flexplate.

Talking Heads
You've probably already skipped ahead to the dyno test numbers, so you know how well our little junkyard dog performed with just a cam and head swap. As we mentioned earlier, the big reason a stock 5.3L does so well with just a cam change is all due to the heads. A stock 5.3L cylinder head uses a similar port configuration to a 5.7L LS1 engine with a slightly smaller (1.89- vs. 2.00-inch) intake valve. As you can see by our flow numbers, even a stone stock 5.3L casting can produce 219 intake cfm at 0.500-inch valve lift. That is more than most production Gen I heads (except for the iron Vortec). On the exhaust side, the story is even better. The stock 5.3L exhaust-port numbers are far higher than all Gen I production heads. In fact, you have to get into expensive aftermarket Gen I heads in order to better the stock exhaust-port flow on this 5.3L head. This is the main reason why this engine makes such good horsepower at the higher engine speeds.

While doing our research, we ran across Richard Reyman's West Coast Racing Cylinder Heads company, which has been quietly working on porting the stock 5.3L cylinder heads. He hand-ports the intake and exhaust bowls and adds slightly larger SSI 1.94/1.57-inch valves. As you can see from our flow bench reports, Reyman's deft touch with the grinder combined with larger valves is worth impressive gains throughout the entire lift curve with a massive intake port flow gain of 37 cfm at 0.500-inch lift to 256 cfm. That's roughly the equivalent flow to a high quality aftermarket Gen I head. Remember, this is just larger valves and a bowl job on otherwise stock heads. On the exhaust side, he gains an impressive 23 cfm for a total of 198 cfm from 175 cfm stock. Those are outstanding exhaust flow numbers.

But Reyman doesn't stop there. He also sells CNC ported heads, which start out as a semi-finished Edelbrock casting and are whittled out using 2.00/1.57-inch valves and a 58cc chamber. All this work pushes the flow up to an impressive 267 cfm at 0.500-inch lift when tested on a 3.780-inch 5.3L bore size. Compared to the stock head, this is a significant gain of 48 cfm (22 percent) at 0.500-inch lift, but the gains are also evidenced again throughout the entire lift curve. Once we evaluated all three sets of cylinder heads, it was time to bolt 'em on the engine and see what they would do.

Evaluation Day
After dragging the used 5.3L truck motor out of the boneyard, dyno guru Ed Taylor stripped and cleaned the long-block and then set it up with a GM Performance Parts single-plane carbureted intake manifold along with a new set of Fel-Pro intake gaskets to seal the carb'd manifold. On top we opted for one of Ed's dyno mule 750-cfm mechanical-secondary Holley carburetors. If you read last month's story on the MSD ignition box, you know how easy the ignition was to set up. We plugged in the sensors and the coil packs and we were ready to go. We bolted on a set of Hooker 13/4-inch coated headers that are actually designed as engine-swap headers for a Gen III engine in an early Camaro.

Our first shot was a simple baseline of the stock 5.3 with the carb and headers, making a credible 337 hp and 351 lb-ft of torque. Next, it was a simple task to strip the accessory drive and timing-gear cover and swap the stock hydraulic roller cam for a new Comp Thumpr cam. We also added a set of Comp beehive valvesprings to the stock heads to ensure adequate valve control at what we figured would be a higher peak horsepower engine speed. That's exactly what happened. Just bolting in this new Thumpr cam (it didn't even have a part number) pushed the peak horsepower from 5,600 to a nose-bleeding 6,800 rpm. But more torque at a higher engine speed is the formula for horsepower, which is what happened when the peak power jumped from 337 to 433 with stock heads. That's a 96hp bump with a cam swap!

Of course, this just made us salivate to bolt on the WCRCH Stage 1 ported production castings. That was also a quick swap, since the Gen III engines only use four head bolts per cylinder, which we replaced with a set of ARP head bolts. With the ported Stage 1 heads in place, the power jumped again, this time up to 449 hp, an improvement of 16 while the torque gained 12 lb-ft with the peak occurring at 5,400 rpm. Our final test was with the CNC-ported castings, where again the little 5.3 spun up to 6,800 rpm cranking out 460 hp with 396 lb-ft of torque. Taken as a whole, this simple head and cam swap on an otherwise stock 5.3L short-block was worth a total of 123 hp and 45 lb-ft of torque.

First off, how many engines can pick up almost 100 hp just from a cam swap? This reveals how good those stock 5.3L heads are, especially the exhaust ports. Combine a small displacement engine with decent exhaust and a relatively stout cam and the result is lots of engine speed. This package does sacrifice torque below 3,000 rpm compared with the base engine, which means this engine would be better matched with a manual transmission or at least an automatic with a high-stall converter. Another important consideration is that a stock 5.3L bottom end won't live forever at 6,800 rpm, so save yourself the trouble of writing to us with that bit of wisdom. Clearly, to keep this combination together, the cam should probably be 5 to 10 degrees smaller on the intake side with a single-pattern design, which would help the bottom end torque and only cost perhaps 10 to 15 hp. Or you could add some forged pistons and rods and have a nice day. Either way, a 100-plus-horsepower gain is reason enough to celebrate. But as impressive as this is, there's a lot more left in this garden-variety truck engine. Next month, we'll push it even harder.

(advertised) (@ 0.050) (inches)
Stock 5.3L, intake N/A 191 0.457 114
Exhaust N/A 190 0.466
Comp Cams, intake 283 227 0.563 109
Thumpr exhaust 303 241 0.546

Go With The Flow
Flow Test 1

Stock 5.3L aluminum head
1.89/1.55-inch valves
60cc chamber
205cc intake port volume

Flow Test 2
West Coast ported Stage 1 5.3L head
1.94/1.57-inch valves
60cc chamber
209cc intake port volume

Flow Test 3
West Coast 5.3L CNC-ported Edelbrock
2.00/1.57-inch valves
58cc chamber
200cc intake port volume

Flow Test 1 Flow Test 2 Flow Test 3
Valve Lift Int. Exh. E/I Int. Exh. E/I Int. Exh. E/I
0.100 65 52 80% 65 57 87% 65 63 97%
0.200 133 101 76% 141 109 77% 132 115 87%
0.300 189 134 71% 187 15 181% 193 164 85%
0.400 208 161 77% 227 175 77% 235 192 82%
0.500 219 175 80% 256 198 77% 267 209 78%
0.600 223 187 84% 252 211 84% 284 217 76%
78% Avg. 80% Avg. 84% Avg.
All three heads were tested on a 3.78-inch test bore fixture.

Dyno Numbers
Test 1
was the baseline test for the stock 5.3L with a GM Performance Parts single-plane intake manifold, a Holley 750-cfm carburetor, and a set of Hooker 13/4-inch headers complete with a 21/2-inch exhaust system and Flowmaster Super 44 mufflers.

Test 2 was the same engine with the only addition being the Comp Thumper hydraulic roller camshaft.

Test 3 used the same configuration as Test 2 with the addition of West Coast Racing Cylinder Heads (WCRCH) mildly ported stock 5.3L heads.

Test 4 was the same as Test 3 with the change to a pair of WCRCH fully CNC-ported Edelbrock castings.

1,600 310 94 273 84 287 88 282 87 - 28 - 7
1,800 320 110 289 101 302 105 300 104 - 20 - 6
2,000 325 124 291 111 304 115 303 115 - 22 - 9
2,200 327 137 290 120 303 125 302 125 - 25 - 12
2,400 328 150 292 132 304 138 303 137 - 25 - 13
2,600 329 163 299 147 311 154 309 153 - 20 - 10
2,800 330 176 310 166 322 172 319 171 - 11 - 5
3,000 332 190 324 186 335 193 332 190 0 0
3,200 335 204 338 207 349 213 345 211 + 10 + 7
3,400 339 219 351 228 360 233 356 230 + 17 + 11
3,600 343 235 360 246 367 251 364 248 + 21 + 13
3,800 347 251 365 263 371 267 368 265 + 21 + 14
4,000 349 266 367 279 372 282 370 280 + 21 + 14
4,200 351 281 368 293 372 297 371 296 + 20 + 15
4,400 351 294 368 308 372 312 372 312 + 21 + 18
4,600 350 306 369 324 374 329 375 329 + 25 + 23
4,800 347 317 371 340 379 348 379 348 + 32 + 31
5,000 342 326 375 358 384 367 385 368 + 43 + 42
5,200 335 332 378 375 390 386 391 387 + 56 + 55
5,400 327 336 381 391 393 403 395 405 + 68 + 69
5,600 316 337 381 405 393 417 396 420 + 80 + 83
5,800 304 336 377 415 388 428 392 432 + 88 + 96
6,000 290 332 369 422 380 435 385 441 + 95 +109
6,200 276 326 360 426 370 439 377 447 +101 +121
6,400 - - 351 429 362 443 370 452 - -
6,600 - - 344 432 357 447 365 457 - -
6,800 - - 336 433 349 449 357 460 - -
Avg. 328.6 242.7 342.6 281.8 352.8 289.8 353.2 291.2
Averages for Tests 2 through 4 were from 1,500 to 6,900 rpm, while Test 1 averages were only for 1,500 through 6,300, so these numbers are lower.
Stock 5.3L engine used local yard $550.00
WCRCH ported 5.3L head 2001 WCRCH 946.90*
WCRCH CNC-ported head   WCRCH 2,150.00
GMPP carb intake manifold 88958675 Scoggin-Dickey 369.75
Comp Thumpr cam NA Summit Racing NA
Comp valvesprings 26918 Summit Racing 189.95
Comp retainers 774-16 Summit Racing 63.95
Holley 750-cfm carb 0-4779C Summit Racing 433.95
Hooker 13/4-inch headers 2288-1HKR Summit Racing 659.95
MSD LS1/LS6 controller 6010 Summit Racing 312.70
Fel-Pro intake gasket 1312-3 Summit Racing 25.88
ARP head bolts 134-3609 Summit Racing 145.95
Flowmaster mufflers 952548 Summit Racing 79.95 ea.
* Priced with supplied core head
Autotronic Controls Corp. (MSD)
El Paso
Holley Performance Products
1801 Russellville Rd.
Bowling Green, KY 42101
KY  42101
Federal-Mogul (Fel-Pro)
Scoggin-Dickey Parts Center
Summit Racing
P.O. Box 909
OH  44309-0909
GM Performance Parts
West Coast Racing Cylinder Heads
18405 Hart St., Dept. GMHTP
CA  91335
  • «
  • |
  • 1
  • |
  • 2
  • |
  • 3
  • |
  • 4
  • |
  • 5
  • |
  • 6
  • |
  • View Full Article
Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!
Tony Lainhart
Tony Lainhart

Im still coming around to center bolt valve covers and serpentine belts...CarCraft is the only mag besides an occasional HotRod Ive stayed with since a kid in the 80s...I need a complete tutorial on the LS series-If you haven't already, Im sure your the mag to do it!

Michael Rapley
Michael Rapley

1955-87 SB Chevy, the New flathead's of this decade & Forever.

Steve Cardot
Steve Cardot

small block chevy's are great engines don't get me wrong but the LS engines are my all time favorite.

Derek van Valen
Derek van Valen

stock ls2 long block, cam,springs,retainers,pushrods, carb and set of headers. 470 to the tire in 94 degree shop. thats about 540 at the crank. was rated 395 stock in the tb ss it came from. glad i didnt have to buy heads lol.

John Smith
John Smith

Pound for pound the LS has has more potential than the Gen 1 as far as a production engine goes based on the heads alone, but I haven't seen a decline in interest for the Gen 1. Mostly because of the price to build difference.

Cam Beaton
Cam Beaton

The small block has gone the way of the flathead. RIP SBC.

Scott Alan Carpenter
Scott Alan Carpenter

Give Mast Motorsports a call at 936-560-2218 or check out They designed their own VVT cams 18 months before anyone else, they have over a dozen intake manifolds and many cylinder head designs which they designed and cast in-house, they engineered their own EFI system, etc. Thanks to Horace Mast's genius and an amazing team, they are THE ones to watch in the LS arena.

Zane Lund
Zane Lund

Beth Tubbs had to take a double take!!

Walt Nothnagel
Walt Nothnagel

I got more out of my 327 by shaveing the heads and decking the block with a cam swap

John Baechtel
John Baechtel

Gen 1 all the way for me. I appreciate LS motors, but don't see any particular advantage beyond all the media hype or if you just have to have an aluminum engine. The current piston powered land speed record is held by an iron block Gen 1 small block thumping about 2300 HP. Nothing really bad about the LS except poor crankcase breathing and that's why Dart makes an LS block configured like a Gen1 engine on the bottom end. Its generally more expensive to build an LS from scratch so why bother if the standard small block will do the job. Cylinder heads are no longer an issue in most cases. so it all depends on what you're building for and what your budget will support.

Josh Lindauer
Josh Lindauer

LS all the way! 6 bolt mains and just an all around better engine! I use to be into the gen 1 style blocks, but after the LS conversion in my ride I'm sold on the these LS setups!

David Leeman
David Leeman

I say fire it up and let those horses run wild

Rafael Torres
Rafael Torres

Ahh thanks. Thought it was an error. Still funny ..I guess he gained what he was looking make me laugh lol..

Brian Reineking
Brian Reineking

Call 847-956-1244! Automotive Engine Specialties is the king of the LS world!

Elijah Rose
Elijah Rose

dont really care at this point. as long as it can growl and it can get me where i need to go

Arnold AutoNtow
Arnold AutoNtow

I sold all my old school sbc stuff to make room for more LS parts lol

Karl Loper
Karl Loper

It's not rocket science is what I say. Modern heads and cam should make 120hp on just about any V8 over OEM heads/cam. Look what AFRs do for an SBF, or Edelbrock Performer RPM on an SBC.

Ricardo Neves
Ricardo Neves

simply with port and polished bored heads and with a more agressive cam angle lobes in the camshaft.

Andy Martin
Andy Martin

Rafael, that's common vernacular when one is joking. It's not an error.

Chris Pirkey
Chris Pirkey

Late model Ls with a carb conversion kit...the original intake is on the back table

Timothy Hickman
Timothy Hickman

LS is the future, but there aint a damn thing wrong with a sbc

Jay Meilleur
Jay Meilleur

Probably gain 100hp just putting a cam in a LS motor.

Connor Doyle
Connor Doyle

It looks like he's got an invisible ratchet about to tighten something.

Rafael Torres
Rafael Torres

Lol what say you ? .... or what do you say? Lol..

Car Craft