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H-Code Changes from 71 to 73?
#1
OK Don't quote me on the numbers but in 1971 the H code Mustangs were rated at 240 HP & 350 lb-ft torque then dropping all the way to 177 HP and 284 lb-ft in 1973. I understand it was emission laws and such that forced them reduce compression. How was this achieved? Are the heads different? Pistons? Also what other things were done that can possibly be reversed? I hope this is not a dumb question and someone can shed some light here.
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#2
of the top of my head:
cylinder heads went from closed chamber (higher compression), to open chamber (lower compression).
the cam was installed slightly retarded.
additional vacuum lines were needed for the EGR system which went into the cars in 1973. this required a Dual vacuum advance for the distributor and a dual vacuum modulator for the transmission this was because the EGR would cause feedback pressure and was screwing up the Shift points and the vacuum advance in the distributor, so they needed a second port going to full manifold vacuum on one side and ported on the other. there were also spark delays and heat sensors that kicked the vacuum system on when the engine was warmed up. cold it acted like a 1971 vacuum system, emissions testing was done with the engine warmed up. when cold the engine needed more fuel to prevent lean backfire. when the engine warmed up then all the emissions crap basically turned on.

to reverse the big changes, you need closed chamber heads, install the cam back to how it was in 1971, replace intake from early cars to ditch the EGR system.
and change over the dual vacuum diaphragms on the transmission and distributor to single type.
this gets you to a simplified vacuum system.

exhaust needs to be changed or welded up to ditch the EGR piping.

there are some other smaller items i'm sure i'm not remembering.

you can still do a lot of tuning just changing the intake manifold to aftermarket, going with an aftermarket carb and simplifying the vaccum system and ditching some vacuum and electrical sensors if the car is equipped with them, things like rpm limiters and such. i have open chamber V4 heads and i spin the tires on a 3:1 rear end ratio with A/c
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#3
Thanks. That has to be the best explanation I have received by a long shot. One question though, for some reason I thought the 2V heads were all open chamber? Thy must have gone to the open in 1972?
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#4
Compression ratio ranged from 9.5:1 in 1970 to 8.0:1 in 1973 and 1974 and all years ran on regular grade fuel. All H-code 351 Cleveland engines used the 2V heads with smaller ports and had open combustion chambers. These engines were produced from 1970 through 1974 and were used on a variety of Ford models, from pony car to full size.

-john
(jbojo)
351C 4V cc heads, 10.5 : 1 CR, 290 Herbert cam, Flat top forged pistons, forged connecting rods, Atomic efi,
C6 with Gear Vendor overdrive, 3.89 Tru Trac, Hooker Super Comp with 2 1/2" Pypes Exhaust.        

Some Mod pictures can be seen at:

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#5
OK so that is my question. If the 2v heads on the H code were all open combustion chambers regardless of year, what changed to bring the compression down from 9.5:1 to 8.0:1?
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#6
i think the deck height changed on the block from 71 to 73.
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#7
73pony;242504 Wrote:OK so that is my question. If the 2v heads on the H code were all open combustion chambers regardless of year, what changed to bring the compression down from 9.5:1 to 8.0:1?

Piston design. At the end they were using dished pistons. On my car I went to closed chamber heads and flat top pistons and went from 8.0:1 to 10.47:1 with these two changes.

-john
(jbojo)
351C 4V cc heads, 10.5 : 1 CR, 290 Herbert cam, Flat top forged pistons, forged connecting rods, Atomic efi,
C6 with Gear Vendor overdrive, 3.89 Tru Trac, Hooker Super Comp with 2 1/2" Pypes Exhaust.        

Some Mod pictures can be seen at:

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#8
Thanks for the info. The reason I am asking is I have a lifter starting to make some noise so I am trying to decide what route to go this winter when I dig into it.

Option I am looking at are....

1. Just having my 2v heads redone with a new cam and 4 bbl intake and carb.

2. Trying to find a set of 4v closed chamber heads to have built, new intake and exhaust manifolds and cam to match.

3. Aftermarket aluminum heads, intake, exhaust manifolds and cam. (I don't really want to go this route except for the benefit of weight reduction)

I am just looking to have a good street cruiser that cam beat a SBC, fun to drive. Not planning on racing or the drag strip but still want to have some fun and be able to smoke the tires if desired. I am running 2 1/2 dual exhaust with turbo mufflers and factory style H-pipe, pertronix III, FMX Trans, currently has a 2.75 open diff, will be changing this out to a 3.25 or 3.5 posi as I still want to be able to run on the highway. The car only has 69, 600 miles on it and runs great except for the new lifter ticking that just showed up in the last two weeks, just lacks in power.

Biggest problem around here is finding someone that works on fords and is not a chevy guy for the machine work. The two engine builders/machine shops I called didn't really give me warm and fuzzys when I told them I had a Cleveland.
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#9
Changing the rear to 3.50 will make a lot of difference for acceleration alone. The 2V heads are good heads and makes power in the lower rpm range (up to 5000rpm) while the 4V heads make the power at higher rpm's. With a gear change I think I would go with your option 1. Cam selection should be made with one that makes power in the lower rpm range. You don't want to pick a cam that starts making power at say 3000-3500rpms for example. Here is the 1973 cam specs for a 2V car:

Intake duration 258, opens 11*30' (BTC), closes 66*30' (ABC), lift .400
Exhaust duration 266, Opens 50*30' (BBC), closes 35*30' (ATC) lift .400

Most important thing to remember is to match the engine components and rear to operate in your desired rpm range. These are just my thoughts here.

-john
(jbojo)
351C 4V cc heads, 10.5 : 1 CR, 290 Herbert cam, Flat top forged pistons, forged connecting rods, Atomic efi,
C6 with Gear Vendor overdrive, 3.89 Tru Trac, Hooker Super Comp with 2 1/2" Pypes Exhaust.        

Some Mod pictures can be seen at:

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#10
As I understand it, 70 2v heads (70 H-code commonly rated at 250hp)did actually have a slightly smaller combustion chamber (by 2 or 3 cc's). 70-71 had flat top pistons and 72-73 had an 8cc dish. Not to mention a possible cam timing change beginning in 72 (a couple degrees retarded).
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