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LS7 Expose'

November 13 2011 at 11:10 PM

  (Login gpence)
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from IP address 75.38.22.73


Response to Clevor Build and Chevy Guys

Jim & all

I don't remember if I've ever explained just how close the LS7 cylinder head duplicates the 351C 4V head in this forum. You guys might find it entertaining or enlightening. There's no winning an argumenmt with a Chevy guy. But they consider the LS7 to be modern, state of the art & hi-tech ... so when you point out how close the 351C 4V head is to the LS7 head, they can't bad mouth the Cleveland any longer.

Performance Goals

The Chevy LS7 is a 427 cubic inch OHV motor making 505 BHP at 6200 rpm (72 bhp per liter) in street trim. The motor has 11.0:1 compression ratio, the hydraulic roller camshaft lifts the intake and exhaust valves 0.591".

The 351C 4V was a 351 cubic inch OHV motor originally designed to make 515 BHP at 7000 rpm (89 bhp per liter) in racing trim. The motor had 11.0:1 compression ratio, the D1ZX-FA racing camshaft lifted the intake valves 0.589" and lifted the exhaust valves 0.612".

The thing to recognize here is that the horsepower output, compression ratio, and valve lift of each OHV motor is very comparable. However, at peak horsepower (6200 rpm) the LS7 must move approximately 10% more air by volume than the 351C 4V race motor at peak horsepower (7000 rpm).
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Cylinder Heads

The LS7 cylinder heads are based upon the heads found on the C5 Corvette race motor, known as the C5R. Both the C5R and LS7 heads differ from other LS series cylinder heads by utilizing raised - rectangular shaped intake ports, larger diameter valves that will only fit in large diameter (at least 4.100") cylinder bores, reduced valve inclination angles and shallower combustion chambers made possible by the reduced valve angles.

The 351C 4V cylinder head was a cylinder head designed for racing having raised - rectangular shaped intake ports, larger diameter valves that would only fit in 4.00" diameter cylinder bores by giving the valves a "side cant", reduced valve inclination angles and shallower combustion chambers made possible by the reduced valve angles.

LS7 verses 351C 4V Cylinder Head Specifications

....................................................LS7 ...................351C 4V

Valve angles ................................ 12° ................... 9-1/2°
Intake valve diameter ................... 2.20" ................. 2.19"
Exhaust valve diameter ................. 1.61" ................. 1.71"
Average intake port cross-section ... 2.97 sq. inches .... 2.91 sq. inches

Look at the size of the LS7 intake ports in the picture below, they're pretty large wouldn't you say?

[linked image]

LS7 verses 351C 4V Cylinder Head Flow Data

.....................LS7 CNC ported.......................351C 4V Home Ported

Valve Lift ... Int Port Flow ... Exh Port Flow ... Int Port Flow ... Exh Port Flow
0.200" ........ 145 cfm .......... 120 cfm ........... 134 cfm ........ 112 cfm
0.300" ........ 222 cfm .......... 159 cfm ........... 198 cfm ........ 148 cfm
0.400" ........ 271 cfm .......... 192 cfm ........... 252 cfm ........ 177 cfm
0.500" ........ 315 cfm .......... 207 cfm ........... 290 cfm ........ 201 cfm
0.600" ........ 348 cfm .......... 219 cfm ........... 328 cfm ........ 219 cfm
0.700" ........ 352 cfm .......... 221 cfm ........... 352 cfm ........ 233 cfm


The LS7 heads are CNC ported by Chevrolet. The LS7 flow data I used is based upon actual flow bench measurements and not upon Chevrolets advertised flow data.

Now look at the LS7 intake port cut away in the next picture. The port does not make a straight line to the valve pocket, it makes a gentle right turn as it enters the valve pocket. This will set the air spinning in the valve pocket above the intake valve. This is identicle to the intake port design of the 351C 4V head, which gently turns left as it enters the valve pocket.

[linked image]

When Chevy engineers undertook the task of designing a 500 bhp head, with a clean sheet of paper, they designed a head with a 2.20" intake valve, 2.97 sq. inch average intake port cross-section, that flows 348 cfm at 0.600" intake valve lift and spins (swirls) the air/fuel mixture inside the valve pocket; i.e. the same general intake port parameters as the 351C 4V intake port. If indeed the LS7 cylinder head is a clean sheet design, then it validates the design of the 351C 4V head, a head often maligned by Chevy enthusiasts. However it is not difficult to imagine that somewhere in the lineage of the C5R/LS7 cylinder heads somebody borrowed a few ideas from the 351C 4V cylinder head. In either case, the LS7 cylinder head design is not modern or hi-tech, its technology can be found in the 30+ year old 351C 4V cylinder head.
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Camshaft

LS7 verses 351 Cobra Jet Camshaft Specifications

.....................................................LS7 .................. 1971 351 CJ

Duration at 0.050 ........................... 211°/230° ........ 206°/221°
Theoretical valve lift ....................... 0.591"/0.591" ... 0.481"/0.490"
Lobe separation angle .................... 120° ................ 117°
Intake lobe center line ................... 123° ATDC ........ 117° ATDC
Exhaust lobe center line ................. 117° BTDC ........ 117° BTDC
Intake valve closes (0.050" spec) .... 48.5° ABDC ....... 40° ABDC


Fords Cobra Jet camshaft is an old school cam design; it dates back to at least the 390 GT of 1966. The lobe centers are spread wide apart to reduce overlap thus providing good drivability in all weather, good vacuum, good fuel economy and low emissions. The intake lobe center line is relatively retarded thus closing the intake valve later. This enabled the 351C Cobra Jet to achieve peak horsepower at about 5800 rpm even though it was equipped with such a small cam (206° intake duration), and imbued the 351 Cobra Jet with a wide torque curve and good over-rev. The exhaust lobe has 15° more duration measured at 0.050" lift than the intake lobe which improves power output with the restrictive factory exhaust system, it opens the exhaust valve sooner and it imbues the motor with the performance of a larger camshaft (280° average advertised duration).

The LS7 camshaft specs compare very closely to those of Ford's Cobra Jet camshaft. Keep in mind the LS7 is a 427 cubic inch engine with a 4.00" stroke crankshaft that makes 500 BHP at 6200 rpm and red-lines at 7000 rpm. All that performance is accomplished with a camshaft having only 211° intake duration. The lobe center separation angle is a very wide 120°, and the intake lobe center line is quite retarded at 123° ATDC. The exhaust lobe has 19° more duration measured at 0.050" lift than the intake lobe. The LS7 roller tappet camshaft commendably lifts the valves a full 1/10th of an inch further off their seats than the Ford flat tappet camshaft, but its indisputable that Chevy's high tech LS7 motor employs camshaft lobe phasing and valve event timing similar to what Ford had used in the 1966 390GT, the 1968 428 Cobra Jet and the 1971 351 Cobra Jet. There's nothing that can be called new, modern, advanced or hi-tech to the LS7 camshaft timing at all.


-G
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Pantera Photos | 351C Historic Information | 351C Technical Information

If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!

 
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