The 350 in Bill Hedekin’s 1967 Corvette Backfires, Stalls, and Uses Oil. Time for HOT ROD to Fix It.
Bill Hedekin’s 1967 Corvette should have been a runner. Once a late-1960s-style SCCA B/Production road racer, the car was returned to street duty, still equipped with a vintage 355ci Mouse with fuelie heads and vintage Edelbrock Torker single-plane intake—but upgraded and rebuilt with Total Seal piston rings, a modern Comp Cams Magnum 280HR hydraulic roller cam, an MSD-6AL ignition, and a FAST EZ-EFI self-tuning, throttle-body fuel injection. Power is transferred back through an M21 Muncie close-ratio, four-speed manual trans to a 3.73:1 Posi IRS.
After the engine refresh, “The car never seemed to be right,” Hedekin says. “It popped and banged whenever I backed off the gas, like a backfire from an old-fashioned carburetor. It always starts fine, but cuts out under throttle. There’s always oil on the [spark] plugs. I’ve changed the plugs myself three or four times, even tried specialty plugs. Several shops tried to tune it. Nothing really helped.” In his rescue request interview, Hedekin primarily blamed the EFI, mentioning the oil-fouled plugs only in passing. When pressed, he allowed, “The car does burn some oil—probably 1 quart every 100 miles.”
We sent the Diamond Bar, California–based car to Rollings Automotive, a full-service shop in Mira Loma, California. Owner Norm Rollings immediately homed in on the oil consumption—not the EFI—as the primary stumbling block. Explains Rollings, “You don’t lose that much oil unless there’s something physically wrong. EFI won’t work right with an oil-use problem. Oil attacks the O2 sensor, which messes up everything, including the system’s self-tuning ability. We needed to find out where the oil was coming from.
“The first step was to look at engine vacuum. A vacuum leak can suck oil into the [intake] manifold runners, from a poor manifold-to-head intake-surface seal or even the wrong intake gasket. That can be tough to find because it can be sucking in from the bottom (the valley side, underneath the manifold). But vacuum was still about 15 inches, even though a vacuum hose had fallen off.
“The disconnected hose did cause the fuel pressure regulator to go to maximum pressure at idle, which will contribute to the driveability issues, but sticking the hose back on produced no significant change in vacuum or driveability for the moment.”
“We then pulled the [spark] plugs. They were really contaminated with oil. We did a leakdown and compression test. The cylinders had good compression and only 8- to 10-percent leakage, which is about ballpark for Total Seal rings on a street car, so it probably wasn’t the rings or valve seats.”
“Next we pulled the valve covers to check the valve-guide slop on the engine’s rare camel-hump factory heads. To measure this, we had to pull the valvesprings. In the process, we found a broken inner damper. This made us question the integrity of all the springs.”
Hedekin says the springs were “an old Z28 design that fit the stock valvespring pocket.” Z28 springs were designed for the old lazy stock solid lifter cams and they don’t really have enough pressure to support today’s hydraulic roller cams.
Springs removed, Rollings observed, “The heads still had old OE umbrella valve seals. Today we machine the guides for positive-stop seals. They offer better oil control in all cases, but with Hedekin’s high-lift cam, positive-stop seals are mandatory: The big lift was crushing the seals into the top of the valveguide; we could visually see there was contact. The old seals could only support around 0.470 lift, but Hedekin’s cam had 0.525 lift. But at least the heads’ bronze valve guides were still nice and tight. So in terms of oil consumption, the only problem was retainer-to-seal interference. The crushed seals let oil run right down the guides. Engine vacuum sucked oil down the valve stem into the engine.”
The minimalist (and more affordable) approach is modifying the existing heads to accept modern springs and positive-stop–style valve seals. The Daddy Warbucks method is a top-end swap to modern aftermarket aluminum heads. We’ll show you both ways to go.
The Fix: Existing Heads
“Original positive-stop or ‘PC seals’ were made from Teflon,” Rollings explains. “They’re OK for the race engines they were originally developed for, but today’s street motors don’t like a pure Teflon-based seal. They’re not malleable enough to conform to the valve stem and can also wear the stem where the seal’s ‘cylinder’ rides. Instead we use a Viton-based seal, with a spring around it for radial tension. It doesn’t harden, and it’s not affected by heat-cycling. It wipes oil off the stem like a windshield-wiper blade wipes water off the windshield.” Modifying the valve guides for positive-stop seals is easily done at home with a hand drill and the appropriate arbor and cutter tooling available through Comp Cams.
The valvespring issue was harder to resolve. Back in the day, the usual approach was machining the spring pockets oversize to accept double (or even triple) springs. But this doesn’t always work for all heads. Hedekin had No. 492 castings; those made before 1972 have thin spring pockets and machining them with a traditional flat seat cutter may break through into the water jacket. Special stepped cutters are available, but Rollings says they’re not compatible with his preferred modern, hydraulic-roller-cam-compatible spring designs. In any event, there was also insufficient material between the outer pockets and head-bolt holes on each end of the head to adequately support larger springs.
What was needed was a spring and retainer that fit the stock pocket while developing adequate pressure for the hydraulic roller cam. Enter a Comp Cams conical spring and retainer combo originally developed for today’s LS engines with their small spring pockets and 8mm valve stems. Conical springs increase the valvetrain rpm limit, reduce resonance, and decrease dynamic spring oscillations. The result: longer spring life and the ability to run more aggressive cams. Comp offers a hybrid 7-degree lock that allows installing this spring and retainer on an old small-block’s 11/32-inch valve stems. On paper, the conicals are designed to generate the desired seat and open pressures at the usual small-block 1.800-inch installed height.
As it turns out, the new springs’ retainers are effectively thicker than the retainers used with conventional springs; with no other changes, the installed height ends up about 0.100-inch shorter, considerably raising spring pressures (table). Even so, Comp Cams’ Valvetrain Group Manager Billy Godbold maintains these higher than “normal” pressures are still OK with a hydraulic roller. With Hedekin’s 0.525-inch-lift cam, there’s still 0.050 inch remaining until coil bind. That’s a little tight according to old-school theory, but again, Godbold isn’t worried: “Due the conical spring’s unique design, it actually likes and performs best when installed close to coil-bind.”
Conical spring PN 7228-16 with Retainer PN 774-16
|Valve Length||Installed Height||Seat Pressure||Open Pressure||Coil-Bind Height||Distance to Coil-Bind|
|With 0.525-inch valve-lift (Hedekin’s existing cam)|
|Stock||1.700″||180 lbs||410 lbs||1.125”||0.050”|
|Plus 0.100″||1.800″||136 lbs||366 lbs||1.125”||0.150”|
|With 0.650-inch valve-lift (max safe valve lift supported)|
|Plus 0.100″||1.800″||136 lbs||421 lbs||1.12”5||0.025”|
If (as is the case on 492 heads) the pocket can’t be deepened and you’re uncomfortable with a shorter installed height, don’t want such high seat pressures, and/or have an even higher-lift cam, the first step is offset valve locks, which offer about 0.050 inch more clearance. The next step is 0.100-inch longer-than-stock valves. On a small-block Chevy, “plus-0.100” valves require about 0.100-inch longer pushrods to restore proper valvetrain geometry—assuming the existing pushrod lengths were correct in the first place. The cam’s base circle diameter, lifter height, head-gasket thickness, and the amount the block and heads were milled also influence pushrod length. Rollings recommends mocking up the motor and using a Manley pushrod checker to positively determine the proper length.
The Fix: Edelbrock Heads
The other solution—and the one Hedekin self-selected—was swapping on modern Edelbrock Performer RPM heads. Perfect for street performance, daily drivers, street rods, and muscle cars, the specific version installed on the Vette (PN 60895) has 64cc combustion chambers and straight spark-plug holes, and comes fully assembled with hydraulic-roller-cam-compatible springs. They bolted right on to the Vette’s existing short-block. The only glitch was due to the new heads’ slightly taller valve-cover rails: One of Hedekin’s existing Chevrolet Performance bow-tie valve covers had to be notched at the rear driver-side to clear the Vette’s power-brake vacuum booster. Hedekin also had Rollings replace his old Torker intake with an Edelbrock Air-Gap dual-plane unit (PN 7501).
The Fix: EZ-EFI
Oil consumption solved and new heads installed, the EZ-EFI was cleared and put into self-tune mode. Idle and part-throttle after warm-up quickly normalized. However, accelerating hard, the engine shut off under load. The fuel pressure was actually declining as rpm rose, when it should have gone up. This indicates either a fuel-pump or fuel-filter problem. Rollings found a clogged fine-mesh screen in a fuel filter poorly located between the fuel tank and fuel pump. As there already was a larger fuel filter installed after the pump, Rollings simply removed the redundant screen filter, then set the fuel pressure regulator to 55 psi. Fuel pressure issue rectified, the only additional manual tweak needed was dialing in a little more cold-start enrichment.
Don’t be quick to blame fuel injection because you’re not familiar with it. This wasn’t a fuel-injection issue; it was a mechanical issue.” — Norm Rollings
Don’t just bolt things together. You must be sure the valvespring is compatible with the cam, the retainer-to-valve guide and valve-seal clearance is right, and the spring’s installed height develops the recommended pressures without going into coil bind. Don’t use a fine-screen filter between the fuel tank and fuel pump. Make sure the engine internals, the electrical system, and the fuel-supply system are up to snuff before casting blame on the fuel injection.
|Parts and Prices|
|Does not include shipping fees, miscellaneous small hardware, standard sealants, or sales taxes. Labor is included only if cannot be performed by the average home mechanic. Priced 11/06/16 and subject to change. All dimensions in linear inches, except as noted.|
|Brand||Part Description||Part No.||Amt.||Price|
|A. Common Parts
These parts are needed with all options (B, C, and D) listed below.
|ARP||BOLT KIT, engine cylinder head, high-performance series, hex-head, most OE and Edelbrock 23° heads||134-3601||1||Summit||$86.39|
|SEALANT, thread, 1.69 fl-oz, PTFE w/ rust & corrosion inhibitors||100-9904||1||Summit||$10.28|
|Fel-Pro||GASKET SET, exhaust header, perforated steel core w/ antistick coating, 1.50 × 1.50 square ports (2/pkg.)||1404||1||RockAuto||$14.18|
|GASKET SET, intake manifold, composite w/ Printoseal, 1.28 × 2.09 port size, no exhaust crossover||1205||1||RockAuto||$14.15|
|GASKET, carburetor or throttle-body mounting, Carter AFB/Holley 4150 dual-pattern, open plenum, 1/32 thk||1914||1||RockAuto||$4.26|
|GASKET, cylinder head, steel-core laminate w/ preflattened steel wire, 4.190-bore × 0.041 thk., 9cc vol.||1004||2||RockAuto||$36.79|
|Gates||HOSE, windshield washer & vacuum, 7⁄32 id, bulk, sold per foot (cut-to-length)||27043||2 ft||O’Reilly||$1.38|
|Lucas||OIL, engine, hydrocarbon, Hot Rod and Classic Car, 20W-50, 5qt jug||10684-1||1||Summit||$35.97|
|O’Reilly||COOLANT, engine, ethylene glycol, concentrate, conventional “green”, 1-gal. jug||GAL||2||O’Reilly||$25.98|
|Prestone||HOSE, power steering return, ⅜ id × 2ft ol package (used by Rollings as fuel hose, cut-to-length)||PS0308||1||O’Reilly||$4.69|
|Summit||TAP SET, thread chasing & cleanup, cadmium-plated steel, UNC, inc. ¼-20, 5⁄16-18, ⅜-16, 7⁄16-14, ½-13, 9⁄16-12||900200||1||Summit||$15.97|
|Wix||FILTER, oil, 5.178 ol × 3.660 id, 1 3⁄16-16 mounting thread, 21-micron particle filtering, 270-psi burst||51060||1||RockAuto||$2.11|
|A. Common Parts Subtotal||$268.94|
|B. Stock Heads With Stock Valves
Also needed: All parts listed in section A (common parts), above.
|ACCEL||PLUG, spark, C-cut electrode, 14mm thread, 13⁄16 hex, 0.375 reach, gasket-seat, projected tip, resistor, short backshell (8/pkg.)||8197||1||Summit||$30.57|
|Comp Cams||ARBOR/PILOT, valve seat & guide cutter, steel, 11⁄32 valve stem||4732||1||Comp||$19.70|
|CUTTER, carbide-tip, cuts 0.530 valve guide||4726||1||Comp||$44.23|
|KEY PKG., engine valvespring, machined steel, single groove, 7°, 11⁄32 valve stem (32/pkg)||648-16||1||Comp||$23.19|
|RETAINER KIT, engine valvespring, steel, 7°, 0.640 id × 1.055 od||774-16||1||Summit||$53.97|
|SEAL PKG., valve stem oil, positive stop, Viton w/ metal body, 11⁄32 valve stem, 0.530 guide, 0.575od||518-16||1||Comp||$16.24|
|SPRING KIT, engine valve, single conical, 1.020/1.290 OD × 0.920 ID, 438 lb/in rate, 1.125 coil-bind height||7228-16||1||Comp||$253.92|
|Dura-Bond||SHIM PKG., engine valvespring, steel, 0.625 id × 1.250 od × 0.015 thk (100/pkg., 16 used)||1055||1||Summit||$16.97|
|B. Stock Heads and Valves Subtotal||$458.79|
|Total, Including Common Parts (A+B)||$747.73|
|C. Add Long Valves to Stock Heads
Also needed: All parts listed in sections A (common parts) and B (stock heads with stock valves), above.
|Comp Cams||ROD SET, engine valve push, Magnum, 0.080-wall, hardened chrome moly steel, 5⁄16 od × 7.400 ol*||7638-16||1||Summit||$104.00|
|Manley||GAUGE, pushrod length checker, Chevrolet, small-block, ⅜ rocker stud||42137||1||Summit||$16.97|
|SI Valves||VALVE, exhaust, Competition Series, stainless steel, 0.250 Stellite tip, 1 lock-groove, 2.020 head od, 0.3415od chromed stem undercut near head, 5.010 ol (+0.100 over stock)||1402SG||8||RPM||$86.16|
|VALVE, intake, Competition Series, stainless steel, 0.250 Stellite tip, 1 lock-groove, 1.600 head od, 0.3415od chromed stem undercut near head, 5.013 ol (+0.100 over stock)||1451SG+100||8||RPM||$86.16|
|C. Stock Heads with Long Valves Subtotal||$293.29|
|Total, Including common Parts & Stock Heads (A+B+C)||$1,041.02|
|D. Full Top-End Conversion
Also needed: All parts listed in section A (common parts), above.
|Champion||PLUG, spark, Copper Plus, 14mm thread, ⅝ hex, 0.750 reach, gasket-seat, projected tip, resistor||RC12YC||8||RockAuto||$10.48|
|Edelbrock||HEAD ASY., engine cylinder, Performer RPM, aluminum, 195/65cc port vol. 2.02/1.60 valves, 64cc chambers, straight plugs, 1.46-od springs for hydraulic-roller cam, 0.575 max lift||60895||2||Summit||$1,459.00|
|MANIFOLD, engine fuel intake, RPM Air-Gap, 1,500–6,500 rpm operating range, aluminum, satin finish||7501||1||Summit||$273.34|
|Mike Cox||LABOR & FABRICATION, notch valve cover for brake vacuum booster clearance||—||—||Rollings||$150.00|
|VHT||PAINT, Wrinkle Plus, black, 11oz aerosol can (used to retouch notched valve cover)||SP201||1||Summit||$12.97|
|*Nominal for small-block Chevy with conventional 23° heads, retrofit hydraulic-roller cam and lifters, and 0.100-inch longer-than-stock valves. Your combo may vary. Be sure and check!||D. Full Top-End Conversion Subtotal||$1,905.79|
|Total , Top-End conversion Plus Common Parts (A+D)||$2,194.73|
Accel, a Holley Performance Brand; Bowling Green, KY; 866.464.6553; Holley.com/brands/accel/
Automotive Racing Products (ARP); Ventura, CA; 800.826.3045 or 805.339.2200; ARP-Bolts.com
Comp Cams; Memphis, TN; 800.999.0853 or 901.795.2400; CompCams.com
Dura-Bond Bearing Co.; Carson City, NV; 775.883.8998; Dura-BondBearing.com
Edelbrock LLC; Torrance, CA; 800.416.8628 (tech) or 310.781.7222 (general); Edelbrock.com
Gates Corp.; Denver, CO; 303.744.5651; Gates.com
Lucas Oil Products Inc.; Corona, CA; 800.342.2512 or 951.270.0154; LucasOil.com
Manley Performance Products Inc.; Lakewood, NJ; 800.526.1362 or 732.905.3366; ManleyPerformance.com
Michael Cox Racing Development; Jurupa Valley, CA; 714.376.6113; Mwc247@gmail.com
O’Reilly Auto Parts; Springfield, MO; 888.327.7153 or 417.829.5727; OReillyAuto.com
Prestone Products Corp.; Danbury, CT; 888.269.0750; Prestone.com
RockAuto LLC; Madison, WI; 866.ROCKAUTO or 608.661.1376; RockAuto.com
Rollings Automotive Inc.; Mira Loma, CA; 951.361.3001; Plus.Google.com/+RollingsAutomotiveIncMiraLoma
RPM—Ron’s Precision Machine, Inc.; Santaquin, UT; 866.700.5877 or 801.754.5338; RpmRons.com
SI Valves; Simi Valley, CA; 800.564.8258 or 805.582.0085; SIvalves.com
Summit Racing Equipment; Akron, OH; 800.230.3030 (orders) or 330.630.0240 (tech); SummitRacing.com
VHT Paints, a Division of Dupli-Color, Inc., a Sherwin-Williams Co.; 800.247.3270; Cleveland, OH; VHTpaint.com
Wix Filters; Gastonia, NC; 704.869.3421 (customer service), 704.864.6748 (sales), or 800.949.6698 (USA, product information); WixFilters.com
NEED JUNK FIXED? If your car has a gremlin that just won’t quit, you could be chosen for Hot Rod to the Rescue. Email us at pitstop@HotRod.com and put “Rescue” in the subject line. Include a description of your problem, your location, a photo of the car, and a daytime phone number.
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