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Mustang Codes H M R Q Deciphering
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#1
Sourced From:

http://en.wikipedia.org/wiki/Ford_335_engine

351 Cleveland engines
Code Engine type Years Compression Notes
H 351C-2V 1970–1974 Low
M 351C-4V 1970–1971 High
R 351C-4V "Boss 351" 1971 High Rare, solid lifters
R 351C-4V HO 1972 Low Very rare, solid lifters, open chamber[1]
Q 351C-4V "Cobra-Jet" May 1971-1974 Low open chamber
====
Additional Info Sourced From:

http://www.mustangsandmore.com/ubb/Forum...00615.html

Originally posted by Hemikiller:

Here's some of the the facts, Jack.

1970, 351C-4V was introduced as an M-code. 300hp, closed chamber heads, 2 bolt main caps, cast flat top pistons for a 10.7:1 compression ratio, non adjustable hydraulic valvetrain, Autolite 4300 square bore carb, dual exhaust. All automatics received the FMX.

1971, M-code continued unchanged, but was rated at 285 for some reason. Probably to be second fiddle to the as-then planned 71 Mustang Boss 302 (290hp). The Boss 302 was dropped (thank god) in favor of the 330hp Boss 351 (R-code). Automatic M-codes received the C-6 trans standard, with a poorly suited torque converter.

R- code (Mustang only) Boss 351 engine was only available in the sportsroofed Boss 351 model, 4-bolt main block, specially selected crank, D1ZX rods of 1041 steel and 180,000 psi rod bolts, closed chamber heads with fully adjustable valvetrain with mechanical cam (290 duration, .477 lift), Boss 302 type aluminum valve covers with "Boss 351" emblem, dual point dizzy, forged pop-up (domed) pistons, larger (2.5" IIRC) exhaust
manifold outlets, aluminum spread bore intake with an Autolite 4300D carb, baffled oil pan with a crank scraper and a re-calibrated dipstick to show 6-quart capacity as full and ram-air, F60-15 tires, wide ratio 4-speed and 3.91 gears as standard.

351CJ (280hp) was introduced in May '71. It was always VIN code "Q", never M. 351CJ was a 4-bolt main block. Pistons were the same cast flat tops as the "M" code. Heads were identical to the M code --except-- that they used an open combustion chamber design like the 351C-2V, reducing compression to around 9:1. Cam was a CJ specific grind, with a split duration and lift (IIRC it was 280/290 duration, .480/.490 lift). 4-bolt main block, dual advance, dual point distributor was standard, oil capacity was 5 quarts, pan had a slosh plate over the sump. Intake was cast iron with a Ford style spreadbore pattern, Autolite 4300D carb. Exhaust manifold outlets were smaller than Boss at 2 1/4", but larger than M's 2" outlets. All CJ's equipped with an automatic got a C-6 with a special hi-stall torque converter (@ 2500rpms).

1972, M-code was gone. Q-code CJ continued essentially unchanged, but was now rated at 266 hp, due to the change from Gross horsepower ratings to SAE Net horsepower ratings. Only other change was to retard the cam 4 degrees for emissions purposes (this was apparently a corporate wide thing, all engines received this mod).

The Mustang-only 1972, R-code was back for a second year, but was now known as the 351HO. It was no longer a specific model, it was just a drivetrain option. Heads on the HO now had the open chamber design, cam was still mechanical, but now had 275 duration and .490 lift.

1973, R-code was gone, Q-code received dished pistons to drop the compression even further. Emissions equipment sprouted from the motor, EGR, spark timing etc. Horsepower was now rated at 264. At some time in the 73 model year, the CJ's received smaller valves in the heads. Valve sizes went from 2.19 I / 1.77" E to 2.05" I / 1.66" E, same as the 2V 351C.

1974, Q-code continued on in the Torino. By this time, it was so masculated by emissions equipment (EGR, TRS, etc) that I don't wanna got here.

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Comments
#2
Wolverine;64847 Wrote:1973, R-code was gone, Q-code received dished pistons to drop the compression even further. Emissions equipment sprouted from the motor, EGR, spark timing etc. Horsepower was now rated at 264. At some time in the 73 model year, the CJ's received smaller valves in the heads. Valve sizes went from 2.19 I / 1.77" E to 2.05" I / 1.66" E, same as the 2V 351C.
Just a note to clarify. I keep reading that it was late in the 73 run that the CJ heads got small valves. I have a pair of open chamber 4v heads with Ford marked 2.05" and 1.66" valves date coded 2H8 which if I am deciphering right would August 8, 1972. This tells me Ford was producing them early in the model year. However, I have a suspicion that the two different HP ratings for the 73 4v 351C (248 and 266) may be the result of that valve difference and that possibly, Ford did not put the small valve heads into Mustangs until late in the year. Perhaps they quit casting the large valve heads before the end of the year and had to use the small valve heads?
The heads I have came out of a 73 Pantera with 65k on the odometer, not a Mustang. The current owner of the car is pretty sure the motor is/was the original motor to the car.
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#3
BUMP - Good Engine (351 C) VIN code info

Ray

1971 Boss 351  
1972 Q code 4 speed convertible 
1971 Mustang Sportsroof  351-2V FMX 
1973 Mach 1 (parts car)
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#4
Yup, every once in a while that guy could be useful. Smile

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Mike

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