difference in 351C-2v heads

[Vinnie]

Hey folks,

Sorry if this has been asked a million times before but the search engine returns nothing for "difference 2v heads" and numerous websites I read didn't not address this.

I'm planning a rebuild of my little 73 351C-2V. The past week I've been reading a lot about all possible upgrades and modifications possible and it lead me to the compression in 351C-2V engines. This website shows compression was changed from 9.5 in 71 to 8.6 in 72/73: http://mustang-s.com/years/1973/1973_engine_351c.htm

It's unclear to me how they did this. Was the 2V head changed from closed chamber to open? (didn't think so) or did the pistons change in some way? Any cam changes?

What were the changes causing the lower compression in 72/73 and can they be reversed?

Thanks,

VIncent.

 

[73pony]

Heads the same. pistons went dished. You can get closed chamber 2v heads though. there were made in Australia for the 302c they had there. I run a set on mine and really like them.

 

[Vinnie]

I know the aussie heads but I'm keeping my original open 2V's. It's just that I can't get my head around a 76 decrease in HP (240 -> 164) just by different pistons. Surely putting back some flat or domed pistons alone is not magically going to bring those horses back?

 

[73pony]

There were other things like measuring Gross Vs Net HP as well as emissions and cam timing that resulted in the loss of HP.

 

[c9zx]

Here is an article that explains the change. https://ateupwithmotor.com/terms-technology-definitions/gross-versus-net-horsepower/

Changing to a flat top piston and getting the deck height near zero will restore enough compression to permit the use of a decent camshaft. The important thing to do is find a competent machine shop to do the machine work. Chuck

 

[Carolina_Mountain_Mustangs]

A cam is probably the biggest HP gain but I think compression would be #2. We use to build them with 15 to one compression. My friends Pontiac has 15 to one right now it is a monster. Has to have special fuel that is very expensive. The sound from a really high compression engine is totally different very crisp.

When I pulled the heads off my H code 73 351 I discovered he had bored the block .060" and had put flat top pistons in and still used the 2-V heads with a 4-V Edelbrock Performer intake to match the 2-V head ports. The car has a cam of what spec I do not know. It has a C-4 automatic with shift kit and higher stall converter. It runs ok but nothing like my Q code did. Would not do a burn out with stock tires without power braking it.

Pic of the pistons he used I do not know brand.

I run 90 octane non methanol fuel. So the flat top pistons will work fine with the 2-V heads.

I do not suggest you bore .060" over that is pushing a Cleveland for sure.

 

[Vinnie]

Thanks for all that info guys! I guess I’ll take the pre 72 ratings with a big pinch of salt! Nevertheless small changes were indeed made for changes in gasoline but it looks like they can be reversed with different pistons, shaving block/heads and a better cam. I’ll take all that in when planning my rebuild.

Cheers!

 

[Vinnie]

I’m trying to learn about compression by doing the math for different setups. I haven’t been able to find the numbers by year for pistons in the 2V as that is the only difference between 71 and 73 H codes?

Also I’m finding chamber volumes ranging from 72-78 cc.

Anybody have the numbers per year on the H-code?

 

[Don C]

From what I can find, the dish and valve cut-outs totaled 13cc for 1972. I haven't found any firm data on how they reduced the compression from 8.6:1 for 1972 to 8.0:1 for 1973, but I believe it was in increasing the piston to deck clearance by .015".

 

[Vinnie]

From what I can find, the dish and valve cut-outs totaled 13cc for 1972. I haven't found any firm data on how they reduced the compression from 8.6:1 for 1972 to 8.0:1 for 1973, but I believe it was in increasing the piston to deck clearance by .015".

Did you find that info online?

The longer I look the more it all starts to become useless. Most pages treat 71-73 H-codes as the same while the 71 version has significantly more HP.

According to this page in 73 it was the 4bbl CJ that has 8.0 comp and the H-code still had 8.6 (like in 72). But other pages suggest there wasn't even a CJ in 73...

Did the block's deck height ever change? or the piston's compression height?

Cheers,

V.

 

[c9zx]

This may help with doing planning calculations. The only way to know what you have is to measure deck height, piston compression distance, and chamber volume at disassembly. With the right piston and head gasket, decking the block, and milling the heads, you can easily reach 9.0+. You can use the Diamond piston calculator to see the effects of changing the various parameters, you may be surprised at what you see. Chuck

351C nominal deck height = 9.215"

351C nominal connecting rod length = 5.78"

351C nominal piston compression height = 1.65"

351C nominal deck clearance = 0.035"

Nominal combustion chamber volume 4V heads:

1970 D0AE heads = 63cc

1971 D1AE heads = 66cc

1971 D1ZE heads = 75cc

1972 D2ZE heads = 75cc

1973 D3ZE heads = 78cc

1974 D3ZE heads = 78cc

Installed spring height = 1.820

 

[Don C]

The Sealed Power catalog shows that the H code replacement pistons have a CD of 1.645 for the higher compression pistons and 1.630 for the lower compression pistons.

 

[Hemikiller]

70-72 all 351C - except the R Codes - all used the same flat top piston.

73-74 all 351C used the same dished piston

IMO, install a set of forged flat top pistons and have them zero decked. You will then have some decent compression and a solid foundation for future upgrades.

FWIW- don't get too hung up on the published data for the 351C. Horsepower ratings and other engine specs were often "fudged" to suit a particular slot in the lineup.

 

[Don C]

I agree, that 76.2cc combustion chamber was a design number, the produced volume is likely +/- 2cc.

Performance specs were always manipulated for various reasons, including insurance and marketing. Having 425HP displayed on the hood of a race car had appeal to a lot of us.

After emission regulations there was one more reason to play with numbers. It still goes on today, with even more reasons, like posted fuel consumption and corporate averge fuel economy standards.

 

[Don C]

I have an M code and a Q code dismantled in the workshop and went out and measured them, the CD is the same for both, and both have flat top pistons, so the compression reduction for the Q codes (at least 1972) is entirely in the heads. I couldn't find a separate listing for Q code pistons, even though the advertised compression ratio for 1973 is less than for 1972. The head volumes are shown as the same for '71 - '74.

 

[Studdley]

If you're rebuilding the heads, I'd go for single groove valves and locks for peace of mind. I've found them at a good price here and the guy who runs is very knowledgeable and cool. https://www.alexsparts.com/351-cleveland-drop-in-valve-springs-kit-580-lift-hyd-flat-tappet/ https://www.alexsparts.com/stainless-steel-valves-ford-351-cleveland-2-041-1-655-2v/

 

[Ron Tanzi]

The 1973 Clevelands had dished pistons combined with the same open chambers as the 72 and previous 2V heads had. This was how the compression dropped. The deck height was the same.

Ron

 

[Vinnie]

Thanks for all those numbers! It’s given me a decent understanding of the whole compression calculating thing.

I filled in a couple of “could be” numbers and got 9.1:1. This is with forged pistons from a fed mogul kit and .02 piston to deck height (so could be less still):

What do you think? Looks correct? (but not optimal perhaps?)

 

[Vinnie]

9.5:1