Bob Lathrop's Modest 351 Windsor - Wicked Little Windsor
Bob Lathrop's Modest 351 Windsor Makes 536 Hp On Pump Gas, And Leaves Plenty Of Big-Blocks In The Dust.
From the April, 2008 issue of Popular Hot Rodding
By Daryl White
Photography by Johnny Hunkins
In the rural farm country of Kalona, Iowa, you'd expect to hear the monotonous drone of a massive John Deere combine working the fields. But on a good day, you can hear the raw horsepower of Bob Lathrop's badass 351 Windsor punching through the air.
Bob built the AFR-headed engine at his shop, Performance Unlimited, where he works on cars during the day and burns the midnight oil building race engines. He says there is just not a big enough market in the area to support a full-time engine shop, so he does what he has to, in order to pay the bills and keep the wolves at bay.
Let's back up a minute, and start at the beginning. Last spring, one of Bob's customers asked him to build an engine for a '67 Fairlane. Bob had wanted to build an engine for the Engine Masters Challenge, so why not build a 351 Windsor that would satisfy both? A project was born.
OEM small-block Ford blocks...
OEM small-block Ford blocks aren't known for being very stable or strong under high-torque loads so Performance Unlimited ran a main girdle from PRW as well as ARP main studs. The girdle effectively joins all the mains together, distributing the torque load and reducing stress in the block.
Bob and his crew, Jim Keifer and Shawn Horras, found a '71 351 Windsor core from an old van that was in pretty decent shape. The early blocks are known to be way beefier than the later stuff, so he figured it would hold the 500-plus horsepower he was shooting for. For that power range, a main girdle from PRW was added for insurance. A call to the Eagle tech line confirmed that a plain-Jane set of SIR 5.959-inch Windsor rods would easily handle that load.
With an OEM Ford crank resting on a set of Clevite bearings, he found the clearance was dead-on out of the box at .0022 inches on the mains and .0020 inches on the rods. Crank thrust was good at .004 inch, and rod side clearance was again spot-on at .020 inch.
A Moroso front-sump pan holds the Pure Power synthetic oil that Bob likes to run. The Melling M-Select pump runs that oil through a Pure Power modular oil filter before feeding the bearings.
A Moroso oil pan (PN 20507)...
A Moroso oil pan (PN 20507) does a great job of keeping the pump supplied with fresh lube. It's been argued that in relatively low-rpm engines like these, an elaborate system of anti-windage devices doesn't really show any benefits. Bob and the crew from Performance Unlimited chose to forgo a windage tray and scraper.
"There really isn't anything trick in the engine," Bob claims, though he admits that the pistons are, in fact, pretty trick. "Besides the heads, the pistons are the single most expensive part of the engine," Bob says. He had the gurus at JE whip up a really nice set of lightweight forgings that gave him plenty of piston-to-valve clearance, a very streetable 10.48:1 compression, and a 1.2mm, 1.5mm, and 3.0mm ring package. That package has a back-cut, moly-faced top ring with a .020-inch gap, a back-cut, napier-style second ring with a .022-inch gap, and a low-tension oil ring. A good portion of the friction in an engine comes from the rings, and using the low-tension oil rings and napier second really helped out, in this case dropping the amount of torque required to rotate the short-block to 16 ft-lb.
COMP Cams ground Bob a hydraulic flat-tappet camshaft with 240/246-at-.050 duration, and .576-/.583-inch lift on a 108 lobe separation. COMP also supplied the 1.6-ratio Magnum series rocker arms for this deal.
Oh yeah, and the heads? How about a set of AFR 205 CNC-ported heads that use a proven porting program and flow big numbers right out of the box? Not wanting to get the runners too big, Bob did almost no work on the heads. A small amount of epoxy was used to blend a variance in the pushrod restriction, and just a few minutes were spent with a cartridge roll on the short-side radius, masking the parting line where the CNC program switched from the intake side of the runner to the chamber side. These little touches brought the flow numbers up a solid 10 cfm on the intake. AFR has some of the most efficient combustion chamber designs on the market, and they were left as-is. The exhaust ports were also left untouched. Bob used the valves and retainers that came with the heads, and only changed the springs to a set from COMP that was set up with 140 pounds on the seat and 380 open. That kind of spring pressure can be rough on a flat- tappet cam, so the break-in was done with care.
The modular Pure Power oil...
The modular Pure Power oil filter uses a CNC-machined aluminum case to hold a fine stainless steel mesh oil screen. The unit flows double the filtered oil compared to a paper element.
Tuning this relatively modest combination is what really made the engine shine. "We ran the Innovate ST-12 wideband system and had an O2 sensor in each header tube," Bob says. Bob claimed earlier that the only trick parts of the engine were the pistons, but it's clear that he spent a bit of time on the Edelbrock Performer RPM intake. "I have about 20 hours of work in that intake, raising roofs and reshaping plenums," Bob says. One thing Bob learned about the Performer RPM is that cylinder number one wanted to be lean, and number eight liked being rich. He played around with various dams, and moved some of the port walls around, managing to even out the fuel distribution and bump up the power curve at the same time. "My goal was to get the cylinders to behave as close to the same as possible," Bob says. Playing with valve lash had almost no effect, and he even tried to run a hotter spark plug in the richest hole to combat a possible fuel-puddling problem. Dyno testing with a Victor Jr. never showed the same broad power that the Performer RPM did. The Victor Jr. was a little higher in peak power, but not enough to justify its use.
The custom JE pistons hang...
The custom JE pistons hang on a set of Eagle SIR rods. Low-tension rings greatly reduced the friction in the engine, freeing up precious horsepower. With many OE auto manufacturers switching over to lower-tension rings, there is a greater number of hot rodders using them without the old-fashioned fears of oiling problems in the combustion chambers.
A good part of the tuning effort was focused on the Charlie Morgan-built 1070 carburetor. Staggering the jets and air bleeds really helped cylinder-to-cylinder variance, and Bob dipped into some emulsion jet changes to even things out. "When I changed the emulsion jets, I got rid of a lean condition I had on all eight cylinders on tip-in," Bob says. Bob tried running an 850 annular booster carb, and it had a little better fuel curve, but just didn't make the power of the 1070. Referring to the ups and downs of power versus efficiency, Bob sighed and said: "That's the game we play. It's a trade-off."
All of Bob's hard work was rewarded with big power numbers, 536 peak horsepower and 488 lb-ft of torque to be exact. Did we mention this was on 91 octane pump gas and all at below 6,500 rpm? Again, with a flat-tappet hydraulic cam.
Bob's philosophy of building engines like this has always been to focus on a combination that's really responsive and torquey. "I've always been a torque guy," Bob confesses. "I think that's what got me into fifth place two years ago," he says, referring to his extremely respectable showing at the 2006 Engine Masters Challenge. That philosophy keeps his customers happy and his business growing.
Bob's engine build sheet sure might not look like it has very many trick parts, but he's proven that he knows how to put together a solid combination that will leave ordinary 351W pump-gas engines in the dust!
An efficient combustion chamber...
An efficient combustion chamber allows the tuner to use less ignition timing to make the most power. Using the least amount of ignition timing that makes the most power frees up the engine, as the rising piston does not have to fight as hard against the growing flame front as the piston approaches top dead center.
COMP ground a relatively short...
COMP ground a relatively short duration for the engine. A tight 108 lobe separation helps draw the intake charge through exhaust scavenging, and the early 102 intake centerline helps to start compressing that charge quickly, building cylinder pressure.
A PRW stud girdle held a good...
A PRW stud girdle held a good set of COMP Magnum 1.6 rocker arms. You can still see the CNC tooling marks on the intake runners indicating how little the Performance Unlimited crew had to do to make these heads really perform. Bob indicated that he thought these heads would also do well on an even smaller engine like a 331.
Using a hydraulic flat-tappet...
Using a hydraulic flat-tappet cam with aggressive lobe designs forced Bob and his team to reevaluate the springs that came standard with the AFR heads. A spring with too little pressure might not control the valve and could lead to valve float problems, while too much pressure could lead to excessive friction and a flat cam lobe.
The intake manifold was the...
The intake manifold was the source of many late work nights. Dyno testing modifications included temporarily epoxying popsicle sticks in various port locations to better control air and fuel movement. These were later taken out and permanently replaced with epoxy dams.
Modest by nature, Bob showed...
Modest by nature, Bob showed his tuning prowess by working the carburetor to its limits. Staggering the jets, staggering the air bleeds, and performing metering block and emulsion jet changes finalized the tuning changes.
The HVH Super Sucker carb...
The HVH Super Sucker carb spacer has proven itself to make power in a number of engine combinations. That was again the case here. Signal to the boosters is increased at a lower rpm, allowing the boosters to activate and get into a stable fuel flow early on.
These Mac long-tube headers...
These Mac long-tube headers were designed to fit a 351 Windsor engine into a Fox-body Mustang. The O2 sensors in each tube allowed Bob to fine-tune each of the cylinders. Final dyno testing was done with MagnaFlow mufflers bolted to the collectors.
Providing an accurate and...
Providing an accurate and powerful spark is important when running an engine with high cylinder pressure. An MSD 6AL fed the Blaster coil, and in turn, pushed the spark to the Pro Billet distributor. An old trick in fine-tuning the Pro Billet distributor is to slightly bend one or more of the reluctor teeth in order to advance or retard timing.
|BY THE NUMBERS |
|PERFORMANCE UNLIMITED 358CI SMALL-BLOCK FORD |
|Bore: ||4.030 inches |
|Stroke: ||3.500 inches |
|Displacement: ||358 cubic inches |
|Compression ratio: ||10.48:1 |
|Camshaft: ||COMP hydraulic, flat tappet |
|Cam duration: ||240/246 degrees |
| ||at .050-inch tappet rise |
|Cam lobe lift: ||.360/.364 inch |
|Rocker ratio: ||COMP Magnum 1.6 ratio |
|Lobe separation: ||108 degrees |
|Installed centerline: ||102 degrees |
|Top ring: ||1.2mm JE |
|Top ring gap: ||.020 inch |
|Second ring: ||1.5mm JE |
|Second ring gap: ||.022 inch |
|Oil ring: ||3mm JE |
|Piston: ||JE, dished top |
|Block: ||OEM Ford 351W |
|Crankshaft: ||OEM Ford 351W |
|Rods: ||Eagle SIR 5.959-inch I-beam |
|Main journal: ||2.620 inches |
|Main bearing clearance: ||.0022 inch |
|Rod journal: ||2.070 inches |
|Rod bearing clearance: ||.0020 inch |
|Cylinder head: ||AFR 205 |
|Peak intake flow: ||295 cfm |
|Intake valve diameter: ||2.080 inches |
|Exhaust valve diameter: ||1.600 inches |
|Intake manifold: ||Edelbrock Performer RPM |
|Carburetor: ||Holley 4150 modified to |
| ||1,070 cfm by Charlie Morgan |
|Header: ||Mac 1 3/4 Fox-body Windsor; |
| ||3-inch collector |
|Ignition: ||MSD 6AL |
|Damper: ||TCI Rattler |
|Water pump: ||Meziere |
|DTS DYNO DATA |
|BEST QUALIFYING PULL |
|RPM ||TQ ||HP |
|2,500 ||364 ||173 |
|2,600 ||374 ||185 |
|2,700 ||383 ||197 |
|2,800 ||393 ||210 |
|2,900 ||404 ||223 |
|3,000 ||413 ||236 |
|3,100 ||419 ||247 |
|3,200 ||423 ||257 |
|3,300 ||421 ||265 |
|3,400 ||416 ||270 |
|3,500 ||411 ||274 |
|3,600 ||410 ||281 |
|3,700 ||412 ||290 |
|3,800 ||418 ||302 |
|3,900 ||427 ||317 |
|4,000 ||439 ||334 |
|4,100 ||451 ||352 |
|4,200 ||461 ||369 |
|4,300 ||469 ||384 |
|4,400 ||477 ||399 |
|4,500 ||481 ||412 |
|4,600 ||485 ||425 |
|4,700 ||486 ||435 |
|4,800 ||487 ||445 |
|4,900 ||488 ||455 |
|5,000 ||487 ||464 |
|5,100 ||486 ||472 |
|5,200 ||486 ||481 |
|5,300 ||484 ||488 |
|5,400 ||481 ||495 |
|5,500 ||479 ||502 |
|5,600 ||477 ||509 |
|5,700 ||474 ||515 |
|5,800 ||470 ||519 |
|5,900 ||466 ||524 |
|6,000 ||463 ||529 |
|6,100 ||457 ||531 |
|6,200 ||451 ||532 |
|6,300 ||445 ||534 |
|6,400 ||439 ||535 |
|6,500 ||433 ||536 |