Tech Articles

Long Rod Chevy 307 = Big Power

(Image/Jim Smart)

Chevrolet’s 307ci small block has long been viewed as a mild-mannered grocery getter. You’ve seen hundreds of thousands of them in Chevelles, Novas, Camaros, and untold other mainstream Chevys. We used to call them “Belly Button Motors” because everyone had one—right? I recall new 1968-73 Chevelle wagons, sedans, and hardtops up and down our Maryland block with 307s. They were good solid dependable engines with a long legacy of service.

Jeff Latimer, a savvy engine builder and racer who used to manage JGM Performance Engineering in Valencia, California, came to us with an intriguing engine build project no one had ever tackled before. This one was the humble Chevrolet 307ci small block, which replaced the venerable 283 that had been in production since the 1950s. Like Ford with the 289 turned 302, Chevy stroked the 283 to 307ci. Chevy borrowed the 327’s crank, which made it easy to increase the 283’s displacement to 307.

“I’ve never understood why Chevrolet never hotrodded the 307, which had the same kind of potential as the 283. Chevrolet produced the 307 from 1968-73, which means there are a lot of cores out there waiting to be rescued and built. Chevrolet never did a high-performance 307 like it did with the 283. Right off the assembly line, the 307 delivered 200 horsepower and 300 lbs.-ft. of torque. Marine versions with four-barrel carburetion made 240,” Jeff commented.

Jeff Thought He Could Double the 307’s Factory Ratings.

The 307 never caught on as a performance engine because it was so easily replaced with a 327 or 350, so why bother? Most opted for the larger 350.

In Jeff’s mind, the 307 was a sleeper. It could be built to make a lot of power and, because cores are so plentiful and cheap, you could get into one for not much money. It could make a lot of power yet be fuel efficient for the daily driver. The 307 has the bore advantage over the more popular 305/5.0L engine—3.875 inches over the 305’s smaller 3.736. Jeff was positive more could be done with the 307. What’s more, the 327’s 3.250 inch stroke made the 307 a clear winner. Plenty of low-end torque.

So, What Can You Do with the 307?

Because this is not a budget build, but more an experiment, not many will be interested in trying this. Jeff tells us he has witnessed many 307 builds, but not to the extent of what he was about to build. He wanted to make the most of 307ci, with a 0.030 inch overbore to 311ci, with 3.905 inch bores.

His original intent was a street/strip engine. Civil for the street and coming on strong at wide-open throttle for the freeway and strip.

Jeff could have easily infused a lot of compression and cam into the little 307 and built a monster mill. He wanted a civilized street engine with a good torque curve and high rpm horsepower. He wanted double that 200 factory horsepower number.

“If we had put this much effort into a 350 or 383 build it would obviously make more power,” Jeff told us, but that’s not the point of this build. The point is how to make real power from 311ci. “To increase the piston dwell time, I have opted for a long 6.250 inch rod combined with custom pistons with a 1.125 inch compression height to get 10.75:1 compression,” Jeff commented.

Jeff opted for Trick Flow (#30310003) cylinder heads and intake, which were ported for a perfect match and improved flow. These fabulous heads could have been used out of the box, however, Jeff wanted maximize the 307’s potential with a good port job. He opted for a small journal 327 forged steel crank with bearing spacers for the large main journal block. The block was align-honed and fitted with ARP studs. The bottom end was balanced and fitted with super light bob weights at 1,550 grams. Jeff quips, “I have seen aluminum rod engines with a heavier weight than that!”

Because these Trick Flow heads are super efficient at more than 75 percent efficiency intake vs. exhaust, Jeff chose a single pattern Crower hydraulic roller cam with 228/228 duration on 108 degree lobe centers. And because this engine will still have more exhaust cam than it needs, Jeff went with 1.6 ratio Trick Flow rockers on intake and 1.5 on the exhaust. Because this is a hydraulic roller cam, it requires a cam button. Jeff opted for a cool aluminum timing cover.  

“The block was going to be close to zero deck with a 0.030 inch thick head gasket,” Jeff comments, “That made for a real tight quench that improved efficiency.” Jeff chose ARP fasteners for added security. Jeff started off with a 750 cfm Holley HP carburetor with vacuum secondaries and a GM HEI distributor modified with MSD ignition components.

The 307’s First Dyno Session

Jeff’s first trip to the dyno netted 401 horsepower at 6,400 rpm and 372 lbs.-ft. of torque at 4,600 rpm, which doubled factory 200 horsepower ratings. His efforts proved out as predicted at twice the factory numbers. Despite Jeff’s best effort, he understood there was a lot of power sleeping in the petite Chevy small block—more than he even knew or expected.

What Jeff did know during that first dyno session was he didn’t have enough cam to make real power. He performed a simple cam swap to a Comp Cams 242/248 duration (0.576/0.562 inch lift intake/exhaust) with 110 lobe centers (#12-443-8) and an intake swap to an Edelbrock single plane Victor Jr. with an in-depth port job along with a Wehr’s carb spacer and a Melling billet one-piece oil pump with Shark Tooth Technology. He managed a whopping 70 horsepower increase. That netted 471 horsepower at 6,500 rpm with torque coming in at 409 lbs.-ft. at 5,500 rpm. That indicated 1.5 horsepower per cubic inch.

A Quest for 500 Horsepower

Being this close to 500 horsepower, Jeff wasn’t lifting now. The 311ci engine was already producing more than 1.5 HP per cubic inch. That’s when he became hell bent to pass the 500-horse mark. Jeff put his years of engine building experience to work. It was his belief we needed more rpm to pass 500 horsepower. He substituted the hydraulic roller tappets with mechanical solid rollers from Comp Cams with the hydraulic roller cam he already had. “The hydraulic cam does not have clearance ramps so I set the lash at zero cold. I installed new valve springs with titanium retainers that would tolerate the higher rpm ranges expected. The greater spring pressure and solid rollers would allow the engine to rev higher than before.”

Jeff admits he became greedy about power. He wanted more and without pulling the heads. He had the intake manifold ported a bit more and added half a ratio to the rockers—1.6 intake and 1.5 exhaust. “Since the cam had lash it became slightly smaller. I was trying to help it to stay closer to the way it was before lifter change. We also added a complete MSD ignition system to keep everything lit at high revs.”

Dyno Run #2

Upon the next trip to the JGM Performance Engineering Dyno, the upgraded 311 came up a bit short at 486 horsepower at 7,200 rpm with a loss in torque to 390 lbs.-ft. peak.

That’s 20 lbs.-ft. off in torque from where it was with the hydraulic lifters on same cam. Horsepower was up 15, but shy of the mark at 486. Jeff was going to close the gap.

“What I feel I learned at this point is that because the cam was essentially the same specs as before, the torque number should not have dropped that much. That’s when I concluded the additional port work on the intake manifold hurt velocity because the engine was small at 311ci,” Jeff commented, “With the goal of 500 horsepower so near, I was always aware we would lose torque in the process.”

Jeff went on to say, “I switched to a different Victor Jr. with only port matching to get velocity and torque up. There was also a last minute cam change from 242/248 on 110 lobe centers to a Comp Cams solid roller cam—254/260 on a 110 lobe centers. I also changed rocker ratio to 1.5/1.5 intake/exhaust.” He added, “I also added a better Canton oil pan with a windage tray and full kick out from Canton.”

Third Dyno’s a Charm

Jeff went back to the dyno with fresh 5w30 Mobil1 synthetic, then warmed up the engine. That first pull was 498 horsepower at 7,200 rpm—mighty close to our goal. Jeff knew it was going to take higher rpms to achieve 500. He dialed in 35 degrees of total timing and pushed it to 7,400 rpm and got 505.5 horsepower and 394 lbs.-ft. of torque at 5,900 rpm.

“Very pleased with the results,” Jeff told us, “We’ve made more than a hundred horsepower from the 307/311 without ever pulling the heads.”

“Throughout the process, we performed several timing test pulls. Total timing of 35 degrees BTDC was all it needed. This, combined with the small chambers, gave us nice tight quench with flat top pistons, which made this engine very efficient, Jeff told us, “The long 6.250 inch Crower rods had the pistons dwelling much longer at both TDC and BDC. We ran two different Holley Carbs to check out the differences. One was a Holley HP 750cfm with vacuum secondaries while the other was an 830 cfm HP.”

He added the two highest horsepower numbers were with the 830.

“In my opinion, the best combination was when we made 471/409 horsepower and torque. That would be best combo to use on the street for both the commute and play,” Jeff commented, “I believe if I put the hydraulic roller back in it with the better valve springs and the Canton oil pan, we would conservatively make 475+ horsepower and 410 lbs.-ft. of torque. Not bad for the little 307 Chevy.”

Jeff went on to say “with the smaller 750 HP with vacuum secondaries, it would use less gas than a 350. Good for the street. The most impressive part of this engine making so much power is that the bore is so small at 3.905 inches and the intake valve equally as small at 1.940.”

Closing Thoughts on the Chevy 307 Engine Build

Jeff reflects on this purely experimental project.

“This project was never meant to be a budget build. It was darned expensive. It goes without saying if I had done a similar build with a larger engine like a 350 or a 383, we would produce much more power. My objective was to make something special out of an odd-duck, no respect engine that has been normally cast aside for one of its bigger brothers. We wound up with 1.625 horsepower per cubic-inch! This is a street engine with only 10.75 to 1 compression, which will run well on pump gas and nobody will ever believe it’s a 307.”

If we hadn’t seen it ourselves, we wouldn’t have believed it.

Jeff’s working with a garden variety #39770020 307ci Chevy block of which millions were produced for pavement duty 50 years ago. Because there are so many 307 cores out there, this was a good core to play with. When Jeff began this effort, his goal was to double factory horsepower ratings. He didn’t expect 500 horsepower. (Image/Jim Smart)
Jeff rounded up a 327 steel crank with a 3.250 inch stroke (same as the 307’s) for this effort. He did not want to trust a cast crank. He wanted a bulletproof bottom end that could withstand a hammering on the dyno. He opted for 6.250 inch Crower I-Beam rods to get piston dwell time up at each end of the bore. RaceTec custom pistons were sourced to work with the Crower rods. (Image/Jim Smart)
This is not a budget build by any means, but instead a research project to see what could be done with Chevrolet’s bread and butter 307. These long rods fit nicely within the 307 block enabling the 307 (bored to 311ci) to make all kinds of power. (Image/Jim Smart)
JGM Performance Engineering in Valencia, California has thoroughly cleaned the block and performed all necessary machine work. ARP main studs have been installed for good structural integrity. Bearing spacers had to be used because the small main bearing 327 crank (prior to 1968) will not just drop into a large main bearing 1968-73 307 block. (Image/Jim Smart)
Jeff’s first cam of choice was a Crower roller hydraulic, which wasn’t as aggressive as he would have liked. A more aggressive Comp Cams roller hydraulic would gain Jeff some 70 horsepower along with induction changes. (Image/Jim Smart)
Two-piece rear main seals can be problematic. Jeff’s approach is a thin bead of Permatex’s The Right Stuff between the seal halves and the block/main cap, then, a daub at the seal ends, delivers leak free performance. Seal lips must be pointed toward the crank. Note the bearing spacer between the bearing and block. (Image/Jim Smart)
Main studs are torqued to specifications, with Jeff checking for freedom of rotation with each cap. Crank endplay is also checked. (Image/Jim Smart)
Crankshaft endplay is checked and checked again. Jeff ran into endplay issues and had to loosen the main caps and work with it again. Ideally, you will have 0.004 to 0.008 inch of endplay. (Image/Jim Smart)
Connecting rod side clearances must always be checked. Side clearances can range significantly. Jeff’s came in at 0.018 inch. (Image/Jim Smart)
Jeff is using a billet piston ring compressor to assemble the short block. Summit Racing stocks a huge variety of piston ring compressors for your home garage. Billet compressors make more sense for a machine shop. Adjustable compressors are more applicable for home use. (Image/Jim Smart)
When it’s time to assemble the short block, Jeff stresses the use of brass or stainless steel block plugs, which tend to be corrosion proof. They outlast steel plugs. For locations like this, opt for wider freeze plugs, which are more secure. Some builders prefer JB Weld for block plugs, which stay secure at high revs. (Image/Jim Smart)
Here’s a comparison of a stock oil filter mount with the bypass versus the Canton billet mount without a bypass. (Image/Jim Smart)
Oil pumps should never be installed right out of the box, though a lot of builders do that. Jeff checks tolerances and the oil pressure relief valve for proper function, then fills the pump cavity with engine assembly lube for a good wet start-up. (Image/Jim Smart)
Jeff opted for a Cloyes dual-roller timing set to reduce internal friction and deliver precision timing. A cam button becomes necessary when you opt for a roller cam in the small block Chevy. (Image/Jim Smart)
True TDC is checked prior to dialing in the cam. Jeff also checks deck height on all eight cylinders. (Image/Jim Smart)
Every cam should be degreed-in, even if it’s the cam that came out of the engine. You will want to see how it measures up to the manufacturer’s cam card. Jeff ran a total of three different cams in this 307 in his quest for 500 horsepower. (Image/Jim Smart)
Jeff stresses using The Right Stuff between the head and block right here to prevent oil leakage. This is a hot spot for oil weep at the valley and block end rails. (Image/Jim Smart)
Because the small block Chevy is a wet deck, Teflon sealer must be applied in the bolt holes to keep coolant where it belongs. (Image/Jim Smart)
This is the Trick Flow Super 23 175 cylinder head for the 307/311. What you get for your hard-earned bucks is performance that closely matches expensive CNC-ported cylinder heads—and for about the same price as cast off-the-shelf heads. These heads are engineered to outperform factory small block Chevy heads on 283 to 350ci high-performance engines. The Super 23 175 heads feature small cross-section intake runners to promote low-rpm torque and high-rpm horsepower on small bore engines like the 283 and 307. Jeff had custom port work done on these heads for even greater performance numbers. (Image/Jim Smart)
Check out the custom port work Jeff had performed on these Trick Flow castings. They perform exceedingly well right out of the box. Jeff wanted to take them to the limit. (Image/Jim Smart)
Jeff tells us the small-block Chevy’s intake/exhaust efficiency is so good he had to reduce exhaust valve lift using 1.5 ratio on exhaust and 1.6 on intakes. (Image/Jim Smart)
We like this StreetBurner Trick Flow dual-plane intake manifold. It offers a broad power band of 1,500 to 6,500 rpm, which means it is optimum for street and strip. The open air design keeps the air/fuel mixture cool for greater power and torque numbers. It also has provisions for nitrous nozzles and additional material for custom porting. You can run carburetion, as Jeff is doing, or throttle body injection. Jeff is running a couple of different carb spacers to see what they do for power. (Image/Jim Smart)
Jeff didn’t limit port work to just the heads, he had the Trick Flow intake custom ported and port matched for smooth abundant flow. Jeff believes in the benefits of carb spacers. (Image/Jim Smart)
We’d like to take this time to talk about gasket technology. Summit Racing stocks a wide variety of terrific silicone gaskets for the small block Chevy. This advanced technology ensures leak-free performance at the valve covers and at the oil pan. (Image/Jim Smart)
Jeff worked with two Holley HP carburetors for this exercise in power. He began with the Holley HP Series 750 cfm with vacuum secondaries followed by the 830 cfm to see what would happen. Remarkably, he was able to close the 500 horsepower gap with the 830. The 750 makes more sense for street use. And honestly, do you need 500 horsepower for the commute? (Image/Jim Smart)
We’ve opted for a stock GM HEI distributor with an MSD Digital HEI Kit. It’s an easy modification to make and it delivers a hot consistent spark for a healthy cold start, keeping the fire lit at high rpm. Jeff did try an MSD ignition system as he neared 500 horsepower, but found he never needed it. (Image/Jim Smart)
To get the 307/311 over the 500 hp mark, Jeff went with a custom ported Edelbrock Victor Jr., a Wehr carb spacer, an 830 cfm Holley HP, along with hotter cam profiles and even mechanical roller tappets. The result in this form was 505.5 horsepower at 7,400 rpm. (Image/Jim Smart)
Jeff will tell you this was not a budget build in light of what it cost to push the 307/311 over 500 horsepower. “I still believe the best combination of parts would be the second hydraulic roller cam we tested back in with the hydraulic roller tappets. I would also keep the Canton pan and the port-matched Victor Jr. intake with the GM/MSD HEI ignition and 750 Holley HP carb, which would make 475 plus horsepower and 410 lbs.-ft. of torque. This would be a better engine combo to actually use on the street,” Jeff commented. (Image/Jim Smart)

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4 Comments

  1. Impressive build.

  2. Loved the article. Very informative. Makes me want to build a 307 just to have someone ask What Are You Running for a engine??? Very nice build. Thank you

  3. Dale Savidge says:

    This hits close to home. My ‘69 Chevy C10 has the original 307, 2 barrel with an HEI system and I’ve always felt like it has a lot more “tiger in its tank” than I’m giving it credit for!! Thanks for sharing

  4. With those heads and a mild cam – .420 lift it would make a great cruiser. That and a freeflowing exhaust with 3.08s and a 4 speed.

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